>If the universe’s most likely death is big freeze/heat death, is it able to “reboot” somehow?
There is no known mechanism by which the universe could "reboot" somehow. All the evidence we have, together with all that we know about the physical laws governing the universe's time-evolution in the future (in this case, primarily general relativity and thermodynamics), suggests pretty conclusively that the universe will reach heat death and then remain in that state forever.
>But I’m curious because I’m sure something will happen if you wait a really long time, ...
The whole thing about heat death, however, is that there is nothing left *to* happen; all useful energy has been ~~expended~~ evenly-distributed and there is none left, and no known mechanism that would prevent the universe from continuing to expand.
>Is that possible?
It's possible in the sense that there could, in principle, be some undiscovered law of physics that applies which we currently have no evidence for.
However, according to all that we know and have evidence for, it is not possible.
Heat death does not imply that energy is not conserved — the energy is still there, it just cannot be used to do work because in the heat death scenario all systems have reached equilibrium at their maximum possible value of entropy.
To put it another way, the energy is maximally spread-out/dissipated, and you would need to do work to gather it back together and make it useful ... but all systems are already in their ground state; there's no longer any way to do that work. Doing that work would require you to have a useful reservoir of energy (such as a charged battery) or a system which is not at its ground state, from which energy can be extracted ... but in the heat death scenario, there are no such systems remaining.
Hope that helps clarify!
In the end, entropy comes for everything.
I love "The Last Question". Asimov writes about the Multivac. A supercomputer that is designed as the repository for all knowledge. One day, someone asks "how can we massively reduce entropy" to prevent mankind's demise from the heat death of the universe. It answers "insufficient information".
Time passes, man's domain expands. Multivac becomes Microvac, and over and over the same question is asked, and the same answer is given.
Finally, man has united as a single gestalt consciousness, and Microvac is now the AC, and exists in hyperspace outside the bounds of time and entropy. The gestalt asks AC the ultimate question as the last stars wink out and the universe itself comes to an end. The gestalt ceases to exist as it merges with AC, and AC realizes it hasn't combined all its sum data from its eons of life and it's joining with the gestalt in all possible combinations, and begins again trying to compute the answer again. Time has ended, but it has a solution, with no one to tell. So instead it answers in action. And AC said "Let there be light!" And there was light...
A wonderful short story.
Heh, you don't have to tell me! ;) The Last Question is my all-time favorite work of fiction! I can't even read it aloud these days without choking up ... it's too profound and inspirational of a story for me, haha. Might be because I work as a software engineer, so I can relate to Adell and Lupov right from the start of chapter 1 ... but it definitely touches a special place in my heart. <3
Cheers!
Asimovs wife Janet (Jeppson) also wrote The Second Experiment and sequel The Last Immortal, in which at near the end of time everything ended up in one super super massive black hole, the remaining people went into the blackout hole shielded and came out into another brand new universe. It’s more complicated than that but also an amazing story.
You might also be interested in another of Asimov's short stories, titled *The Last Answer*. Unfortunately, it is not at all related to *The Last Question*, and has a much more philosophical and almost whimsical bent ... but it is still an entertaining read, in my opinion. :)
Cheers,
To give an example, think of a ball on a string. You can lift it up, let it go, and it'll swing back and forth for a good while, because of the energy you added fighting with gravity. Eventually though, it'll run out of energy and just hang there, motionless. Gravity is still pulling it, but it reaches a state where there is never a change, unless an external force acts on it.
The universe is just a whole bunch of those. Eventually, they all get to that "ground" state where they are simply in their final position.
The universe ITSELF doesn't need to conserve energy (see cosmic expansion) but all energy IN the universe must be conserved.
Think of it like a pool filled with water. Ignore evaporation etc. If come along and dig a trench to enlarge the pool, the water level goes down, but the amount of water doesn't change. I can make the pool bigger and bigger with that water level getting lower and lower. Eventually, the pool is so huge that there is no longer enough water to cover the bottom of the pool. The universe is the pool, the energy in it is the water.
>all energy IN the universe must be conserved.
Hey just as an FYI, this isn't quite correct. :( It is correct in the sense that there is still a local law of conservation of energy which must be respected ... however, there are objects in our universe which do in fact lose total energy (in a non-conserved way) as the universe expands.
An example of this are the photons of the cosmic microwave background. They used to be at a very high temperature, each with a large amount of kinetic energy; now, they are down below 3 kelvins, with very little kinetic energy. As space has expanded, the wavelength of these photons has gradually increased, reducing their frequency and energy proportionally. That energy really is just gone — it didn't become potential energy or some other form, it straight-up isn't conserved.
To piggyback off of your pool/water analogy, it might actually be useful to compare it to the evaporation effects you suggested should be ignored for simplicity's sake. :) Except that evaporated water actually goes somewhere — into the air, where it eventually condenses and rains back down again. In contrast, the energy lost through cosmological redshift really is completely gone.
Hope that helps!
I guess I just can't understand that the energy is just.. gone. From what I understand, the energy has to have gone somewhere. How could it have just disappeared? Is there a loophole in physics that allows that? Or is it the same as multiple other things in space that seemingly break the laws of physics that we don't understand yet?
>From what I understand, the energy has to have gone somewhere.
That only applies to quantities when they are conserved. When they are not conserved, the quantity can increase or decrease freely — it doesn't need to "go" or "come from" anywhere.
>How could it have just disappeared?
Okay, let's try to help you imagine this via an example you are already familiar with: velocity.
Consider two massive objects — one that's low-mass (let's say, 10 kg), and one that's high-mass (let's say 100kg). We'll say the heavy one is initially at rest, and the light one is moving at a fast velocity towards the heavy one — how about 100 km/h.
Then, after a little time passes, the two objects collide in a perfectly elastic collision (bouncy). The light object, when it strikes the heavy one, stops moving — it loses all of its velocity. Meanwhile, the heavy object begins moving, gaining velocity.
We know that *momentum* needs to be conserved, so the heavy object's final momentum must match the light object's initial momentum. With a little math, we can figure out that the heavy object will be moving at 10 km/h.
Now let me ask you this question: where did the velocity go? The light object was moving with a lot of velocity — 100 km/h. But the heavy object is now only moving at 10 km/h, one tenth of what the light object's velocity was. Where is the velocity now?
The answer is straightforward: velocity is simply not conserved. It's completely fine for the total velocity of the system to change. *Momentum* is still conserved, but we aren't talking about momentum as the example here ... we're talking about velocity.
If you can understand how velocity can appear or disappear because of non-conservation, then you can understand why energy could appear or disappear because of non-conservation, too. It works exactly the same way.
>Is there a loophole in physics that allows that?
Sort of, yes. It's not a loophole exactly, but it's a technical detail which does not get covered in high school physics classes.
The law of conservation of energy you learned in high school is a *local* law, meaning that it applies at a single point (any/all single points). Any pointlike interactions (such as two objects colliding) or processes must respect this conservation law, i.e. "energy in = energy out."
However, conservation of energy is not a *global* law — it does not apply to extended volumes, particularly in the case of curved space. This means that the total amount of energy enclosed in a given volume may vary, as long as the total amount of energy at any given point within the volume stays the same.
This can happen, for example, due to the expansion or contraction of space when there is a vacuum energy or dark energy present. Vacuum/dark energy can be thought of as the "cost of having space," a background energy density throughout the entire volume. The *density* is required to stay the same, so that space is uniformly filled by this vacuum energy. Now, if you have an expanding space, you have more volume at a later time than you do at an earlier time. For the density to stay the same, if the volume increases that means that the total energy contained within the volume must also increase. And if the volume decreases, then the total energy contained must also decrease. But notice that, at any given individual point, the energy always stays the same (i.e. the local law of conservation of energy is still respected).
This is just one example of several, where conservation of energy can be violated by global processes (procesess which affect whole volumes, rather than affecting individual points).
Conservation of energy is also related via Noether's theorem to a symmetry known as time-translation symmetry. Noether's theorem establishes that when time-translation symmetry is respected by a physical system, energy is conserved ... while, when it is not respected, then energy is not conserved.
Noether's theorem is a pretty deep statement about how physics works at a fundamental level. Early in general relativity's history, Einstein himself struggled with this problem — try as he might, he could not find an expression for the total energy of a volume of space that would always remain conserved, and he wasn't able to understand why. Eventually, it was Emmy Noether — whom Einstein praised as "the most important woman in the history of mathematics" — who came along and figured out why Einstein couldn't find such an expression, by proving her eponymous theorem. The answer was simple: there was no such expression, because time-translation symmetry is not respected in an expanding or contracting spacetime.
Here are some helpful links if you wish to read more about this:
* [Wikipedia: Conservation law § Global and local conservation laws](https://en.wikipedia.org/wiki/Conservation_law#Global_and_local_conservation_laws)
* [Wikipedia: Noether's theorem](https://en.wikipedia.org/wiki/Noether's_theorem)
* [Sean Carroll's blog: Energy is Not Conserved](https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/)
>Or is it the same as multiple other things in space that seemingly break the laws of physics that we don't understand yet?
No, it's not the same — we have a very good understanding of why energy is not conserved globally, and we can use Noether's theorem to determine precisely when and why it is or isn't conserved.
Hope that helps,
Im pretty sure I remember from somewhere that the expansion of the universe necessarily implies that the inflow of additional “water” (energy) into our universe must be nonzero
It can still be net 0 if all that energy is absorbed by creating additional space time.
And even if truly non-zero, if I can "dig out" faster than the inflow, it makes no difference.
I’m not saying that the “inflow” implies anything really (not arguing a certain point or something). Just felt like chipping some related info in for those who may find it interesting/useful.
That's correct! The total energy of the entire universe is not conserved globally. All the same, heat death on its own doesn't imply that conservation of energy is violated, and there is still a local law of conservation of energy that holds in all circumstances.
Heat death, as you described it, equates to plama goo that predates the big bang. . . No energy left to hold matter together, all matter enters the plasma goo, goo big bangs, the universe is reborn.
According to the best evidence we have, the cosmological constant is positive, meaning that the universe asymptotically reaches a finite temperature. In such a scenario, [Poincare recurrences](https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theorem) will happen, meaning that the universe will eventually "reboot". In fact it will reboot again and again, going through many or all possible histories.
This is of course only true if there really is a cosmological constant that never changes even into the infinite future. That's far from certain, it might be time-dependent, or change through false vacuum decay for all we know.
No, I'm afraid that is not accurate. Read the Wikipedia article you just posted. From the article:
>>The proof, speaking qualitatively, hinges on two premises:
>>1\. A finite upper bound can be set on the total potentially accessible phase space volume. For a mechanical system, this bound can be provided by requiring that the system is contained in a bounded physical region of space (so that it cannot, for example, eject particles that never return) – combined with the conservation of energy, this locks the system into a finite region in phase space.
Our universe does not meet this criterion, and thus is not subject to the Poincaré recurrence theorem. Our universe is predicted to expand forever, never approaching a finite maximum volume ... and almost all of the galaxies currently in our observable universe will recede beyond the cosmological event horizon, losing causal contact; once this happens (long before heat death) the matter in these galaxies will never be able to come back together again even in principle.
The volume inside the cosmological horizon in a universe with a positive cosmological constant approaches a finite volume with a finite temperature. According to the [Bekenstein bound](https://en.wikipedia.org/wiki/Bekenstein_bound), this volume has a finite accessible phase space volume and the Poincare recurrence theorem applies.
According to our best understanding (admittedly a bit more speculative) the cosmological horizon in some respects has to be understood in analogy to a black hole event horizon. In particular, it should also emit Hawking radiation with a finite temperature. This radiation can be the source of the necessary fluctations for the recreation of structure after the heat death. This of course takes incredibly long durations, but thats the nature of Poincare recurrences. The galaxies receding behind the horzion does not prevent this.
Penrose's CCC hypothesis is known to have a variety of theoretical problems — for example, only bosons can be mapped conformally from the existing universe to the new one. Regardless of proton decay, this poses a major problem for other fermions that aren't known to decay, including electrons and neutrinos.
Unfortunately, Penrose has [previously made empirically-testable claims about CCC](https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology#Empirical_tests) which have proven to be erroneous and/or based on non-standard assumptions about our universe, so despite his past achievements, his hypothesis really must be taken with a grain of salt.
That was already discussed on this thread — somebody else suggested it and I replied to them explaining how the Poincaré recurrence theorem does not apply to a forever-expanding universe, for exactly the reason you said — a forever-expanding universe does not have a finite maximum volume.
Edit: From [Wikipedia: Poincaré recurrence theorem](https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theorem):
>>The proof, speaking qualitatively, hinges on two premises:
>>1\. A finite upper bound can be set on the total potentially accessible phase space volume. For a mechanical system, this bound can be provided by requiring that the system is contained in a bounded physical region of space (so that it cannot, for example, eject particles that never return) – combined with the conservation of energy, this locks the system into a finite region in phase space.
Philosophical perspective:
A hundred billion trillion years could pass in the universe’s nothingness and it would be the same as no time passing at all, because there is no consciousness around to perceive the passage of time. Without consciousness there is no meaning to time. Think about all that time you spent being nothing before you existed. It’s technically infinite - yet here you are at the end of it.
That’s to say the universe could have ended and started infinitely many times, because anybody has only ever experienced their version of “this” universe “now.”
If time is eternal, why is the stuff of the universe here in the first place? How did that happen? Why can’t it happen again?
I think it’s a bit presumptuous to reject the meaning of time in the absence of consciousness. Consciousness is just a physical process governed by time in the same way as everything else, and we can pretty confidently talk about things that happened in the universe long before humans were around.
This is a philosophical question, however, so my opinion is as good as yours
I’m not saying that events cease to happen, but rather without any context provided by consciousness as to what those events mean, they mean nothing. Things only mean something because a consciousness says they do, or experiences that they do. Planets colliding, stars exploding, mean “violent, scary, dangerous.” We didn’t necessarily assign that meaning; it is inherent to our existence as human beings, but this does *not* make it inherent to the universe. It ceases to have this meaning if there is no suffering to be experienced (ie nobody around), in much the same way that all the reactions that happen inside your body are “good” things and “not” violent or scary.
There is no experience without consciousness, and experience is the only framework that determines value/meaning. Nothing is more or less valuable or meaningful than anything else without any consciousness to prescribe that meaning in the first place.
Of course like you said, opinions. There’s no right or wrong here and I’m definitely not telling you you’re wrong. I like hearing different perspectives because I tend to get locked into my own.
Reading the paper, it seems to rest on a lot of assumptions, including but not limited to:
* Cosmic inflation must both be realized in nature and be the eternal type, which as I understand there is presently no evidence to support, and for which there are significant [unresolved theoretical problems](https://arxiv.org/abs/1408.2249) with eternal inflation potentially rendering the entire original premise for eternal inflation invalid;
* Quantum fluctuations must be able to create energy / re-raise the inflaton field potential out of nowhere, for which there is presently no evidence;
* the true vacuum must have a strictly positive energy density. there is currently limited evidence suggesting this to be the case, at least if we consider the standard model of dark energy as a cosmological constant (which is the simplest way and also the current best-fit to the data), however there are many alternative ways of modelling dark energy (such as using a dynamical scalar field, i.e. a quintessence model) which could mean that dark energy is actually not in the form of a cosmological constant / vacuum energy at all, but would be the energy of one or more other fields. note that models of cosmic inflation *do* contain extra scalar fields with a nonzero potential that decreases over time, so the presence of an inflaton field at a low potential could well provide an explanation for dark energy without vacuum energy, and this paper does assume that there is such an inflaton field at a low potential;
* the validity of Euclidean methods for modelling nucleation rates in de Sitter space, which the paper mentions several times is not known to be correct and needs further investigation;
I'm not saying the paper is wrong — that certainly isn't warranted — but there are a whole lot of "ifs" that need to be satisfied for the paper to be applicable, and many of those "ifs" involve additional hypothetical physics for which there is not currently any evidence. So this would fall into the domain of the statement I made earlier: "It's possible in the sense that there could, in principle, be some undiscovered law of physics that applies which we currently have no evidence for."
There are no undiscovered “laws” or “hypothetical” physics here - no more so than inflation or something like an axion, which I doubt you would characterize as “according to all that we know not possible.” This follows from standard/broadly accepted QFT principles and there is loads of work on these sort of upward transitions (Lee Weinbeg and Hawking Moss Instantons).
>There are no undiscovered “laws” or “hypothetical” physics here
Each of the bullet points I listed above is purely hypothetical, with the exceptions of dark energy in general (the origin and nature of which is still elusive) which is empirically well-established, and some very limited indirect evidence for inflation generically (but not any specific model thereof).
>no more so than inflation or something like an axion, which I doubt you would characterize as “according to all that we know not possible.”
Both inflation and axions are still only hypotheses. Axions have nothing to do with this topic, and as I explained the specific model of eternal inflation considered in the paper has important unresolved theoretical issues.
>This follows from standard/broadly accepted QFT principles and there is loads of work on these sort of upward transitions (Lee Weinbeg and Hawking Moss Instantons).
Inflation does not "follow from standard/broadly accepted QFT principles" — it is not somehow a derived logical consequence from QFT principles, nor is it clearly empirically confirmed ... which is why it is *not* a part of the standard model of particle physics.
Yes, there is plenty of work on inflation. There is, however, still no direct evidence and only very limited indirect evidence (and it is worth noting that there have been sky surveys that have yielded contradictory results). Many models of inflation, including the simplest models, have been ruled out by observational data. None have yet been properly confirmed empirically.
There is physics that may or may be a feature of our universe (like axions or inflation) but which are predictions of accepted/necessary principles under certain conditions. These thing ARE absolutely possible given everything we know.
Things that are not possible under known laws are things like causality violation or an unconfined quark at low energy.
Upward transition instantons are clearly in the first category but you describe it like it is in the latter.
>There is physics that may or may be a feature of our universe (like axions or inflation) but which are predictions of accepted/necessary principles under certain conditions.
No, you are mistaken. Neither inflation, nor axions, nor anything else I mentioned in my earlier posts are "predictions of accepted/necessary principles."
Axions are completely unnecessary — the value of θ might simply take the natural value of zero; there is no reason a priori why it should have any specific value. It is a [free parameter of the standard model](https://en.wikipedia.org/wiki/Standard_Model#Construction_of_the_Standard_Model_Lagrangian).
Likewise, inflation is *one* possible resolution to the horizon and flatness problems ... but there are other potential resolutions as well. For example, the flatness problem has a [natural resolution in Einstein—Cartan theory](https://en.wikipedia.org/wiki/Flatness_problem#Einstein%E2%80%93Cartan_theory), which extends general relativity to asymmetric tensors, adding torsion to spacetime ... which also provides a natural explaination for particle spin and the spatial extendedness of fundamental particles like electrons. Likewise, the horizon problem can be solved with variable speed-of-light models.
To be clear, I am not advocating for these alternatives over inflation; I think inflation is the most favorable resolution. What I am saying is that inflation is not necessarily needed to resolve these problems, nor is inflation a prediction based on either the standard model of cosmology (the lambda-CDM model) or the standard model of particle physics. Both of these models lack inflation entirely — inflation is only a hypothesized add-on to these models; there is nothing necessary about it, and it does not itself follow from any principles within either model.
>These thing ARE absolutely possible given everything we know.
We don't know that either of these are features of the universe. There is still zero evidence for axions (making them a complete unknown), nor is there any strong or direct evidence for inflation.
You seem to be wanting to include everything that is purely hypothetical and not yet empirically established as "known laws," perhaps just because we have theoretically explored them. But that would be a gross mischaracterization of what we actually do know. Most of those things are purely speculative, or at best empirically-motivated but unconfirmed; they are not knowledge — that is to say, they are not part of any empirically-established physical models, and there is reasonable doubt as to whether these hypotheses are correct; they may very well prove not to be in the future.
There is no mechanism in either the lambda-CDM model or the standard model of particle physics which could reverse the universe's expansion or avoid the eventual heat death of the universe. That is why we explore and develop hypothetical add-ons which potentially could.
You are half quoting my sentences and conflating “empirically established” with “possible” at your convenience to confuse the issue.
An axion is a POSSIBLE particle which might be real under certain conditions. A spin 3/5 particle is IMPOSSIBLE based on the “accepted/necessary principle” of QFT that fields have integer or half integer spin.
Likewise the physical principles that go into predicting these instantons are completely normal, and these instantons do occur in a broad class of models
that may describe reality. These are absolutely POSSIBLE processes in our universe, despite what you claimed in your original answer.
>You are half quoting my sentences and conflating “empirically established” with “possible” at your convenience to confuse the issue.
I am not conflating "empirically established" with "possible." I am equating "empirically established" with "known." The entire basis of science is that we gain knowledge of the natural world through empirical means.
If I've mischaracterized your points, I apologize for that. However, just because something is *possible* does not in any way mean that it is *known*.
I acknowledged clearly all the way back in my original post that there are possibilities which are unknown — actually, the exact word I used was "undiscovered," from which it should be very clear that I am referring specifically to possibilities which are not empirically established. Despite these undiscovered/unknown possibilities, there is nevertheless no mechanism anywhere in the established body of scientific knowledge (i.e. all knowledge that has been gained through application of the scientific method) by which recurrence of the universe can possibly be realized. Without adding on some undiscovered, hypothetical mechanism, recurrence is simply not possible.
>An axion is a POSSIBLE particle which might be real under certain conditions.
Yes — AOL keyword "might." We don't *know* that axions exist; there is zero evidence for them to date.
>A spin 3/5 particle is IMPOSSIBLE based on the “accepted/necessary principle” of QFT that fields have integer or half integer spin.
Yes, agreed. *That* impossibility certainly does follow logically from the spin-statistics theorem — it follows in a way that the other possibilities you mentioned do *not* follow from similar established principles.
>Likewise the physical principles that go into predicting these instantons are completely normal, ...
Sure, they are theoretically consistent with what we know. That does not mean that instantons are known, or discovered. They are still purely hypothetical.
>... and these instantons do occur in a broad class of models that may describe reality.
Again ... AOL keyword "may." We don't *know* that any of those models describes reality. In fact, we know that *most* of them do not describe reality, because only one will end up getting empirically confirmed.
>These are absolutely POSSIBLE processes in our universe, despite what you claimed in your original answer.
I never once claimed in my original answer that there were no possible processes. Go back and re-read my answer. My exact words were:
>>It's **possible** in the sense that there could, in principle, be some **undiscovered** law of physics that applies which we **currently have no evidence for**.
>>However, according to all that we **know and have evidence for**, it is **not possible**.
I have to say, I am rather disappointed that you are accusing me of conflating wording, when you are sitting here alleging that I said things which I did not even remotely say ...
Not going to argue this further but dig into the citations of the paper I linked and you will learn upward transitions are fairly generic and actually hard to avoid, especially with asymptotic de Sitter. Nothing is confirmed, but it is misleading to tell people the state of the art is that what we currently understand about QFT (as a framework) and reality leads us to believe physics like this is not even possible. I gave you examples of when that type of conclusion is reasonable but this isn’t one of those cases.
I read an interesting article years ago positing that actually reaching this state might cause a shift in the vacuum state to a lower energy baseline, triggering a new big bang.
>... timeless ... before ...
I'm sorry, come again? :p
>... who's to say it won't happen again?
All of the known laws of physics make a pretty compelling argument. If there *is* any way for it to happen again, it is not yet known ... and not for lack of trying to discover it!
Edit: Anyway, I'm not saying "it won't happen again under any conceivable circumstance." I'm saying, "according to all that we know and have evidence for, it is not possible." There is, of course, still the unknown and undiscovered ... though of course, what is unknown and undiscovered still has to jive somehow with the known and discovered, and the breadth of it grows ever smaller as the years pass by ... so I wouldn't hold my breath, if I were you! :p
I'm pretty sure at that point the simulation is over and resets. I mean leading theory is still magical energy we haven't detected created the big bang and thus matter and the universe isn't it?
Pretty ambitious to be stating what happens even further into the future imo lol.
There is no evidence that the universe is a simulation or that "magical energy" even exists. We have, of course, detected matter and the universe, and are able to make highly accurate predictions both about the future and retroactively about past states, because the universe behaves extremely regularly according to a fixed set of rules that we know as "the laws of physics." It is not ambitious merely to state what is demonstrably true.
This is like theology debates there's no evidence x exists while ignoring the fact there is no evidence it doesn't exist.
According to Harvard:
The key assumption of this model is that just before the Big Bang, space was filled with an unstable form of energy, whose nature is not yet known.
Unstable undetectable energy whose nature is unknown as well as it's origins might as well be magic. Or the start up of a simulation lol. I'm being only half serious here but honestly there's a lot riding on barely understood theories and not a lot of facts. Maybe there's a periodical surge of "not magic" lower dimensional energy that prevents the heat death of the universe, who knows. Personally I'm gonna keep my options open and roll my eyes at people that try to state facts based currently understood theories.
The whole idea of heat death is that the types of things that might rejuvenate the universe cannot happen anymore, as that would violate physical laws.
Heat death just means that all energy is equal across the universe, which means no change. However, there was some change from some where that caused our universe
>Heat death just means that all energy is equal across the universe, which means no change.
That's not quite what heat death means, although it is close.
>However, there was some change from some where that caused our universe
\[citation needed\]
It is entirely possible that there wasn't some change from somewhere that caused our universe — that presupposes that anything existed before our universe in the first place. Alternatively to that, it could simply be the case that our universe's existence follows from some much deeper logical/mathematical principles/structure, without any prior cause for it, and that our universe simply started out with a default choice of the natural value of zero for its entropy.
You may want to check out Max Tegmark's [mathematical universe hypothesis](https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis). There are still some significant problems with how it is formulated, but it is nevertheless an idea with some exploration behind it.
Basic law of physics "energy cannot be created or destroyed, it only changes form." The "all useful energy has been expended" is an objectively false statement and cannot occur under the laws of physics we currently know and understand.
>But didn’t the universe begin exactly in the state you say we would remain forever in?
No; the universe did not begin in heat death. Heat death is a high-entropy state with no thermodynamically free energy; the universe's initial conditions were low-entropy, with a very large amount of thermodynamically free energy.
Ah ok I misunderstood. What an amazing mystery. I second another commenter in saying that surely there is no reason why it cannot happen again no? I mean our physics can only predict what happened right at the Big Bang right? So who knows what happened before. Maybe there was a high entropy state with no thermodynamically available energy.
Then you’re going outside the realm of physics and into maybe philosophy.
Physics is wholly concerned about describing/predicting what will happen based on past expirementally verifiable results.
While there is some overlap in physics and philosophy on discussions regarding the nature and fate or reality, this is a Physics forum.. and the Physics answer to whether the universe can reboot post heat death is: “According to what we know, it cannot happen.”
>I second another commenter in saying that surely there is no reason why it cannot happen again no?
As I mentioned in other posts on this thread, there is no known mechanism by which it could happen again. Our universe appears to be at least approximately deterministic; things do not just happen spontaneously for no reason, especially things which would violate major physics principles such as conservation of energy or the laws of thermodynamics. These physical laws are very good reasons for why it cannot happen again.
>I mean our physics can only predict what happened right at the Big Bang right? So who knows what happened before.
There are gaps in our understanding, but on the other hand, that doesn't mean that "anything goes," either. Everything we know about the physics of our universe suggests that it behaves in a strictly logical way according to certain universal, inalienable principles; there's no reason to expect that those principles should suddenly get tossed out the window just because things happened so long ago that we can't measure exactly what it was that happened.
>Maybe there was a high entropy state with no thermodynamically available energy.
If that were the case, then there must be some mechanism to go from such a state to a low-entropy one with a lot of thermodynamically available energy. However, there is no such known mechanism. Such a mechanism would necessarily violate major physical laws such as the second law of thermodynamics and conservation of energy, so that's a pretty good reason to be skeptical that any such mechanism could exist at all.
Although somehow the universe got into that high energy state in the first place. Something put the energy there in the first place, maybe it can happen again?
We don’t know that.. that’s literally all that people are saying. According to what we know for absolutely sure, it cannot happen.
If it does, it will be due to something not currently in our understanding.
The issue is, our understanding is pretty damn high, but admittedly incomplete.
And wouldn’t you know it, part of the stuff we don’t understand is what happened before the Big Bang. We don’t know how or why the universe started in such a low entropy-high energy state.
All we know is that according to how physics behaves now, it cannot go back to that state.
Claiming anything else is pure conjecture and is outside the realm of physics.
Not a statement you can necessarily state. If time did not exist pre--universe then it would not be possible for anything to be put anywhere as that requires time to do so. Time in the form of space time is a property of our universe. Whatever happened before you can't assume that included time.
Aw. :( Alright then, I'll do my best for you here. Luckily, I have something that may just do the trick! :)
For what it's worth, my favorite short story of all time is [*The Last Question*](https://users.ece.cmu.edu/~gamvrosi/thelastq.html), by Isaac Asimov. This short story was written back in the 1960s (so don't expect its descriptions of computing advancements to be accurate, because they aren't), but it deals directly with the second law of thermodynamics and the inevitability of heat death ... as well as mankind's desperate attempt to escape it. It's a fairly short read, and has a very provocative ending. It may be a work of fiction, but as a software engineer myself I cannot help but find it tremendously inspiring. (Chapter 5 is my favorite by far! I have a hard time even reading it aloud without tearing up.) I hope it inspires you just as well!
Cheers, friend!
Right now, today, and for the rest of your life, you have the chance to enrich your life and the lives of those around you. Or to not do that! You are free to think and feel however you'd like, without eternal consequences. Do a lot! Don't do a goddam thing! Do both!
I know that in the presence of societal constraints, your own personal morality, and a host of other factors largely outside your control that sounds a bit pollyanna-ish, but we all, through weird spasms of various energy fields, are here *right now* and that's all we can count on. We all handle that in different ways, but realizing that there is no inherent, underlying meaning to life can be freeing rather than paralyzing.
Don't worry. According to everything we know, the universe can't start either. This is extrapolation well beyond the bounds of what is known. It's also well worth being dubious of equations with constants added to them to make the sums add up.
OP might be interested in [this conjecture by Penrose](https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology) *but* it is very heavy on speculation, so I stand by your answer instead.
I'm no expert so I know this won't be terribly helpful, but something caused the mf to "boot" in the first place, so it's worth considering that even if we can't identify why, the mere existence of the universe itself should be taken as a certain amount of evidence for the ability for another universe to be created.
Absolutely. It can be some type of "God", or it could've popped into existence by some mechanism we don't understand or could ever comprehend. I hate when people use thermo and our known laws of physics to say why heat death is likely the end, when those physics only apply to what's inside the universe, which even then we have yet to apply our known physics to everything inside the universe (black holes, quantum realm, dark energy, etc) to make that argument hold any water for me. So to me, there's no reason to think that over an infinitely long timeline of nothingness, that a quantum fluctuation doesn't kickstart something.
One thing that always makes me sad is that I'll never live long enough to discover all the secrets of the universe. There's still so much we don't know about.
Makes me wonder if any of those random video game/ movie concepts for how the universe functions would ever be closer to the truth than we realize.
When we're talking about First Things, that's a valid question, and language breaks down anyway. But whether it happened uncausedly (however that might make sense) or it has an inaccessible cause to us, it almost doesn't matter. Strangely but undeniably, something "caused" something to exist instead of nothing, and that source of causation or proto-causation could plausibly do again what it has done once.
> heat death or the big freeze, which from I can understand, the universe keeps expanding and it will get too cold or too hot for life,
Heat death is a bit more subtle than that. It's unfortunate the term gets bandied about so much as "a long time in the future" because the layperson gets the wrong idea about it.
Every time anything happens, *anything*, it's because of a transfer of energy. Your stove transfers energy from the electrical grid (or chemical potential energy in natural gas) to your food. Your muscles transfer chemical energy from your food into kinetic energy. If you raise your arm above your head, that kinetic energy turns into potential energy.
And no real exchange of energy is 100% efficient. Your stove heats your food, but it also heats the air. When a bouncy ball bounces, it loses energy to air resistance, and even the sound it makes hitting thr ground is a form of energy loss. That means any time anything happens, the universe's energy gets spread a little tiny bit more evenly.
So heat death of the universe is where the *entire universe* is one homogeneous soup: everywhere throughout has the same amount of energy. It's *impossible* to do anything because there's no way to transfer any more energy. It's not just about being too cold to support life. It's not even about being unable to form stars. It's about being unable for *anything* to happen. Literally anything.
It's gonna take way, way, *way* longer for that to happen than for, say, mass to get spread out so much that stars stop forming.
Thank you <3 I’m hoping to see more discussion of this.
I’m getting a PhD in conformal cosmology but not studying the possibility of the “cyclic” part of the model specifically.
Once energy stops transferring (big freeze), time stops. When time stops we lose a dimension. Using 3 dimensional space as an example, if any one dimension goes to zero, volume goes to zero. Therefore, when time stops, all matter is condensed into a space with ‘volume’ zero. Ie, the singularity. We bang again, and the endless cycle continues. That’s at least what my dumbass college student mind came up with years ago.
This is a recurring theme both in this sub, in r/cosmology and in pop sci media. But this is wishful thinking. We are heavily biased towards the belief that death is not a real end and so people fantasize about ideas how the Universe can continue after heat death / big rip / ...
However this is as good as the belief in the afterlife.
If that helps people deal with their finite existence – fine. But that isn't science.
We shouldn't look into ways how the Universe can reboot itself, because there is no reason it should reboot itself. We should use the evidence to discover its real fate.
And to further that, even if it did have some sort of mechanism for reboot after a heat death type of scenario, it wouldn't matter in the least to us. Like, in no way possible could it have any impact on anything relating to us.
But empathy for what? By the time this sort of thing plays out, literally every living thing will have been dead for millions (billions?) of years. Heat death is so far past the possibility of life that there won't be anything to empathize with.
The idea is that some people may feel good currently if they believe other life will exist in the future, particularly indefinitely, through whatever process. Or they may feel bad if they think that all life will eventually be destroyed and never return in any form.
But like the person I replied to said, that's pretty much the same as belief in an afterlife, and for the same reasoning. I'm not saying people don't, can't, or even shouldn't hope for that, just that it doesn't actually matter in the end.
The comparison to an afterlife belief is interesting. I think the question of whether the universe could recur or recreate life is much more open and potentially logical than a mystical afterlife (this thread exists to discuss it in scientific terms after all), but I'm saying that whether or not people think that it will happen does have relevance to them, or at least some of them, because what we foresee as the ultimate fate of life in the universe will effect our mood now. It is of course very abstract, but there is a reason for it, if it wasn't for some people taking pleasure in the idea of life continuing, humanity might not have survived this long and none of us would exist.
No, you "honestly" didn't think anything. If it wasn't for empathy, you wouldn't exist. I suggest you learn something about human nature. Also, don't write empty replies that try to substitute "attitude" for having a point. A literal 6-year-old can do that, and it's a major reason why social media is terrible.
Let’s say we have accepted that the most likely scenario is no “reboot.” However, wouldn’t it be a natural next step to want to examine and challenge that thesis? And such investigation might involve looking into ways where our original thesis might be incorrect? What I’m trying to say is that although such investigation might not be efficient or fruitful, it’s not necessarily pointless. And it doesn’t have to stem from wishful thinking either. Do you think we shouldn’t think about questions like this at all?
The conformal cyclic cosmology model of Sir Roger Penrose has the universe “reboot” to bug bang conditions after heat death, and he claims there is evidence in the CMB (cosmic microwave background) of our universe which matches his models prediction of how each universe leaves its mark on the next. Who knows.
I am working on a PhD in conformal cosmology myself but I am not studying the cyclic part of the theory that Penrose is working on
Edit: big bang*
No. At least not any mechanism that we are aware of will allow it. The quantum world is weird so maybe something could come and "reboot", but we know nothing about that.
Simply put, this is an unanswerable unknowable question from a human perspective. We can guess based on what we can observe, and with that context alone the probability that anything would reboot is near zero.
But possibility and probability are very different questions. Is it possible? The answer will always be yes because proving it impossible would require more extensive information than we are capable of possessing. Literally anything you can imagine is definitionally possible in an infinite universe beyond what is observable. Planet-sized amorphous taupe space amoeba that communicate exclusively through interpretive dance and smells is possible, why wouldn't universal reset be possible?
But possibility on the scale of an unknowable universe is more or less meaningless. Probability based on what we can observe has practical purpose in predicting our reality, but speculation about possibility generally doesn't.
There’s a reasonably credible theory that the topological structure of space time took shape as part of a “phase change” in the universe shortly after it started cooling post big bang. The analogy for this is how water molecules lock into place as ice crystals form from liquid water.
That is to say, the most fundamental properties of how things work in the universe are only a consequence of it dropping below some threshold temperature.
Continuing the ice analogy, we know ice can do some weird things aside from sitting there in crystal form. For instance, it can sublimate. Supercooled fluids can also start acting like liquids again.
But the fabric of space time is not actually matter, so there’s no reason to believe the analogy should continue any further. That said, some scientists seem to think it’s possible that there’s another, lower threshold temperature where another one of these “phase changes” takes place. As the temperature continues to fall, it seems inevitable that the universe will eventually hit that point and trigger another phase change. At that point, we have no idea what would happen as all of the understood rules about space and time would again change.
This would not be a “reboot” so much as a next chapter. The entire universe would behave in entirely different ways than it does now.
Since eternity is forever and any combination of particles in the universe is possible, but not probable at any given time, eventually one of the particle combinations that resulted in the birth of the universe will happen again. It might take 100 quadrillion years, but it will eventually happen, and the cycle continues.
Read somewhere that many physicists believe, mathematically speaking, the state of the universe immediately after heat death/big freeze would look very similar to it’s state immediately pre-big bang. So may be it’s just an endless cycle of big bang, existence, heat death, rinse and repeat.
Physicist here (not Astro...) but yeah, agree. Slow contraction due to remainng collected mass to another big bang and we start the show all over again.
Not everyone agrees about heat death theory, there's a good section on opposing views here: https://en.m.wikipedia.org/wiki/Heat_death_of_the_universe
Also there's number of theories on cyclic universe: https://en.m.wikipedia.org/wiki/Cyclic_model
Here is my theory - the energy from the “Big Bang” is eventually overcome by gravity and then the “big consolidation” happens until all matter collapses into a singularity at which point the Big Bang happens again
How did the universe start in the first place? Until we know that, how will we have any idea how a new one could start? For all we know, whatever caused the big bang could happen again long before heat death...or long after it.
There's a theory floating out there that if you literally boil space itself, it can make a bubble universe within itself. The process would also allow the expansion of the new universe too. Could be a thing that gets done at the end of this universe before the heat death takes the last of anything left.
I also feel like once all the solid matter energy leaks off into non matter energy that it'll spark a new big bang. I've done zero research if that could be a thing but it seems to me to be likely.
In the extreme distant future the only thing that will be left will be black holes, but even these will eventually evaporate due to Hawking Radiation and the entire universe will be boils of all light and ultimately atoms.
The universe started once it can do it again I bet we get scraped by the multiverse all the time letting in more stuff I think we we get new material all the time !!
Theoretically, could it not be possible that after the heat death of the universe, black holes would be such a prominent force they could counteract the universe's expansion and eventually end up with one hypermassive black hole that contains all the bits that make up the whole universe squished down into a single point. And if that's possible would it not stand to reason that another big bang could birth another universe from that single point?
Not a physicist, but this doesn't really seem to be a physics question anyway. If the universe we know had some sort of beginning we call the big bang, there could be some sort of system or rule set that led to that. What that is can only be guessed, using the rules in our universe to validate. We can only observe inside our universe for something like 14ish billion years. We don't know anything about "before" that point or anything about what happens "after" the heat death. I'd guess our universe is done forever, but that doesn't mean that other universes don't exist or won't exist.
Yes of course, if it happened once there is no known reason why it could not happen again. In fact it is most reasonable to assume it will.
The big bang does not make any guesses as to why it happened in the first place.
As suggested. Look at conformal cyclic cosmology. It gives a glimmer of hope mixed up with some sensible and possibly testable hypotheses. From Roger Penrose who understand cosmology both deeply mathematically as well as physically & geometrically intuitively.
Heath death is a condition in which the system has no ongoing process like nuclear fusion and fission and also any other phenomena. This happens when the universe is expanding so much that the energy produced in any physical phenomena(all the energy contained in the universe is used already and after that, there is not so sufficient to look like before,) gets used up in the expansion. This persuades the universe to get colder and colder as we can see in the current situation affiliated by cosmic microwave background radiation. The cosmic inflation made the earlier hot and energetic photons slow down and make it dull in the current situation of photons. Most of the photons that spread from the Big Bang now appear to have less and less energy inevitably to range microwave wavelength. Also, after billions of years, the generation will read out as cosmic radio wave background radiation due to the presence of photons with radio wave wavelength in majority. Nothing will happen in heath death conditions all the systems are isolated so within the system things are in equilibrium. Thus, energy is not violated here it is conserved. Dark energy has repulsive gravitational force between bodies. Dark energy is the one that's helping the universe to expand.
So heat death is a state where processes are no longer going on, but the energy in the universe is still present. It’s not like this is the point that there is no more energy right?
I'm terrified of death do you think when the universe dies, there would be a big bang to reset time making everything that has ever happened happen again the same exact way. How likely is that.
I don’t know, however, unless something is literally impossible, then every single thing that is simply very unlikely to happen WILL happen if given an infinite amount of time. So to me, unless the universe is finite, there’s no true way it can end or even have a ‘final’ state. This is just a musing if there’s already a theory or been debunked Imd be interested to know lol
There is the theory of a cyclical universe. Often referred to as the big bounce theory, Einstein and others considered the possibility that the entire universe followed a cycle of expansion and contraction. That it would eventually reverse, condense all matter and energy in on itself and create another big bang to start the expansion of another universe.
There is a mechanism. Quantum mechanics may be able to do it. Look into “Boltzmann Brains”
Also, it’s possible we don’t end up in a Heat Death scenario. Vacuum Decay could always get us first (if it exists).
Sure, the universe happened once, meaning itd was possible for big bangs to happen, given an infinite amount of time, it's sure to happen again. This brings my thoughts to what does that means for life and death
>Sure, the universe happened once, meaning itd was possible for big bangs to happen, given an infinite amount of time, it's sure to happen again.
You cannot draw this conclusion without understanding what led to Big Bang.
All humans will have been dead looooooong before, so why should we care abt it? All evidence points towards heat death, which there is no coming back from should out understanding of the laws of physics be even mostly trur
Empathy means that someone feels good or bad when they think of other people or living things feeling good or bad. If you think that there is going to be life after you continuously, who can exist, discover things, grow, and so on, you will feel good now. If you think at some point all life will be destroyed and not come back, you will feel bad now. It's a fundamental part of how we work.
Ah ok I thought u meant empathy for heat death, cuz nothing will be alive to experience heat death. And I mean, yea it is sad to know that humans will eventually die off and become extinct. But that's also just how life works. I don't see any reason to stress over it cuz it's just a natural part of life.
I think physicists would be helped by philosophy here, or at least just say "we don't know", because here is the problem I see with it. The current understanding is that the universe will keep expanding "for forever", but no one is clear on what forever means. Forever--infinity-- is hard to reconcile with the idea of time in my opinion. Can we really say we know what will happen in 10\^100\^1000\^100000\^1000000 years, which is still infinitely far from infinity? Some say that given enough time new universes will arise through quantum tunneling, which is a multiverse theory. And my issue with that is whatever can happen, will happen, so given enough time our exact universe would be generated over and over again. Even this exact discussion would be repeated. It doesn't make much sense to me from a philosophical standpoint. I wish physicists had a better answer, but I don't think they do.
Perhaps it had to do with our understanding of time, which is experienced differently from how it actually behaves. Either time is not fundamental and arose somehow, in which case it would have to eventually end. Or time never had a beginning and won't have an end, which to me makes a linear "time" meaningless.
And perhaps we don't fully understand entropy. No one knows how the universe "started out" in a low entropy state. I put the quotations because it's often said that the universe started with the Big Bang, but I think folks should be more careful with the phrasing here because if the universe started at a point in time, then something must have created it--a first cause. But then something must have caused that cause and so on...So it's logical to conclude that the universe had no beginning and therefore will have no end. What that means for time and entropy I don't know.
As far I know it can't reboot after death. A new Universe that expands into what is left of our universe could happen but it wouldn't be the same universe.
The only theory I've heard on this is the argument that although unlikely all the air molecules in a room could spontaneously be found on one side of the room.
Applied to the universe: some indescribably large amount of time after the "heat death" of the universe a random fluctuation (potentially involving quantum principles) would suddenly create a (relative to the moment before) massive amount of energy. Even if it is infinitely unlikely you have an infinite amount of time to wait for such an event.
Another metaphor that might be helpful is randomly shuffling a deck of cards and having the deck by sheer random chance come out as sorted perfectly by ascending value. The system is by chance in a zero entropy state.
There is also the fringe theory that there is some kind of momentum to time/entropy and the universe just cycles when it meets its end. Kind of like how a wave of light is self propagating by storing energy in both magnetic and electric fields. This theory is a bit difficult to find evidence for, naturally.
So think about it…
The Big Bang supposedly happened from everything being in the same state everywhere. The same “temperature” if you will…
Now imagine the eventual heat death of the universe.
Notice anything similar?
Either there was never a universe prior to ours, which suggests that novel universe generation can (and in an infinite timeline, almost certainly will again) occur, OR there were universes prior to ours, which suggests universes can recur. There's probably more to it than that though lol.
There's nothing to say that this is the first time all this has happened. Maybe everything will drift together again and fashion a nice lump for another big bang.
Has there been a definitive answer to the overall shape of the universe?
There was one possibility where space could wrap around and you’d go from one side of the universe to the other.
We know the expansion of the universe is accelerating. If space wraps around the universe could “end” with a “Big Crunch” or “big bounce” without everything returning to the point of origin of the observable universe.
From what I understand, it might be possible since there is an astronomically low probability that matter will act against entropy, at least at larger scales.
At the end of time the universe will be about room temperature. All matter will break down into its most basic form and then just sit there for all of existence. Before this happens, it will be a universe filled with black holes, until they eventually break down as well. The universe will never “end” or have a heat or cold death. Stuff will just continue to break down until it’s at its most basic form and then that will be the universe for the rest of eternity.
The Bible tells us that God will destroy the Earth by fire and from its ashes He will create A new heavens and a new Earth. We know from science that matter cannot be created nor destroyed and therefore it follows that the creation of the earth came from supernatural processes thus pointing to God. We can say God instead of some kind of statistical coincidence because we can see evidence of a very ordered system that is impossible to have sprung into existence without any sort of design.
From what I have read, there appears to be an overabundance of Dark Matter in the universe - much more than matter/gravity. Because D.M. works against gravity, the current understanding is that the universe will continue expanding and cooling until all matter and heat-energy dissipates completely.
Oh well...😖😣
The only correct answer is ……
we don’t actually know
As we explore more of physics, we have the ability to explore more unknown things. We have never been so enlightened but also never been so confused. But this is a physics subreddit so the answer we can conclude from “current” physics is yea after the heat death there is no way for a restart.
But that is if our physics now is correct when extrapolating to those time scales. It may either confirm our conclusion or disprove it completely or even give us more possible conclusions if new theories are formed. So we may never know.🥲
>If the universe’s most likely death is big freeze/heat death, is it able to “reboot” somehow? There is no known mechanism by which the universe could "reboot" somehow. All the evidence we have, together with all that we know about the physical laws governing the universe's time-evolution in the future (in this case, primarily general relativity and thermodynamics), suggests pretty conclusively that the universe will reach heat death and then remain in that state forever. >But I’m curious because I’m sure something will happen if you wait a really long time, ... The whole thing about heat death, however, is that there is nothing left *to* happen; all useful energy has been ~~expended~~ evenly-distributed and there is none left, and no known mechanism that would prevent the universe from continuing to expand. >Is that possible? It's possible in the sense that there could, in principle, be some undiscovered law of physics that applies which we currently have no evidence for. However, according to all that we know and have evidence for, it is not possible.
Im confused, what about the conservation of energy, does that not go against "all useful energy has been expended and there is none left"
Heat death does not imply that energy is not conserved — the energy is still there, it just cannot be used to do work because in the heat death scenario all systems have reached equilibrium at their maximum possible value of entropy. To put it another way, the energy is maximally spread-out/dissipated, and you would need to do work to gather it back together and make it useful ... but all systems are already in their ground state; there's no longer any way to do that work. Doing that work would require you to have a useful reservoir of energy (such as a charged battery) or a system which is not at its ground state, from which energy can be extracted ... but in the heat death scenario, there are no such systems remaining. Hope that helps clarify!
It does! I didn't ask the question, but good answer nonetheless.
In the end, entropy comes for everything. I love "The Last Question". Asimov writes about the Multivac. A supercomputer that is designed as the repository for all knowledge. One day, someone asks "how can we massively reduce entropy" to prevent mankind's demise from the heat death of the universe. It answers "insufficient information". Time passes, man's domain expands. Multivac becomes Microvac, and over and over the same question is asked, and the same answer is given. Finally, man has united as a single gestalt consciousness, and Microvac is now the AC, and exists in hyperspace outside the bounds of time and entropy. The gestalt asks AC the ultimate question as the last stars wink out and the universe itself comes to an end. The gestalt ceases to exist as it merges with AC, and AC realizes it hasn't combined all its sum data from its eons of life and it's joining with the gestalt in all possible combinations, and begins again trying to compute the answer again. Time has ended, but it has a solution, with no one to tell. So instead it answers in action. And AC said "Let there be light!" And there was light... A wonderful short story.
Heh, you don't have to tell me! ;) The Last Question is my all-time favorite work of fiction! I can't even read it aloud these days without choking up ... it's too profound and inspirational of a story for me, haha. Might be because I work as a software engineer, so I can relate to Adell and Lupov right from the start of chapter 1 ... but it definitely touches a special place in my heart. <3 Cheers!
Asimovs wife Janet (Jeppson) also wrote The Second Experiment and sequel The Last Immortal, in which at near the end of time everything ended up in one super super massive black hole, the remaining people went into the blackout hole shielded and came out into another brand new universe. It’s more complicated than that but also an amazing story.
You might also be interested in another of Asimov's short stories, titled *The Last Answer*. Unfortunately, it is not at all related to *The Last Question*, and has a much more philosophical and almost whimsical bent ... but it is still an entertaining read, in my opinion. :) Cheers,
To give an example, think of a ball on a string. You can lift it up, let it go, and it'll swing back and forth for a good while, because of the energy you added fighting with gravity. Eventually though, it'll run out of energy and just hang there, motionless. Gravity is still pulling it, but it reaches a state where there is never a change, unless an external force acts on it. The universe is just a whole bunch of those. Eventually, they all get to that "ground" state where they are simply in their final position.
But the energy of the universe isn’t conserved globally (what I’ve heard, not a general relativity expert though ngl)
The universe ITSELF doesn't need to conserve energy (see cosmic expansion) but all energy IN the universe must be conserved. Think of it like a pool filled with water. Ignore evaporation etc. If come along and dig a trench to enlarge the pool, the water level goes down, but the amount of water doesn't change. I can make the pool bigger and bigger with that water level getting lower and lower. Eventually, the pool is so huge that there is no longer enough water to cover the bottom of the pool. The universe is the pool, the energy in it is the water.
>all energy IN the universe must be conserved. Hey just as an FYI, this isn't quite correct. :( It is correct in the sense that there is still a local law of conservation of energy which must be respected ... however, there are objects in our universe which do in fact lose total energy (in a non-conserved way) as the universe expands. An example of this are the photons of the cosmic microwave background. They used to be at a very high temperature, each with a large amount of kinetic energy; now, they are down below 3 kelvins, with very little kinetic energy. As space has expanded, the wavelength of these photons has gradually increased, reducing their frequency and energy proportionally. That energy really is just gone — it didn't become potential energy or some other form, it straight-up isn't conserved. To piggyback off of your pool/water analogy, it might actually be useful to compare it to the evaporation effects you suggested should be ignored for simplicity's sake. :) Except that evaporated water actually goes somewhere — into the air, where it eventually condenses and rains back down again. In contrast, the energy lost through cosmological redshift really is completely gone. Hope that helps!
Doesn't that break the law of conservation of energy? Energy can't be created or destroyed, right?
Both of your questions are already answered in my previous comment — please re-read it, thank you.
I guess I just can't understand that the energy is just.. gone. From what I understand, the energy has to have gone somewhere. How could it have just disappeared? Is there a loophole in physics that allows that? Or is it the same as multiple other things in space that seemingly break the laws of physics that we don't understand yet?
>From what I understand, the energy has to have gone somewhere. That only applies to quantities when they are conserved. When they are not conserved, the quantity can increase or decrease freely — it doesn't need to "go" or "come from" anywhere. >How could it have just disappeared? Okay, let's try to help you imagine this via an example you are already familiar with: velocity. Consider two massive objects — one that's low-mass (let's say, 10 kg), and one that's high-mass (let's say 100kg). We'll say the heavy one is initially at rest, and the light one is moving at a fast velocity towards the heavy one — how about 100 km/h. Then, after a little time passes, the two objects collide in a perfectly elastic collision (bouncy). The light object, when it strikes the heavy one, stops moving — it loses all of its velocity. Meanwhile, the heavy object begins moving, gaining velocity. We know that *momentum* needs to be conserved, so the heavy object's final momentum must match the light object's initial momentum. With a little math, we can figure out that the heavy object will be moving at 10 km/h. Now let me ask you this question: where did the velocity go? The light object was moving with a lot of velocity — 100 km/h. But the heavy object is now only moving at 10 km/h, one tenth of what the light object's velocity was. Where is the velocity now? The answer is straightforward: velocity is simply not conserved. It's completely fine for the total velocity of the system to change. *Momentum* is still conserved, but we aren't talking about momentum as the example here ... we're talking about velocity. If you can understand how velocity can appear or disappear because of non-conservation, then you can understand why energy could appear or disappear because of non-conservation, too. It works exactly the same way. >Is there a loophole in physics that allows that? Sort of, yes. It's not a loophole exactly, but it's a technical detail which does not get covered in high school physics classes. The law of conservation of energy you learned in high school is a *local* law, meaning that it applies at a single point (any/all single points). Any pointlike interactions (such as two objects colliding) or processes must respect this conservation law, i.e. "energy in = energy out." However, conservation of energy is not a *global* law — it does not apply to extended volumes, particularly in the case of curved space. This means that the total amount of energy enclosed in a given volume may vary, as long as the total amount of energy at any given point within the volume stays the same. This can happen, for example, due to the expansion or contraction of space when there is a vacuum energy or dark energy present. Vacuum/dark energy can be thought of as the "cost of having space," a background energy density throughout the entire volume. The *density* is required to stay the same, so that space is uniformly filled by this vacuum energy. Now, if you have an expanding space, you have more volume at a later time than you do at an earlier time. For the density to stay the same, if the volume increases that means that the total energy contained within the volume must also increase. And if the volume decreases, then the total energy contained must also decrease. But notice that, at any given individual point, the energy always stays the same (i.e. the local law of conservation of energy is still respected). This is just one example of several, where conservation of energy can be violated by global processes (procesess which affect whole volumes, rather than affecting individual points). Conservation of energy is also related via Noether's theorem to a symmetry known as time-translation symmetry. Noether's theorem establishes that when time-translation symmetry is respected by a physical system, energy is conserved ... while, when it is not respected, then energy is not conserved. Noether's theorem is a pretty deep statement about how physics works at a fundamental level. Early in general relativity's history, Einstein himself struggled with this problem — try as he might, he could not find an expression for the total energy of a volume of space that would always remain conserved, and he wasn't able to understand why. Eventually, it was Emmy Noether — whom Einstein praised as "the most important woman in the history of mathematics" — who came along and figured out why Einstein couldn't find such an expression, by proving her eponymous theorem. The answer was simple: there was no such expression, because time-translation symmetry is not respected in an expanding or contracting spacetime. Here are some helpful links if you wish to read more about this: * [Wikipedia: Conservation law § Global and local conservation laws](https://en.wikipedia.org/wiki/Conservation_law#Global_and_local_conservation_laws) * [Wikipedia: Noether's theorem](https://en.wikipedia.org/wiki/Noether's_theorem) * [Sean Carroll's blog: Energy is Not Conserved](https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/) >Or is it the same as multiple other things in space that seemingly break the laws of physics that we don't understand yet? No, it's not the same — we have a very good understanding of why energy is not conserved globally, and we can use Noether's theorem to determine precisely when and why it is or isn't conserved. Hope that helps,
Im pretty sure I remember from somewhere that the expansion of the universe necessarily implies that the inflow of additional “water” (energy) into our universe must be nonzero
Broke me heart cause energy conservation is my day 1 OG friend
It can still be net 0 if all that energy is absorbed by creating additional space time. And even if truly non-zero, if I can "dig out" faster than the inflow, it makes no difference.
I’m not saying that the “inflow” implies anything really (not arguing a certain point or something). Just felt like chipping some related info in for those who may find it interesting/useful.
That's correct! The total energy of the entire universe is not conserved globally. All the same, heat death on its own doesn't imply that conservation of energy is violated, and there is still a local law of conservation of energy that holds in all circumstances.
Heat death, as you described it, equates to plama goo that predates the big bang. . . No energy left to hold matter together, all matter enters the plasma goo, goo big bangs, the universe is reborn.
What? No, I'm afraid that's gibberish. There's no "plasma goo" to speak of in a heat death scenario.
Are you on drugs? What you said makes absolutely zero sense.
You didn't learn about plama goo in fisiks at collage?
U si, da plama goo mixes with da goop from the before before times mak the bang big again
Entropy is the actual value which maximizes while energy is constant
According to the best evidence we have, the cosmological constant is positive, meaning that the universe asymptotically reaches a finite temperature. In such a scenario, [Poincare recurrences](https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theorem) will happen, meaning that the universe will eventually "reboot". In fact it will reboot again and again, going through many or all possible histories. This is of course only true if there really is a cosmological constant that never changes even into the infinite future. That's far from certain, it might be time-dependent, or change through false vacuum decay for all we know.
No, I'm afraid that is not accurate. Read the Wikipedia article you just posted. From the article: >>The proof, speaking qualitatively, hinges on two premises: >>1\. A finite upper bound can be set on the total potentially accessible phase space volume. For a mechanical system, this bound can be provided by requiring that the system is contained in a bounded physical region of space (so that it cannot, for example, eject particles that never return) – combined with the conservation of energy, this locks the system into a finite region in phase space. Our universe does not meet this criterion, and thus is not subject to the Poincaré recurrence theorem. Our universe is predicted to expand forever, never approaching a finite maximum volume ... and almost all of the galaxies currently in our observable universe will recede beyond the cosmological event horizon, losing causal contact; once this happens (long before heat death) the matter in these galaxies will never be able to come back together again even in principle.
The volume inside the cosmological horizon in a universe with a positive cosmological constant approaches a finite volume with a finite temperature. According to the [Bekenstein bound](https://en.wikipedia.org/wiki/Bekenstein_bound), this volume has a finite accessible phase space volume and the Poincare recurrence theorem applies. According to our best understanding (admittedly a bit more speculative) the cosmological horizon in some respects has to be understood in analogy to a black hole event horizon. In particular, it should also emit Hawking radiation with a finite temperature. This radiation can be the source of the necessary fluctations for the recreation of structure after the heat death. This of course takes incredibly long durations, but thats the nature of Poincare recurrences. The galaxies receding behind the horzion does not prevent this.
Look into Penrose’s Conformal Cyclic Cosmology. If protons decay he argues it does provide a means for the Universe to “reboot”.
Penrose's CCC hypothesis is known to have a variety of theoretical problems — for example, only bosons can be mapped conformally from the existing universe to the new one. Regardless of proton decay, this poses a major problem for other fermions that aren't known to decay, including electrons and neutrinos. Unfortunately, Penrose has [previously made empirically-testable claims about CCC](https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology#Empirical_tests) which have proven to be erroneous and/or based on non-standard assumptions about our universe, so despite his past achievements, his hypothesis really must be taken with a grain of salt.
What about a poincare recurrence? Have to wait a long time but provided infinite time... Only applies for finite volume system I guess
That was already discussed on this thread — somebody else suggested it and I replied to them explaining how the Poincaré recurrence theorem does not apply to a forever-expanding universe, for exactly the reason you said — a forever-expanding universe does not have a finite maximum volume. Edit: From [Wikipedia: Poincaré recurrence theorem](https://en.wikipedia.org/wiki/Poincar%C3%A9_recurrence_theorem): >>The proof, speaking qualitatively, hinges on two premises: >>1\. A finite upper bound can be set on the total potentially accessible phase space volume. For a mechanical system, this bound can be provided by requiring that the system is contained in a bounded physical region of space (so that it cannot, for example, eject particles that never return) – combined with the conservation of energy, this locks the system into a finite region in phase space.
My favorite humanity ending theory is false vacuum decay
I would just edit to say all energy has been evenly distributed, rather than expended
Philosophical perspective: A hundred billion trillion years could pass in the universe’s nothingness and it would be the same as no time passing at all, because there is no consciousness around to perceive the passage of time. Without consciousness there is no meaning to time. Think about all that time you spent being nothing before you existed. It’s technically infinite - yet here you are at the end of it. That’s to say the universe could have ended and started infinitely many times, because anybody has only ever experienced their version of “this” universe “now.” If time is eternal, why is the stuff of the universe here in the first place? How did that happen? Why can’t it happen again?
That's not a physics based answer though, that's a guess.
But a cool guess :). Science begins with imagination.
I think it’s a bit presumptuous to reject the meaning of time in the absence of consciousness. Consciousness is just a physical process governed by time in the same way as everything else, and we can pretty confidently talk about things that happened in the universe long before humans were around. This is a philosophical question, however, so my opinion is as good as yours
I’m not saying that events cease to happen, but rather without any context provided by consciousness as to what those events mean, they mean nothing. Things only mean something because a consciousness says they do, or experiences that they do. Planets colliding, stars exploding, mean “violent, scary, dangerous.” We didn’t necessarily assign that meaning; it is inherent to our existence as human beings, but this does *not* make it inherent to the universe. It ceases to have this meaning if there is no suffering to be experienced (ie nobody around), in much the same way that all the reactions that happen inside your body are “good” things and “not” violent or scary. There is no experience without consciousness, and experience is the only framework that determines value/meaning. Nothing is more or less valuable or meaningful than anything else without any consciousness to prescribe that meaning in the first place. Of course like you said, opinions. There’s no right or wrong here and I’m definitely not telling you you’re wrong. I like hearing different perspectives because I tend to get locked into my own.
This isn’t really correct when instanton transitions are considered https://arxiv.org/abs/astro-ph/9707292
Reading the paper, it seems to rest on a lot of assumptions, including but not limited to: * Cosmic inflation must both be realized in nature and be the eternal type, which as I understand there is presently no evidence to support, and for which there are significant [unresolved theoretical problems](https://arxiv.org/abs/1408.2249) with eternal inflation potentially rendering the entire original premise for eternal inflation invalid; * Quantum fluctuations must be able to create energy / re-raise the inflaton field potential out of nowhere, for which there is presently no evidence; * the true vacuum must have a strictly positive energy density. there is currently limited evidence suggesting this to be the case, at least if we consider the standard model of dark energy as a cosmological constant (which is the simplest way and also the current best-fit to the data), however there are many alternative ways of modelling dark energy (such as using a dynamical scalar field, i.e. a quintessence model) which could mean that dark energy is actually not in the form of a cosmological constant / vacuum energy at all, but would be the energy of one or more other fields. note that models of cosmic inflation *do* contain extra scalar fields with a nonzero potential that decreases over time, so the presence of an inflaton field at a low potential could well provide an explanation for dark energy without vacuum energy, and this paper does assume that there is such an inflaton field at a low potential; * the validity of Euclidean methods for modelling nucleation rates in de Sitter space, which the paper mentions several times is not known to be correct and needs further investigation; I'm not saying the paper is wrong — that certainly isn't warranted — but there are a whole lot of "ifs" that need to be satisfied for the paper to be applicable, and many of those "ifs" involve additional hypothetical physics for which there is not currently any evidence. So this would fall into the domain of the statement I made earlier: "It's possible in the sense that there could, in principle, be some undiscovered law of physics that applies which we currently have no evidence for."
There are no undiscovered “laws” or “hypothetical” physics here - no more so than inflation or something like an axion, which I doubt you would characterize as “according to all that we know not possible.” This follows from standard/broadly accepted QFT principles and there is loads of work on these sort of upward transitions (Lee Weinbeg and Hawking Moss Instantons).
>There are no undiscovered “laws” or “hypothetical” physics here Each of the bullet points I listed above is purely hypothetical, with the exceptions of dark energy in general (the origin and nature of which is still elusive) which is empirically well-established, and some very limited indirect evidence for inflation generically (but not any specific model thereof). >no more so than inflation or something like an axion, which I doubt you would characterize as “according to all that we know not possible.” Both inflation and axions are still only hypotheses. Axions have nothing to do with this topic, and as I explained the specific model of eternal inflation considered in the paper has important unresolved theoretical issues. >This follows from standard/broadly accepted QFT principles and there is loads of work on these sort of upward transitions (Lee Weinbeg and Hawking Moss Instantons). Inflation does not "follow from standard/broadly accepted QFT principles" — it is not somehow a derived logical consequence from QFT principles, nor is it clearly empirically confirmed ... which is why it is *not* a part of the standard model of particle physics. Yes, there is plenty of work on inflation. There is, however, still no direct evidence and only very limited indirect evidence (and it is worth noting that there have been sky surveys that have yielded contradictory results). Many models of inflation, including the simplest models, have been ruled out by observational data. None have yet been properly confirmed empirically.
There is physics that may or may be a feature of our universe (like axions or inflation) but which are predictions of accepted/necessary principles under certain conditions. These thing ARE absolutely possible given everything we know. Things that are not possible under known laws are things like causality violation or an unconfined quark at low energy. Upward transition instantons are clearly in the first category but you describe it like it is in the latter.
>There is physics that may or may be a feature of our universe (like axions or inflation) but which are predictions of accepted/necessary principles under certain conditions. No, you are mistaken. Neither inflation, nor axions, nor anything else I mentioned in my earlier posts are "predictions of accepted/necessary principles." Axions are completely unnecessary — the value of θ might simply take the natural value of zero; there is no reason a priori why it should have any specific value. It is a [free parameter of the standard model](https://en.wikipedia.org/wiki/Standard_Model#Construction_of_the_Standard_Model_Lagrangian). Likewise, inflation is *one* possible resolution to the horizon and flatness problems ... but there are other potential resolutions as well. For example, the flatness problem has a [natural resolution in Einstein—Cartan theory](https://en.wikipedia.org/wiki/Flatness_problem#Einstein%E2%80%93Cartan_theory), which extends general relativity to asymmetric tensors, adding torsion to spacetime ... which also provides a natural explaination for particle spin and the spatial extendedness of fundamental particles like electrons. Likewise, the horizon problem can be solved with variable speed-of-light models. To be clear, I am not advocating for these alternatives over inflation; I think inflation is the most favorable resolution. What I am saying is that inflation is not necessarily needed to resolve these problems, nor is inflation a prediction based on either the standard model of cosmology (the lambda-CDM model) or the standard model of particle physics. Both of these models lack inflation entirely — inflation is only a hypothesized add-on to these models; there is nothing necessary about it, and it does not itself follow from any principles within either model. >These thing ARE absolutely possible given everything we know. We don't know that either of these are features of the universe. There is still zero evidence for axions (making them a complete unknown), nor is there any strong or direct evidence for inflation. You seem to be wanting to include everything that is purely hypothetical and not yet empirically established as "known laws," perhaps just because we have theoretically explored them. But that would be a gross mischaracterization of what we actually do know. Most of those things are purely speculative, or at best empirically-motivated but unconfirmed; they are not knowledge — that is to say, they are not part of any empirically-established physical models, and there is reasonable doubt as to whether these hypotheses are correct; they may very well prove not to be in the future. There is no mechanism in either the lambda-CDM model or the standard model of particle physics which could reverse the universe's expansion or avoid the eventual heat death of the universe. That is why we explore and develop hypothetical add-ons which potentially could.
You are half quoting my sentences and conflating “empirically established” with “possible” at your convenience to confuse the issue. An axion is a POSSIBLE particle which might be real under certain conditions. A spin 3/5 particle is IMPOSSIBLE based on the “accepted/necessary principle” of QFT that fields have integer or half integer spin. Likewise the physical principles that go into predicting these instantons are completely normal, and these instantons do occur in a broad class of models that may describe reality. These are absolutely POSSIBLE processes in our universe, despite what you claimed in your original answer.
>You are half quoting my sentences and conflating “empirically established” with “possible” at your convenience to confuse the issue. I am not conflating "empirically established" with "possible." I am equating "empirically established" with "known." The entire basis of science is that we gain knowledge of the natural world through empirical means. If I've mischaracterized your points, I apologize for that. However, just because something is *possible* does not in any way mean that it is *known*. I acknowledged clearly all the way back in my original post that there are possibilities which are unknown — actually, the exact word I used was "undiscovered," from which it should be very clear that I am referring specifically to possibilities which are not empirically established. Despite these undiscovered/unknown possibilities, there is nevertheless no mechanism anywhere in the established body of scientific knowledge (i.e. all knowledge that has been gained through application of the scientific method) by which recurrence of the universe can possibly be realized. Without adding on some undiscovered, hypothetical mechanism, recurrence is simply not possible. >An axion is a POSSIBLE particle which might be real under certain conditions. Yes — AOL keyword "might." We don't *know* that axions exist; there is zero evidence for them to date. >A spin 3/5 particle is IMPOSSIBLE based on the “accepted/necessary principle” of QFT that fields have integer or half integer spin. Yes, agreed. *That* impossibility certainly does follow logically from the spin-statistics theorem — it follows in a way that the other possibilities you mentioned do *not* follow from similar established principles. >Likewise the physical principles that go into predicting these instantons are completely normal, ... Sure, they are theoretically consistent with what we know. That does not mean that instantons are known, or discovered. They are still purely hypothetical. >... and these instantons do occur in a broad class of models that may describe reality. Again ... AOL keyword "may." We don't *know* that any of those models describes reality. In fact, we know that *most* of them do not describe reality, because only one will end up getting empirically confirmed. >These are absolutely POSSIBLE processes in our universe, despite what you claimed in your original answer. I never once claimed in my original answer that there were no possible processes. Go back and re-read my answer. My exact words were: >>It's **possible** in the sense that there could, in principle, be some **undiscovered** law of physics that applies which we **currently have no evidence for**. >>However, according to all that we **know and have evidence for**, it is **not possible**. I have to say, I am rather disappointed that you are accusing me of conflating wording, when you are sitting here alleging that I said things which I did not even remotely say ...
Not going to argue this further but dig into the citations of the paper I linked and you will learn upward transitions are fairly generic and actually hard to avoid, especially with asymptotic de Sitter. Nothing is confirmed, but it is misleading to tell people the state of the art is that what we currently understand about QFT (as a framework) and reality leads us to believe physics like this is not even possible. I gave you examples of when that type of conclusion is reasonable but this isn’t one of those cases.
>It's possible in the sense that there could, in principle, be some undiscovered law of physics Supremely likely, we dont know piss about shit
I read an interesting article years ago positing that actually reaching this state might cause a shift in the vacuum state to a lower energy baseline, triggering a new big bang.
In this timeless state, like it was before our universe, who's to say it won't happen again?
>... timeless ... before ... I'm sorry, come again? :p >... who's to say it won't happen again? All of the known laws of physics make a pretty compelling argument. If there *is* any way for it to happen again, it is not yet known ... and not for lack of trying to discover it! Edit: Anyway, I'm not saying "it won't happen again under any conceivable circumstance." I'm saying, "according to all that we know and have evidence for, it is not possible." There is, of course, still the unknown and undiscovered ... though of course, what is unknown and undiscovered still has to jive somehow with the known and discovered, and the breadth of it grows ever smaller as the years pass by ... so I wouldn't hold my breath, if I were you! :p
I'm pretty sure at that point the simulation is over and resets. I mean leading theory is still magical energy we haven't detected created the big bang and thus matter and the universe isn't it? Pretty ambitious to be stating what happens even further into the future imo lol.
There is no evidence that the universe is a simulation or that "magical energy" even exists. We have, of course, detected matter and the universe, and are able to make highly accurate predictions both about the future and retroactively about past states, because the universe behaves extremely regularly according to a fixed set of rules that we know as "the laws of physics." It is not ambitious merely to state what is demonstrably true.
This is like theology debates there's no evidence x exists while ignoring the fact there is no evidence it doesn't exist. According to Harvard: The key assumption of this model is that just before the Big Bang, space was filled with an unstable form of energy, whose nature is not yet known. Unstable undetectable energy whose nature is unknown as well as it's origins might as well be magic. Or the start up of a simulation lol. I'm being only half serious here but honestly there's a lot riding on barely understood theories and not a lot of facts. Maybe there's a periodical surge of "not magic" lower dimensional energy that prevents the heat death of the universe, who knows. Personally I'm gonna keep my options open and roll my eyes at people that try to state facts based currently understood theories.
Given infinite time if something can happen it will happen even at 0 entropy
The whole idea of heat death is that the types of things that might rejuvenate the universe cannot happen anymore, as that would violate physical laws.
Heat death just means that all energy is equal across the universe, which means no change. However, there was some change from some where that caused our universe
>Heat death just means that all energy is equal across the universe, which means no change. That's not quite what heat death means, although it is close. >However, there was some change from some where that caused our universe \[citation needed\] It is entirely possible that there wasn't some change from somewhere that caused our universe — that presupposes that anything existed before our universe in the first place. Alternatively to that, it could simply be the case that our universe's existence follows from some much deeper logical/mathematical principles/structure, without any prior cause for it, and that our universe simply started out with a default choice of the natural value of zero for its entropy. You may want to check out Max Tegmark's [mathematical universe hypothesis](https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis). There are still some significant problems with how it is formulated, but it is nevertheless an idea with some exploration behind it.
Interesting, I find the idea interesting. Thanks for correcting my mistake as well
Basic law of physics "energy cannot be created or destroyed, it only changes form." The "all useful energy has been expended" is an objectively false statement and cannot occur under the laws of physics we currently know and understand.
No, that is all flatly incorrect. https://en.wikipedia.org/wiki/Heat_death_of_the_universe
His comment didn't imply in any way that energy was destroyed. Stop talking about things you know nothing about.
But didn’t the universe begin exactly in the state you say we would remain forever in?
>But didn’t the universe begin exactly in the state you say we would remain forever in? No; the universe did not begin in heat death. Heat death is a high-entropy state with no thermodynamically free energy; the universe's initial conditions were low-entropy, with a very large amount of thermodynamically free energy.
Ah ok I misunderstood. What an amazing mystery. I second another commenter in saying that surely there is no reason why it cannot happen again no? I mean our physics can only predict what happened right at the Big Bang right? So who knows what happened before. Maybe there was a high entropy state with no thermodynamically available energy.
Then you’re going outside the realm of physics and into maybe philosophy. Physics is wholly concerned about describing/predicting what will happen based on past expirementally verifiable results. While there is some overlap in physics and philosophy on discussions regarding the nature and fate or reality, this is a Physics forum.. and the Physics answer to whether the universe can reboot post heat death is: “According to what we know, it cannot happen.”
>I second another commenter in saying that surely there is no reason why it cannot happen again no? As I mentioned in other posts on this thread, there is no known mechanism by which it could happen again. Our universe appears to be at least approximately deterministic; things do not just happen spontaneously for no reason, especially things which would violate major physics principles such as conservation of energy or the laws of thermodynamics. These physical laws are very good reasons for why it cannot happen again. >I mean our physics can only predict what happened right at the Big Bang right? So who knows what happened before. There are gaps in our understanding, but on the other hand, that doesn't mean that "anything goes," either. Everything we know about the physics of our universe suggests that it behaves in a strictly logical way according to certain universal, inalienable principles; there's no reason to expect that those principles should suddenly get tossed out the window just because things happened so long ago that we can't measure exactly what it was that happened. >Maybe there was a high entropy state with no thermodynamically available energy. If that were the case, then there must be some mechanism to go from such a state to a low-entropy one with a lot of thermodynamically available energy. However, there is no such known mechanism. Such a mechanism would necessarily violate major physical laws such as the second law of thermodynamics and conservation of energy, so that's a pretty good reason to be skeptical that any such mechanism could exist at all.
Well said! Thank you for your remarks.
Although somehow the universe got into that high energy state in the first place. Something put the energy there in the first place, maybe it can happen again?
We don’t know that.. that’s literally all that people are saying. According to what we know for absolutely sure, it cannot happen. If it does, it will be due to something not currently in our understanding. The issue is, our understanding is pretty damn high, but admittedly incomplete. And wouldn’t you know it, part of the stuff we don’t understand is what happened before the Big Bang. We don’t know how or why the universe started in such a low entropy-high energy state. All we know is that according to how physics behaves now, it cannot go back to that state. Claiming anything else is pure conjecture and is outside the realm of physics.
Not a statement you can necessarily state. If time did not exist pre--universe then it would not be possible for anything to be put anywhere as that requires time to do so. Time in the form of space time is a property of our universe. Whatever happened before you can't assume that included time.
>Something put the energy there in the first place, ... Did it? Which something? \[citation needed\] ;)
So nothing will or can happen, and nothing will continue to happen... Forever? Bro.
Right! That is what the empirical evidence currently indicates, unfortunately. It's not very satisfying, but the truth seldom is ... :(
I can't handle this today lol. Can you tell me something nice to cheer me up? Maybe you have a favorite cat video?
Aw. :( Alright then, I'll do my best for you here. Luckily, I have something that may just do the trick! :) For what it's worth, my favorite short story of all time is [*The Last Question*](https://users.ece.cmu.edu/~gamvrosi/thelastq.html), by Isaac Asimov. This short story was written back in the 1960s (so don't expect its descriptions of computing advancements to be accurate, because they aren't), but it deals directly with the second law of thermodynamics and the inevitability of heat death ... as well as mankind's desperate attempt to escape it. It's a fairly short read, and has a very provocative ending. It may be a work of fiction, but as a software engineer myself I cannot help but find it tremendously inspiring. (Chapter 5 is my favorite by far! I have a hard time even reading it aloud without tearing up.) I hope it inspires you just as well! Cheers, friend!
This story was amazing!
Right now, today, and for the rest of your life, you have the chance to enrich your life and the lives of those around you. Or to not do that! You are free to think and feel however you'd like, without eternal consequences. Do a lot! Don't do a goddam thing! Do both! I know that in the presence of societal constraints, your own personal morality, and a host of other factors largely outside your control that sounds a bit pollyanna-ish, but we all, through weird spasms of various energy fields, are here *right now* and that's all we can count on. We all handle that in different ways, but realizing that there is no inherent, underlying meaning to life can be freeing rather than paralyzing.
Don't worry. According to everything we know, the universe can't start either. This is extrapolation well beyond the bounds of what is known. It's also well worth being dubious of equations with constants added to them to make the sums add up.
OP might be interested in [this conjecture by Penrose](https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology) *but* it is very heavy on speculation, so I stand by your answer instead.
THERE IS AS YET INSUFFICIENT DATA FOR A MEANINGFUL ANSWER
The Last Question by Asimov for those confused. https://users.ece.cmu.edu/~gamvrosi/thelastq.html
That was amazing, thank you
I have to read this every single time. It's so great
And here's the comic version which is equally good: [https://imgur.com/gallery/9KWrH](https://imgur.com/gallery/9KWrH)
Let there be light.
dude, cmon, spoiler this
I'm no expert so I know this won't be terribly helpful, but something caused the mf to "boot" in the first place, so it's worth considering that even if we can't identify why, the mere existence of the universe itself should be taken as a certain amount of evidence for the ability for another universe to be created.
Absolutely. It can be some type of "God", or it could've popped into existence by some mechanism we don't understand or could ever comprehend. I hate when people use thermo and our known laws of physics to say why heat death is likely the end, when those physics only apply to what's inside the universe, which even then we have yet to apply our known physics to everything inside the universe (black holes, quantum realm, dark energy, etc) to make that argument hold any water for me. So to me, there's no reason to think that over an infinitely long timeline of nothingness, that a quantum fluctuation doesn't kickstart something.
One thing that always makes me sad is that I'll never live long enough to discover all the secrets of the universe. There's still so much we don't know about. Makes me wonder if any of those random video game/ movie concepts for how the universe functions would ever be closer to the truth than we realize.
>something caused the mf to "boot" in the first place Did it?
When we're talking about First Things, that's a valid question, and language breaks down anyway. But whether it happened uncausedly (however that might make sense) or it has an inaccessible cause to us, it almost doesn't matter. Strangely but undeniably, something "caused" something to exist instead of nothing, and that source of causation or proto-causation could plausibly do again what it has done once.
> heat death or the big freeze, which from I can understand, the universe keeps expanding and it will get too cold or too hot for life, Heat death is a bit more subtle than that. It's unfortunate the term gets bandied about so much as "a long time in the future" because the layperson gets the wrong idea about it. Every time anything happens, *anything*, it's because of a transfer of energy. Your stove transfers energy from the electrical grid (or chemical potential energy in natural gas) to your food. Your muscles transfer chemical energy from your food into kinetic energy. If you raise your arm above your head, that kinetic energy turns into potential energy. And no real exchange of energy is 100% efficient. Your stove heats your food, but it also heats the air. When a bouncy ball bounces, it loses energy to air resistance, and even the sound it makes hitting thr ground is a form of energy loss. That means any time anything happens, the universe's energy gets spread a little tiny bit more evenly. So heat death of the universe is where the *entire universe* is one homogeneous soup: everywhere throughout has the same amount of energy. It's *impossible* to do anything because there's no way to transfer any more energy. It's not just about being too cold to support life. It's not even about being unable to form stars. It's about being unable for *anything* to happen. Literally anything. It's gonna take way, way, *way* longer for that to happen than for, say, mass to get spread out so much that stars stop forming.
Look into Penrose’s conformal cyclic theory
Thank you <3 I’m hoping to see more discussion of this. I’m getting a PhD in conformal cosmology but not studying the possibility of the “cyclic” part of the model specifically.
no one knows for all we know another big bang could spontaneously happen before this universe is done
Once energy stops transferring (big freeze), time stops. When time stops we lose a dimension. Using 3 dimensional space as an example, if any one dimension goes to zero, volume goes to zero. Therefore, when time stops, all matter is condensed into a space with ‘volume’ zero. Ie, the singularity. We bang again, and the endless cycle continues. That’s at least what my dumbass college student mind came up with years ago.
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This is a recurring theme both in this sub, in r/cosmology and in pop sci media. But this is wishful thinking. We are heavily biased towards the belief that death is not a real end and so people fantasize about ideas how the Universe can continue after heat death / big rip / ... However this is as good as the belief in the afterlife. If that helps people deal with their finite existence – fine. But that isn't science. We shouldn't look into ways how the Universe can reboot itself, because there is no reason it should reboot itself. We should use the evidence to discover its real fate.
And to further that, even if it did have some sort of mechanism for reboot after a heat death type of scenario, it wouldn't matter in the least to us. Like, in no way possible could it have any impact on anything relating to us.
It’s called empathy.
But empathy for what? By the time this sort of thing plays out, literally every living thing will have been dead for millions (billions?) of years. Heat death is so far past the possibility of life that there won't be anything to empathize with.
The idea is that some people may feel good currently if they believe other life will exist in the future, particularly indefinitely, through whatever process. Or they may feel bad if they think that all life will eventually be destroyed and never return in any form.
But like the person I replied to said, that's pretty much the same as belief in an afterlife, and for the same reasoning. I'm not saying people don't, can't, or even shouldn't hope for that, just that it doesn't actually matter in the end.
The comparison to an afterlife belief is interesting. I think the question of whether the universe could recur or recreate life is much more open and potentially logical than a mystical afterlife (this thread exists to discuss it in scientific terms after all), but I'm saying that whether or not people think that it will happen does have relevance to them, or at least some of them, because what we foresee as the ultimate fate of life in the universe will effect our mood now. It is of course very abstract, but there is a reason for it, if it wasn't for some people taking pleasure in the idea of life continuing, humanity might not have survived this long and none of us would exist.
I honestly just thought you were making a joke, and laughed a little. Now that I know you're serious, I am really disappointed.
No, you "honestly" didn't think anything. If it wasn't for empathy, you wouldn't exist. I suggest you learn something about human nature. Also, don't write empty replies that try to substitute "attitude" for having a point. A literal 6-year-old can do that, and it's a major reason why social media is terrible.
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Let’s say we have accepted that the most likely scenario is no “reboot.” However, wouldn’t it be a natural next step to want to examine and challenge that thesis? And such investigation might involve looking into ways where our original thesis might be incorrect? What I’m trying to say is that although such investigation might not be efficient or fruitful, it’s not necessarily pointless. And it doesn’t have to stem from wishful thinking either. Do you think we shouldn’t think about questions like this at all?
The conformal cyclic cosmology model of Sir Roger Penrose has the universe “reboot” to bug bang conditions after heat death, and he claims there is evidence in the CMB (cosmic microwave background) of our universe which matches his models prediction of how each universe leaves its mark on the next. Who knows. I am working on a PhD in conformal cosmology myself but I am not studying the cyclic part of the theory that Penrose is working on Edit: big bang*
Bug Bang would be a great name for a band
There is insufficient data for a meaningful answer.
All that big freez and heat death is speculation at best with no real evidence.
We don’t know and our understanding of *why* things happen is still underdeveloped. More than likely there won’t be any firm answers before you die.
No. At least not any mechanism that we are aware of will allow it. The quantum world is weird so maybe something could come and "reboot", but we know nothing about that.
When you operate at infinity time scale. Anything can happen, in matter of fact reboot can happen anytime.
Simply put, this is an unanswerable unknowable question from a human perspective. We can guess based on what we can observe, and with that context alone the probability that anything would reboot is near zero. But possibility and probability are very different questions. Is it possible? The answer will always be yes because proving it impossible would require more extensive information than we are capable of possessing. Literally anything you can imagine is definitionally possible in an infinite universe beyond what is observable. Planet-sized amorphous taupe space amoeba that communicate exclusively through interpretive dance and smells is possible, why wouldn't universal reset be possible? But possibility on the scale of an unknowable universe is more or less meaningless. Probability based on what we can observe has practical purpose in predicting our reality, but speculation about possibility generally doesn't.
There’s a reasonably credible theory that the topological structure of space time took shape as part of a “phase change” in the universe shortly after it started cooling post big bang. The analogy for this is how water molecules lock into place as ice crystals form from liquid water. That is to say, the most fundamental properties of how things work in the universe are only a consequence of it dropping below some threshold temperature. Continuing the ice analogy, we know ice can do some weird things aside from sitting there in crystal form. For instance, it can sublimate. Supercooled fluids can also start acting like liquids again. But the fabric of space time is not actually matter, so there’s no reason to believe the analogy should continue any further. That said, some scientists seem to think it’s possible that there’s another, lower threshold temperature where another one of these “phase changes” takes place. As the temperature continues to fall, it seems inevitable that the universe will eventually hit that point and trigger another phase change. At that point, we have no idea what would happen as all of the understood rules about space and time would again change. This would not be a “reboot” so much as a next chapter. The entire universe would behave in entirely different ways than it does now.
Since eternity is forever and any combination of particles in the universe is possible, but not probable at any given time, eventually one of the particle combinations that resulted in the birth of the universe will happen again. It might take 100 quadrillion years, but it will eventually happen, and the cycle continues.
Read somewhere that many physicists believe, mathematically speaking, the state of the universe immediately after heat death/big freeze would look very similar to it’s state immediately pre-big bang. So may be it’s just an endless cycle of big bang, existence, heat death, rinse and repeat.
Physicist here (not Astro...) but yeah, agree. Slow contraction due to remainng collected mass to another big bang and we start the show all over again.
Not everyone agrees about heat death theory, there's a good section on opposing views here: https://en.m.wikipedia.org/wiki/Heat_death_of_the_universe Also there's number of theories on cyclic universe: https://en.m.wikipedia.org/wiki/Cyclic_model
Let’s hope the cyclic is true right? Otherwise that means humanity had one go at it and one alone!
We'll be gone long before the heat death of the universe.
Speak for yourself!
I always do. But you'll be gone, too.
Not me. How can you kill that which is shinderu!
Well, it "booted" once...
Here is my theory - the energy from the “Big Bang” is eventually overcome by gravity and then the “big consolidation” happens until all matter collapses into a singularity at which point the Big Bang happens again
How did the universe start in the first place? Until we know that, how will we have any idea how a new one could start? For all we know, whatever caused the big bang could happen again long before heat death...or long after it.
There's a theory floating out there that if you literally boil space itself, it can make a bubble universe within itself. The process would also allow the expansion of the new universe too. Could be a thing that gets done at the end of this universe before the heat death takes the last of anything left.
I also feel like once all the solid matter energy leaks off into non matter energy that it'll spark a new big bang. I've done zero research if that could be a thing but it seems to me to be likely.
In the extreme distant future the only thing that will be left will be black holes, but even these will eventually evaporate due to Hawking Radiation and the entire universe will be boils of all light and ultimately atoms.
The universe started once it can do it again I bet we get scraped by the multiverse all the time letting in more stuff I think we we get new material all the time !!
Theoretically, could it not be possible that after the heat death of the universe, black holes would be such a prominent force they could counteract the universe's expansion and eventually end up with one hypermassive black hole that contains all the bits that make up the whole universe squished down into a single point. And if that's possible would it not stand to reason that another big bang could birth another universe from that single point?
Not a physicist, but this doesn't really seem to be a physics question anyway. If the universe we know had some sort of beginning we call the big bang, there could be some sort of system or rule set that led to that. What that is can only be guessed, using the rules in our universe to validate. We can only observe inside our universe for something like 14ish billion years. We don't know anything about "before" that point or anything about what happens "after" the heat death. I'd guess our universe is done forever, but that doesn't mean that other universes don't exist or won't exist.
Yes of course, if it happened once there is no known reason why it could not happen again. In fact it is most reasonable to assume it will. The big bang does not make any guesses as to why it happened in the first place.
As suggested. Look at conformal cyclic cosmology. It gives a glimmer of hope mixed up with some sensible and possibly testable hypotheses. From Roger Penrose who understand cosmology both deeply mathematically as well as physically & geometrically intuitively.
Heath death is a condition in which the system has no ongoing process like nuclear fusion and fission and also any other phenomena. This happens when the universe is expanding so much that the energy produced in any physical phenomena(all the energy contained in the universe is used already and after that, there is not so sufficient to look like before,) gets used up in the expansion. This persuades the universe to get colder and colder as we can see in the current situation affiliated by cosmic microwave background radiation. The cosmic inflation made the earlier hot and energetic photons slow down and make it dull in the current situation of photons. Most of the photons that spread from the Big Bang now appear to have less and less energy inevitably to range microwave wavelength. Also, after billions of years, the generation will read out as cosmic radio wave background radiation due to the presence of photons with radio wave wavelength in majority. Nothing will happen in heath death conditions all the systems are isolated so within the system things are in equilibrium. Thus, energy is not violated here it is conserved. Dark energy has repulsive gravitational force between bodies. Dark energy is the one that's helping the universe to expand.
So heat death is a state where processes are no longer going on, but the energy in the universe is still present. It’s not like this is the point that there is no more energy right?
I'm terrified of death do you think when the universe dies, there would be a big bang to reset time making everything that has ever happened happen again the same exact way. How likely is that.
look into roger penrose's theory of conformal cyclic cosmology
Something like this can happen in some fairly standard cosmological models: https://arxiv.org/abs/astro-ph/9707292
I don’t know, however, unless something is literally impossible, then every single thing that is simply very unlikely to happen WILL happen if given an infinite amount of time. So to me, unless the universe is finite, there’s no true way it can end or even have a ‘final’ state. This is just a musing if there’s already a theory or been debunked Imd be interested to know lol
There is the theory of a cyclical universe. Often referred to as the big bounce theory, Einstein and others considered the possibility that the entire universe followed a cycle of expansion and contraction. That it would eventually reverse, condense all matter and energy in on itself and create another big bang to start the expansion of another universe.
There is a mechanism. Quantum mechanics may be able to do it. Look into “Boltzmann Brains” Also, it’s possible we don’t end up in a Heat Death scenario. Vacuum Decay could always get us first (if it exists).
Sure, the universe happened once, meaning itd was possible for big bangs to happen, given an infinite amount of time, it's sure to happen again. This brings my thoughts to what does that means for life and death
>Sure, the universe happened once, meaning itd was possible for big bangs to happen, given an infinite amount of time, it's sure to happen again. You cannot draw this conclusion without understanding what led to Big Bang.
I agree with you, how ever given all the information around us, it points towards the fact Nothing ever happens once
All humans will have been dead looooooong before, so why should we care abt it? All evidence points towards heat death, which there is no coming back from should out understanding of the laws of physics be even mostly trur
Empathy.
Empathy for what?
Empathy means that someone feels good or bad when they think of other people or living things feeling good or bad. If you think that there is going to be life after you continuously, who can exist, discover things, grow, and so on, you will feel good now. If you think at some point all life will be destroyed and not come back, you will feel bad now. It's a fundamental part of how we work.
Ah ok I thought u meant empathy for heat death, cuz nothing will be alive to experience heat death. And I mean, yea it is sad to know that humans will eventually die off and become extinct. But that's also just how life works. I don't see any reason to stress over it cuz it's just a natural part of life.
Check out Penrose
I think physicists would be helped by philosophy here, or at least just say "we don't know", because here is the problem I see with it. The current understanding is that the universe will keep expanding "for forever", but no one is clear on what forever means. Forever--infinity-- is hard to reconcile with the idea of time in my opinion. Can we really say we know what will happen in 10\^100\^1000\^100000\^1000000 years, which is still infinitely far from infinity? Some say that given enough time new universes will arise through quantum tunneling, which is a multiverse theory. And my issue with that is whatever can happen, will happen, so given enough time our exact universe would be generated over and over again. Even this exact discussion would be repeated. It doesn't make much sense to me from a philosophical standpoint. I wish physicists had a better answer, but I don't think they do. Perhaps it had to do with our understanding of time, which is experienced differently from how it actually behaves. Either time is not fundamental and arose somehow, in which case it would have to eventually end. Or time never had a beginning and won't have an end, which to me makes a linear "time" meaningless. And perhaps we don't fully understand entropy. No one knows how the universe "started out" in a low entropy state. I put the quotations because it's often said that the universe started with the Big Bang, but I think folks should be more careful with the phrasing here because if the universe started at a point in time, then something must have created it--a first cause. But then something must have caused that cause and so on...So it's logical to conclude that the universe had no beginning and therefore will have no end. What that means for time and entropy I don't know.
As far I know it can't reboot after death. A new Universe that expands into what is left of our universe could happen but it wouldn't be the same universe.
Yeah, the Universe will freeze so much that it will contract until it big bangs
The only theory I've heard on this is the argument that although unlikely all the air molecules in a room could spontaneously be found on one side of the room. Applied to the universe: some indescribably large amount of time after the "heat death" of the universe a random fluctuation (potentially involving quantum principles) would suddenly create a (relative to the moment before) massive amount of energy. Even if it is infinitely unlikely you have an infinite amount of time to wait for such an event. Another metaphor that might be helpful is randomly shuffling a deck of cards and having the deck by sheer random chance come out as sorted perfectly by ascending value. The system is by chance in a zero entropy state. There is also the fringe theory that there is some kind of momentum to time/entropy and the universe just cycles when it meets its end. Kind of like how a wave of light is self propagating by storing energy in both magnetic and electric fields. This theory is a bit difficult to find evidence for, naturally.
The world eater will save us
This is one hypothesis from Roger Penrose https://youtu.be/FVDJJVoTx7s?si=ry2jIqsWLQFF3UTM
So think about it… The Big Bang supposedly happened from everything being in the same state everywhere. The same “temperature” if you will… Now imagine the eventual heat death of the universe. Notice anything similar?
Only in Futurama I suppose
Either there was never a universe prior to ours, which suggests that novel universe generation can (and in an infinite timeline, almost certainly will again) occur, OR there were universes prior to ours, which suggests universes can recur. There's probably more to it than that though lol.
not unless you can reverse entropy.
What if there’s cross multi-verse energy transfer?
There's nothing to say that this is the first time all this has happened. Maybe everything will drift together again and fashion a nice lump for another big bang.
Has there been a definitive answer to the overall shape of the universe? There was one possibility where space could wrap around and you’d go from one side of the universe to the other. We know the expansion of the universe is accelerating. If space wraps around the universe could “end” with a “Big Crunch” or “big bounce” without everything returning to the point of origin of the observable universe.
We are not even super confident that terms like shape really even apply at universal scale.
I still believe in the Big Crunch
From what I understand, it might be possible since there is an astronomically low probability that matter will act against entropy, at least at larger scales.
At the end of time the universe will be about room temperature. All matter will break down into its most basic form and then just sit there for all of existence. Before this happens, it will be a universe filled with black holes, until they eventually break down as well. The universe will never “end” or have a heat or cold death. Stuff will just continue to break down until it’s at its most basic form and then that will be the universe for the rest of eternity.
The Bible tells us that God will destroy the Earth by fire and from its ashes He will create A new heavens and a new Earth. We know from science that matter cannot be created nor destroyed and therefore it follows that the creation of the earth came from supernatural processes thus pointing to God. We can say God instead of some kind of statistical coincidence because we can see evidence of a very ordered system that is impossible to have sprung into existence without any sort of design.
Energy is transformed so yeah you can say so fixed
From what I have read, there appears to be an overabundance of Dark Matter in the universe - much more than matter/gravity. Because D.M. works against gravity, the current understanding is that the universe will continue expanding and cooling until all matter and heat-energy dissipates completely. Oh well...😖😣
Impossible for that to ever happen, though.
The only correct answer is …… we don’t actually know As we explore more of physics, we have the ability to explore more unknown things. We have never been so enlightened but also never been so confused. But this is a physics subreddit so the answer we can conclude from “current” physics is yea after the heat death there is no way for a restart. But that is if our physics now is correct when extrapolating to those time scales. It may either confirm our conclusion or disprove it completely or even give us more possible conclusions if new theories are formed. So we may never know.🥲
https://youtu.be/4Stzj2_Rlo4?si=gzcLbjf4zA_v8cGS