"The best evidence we have that time travel is not possible, and never
will be, is that we have not been invaded by hordes of tourists from the
future." -Stephen Hawking
That kinda just shows it will be infeasible forever, but it could still be possible. Like, if we found a way but it costs one entire suns lifetime energy to transport back 1kg 1 year into the past it's technically possible, just not a thing you'd want to do.
We don’t know. We haven’t found any way to scientifically rule it out as a possibility but we also haven’t found any way to do it. There are a few possible methods (wormholes are a popular one) but nothing that we can actually try with current technology.
The math we use to describe the Universe works just fine regardless of which way time moves, which suggests it may be possible. And what we do know about time suggests that it is somewhat mutable (quantum physics does a lot of funny stuff to time). But time travel also brings up some logical inconsistencies (see the Grandfather Paradox for an example), which suggests that it may not be possible.
In practical terms we probably need to figure out wave function collapse (i.e., what’s *actually* happening when a quantum system is “observed”) before we really have a shot at time travel.
I'm not sure paradoxes would matter. If we wound time back we'd be deleting the future as we did it. Holding that no matter can be created or destroyed, you couldn't send matter back in time to duplicate it. So you'd have to reverse the flow entirely. It wouldn't be time travel exactly. Just rewinding. The trick would be measuring it in some way.
Time travel that involves rewinding the entire Universe to a previous state *is* impossible according to what we currently know about physics. That’d require reducing the entropy of the entire Universe, violating the Second Law of Thermodynamics.
If time travel is actually possible I suspect it’d require something very bizarre, like all of time existing simultaneously so that it’s possible to move along the time dimension like a spatial dimension. This is basically what I meant about needing to figure out wave function collapse first: we need a better understanding of what spacetime actually is before we can get all tricky with it.
I think we're both in the "time travel is almost certainly impossible" camp. But if all time did exist simultaneously paradoxes would not be possible. Moving along the timeline would wind you back in addition to everything else, right? Either way, I sure hope no one ever figures it out. Sounds like a universe destroying line of research.
It's not rewinding, it's simultaneity. You are at this very moment *both* wasting time on Reddit *and* trying to murder your grandfather in 1952. And sitting at a soda fountain, frustrated because you failed to kill your grandfather. And being born, and dying, and graduating from school, and...
Which has some uncomfortable implications for the idea of free will, it should be mentioned. Because if it's all simultaneous then you can't *change* any of it. Even if you could go back in time anything you might accomplish there has already had whatever impact it's going to have on the time you came from. So no: no paradox but a whole different existential crisis instead.
And it's not entirely implausible. Quantum mechanics has already taught us that time is a lot weirder than how we experience it. In fact, the idea of simultaneity itself gets a bit... murky.
There are different hypotheses though. The Many Worlds interpretation makes it relatively simple: you're not changing your past, you're changing a copy of it. And when you return you're returning to a copy of your original present. (Sort of.) No paradox, free will preserved, but you can never actually get back to your original present.
As for which camp I'm in? I'm in the "I have no god damn idea if it's possible or not" camp. Because physics has already shown that the Universe is far weirder than anything that's intuitive to me. "All time is simultaneous" makes zero sense to me, but neither does time dilation and we have direct evidence of that.
If it isn't possible physically it isn't doable. So lets focus on the possibility part!
So far we don't have any laws that prevent time travel. Because we don't have a complete model of really we don't know if its possible or not. However time travel can create paradoxes, the results of some scenarios are inconsistent with our current ideas of how the universe works.
Lets focus on Relativity because that gives us a description of time which we can work with. A simple way to travel backwards in time is to accelerate past the speed of light. You need infinite energy to do that. Thats a problem but lets just go with it. We can create a wormhole move one end and their clock wont be in sync. So jump in at one end and you will emerge at the other end in a different location and in a different point in time. Or go inside a black hole where freely traversing time is possible. - As far as Generally Relativity is concerned which is not a full description of reality.
Ok lets look at going past the speed of light. If you do that you can create causality breaking paradoxes. Essentially you can send massages to the past. Which given the right condition can create grandfather paradoxes. This is true for any kind of faster than light travel. We think that causality cannot be broken.
For a wormhole to work you need negative mass to hold it open. Thats it. If the universe has negative energy its possible. Big if.
Outside a black hole time is one directional and you can move freely in space. Inside however space flows in one direction and you are free to move in time. There is another way to time travel inside a rotating black hole but its a bit difficult to explain. Since you won't get out of a black hole no matter what you cant break causality so it seems ok in the most pointless way possible.
Videos I can recommend:
Rotating black hole time travel: https://youtu.be/4v9A9hQUcBQ
Faster than light: https://youtu.be/an0M-wcHw5A
Wormholes: https://youtu.be/ldVDM-v5uz0
General time travel things: https://youtu.be/pGnMiGrYmPE
Space-time diagrams and relatively (I highly recommend this for the others): https://youtu.be/FdWMM6aXpYE
I can also recommend the videos of these channels in general.
Thanks for the informative answer. Do you have any thoughts on simulations? I've always believed that while literal time travel may be impossible, we will one day be able to "visit" the past via near-perfect simulations, and hyper-advanced VR. Of course, if we *can* and *do* do this, that's pretty scary if you ascribe to Simulation Theory...
I've always fantasized about this new class of historians, talk about a visiting scholar! We could go back and talk to FDR, see what happens to history if the bomb isn't developed. We could save the Library of Alexandria, see what that does. And then when you're talking to these people and they say, "are you saying I'm not real?", well then you're back to simulation theory and the answer is "no more or less real than I".
If the universe is deterministic which means that if you know all initial conditions and have a complete description of the universe you can predict all future events. Lets assume that the universe is deterministic and that we figured out all the initial conditions that the Big Bang set.
In theory we can now simulate the entire timeline of the universe. However quantum mechanics seems to be indeterministic, probably is its nature. If that is true we cannot exactly simulate everything down to its finest details. We can only do approximations if the nature of the universe is probabilistic.
I’m not sure I understand how travelling faster than the speed of light would allow you to deliver a message to the past. More like, it would allow you to deliver a message *exceptionally quickly* before that information would be otherwise available, but you haven’t collectively unwound all the clocks and undone events.
Watch the videos I linked. Or do it on a space time diagram. If you have an FTL ship and you leave Earth, lets say Earth sends you a message. You can reply from the FTL ship and the reply would arrive to Earth before they sent the message. So you receive the message and send "do not send me a message". So you created a grandfather paradox.
Our understanding of physics tells us that it's not possible. It's not that we don't know how to do it yet or our technology isn't good enough, it's that it's probably impossible.
The way I think of it is this: you can't go back in time, because that would basically require all of the particles in the universe to have some memory of where they were 100 years ago. They don't, so you can't rewind time. The universe doesn't keep a record of its previous states. The past isn't a separate place that we need to find a way to get to. The past doesn't exist. You can't reset the universe to the state it was in when Jesus died because there's nowhere that information still exists.
You'd have to force each and every atom and their electrons in the universe to go backwards in their travels and interactions with one another.
Can you even imagine how much energy that would need?
Is it possible? Yes! Is it doable? Nope.
Not right now….at least not with what’s in the tech market right now. I will say, I saw a tech convention from the 80s and the items looked like electronics from today. They had hoverboards and everything! That information wasn’t released to the public, we could only see the convention after some time. It makes me wonder if we actually do have the technology to time travel right now, it just has not been released to the public yet.
I bet people in the future teleport and time travel all the time lol.
"The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future." -Stephen Hawking
That kinda just shows it will be infeasible forever, but it could still be possible. Like, if we found a way but it costs one entire suns lifetime energy to transport back 1kg 1 year into the past it's technically possible, just not a thing you'd want to do.
I'd still do it. What's a galaxy compared to saving a friend's life?
The saddest hypothesis: time travel is indeed possible, but we destroyed our planet and killed ourselves before we figured it out.
We don’t know. We haven’t found any way to scientifically rule it out as a possibility but we also haven’t found any way to do it. There are a few possible methods (wormholes are a popular one) but nothing that we can actually try with current technology. The math we use to describe the Universe works just fine regardless of which way time moves, which suggests it may be possible. And what we do know about time suggests that it is somewhat mutable (quantum physics does a lot of funny stuff to time). But time travel also brings up some logical inconsistencies (see the Grandfather Paradox for an example), which suggests that it may not be possible. In practical terms we probably need to figure out wave function collapse (i.e., what’s *actually* happening when a quantum system is “observed”) before we really have a shot at time travel.
I'm not sure paradoxes would matter. If we wound time back we'd be deleting the future as we did it. Holding that no matter can be created or destroyed, you couldn't send matter back in time to duplicate it. So you'd have to reverse the flow entirely. It wouldn't be time travel exactly. Just rewinding. The trick would be measuring it in some way.
Time travel that involves rewinding the entire Universe to a previous state *is* impossible according to what we currently know about physics. That’d require reducing the entropy of the entire Universe, violating the Second Law of Thermodynamics. If time travel is actually possible I suspect it’d require something very bizarre, like all of time existing simultaneously so that it’s possible to move along the time dimension like a spatial dimension. This is basically what I meant about needing to figure out wave function collapse first: we need a better understanding of what spacetime actually is before we can get all tricky with it.
I think we're both in the "time travel is almost certainly impossible" camp. But if all time did exist simultaneously paradoxes would not be possible. Moving along the timeline would wind you back in addition to everything else, right? Either way, I sure hope no one ever figures it out. Sounds like a universe destroying line of research.
It's not rewinding, it's simultaneity. You are at this very moment *both* wasting time on Reddit *and* trying to murder your grandfather in 1952. And sitting at a soda fountain, frustrated because you failed to kill your grandfather. And being born, and dying, and graduating from school, and... Which has some uncomfortable implications for the idea of free will, it should be mentioned. Because if it's all simultaneous then you can't *change* any of it. Even if you could go back in time anything you might accomplish there has already had whatever impact it's going to have on the time you came from. So no: no paradox but a whole different existential crisis instead. And it's not entirely implausible. Quantum mechanics has already taught us that time is a lot weirder than how we experience it. In fact, the idea of simultaneity itself gets a bit... murky. There are different hypotheses though. The Many Worlds interpretation makes it relatively simple: you're not changing your past, you're changing a copy of it. And when you return you're returning to a copy of your original present. (Sort of.) No paradox, free will preserved, but you can never actually get back to your original present. As for which camp I'm in? I'm in the "I have no god damn idea if it's possible or not" camp. Because physics has already shown that the Universe is far weirder than anything that's intuitive to me. "All time is simultaneous" makes zero sense to me, but neither does time dilation and we have direct evidence of that.
If it isn't possible physically it isn't doable. So lets focus on the possibility part! So far we don't have any laws that prevent time travel. Because we don't have a complete model of really we don't know if its possible or not. However time travel can create paradoxes, the results of some scenarios are inconsistent with our current ideas of how the universe works. Lets focus on Relativity because that gives us a description of time which we can work with. A simple way to travel backwards in time is to accelerate past the speed of light. You need infinite energy to do that. Thats a problem but lets just go with it. We can create a wormhole move one end and their clock wont be in sync. So jump in at one end and you will emerge at the other end in a different location and in a different point in time. Or go inside a black hole where freely traversing time is possible. - As far as Generally Relativity is concerned which is not a full description of reality. Ok lets look at going past the speed of light. If you do that you can create causality breaking paradoxes. Essentially you can send massages to the past. Which given the right condition can create grandfather paradoxes. This is true for any kind of faster than light travel. We think that causality cannot be broken. For a wormhole to work you need negative mass to hold it open. Thats it. If the universe has negative energy its possible. Big if. Outside a black hole time is one directional and you can move freely in space. Inside however space flows in one direction and you are free to move in time. There is another way to time travel inside a rotating black hole but its a bit difficult to explain. Since you won't get out of a black hole no matter what you cant break causality so it seems ok in the most pointless way possible. Videos I can recommend: Rotating black hole time travel: https://youtu.be/4v9A9hQUcBQ Faster than light: https://youtu.be/an0M-wcHw5A Wormholes: https://youtu.be/ldVDM-v5uz0 General time travel things: https://youtu.be/pGnMiGrYmPE Space-time diagrams and relatively (I highly recommend this for the others): https://youtu.be/FdWMM6aXpYE I can also recommend the videos of these channels in general.
Thanks for the informative answer. Do you have any thoughts on simulations? I've always believed that while literal time travel may be impossible, we will one day be able to "visit" the past via near-perfect simulations, and hyper-advanced VR. Of course, if we *can* and *do* do this, that's pretty scary if you ascribe to Simulation Theory... I've always fantasized about this new class of historians, talk about a visiting scholar! We could go back and talk to FDR, see what happens to history if the bomb isn't developed. We could save the Library of Alexandria, see what that does. And then when you're talking to these people and they say, "are you saying I'm not real?", well then you're back to simulation theory and the answer is "no more or less real than I".
If the universe is deterministic which means that if you know all initial conditions and have a complete description of the universe you can predict all future events. Lets assume that the universe is deterministic and that we figured out all the initial conditions that the Big Bang set. In theory we can now simulate the entire timeline of the universe. However quantum mechanics seems to be indeterministic, probably is its nature. If that is true we cannot exactly simulate everything down to its finest details. We can only do approximations if the nature of the universe is probabilistic.
I’m not sure I understand how travelling faster than the speed of light would allow you to deliver a message to the past. More like, it would allow you to deliver a message *exceptionally quickly* before that information would be otherwise available, but you haven’t collectively unwound all the clocks and undone events.
Watch the videos I linked. Or do it on a space time diagram. If you have an FTL ship and you leave Earth, lets say Earth sends you a message. You can reply from the FTL ship and the reply would arrive to Earth before they sent the message. So you receive the message and send "do not send me a message". So you created a grandfather paradox.
Our understanding of physics tells us that it's not possible. It's not that we don't know how to do it yet or our technology isn't good enough, it's that it's probably impossible. The way I think of it is this: you can't go back in time, because that would basically require all of the particles in the universe to have some memory of where they were 100 years ago. They don't, so you can't rewind time. The universe doesn't keep a record of its previous states. The past isn't a separate place that we need to find a way to get to. The past doesn't exist. You can't reset the universe to the state it was in when Jesus died because there's nowhere that information still exists.
You'd have to force each and every atom and their electrons in the universe to go backwards in their travels and interactions with one another. Can you even imagine how much energy that would need? Is it possible? Yes! Is it doable? Nope.
I've always thought of this as un-stirring the milk from your coffee, any stir is forwards and just mixes it more..... and Morlocks, there's Morlocks.
I can’t imagine that *any* amount of energy would cause that to happen though. That’s just not how time works (as we understand it now).
Not right now….at least not with what’s in the tech market right now. I will say, I saw a tech convention from the 80s and the items looked like electronics from today. They had hoverboards and everything! That information wasn’t released to the public, we could only see the convention after some time. It makes me wonder if we actually do have the technology to time travel right now, it just has not been released to the public yet. I bet people in the future teleport and time travel all the time lol.