# Is time travel possible?

is time travel possible?

If yes, how is it possible to travel through time?

Can we travel in the future?

Can we travel back in the past?

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Dale
Mentor
Can we travel in the future?
Yes, I am doing so right now.

Are you joking here?

I'm serious.

HallsofIvy
Homework Helper
So is DaleSpam. He, and everyone, is moving into the future at 1 second per second. Your question is really "Can we move into the future faster than that?"

yes, we are all traveling through time, including you.
however it is impossible to travel backwards in time, we can only go forwards. You can go forwards more slowly than everyone else by changing your velocity to near the speed of light(so it appears that everyone else is aging faster than you are).

Einstein said that we can travel in the future.

That's what i meant, if it is really possible.

jack action
Gold Member
Let's try a "thought" experiment. Let's assume that time traveling is possible. Let's say it is possible to travel into the future. Even though the future doesn't exists yet, somehow you can go there.

If this is true, then time travel to the past should be easier, since the past already happened and it should be easier to "locate".

Let's say that in the future, someone discover how to travel through time. Well, it means that this person would be able to travel to the past (meaning here, at this time) and tell us that it is possible.

Since it never happened, that probably means that nobody will ever discover time travel.

Wouldn't the laws of thermodynamics prevent travel into the past? If you leave the present and travel to the past then you are decreasing the total mass and energy of the universe at this point in time and increasing the total mass and energy of the universe at some point in the past. And you couldn't say that the two cancel out because they don't occur simultaneously because of the relativity of simultaniety right? Or is that reasoning just off the wall?

You can slow down the processes in your body so that you live longer. You also experience things around you happening at a quicker rate because your brain function is slowed down.

You do this by increasing your relative velocity so that the interactions between elementary particles take longer to take place (relatively) because light speed is a constant and so electromagnetic forces have to travel a longer distance within the molecules that make up your body.

This is what special relativity proposed about watches and it translates to living things. We can live longer, though we do not experience a longer life.

f95toli
Gold Member
First of all, time travel into the future is definitely possible. All you need to do is to travel at some speed v relative to whatever you use as your reference frame. This is a basic result from special relativity. The effect is small but easy to measure if you e.g. put an atomic clock on an aircraft and fly around reasonably fast for a few hours (an experiment that has been done a few times). This effect is also routinely compensated for in the GPS system (since the satellites are moving pretty fast with respect to you). Or, you can just live on the second floor; you will age slightly faster (according to your neighbors, remember that this is a relative effect) than the people on the ground floor (again, easy to measure with a good clock)...This is one prediction by general relativity.

Now, we don't know if it possible to travel back in time or not. Some physicists believe that it is impossible, but no one has been able to show that this is really the case. The are a number of suggested possible mechanisms (possible as in "no one has shown that they won't work) for traveling backwards in time, but all of them are extremely impractical; they e.g. require enormous amounts of energy (the equivalent of several stars etc),exotic matter and so on.
Hence, it might turn out that time travel might be possible in principle but extremely impractical (it could e.g. turn out that you would require all the energy in the universe to travel 1s back in time)

Now, we don't know if it possible to travel back in time or not. Some physicists believe that it is impossible, but no one has been able to show that this is really the case. The are a number of suggested possible mechanisms (possible as in "no one has shown that they won't work) for traveling backwards in time, but all of them are extremely impractical; they e.g. require enormous amounts of energy (the equivalent of several stars etc),exotic matter and so on.
Hence, it might turn out that time travel might be possible in principle but extremely impractical (it could e.g. turn out that you would require all the energy in the universe to travel 1s back in time)
There is a fundamental problem with these theories beyond just that of FTL travel. You must keep track of which observer is accelerating up to the relative velocity for that is the observer that is invariantly changing reference frames. This means that for a back to the future, observer existing in history type scenario the entire universe would need to accelerate to an FTL velocity relative to the observer and not the other way around.

f95toli
Gold Member
There is a fundamental problem with these theories beyond just that of FTL travel. You must keep track of which observer is accelerating up to the relative velocity for that is the observer that is invariantly changing reference frames. This means that for a back to the future, observer existing in history type scenario the entire universe would need to accelerate to an FTL velocity relative to the observer and not the other way around.
None of the "serious" suggestions(=published in proper journals) that I am aware of rely on FTL travel (but then, I am most definitly not an expert, I just look at stuff I stuble across in PRL etc), they are all based on what I guess one could call various "GR effects": worm holes, rmassive otating cylinders (which was the "original" idea, developed by Wheeler(?) ) etc.
The basic idea is always to play with spacetime itself somehow (which is why you need so much energy).

What I've never liked about the past time travel thought experiments is that they assume that time is same going backwards as forwards. In other words, they assume that going back in time is equivalent to rewinding a movie, playing it again, and seeing the same thing. We don't necessarily know that this is a valid assumption. In fact, it's an ongoing endeavor in particle physics to try to understand what role the time operator plays. We know that there is no symmetry from the T operator. Beyond that even, what we thought gave a great unbreakable symmetry from the combined CP operator has been broken itself. The combination of it all, CPT may hold for now, but we don't really know if it will be broken too. When we know that nature doesn't always even obey seemingly intuitive rules such as parity, that when we look at a particle in a mirror and the mirror doesn't mimic the original object, we have to realize that physics can easily jump out of the realm of our everyday experience.

Look at the reason why future travel is possible. Special relativity, and general relativity, are mind boggling concepts that have very little rationale in our everyday lives. They break seemingly intuitive concepts in our mind. Yet, we find that from this broken intuition of relative speeds that future time travel is possible. Time travel to the past might be possible for other non-inuitive reasons, just as has happened with future time travel.

In my opinion, we can't really fully rule out time travel to the past just because it fails our current intuition and reasoning.

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the faster you get to the speed of light the slower time flows around you. so yes you can

Interesting that you ask. In the Philosophy of Physics class I took this semester (yes, grad students goof off too), we addressed that issue. I'd recommend "The Paradoxes of Time Travel" by David Lewis. It should be easily located with Google. In this paper, he addresses the question of whether or not a time traveler can change history. It's a rather fascinating read. He even starts with "I maintain that time travel is possible." So he'd say yes.

Of course I'm directing you to a philosophy paper because, for better or worse, we don't take time travel all that seriously in physics. There is the theoretical possibility of traveling through a wormhole with one end held at a fixed time. But since wormholes themselves are only mathematical oddities of General Relativity, even this is quite far fetched.

the faster you get to the speed of light the slower time flows around you. so yes you can
Actually "time" speeds up around you, you slow down. It is your inner workings that have a longer distance to travel now that you have a new found velocity.

There is no magic behind this, it's just that x' = x + vt so the distance is longer for an electromagnetic wave or any other force with an invariant speed to travel.

@arunma Thanks I got it in PDF and I'm going to read it.

If you traveled at the speed of light, you would go back in time. However, matter traveling at the speed of light(in a vacuum) is impossible. And you have to remember that time is relative, and also the fact that because you are in the fourth dimension, you have the rights of time and space travel.

diazona
Homework Helper
However, matter traveling at the speed of light(in a vacuum) is impossible.
That's true, but I don't think the rest of what you said makes much sense... certainly traveling at the speed of light wouldn't allow you to travel back in time. (It's not even a well-defined situation, really) Only exceeding the speed of light (in a vacuum) is associated with going back in time.

If time travel to the past was possible then somebody would have done it. Though, it is known to be impossible now but if 20,000 years in the future somebody figures out how to do it, then our current now would already be altered.

Traveling to the future in the sense of faster then 1 second at a time is indeed possible.

Because it is impossible to go faster then the speed of light, once your velocity approaches it, time in for you would slow down.

Dale
Mentor
If time travel to the past was possible then somebody would have done it. Though, it is known to be impossible now but if 20,000 years in the future somebody figures out how to do it, then our current now would already be altered.
All of the GR time machines of which I am aware cannot travel back earlier than when they were built. So if someone were to build a time machine 20,000 years in the future then they would not be able to come back and visit us.

By what I've learnt is that only the speed of light is constant.

Is that correct?

By what I've learnt is that only the speed of light is constant.

Is that correct?
Light speed is invariant (IE: constant) under coordinate transformations (IE: in all reference frames). This is what leads to space-time variance under coordinate transformations (IE: between reference frames).

Actually "time" speeds up around you, you slow down. It is your inner workings that have a longer distance to travel now that you have a new found velocity.

There is no magic behind this, it's just that x' = x + vt so the distance is longer for an
electromagnetic wave or any other force with an invariant speed to travel.[/QU

yeah thats what i meant to put was just talking while typing

I dont think time travel is possible... if it is.. you can only travel into the future..