[EvaH (OP)]

D wants to know:

Is time travel possible?

What do you think?

[Kryptid]

In a sense, time travel into the future is possible utilizing time dilation. Time travel into the past is far less certain. As we understand it, it would require unconfirmed phenomena such as faster-than-light travel or wormholes.

[alancalverd]

Push the button. The machine transports you 10 minutes into the future then explodes. No problem, as you are now well clear of the present. There are lots of dead machines in the world.

If the machine transports you 10 minutes into the past, we have a problem because the machine that did the job doesn't exist 10 minutes later!

[CliffordK]

In a sense, time travel into the future is possible utilizing time dilation. Time travel into the past is far less certain. As we understand it, it would require unconfirmed phenomena such as faster-than-light travel or wormholes.

I don't believe so.

Say you could instantaneously jump to Pluto.  You will have moved forward in time, but you will simply be in a different location in the universe.  Looking through your telescope at Earth,and you might view events that occurred 5 1/2 hours earlier. 

Watching a supernova from the direction of Earth, and observers on Earth will see the event about 5 1/2 hours before you do. 
Watching a supernova from the opposite direction of Earth, and observers on Earth will see it about 5 1/2 hours after you do.

Now, send a "Hello" message back to Earth via radio waves.  Then wait an hour and jump back.

You'll rejoin Earth time an hour later as if nothing happened.  Then once the light waves reach you, you'll also observe the Hello message.

You may be able to recount the supernova you observed before Earth observed it.  But,that is simply a transmission delay problem, somewhat like being at a track meet, and seeing the puff of smoke and runners take off before you hear the sound of the starter's pistol.

[Kryptid]

Instantaneous in whose reference frame? The very act of such faster-than-light movement would cause you to time travel. Look up the Andromeda paradox and you see that "now" and "simultaneous" are relative terms.

[jeffreyH]

Mathematics might say you can travel into the past but it doesn't make it true. Where are the observations of those time travellers? This would be a huge tourist industry. A real money making opportunity for those future capitalists.

So where are they?

[jeffreyH]

Say you had a rival in business. You send an assassin back in time to kill your rivals grandfather. Now the rival is never born so you don't have the reason to send the assassin in the first place.

You have invented Schrödinger's grandfather. Alive and dead at the same time.

[Jarek Duda]

We live in 4D spacetime - time is just 4th dimension, past and future are already there: https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)
The question is if causality works in both time directions, and CPT theorem ( https://en.wikipedia.org/wiki/CPT_symmetry ) suggests that it should ... ?

For example laser stimulates photon emission, which cause target excitation later.
So could there be built CPT analogue of laser? Stimulating photon absorption - causing deexcitation of target earlier?

For FEL ( https://en.wikipedia.org/wiki/Free-electron_laser ) it seems doable: electrons in sinus-like trajectories, so after CPT we get just positrons traveling the same trajectories in opposite direction.
So could it cause e.g. faster deexcitation of constantly excited target (e.g. sodium lamp for narrow spectrum)?

[yor_on]

https://en.wikipedia.org/wiki/Eternalism_%28philosophy_of_time%29 is interesting Jarek. And a great consolation for those thinking that something is lost for ever. At the same time as it might imply a sort of 'clock work universe' read one way. If it doesn't, if we assume that we have 'free will' to choose, then it must be lost in its outcomes. Or maybe you could get both, but only by assuming that once those choices are made they are stamped into stone, a mold creating a static 'block universe' that you then might be able to traverse in both 'directions'. I don't like time reversals by several reasons, one being that this 'universe' must bifurcate into a new one each time you reverse direction.

[Jarek Duda]

While it is highly nonintuitive for us, there are lots of arguments for living in spacetime, time being only 4th dimension, fundamentally there it time/CPT symmetry, e.g.:
- in special relativity we literally change time direction by changing velocity,
- general relativity discusses behavior of entire 4D spacetime as kind of "4D jello",
- there are lots of arguments for time symmetry in quantum mechanics, e.g. path ensembles, unitary evolution, Wheeler, delayed choice experiments ... Bell violation: of inequalities derived assuming time asymmetry (hidden variables in the past) - no problem to violate them in symmetric models starting with Ising ( https://physics.stackexchange.com/questions/524856/violation-of-bell-like-inequalities-with-spatial-boltzmann-path-ensemble-ising ).

Talk with discussion and gathered experiments like Wheeler, delayed choice quantum eraser etc.:

Regarding free will, in spacetime "the future is already there" based also on our decisions ... which from our perspective were made by us - perspective with limited knowledge: "physics knows the future", but we don't - we are systems of atoms ruled by the same physics, making decisions based on our history, knowledge and current situation.

[evan_au]

Is time travel possible?

Yes, of course it's possible! I do it all the time.

At present, I am traveling into the future at 1 second per second.

If I had a tame black hole, or a fusion rocket, I could travel into the future a bit slower.
- If it were possible for me to get out of the local galaxy cluster, I could travel into the future slightly faster (relative to someone on Earth)

The challenge is to travel anything but 1 second per second into the future!

[charles1948]

Is time travel possible?

Yes, of course it's possible! I do it all the time.

At present, I am traveling into the future at 1 second per second.

If I had a tame black hole, or a fusion rocket, I could travel into the future a bit slower.
- If it were possible for me to get out of the local galaxy cluster, I could travel into the future slightly faster (relative to someone on Earth)

The challenge is to travel anything but 1 second per second into the future!

As you say, that's the real challenge.  Travelling back into the past is easy. We do it through media such as human memory, written records, photographs, movies, gramophone records, CD's and so on.

These media simply record events which have already happened.

But how you could travel into the future and record events which haven't yet happened, seems more difficult.

[jeffreyH]

The trouble with the idea of time travel to the past is, there is nothing to go back to. We invented time as a way of measuring change. You could argue that time dilation shows time is real. However, that is only a slowing down of physical processes. The slowing down of the change of states.

[charles1948]

The trouble with the idea of time travel to the past is, there is nothing to go back to. We invented time as a way of measuring change. You could argue that time dilation shows time is real. However, that is only a slowing down of physical processes. The slowing down of the change of states.

A very perceptive and excellent post.   There's really no such thing as "Time".  It's just a word we've invented in an attempt to account for the way things change.

So we say that things change because of the passage of "Time". Then try to investigate the properties of "Time".
And whether it's possible to travel through "Time" in a "Time Machine"

But we could say - things change because of the passage of "Change"
Then attempt to investigate the properties of "Change", and whether it's possible to travel through "Change",
In a "Change Machine".

You know, the more I think about that, it doesn't seem so silly as I first thought!

[jeffreyH]

Time is still a mathematical dimension. Change, however, is not. Try looking at a calculus book.

Especially look up what a function is. It'll be hours of fun for you.

[gem]

Hi all,
so in regards to the original question, and some of the responses, are we not coming up against the laws of thermodynamics and entropy ? and arguments as to whether physical occurrences, in reality can be symmetric in time.

Yes we know atomic clocks are variously affected by relative velocity and gravitational fields strength, and can be and indeed need to calculated, but I don't believe I have seen an example of a atomic clock going backwards. 

[Halc]

Time is still a mathematical dimension.

This is an interpretation (and one with which I agree), but the prevailing view (possibly not among physicists) is that time is something other than a dimension.
Time travel is expressed very differently in the two interpretations of time.
If time is a dimension, then there is spacetime through which nothing actually travels at all. There are only worldlines, so time travel might represented as a discontinuous worldline.
Time travel under presentism seems an incoherent concept since it implies travel to a state of things that isn't at the present and hence doesn't exist. Most people suggesting time travel assume the naive presentist view, and yet do not address the obvious contradiction with it.

Yes we know atomic clocks are variously affected by relative velocity and gravitational fields strength

All clocks are affected by these things. Atomic clocks are in no way special in this way. Paint peels slower relative to a frame in which it moves faster.

The coordinate rate of clocks can indeed be negative given a non-inertial coordinate system. Ask Mike Fontenot who seems to lose untold sleep about such things, and who has coincidentally posted for the first time in a while just a while ago.

[gem]

Hi all
Halc can I please just clarify your points in regards to;

All clocks are affected by these things. Atomic clocks are in no way special in this way. Paint peels slower relative to a frame in which it moves faster.

The coordinate rate of clocks can indeed be negative given a non-inertial coordinate system. Ask Mike Fontenot who seems to lose untold sleep about such things, and who has coincidentally posted for the first time in a while just a while ago.

In regards to your first point does direction of relative away/towards have any bearing, and your second point are you stating rate of atomic clocks as negative as going in reverse or just slowing down ?.
Because if negative = reverse       
are we not setting the scenario of the paint analogy going from peeling/flaking into the wind to.  returning to the painted surface becoming shiny and new then liquid in the brush/sprayer to been back in the paint tin ?

[Halc]

Hi all
Halc can I please just clarify your points in regards to;

All clocks are affected by these things. Atomic clocks are in no way special in this way. Paint peels slower relative to a frame in which it moves faster.

In regards to your first point does direction of relative away/towards have any bearing

"Direction of relative" does not parse. If you mean direction of motion, the answer is no, it matters not. In SR, time dilation relative to inertial frame F is a function of your speed relative to F, and speed, being a scalar, doesn't have a direction.

The coordinate rate of clocks can indeed be negative given a non-inertial coordinate system.

are you stating rate of atomic clocks as negative as going in reverse or just slowing down ?.
Because if negative = reverse

I meant reverse. We're talking about a coordinate effect, and one can create a non-inertial coordinate system where some clocks run backwards. Look up the Andromeda 'paradox' as an illustration of this.

are we not setting the scenario of the paint analogy going from peeling/flaking into the wind to.  returning to the painted surface becoming shiny and new then liquid in the brush/sprayer to been back in the paint tin ?

If my coordinate system orders the paint events that way, then yes. It's just a coordinate system.

[charles1948]

Say you had a rival in business. You send an assassin back in time to kill your rivals grandfather. Now the rival is never born so you don't have the reason to send the assassin in the first place.

You have invented Schrödinger's grandfather. Alive and dead at the same time.

Your post raises a point which has always puzzled me.

In these discussions about paradoxes resulting from Time Travel into the past, it's always the "grandfather" that gets killed.

Why isn't it just the "father"?    I mean, if your father was killed before he had a chance to engender you, you'd never have been born.  And the temporal paradoxes flowing from that, given that you do actually exist, would still apply.

So why the one-generational back-step to grandfather?  Has it any significance?