Exploring the Possibilities of Time Travel

[JesseM]

There are those in the science community who fancy the idea of time travel to be a reality.

Not really, they just note that general relativity seems to allow it in certain unusual spacetimes, but I think everyone would also acknowledge that we need to understand more about the relationship between GR and quantum mechanics in order to judge if these scenarios are actually possible.

Simultaneous action between two objects is a reality in the quantum realm.

No, quantum field theory respects relativity, and therefore you can analyze the same physical situation from the perspective of different frames which disagree about simultaneity, and the laws of physics will work exactly the same in each of these frames.

When the "theory of everything" is perfected I do believe it will include a form of absolute time while still honoring SR.

Impossible by definition to have absolute time yet still honor SR, because the only way to establish a preferred definition of simultaneity by physical experiment would be to show that the laws of physics don't work precisely the same way in every frame, that one frame is "preferred" physically, which violates the first postulate of SR.

 

[peter0302]

No, quantum field theory respects relativity, and therefore you can analyze the same physical situation from the perspective of different frames which disagree about simultaneity, and the laws of physics will work exactly the same in each of these frames.

The laws of physics may work the same in every frame but every interpretation of QM other than many-worlds requires faster-than-light influence. You can argue with the semantics but at the interprative level there's no getting around it.

 

[Chip Orr]

No, quantum field theory respects relativity, and therefore you can analyze the same physical situation from the perspective of different frames which disagree about simultaneity, and the laws of physics will work exactly the same in each of these frames.
You might disagree as an observer about simultaneity from the perspective of different frames but after analyzing the event properly by compensating for info travel distance, velocity of observer in said frame etc. you would agree on the simultaneity of the event. On the other point there is not a "preffered frame" where physics doesn't work the same as in other frames. Absolute time would be a moment shared by all frames but not bound by the same apparent time.

 

[JesseM]

You might disagree as an observer about simultaneity from the perspective of different frames but after analyzing the event properly by compensating for info travel distance, velocity of observer in said frame etc. you would agree on the simultaneity of the event.

Simultaneity doesn't refer to seeing events at the same time, it is already based on "compensating for info travel distance" (with each observer assuming that info travels at c in their own rest frame). For example, if in the year 2010 as measured by my clocks I see the light from an event which is 10 light-years away as measured by my rulers, and then in the year 2020 I see the light from an event which is 20 light years away, I will label these two events as "simultaneous" in my frame. But if each observer follows a similar procedure for "compensating for info travel distance", they will disagree about whether events are simultaneously. Take a look at the thought-experiment Einstein uses to illustrate this here and here, which is also illustrated in a little animation http://www.cord.edu/dept/physics/credo/etrain_credo.html , and discussed on pp. 62-63 of Taylor and Wheeler's book spacetime physics as follows:

The Principle of Relativity directly predicts effects that initially seem strange--even weird. Strange or not, weird or not; logical argument demonstrates them and experiment verifies them. One effect has to do with simultaneity: Let two events occur separated in space along the direction of relative motion between laboratory and rocket frames. These two events, even if simultaneous as measured by one observer, cannot be simultaneous as measured by both observers.

Einstein demonstrated the relativity of simultaneity with his famous Train Paradox. (When Einstein developed the theory of special relativity, the train was the fastest common carrier.) Lightning strikes the front and back ends of a rapidly moving train, leaving char marks on the train and on the track and emitting flashes of light that travel forward and backward along the train (Figure 3-1). An observer standing on the ground halfway between the two char marks on the track receives the two light flashes at the same time. He therefore concludes that the two lightning bolts struck the track at the same time--with respect to him they fell simultaneously.

A second observer rides in the middle of the train. From the viewpoint of the observer on the ground, the train observer moves toward the flash coming from the front of the train and moves away from the flash coming from the rear. Therefore the train observre receives the flash from the front of the train first.

This is just what the train observer finds: The flash from the front of the train arrives at her position first, the flash from the rear of the train arrives later. But she can verify that she stands equidistant from the front and rear of the train, where she sees char marks left by the lightning. Moreover, using the Principle of Relativity, she knows that the speed of light has the same value in her train frame as for the ground observer (Sectin 3.3 and Box 3-2), and is the same for light traveling in both directions in her frame. Therefore the arrival of the flash first from the front of the train leads her to conclude that the lightning fell first on the front end of the train. For her the lightning bolts did not fall simultaneously. (To allow the train observer to make only measurements with respect to the train, forcing her to ignore Earth, let the train be a cylinder without windows--in other words a spaceship!)

Did the two lightning bolts strike the front and the back of the train simultaneously? Or did they strike at different times? Decide!

Strange as it seems, there is no unique answer to this question. For the situation described above, the two events are simultaneous as measured in the Earth frame; they are not simultaneous as measured in the train frame. We say that the simultaneity of events is, in general, relative, different for different frames. Only in the special case of two or more events that occur at the same point (or in a plane perpendicular to the line of relative motion at that point--see Section 3.6) does simultaneity in the laboratory frame mean simultaneity in the rocket frame. When the events occur at different locations along the direction of relative motion, they cannot be simultaneous in both frames. This conclusion is called the relativity of simultaneity.

The relativity of simultaneity is a difficult concept to understand. Almost without exception, every puzzle and apparent paradox used to "disprove" the theory of relativity hinges on some misconception about the relativity of simultaneity.

On the other point there is not a "preffered frame" where physics doesn't work the same as in other frames.

I agree, but if there was a preferred definition of simultaneity, that would require a preferred frame. If we assume there are no preferred frames, there can be no preferred definition of simultaneity.

Absolute time would be a moment shared by all frames but not bound by the same apparent time.

What do you mean by "shared by all frames" and "not bound by the same apparent time"? Can you give an example?

 

[Chip Orr]

Simultaneity doesn't refer to seeing events at the same time, it is already based on "compensating for info travel distance" (with each observer assuming that info travels at c in their own rest frame). For example, if in the year 2010 as measured by my clocks I see the light from an event which is 10 light-years away as measured by my rulers, and then in the year 2020 I see the light from an event which is 20 light years away, I will label these two events as "simultaneous" in my frame. But if each observer follows a similar procedure for "compensating for info travel distance", they will disagree about whether events are simultaneously. Take a look at the thought-experiment Einstein uses to illustrate this here and here, which is also illustrated in a little animation http://www.cord.edu/dept/physics/credo/etrain_credo.html , and discussed on pp. 62-63 of Taylor and Wheeler's book spacetime physics as follows:


I agree, but if there was a preferred definition of simultaneity, that would require a preferred frame. If we assume there are no preferred frames, there can be no preferred definition of simultaneity.

What do you mean by "shared by all frames" and "not bound by the same apparent time"? Can you give an example?

The world map of the universe is the now, the world picture is the universe as it appears to be but isn't. The simultaneity of an event has nothing to do with the time an observer receives information or the amount of time dilation they are experiencing. Most events that seem to be simultaneous are not and those that don't seem to be might be. The now is the same for all frames regardless of perspective. It seems to me that man has arrogantly put himself at the center of a system once again by thinking his detection of an event has any bearing or relivance on the actual now of the event. Thank you for your time!

 

[vsandel]

What do you mean by the changes being reversed? If I travel from Los Angeles to New York, it's not that the Los Angeles around me has to be transformed into New York, it's that I move from one preexisting city to another. Similarly, a time traveler just has a worldline that moves from a future region of spacetime to a past region, but the future region doesn't "become" the past region.

Two assumptions I make are that time is just the rate at which things change, and that to visit the past the past or future they must both exist. The definition of time travel as popularly conceived is that a person visits either an earlier or later "now". If my assumptions are correct, either the earlier and later "now" has to exist, or if they don't, then to go back to an earlier "now" the changes that have taken place between the present "now" and the earlier "now" would have to be reversed. The latter possibility is ridiculous, so that leaves a view of the cosmos that physically preserves intact all previous "nows" The proposed cosmological models that I have read about that preserve the past "nows" are the infinite universes models. I reject them on the basis of Occam's Razor. Therefore I do not believe time travel as popularly conceived is possible.

 

[JesseM]

Two assumptions I make are that time is just the rate at which things change, and that to visit the past the past or future they must both exist. The definition of time travel as popularly conceived is that a person visits either an earlier or later "now". If my assumptions are correct, either the earlier and later "now" has to exist. If they don't, then to go back to an earlier "now" the changes that have taken place between the present "now" and the earlier "now" would have to be reversed. The latter possibility is ridiculous, so that leaves a view of the cosmos that physically preserves intact all previous "nows" The proposed cosmological models that I have read about that preserve the past "nows" are the infinite universes models. I reject them on the basis of Occam's Razor. Therefore I do not believe time travel as popularly conceived is possible.

But in relativity there isn't even a single "now", each frame has a different definition of now (two events that happen at the same time in one frame happen at different times in another), and all frames are equally valid physically. This is what leads you to the "spacetime" picture where what really exists is a 4-dimensional structure containing every event in every time, and different frame's "nows" represent different ways of slicing this structure up into 3-dimensional slices (sort of like if you sliced up the block of ice in my analogy into 2D slices, with different frames slicing it up at different angles). I guess this is somewhat similar to your notion of "physically preserving intact all previous nows", except "preserving" suggests that you are imagining some genuine "flow of time" with past states having to be "preserved" after they are no longer in the now, whereas the 4-dimensional view just suggests all events in history coexist in a single static 4D spacetime, and there is no objective "now" moving along it in the first place. And this 4D spacetime perspective has nothing to do with any cosmological model of infinite universes as you suggest, it's just the standard way of imagining a single universe in general relativity...the very words "the universe" in GR are usually taken to mean a 4D spacetime rather than a 3D space which changes over time.

 

[vsandel]

But in relativity there isn't even a single "now", each frame has a different definition of now (two events that happen at the same time in one frame happen at different times in another), and all frames are equally valid physically. This is what leads you to the "spacetime" picture where what really exists is a 4-dimensional structure containing every event in every time, and different frame's "nows" represent different ways of slicing this structure up into 3-dimensional slices (sort of like if you sliced up the block of ice in my analogy into 2D slices, with different frames slicing it up at different angles). I guess this is somewhat similar to your notion of "physically preserving intact all previous nows", except "preserving" suggests that you are imagining some genuine "flow of time" with past states having to be "preserved" after they are no longer in the now, whereas the 4-dimensional view just suggests all events in history coexist in a single static 4D spacetime, and there is no objective "now" moving along it in the first place. And this 4D spacetime perspective has nothing to do with any cosmological model of infinite universes as you suggest, it's just the standard way of imagining a single universe in general relativity...the very words "the universe" in GR are usually taken to mean a 4D spacetime rather than a 3D space which changes over time.

If there is no "now", only 4D spacetime, then would the future not be fixed as well as the past? This would be a predetermined universe just playing out as a movie.

I am not acquainted with relativity mathematically, but in my way of thinking the relativity of simultaneity does not rule out an absolute "now". I look at it as a measurement problem much like the Heisenberg uncertainty principle. The accuracy of simultaneous measurement of the momentum and position of a particle is limited because measuring the momentum disturbs the position and visa verse. That does not imply that the particle does not have momentum and position simultaneously, just that we can't measure it. Looking at time as change, the "now" would be the state and position of every particle in the universe at a given instant. Although we can't measure it, who is to say it doesn't exist?

Furthermore, general relativity is just a mathematical model of reality, not the reality itself. It correctly predicts some things and is the best current model of reality, but as soon as it makes a wrong prediction it will be "back to the drawing board". To completely dismiss ideas that are not based upon GR seems to me to be to be unwise until there is a lot more evidence that the theory truly predicts reality accurately.

 

[JesseM]

If there is no "now", only 4D spacetime, then would the future not be fixed as well as the past? This would be a predetermined universe just playing out as a movie.

Sure, all events would be fixed in spacetime (though this is not the same as 'determinism' which assumes that complete knowledge of the present is enough to uniquely predict the future). In the 4D view it wouldn't even really be "playing" out, since that would imply some notion of a "moving present" passing through spacetime...instead all of spacetime would just exist like a static geometric structure.

I am not acquainted with relativity mathematically, but in my way of thinking the relativity of simultaneity does not rule out an absolute "now".

You're right that it doesn't rule it out in a metaphysical sense, but it rules it out as a notion that could have any physical correlates whatsoever--if relativity is correct there would be absolutely no experiment that could distinguish the "correct" definition of simultaneity from the "false" definitions of other frames, so true simultaneity would be just a sort of phantom, a bit like the idea of entities "outside" the physical universe.

I look at it as a measurement problem much like the Heisenberg uncertainty principle. The accuracy of simultaneous measurement of the momentum and position of a particle is limited because measuring the momentum disturbs the position and visa verse. That does not imply that the particle does not have momentum and position simultaneously, just that we can't measure it.

Sure, this is a decent analogy, as long as you accept that quantum mechanics means that it is impossible in principle to ever measure them simultaneously, not just that it's something we can't do at present with existing experimental techniques (of course quantum mechanics could be wrong, but the same is true of relativity). And there are some serious problems with imagining that values for noncommuting variables exist simultaneously, it seems that you'd have to accept the possibility of hidden FTL effects as well--see this thread.

Furthermore, general relativity is just a mathematical model of reality, not the reality itself. It correctly predicts some things and is the best current model of reality, but as soon as it makes a wrong prediction it will be "back to the drawing board". To completely dismiss ideas that are not based upon GR seems to me to be to be unwise until there is a lot more evidence that the theory truly predicts reality accurately.

General relativity the physical theory will have to be replaced by a theory of quantum gravity, but the relativity of simultaneity is a consequence of Lorentz-invariance, which is not a specific physical theory but rather a symmetry in the local laws of physics, like translational symmetry, which can be true of many possible theories (for example, although quantum field theory was discovered long after special relativity, it is compatible with SR because it is a Lorentz-invariant theory). This post from sci.physics.relativity argues for why "aether theories" which postulate a single preferred frame should be considered very unlikely, and some of the given reasons would also make good arguments against the idea that some future theory will end up violating Lorentz-symmetry (especially reason #6). The basic question is, if fundamentally the laws of physics don't respect this symmetry, why have all observed phenomena so far respected it? Is it just a gigantic coincidence?

 

[vsandel]

The Twin Paradox

Yes, and so is "v". The negatives therefore cancel out to a positive.

In an earlier submission I stated that concerning the twin paradox, when a spaceship travels away from the Earth at a velocity near the speed of light time in the spaceship slows down relative to the time on earth, but on the return trip time in the spaceship speeds up relative to Earth time. Therefore the returning twin has aged exactly as much as the twin who stayed on earth. I stand by my conclusion in spite of the fact that I wrongly stated that x in the equation is negative on the way back:

t'=(t-vx/c^2)/sqrt(1-v^2/c^2)​

In Einstein's train illustration, the origin is placed on the embankment (earth) and both the train and light beam travel along the x axis. When the train (or space ship) is traveling in the positive x direction, x is positive and so is v. However, if at some point far from Earth the train (space ship) is turned around and now travels toward the light source, x is still positive but v is now negative as Peter points out. Therefore the traveler's time speeds up relative to earth.

Using Einstein's reasoning in explaining relativity, traveling toward the light source would cause the expected speed of light measured on the train (space ship) to be the sum of c+v where v is the velocity toward the light source. Since c must be constant, time must speed up in the train (spaceship) relative to the embankment (earth). Thus the slowing of time in a spaceship going away from Earth would be compensated by the speeding up of time on the way back. Therefore the twin paradox is false, and you cannot travel to the future by going on a space journey at near the speed of light. Is my reasoning correct?

 

[peter0302]

I stand by my conclusion in spite of the fact that I wrongly stated that x in the equation is negative on the way back:

t'=(t-vx/c^2)/sqrt(1-v^2/c^2)​

I should have pointed out in the other thread that that formula really is about coordinate transformations, not figuring out propert time. The formula you should be using is t'=t/gamma, which is independent of direction.

 

[vsandel]

I should have pointed out in the other thread that that formula really is about coordinate transformations, not figuring out propert time. The formula you should be using is t'=t/gamma, which is independent of direction.

Like I said earlier, I am not a physicist. What is gamma? The formula I mention is taken from Einstein's book "Relativity", which is high school level reading.

 

[JesseM]

Like I said earlier, I am not a physicist. What is gamma? The formula I mention is taken from Einstein's book "Relativity", which is high school level reading.

"gamma" (also written as the greek letter $$\gamma$$) is used as shorthand for the factor $$\frac{1}{\sqrt{1 - v^2/c^2}}$$ which appears in a lot of different equations in relativity.

For example, for v=0.6c, gamma would be 1.25.

 

[vsandel]

Thanks, JesseM. I appreciate your comments to my posts. Is my reasoning correct that on the return trip from a space journey time relative to Earth time speeds up?

 

[JesseM]

Thanks, JesseM. I appreciate your comments to my posts. Is my reasoning correct that on the return trip from a space journey time relative to Earth time speeds up?

No, as long as the speed relative to Earth on both legs of the journey is the same, the time dilation factor (which depends only on the value of the speed v) will be the same on both legs too. Your argument seems to be similar to the one made by joey_m on this thread, so have a look at my comments there, especially the numerical example I gave in post #6 (and elaborated on in later posts).

 

[ben328i]

if hasn't already been said. which i think it might have
time travel to the future is possible.
you can not go backwards
you have to go faster than light to actually achieve time travel.
that is millions of millions miles and hour.
what have we hit recently ? maybe 2 grand maybe 4

 

[W3pcq]

we hit excess of 20k in space.

 

[W3pcq]

If you define time using a photon emitter, then how would you measure a reversal of time with the same device?
If no photons emit, then I suppose T=0. How can a negative amount of photons be emitted? I guess if the emitter stops emitting and begins to take in photons, then "time" has reversed?

Maybe if you could somehow absolutely eliminate all velocity and maximize gravity, then you could never age? Maybe that is what black hole is. In black whole T=0, maybe <0? There is a debate over wether or not time can exist without matter, if time can't exist without matter, then could matter exist without time. If time was slowed to zero would we cease to exist? If time could go from 0 to -n, would we have already been crushed out of existence by the time we reached t=0?

 

[basePARTICLE]

If you define time using a photon emitter, then how would you measure a reversal of time with the same device?If no photons emit, then I suppose T=0. How can a negative amount of photons be emitted? I guess if the emitter stops emitting and begins to take in photons, then "time" has reversed?

Maybe if you could somehow absolutely eliminate all velocity and maximize gravity, then you could never age? Maybe that is what black hole is. In black whole T=0, maybe <0? There is a debate over wether or not time can exist without matter, if time can't exist without matter, then could matter exist without time. If time was slowed to zero would we cease to exist? If time could go from 0 to -n, would we have already been crushed out of existence by the time we reached t=0?

As soon as someone says define time as XXX we usually encounter new physics, unless there is irreducible correspondence.

 

[Ian Davis]

The focus on whether time travel is possible seems to presume that the past continues to exist. I would be interested is strictly mathematical devices demonstrating that there was a mathematical theory that supported the notion that there was some past to go back to in the first place.

http://www.rochester.edu/news/show.php?id=2544
http://seattlepi.nwsource.com/local/292378_timeguy15.html
http://cs.astronomy.com/asycs/forums/p/29882/361586.aspx#361586

I've not seen the problem raised here that were one to go backwards in time, short of finding a very auspicious route that avoided the problem altogether, one would at the
moment in time when one ceased moving forward in time collide with that former self
that was still moving forward in time. If charges are reversed for matter moving backwards in time this would I suspect (feel free to contradict me -- I'm no expert) be a case of a very large quantity of matter meeting an equally large quantity of anti-matter. Not a good recipe for surviving long enough to declare that time travel works. Even circumventing this issue, one would create a nightmare in the book-keeping department where all sorts of physical properties are supposed to be preserved.

 

[oliverswanshu]

Hello everyone,
I am new to this forum and would greatly appreciate your expertise in answering (or attempting to) a few questions for me. I don't care if you focus on one or all of them, but any answers are appreciated! These are about time travel, with a focus on traveling BACK in time...

1. Will time travel ever be possible?
2. If so, how soon? Also, how far back would we be able to travel?
3. Would our bodies be able to survive time travel?
4. Would time travellers be able to communicate with other humans, or would they be on a separate plane of time/dimension?
5. Is it possible that spaceship sitings are time travel machines from the future?

Also, if anyone can answer #4 (at least in theory), I'd be grateful.

That simple statement of fact is the key to understanding why we cannot nor will not- ever- physically travel, either forward or backward, in time. We, the computer keyboard I am using, my house, the planet, all the planets in our solar system not to mention all the galaxies; in fact the universe and everything in it; only PHYSICALLY exists in the present- in the NOW. There is no future- physically now. We and the whole universe does not PHYSICALLY exist in ten minutes AT PRESENT. We can be confident that it will because it DID ten minutes ago.
And that's the point. Thats the simple truth of it all. Because we remember the past we therefore anticipate the future.
We (and everything) physically exist only in the NOW. The PRESENT. We and everything (and I mean everything) do not physically still exist ten minutes- one nano second ago. Nor do we physically exist at present, one minute from NOW.
Simple eh?