The discussion centers on the theoretical possibility of time travel, particularly traveling back in time. Participants highlight that current scientific understanding, particularly within the framework of General Relativity, does not definitively support the feasibility of time travel. Key points include the necessity of reversing all changes in the universe to enable backward time travel, the implications of energy requirements, and the potential existence of wormholes as theoretical conduits for time travel. The conversation also touches on the philosophical implications of time and the nature of reality.
PREREQUISITESPhysicists, philosophers, science fiction writers, and anyone interested in the theoretical aspects of time travel and its implications on reality.
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.vsandel said:My assumption is that to visit the past, the past must still exist. If the object's "worldline" curves back on itself to revisit a region of spacetime it already crossed through before, would that space time not have the identical state and energy as when first crossed? If so, wouldn't the changes have to be reversed?
Where did I assume anything about infinite universes? I am imagining that there is no objective "now", any more than there's an objective "here". There's just 4D spacetime, which is represented in my analogy by the entire block of ice (in philosophy this idea is known as eternalism, or the B-theory of time, and is opposed to presentism which says that only the present exists). Even without time travel, relativity calls the notion of an objective "now" into question, because different reference frames will disagree about whether two spatially separated events happened "at the same time" or "at different times" (this is a feature known as the relativity of simultaneity), and relativity says that as far as physics goes all these reference frames are on equal footing.vsandel said:It seems that you are assuming an infinite universes model of the cosmos in which the "now" travels from one universe to another.
That assumes that visitors from the future would allow their presence to be known.A very simple "proof" for time travel not possible is that nobody from the future had ever been traveled back to meet us.
Michael Jackson.Further, there is no solid proof ever for any extra terrestial intelligents had ever visited our solar system.
bchui said:A very simple "proof" for time travel not possible is that nobody from the future had ever been traveled back to meet us. Further, there is no solid proof ever for any extra terrestial intelligents had ever visited our solar system. I doubt that even space travel is more difficult tahn ever imagined, if not impossible!
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.Chip Orr said:There are those in the science community who fancy the idea of time travel to be a reality.
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.Chip Orr said:Simultaneous action between two objects is a reality in the quantum realm.
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.Chip Orr said:When the "theory of everything" is perfected I do believe it will include a form of absolute time while still honoring SR.
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.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.
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.
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:Chip Orr said: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.
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.
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.Chip Orr said:On the other point there is not a "preffered frame" where physics doesn't work the same as in other frames.
What do you mean by "shared by all frames" and "not bound by the same apparent time"? Can you give an example?Chip Orr said:Absolute time would be a moment shared by all frames but not bound by the same apparent time.
JesseM said: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?
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.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.
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 said: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.
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.JesseM said: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.
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.vsandel said: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.
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.vsandel said:I am not acquainted with relativity mathematically, but in my way of thinking the relativity of simultaneity does not rule out an absolute "now".
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.vsandel said: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.
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 said: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.
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:peter0302 said:Yes, and so is "v". The negatives therefore cancel out to a positive.
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 said: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)
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.peter0302 said: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.
"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.vsandel said: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.
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).vsandel said: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?
As soon as someone says define time as XXX we usually encounter new physics, unless there is irreducible correspondence.W3pcq said: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?
mlw said: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.