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TheQuestionGuy14
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Is there any way to travel back in time in reality according to GR? Let me know!
TheQuestionGuy14 said:Is there any way to travel back in time in reality according to GR?
In reality I mean, not in the movies, through our understanding of physics.PeterDonis said:What does "in reality" mean?
As a matter of mathematics, the Einstein Field Equation, the central equation of GR, admits solutions with closed timelike curves, which is the technical way of saying "allows travel back in time". But AFAIK nobody thinks those solutions are physically realistic.
TheQuestionGuy14 said:In reality I mean, not in the movies, through our understanding of physics.
TheQuestionGuy14 said:Is there any way to travel back in time in reality according to GR? Let me know!
Chris Miller said:traveling backwards in time would be relative
Chris Miller said:since the universe is a 4D construct, it has no "thickness" in its highest 4th (t) dimension (just as an expanding 3D balloon has no 3rd (z) dimensional thickness. So in the universe, a point's past does not exist. So, to travel back you would have to leave the universe
Chris Miller said:So traveling backwards in time would be relative.
Chris Miller said:So, to travel back you would have to leave the universe.
DrStupid said:Yes, in the sense that t2 < t1 for one observer doesn't necessarily imply t2' < t1' for any other.
Going from (x,y,z,t1) to (x,y,z,t2) with t2 < t1 within the same block universe would be a time-travel into the past. Why do I need to leave the universe to do that?
Leaving the universe would be required for different pasts (e.g. in the grandfather paradox with one past where the grandfather survives and another where he dies). But that's another topic.
Chris Miller said:I guess I was assuming that for an observer at any x,y,z there is only one t:
Thanks, PD. Yes, I understand that x,y,z,t are just coordinates in 4D spacetime (the universe). Not sure what you mean by "an observer's worldline" though. Especially "worldline."PeterDonis said:Why would you assume that? Do you understand that ##t## is a coordinate just like ##x, y, z##, and that an observer's worldline is a curve in spacetime, i.e., a continuous set of points, not just a single point?
Chris Miller said:Not sure what you mean by "an observer's worldline" though
Chris Miller said:Local time travel is loaded with paradoxes.
Thanks. So, according to GR/SR, it's theoretically possible for an observer to return to some x,y,z,t point at a t' that is < t?PeterDonis said:It is the curve in spacetime that describes the observer's history. The usual way of expressing it mathematically is as a set of four functions ##x(\tau)##, ##y(\tau)##, ##z(\tau)##, ##t(\tau)## that parameterize the curve by the observer's proper time ##\tau##.
Thanks again, Dr. S. By "local time travel" I just meant traveling from x,y,z,t to x,y,z,t' where t' < t.DrStupid said:I don't know, what you mean by "local time travel", but due to the Novikov self-consistency principle paradoxes wouldn't be an issue.
Very interesting. To me, this translates to "Traveling back in time is impossible," because even just the appearance of an atom changes something. Also, I'd argue that, even were backwards time travel common, it would be impossible to change the past regardless of one's actions. Whatever "new" past then becomes the only past, along with all memory and record of it. It would be no different than how we "change" the future. (E.g., I could assert that a time travel technology was invented, the result of which was so disastrous, someone went back and killed the inventor so that it never happened after all.)The principle asserts that if an event exists that would cause a paradox or any "change" to the past whatsoever, then the probability of that event is zero.
Chris Miller said:To me, this translates to "Traveling back in time is impossible," because even just the appearance of an atom changes something. Also, I'd argue that, even were backwards time travel common, it would be impossible to change the past regardless of one's actions. Whatever "new" past then becomes the only past, along with all memory and record of it.
Chris Miller said:according to GR/SR, it's theoretically possible for an observer to return to some x,y,z,t point at a t' that is < t?
Chris Miller said:Each point in the 4D universe is mapped onto to a unique x,y,z,t coordinate
Chris Miller said:By "local time travel" I just meant traveling from x,y,z,t to x,y,z,t' where t' < t.
Chris Miller said:To me, this translates to "Traveling back in time is impossible,"
Chris Miller said:even just the appearance of an atom changes something
Chris Miller said:The math and QM, though over my head, seem mis-applied
PeterDonis said:It is the curve in spacetime that describes the observer's history. The usual way of expressing it mathematically is as a set of four functions ##x(\tau)##, ##y(\tau)##, ##z(\tau)##, ##t(\tau)## that parameterize the curve by the observer's proper time ##\tau##.
Arkalius said:I think it is accurate to say that in flat spacetime, for any ##\tau_1 \lt \tau_2##, ##t(\tau_2)^2 - t(\tau_1)^2 \gt x(\tau_2)^2 - x(\tau_1)^2 + y(\tau_2)^2 - y(\tau_1)^2 + z(\tau_2)^2 - z(\tau_1)^2##, correct?
Arkalius said:Would it also be true in general relativity in curved spacetime geometries with which we are currently familiar?
Arkalius said:would it also be accurate that there exist solutions to the field equations that create geometries wherein this inequality no longer holds true, including closed timelike curves?
Sure, if you have access to a wormhole and don't mind the risk of getting stuck in an infinite time loop. From a nature.com article:TheQuestionGuy14 said:Is there any way to travel back in time in reality according to GR? Let me know!
stoomart said:if you have access to a wormhole
I assume there aren't any realistic conditions where CTCs are possible (maybe inside black holes rotating at relativistic speeds?).PeterDonis said:This requirement is highly likely to mean you can't do this "in reality" in the usual sense
stoomart said:I assume there aren't any realistic conditions where CTCs are possible
stoomart said:(maybe inside black holes rotating at relativistic speeds?)
According to General Relativity, time travel is possible by creating a closed timelike curve, also known as a time loop. This can be achieved through the use of extreme gravitational forces or by warping space-time using a massive object like a black hole.
No, General Relativity only allows for time travel to points in the past that are within the existence of the time machine. This means that you cannot travel back to a time before the time machine was created.
According to General Relativity, changing the past is not possible. The past is already set and any actions taken in the past will have already happened in the present. This concept is known as the "Novikov self-consistency principle".
The speed required to travel back in time depends on the strength of the gravitational forces or the mass of the object used for warping space-time. However, it is estimated that the speed needed would be close to the speed of light.
General Relativity predicts that time travel could create paradoxes, where the past is changed in a way that contradicts the present. This could have serious consequences and is one of the reasons why time travel is still just a theoretical concept.