Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Space time physics question

  1. May 15, 2005 #1

    Well I'm not very knowledgable in this area - or in fact any area of physics but I would like to learn and I often find that to ask the questions will help you learn so - here I go.

    I have read and heard, that if a person A moves away from person B with increasing velocity, the time of person A will become less relative to person B.

    So - as A(v) increases, A(t)decreases, relative to B(t).

    This led me to think about the concept of a constant, i.e. if one multiplies some constant, velocity and relative time, one emerges with a constant that is true for the velocity for person B.

    i.e. A(v) * A(t) * C = B(v) * B(t) * C

    After this, i became wary, if person travels in the opposite direction to person A, then their Velocity becomes negative in reference to person A. Therefore, for a relationship to exist, time must also become negative, therefore time for person B, is running in reverse, with reference to person B, on one of them is going back in time.

    this is where i began to think, I must be wrong ada so dove to my computer to ask your advice and knowledge, I have some other ideas too however but they can wait - this is one i'm very interested about.


  2. jcsd
  3. May 15, 2005 #2


    User Avatar
    Science Advisor

    Does B(t) represent the length of B's clock-tick compared to A's clock tick from A's perspective, or from B's perspective? They'll be different. If the relative velocity of A and B is 0.6c, then from B's perspective, A's clock will only tick 0.8 seconds in the same time that B's clock ticks 1 second. But from A's perspective, it's B's clock that's slowed down by a factor of 0.8, so that A's own clock ticks 1/(0.8) = 1.25 seconds in the time that B's clock ticks 1 second.
    It's only the magnitude of the velocity (the speed) that determines the amount the clock slows down, not the direction. There's no such thing as negative speed, of course.
  4. May 15, 2005 #3
    B(t) is the clock tick of B relative to A yes.

    Okay i understand now - I just confused myself by saying. 'You cannot have velocity without direction. it is a vector quantity not a scalar.'

    on a tangent, I also heard, that a clock on the surface of the Earth, will be 'faster' than a clock 1 mile above the Earth, as gravity 'bends' space time less. Is this correct? And is there a better explanation?

    Also, as we can travel through space, why cannot we travel through time?

    Finally - if one considers a 1 Dimensional (1D) being, that beign could not comprehend, nor see 2D. And in a similar fashion a (2D) being comprehend 3D and so we can comprehend 4D. We cannot move through time, as a dot on a page can move out towards us - does this imply time is the fourth direction - or that we just figured out how to travel through it yet?

    sorry for all the questions!


    Last edited by a moderator: May 15, 2005
  5. May 16, 2005 #4
    It is true that a gravitational field affects time, however you've got it backwards: a clock close to a large gravitational field will tick slower than an identical clock which is not exposed to a gravitational field. One very easy way to explain this is to think of a "gravitational field" in terms of motion. Here's a brief summary of a description Einstein used.

    Imagine you are out in "empty" space, far away from any other stars or bodies. Now imagine a large cage out there in that empty space with a man in it. Since the man is not subject to any gravitational fields, he has no way of orienting himself so that he can call one direction "down" and another "up", but imagine also that there is a hook on one end of the cage and a rope tied to that hook is pulled by some large, unseen giant far away. The giant pulls with a constant force on the rope, accelerating the cage in one direction. The man will now have a reason to call the direction of his acceleration "up" and the opposite direction "down", and he will be able to stand up planting his feet in the down direction. If this man has grown up in this cage all his life, he will assume himself to be "still". If he is inquisitive about the state of his universe, he may then perform experiments in order to learn. Upon letting go of a ball he will always see it fall down, and thus will come to the conclusion that there is a gravitational force in his universe pulling all bodies in the downward direction. Shortly after this realisation, he may wonder, "why then doesn't my cage fall as well?" But it won't take him long to discover the rope which is tied to the top of the cage, thus he will conclude that the cage doesn't fall because it is being held up by the rope. In accordance with the general principle of relativity, the man is justified in all of his conclusions.

    The previous example is one Einstein uses to familiarize you with the idea of using acceleration in one person's perspective as a gravitational field in another person's perspective. To help in understanding a gravitational field's affect on time, he uses an example of a spinning disc. Let's assume again that we are out in "empty space", watching a large, flat disc spinning. A man living on this disc, calling himself "at rest", will come to the conclusion that there is a gravitational field pulling on him and other bodies in all directions within the plane of the disc and with increasing magnitude as he walks away from the center of the disc. We (watching the disc spin) of course conclude that he's actually feeling a centrifugal force as a result of the disc's spin (just like the force you feel on a ride at the fair or a theme park that moves in a fast circular motion), but he is justified in his belief that he and his disc are stationary and that the effects are due to a gravitational field. If this man, also being inquisitive about the universe, were to place one clock at the center of the disc and a clock of identical construction at the edge of the disc, he would find that the clock at the edge (in a stronger gravitational field) would tick slower than the clock at the center of the disc. We can easily come to this conclusion because we know that as you speed up, time slows down, and from our perspective (watching the disc spin) we see that the clock at the edge is moving faster than the clock at the center, while the man living on the disc is perfectly justified in saying that it's not motion that affects the clock's speed, but the gravitational field.

    This explanation doesn't exactly answer all questions of "why", but it's a very easy to understand explanation, and it doesn't require any math, just concepts.

    We're always traveling through time. What we can't do is travel freely through time, atleast in the same sense that we can travel freely through space.
    Last edited: May 16, 2005
  6. May 16, 2005 #5
    With reference to the clocks - you are saying that A clock in a gravitational field runs more slowly according to the gravitational time dilation relationship from general relativity?

    Is there something stopping us? Or is it just we haven't figured out how yet?


  7. May 16, 2005 #6
    Yes, a clock in a gravitational field runs more slowly respect to an observer in a less-strong gravitational field (or less acelerated, what is the same).

    And actually there is something that is stopping us in the diference in the way we can travel along the space and along time: the principle of casuality. See, you can "travel" to the future going at great speed respect to the earth, for example, and come back (to earth); however, you cannot travel back in the time cause then you can change the future. Although is factible (in principle) acording to certain solutions in GR abpout formation of singularities, it has to do with the hipotesis from Hawking, that there cannot be open singularities that allow travel to past, and maybe with the posibilities of the existence of multiple universes, but maybe you dont want to come back just to be killed by a car, or be switched to space by the uncertainity in the predictions about where to go. There is the possibility, if it can be, to use negative energy to travel to past; however, to take you this way, you have to use (and waste) an energy in the order of energy in the observable universe. :wink:
  8. May 16, 2005 #7
    To be perfectly honest I didn't understand that initially but I think i'm starting to understand it!!

    On a bit of a tangent, if one did travel back in time, isn't there a massive chance (due to chaos theory) that one will change the 'future' resulting in you not existing, or existing in a different way, such that you didn't trael back in time.

    If you did not travel back in time, then you wouldn't have changed anything so nothing would change.

    Essentially, if one went back in time, and resulted in the future being affected to the extent where you never have gone back in time, you get booted back to your own time.

    OR, does the future change due to your actions? I ask because I have heard of a theory that every possibility is played out in a different reality and i wondered, if you changed the past would you move back to a changed future?


  9. May 17, 2005 #8
    The second should be the only logical answer, if there can be travel to past, you should go to a changed future . The first one is equivalent to the mentioned hipotesis of Hawking: there should be a physical principle that do not allow travel to past (not open singularities), cause the fact of observing and being in the past, is to change it (so the future). Regards.
  10. May 17, 2005 #9


    User Avatar
    Science Advisor

    There's another possibility, which most physicists seem to favor over the "changing the future" idea when speculating about time travel, and that's that only self-consistent histories are allowed, so anything you do in the past was already a part of history all along, even before you made your trip in time. There was a long discussion of this here:

  11. May 17, 2005 #10
    Thank you Jesse. However, as Hawking points over and over, we have not been visited from the future (at least we dont know), maybe related to the limits of going to practice in it. I find the idea that you mentioned very interesting and has directly relation (i guess) with the ideas of Julian Barbour and others about the non-existence of time. That should be developed too by people relating tachyons with dark energy, i guess, but i find that too little people makes contribution to the area of time travel directly, and Thorne is the most representative. :smile:

  12. May 17, 2005 #11


    User Avatar
    Science Advisor

    Well, in the wormhole method of time travel suggested by general relativity, it wouldn't be possible to go back to a date before the wormhole was created--I think Hawking's comment about time travelers was meant to be somewhat facetious. But I do think that most physicists would favor Hawking's "chronology protection hypothesis" that the ultimate laws of physics will make time travel impossible, that even though general relativity seems to allow it, quantum gravity will rule it out (there are some hints that this is true from string theory--see this New Scientist article).
  13. May 18, 2005 #12
    This is a predestinational paradox - where you hav eto go back in time and do something for your past, and indeed the future to occur as it has/will do.


  14. May 18, 2005 #13
    Time travel has been discussed so many times in so many forums that, if it was possible at all, somebody most likely should have hit the bull's eye by now, shouldn't it?

    Strictly speaking, travelling into the past would mean exactly reversing the motion of every individual particle in the universe. Reversing time would do that but it would not be noticeable since also your own thoughts etc. would follow this reversed time.

    The sign (+ or -) of the flow of time is a relative notion, comparable with e.g. relative distance. Every moving thing around "me" (the observer) has a different (slower) timeflow than myself. The scalar value of the timeflow depends on who measures it. If I call the speed of my timeflow [tex]X[/tex], the timeflow of moving things that surround me is [tex]<X[/tex]. But from the perspective of something else my timeflow is the slower one. An invariant value is the delta in the timespeeds, which is always a positive value and can be compared with the positive definite value of e.g. "distance". So one can not speak of reversing the timeflow as such. Asking to reverse timeflow would be similar to e.g. asking for a negative distance.

    Since the timeflow as such can not be reversed, the only possibility would be to exactly reverse the momentum of all particles in any chosen environment which would on its turn involve the reversing of all bosons of all forcefields involved etc. etc.
    This all seems to me an extremely complex operation, if not impossible at all.

    But there is another indication that "traveling through time" is a non-issue. Whenever something moves relative to me, non-simultaneities start to exist in points of the moving object, which simply means that not all points of that object share the same time coordinate any more. In the course of the evolution of the universe this has led to the situation that virtually all timecoordinates exist in the universe as it is today. Photon's created during the Big Bang still have the timecoordinate from that event while particles that have moved at velocities close to lightspeed since the Big Bang will have timecoordinates from the very early universe (all this from the perspective of me as an observer of course). The fact that we are still able to observe all this as if it was "right now" proves that Nature has a way of making us observe any timecoordinate, no matter how far "away". So our environment is truly four-dimensional, i.e. all timecoordinates are observable in every single point in 3D space all "at once".
    You can translate this to the world of a "Flatlander". He lives on a 2D plane and dimension 3 is his timedimension. That third dimension makes his universe 3D. The Flatlander observes this whole 3D universe in his 2D plane which means that it is fully contracted into that plane, making all coordinates of the third dimension be observable in every single point in his 2D plane.
    The contraction could possibly be related to the Flatlander moving with velocity [tex]c[/tex] in his third (time) dimension, like we do in our 4th dimension.
  15. May 18, 2005 #14


    User Avatar
    Science Advisor
    Gold Member

    I missed the part where you need imaginary mass to move backwards through time. That implies the consequences are also imaginary.
  16. May 18, 2005 #15


    User Avatar
    Science Advisor

    Only for tachyons though (and if you suppose that tachyons have imaginary rest mass, all their measurable properties will be real-valued). In general relativity, you don't need imaginary mass for traveling back in time via a wormhole, although you do need negative energy to create the wormhole (but quantum effects seem to allow regions of space with negative energy, as in the http://www.absoluteastronomy.com/encyclopedia/C/Ca/Casimir_effect.htm [Broken]).
    Last edited by a moderator: May 2, 2017
  17. May 18, 2005 #16


    User Avatar
    Science Advisor

    Not true, general relativity allows you to actually travel to a past region of spacetime in certain circumstances, which is not the same as recreating the conditions of the past in the present. You have to get rid of the idea that the past has "ceased to exist" and the future has "yet to exist", and just think in terms of the "block time" view where past, present and future are different parts of a single 4D spacetime manifold--this view is supported by the fact that in relativity different observers can have different definitions of simultaneity, so that two events which both happen "at the same time" in one observer's coordinate system may have happened at "different times" in another's, there's no single universal present. See this article by physicist Paul Davies on block time vs. our ordinary notion of time flowing forward:


    Think of a block of solid ice with various 1-dimensional strings embedded in it--if you cross-section this block, you will see a collection of 0-dimensional points (the strings in cross-section) arranged in various positions on a 2-dimensional surface, and if you take pictures of successive cross-sections and arrange them into a movie, you will see the points moving around continuously relative to one another (in terms of this metaphor, the idea that there is no single universal present means you have a choice of what angle to slice the ice when you make your series of cross-sections). You shouldn't think of time travel as the points returning to precisely the same configuration they had been in at an earlier frame of the movie; instead, you should just imagine one of the strings curving around into a loop within the 3-dimensional block, what in general relativity is known as a "closed timelike curve".
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook