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Einstein and Time Machines

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  1. Aug 7, 2018 #1
    Does anybody have any references where Einstein speaks about Time Machines?

    From collected sources I found, he strongly rejected the idea of Time Machine but through his theory implied that time travel may be possible if wormholes existed.

    Something needs to be infinitely fast to do this? Have their own gravitational field? Answers to these also please.
    If something goes at infinite velocity doesn't wave nature becomes superior than their particle nature? So that means even if time travel is possible only wave packets of existing particles will reach the future or past?



    Any definite sources such as books/journals/research papers etc.?
     
    Last edited: Aug 7, 2018
  2. jcsd
  3. Aug 7, 2018 #2
    Not that this really answers your question, but from what we know about physics and how its applied in our world time travel is 100% possible. Move fast enough and everything else slows down around you. Once you slow back down a great deal of time has passed for everyone/thing else, but not for you.
     
  4. Aug 8, 2018 #3

    Ibix

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    Some solutions to Einstein's field equations include "loops" in spacetime which you can travel around and meet yourself in the past. As far as I'm aware they're all either not descriptions of our universe, require stuff we've never seen (like concentrations of negative energy), or are idealisations that we don't expect to really exist in nature (the interior of Kerr black holes, for example). So we can describe such spacetimes, but we don't think they can exist.

    You can communicate backwards in time if you can find particles that can exceed light speed. Again we've never seen such things and they lead to paradoxes (google for the tachyonic anti-telephone, which is a less bloodthirsty version of the shoot-your-own-grandad paradox).

    I would assume it's the apparent potential for paradoxes that Einstein disliked. I'm afraid I don't have references to support that assumption, however.
     
  5. Aug 8, 2018 #4

    Ibix

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    There are caveats around this statement. It's broadly true, but there are quite a lot of ways that taking it as strictly true could come back to bite. There are a billion threads on the twin paradox in the relativity forum if you wish to know more.
     
  6. Aug 9, 2018 #5
    I heard somewhere that if we move at speeds greater than light, then the effects would occur much before the cause(Law of causality).
    I think it would be in a light like world. (Minkowski's cone and Space time geometry).
     
  7. Aug 9, 2018 #6

    Ibix

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    That's not really correct (which is probably something to do with "I heard somewhere" not really meeting PF's referencing standards). Anything moving faster than light is moving backwards in time in some choice of reference frame, but is also moving forwards in time in other reference frames. So from some perspectives, effect would precede cause, while from others it would follow. The order of events wouldn't be clearly defined in relativity - which doesn't matter because we've never seen anything travelling faster than light.
    I've never heard the term "light like world" - I think you are confusing a number of things.

    The light cone is the surface that separates all events that you can reach travelling at or below the speed of light, starting from some given event (such as where you are now). You can affect events inside your future light cone, and can have been affected by events in your past light cone. Events outside the light cone can't affect you (yet) and can't be affected by you, so whether they happen before or after now isn't important (within limits). Those events are space-like separated from you. Events actually lying on the light cone are light-like separated from you.
     
  8. Aug 9, 2018 #7
    How is the order of precedence/succession of events decided?

    Light like separated? Meaning....?

    You are right. I used the line "I heard somewhere" because it is mostly from a combination of popular science books and videos+little memory that would not serve as a reliable source.
     
  9. Aug 9, 2018 #8

    Ibix

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    If you can get from one event to the other without exceeding the speed of light, the order of the events is fixed (and these events are "time-like" separated if you could do it below the speed of light and "light-like" or "null" separated if you could only get from one to the other at the speed of light). The order of events that are far enough apart that you can't get from one to the other ("space-like" separated) even at the speed of light is a matter of choice.

    Of course, two events that are space-like separated from you now may not be space-like separated from each other. Their relative order is fixed - but whether either or both happened before "now" is a matter of choice.
     
  10. Aug 9, 2018 #9
    Am I saying right?

    Time like separated if i can proceed from one event to another at speed below c. That means in this dimension. Should we use speed or velocity? Events have space-time coordinates.

    Light Like if I can go at speed of light.

    Space Like if they are widely separated.

    How can two events be space like separated from me and not from themselves?
     
  11. Aug 9, 2018 #10
    Say I map the entire timeline of the universe into the number line, starting from t=0 to infinity. Then each point will refer to a particular time in the whole timeline. Say we are at the 1000th position at this instant. Then taking that position as T=0 and as the present instant I measure the time that passed(in positions) in writing this post. Say I arrive at 1002th position. Now the choice “now“ is not abstract. We are referring to a particular point in the whole timeline.

    Is this feasible?
     
  12. Aug 9, 2018 #11

    jbriggs444

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    When you mark down time 1000 at sunrise on August 10 over here, what event over there in the Andromeda galaxy are you considering as occurring at time 1000?

    You have a wide variety of choices available.
     
  13. Aug 9, 2018 #12

    Ibix

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    Yes.
    No. Spacetime is four dimensional, meaning, roughly speaking, that there are four basic directions - forward/backward, left/right, up/down, future/past. You seem to be using it in the science fiction sense of "a different dimension".
    Two events are light-like separated if the only thing that could get from one to the other is something travelling at the speed of light. You cannot do this.
    Two events are light-like separated if the distance between them is more than the speed of light times the time between them.
    They're close to each other but not to you. For example, the Sun is eight light minutes away. But a couple of spaceships near the Sun and a few hundred metres apart from each other don't care that you are eight light minutes away.
    You can do this unambiguously for an individual person or a clock or something. And you can do it unambiguously for another clock. But you can't unambiguously specify that both clocks read 1000 at the same time - this is the key difference between relativity and Newtonian physics. Time is not an absolute quantity that everyone agrees on.
    You are referring to a point on a timeline. But time is not a line - it's a direction in spacetime. And you have a lot of freedom to choose which direction you call "time", and you don't have to use the same direction here as someone else does elsewhere. Hence you have a lot of freedom to define what you call "all of space now".

    There are natural choices of what you mean by "all of space now" in many circumstances, but there's never only one possible choice.
     
  14. Aug 9, 2018 #13
    What about this?
     
  15. Aug 9, 2018 #14
    If second is the base unit. Then. Some formula like. 1 earth seconds=..... Other galaxy seconds that can be used to scale the earth-timeline.
     
    Last edited: Aug 9, 2018
  16. Aug 9, 2018 #15

    Maybe a directed line segment.
     
  17. Aug 9, 2018 #16
    Excuse my choice of word. Maybe the classical/Newtonian world.
     
  18. Aug 9, 2018 #17
    What about synchronization and simultaneity? I am referring to SR.

    What if I use t=o for both clocks at the start of the universe as base.

    Why 'a' timeline? Why can't I scale the whole galaxy into one timeline?
     
    Last edited: Aug 9, 2018
  19. Aug 9, 2018 #18

    jbriggs444

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    No. You are not getting it. This is a relativity of simultaneity thing. Not a scaling thing.
     
  20. Aug 9, 2018 #19
    Please explain why?
     
  21. Aug 9, 2018 #20
    Off topic

    Why is each post duplicating?Happening to me also.
     
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