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Possibility of time travel

  1. Aug 15, 2010 #1
    Hello. I've been on this forum for a while. Though I never really bothered clicking the "register" button until now.

    I am no scientist (In fact I still haven't reached university yet), so this might be a bit, well, simple-minded.

    Recently I've had some thoughts and discussions with my friends about time-travel. While no one has ever drawn any conclusion, I want to bring an end to the discussion:

    How is it possible? I mean, if you traveled somewhere in the past and changed it, while still being 'you', how can you really change the past? And example: If you shoot Hitler in 1937, but you still remember the events like holocaust and everything else that happened in and since the second world war (Before the time travel and after the murder), it could not possibly mean that you changed the past since there would be two different pasts for you. Will you create an instance of some sort, or how does this work?

    I tried google-fu. But I ended up with many (interesting) theories - way over my head at times.

    I am sorry if I'm unclear. I tried my best at 11:25 PM.

    Thank you very much for reading this!

    Best regards,
    Last edited: Aug 15, 2010
  2. jcsd
  3. Aug 15, 2010 #2


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    The Stanford Encyclopedia of Philosophy contains a fairly confusing but comprehensive introduction to paradoxes of closed causal loops. It's a frustrating discussion that isn't going to end and you're certainly not going to end it, but it's interesting to think about if you just want to pass time or write a science fiction story or something. Part of the entry suggests that physics other than relativity might make backwards causation impossible:

  4. Aug 16, 2010 #3


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    I think, were time travel possible, you would probably erase the future from which you departed, effectively taking a one way trip to an alternative universe. There are, of course, other problems. I suspect the universe would object to losing mass without compensation. The gravitational effects alone would have untold consequences. Let's say, for the sake of argument, you could send the sun backwards in time. This would be catastrophic for the 'present' solar system. The planets would immediately wander off into space. And injecting another sun anywhere near the 'past' solar system would be similarly disruptive - not unlike killing everyone's grandfather. It is at best paradoxical, and probably violates any number of physical laws.
  5. Aug 17, 2010 #4
    I dont think time travel would work! How would you travel back in time? how would it be physically possible? I dont really understand worm holes... how can a black hole become a worm hole?
    and I think if time travel did happen... then every time something travelled back in tie, then whole universe would change - its chaos! right? like a small change in the past will trigger who-knows-what in the future cos of chance?
  6. Aug 17, 2010 #5
    If you don't believe that travel in time (to a relative past) isn't possible in this universe at this time, then you're in good company. There are many conjectures and theories to explain this, ranging from the philosophical, to quantum physical explanations.


    How you travel backwards in time would require FTL travel of some kind, or exotic geometries which achieve a similar result such as wormholes or "warp". In the end, you're still moving from point A-B-A faster than light could make the same trip.

    For the rest, a black hole does not BECOME a wormhole, but in theory it could be an environment in which one could exist. That wormhole shouldn't be traversable however. http://en.wikipedia.org/wiki/Einstein-Rosen_Bridge

    An example of a test particle falling into one: http://www.physics.indiana.edu/~nipoplaw/PLB_687_110.pdf [Broken]
    Last edited by a moderator: May 4, 2017
  7. Aug 18, 2010 #6


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    Consider, nucleargirl, the initial universe was fine tuned to within less than one part in a septillion. Injecting [or subtracting] almost any amount of mass into the very early universe would have been catastrophic. It would have either collapsed, or been rendered incapable of producing stars, galaxies, etc.
  8. Aug 18, 2010 #7
    I've heard an idea saying that, if one actually traveled back in time to say, kill their grandparents, they(the person) would still exist, just the act of changing the past would cause another multiverse to form. Meaning you would no longer be in your universe/[specific multiverse]. This may not be completely veritable, so excuse me if it's inaccurate since it's based on a theory.
  9. Aug 18, 2010 #8
    That would be a kind of MWI (Many Worlds Interpretation). There is thinking that: we live in the present, and that includes any influence in the past including travel from the future. There is Novikov's principle, and others that are similar in stating that traveling back in time is like rewinding a video; you can participate in it, but being "written" you cannot change anything. In that sense, traveling in time is the same as making a puppet of yourself. Then you have Hawking's view that energy densities of certain transiting particles would cause a time machine to "blow up" upon activation.

    The explanation that holds the most water with me is just that the curvature needed to achieve backwards travel in time doesn't exist in the universe, outside of theory. I'm not saying you can't put the design for that topology on paper, just that it doesn't have a physical reality.
  10. Aug 18, 2010 #9
    Well the way I see it, anything relating to the past and other universe is all theories. There's no mainstream science Time-Travel devices, let alone any devices which would allow for visiting other potential multiverses. Personally, if there is other universes existent other than ours with life just as intelligent as us, then I don't think it's our business to be screwing with them.. Since we don't know for a fact what would happen if we actually did travel back in time and change the past.
  11. Aug 18, 2010 #10
    I think that's a fine mix of the practical and the ethical, and it works for me.
  12. Jul 9, 2011 #11
    If you had a time machine which you could sit in and press a panel which read : move me backward in time... you could press the panel and begin to move back in time.. until you reached the time of : before you had pushed the panel. ! Ad infinitum.. meanwhile, ' time' would be marching on.
  13. Jul 9, 2011 #12


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    Understanding first that this is rank speculation...

    I had a fanciful thought once about the nature of time which I developed a bit and it offered one resolution to time travel paradoxes.

    First imagine the evolution of the universe over time, and the division of time into fixed past, active present, and possible future, in the analogue of a crystal --say ice-- forming around a nucleus (the big bang).

    One can view matter particles as defects in the crystal propagating according to rules but with a certain randomness (intrinsic quantum uncertainty). The surface where the solid is forming is the "now" and you might note that after freezing one can slice across an event in many different ways, the plane of simultaneous events is relative.

    If any inconsistencies develop in the history then either they are isolated by being frozen over (ambiguous past quantum states never observed) or, if not, they produce stresses on the crystal instigating a thawing and refreezing until a consistent history manifests.

    So given this model of history, for one to go back into the past one would have to propagate (as a collection of defects) back into past layers which necessitates thawing history back to the target time. Once this occurs then one would be propagating within a free floating piece of crystal which must remerge into the history consistently... or more likely one's personal history would melt and the past refreeze in a consistent but alternative way, or one never makes it back and spawns an alternative universe forming around ones free floating crystalline history.

    Again this is beyond speculation, more a fantasy model, suitable for premises in a science fiction novel. But there's a certain beautiful consistency to it. One can imagine the God-like power of being able to swim in the ocean of possibility selectively thawing history until it happens the way you want then rejoining the evolving now.
  14. Jul 25, 2011 #13
    why time travel is impossible:
    the mass of the universe Ma-z is being left out of any possible equations when it comes to moving along spacetime lines... you may find a way to move your mass Mx to a prior space time coordinate XYZTp however you will not find a way to move the Ma-z to XYZTp because it will be where it is now.. .XYZTnow... and no machine could ever collapse the universe to a prior moment.

    [XYZTnow]Mx --> XYZTp =! Ma-z --> XYZTp
  15. Jul 25, 2011 #14
    I woke up this morning and tuned in CNN television. I caught the end of a story about how a scientist in Hong Kong proved that time travel is impossible. I searched recent news stories on line and came up with this posted July 24, 2011: "The Hong Kong University of Science and Technology research team led by Du Shengwang said they had proved that a single photon, or unit of light, "obeys the traffic law of the universe."

    I suppose that is what the CNN news was referring to. Does anyone out there have any knowledge of this experiment or area of study? I am thinking that he must have demonstrated that a single photon is subject to the same limitation as electromagnetic waves. Is this new information?
    Last edited: Jul 25, 2011
  16. Jul 25, 2011 #15
    Here's an article about it: -

    http://www.physorg.com/news/2011-07-hk-physicists-photons.html" [Broken]
    Last edited by a moderator: May 5, 2017
  17. Jul 25, 2011 #16

    say what? Can you explain that in layman's terms. I just read Einstein's 70 page explanation of Special and General relativity last night and it was loaded with metaphor and easy to grasp visual models (train tracks and lanterns etc). He even apologized in advance for repeating himself. But his discourse was beautifully written. It made it all seem at once accessible, exciting and mysterious. What do you mean by "collapse the universe to a prior moment".
  18. Jul 25, 2011 #17
    think of matter/energy as billiard balls upon a table... all the balls in motion... you can take pictures at intervals and make a grid and document where each ball was in each frame... but if you somehow grab one of the balls and place it where it as say a hundred frames ago, how will that effect all the other balls in motion? My point is that it will only move that one ball to a location it used to be in without changing the CURRENT location of any of other balls... it will not cause all of the other balls to magically move to where they were a hundred frames ago... and there can be no machine powerful enough to move all the balls back to where they were one hundred frames ago... which is what time travel would necessitate, because as far as all rational science indicates, the balls representing the energy of the universe do not leave copies of themselves where they used to be... the energy does not exist in all moments, but only in the present moment... otherwise there is no explanation for motion at all.
  19. Jul 25, 2011 #18
    jiohdi, I think that is a good intuitive analysis of what moving back in time would require. I think you are saying that going back in time is the equivalent of rewinding the big bang itself. Since the energies are unspeakably massive, it does indeed appear that it is impossible.

    I think most of our fantasies about time travel implies that we are somehow slipping through some fanciful time wrinkle. The only way I can imagine approximating traveling back in time would be as a time tourist. Certainly traveling to the past as a time tourist will be possible for generations after us as long as we store the evidence of our lives. Maybe we will develop some kind of super surveillance where virtually everything is recorded, stored and perpetually kept energized in cyber space. But I travel back in time now every time I open my old yearbook, watch an old movie or watch the news for that matter.
  20. Aug 4, 2011 #19

    Now, we all know there is no evidence to support time travel. However ...

    The Many Worlds theory assumes all possible outcomes at any instant always happen. We often see sci-fi flicks that show someone traveling back in time to their past, eg in the case of Back To The Future for example. Then, because he traveled back and interacted, when he returns to his present, everything is much different because he altered the past (a new World of many). However, if Many Worlds is the model of reality, and assuming backwards time travel possible, then shouldn't it be a foregone conclusion that when you get there "that said past would be totally different from the past that gave rise to you"? I mean, why should you return to a prior point wrt your own timeline, versus some other timeline altogether?

  21. Aug 4, 2011 #20
    parallel universe !!
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