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

Any arguments for time travel back in time (the past)?

  1. Feb 10, 2009 #1
    Theres the Butterfly effect and Grandfather paradox which makes logical time travel into the past difficult. Is there any theory or anything which suggests it may be possible?
  2. jcsd
  3. Feb 10, 2009 #2
    I don't know about any theories, but I can think of two possibilities that could happen if time travel into the past was possible.

    1. If you kill your grandfather, then you can't travel back into the future.
    2. If you kill your grandfather, then you cease to exist.

    But, perhaps, if you kill your grandfather, it wouldn't matter if you killed him in the past, because you already exist in the future.
    Last edited: Feb 10, 2009
  4. Feb 12, 2009 #3

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    By the butterfly effect I assume that you mean the multiplicative effects of changing history. This was misinterpreted the first time which is why the thread was deleted.

    I think it is fair to say that while the door hasn't been closed entirely on travel to the past, it may be that the energy requirements would far exceed anything possible for the foreseeable future - maybe beyond anything that will ever be possible. There is also the [physics based] argument that one could only travel back to the point in time that the time machine is first turned on.

    The Future of Spacetime [2002] has a nice series of essays on this subject.
  5. Feb 12, 2009 #4
    One possible scenario which "might" permit this would be such that the "travel back" demands "observable only" with no possible actual interaction.
    Just my thoughts...
  6. Feb 12, 2009 #5
    Travelling to the past is simple; just move all particles back to the way they were at the time you want to be.
  7. Feb 13, 2009 #6
    Yep thats what i meant (the multiplicative effects). What else does it mean?
    I got a warning and email saying "Crackpot discussions are not allowed here on ther PF" and it was my third post , I was going to leave the forum if that was a crackpot discussion. (adj.Foolish; harebrained: a crackpot notion).

    Anyway I found this, http://en.wikipedia.org/wiki/Novikov_self-consistency_principle .

    Quoting the Wikipedia Encyclopedia (which I can legally do - It is for the public by the public. Although we should not allways rely on it I checked out some other sites and the Novikov self consistency principle is real.)

    The Novikov Principle is able to circumvent most commonly-cited paradoxes which are often alleged to exist should time travel be possible (and are often claimed to make it impossible). A common example of the principle in action is the idea of preventing disasters from happening in the past and the potential paradoxes this may cause (notably the idea that preventing the disaster would remove the motive for the traveler to go back and prevent it and so on). The Novikov self-consistency principle states that a time traveler would not be able to do so.

    An example is the Titanic sinking; even if there were time travelers on the Titanic, they obviously failed to stop the ship from sinking. The Novikov Principle does not allow a time traveler to change the past in any way, but it does allow them to affect past events in a way that produces no inconsistencies—for example, a time traveler could rescue people from a disaster, and replace them with realistic corpses seconds before it occurs. Providing that the rescuees do not re-emerge until after the time traveler first journeyed into the past, his/her motivation to create the time machine and travel into the past will be preserved. (See Millennium.) In this example, it must always have been true that the people were rescued by a time traveler and replaced with realistic corpses, there was no "original" history where they were actually killed, since the notion of "changing" the past is ruled out completely by the self-consistency principle.
  8. Feb 13, 2009 #7
    Thanks for the essay recommendation I will look them up.

    Ahh the energy requirments that would/could make it impossible. But even before we try because of the butterfly effect it makes it "logically impossible". Like how in the cosmological argument a being that can exist before it existed to make it self exist is logically impossible as it would have to exist before it existed lol.
  9. Feb 13, 2009 #8
    Aren't causality and the 2nd Law of Thermodynamics two doors that are closed pretty tightly against time travel to the past?
  10. Feb 14, 2009 #9
    Your proposition is correct if one only wants to travel to an instantaneous moment in the past. After that moment, though, the particles wouldn't necessarily behave in the same way as they did in the past.
  11. Feb 14, 2009 #10
    There is a simple conflict to think about when traveling time.

    Two objects cannot be at the same space and time. So when you travel back to time, the volume position where your body us at the present must be completely void on the specific time of past you will travel in; otherwise your body will be conflicting with the mass on that position at that time.

    Edit: There is another one i just thought of. Let's say that time travel is instantaneous and you travel to the past today at 1:00PM back to the time you started to travel at 1:00PM. So you will not be in a future anymore and your time is completely cut off up to 1:00PM. What will become of the volume which your body used to occupy? Your body is not there in the future because you traveled back in time. But there cannot be anything else because your body is occupying it for all eternity since 1:00PM.

    Edit2: I mean travel back to when you finished traveling to the past; since that is the exact time when that space is available. In other words, you exist but do not exist paradox with time traveling.
    Last edited: Feb 14, 2009
  12. Feb 14, 2009 #11
    When you travel back in time, according to relativity you must be able to go at or faster than the speed of light, 300,000 km/s. If theoretically we are able to achieve this it is still impossible to go back in time because when we travel faster than light then effect precedes cause, which means that is things happen before they are actually seen by other like the athletes running before the race has even started. Since we live in a world of effect coming after cause i.e; the athletes running after the race has started, the notionthat the reverse should happen is impossible. Hence it is impossible to travel back in time according to me.
  13. Feb 14, 2009 #12
    I think that there certain concepts that only exist in the human imagination, they don't exist in reality. One of them is the concept of infinity. Nothing is infinite. As we learn more and more almost everything that we ever thought was infinite has turned out not to be. When infinity creeps into math it is usually a sign of a mistake isn't it?

    Time travel to the past would require infinity in various forms. I believe that for this reason alone it isn't even remotely possible.

    It would require every possible (as opposed to the actual configuration that supposedly could be deduced at the end of time) configuration (position, energy, etc.) of every particle in the universe at every instant from the big bang to the end of time to exist simultaneously. The actual configuration is the way things are without time travel, the possible would encompass changes that would inevitably occur with travel to the past, it WOULD change the future.

    In actuality there may not be that much difference between the 2 since the only thing that can change anything from it's normal path as governed by the laws of physics is life. Life introduces a random factor so anything involved with life can't be deduced by the laws of physics. You can be a physics god but the laws of physics will not explain the existence of a lunar lander on the moon or one particular crater on a certain comet.
    Last edited: Feb 14, 2009
  14. Feb 14, 2009 #13
    Most discussions of time travel are elementary at best, and nonsense at worst. There are, however, solutions in General Relativity that contain closed-time-like curves. We cannot take these solutions too literally, as they tend to occur in artificial situations e.g. von Neumann discovered such solutions in a rotating-universe model , and because GR, as a non-quantum theory, is not the final word on spacetime.
  15. Feb 14, 2009 #14
    It seems that the conception of most of us have about traveling back in time is similar to the movie Back to the Future in which a person from the present travels back to the past but remaining as he is in the present. This view presumes that somehow the time traveler continues to travel forward in time while the rest of the universe has traveled backwards.

    If travel back in time were possible it would mean rewinding the whole universe to that time. The earth would have to orbit backwards and even air molecules would have to retrace their exact paths from the present to the past. All aging and knowledge gained would also have to be reversed. Anybody arriving in the past from the present would do so naively without any of the knowledge time from which he came.

    The whole point of returning to the past of course, is doing so with the knowledge of the present. However this seems to always involve the potential for paradoxes or absurdities - great for movies but very problematic in practice.
  16. Feb 14, 2009 #15
    Exactly the way I have always thought of time travel. Maybe we are all time travelling now and then, we just don't know it because our memories of 'the future' don't exist in 'the past'.
  17. Feb 14, 2009 #16
    I get what you mean but I don't understand it. How can we be time travelling now and how can we not know because our memories of the future don't exist in the past.

    Please can you explain this because it sounds really intresting and I don't get it.
  18. Feb 14, 2009 #17
    Say our lives are being played out from a recording like a DVD. Everything is there, past present and future, but we are only seeing the moment being played. Who is to say what we think is playing forward really isn't playing backwards?
  19. Feb 14, 2009 #18
    ...and he's right, of course - it's simple :rolleyes:
  20. Feb 14, 2009 #19
    I think it was Steven Hawking who asked, if time trvel is possible, "where are all the tourists from the future?"
  21. Feb 15, 2009 #20
    Originally Posted by Lambda3 View Post

    "Your proposition is correct if one only wants to travel to an instantaneous moment in the past. After that moment, though, the particles wouldn't necessarily behave in the same way as they did in the past."

    ...and he's right, of course - it's simple"

    He is? If you go into THE past, and every particle has EXACTLY the same properties, every particle that isn't being directly affected by your instantaneous presence in the past would act exactly as it did before. The longer you spend in the past the more particles you will affect which will in turn affect particles on their own creating a growing area of effect. If you were to appear for just an instant in the past the effect would be almost unnoticeable, butterfly effect notwithstanding, if you stayed there the effect would grow but it wouldn't change anything at all on Alpha Centauri.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook