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A User's Guide to Time Travel

  1. Jul 11, 2003 #1

    Ivan Seeking

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    "All it takes is a grasp of theoretical physics, control of the space-time continuum, and maybe a ball of cosmic string."


    I know we have objections to this statement:
    "Once confined to fantasy and science fiction, time travel is now simply an engineering problem"
    Last edited: Jul 11, 2003
  2. jcsd
  3. Jul 18, 2003 #2
    The time travel is possible.

    You are now treviling in time. Forward in time.

  4. Jul 19, 2003 #3

    Ivan Seeking

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    I like to joke that except for clocks and sundials, and then bicycles, cars, planes, rockets, planets, blackholes, and maybe wormholes, there are no such things as time machines.

    This statement by Kaku is a little overboard though.

    I think this remains to be shown. My understanding is that we have models that may/could work but that can't be tested. Therefore we can't know if they would work.
  5. Jul 22, 2003 #4
    Would there not be a problem with the rule "Energy can't be created or destroyed". Time travel as the books use it would violate that law. You Will be adding to the time you are going to and taking from the one your leaving. Just broke both ends and now both are out of balance. I am not a student. Just like to read science books, so my knowledge base is weak. Just posting my thought on it.
    Last edited by a moderator: Jul 22, 2003
  6. Jul 23, 2003 #5
    I don't know about those models for traveling in time, however I don't think a time travel can be done with success. If the time travel can be done. Why don't we have visitors from the future?
  7. Jul 23, 2003 #6
    re: Traveling Backward in Time

    Quantum mechanics tells us that traveling backwards in time is completely impossible. Not only has the past expired, but it has been completely erased!

    The mere fact that quantum events are random tells us unequivocally that the past has been erased for all time. There is simply no way to retrace events backward in time. To trace them backwards we would need a trail. But the random nature of quantum mechanics assures us that the trail to previous events has been forever erased.

    So we have a theory that unequivocally tells us that travel into the past is impossible. It's called Quantum Mechanics. No need to guess about that one anymore.
  8. Jul 23, 2003 #7

    Ivan Seeking

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    What would one look like? How do we know they have never been here? Maybe this is a boring millenium.

    A rather suspect question in light of all the UFO reports.

    “Of course it is possible that UFO's really do contain aliens, as many people believe, and the government is hushing it up. I couldn't possibly comment!"

    Stephen Hawking: Millennium speech at the White House.
    Last edited: Jul 23, 2003
  9. Jul 23, 2003 #8

    Ivan Seeking

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    Re: re: Traveling Backward in Time

    Until we can unify QM with GR it is called an incomplete theory.
  10. Jul 23, 2003 #9
    Borring? I don't think so...

    Really in my humble opinion is impossible I think there are much more arguments against the time travel than in favour.
  11. Jul 23, 2003 #10

    Ivan Seeking

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    I just ask questions and make trouble. IMHO, I dunno
    But oh what fun! Really though, some physicists really think this could be done. I will argue the position of others, but I am really in no position to have my own opinion.

    But to guess...I say bring on the temporal paradoxes! I will many worlds my way out every time.
  12. Jul 23, 2003 #11
    Well, this is certainly true. My statement was only given in the spirit of believing that the random nature of QM the correct. I do agree that it is still possible that this interpretation of QM is incorrect. So with that in mind I supposed that time travel may still be a possibility (at least in the sense that a pathway might possibly exist)

    However, I would like add that your quote here could very well be written backwards:

    Until we can unify GR with QM then GR is called an incomplete theory!

    In fact, until we have a grand unified theory every theory must be thought of as incomplete !

    However, I would argue that no matter what theory wins out, an underlying fundamental truth must remain:

    We either live in a universe that has the possibility of time travel, or we live in a universe where we have free will, but we can't live in a universe that maintains both of these properties simultaneously

    Why? Because the very thing that would give us free will (quantum randomness) would prevent time travel, and the very thing that would allow time travel (quantum determinism) would prevent free will.

    So take your pick. Free will, or the possibility of time travel.

    By the way, even if quantum events are completely determined, that still doesn't guarantee that time travel would be possible. Other properties of the universe could still prevent it from being accomplished.

    However, if the random nature of QM is correct then backward travel in time would be ruled out (even if the other properties of the universe would allow it! There simply wouldn't be any trail to follow. The past is being continuously erased (assuming that quantum events are truly random). All that could possibly exist is now, even a predetermined future would be out of the question.
  13. Jul 23, 2003 #12

    Ivan Seeking

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    I completely agree. And I don't think QM will ever be wrong, but I do allow for a synthesis that changes our expectations [you might say].

    I don't completely agree here. I surely admit that you could be right, but the holes that GR seem to leave open here for time travel makes me suspect that this randomness is only a limitation as viewed from our limited perspective. For example, before Einstein, upper speed limit made no sense; unless mandated by God. Also, before QM, tunneling might have seemed equally magical.

    Finally, though I know the theory is outrageous, the Many Worlds theory has the bothersome quality that it seems to resolve most if not all temporal paradoxes. It time travel is possible then I suspect this theory will be needed to explain what we observe. I don't see how one could be possible without the other.

    In one interesting lecture from Igor Novikov in "The Future of Spacetime", we find that we can affect history only to the extent that we don't create a temporal paradox. This again points to a proof that leave the door open for such realities.
  14. Jul 23, 2003 #13
    This is interesting. I do wish I had more time to devote to studying GR. Unfortunately I'm currently studying QM and I barely have time to do this.

    I'm also not sold entirely that QM necessarily implies randomness. This is just what I am being taught (brainwashed to believe).

    It is claimed that we can't know otherwise therefore we must conclude randomness. That's a pretty shaky way to come to a conclusion if you ask me. I mean, if we can't prove that it's random, then we can't prove that it's not right? So we do we make any conclusions at all?

    The main drive is that probability theory in mathematics is based on randomness and it seems to work, so the mathematicians want to conclude that this means that quantum theory is random. The physicists just follow suit because they have no choice in the matter.

    I personally have a lot of bones to pick with mathematical formalism. It is definitely not a complete science. As a matter of fact, mathematics is not even a science at all. It doesn't use the scientific method, and therefore cannot claim to be a science. I wish that the scientific community would create their own formalism of quantity based on the scientific method, and use it instead of pure abstract mathematics. It would necessarily be quite different to be sure.

    It actually amazes me that scientists have such faith in pure mathematics when it isn't even based on the scientific method. I think that this is probably due to the fact that they have been able to use it in an applied fashion over the years by simply ignoring the underlying abstract formalism associate with it.

    By the way Ivan, I like your signature line. Good thinking. :wink:
  15. Jul 24, 2003 #14
    Bonjour Dr Luz,

    It's not a joke. We are actually travelling forward in time, but, at the opposite, that's the way we define time. Egg-Chicken paradox?

    Upon our current definition, if you increase your speed (work faster?), your sun-tanned friends on the beach will travel faster in time.

    Shall you speed you up "minus v2" [?]
  16. Jul 24, 2003 #15
    Here's an interesting website:


  17. Jul 24, 2003 #16
    Here's a transcript of an interview with Sagan on time travel:


    Carl Sagan

    Hear Sagan via RealAudio
    Get RealAudio software

    Sagan on Time Travel

    Carl Sagan, the astronomer, Pulitzer Prize-winning author, and legendary popularizer of science, gave this interview during the making of "Time Travel." True to form, he discusses arcane aspects of the field -- from how you define time to what it might look like inside a wormhole -- with flair and a refreshing dash of humor. Sagan was David Duncan Professor of Astronomy and Space Sciences and director of the Laboratory for Planetary Studies at Cornell University when he died in 1996.


    NOVA: Let's start with the crux of the matter. What for you is time?

    Sagan: Ever since St. Augustine, people have wrestled with this, and there are all sorts of things it isn't. It isn't a flow of something, because what does it flow past? We use time to measure flow. How could we use time to measure time? We are stuck in it, each of us time travels into the future, one year, every year. None of us to any significant precision does otherwise. If we could travel close to the speed of light, then we could travel further into the future in a given amount of time. It is one of those concepts that is profoundly resistant to a simple definition.

    NOVA: Do you think that backwards time travel will ever be possible?

    Hear Sagan via RealAudio

    Sagan: Such questions are purely a matter of evidence, and if the evidence is inconsistent or insufficient, then we withhold judgment until there is better evidence. Right now we're in one of those classic, wonderfully evocative moments in science when we don't know, when there are those on both sides of the debate, and when what is at stake is very mystifying and very profound.

    If we could travel into the past, it's mind-boggling what would be possible. For one thing, history would become an experimental science, which it certainly isn't today. The possible insights into our own past and nature and origins would be dazzling. For another, we would be facing the deep paradoxes of interfering with the scheme of causality that has led to our own time and ourselves. I have no idea whether it's possible, but it's certainly worth exploring.

    NOVA: Would you like it to be possible?

    Hear Sagan via RealAudio

    Sagan: I have mixed feelings. The explorer and experimentalist in me would very much like it to be possible. But the idea that going into the past could wipe me out so that I would have never lived is somewhat disquieting.

    NOVA: On that note, can you describe the "grandfather paradox?"

    Hear Sagan via RealAudio

    Sagan: The grandfather paradox is a very simple, science-fiction-based apparent inconsistency at the very heart of the idea of time travel into the past. It's very simply that you travel into the past and murder your own grandfather before he sires your mother or your father, and where does that then leave you? Do you instantly pop out of existence because you were never made? Or are you in a new causality scheme in which, since you are there you are there, and the events in the future leading to your adult life are now very different? The heart of the paradox is the apparent existence of you, the murderer of your own grandfather, when the very act of you murdering your own grandfather eliminates the possibility of you ever coming into existence.

    Among the claimed solutions are that you can't murder your grandfather. You shoot him, but at the critical moment he bends over to tie his shoelace, or the gun jams, or somehow nature contrives to prevent the act that interrupts the causality scheme leading to your own existence.

    NOVA: Do you find it easy to believe the world might work that way -- that is, self-consistently -- or do you think it's more likely that that there are parallel universes?

    Hear Sagan via RealAudio

    Sagan: It's still somewhat of a heretical ideal to suggest that every interference with an event in the past leads to a fork, a branch in causality. You have two equally valid universes: one, the one that we all know and love, and the other, which is brought about by the act of time travel. I know the idea of the universe having to work out a self-consistent causality is appealing to a great many physicists, but I don't find the argument for it so compelling. I think inconsistencies might very well be consistent with the universe.

    NOVA: As a physicist, what do you make of Stephen Hawking's chronological protection conjecture [which holds that the laws of physics disallow time machines]?

    Hear Sagan via RealAudio

    Sagan: There have been some toy experiments in which, at just the moment that the time machine is actuated, the universe conspires to blow it up, which has led Hawking and others to conclude that nature will contrive it so that time travel never in fact occurs. But no one actually knows that this is the case, and it cannot be known until we have a full theory of quantum gravity, which we do not seem to be on the verge of yet.

    One of Hawking's arguments in the conjecture is that we are not awash in thousands of time travelers from the future, and therefore time travel is impossible. This argument I find very dubious, and it reminds me very much of the argument that there cannot be intelligences elsewhere in space, because otherwise the Earth would be awash in aliens. I can think half a dozen ways in which we could not be awash in time travelers, and still time travel is possible.

    NOVA: Such as?

    Hear Sagan via RealAudio

    Sagan: First of all, it might be that you can build a time machine to go into the future, but not into the past, and we don't know about it because we haven't yet invented that time machine. Secondly, it might be that time travel into the past is possible, but they haven't gotten to our time yet, they're very far in the future and the further back in time you go, the more expensive it is. Thirdly, maybe backward time travel is possible, but only up to the moment that time travel is invented. We haven't invented it yet, so they can't come to us. They can come to as far back as whatever it would be, say A.D. 2300, but not further back in time.

    Then there's the possibility that they're here alright, but we don't see them. They have perfect invisibility cloaks or something. If they have such highly developed technology, then why not? Then there's the possibility that they're here and we do see them, but we call them something else -- UFOs or ghosts or hobgoblins or fairies or something like that. Finally, there's the possibility that time travel is perfectly possible, but it requires a great advance in our technology, and human civilization will destroy itself before time travelers invent it.

    I'm sure there are other possibilities as well, but if you just think of that range of possibilities, I don't think the fact that we're not obviously being visited by time travelers shows that time travel is impossible.
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