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Keen amateur, could someone help me

  1. May 25, 2008 #1
    Hi everyone,
    I was wondering if you clever chaps could help me out. I am very interested in physics i only wish i was clever enough to do my degree in it but never mind. Anyway i try and pick up on what i can i was just looking into time travel recently and i have a problem getting my head around something. Someone stated that if it were possible to go back in time then we could never go back to before the time machine was invented. I do not understand why this is the case as surely once it had been built it could go back to where it wanted and still exist?

    Anyway i hope this isnt in the wrong section.


  2. jcsd
  3. May 25, 2008 #2
    Perhaps because if one goes back in time, before the time machine has been invented, they could end up causing a chain of events that causes the machine to not be invented. If the machine was never invented, the time-traveler never moved back in time, and you get a paradox.

    It's a lot like the "killing my grandpa thing" (Disclaimer: I am in no way responsible for what I said. I blame somebody else).
  4. May 25, 2008 #3
    Actually, I won't there still be paradoxes for time traveling backwards regardless?

    So you invent the time machine in January 1st, 2000. You wait until January 1st, 2001 to time travel back to say, February 1st, 2000 and say hi to yourself. Well okay, so if that happened, then after you invent the time travel machine in Jan 1st, 2000 and see your future self visit you a month later. How would that work if after that encounter, you never swore to yourself to never use the time machine? Unless we go by the idea of each time you time travel, you create a new parallel universe or something...I don't see how that works?
  5. May 25, 2008 #4
    If this was to happen then would the you of the future just sease to exist instanty? Is this what a paradox means? Would that not imply that there was an infinate number of 'you's' if one could suddenly sease to eist and the other carried on living or is there something about time that i do not know about?
  6. May 25, 2008 #5

    Right, that's the paradox I came up with.

    In order for you to satisfy the condition of you seeing your future self, you MUST embark on this path of well, visiting yourself in the past. What if you are prevented from entering the time machine? Then if that happens...this scenario ceases to exist. So then you end up back to square one.

    What CAN happen is for people to uh..."time travel in the future". We're already doing that as it is.
  7. May 25, 2008 #6


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    I think the line of reasoning that you present in the OP arises to combat the question "why hasn't anyone ever met a time traveller?" It's a fair point that, if time travellers could travel back to whenever they wanted, they would have done with very close to 100% probability, so then why haven't we seen one? The simplest explanation (excluding the "time travel is impossible" explanation) is that time travellers can only go back so far as a time when a device had been built. Of course by device here I just mean warping of spacetime in such a way to permit time travel.
  8. May 25, 2008 #7

    But what about paradoxes...like the one i just mentioned? how do you get around something like that?
  9. May 25, 2008 #8
    Sorry for firing all these questions but ive never had the chance to ask peopel in the know before, but as we are traveling forward in time whenever we travel even by the slightest amount deos this mean that there is an infinate amount of timezones and we are jumping from one to other or are we simply moving in the same time zone, and what is the science behind this. Again sorry for all the questions but i have so much that i want to know and i find it difficult to understand what is written in a lot of books.
  10. May 25, 2008 #9


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    That's one of the solutions to the paradox...
    The second answer to the paradox was called something like the "consistent history" solution, and basically said that we need to find a solution to the equations that permitted travel to the past, but remained consistent. Thus, in this solution to the paradox, we would just say that you cannot go into the time machine and go back and visit yourself until you had come back from visiting yourself in the past (confusing, I know!)
  11. May 25, 2008 #10
    I'm not exactly sure what you mean by timezones...but time travel forward works with Einstein's theory of relativity...

    Look up something called time dilation. It's already been experimented with the famous two atomic clock mounts on a plane and on the ground
  12. May 25, 2008 #11
    ok thanks ill have a search for that.
  13. May 25, 2008 #12
    One final question then i read a bit about that i understand the premise although i just want to clarify one thing. The time for the person traveling seems to go slower, deos this mean that you could travel at huge speeds for a long time and come back to earth to find that you have aged less for example, or is it just that the clock you took onboard went round really slowly, like each secpnd took more like ten seconds to change on the clock, or would it carry on at the dame pace? It is so hard for me to get my around the idea that we could say live for ten years on earth but an astronought who was traveling fast enough could only be away for one year.

  14. May 25, 2008 #13

    Problem is, that requires some incredibly high speeds in order to see significant effects of time dilation.

    Do you know what GPS is? The way that system works also relies on the fundamentals of the Theory of Relativity because those satellites in orbit must account for the time differences between themselves and the ground receiver, so...there's one practical application that is used on a regular basis for some people.
  15. May 25, 2008 #14
    Wanna have some fun with your time machine? How 'bout throwing a hand-grenade with the pin pulled into the time machine and sending it back one minute into the past? I hope for the universe's sake you're quick with the repairs, heh heh.

    Oh, and to see yourself in the future (or the past for that matter), would be a major violation of the first law of thermodynamics...when you leave the present, there's an awful lot of energy (the atoms you're made of) disappearing from the universe, and alot being created when there's suddenly two of you in the future or past...unless there's some loophole I don't know about.
  16. May 29, 2008 #15
    I always think about this.

    The way I see it: If time-travel is the movie sense is possible, we can not change the past - becuase NO MATTER WHAT HAPPENS, its already happened. In the past of the person before he timetravelled.

    So like, If a guy travelled to the past and killed his grandfather, somehow the grandfather survived, they froze his sperm, he was a sperm donor, etc etc but somehow he didn't die becuase he would have had to have that same experience for the exact events to take place for the grandson to time travel in the first place.

    You see in the movies people going back in time and saving people, but since it is the past they actually didn't change anything, because that event had to take place for the events to unfold to allow them to time travel in the first place.

    Anyway, I am not a physicist (spelling? lol) but I don't think it is possible. To me for there to be time travel into the past - there would have to be a universal memory of the past stored somehow, and I can't comprehend how that would be possible.

    As for future time travel, as someone said we are always 'time travelling'. Just depends how you define it. Does time travel mean going from one time to another without ageing? Or does it mean going from one time to another without being aware of it. Because technically we time travel 8 hours (or so :) ) each night when we sleep if the latter was true.

    Summary: my answer to the paradox of killing grandpa is that 'well it already has happened, so go ask your grandpa for the story of the guy he met once that tried to kill him'.
  17. May 29, 2008 #16
    In my dreams

    Based on no physics whatsoever...

    If the unfolding of events is deterministic you could imagine history/time as one big movie. Currently we are both in and watching the 'movie'. But an actual movie, say in film format, exists all at once - you can hold it in your hands - its just that when it plays we only experience one bit at a time. So the movie itself exists outside of its own internal sense of time.

    We could view history/spacetime as a big movie we are in and only seeing as it plays out, but, in another realm (I won't say dimension) it all exists at once.

    Now, stretching an analogy to breaking point, if we could take a character in a movie from near the end and put him (with his future movie knowledge) in near the start. And what I can imagine is that the plot of the movie would change as the new effect of the character takes place - I hope this makes some kind of sense. So that you would would end up with a new movie. Now, in this movie, the things that happened to the character before may not even happen at all, since he decides to forget joining the army and losing his legs, and takes up the tuba instead.

    So what I imagine is a realm in which the course of history can change. Thus, time travellers CAN kill their own Fathers or have experienced events in the future that don't happen second time round. The narative integrity is maintained by the flow of one 'movie' into another 'movie'.

    As I said, based on no physics whatsoever.
  18. May 29, 2008 #17
    I have written a reply to this question below, "Is Time Travel Possible" The answers here are all pretty good and most of them have been thought out pretty clearly. Rather then keep answering this question again and again I copied the answer I wrote from another question to this area. I sure would be interested to continuing this question with anyone who cares to answer my response.

    I for one do not know the answer. Like you I have pondered this very same question is time travel possible. I initially said no way but as I kept reading a unique set of words kept popping up again and again and here is what I am reading...

    A growing number of physicists are convinced that the thing we call ‘the universe’ — namely space, with all the matter and energy it contains — is not the whole of reality. According to quantum theory — the deepest theory known to physics — our universe is only a tiny facet of a larger multiverse, a highly structured continuum containing many universes.

    I could easily write some deep explanation with scientific clarity but to answer your hypothesis of cause and effect and tell you that the laws that govern our universe can not be broken, but what if when you go back in time and make a change to the time line the change does not effect your future but rather a alternate future in the multiversity.

    Everything in our universe — including you and me, every atom and every galaxy — has counterparts in these other universes. Some counterparts are in the same places as they are in our universe, while others are in different places. Some have different shapes, or are arranged in different ways; some are so different that they are not worth calling counterparts. There are even universes in which a given object in our universe has no counterpart

    On large scales, universes obey the laws of classical physics, and so each behaves as though the others were not there. But on microscopic scales, quantum mechanics becomes dominant and the universes are far from independent. Universes that are very alike are close together in the multiversity and affect each other strongly, though only in subtle, indirect ways — a phenomenon known as quantum interference.

    Without quantum interference, electrons would spiral into atomic nuclei, destroying every atom literally in a flash. Solid matter would be unstable, and the phenomena of biological evolution and human thought would be impossible. And as I shall explain, it is quantum interference that provides our evidence for the existence of the multiversity.

    Through interference, its counterparts in other universes can affect each particle in our universe. What we see as a single subatomic particle is really a sprawling trans-universe structure, spanning a large region of the multiversity. Although we cannot see the parts of this structure that are outside our universe, we can infer their presence from the results of experiments.

    As I said scientists are going to experiment with high speed partials at CERN so the rules have not been written yet, we have no unified theory yet. We just do not know what is possible. I for one hope for time travel, can you just imagine knocking on your families door in say 1941 would they believe you came from the future or just call the cops saying you are a madman. No one would ever believe you. I remember an old saying.... we have met the aliens and they are us…. Have a great day Big Walt
  19. May 29, 2008 #18
    You could go back before the machine was invented, but you could not affect anything or anyone related to yourself, or the time machine; anything done that may jeopardize the creation of the time machine would create a paradox, as well as anything that would jeopardize your birth (i.e. killing your grandfather, aptly called "The grandfather paradox"). Then there is the "Time Cop" paradox- stating that you can't go into the future because it hasn't happened yet, you would have to be brought into one possible future by someone from that future.

    My question is this- if you did create a paradox, would the timeline change instantly, or in 'waves' such as in "A Sound of Thunder" (a controversially crappy B-movie)?
  20. May 29, 2008 #19
    Another thing that bothers me about time travel in the movies (other than the whole first law of thermodynamics thing...two of you at the same time, or you appearing anywhere in another time for that matter, is creating energy): Why is it that people always assume that when you zap yourself 100 years into the past or the future, somehow gravity is going to keep you firmly planted on terra-firma? The planet we are on now was nowhere near where it is 100 years ago. When you take into account the motion of the planet around the sun, and the motion of the sun around the galaxy, and the motion of the galaxy etc..., odds are you'd probably appear floating in space doing the funky-chicken suffocating with no air to breathe....far far away from earth. Could be a great way to avoid paradoxes though! Could also be a great way to avoid a catastrophic nuclear explosion of some kind when the matter you're made of suddenly merges with the air molecules you materialize in the midst of...or are we to believe that the air will just "woosh" away when we materialize in another time? I guess that's a good argument for only being able to time travel inside the machine only during a period the machine actually existed, though you could only ever use the machine once as you could materialize at the same time someone else (or yourself) was in the machine, and the machine would have to contain a perfect vaccuum.

    Personally I find time-travel ridiculous, though it does make for great fiction (I loved Timeline...both book and movie).
    Last edited: May 29, 2008
  21. Jul 15, 2008 #20
    I feel time travel specially in the past as ridiculous.If you can go 1 year into the past by laws of physics , you could also go 100 years into the past by the same laws.Hence by the same logic you could also go million years back or billion years back.Even if the parallel universe theory is true , going back 13 billion years back would land us to the time big bang happened.In which state would you be then?Is it possible that the entire universe then had collapsed in something less than the size of proton and u r standing somewhere in space in your current form.
  22. Jul 15, 2008 #21


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    Why is this true? See my post #6: it is widely believed that, even if time travel into the past is viable, one can only go back so far as a time when a time machine was invented.
  23. Jul 15, 2008 #22
    i have an interesting point about time travel if ever could be possible... time and space are linked deeply. If i were to move back in time in my machine at this location i am currently at in the universe... i would die because when i came out at the next time the earth would no longer be at that coordinate and i would be in space... and if i went back in time the matter that the time machine and I are made of have entered the univers where they already exist which violates laws of physics by creating energy and matter. It cant be done!!!
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