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

Time travel and paradoxs

  1. Jun 25, 2011 #1
    so they say we cant travel back n time because it would create a paradox. Theres a line of thought thats says it may be possible i we select the history that does not cause a paradox. However it seems to me that quantum physcics also says that all historys exsist until we collapse the wave. This means we cannot create a paradox that doesnt already exsist as a possible history. The paradox problem also seems to imply that a paradox is only a paradox because it creates a new timeline, but given that we know a timeline only exsists because its the line of entropy, then theres nothing special about timeline paradoxs, its more to do with our abaility to think past that timeline we cannot currently move out of.
  2. jcsd
  3. Jun 25, 2011 #2
    There are many examples of time travel paradoxes. I implore you to read up on Hawking's Chronological Protection Conjecture. I think his conjecture is closest to the truth of time travel.
  4. Jul 4, 2011 #3
    The Granfather Paradox. This one is AGAINST time travel. Suppose you travel back in time and kill your grandfather BEFORE your father is CONCIEVED. Where does that leave you? If your father was never here how can you be here able to go back etc...Now heres a different idea...Time travel should be possible so that your grandfather can travel FORWARD in time and kill you just BEFORE you go back in time to kill him. This one is in favor of time travel. This can be called THE GRANDFATHERS REVENGE THEORY. This theory is in reply to Steven Hawkins theory that the past must be made safe for historians (chronology protection conjecture). This alternate view makes the future safe for grandfathers.
  5. Jul 4, 2011 #4
    Yes my point really was that theres nothing scientific in calling something such as the grandfather paradox a paradox. The implication is that a paradox prohibits time travel but the idea of a paradox is something humanistic and is a paradox to us because we are still trapped into thinking in terms of a single time line and cannot conceive of travel to a point in that line and changing its history. Clearly we have never done this in a laboratory. However my own thinking was that time is not fixed, we are living in a collapsed wave, and all possible histories still exist as probability waves. Just because we cannot conceive of what happens when we change the linear nature of a collapsed wave does not to me make much sense to assume it would be a paradox if our definition of a paradox is something that prohibits time travel. I am trying to say that we could not change anything if all possible histories already exists as uncollapsed waves, they would already contain so called paradoxes as a possibility.
  6. Jul 4, 2011 #5
    Yes, but do these alternate histories exist as physical realities? If they do I would agree with you that they are full of paradoxes. The simplest example I can think of is an alternate physically real history with a law for existence that postulates the impossibility of alternate histories. That would make this future impossible because we postulate that alternate histories exist...unless we are the physical evolution of that history (the one that prohibits alternate histories)

    If these alternate histories are merely 'thought experiments' meaning they don't physically exist then what is the point of the underlying philosophy of Quantum Mechanics? You don't need Q.M. to explain that an experiment can have many outcomes. In my opinion, what Q.M. contributes is the idea that an experiment can have ANY outcome and all those outcomes EXIST untill you collapse the wave function by actually performing the experiment. In my opinion this is an absurd idea filled with paradoxes.

    One more thing. Popularizers of Q.M. always mention the POSITIVE aspects of multiple realities. You are rich, beautiful, healthy, smart, safe, etc. They never mention the negative. You're missing a leg, an eye, you're constantly tortured, murdered, etc.

    If you made a list of the positive and negative aspects of multiple realities, the NEGATIVE would far outweigh the positive simply because there are more ways to cause chaos than there are ways to cause order.

    Personally I wouldn't like to be a rock. I like the fact that billions of years of evolution have 'steered' me to complexification and greater order as a concious being able to think my own thoughts.

    These multiple realities Q.M. is allowing... most of them would be quark soup...if you follow the logical consequences of the reasoning of popularizers beyond their first few sentences.:biggrin:
  7. Jul 4, 2011 #6
  8. Jul 4, 2011 #7
    Wait a minute, I think I just got your point which I find very interesting. You are saying it is not a paradox if you move into an alternate history that allows paradoxes?

    Hmm... then so called 'paradoxes' may be allowed in this reality. I am not against this idea, but clearly paradoxical situations are not 'easy' to create, on the other hand there are examples such as quantum eraser, quantum tunnelling, quantum entanglement, EPR. etc. (Please add to the list)

    Thank you for the post, but i am still in favor of making Q.M. a bit more logical. For instance you can have an infinite number of realities as long as some of them are missing (The absurd ones) The same way you can have an infinite amount of numbers between zero and 1 but 2 is absent.

    This way the absurd reality where I am a conscious bowling ball DNE
  9. Jul 4, 2011 #8
    The all possible histories i believe is no more and no less real than the particle probability wave and requires some form of interaction for a possibility to become a reality. Yes its really interesting to think of alternate universes and all possible laws governing them but im not really thinking past this one. Do the laws of physics allow for different history's to have different laws? Very interesting but i thought those histories themselves are governed by our current laws of physics since its the current laws that allow for them in the first place.
    I dont see how a paradox prohibits time travel, the laws of known physics dont, so why does our inability to conceive the result of a paradox make time travel impossible? To me its nothing more than infinity, does anyone really know what it is? But it doesnt stop us using it to describe the universe, as long as the maths works. My point about paradoxes being contained or not contained in all possible histories is that its not relevant. Because although we live apparently one linear life, its just part of all possible histories, so breaking the linear line in our reality, which we would have already done by time travelling, would now mean the paradox is just part of the new history.
    The all possible histories is as real as the electron taking all possible paths in the double slit experiment, and the electron does eventually appear real to us when their wave is collapsed. QM doesnt just say an experiment can have any outcome, actually i dont interpret it that way, its more an possible outcome, it must be paths that the electron could have taken. And even then some paths are more likely than others. While it may seem absurd its real. The maths works and the experiments confirm the predictions.

    does one thing suppose the other? Much of the negative and positive would be very subjective, i wonder how you would actually calculate the balance. The implication in what you say is that chaos must give rise to more negative possibilities, but why must they outweigh the positive? More weather patterns for example means more ways to have a good day as well as bad. For many of the negative paths of this increased chaos would lead to such a calamity that the universe would be blown apart, so that would end that history. But for the positive lines they would go on and on, i wonder once you calculate the amount of information within the positive histories to the negative ones if they would not balancer out and be consistent with our symmetrical universe?
    Im not sure that qm allows for multiple realities unless you mean multiple universes and parralel universes? the maths isnt that proven in these areas i believe. Much is still theoritcal and allows for explanation of other things such as the apparent tight laws that allow for us to exist.

    Its interesting here you seem to be making a case for the positive benefits of chaos, entropy over billions of years.
  10. Jul 4, 2011 #9
    Yes ive looked at this before, i love all this apparent non-real reality :)
  11. Jul 4, 2011 #10
    In a way, im saying that paradox is already contained in that history or you could not have moved into that history to start with. But i think this is a slight mis-interpretation? As i understand it its not that anything is possible, that you could wake up as a bowling ball, its that anything is possible if there was a way for it to happen, no matter how remote.
  12. Jul 4, 2011 #11
    I like how you mention 'tight' laws that allow for our existence. I take it then you would agree that most of these other universes would not have these laws 'tight' enough to allow conscious beings capable of asking these questions? Those are the ones i would call ABSOLUTELY negative. I tried to use an example that anyone could relate but i like your idea better, it is less subjective.

    Be very careful!! I am not saying other universes don't exist, i'm not saying they do. i can't prove either. What i am saying is that if other universes exist it is a bad thing (in my subjective opinion) Popularizers of Q.M. say it is a good thing (in their subjective opinion)

    They make their argument for positive aspects of infinite realities, i make my argument for negative aspects of infinite realities.

    I would like to adress your last remark. Perhaps i should have used dis-order instead of chaos? i am not using some 'fancy' chaos theory here, i am saying dis-order is an enemy of greater complexity, greater complexity is needed for consciousness, therefore dis-order is an enemy of consciousness.:biggrin:
  13. Jul 4, 2011 #12
    I certainly do not want to give chaos credit for consciousness even though i think there are thinkers out there that do:biggrin::smile:
  14. Jul 5, 2011 #13
    [I can't accept quantum mechanics because] "I like to think the moon is there even if I am not looking at it."- Albert Einstein

    I think Einstein made this negative quote about Q.M. as it was understood in his day. Has this consequence of quantum mechanical philosophy changed since then?

    "As far as the laws of mathematics refer to reality, they are not certain, and as far as they are certain, they do not refer to reality." (A. Einstein)

    I think this is an interesting warning by Dr, Einstein.

    "Had I known that we were not going to get rid of this damned quantum jumping, I never would have involved myself in this business!"- Erwin Schrödinger

    Again, very interesting insight from one of the founders of Q.M.

    http://www.integrated-magic.com/faqs/famous-physcists [Broken]
    Last edited by a moderator: May 5, 2017
  15. Jul 5, 2011 #14


    User Avatar
    Science Advisor
    Gold Member

    Yes, it is generally considered - with all due respect to this great man - wrong. Read up on the related papers EPR (Einstein, Podolsky, Rosen), Bell's Theorem (1965) and Aspect (1981).


    We live in an observer dependent reality (i.e. a contextual world), which goes against the quote above (which argues for observer independence). Although this conclusion is somewhat interpretation dependent.
  16. Jul 5, 2011 #15


    User Avatar
    Gold Member

    And the intriguing reply from Niels Bohr was:
    Can you prove that the moon is there if nobody is looking at it?

    Personally I love Einstein; he was a great genius, but to make him a scientific "God", which never made any mistakes, doesn’t benefit anyone...
  17. Jul 5, 2011 #16
    If nobody is looking at it then everyone Must look somewhere ELSE. This requires a systematic search of the entire universe to complete the proof Bohr wants.

    His rquest is equivalent to asking 'prove unicorns do not exist':smile:

    I do not make him a "God" His superiors made him a Dr. and his peers made him famous.
  18. Jul 5, 2011 #17
    Thanks for the links. I am aware of these and mention them in post #7. Still, it benefits everyone to learn more.

    If the observed effect is not a consequence of experimental apparatus interference, then what is the conclusion? Space and particles have mystical properties?

    Should we trust Q.M. to make predictions about large objects, like THE MOON!!?? Or should we confine it to the sub-atomic level where things behave in counterintuitive ways because of imperfections in the experimental apparatus and lack of understanding by us.

    Einstein was right-with all respect to great thinkers against him-Q.M. does NOT describe the behavior of the moon, Relativity does.:smile:
    Last edited: Jul 5, 2011
  19. Jul 5, 2011 #18


    User Avatar

    Staff: Mentor

    I've never understood the question "Is the moon still there if no one is looking?"
    As far as I understood an observer doesn't have to be a person, it can be anything that interacts with the object somehow. If so, then everything in the universe is interacting with something all the time, correct? Am I misunderstanding something here?
  20. Jul 5, 2011 #19
    I thought CONSCIOUSNESS collapses the wave function. Is a mechanical detector conscious if your eye is not on it?

    I thought that any conscious sensory perception is needed in order to collapse the wave function. This perception can happen now, in the past, or in the future long after the experiment itself is over.

    This is what 'delayed quantum eraser' shows. :smile:

    Look at post # 7 for lnk
  21. Jul 5, 2011 #20


    User Avatar
    Gold Member

    Nope. You're perfectly correct.

    The Moon thing is not a practical thought experiment, but it is one about principle.

    In principle there is a possibility that the Moon mgiht not be interacting with anything for some brief moment in time. In that time, we cannot be certain it is not there.

    No. It is simply a measurement. Consciousness is not part of it.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Time travel and paradoxs
  1. Time travel (Replies: 1)