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Is the future undetermined or simply undeterminable?

  1. Nov 16, 2015 #1
    I'm not entirely sure how to clarify this question, so instead I'll pose another one that should serve the same purpose.

    If, hypothetically, someone in the future devised a way to travel back in time (which to my understanding has never been shown to violate any laws of physics) after conducting many experiments on subatomic particles. He proceeds to travel back to just before he conducted those experiments, at a sufficient distance away from his past self to not to affect the outcome of the experiments.

    then, according to the currently accepted theories, would the results be the same as the version of the person from the future remembers, or would those experiments be just as likely to have different results as the same results.
  2. jcsd
  3. Nov 16, 2015 #2


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    This is philosophy or metaphysics, not science since if you are going to allow violations of causality, you can pretty much make up your own rules.
  4. Nov 16, 2015 #3
    It would violate the special theory of relativity for any particle with real mass, or massless. If such a particle traveled back in time in some frame, there would always be another frame where the particle travels faster than light, which is prohibited by SR. The only particles that could travel back in time are hypothetical particles with imaginary mass, called tachyons: https://en.m.wikipedia.org/wiki/Tachyon.
  5. Nov 16, 2015 #4
    Could there also be problems with conservation of mass-energy as well?
    When the person travels from the future to the past, then in the future frame the mass-energy which constitutes the traveler is disappearing,
    while in the past frame, a second copy of the traveler, mass and all, pops into existence.
    This creates a situation where total mass energy of the universe is greater in the past and less in the future.
  6. Nov 16, 2015 #5


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    This always used to bug me about time travel. Say you have a barrel of oil. At t = 0, you send the barrel to t = -10. This means that at t = 0, you "now" have the original barrel of oil plus the time-traveled barrel. Rinse and repeat; oil crisis solved! In reality, I don't understand enough about GR and closed timelike curves to see where the disconnect is (I'm veeeerrry slowly working my way through Wald), but I know that mass-energy is only locally conserved on a general manifold, and I suspect CTC is one of those cases where the strict conservation of energy breaks down.
  7. Nov 17, 2015 #6
    I looked into it a little bit, and two sources that I found seem to suggest that in a sufficiently advanced society, and assuming that a time machine of sorts was built before the experiments from my initial hypothetical experiment took place, time travel back in time would be possible. I could be misinterpreting the text, but it seems like both websites affirm that if a time machine is build to take advantage of worm holes, it would be possible to travel back to any time after it was built.


  8. Nov 17, 2015 #7


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    Boolean correctness. If pigs had wings, they could fly.
    Both websites bring the level of this thread back to "B" .
    And it will be ruddy difficult to take advantage of wormholes that allow time travel when they instantly collapse upon formation (same dr Who ? webpage).
  9. Nov 17, 2015 #8
    Another complication that arises is this:
    Let's say the traveler travels from Friday back to the previous Monday.
    On the Monday originally before he traveled back, he and everything else in the Universe is a given state and is in principle fully describable.
    After he has traveled back, that original description of the Universe on Monday is no longer valid, (because now the Universe also contains his other future self as well as the original him.)
    Therefore 'last Monday' as he recalls it does not exist, and wherever he is traveling to is not actually the original Monday at all, just something that looks similar.
  10. Nov 17, 2015 #9
    that is why I explicitly asked about what is said in currently accepted theories. I know that

    That is why I specified that he traveled to the past "at a sufficient distance away from his past self to not to affect the outcome of the experiments."
  11. Nov 17, 2015 #10


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    As far as I know there are no accepted theories that say time travel of the sort you propose is possible.

    Proximity is irrelevant. Its the total energy measure that means you are in a different "universe". It can't be the same universe because the other one didn't have a second copy of you present (regardless of proximity to the then-current you).
  12. Nov 17, 2015 #11


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    Neither I am aware of any mainstream theory that allows time travel to the past.
    In my personal opinion, this is not a very fruitful premise for a discussion on determinism in neither general physics or quantum mechanics.
    Why? Because I could also ask the following question: "If you would be able to travel to the past, would you be able to remember that you had travelled to the past?", and that question would be equally impossible to answer, since no-one knows what would happen... etc etc.
  13. Nov 17, 2015 #12
    No need to invoke time travel. You can go a long way by asking yourself, "Looking at things from today's perspective, was what I did yesterday determined? What about what I'm I am doing right now? How will I think about that when I recall back on it tomorrow?"
  14. Nov 20, 2015 #13
    By introducing the time travel, I think you have distracted almost everyone (perhaps even yourself) from your basic question. Instead of time travel, let's just presume you have an identical twin. Since I don't know what your purpose is in making this the same person as you, I don't know how "identical" you want this twin to be. But this twin can certainly be more identical to you now than you will be to a version of yourself that has spend years in interplanetary or interstellar travel.

    So now your question becomes this: While I am on Earth conducting many experiments on subatomic particles, my twin is on Pluto performing the same set of experiments. When we compare the results, will they be the same?

    At some level of detail, the answer will obviously be "no".

    At this point I would guess that you still have some unaddressed question. And now you have a renewed opportunity to rephrase it.
  15. Nov 20, 2015 #14
    Answering your title question, the future is clearly determinable. If you want to know something about what will happen tomorrow, give me 24 hours and I'll tell you.
  16. Nov 22, 2015 #15
    The future is definitely NOT DETERMINABLE since quantum physics is based on 'unobserved probabilities' as opposed to 'observed' classical physics. If there were no intelligences in the universe with free will to act however they please, then perhaps once could somehow calculate the future end results from past states. But if on a whim some intelligence decides to create a nuclear device and obliterate a small part of the universe, then the future changes in a totally new direction. (Just my rambling muses.)
  17. Nov 22, 2015 #16


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    You are somewhat overstating the situation.

    Although quantum mechanics is indeed probabilistic at its core there remains the possibility that QM is not the last word. Imagine a black box that flashes a red or green light when a button is pushed - we could construct a perfectly satisfactory probabilistic theory of the box (red light 30% of the time, green the other 70%). However, if we were to take the box apart and examine its inner workings, we might learn enough more to be able to predict a deterministic result for any given button press (and also to calculate that 70/30 green/red expectation values for a large number of random presses).

    What we do know is that quantum mechanics is the best theory we have so far, and as long as that is the case the title question is undecidable. (With apologies to .Scott whose answer I really like).
  18. Nov 22, 2015 #17
    The evolution of the wave function is deterministic at all times. The future is therefore fully determined, but undeterminable optically.
  19. Nov 22, 2015 #18
    The future is not set upon a rigid stone body for what each person has to do individually, but what has been set upon a rigid steel starship superstructure is that humanity must colonize space and explore or face extinction with the possibility of causing the extinction of all life in the Universe for exploring and colonizing space.
  20. Nov 22, 2015 #19
    If the future has already taken place and like you stated someone blows something up in the past they really haven't affected the future that they came from. They merely created a new reality. Think of it this way. You have three train tracks ahead of you that represent the current historical time line and the other two representing possible realities. You are approaching the three from a single track. No matter which track you take history remains the same. Suddenly a nuclear bomb explodes in the middle track time line. As soon as the bomb explodes affecting the history of the original timeline the original timeline splits from the affected time line that allows the original time line to remain unaffected. If you were to travel back in time again to the point of where the bomb was exploded and explode another bomb an entirely new timeline separate from the original time line created when the first bomb was detonated would be created. Each causality within a changed time line creates another new time line. This is because of the emotions involved. The that the time line changed is affected the more the emotions of the people are affected thus creating new realities.

    If you were to return to the single track and changed the switch to the very first original time line and compared it to the second and third time lines parallel to one another they would function separate of each other and would not overlap.

    But what if one person from all three time lines functioned the same in all three time lines whose personal events did not change but remained the same, would that person be able to see into all three timelines even though they had not been involved with the actual timeline shift to begin with?

    What I have noticed from people is that they try and make theirself believe that they can change time simply by using words or sounds. For example I have noticed that when I say something on the internet that corresponds with my real name those who think they have control over time and events will use word associations to try and trip my brain up. They use their word associations of what I have typed but in a different sentence structure where what they are talking about might be close to the same thing that I was talking about but it really wasn't.

    These fake "Time Changers" can be easily identified because they will talk about what you have differently usually the next day or three to four days after I have posted something. To them talking in semantics the day after creates a fast state of time change in the persons mind where as the longer they go without making any references to them at least once again allows the for the thought to the stored in my mind where they later talk about the topics hoping to make me think that we have relapsed into the past and that time has changed. Basically this is the base for what is De Je Vu. De Je Vu is used to try and create false realities in the past such as someone else writing a book that they didn't but have instead hacked into the persons computer and by reading what seen on the persons computer screen the person being attacked with De Je Vu would wonder how did the person get that information. Then the attacker attributes their knowledge as coming from God and that it was them that wrote the book and not you. The attacker will then try and convince others around him of his truth by promising riches and fame if they believe him.

    De Je Vu is not even wrong.

    Words therefore really do not have any impact on the creation of or deletion of reality. A person say that they are going bomb a certain place over and over and over again and most people would ignore that person going about their daily lives how they normally would. Its not until the person actually bombs that place do the people actually change their time line of events to move around the explosion. But once the explosion has taken place and systems repaired the time line shift usually collapses back into the normal time line with minor shifts taking place that really do not adjust the overall continuity of the time line itself.
    Last edited: Nov 22, 2015
  21. Nov 22, 2015 #20


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    I know this is not what the OP starter had in mind, but I can't resist throwing in my simple-minded information theory. Suppose a coin has been tossed but you have not looked at the result yet. The future is determined since the heads/tails that you will see has already happened. But you do not know enough to determine it till you look. So there can be limits to the information available that make things indeterminable even if they are already determined. I imagine that there are more sophisticated examples of this in information theory.
  22. Nov 22, 2015 #21
    That's a good enough analogy.
  23. Nov 22, 2015 #22


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  24. Nov 22, 2015 #23


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    This is incorrect, for two reasons.
    First, collapse is defined to be a non-deterministic change in the wave function so the evolution of the wave function is deterministic only in those interpretations that do not include collapse.
    Second, even in those interpretations that do not include collapse the wave function determines only the probability of particular outcomes, so the deterministic evolution of the wave function does not mean that the future is determined.
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