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Question about determinism

  1. May 27, 2014 #1
    Hi, some time ago I read the book “The black hole war” by Leonard Suskind and I became aware of the issue of the lose of information in the black holes. Since then there is a question that is racking my mind but I am not smart enough to find the answer. Is about determinism.
    Lets say that we have an isolated system of particles bouncing and colliding with each other, etc..
    If we know the information of each particle and the physical laws, we can reverse the movie in time and know how the system was at a given point in the past.
    Now we put a brain in the system and the brain can move a lever and push a particle to the right or to the left side. If the brain is a mechanism like any other, we can still reverse the movie, but if the brain can really choose what to do, doesn't it means that we cannot reverse the movie and the information chain is broken? This would go against the principle of conservation of the information.
     
    Last edited: May 27, 2014
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  3. May 27, 2014 #2

    adjacent

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    How do you put a brain there?
     
  4. May 27, 2014 #3
    Please, joking is nice but I really want to know the answer to this.
     
  5. May 27, 2014 #4

    Fredrik

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    It looks like you have already found the answer. If the brain's actions are correctly described by a deterministic theory, then the choice it will make is determined by the initial conditions. Does this mean that it can't "really" choose? It depends on what you mean by "really". The brain is choosing, and the choice is a physical process like any other. That sounds pretty real to me. But your idea about what a "real choice" is, seems to include that the choice isn't determined by the initial conditions. This would mean that the brain can't be described by a deterministic theory. What would the alternative be? It could be described by a theory where an initial condition corresponds to many possible outcomes, and only determines the probabilities of those outcomes. But you probably wouldn't consider this a real choice either, because now your brain is making random decisions instead of predetermined ones.

    So it appears that what you mean by a "real choice" can only exist if brains are fundamentally different from anything that we have so far been able to describe with theories of physics. There is no evidence that they are.
     
  6. May 27, 2014 #5

    stevendaryl

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    Well, if the laws of physics are deterministic, then so is the brain.

    However, the issue of determinism in physics is a little complicated when you take into account quantum mechanics (which Suskind surely is). The usual way that quantum mechanics is applied involves a deterministic aspect, where the wave function evolves deterministically, and a nondeterministic aspect, where a measurement of a physical quantity gives a nondeterministic result, with probabilities computed from the wave function. The measurement process seems to be an irreversible process, although there is no consensus about that.
     
  7. May 27, 2014 #6

    AlephZero

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  8. May 27, 2014 #7

    sophiecentaur

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    The concept of 'free will' is an interesting one and not part of Physics (which has quite enough to think of, without delving into philosophy). There is lots of evidence to suggest that we have made our 'choices' long before we are actually conscious of working things out.
     
  9. May 27, 2014 #8
    Fredrik: Thanks for your very nice answer. By really I mean that we can really change the future and it is not already determined by the physical laws. For what I know, my opinion is that we cannot, but I would really want to know if there is a solution to this dilemma.

    Stevendary: “a measurement of a physical quantity gives a non deterministic result” that was nice. I am going to make a search about this. Quantum fluctuations or quantum uncertainty are what supposedly would allow a non all determined reality, but the fact that the future can’t be predicted doesn't mean that we can change it, in my humble opinion.

    Thanks alephzero and sophiecentaur for your answers.
     
  10. May 27, 2014 #9

    stevendaryl

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    When it comes to quantum mechanics, almost any time you express something in words, rather than formulas, then you're likely to be misleading in some way. The phrase "quantum fluctuations" seems to be about intrinsic fluctuations in some physical property. For certain purposes, we can think of it that way, but the equations don't really say that, explicitly.

    To give an example: The harmonic oscillator--a mass connected to a spring whose force is proportional to how much it is stretched or compressed. In classical mechanics, the minimum energy would be zero--corresponding to the case where the spring is neither compressed nor stretched, but is at its equilibrium length. If you do a quantum-mechanical analysis, you will find that it has a minimum possible energy level that is nonzero--the so-called "ground state". This ground state is sometimes interpreted as due to fluctuations. You can visualize the mass randomly jerking back and forth near the equilibrium point. But the wave function for a particle in the ground state doesn't show any kind of random motion. To reconcile these two pictures, you can introduce the measurement process: If the mass is in the ground state, and you measure its position, you will get a nondeterministic value (it is most likely to be in the equilibrium position, but has a nonzero probability of being far away from the equilibrium position). So when you try to observe the position or momentum of the mass, you get a random result as if those quantities were fluctuating.

    But quantum mechanics doesn't really say that the quantities are fluctuating. Fluctuation suggests that the mass always has a definite (if unknown) position and a definite (if unknown) momentum, but those quantities are changing unpredictably, so the best we can do is give probabilistic descriptions. But quantum mechanics is weirder than that. Bell's theorem shows that there is no simple way to understand quantum uncertainty in terms of quantities that have definite, if unknown, values.
     
  11. May 27, 2014 #10

    WannabeNewton

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    "Deterministic" means if you give me the necessary initial conditions I can provide you with all possible information about a system for all time. In QM if you give me an initial state of a system I can evolve it deterministically for all time using the Schrodinger equation, which is a first order dynamical differential equation for the state. The fact that these states contain fluctuations of observables from their expectation values does not mean the theory is non-deterministic. In QM a pure state contains all possible information about a system at any given instant of time, which is what we need for determinism. QM, or at least the most viable interpretations of it, state that the fluctuations of observables from their expectation values and the probabilistic rules given to the state are intrinsic to nature and as such we can't do any better than pure states as far as information and determinism are concerned.
     
  12. May 27, 2014 #11

    Dale

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    If a brain "can really change the future and it is not already determined by the physical laws" then tautologically you cannot use physical laws to determine the future state of any system containing a brain.

    Any such system is, by your assumption, not deterministic and cannot be analyzed using physical laws. As such, it is a conversation for the local pub, and not for physics forums. This is thread is on a short leash and will only remain open as long as the conversation remains limited to systems that obey the laws of physics, not hypothetical brains that by assumption violate them.
     
  13. May 27, 2014 #12
    Stevendary: Thanks for the clarifying. I was also thinking about those particles that can appear temporarily out of empty space.
    In the quote I was expressing an opinion, not arguing something you said, as it looks like. Sorry by that.

    Thanks wannabenewton and mentor.

    Mentor: before posting here I took a look at some forums about philosophy and there are even about determinism, but what i would like to know is what physics says about this.
     
    Last edited: May 27, 2014
  14. May 27, 2014 #13

    sophiecentaur

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    Determinism and free will are both matters of faith and have nothing to do with Physics, imo. Probability works as a way of working things out - that's all you can say.
     
  15. May 27, 2014 #14
    This. Very nicely put.
     
  16. May 27, 2014 #15

    Dale

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    Physics cannot say anything about a system whose behavior you assume is not determined by physical laws. You are literally asking "what does physics say about situations where physics doesn't say anything?". The answer is tautologically "nothing".

    If you want to talk about what physics says about a scenario then you must posit that the scenario is governed by the laws of physics.
     
  17. May 27, 2014 #16

    adjacent

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    I feel like an idiot now. Sorry :frown:
     
  18. May 27, 2014 #17

    sophiecentaur

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    No need to feel bad. There is a long queue of us who feel that way on PF, from time to time. :smile:
     
  19. May 27, 2014 #18
    I do think that these are matters of faith but I do not think they have "nothing to do with physics". After all, Determinism is based on the laws of physics. If these laws say a+b=c, and we know a and b, we can determine c. Determinism says that if the brain works the same way as the laws of physics, we can predict what one will do given the circumstances.
     
  20. May 27, 2014 #19
    Thanks for your answers, but I refresh the original question:
    It was if the act of being able to choose and therefore to change the future, is meaning a loss of information, going against the fundamental principle of physics that says that the information can't be lost.
    If we can reverse the movie, this means that everything is "automatic", If we have free will we lose information because the chain of events is broken by the act of choosing, we cannot reverse the movie.
    I don't know this for sure, is what I am asking.
     
    Last edited: May 27, 2014
  21. May 27, 2014 #20
    Really, this is the best answer:

     
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