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Fascinating. Do physicists generally believe this? The idea that both the infinite future and the infinite past are all encoded into any snapshot of the unive

Fascinating. Do physicists generally believe this? The idea that both the infinite future and the infinite past are all encoded into any snapshot of the universe is blowing my mind. My layperson's understanding of QM always gave me the impression that it does the opposite: that it gives non-determinism a place to "hide," so-to-speak. My impression was that religious believers could use QM to argue that their unseen God, for example, does actually have a physically plausible mechanism by which he/it could intervene in the physical world. But if I'm understanding this theory correctly, unless God is a physical thing, there is no way for God or any other supernatural force to affect the physical world without itself being physical. This theory also would seem to deny the existence of free will.

solipsism 11y ago
QM does away with any notion of conservation of information. Time reversibility (T symmetry) is usually talked about at macro scales where QMs effects are vanishingly small. IANAP Regarding free will -- rigorously define it for us, without evoking the supernatural, and as distinct from pure randomness and absolute determinism, and it should be easy to tell whether the law of conservation of information allows for it. Before spending too much time in e it, though, note that such a definition is widely considered impossible. IANAP
- haberman 11y ago
  To me, a lack of free will would mean that a mind's future state is entirely determined by its current state. "Free will" would mean that its future state can be altered by some nonphysical entity (whether you call that a soul, consciousness, or whatever).
  - RobertoG 11y ago
    altered how? at random? randomness doesn't look 'free' to me, looks.. random. following some rules? not 'free' anymore. This "free will" thing doesn't support well scrutiny.
    - haberman 11y ago
      Altered according to the agency of the nonphysical entity. Not randomly or following rules.
      - solipsism 11y ago
        Not randomly or following rules There is no other choice. By definition, random means "not following any rule or pattern".
        haberman 11y ago
        I think it depends on your perspective. From the perspective of my keyboard, my input is random/unpredictable (it doesn't have perfect entropy, but it doesn't follow rules that make it completely predictable). From the perspective of my body, I have complete control over what I am typing, and there is nothing random about it.
        solipsism 11y ago
        No it doesn't depend on your perspective. If it did the conversation would make no sense. The only perspective worth considering is the global objective perspective. Quantum events are globally, objectively random. Nothing else seems to be, which means, by definition, that everything else is deterministic.
        haberman 11y ago
        The question I'm trying to get at is: from the perspective of the physical realm (which may be, even likely is, all that exists) is there any possible mechanism by which something nonphysical could influence events in the physical realm? For example, is there any physical property that is not "locked into place" by the laws we know and the observations we can make? Is there any fundamental impossibity (not practical impossibility) of snapshotting the state of the universe and using that snapshot to determine every past and future state of the universe?
      - 11y ago
  - solipsism 11y ago
    I said without evoking the supernatural. The concept of a soul is supernatural. There's no reason to think consciousness is anything other than physical in nature -- but to the extent that you mean it in some kind of supernatural, extra-physical way, it's not part of this conversation. By all means go on thinking there might be some supernatural thing called a "free will" if you want. But if it's supernatural, then the utterly natural laws of QM wouldn't have much to say about it.
    - haberman 11y ago
      > There's no reason to think consciousness is anything other than physical in nature Consciousness is not the same thing as agency. It is possible that we are conscious but do not actually have free will / agency. > By all means go on thinking there might be some supernatural thing called a "free will" if you want. But if it's supernatural, then the utterly natural laws of QM wouldn't have much to say about it. I am not sure I agree. If QM implies a strict determinism, to me that rules out free will. For free will to be plausible, there has to be a place for non-determinism to "hide." If QM can rule that out, to me that rules out free will.
  - jonsen 11y ago
    Free will is an obvious idea for a self aware consciousness to adhere to. The purpose of consciousness is to keep its organism viable. It does that by collecting an essence of past and from that predict an essence of the future. This expectation of the immediate future is then corrected according to actual perceptions and transformed into appropriate actions. The full past is abstracted into an essence state, lots of information is lost to elsewhere, so there is no way to determine what details of the past went into forming the precise state from which the brain makes its actions. Of course it has to feel like at least some of our choices emerges from nowhere. But the only way to know if a brains thoughts are really free is to replay its entire brain state building past in every minute detail and look for alternative actions. An impossible experiment.
- Dn_Ab 11y ago
  Time reversibility exists in quantum mechanics because observables are self adjoint operators. Closed systems evolve unitarily. In simpler terms, you can think of it as the requirement that maps preserve distances and are easily invertible. We need this so that the information describing a system (which we can still talk about in terms of traces), remains invariant with time. In the classical sense, the corresponding violation leads to probabilities not summing to 1! We clearly can't have information shrink and for pure systems, dropping distance preserving maps leads to a really awesome universe (I believe this also ends up highly recommending L2). We literally go from a universe that is almost certainly near the bottom end of the Slow Zone of Thought to the Upper Beyond (https://en.wikipedia.org/wiki/A_Fire_Upon_the_Deep#Setting https://en.wikipedia.org/wiki/A_Fire_Upon_the_Deep#Setting). We gain non-locality, causality violations and powerful computational ability. In practice, our confusion about a system does increase with time as classical systems become ever more correlated, losing distinguishability, aka decoherence.
danbruc 11y ago
It is the same for quantum mechanics, in technical terms the time evolution of a quantum system is unitary. It is the uncertainty principle that prevents you from learning the exact state of a system and therefore perfectly predicting its evolution into the future or past. The underlying mathematical structure nonetheless allows you to do exactly this assuming you exactly knew the state of the system at one point in time. There is of course the infamous collapse of the wave function that seems to turn a quantum state into a classical state during a measurement and this with different probabilities for different outcomes. This collapse of the wave function is not unitary and there are no good reasons to assume that a measurement is in any way special and should violate the laws of quantum mechanics which have otherwise been shown to work with incredible precision. Therefore the collapse of the wave function is probably not a thing but just an apparent artifact of our ignorance of the details of the measurement apparatus which just is a gigantic quantum mechanical systems with billions and billions of particles. Another way to think about that are symmetries and conserved quantities. The physical laws are believed to have certain symmetries, for example time and space translation symmetry or rotation symmetry, i.e. a physical experiment should yield the exact same result no matter whether you do it here or five meters to the left, today or in three weeks, with the laser beam pointing north or east. Space translation symmetry, for example, means that if you change all you initial say x coordinates by the same amount the calculations should nonetheless yield exactly the same result as a calculation with the unchanged coordinates besides that the final state will of course have the same offset in its x coordinates. If you demand that your laws of physics obey such symmetries and do some math you will find that each such symmetry inevitably gives you a quantity that does not change over time, i.e. that is conserved. That is Noether's theorem and extremely general, i.e. (almost) no matter what laws you imagine, a symmetry will always give you a conserved quantity. The conserved quantity for space translation symmetry is what we call momentum, for rotation in space it is angular momentum and for time translation it is energy. But now that we have conserved quantities it is now impossible that two states differing in any of the conserved quantities will ever evolve into the same state or have ever been in the same state because that would require the change of a conserved quantity. So to summarize, very basic assumptions like isotropy and homogeneity of space and time inevitably lead to conserved quantities which in turn prevent certain state evolutions that would violate those conservation laws. I am not sure if those constraints force time reversibility onto you but I assume they do. Another commenter mentioned T-symmetry, that is that the physical laws remain the same if you reverse the time axis, i.e. multiply the time coordinates by minus one flipping past and future. This is of course not the same as time translation symmetry which is about adding to the time coordinates and it is also different from the reversibility I mentioned before, i.e. the ability to retrodict past states which does not require that the evolution into the past is symmetric with the evolution into the future. T-symmetry is believed to be violated for very good reasons but that has not yet been shown experimentally although it follows from the observed CP-violation and the assumed CPT-symmetry. And I am not a physicist, so while I believe everything I said is accurate or marked as me not being sure or not knowing it may still contain some - hopefully minor - errors.

solipsism 11y ago

QM does away with any notion of conservation of information. Time reversibility (T symmetry) is usually talked about at macro scales where QMs effects are vanishingly small. IANAP Regarding free will -- rigorously define it for us, without evoking the supernatural, and as distinct from pure randomness and absolute determinism, and it should be easy to tell whether the law of conservation of information allows for it. Before spending too much time in e it, though, note that such a definition is widely considered impossible. IANAP

haberman 11y ago
To me, a lack of free will would mean that a mind's future state is entirely determined by its current state. "Free will" would mean that its future state can be altered by some nonphysical entity (whether you call that a soul, consciousness, or whatever).
- RobertoG 11y ago
  altered how? at random? randomness doesn't look 'free' to me, looks.. random. following some rules? not 'free' anymore. This "free will" thing doesn't support well scrutiny.
  - haberman 11y ago
    Altered according to the agency of the nonphysical entity. Not randomly or following rules.
    - solipsism 11y ago
      Not randomly or following rules There is no other choice. By definition, random means "not following any rule or pattern".
      - haberman 11y ago
        I think it depends on your perspective. From the perspective of my keyboard, my input is random/unpredictable (it doesn't have perfect entropy, but it doesn't follow rules that make it completely predictable). From the perspective of my body, I have complete control over what I am typing, and there is nothing random about it.
        solipsism 11y ago
        No it doesn't depend on your perspective. If it did the conversation would make no sense. The only perspective worth considering is the global objective perspective. Quantum events are globally, objectively random. Nothing else seems to be, which means, by definition, that everything else is deterministic.
        haberman 11y ago
        The question I'm trying to get at is: from the perspective of the physical realm (which may be, even likely is, all that exists) is there any possible mechanism by which something nonphysical could influence events in the physical realm? For example, is there any physical property that is not "locked into place" by the laws we know and the observations we can make? Is there any fundamental impossibity (not practical impossibility) of snapshotting the state of the universe and using that snapshot to determine every past and future state of the universe?
    - 11y ago
- solipsism 11y ago
  I said without evoking the supernatural. The concept of a soul is supernatural. There's no reason to think consciousness is anything other than physical in nature -- but to the extent that you mean it in some kind of supernatural, extra-physical way, it's not part of this conversation. By all means go on thinking there might be some supernatural thing called a "free will" if you want. But if it's supernatural, then the utterly natural laws of QM wouldn't have much to say about it.
  - haberman 11y ago
    > There's no reason to think consciousness is anything other than physical in nature Consciousness is not the same thing as agency. It is possible that we are conscious but do not actually have free will / agency. > By all means go on thinking there might be some supernatural thing called a "free will" if you want. But if it's supernatural, then the utterly natural laws of QM wouldn't have much to say about it. I am not sure I agree. If QM implies a strict determinism, to me that rules out free will. For free will to be plausible, there has to be a place for non-determinism to "hide." If QM can rule that out, to me that rules out free will.
- jonsen 11y ago
  Free will is an obvious idea for a self aware consciousness to adhere to. The purpose of consciousness is to keep its organism viable. It does that by collecting an essence of past and from that predict an essence of the future. This expectation of the immediate future is then corrected according to actual perceptions and transformed into appropriate actions. The full past is abstracted into an essence state, lots of information is lost to elsewhere, so there is no way to determine what details of the past went into forming the precise state from which the brain makes its actions. Of course it has to feel like at least some of our choices emerges from nowhere. But the only way to know if a brains thoughts are really free is to replay its entire brain state building past in every minute detail and look for alternative actions. An impossible experiment.
Dn_Ab 11y ago
Time reversibility exists in quantum mechanics because observables are self adjoint operators. Closed systems evolve unitarily. In simpler terms, you can think of it as the requirement that maps preserve distances and are easily invertible. We need this so that the information describing a system (which we can still talk about in terms of traces), remains invariant with time. In the classical sense, the corresponding violation leads to probabilities not summing to 1! We clearly can't have information shrink and for pure systems, dropping distance preserving maps leads to a really awesome universe (I believe this also ends up highly recommending L2). We literally go from a universe that is almost certainly near the bottom end of the Slow Zone of Thought to the Upper Beyond (https://en.wikipedia.org/wiki/A_Fire_Upon_the_Deep#Setting https://en.wikipedia.org/wiki/A_Fire_Upon_the_Deep#Setting). We gain non-locality, causality violations and powerful computational ability. In practice, our confusion about a system does increase with time as classical systems become ever more correlated, losing distinguishability, aka decoherence.

danbruc 11y ago

It is the same for quantum mechanics, in technical terms the time evolution of a quantum system is unitary. It is the uncertainty principle that prevents you from learning the exact state of a system and therefore perfectly predicting its evolution into the future or past. The underlying mathematical structure nonetheless allows you to do exactly this assuming you exactly knew the state of the system at one point in time. There is of course the infamous collapse of the wave function that seems to turn a quantum state into a classical state during a measurement and this with different probabilities for different outcomes. This collapse of the wave function is not unitary and there are no good reasons to assume that a measurement is in any way special and should violate the laws of quantum mechanics which have otherwise been shown to work with incredible precision. Therefore the collapse of the wave function is probably not a thing but just an apparent artifact of our ignorance of the details of the measurement apparatus which just is a gigantic quantum mechanical systems with billions and billions of particles. Another way to think about that are symmetries and conserved quantities. The physical laws are believed to have certain symmetries, for example time and space translation symmetry or rotation symmetry, i.e. a physical experiment should yield the exact same result no matter whether you do it here or five meters to the left, today or in three weeks, with the laser beam pointing north or east. Space translation symmetry, for example, means that if you change all you initial say x coordinates by the same amount the calculations should nonetheless yield exactly the same result as a calculation with the unchanged coordinates besides that the final state will of course have the same offset in its x coordinates. If you demand that your laws of physics obey such symmetries and do some math you will find that each such symmetry inevitably gives you a quantity that does not change over time, i.e. that is conserved. That is Noether's theorem and extremely general, i.e. (almost) no matter what laws you imagine, a symmetry will always give you a conserved quantity. The conserved quantity for space translation symmetry is what we call momentum, for rotation in space it is angular momentum and for time translation it is energy. But now that we have conserved quantities it is now impossible that two states differing in any of the conserved quantities will ever evolve into the same state or have ever been in the same state because that would require the change of a conserved quantity. So to summarize, very basic assumptions like isotropy and homogeneity of space and time inevitably lead to conserved quantities which in turn prevent certain state evolutions that would violate those conservation laws. I am not sure if those constraints force time reversibility onto you but I assume they do. Another commenter mentioned T-symmetry, that is that the physical laws remain the same if you reverse the time axis, i.e. multiply the time coordinates by minus one flipping past and future. This is of course not the same as time translation symmetry which is about adding to the time coordinates and it is also different from the reversibility I mentioned before, i.e. the ability to retrodict past states which does not require that the evolution into the past is symmetric with the evolution into the future. T-symmetry is believed to be violated for very good reasons but that has not yet been shown experimentally although it follows from the observed CP-violation and the assumed CPT-symmetry. And I am not a physicist, so while I believe everything I said is accurate or marked as me not being sure or not knowing it may still contain some - hopefully minor - errors.