7 ms·

>Does it account, for instance, for the fact that we know more about the past than the future? Reminded me of a side remark from another context: The past onl

by jonsen 9y ago

>Does it account, for instance, for the fact that we know more about the past than the future? Reminded me of a side remark from another context: The past only exists in our memory, therefore in some sense we can't recall incorrectly.

lisper 9y ago
That depends on what you mean. The past state of the universe is faithfully encoded in the current state of the universe. But that does not mean that a subset of that state (a single human, say, or even the entire population of a single planet) cannot have false memories.
- semi-extrinsic 9y ago
  What do you mean "faithfully"? For sure we can all agree that this encoding is irreversible - i.e. we can't reconstruct the past state from the present. And due to the combination of chaos in macroscopic systems and quantum uncertainty, I think it's highly likely that we can conceive of two slightly different past states that give present states indistinguishable even to $deity_of_unlimited_power. So how is it "faithful"?
  - lisper 9y ago
    > What do you mean "faithfully"? A good question! I mean that future states are produced by past states according to physical laws. > For sure we can all agree that this encoding is irreversible - i.e. we can't reconstruct the past state from the present. Not with full accuracy, no. But past states are constrained by present states in useful and interesting ways.
    - kmill 9y ago
      Usually "encoded faithfully" is used to mean that the encoded thing can be recovered perfectly (mathematically, not necessarily physically). I thought you were saying that both the past and the future are functions of the present, but we can't calculate those functions ourselves with full accuracy.
      - lisper 9y ago
        > I thought you were saying that both the past and the future are functions of the present Yes, that is exactly what I'm saying. > but we can't calculate those functions ourselves with full accuracy. Yes, that is also true. The universe as a whole is not constrained by the limits of what we humans can compute.
        kmill 9y ago
        > Yes, that is exactly what I'm saying. Sorry to be a stickler for detail, but earlier you defined "faithful" to mean the future is a function of the present, and now you're saying that "the past and future are functions of the present" is exactly what you are saying. Perhaps you mean that it's exactly what you intended to say? I'm not intending to undermine anything with "gotchas." It just seems to me that more precision will help your arguments. One thing I do not see, in your essay, is how reducing "memory" to "entanglement" means time is asymmetric. What is it about entanglement that is asymmetric that isn't just "that's how entanglement is defined"? I also do not understand how the thought experiment implies time travel cannot happen. Has anyone proved that the wave equation has no solutions when there are closed timelike curves? (I assume not since there are papers studying quantum computations possible in the presence of closed timelike curves, for instance.)
        lisper 9y ago
        All of those statements are consistent with the concept that I am attempting to communicate through the imprecise medium of natural language. > What is it about entanglement that is asymmetric that isn't just "that's how entanglement is defined"? Entanglement is symmetric. You can reverse an entanglement. But only by returning the entangled particles to the same physical location. The reason that this symmetric quantum process gives rise to (what appears to us to be) a time-asymmetric classical universe is that what we classical entities actually are is massively entangled systems of vast numbers of quantum particles (or, to be as precise as I can, we are quantum systems with a vast number of mutually entangled degrees of freedom). All those entanglements can be reversed, but if you reverse them, the result is not "moving backwards in time" as humans commonly conceptualize it. The result is rewinding the whole universe to a previous state. That is actually possible in principle. As I note at the end of the essay, there is no way to distinguish a universe that is constantly being rewound and replayed from the universe we live in. But what most people think of as going backwards in time is rewinding the whole universe with the exception of themselves to a previous state. That's not possible. I don't actually know what the consequences are of applying this idea to CTCs. I don't understand CTCs well enough to do it myself. There's probably a thesis or two in there somewhere. Mixing QM and GR is a rare skill.
  - JumpCrisscross 9y ago
    > For sure we can all agree that this encoding is irreversible Isn't the loss of information, i.e. the breaking of quantum determinism, at the centre of the black hole information paradox [1]? It's a paradox because quantum determinism and reversibility imply information can never be destroyed. [1] https://en.wikipedia.org/wiki/Black_hole_information_paradox https://en.wikipedia.org/wiki/Black_hole_information_paradox
- jonsen 9y ago
  "The past ..." — If there is only one.
- rst 9y ago
  The past state of the universe is encoded in the present state only in the same way, and to the same extent, that the future is.
  - lisper 9y ago
    Only in a classical universe, not a quantum one. The past is encoded in a way that the future is not because our classical universe emerges from a quantum universe through the creation of entanglements, which is what defines the arrow of time.
    - eru 9y ago
      Why? The wave function evolves symmetrically and deterministically, doesn't it?
      - lisper 9y ago
        Yes, it does. But the classical universe emerges because sub-systems of the wave function become classically correlated as a result of entanglements [1]. These entanglements are what define the arrow of time in the (or perhaps it would be more accurate to say "a" instead of "the" here) classical universe [2]. Information about the past is the classical correlations that arise from entanglements. Entanglements can only arise in (sub-)systems that were initially co-located, so relativity insures that these propagate only from the past into the future. [1] http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf [2] http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
        eru 9y ago
        Do we really understand quantum mechanics well enough to say how our classic approximation arises from it? Edit: interesting paper on quantum information theory and measurement vs entanglement!
        9y ago
- leovailati 9y ago
  You are incorrect. According to systems theory, the current state of a system holds the information you need to go forward, not necessarily backwards. Imagine a bouncing basketball: if you can only see its current state, you don't necessarily know when the ball started bouncing. It could be that it started years ago, bouncing all the way from the stratosphere. You can't know the initial conditions of a system just from it's current state. After the ball stops bouncing it can rest on the floor for a million years and you will never know if it stopped bouncing a million years ago or five seconds ago, that is because a real bouncing ball is a non-linear system. So no, the history of a system is not encoded on its current state. On the other hand, you can always predict the future state of a bouncing ball system if you know its current state.
  - lisper 9y ago
    Nope. The laws of classical mechanics are fully time-reversible. All you have to do to go backwards is invert the sign on all the velocities.
    - leovailati 9y ago
      Right, but that was not my point. I said two things: you can't always know the initial conditions and you can't transpose non-linearities. Yes, you can invert the sign of all velocities and trace the path of a particle in reverse, but can you tell where it started moving if its current velocity is constant? What if that particle hits a wall and stays there? How can you know when in the past it stopped moving?
      - lisper 9y ago
        > you can't always know the initial conditions Just because you can't know them doesn't mean they aren't physical. God can know the initial conditions. In a classical world, that's good enough. > and you can't transpose non-linearities So? Classical mechanics is linear. > can you tell where it started moving if its current velocity is constant? Yes, of course. Just as easily -- and by the exact same method -- as you can tell where it will stop moving. > What if that particle hits a wall and stays there? How can you know when in the past it stopped moving? That's a question that is answered in a first-year physics course, but the TL;DR is that any collision must conserve both momentum and energy. If a ball hits a wall and "sticks" then some of its energy must be dissipated as heat. That is the thermodynamic arrow of time, which is exactly what the original post was about.
- deleted 9y ago
  [deleted]
hinkley 9y ago
So you didn't post this message three hours ago?

by jonsen 9y ago

lisper 9y ago

That depends on what you mean. The past state of the universe is faithfully encoded in the current state of the universe. But that does not mean that a subset of that state (a single human, say, or even the entire population of a single planet) cannot have false memories.

semi-extrinsic 9y ago
What do you mean "faithfully"? For sure we can all agree that this encoding is irreversible - i.e. we can't reconstruct the past state from the present. And due to the combination of chaos in macroscopic systems and quantum uncertainty, I think it's highly likely that we can conceive of two slightly different past states that give present states indistinguishable even to $deity_of_unlimited_power. So how is it "faithful"?
- lisper 9y ago
  > What do you mean "faithfully"? A good question! I mean that future states are produced by past states according to physical laws. > For sure we can all agree that this encoding is irreversible - i.e. we can't reconstruct the past state from the present. Not with full accuracy, no. But past states are constrained by present states in useful and interesting ways.
  - kmill 9y ago
    Usually "encoded faithfully" is used to mean that the encoded thing can be recovered perfectly (mathematically, not necessarily physically). I thought you were saying that both the past and the future are functions of the present, but we can't calculate those functions ourselves with full accuracy.
    - lisper 9y ago
      > I thought you were saying that both the past and the future are functions of the present Yes, that is exactly what I'm saying. > but we can't calculate those functions ourselves with full accuracy. Yes, that is also true. The universe as a whole is not constrained by the limits of what we humans can compute.
      - kmill 9y ago
        > Yes, that is exactly what I'm saying. Sorry to be a stickler for detail, but earlier you defined "faithful" to mean the future is a function of the present, and now you're saying that "the past and future are functions of the present" is exactly what you are saying. Perhaps you mean that it's exactly what you intended to say? I'm not intending to undermine anything with "gotchas." It just seems to me that more precision will help your arguments. One thing I do not see, in your essay, is how reducing "memory" to "entanglement" means time is asymmetric. What is it about entanglement that is asymmetric that isn't just "that's how entanglement is defined"? I also do not understand how the thought experiment implies time travel cannot happen. Has anyone proved that the wave equation has no solutions when there are closed timelike curves? (I assume not since there are papers studying quantum computations possible in the presence of closed timelike curves, for instance.)
        lisper 9y ago
        All of those statements are consistent with the concept that I am attempting to communicate through the imprecise medium of natural language. > What is it about entanglement that is asymmetric that isn't just "that's how entanglement is defined"? Entanglement is symmetric. You can reverse an entanglement. But only by returning the entangled particles to the same physical location. The reason that this symmetric quantum process gives rise to (what appears to us to be) a time-asymmetric classical universe is that what we classical entities actually are is massively entangled systems of vast numbers of quantum particles (or, to be as precise as I can, we are quantum systems with a vast number of mutually entangled degrees of freedom). All those entanglements can be reversed, but if you reverse them, the result is not "moving backwards in time" as humans commonly conceptualize it. The result is rewinding the whole universe to a previous state. That is actually possible in principle. As I note at the end of the essay, there is no way to distinguish a universe that is constantly being rewound and replayed from the universe we live in. But what most people think of as going backwards in time is rewinding the whole universe with the exception of themselves to a previous state. That's not possible. I don't actually know what the consequences are of applying this idea to CTCs. I don't understand CTCs well enough to do it myself. There's probably a thesis or two in there somewhere. Mixing QM and GR is a rare skill.
- JumpCrisscross 9y ago
  > For sure we can all agree that this encoding is irreversible Isn't the loss of information, i.e. the breaking of quantum determinism, at the centre of the black hole information paradox [1]? It's a paradox because quantum determinism and reversibility imply information can never be destroyed. [1] https://en.wikipedia.org/wiki/Black_hole_information_paradox https://en.wikipedia.org/wiki/Black_hole_information_paradox
jonsen 9y ago
"The past ..." — If there is only one.
rst 9y ago
The past state of the universe is encoded in the present state only in the same way, and to the same extent, that the future is.
- lisper 9y ago
  Only in a classical universe, not a quantum one. The past is encoded in a way that the future is not because our classical universe emerges from a quantum universe through the creation of entanglements, which is what defines the arrow of time.
  - eru 9y ago
    Why? The wave function evolves symmetrically and deterministically, doesn't it?
    - lisper 9y ago
      Yes, it does. But the classical universe emerges because sub-systems of the wave function become classically correlated as a result of entanglements [1]. These entanglements are what define the arrow of time in the (or perhaps it would be more accurate to say "a" instead of "the" here) classical universe [2]. Information about the past is the classical correlations that arise from entanglements. Entanglements can only arise in (sub-)systems that were initially co-located, so relativity insures that these propagate only from the past into the future. [1] http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf [2] http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
      - eru 9y ago
        Do we really understand quantum mechanics well enough to say how our classic approximation arises from it? Edit: interesting paper on quantum information theory and measurement vs entanglement!
        9y ago
leovailati 9y ago
You are incorrect. According to systems theory, the current state of a system holds the information you need to go forward, not necessarily backwards. Imagine a bouncing basketball: if you can only see its current state, you don't necessarily know when the ball started bouncing. It could be that it started years ago, bouncing all the way from the stratosphere. You can't know the initial conditions of a system just from it's current state. After the ball stops bouncing it can rest on the floor for a million years and you will never know if it stopped bouncing a million years ago or five seconds ago, that is because a real bouncing ball is a non-linear system. So no, the history of a system is not encoded on its current state. On the other hand, you can always predict the future state of a bouncing ball system if you know its current state.
- lisper 9y ago
  Nope. The laws of classical mechanics are fully time-reversible. All you have to do to go backwards is invert the sign on all the velocities.
  - leovailati 9y ago
    Right, but that was not my point. I said two things: you can't always know the initial conditions and you can't transpose non-linearities. Yes, you can invert the sign of all velocities and trace the path of a particle in reverse, but can you tell where it started moving if its current velocity is constant? What if that particle hits a wall and stays there? How can you know when in the past it stopped moving?
    - lisper 9y ago
      > you can't always know the initial conditions Just because you can't know them doesn't mean they aren't physical. God can know the initial conditions. In a classical world, that's good enough. > and you can't transpose non-linearities So? Classical mechanics is linear. > can you tell where it started moving if its current velocity is constant? Yes, of course. Just as easily -- and by the exact same method -- as you can tell where it will stop moving. > What if that particle hits a wall and stays there? How can you know when in the past it stopped moving? That's a question that is answered in a first-year physics course, but the TL;DR is that any collision must conserve both momentum and energy. If a ball hits a wall and "sticks" then some of its energy must be dissipated as heat. That is the thermodynamic arrow of time, which is exactly what the original post was about.
deleted 9y ago
[deleted]

hinkley 9y ago

So you didn't post this message three hours ago?