4 ms·

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

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.
    - posterboy 9y ago
      > God https://en.wikipedia.org/wiki/Laplace's_demon https://en.wikipedia.org/wiki/Laplace's_demon
      - leovailati 9y ago
        This is a great link, thank you for the reference. @lisper, check this out.
    - leovailati 9y ago
      Even a high school level equation of motion has a t squared term in it. Classical mechanics is not linear. And even if it were, the real world is obviously not. The basket ball will stop bouncing at some point, even though the model for the bouncing movement is an infinite exponentially decaying curve. After it stops bouncing it will stay there, resting, for ever. Good luck trying to determine when and if it moved at all. The fact that you have to invoke God to reverse a system back into its initial conditions proves my initial point. Remember, my whole point was the current state of a system does not necessarily encode its past. You cannot know where or when a particle with constant velocity started or will stop moving. Unless you are God, of course. And you cannot know when a particle hit a wall. All you can know is that it happened some time in the past. What did I say that made you think that initial conditions are not "physical" and what does that even mean? I feel like we are talking about different things...
      - lisper 9y ago
        You don't understand what a linear theory means. It does not mean that higher order polynomials don't exist in the theory, it means that the dynamics of the system are described by linear differential equations. And your bouncing basketball example is dissipative (look it up). > Remember, my whole point was the current state of a system does not necessarily encode its past. Yes, I know. You're wrong about that. The state of a bouncing basketball is not described by the position and velocity of the basketball, it's described in classical mechanics by the position and velocity of every elementary particle that the basketball comprises, and in quantum mechanics by the wave function of every such particle. That information, together with the same information about the environment, encodes the basketball's past and its future.
unixhero 9y ago
You cannot necessarily climb down a mountain the same way you climbed up. Or swim backwards.
- leovailati 9y ago
  I agree, but I don't understand what your point is.
  - unixhero 9y ago
    I am working on understanding the paper itself. I am reflecting, that's all:)
    - leovailati 9y ago
      Me too! Sorry about that.

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.
  - posterboy 9y ago
    > God https://en.wikipedia.org/wiki/Laplace's_demon https://en.wikipedia.org/wiki/Laplace's_demon
    - leovailati 9y ago
      This is a great link, thank you for the reference. @lisper, check this out.
  - leovailati 9y ago
    Even a high school level equation of motion has a t squared term in it. Classical mechanics is not linear. And even if it were, the real world is obviously not. The basket ball will stop bouncing at some point, even though the model for the bouncing movement is an infinite exponentially decaying curve. After it stops bouncing it will stay there, resting, for ever. Good luck trying to determine when and if it moved at all. The fact that you have to invoke God to reverse a system back into its initial conditions proves my initial point. Remember, my whole point was the current state of a system does not necessarily encode its past. You cannot know where or when a particle with constant velocity started or will stop moving. Unless you are God, of course. And you cannot know when a particle hit a wall. All you can know is that it happened some time in the past. What did I say that made you think that initial conditions are not "physical" and what does that even mean? I feel like we are talking about different things...
    - lisper 9y ago
      You don't understand what a linear theory means. It does not mean that higher order polynomials don't exist in the theory, it means that the dynamics of the system are described by linear differential equations. And your bouncing basketball example is dissipative (look it up). > Remember, my whole point was the current state of a system does not necessarily encode its past. Yes, I know. You're wrong about that. The state of a bouncing basketball is not described by the position and velocity of the basketball, it's described in classical mechanics by the position and velocity of every elementary particle that the basketball comprises, and in quantum mechanics by the wave function of every such particle. That information, together with the same information about the environment, encodes the basketball's past and its future.

unixhero 9y ago

You cannot necessarily climb down a mountain the same way you climbed up. Or swim backwards.

leovailati 9y ago
I agree, but I don't understand what your point is.
- unixhero 9y ago
  I am working on understanding the paper itself. I am reflecting, that's all:)
  - leovailati 9y ago
    Me too! Sorry about that.