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While most laws of physics may be time symmetric, the laws of thermodynamics are not - wherever there is a gradient in entropy. In very informal terms, anywher

While most laws of physics may be time symmetric, the laws of thermodynamics are not - wherever there is a gradient in entropy. In very informal terms, anywhere where there is very low entropy (higher order), any direction away from that will likely (to the point of certainty) be in a direction of higher entropy (lower order). For instance, if you have a jar of red & blue marble, with high order (such as perfect alternating layers of red vs blue marbles) then any disturbances (such as shaking the jar, reaching in and moving a handful, …) will almost certainly reduce order. And no practical amount of shaking the jar will ever return the marbles to a high order state again. Even though any close up video of the marbles being jostled will reveal the same physical properties and behavior for the marbles, whether played forward or backward. So at the individual marble level, laws are symmetric. But at the jar of marbles level, one direction of time looks very different than the other. We, and the particles that make us, and our environment, are the marbles whose configuration is more disordered the further in time we get from the Big Bang. So we perceive the time direction away from the Big Bang as the future, toward the Big Bang as the past. And the statistical “force” of increasing entropy provides the useful energy we use to survive, learn, create useful islands of order, in the greater sea of increasing disorder.

soperj 4y ago
> And no practical amount of shaking the jar will ever return the marbles to a high order state again. This can't be true. Take the case where there's only 2 marbles, it's likely that at some point that shaking them will return them to the original state. For higher numbers of marbles the probability becomes lower and lower, but is never 0.
- a257 4y ago
  With enough marbles, the discrete distribution behaves like a continuous distribution. The point probability of a continuous distribution is infinitely improbable -- in other words it is 0. (0.00..0..1 = 0 just as 0.99 repeating equals 1)
  - TechBro8615 4y ago
    > infinitely improbable -- in other words it is 0 This sounds like a rather bold statement to make, as long as we're already speaking so metaphysically.
    - a257 4y ago
      The statement is mathematically correct [0]. [0] https://stats.stackexchange.com/questions/142730/px-x-0-when-x-is-a-continuous-variable https://stats.stackexchange.com/questions/142730/px-x-0-when...
      - TechBro8615 4y ago
        We're talking about physics, not mathematics. We don't have the luxury of hand-waving away fundamental questions about the nature of the continuum. See: https://en.wikipedia.org/wiki/Continuum_hypothesis https://en.wikipedia.org/wiki/Continuum_hypothesis
        a257 4y ago
        Please elaborate.
        TechBro8615 4y ago
        I'm neither a mathematician nor a physicist, so I'm not nearly informed enough to elaborate on the subject properly, but I would note that the axiom of choice is not proven, and while you can mathematically divide a continuum into infinitesimally small sets, you cannot do the same to physical matter. From my understanding, many of the contradictions between classical and quantum physics arise at this boundary between the discrete and the continuous, where classical physics generally assumes continuity while quantum physics is constructed around discrete quantization mostly independent of time. Again, I'm not a physicist, but I think it's telling that the validity of continuum mechanics [0] depends on a model and multiple assumptions. I have no trouble agreeing with your original statement, "with enough marbles, the discrete distribution behaves like a continuous distribution," when speaking mathematically, but mathematics by its nature is an idealized model of the world - I'm not willing to accept that it's objectively representative of physical reality. The discussion we're having here is one of metaphysics, so it feels a bit like the height of hubris to use mathematics as the tool for describing objective reality, because metaphysically, we cannot say that mathematics is anything other than a tool we've constructed for approximating the model of the world as we understand it. Considering the subject of this post is about retrocausality, we're already throwing out some pretty wild ideas, so I think it's a bit hubristic to dismiss them by citing a branch of mathematics that assumes the existence of countably infinite sets [1]. [0] https://en.wikipedia.org/wiki/Continuum_mechanics#Validity https://en.wikipedia.org/wiki/Continuum_mechanics#Validity [1] https://en.wikipedia.org/wiki/Constructivism_(philosophy_of_mathematics)#Axiom_of_choice https://en.wikipedia.org/wiki/Constructivism_(philosophy_of_...
- dekhn 4y ago
  you're correct. In stats/thermo class there's a commonly taught "what's the probability of all the molecules of air in a room spontaneously moving to <extremely small location>". The problem is that shoving all those molecules into a tiny location woudl increase the pressure tremendously, sending all the particles in directions that would eventually return to a uniform distribution. IIUC my professor right the probability is non zero but is practically impossible for a large number of incompressible spheres.
- Nevermark 4y ago
  > For higher numbers of marbles the probability becomes lower and lower, but is never 0. Exactly. It doesn’t take a very large jar to make a return to an original state practically impossible. But even for two marbles, shaking them to return them to their original locations costs a fair amount of organized energy. And that is organized chemical energy in your muscles, which you are dissipating as disorganized heat energy due to your efforts, increasing the total disorganization. And the marbles are microscopically wearing down, shedding compounds. The jar is also wearing down. Both becoming less organized. So even with two marbles, overall entropy continues increasing. We can locally reverse entropy, only at the cost of actually increasing entropy in total. - Organizing your day takes effort and uses up useful organized energy. - Computation uses up useful organized energy. - Your cells, maintaining their metabolism, uses up useful organized energy. No matter what we do, entropy still increases.
lordnacho 4y ago
That all makes sense, but why does cause and effect seem to go one way with the entropy arrow?
- Nevermark 4y ago
  Because cause and effect is more understandable and controllable in one direction. Everything we do takes advantage of the higher to lower entropy gradient. We get relatively organized photons from the sun, a percentage get stored in wood of trees, so we can burn that for heat which can drive an engine or generate electricity, with the end result is doing some useful work. And we don’t actually lose any energy, due to conservation of energy. But we have now lost USABLE energy, because all our captured energy is now heat diffusely spreading out from our engine into the atmosphere where it becomes uncontrolled DISORGANIZED and therefore no longer usable. So clear causes tend to occur in past of clear effects. For large objects. But again, if you watch videos of individual marbles bouncing around, forward or backward, you see just as clear chains of cause and effect in either direction.
dekhn 4y ago
Not sure if you appreciate this, but if you repeat the shaking experiment enough times, you will see it return to perfect order- at some exceptionally tiny probability. I'm pretty sure you are saying that but instead of "practical amount", it's easier to say it's just exceptionally unlikely.
- Nevermark 4y ago
  Well lifetimes of shaking is not practical is it. Marbles in a jar are not arranged in a neat spacial grid, but in all kinds of configurations. The chance of recovering any near perfect starting symmetry, depending on the size of a jar, might take decades or millions, or more years. In the meantime, all that energy being used to shake the jar is generating huge amounts of heat entropy.
mftb 4y ago
So I think I understand this argument and it makes sense to me (at the moment). To me the (to paraphrase), "increasing as we move away from the big bang" stuff seems to lead right to the idea that the expansion of the universe is what we perceive as the passing of time, but I never hear any one knowledgeable say that. Is there some obvious reason that it's not that succinct?
- Nevermark 4y ago
  No, the expansion of the universe isn’t the same thing as increasing disorder of particles. It is possible that that the original expansion, inflation, and even dark energy are also driven by thermodynamics, but we just don’t know that.
  - mftb 4y ago
    I see, I was conflating the expansion with disorder. Thanks.
sbaiddn 4y ago
"And no practical amount of shaking the jar will ever return the marbles to a high order state again." Im not saying you're wrong, but then there's Poincare's recurrence theorem. Entropy is funky. (Certainly not practical amounts anyway)

soperj 4y ago

> And no practical amount of shaking the jar will ever return the marbles to a high order state again. This can't be true. Take the case where there's only 2 marbles, it's likely that at some point that shaking them will return them to the original state. For higher numbers of marbles the probability becomes lower and lower, but is never 0.

a257 4y ago
With enough marbles, the discrete distribution behaves like a continuous distribution. The point probability of a continuous distribution is infinitely improbable -- in other words it is 0. (0.00..0..1 = 0 just as 0.99 repeating equals 1)
- TechBro8615 4y ago
  > infinitely improbable -- in other words it is 0 This sounds like a rather bold statement to make, as long as we're already speaking so metaphysically.
  - a257 4y ago
    The statement is mathematically correct [0]. [0] https://stats.stackexchange.com/questions/142730/px-x-0-when-x-is-a-continuous-variable https://stats.stackexchange.com/questions/142730/px-x-0-when...
    - TechBro8615 4y ago
      We're talking about physics, not mathematics. We don't have the luxury of hand-waving away fundamental questions about the nature of the continuum. See: https://en.wikipedia.org/wiki/Continuum_hypothesis https://en.wikipedia.org/wiki/Continuum_hypothesis
      - a257 4y ago
        Please elaborate.
        TechBro8615 4y ago
        I'm neither a mathematician nor a physicist, so I'm not nearly informed enough to elaborate on the subject properly, but I would note that the axiom of choice is not proven, and while you can mathematically divide a continuum into infinitesimally small sets, you cannot do the same to physical matter. From my understanding, many of the contradictions between classical and quantum physics arise at this boundary between the discrete and the continuous, where classical physics generally assumes continuity while quantum physics is constructed around discrete quantization mostly independent of time. Again, I'm not a physicist, but I think it's telling that the validity of continuum mechanics [0] depends on a model and multiple assumptions. I have no trouble agreeing with your original statement, "with enough marbles, the discrete distribution behaves like a continuous distribution," when speaking mathematically, but mathematics by its nature is an idealized model of the world - I'm not willing to accept that it's objectively representative of physical reality. The discussion we're having here is one of metaphysics, so it feels a bit like the height of hubris to use mathematics as the tool for describing objective reality, because metaphysically, we cannot say that mathematics is anything other than a tool we've constructed for approximating the model of the world as we understand it. Considering the subject of this post is about retrocausality, we're already throwing out some pretty wild ideas, so I think it's a bit hubristic to dismiss them by citing a branch of mathematics that assumes the existence of countably infinite sets [1]. [0] https://en.wikipedia.org/wiki/Continuum_mechanics#Validity https://en.wikipedia.org/wiki/Continuum_mechanics#Validity [1] https://en.wikipedia.org/wiki/Constructivism_(philosophy_of_mathematics)#Axiom_of_choice https://en.wikipedia.org/wiki/Constructivism_(philosophy_of_...
dekhn 4y ago
you're correct. In stats/thermo class there's a commonly taught "what's the probability of all the molecules of air in a room spontaneously moving to <extremely small location>". The problem is that shoving all those molecules into a tiny location woudl increase the pressure tremendously, sending all the particles in directions that would eventually return to a uniform distribution. IIUC my professor right the probability is non zero but is practically impossible for a large number of incompressible spheres.
Nevermark 4y ago
> For higher numbers of marbles the probability becomes lower and lower, but is never 0. Exactly. It doesn’t take a very large jar to make a return to an original state practically impossible. But even for two marbles, shaking them to return them to their original locations costs a fair amount of organized energy. And that is organized chemical energy in your muscles, which you are dissipating as disorganized heat energy due to your efforts, increasing the total disorganization. And the marbles are microscopically wearing down, shedding compounds. The jar is also wearing down. Both becoming less organized. So even with two marbles, overall entropy continues increasing. We can locally reverse entropy, only at the cost of actually increasing entropy in total. - Organizing your day takes effort and uses up useful organized energy. - Computation uses up useful organized energy. - Your cells, maintaining their metabolism, uses up useful organized energy. No matter what we do, entropy still increases.

lordnacho 4y ago

That all makes sense, but why does cause and effect seem to go one way with the entropy arrow?

Nevermark 4y ago
Because cause and effect is more understandable and controllable in one direction. Everything we do takes advantage of the higher to lower entropy gradient. We get relatively organized photons from the sun, a percentage get stored in wood of trees, so we can burn that for heat which can drive an engine or generate electricity, with the end result is doing some useful work. And we don’t actually lose any energy, due to conservation of energy. But we have now lost USABLE energy, because all our captured energy is now heat diffusely spreading out from our engine into the atmosphere where it becomes uncontrolled DISORGANIZED and therefore no longer usable. So clear causes tend to occur in past of clear effects. For large objects. But again, if you watch videos of individual marbles bouncing around, forward or backward, you see just as clear chains of cause and effect in either direction.

dekhn 4y ago

Not sure if you appreciate this, but if you repeat the shaking experiment enough times, you will see it return to perfect order- at some exceptionally tiny probability. I'm pretty sure you are saying that but instead of "practical amount", it's easier to say it's just exceptionally unlikely.

Nevermark 4y ago
Well lifetimes of shaking is not practical is it. Marbles in a jar are not arranged in a neat spacial grid, but in all kinds of configurations. The chance of recovering any near perfect starting symmetry, depending on the size of a jar, might take decades or millions, or more years. In the meantime, all that energy being used to shake the jar is generating huge amounts of heat entropy.

mftb 4y ago

So I think I understand this argument and it makes sense to me (at the moment). To me the (to paraphrase), "increasing as we move away from the big bang" stuff seems to lead right to the idea that the expansion of the universe is what we perceive as the passing of time, but I never hear any one knowledgeable say that. Is there some obvious reason that it's not that succinct?

Nevermark 4y ago
No, the expansion of the universe isn’t the same thing as increasing disorder of particles. It is possible that that the original expansion, inflation, and even dark energy are also driven by thermodynamics, but we just don’t know that.
- mftb 4y ago
  I see, I was conflating the expansion with disorder. Thanks.

sbaiddn 4y ago

"And no practical amount of shaking the jar will ever return the marbles to a high order state again." Im not saying you're wrong, but then there's Poincare's recurrence theorem. Entropy is funky. (Certainly not practical amounts anyway)