9 ms·

I've wondered why we don't just assume the arrow of time was reversed in the past? We know that in the past the universe was more organized (had less entropy),

by atx42 4y ago

I've wondered why we don't just assume the arrow of time was reversed in the past? We know that in the past the universe was more organized (had less entropy), so it doesn't seem like much of a stretch from believing entropy started out that way. Wouldn't that make it easier things like life to form, etc. and simpler than reaching for alien life / multiverse theories? Or does it make it just as difficult? Just a lazy armchair physicist here, but maybe evening TO morning was the first day?

leephillips 4y ago
I can’t grasp the meaning of all of your comment, but I think it’s worth pointing out that there’s nothing in the Second Law that prevents life from evolving. We certainly don’t need to invoke multiverses for that. For systems out of thermal equilibrium, such as a planet embedded in a flux of energy from its star, entropy can and does decrease all the time. An air conditioner (heat pump), for example, causes heat to flow opposite to the temperature gradient, pushing heat from your house into the outside, and increasing the temperature difference, thereby decreasing the entropy in your area. But you have to expend energy to do so, hence your electricity bill.
- atx42 4y ago
  Does reversing time makes it "easier" to get life, than the current direction?
  - leephillips 4y ago
    The direction of time is just a convention, and the laws of physics are (still, as far as I know) time-reversible, so this is really irrelevant. You need an energy source to make anything, including life.
    - pfortuny 4y ago
      All except for (precisely) the heat equation, which is not time-reversible (there is a “loss of information” as time passes, very badly and roughly speaking). See https://math.stackexchange.com/questions/628720/why-heat-equation-is-not-time-reversible-time-arrow-in-mathematics https://math.stackexchange.com/questions/628720/why-heat-equ...
      - leephillips 4y ago
        The heat equation is not a law of physics.
  - willis936 4y ago
    No one can be sure, but I can effectively time reverse my AC in the winter for efficient heating. :) Metaphysics isn't necessary (or helpful) in figuring out how we got here.
    - atx42 4y ago
      Interesting. I guess it gets back to "first cause" argument. If life is organizational, does it require an organized process to form. Isn't metaphysics the only thing we have to figure out how we got here? Provable or not? Even science agrees that there are true statements that are not provable.
      - leephillips 4y ago
        We don’t need an organized process, intelligence, or intentionality to create order. We just need a flux of energy. We observe order arising without the intervention of an organizational force all the time, all around us. If you, like me, enjoy observing clouds, you may have noticed that they often form regular geometric patterns, often an area of stripes with a definite wavelength. This is a form of convection pattern that you can also observe in your kitchen. Nobody arranged the clouds that way; the order appeared “naturally”.
      - willis936 4y ago
        Math says there are statements that are not provable. Scientific intellect is an entirely empirical set of descriptions of observations. All of its models have finite domains. Speculating outside of determined physical limits is metaphysics: pure fiction. It adds nothing of value and certainly has no bearing on important questions like the origin of life or the universe.
- atx42 4y ago
  Wouldn't the Big Bang represent a much higher level of entropy than now?
  - mensetmanusman 4y ago
    It might be counter intuitive, but no. When you confine systems, the number of states that they can experience goes down quickly, so the Big Bang was an extremely low entropy point. Ever since then the entropy of the universe has been increasing overall. We are about half way to the heat death of the universe from an entropy calculation point of view.
  - snowwrestler 4y ago
    If the Big Bang actually happened, just prior to the bang would have been lowest state of entropy ever, as the entire universe existed in one single configuration.
    - zmgsabst 4y ago
      Can you elaborate? The universe has a collective state now, so I’m not seeing how scaling changes that.
      - dekhn 4y ago
        They mean there was only one possible configuration of matter, like a die that could only ever roll to "1". I like to think of entropy as "the universe prefers to have the greatest degrees of freedom available to it and there is a cost to reducing degrees of freedom". For example, to make a perfect crystal you have to cool to absolute zero (and even then there can be residual entropy!). Effectively, entropy = the logarithm of the number of ways a thing can be ordered. If a thing can only be ordered in one way, then its entropy is minimized.
        zmgsabst 4y ago
        Well, there isn’t matter at that time — and your analogy to dice is exactly my point. That instant could roll to anything from its current position (and according to many worlds, did) — the phase space of that object is huge, far beyond anything we normally see. That single object has all the complexity of every possible universe stacked into an infinitesimal region! We’re in a colder universe than it was at that compacted moment, which I’m having trouble aligning with your explanation about crystals: we’re seeing the cooling towards absolute zero as a superheated droplet spreads out and freezes. Defects forming in the crystallization of iron doesn’t imply the cold, defect laden iron has more entropy than the liquid drop it cooled from.
lrkr097 4y ago
Flying around the Earth super fast won’t literally rewind its state. It will erode current state but not restore old state. An accurate physical theory needs to account for events we can’t recreate influencing present state. That’s hard when objects and forces of influence can be separated by millions of light years at this point. What these physicists are working on though is a math framework that might help simulate old state by being able to account for eons of time and vast distances traversed. So our intuition is as you say it’s just hard to accurately model what things would look like if such a reversing was to happen.
jonsen 4y ago
I think you might enjoy this lecture on the meaning of time: https://m.youtube.com/watch?v=-6rWqJhDv7M https://m.youtube.com/watch?v=-6rWqJhDv7M

by atx42 4y ago

leephillips 4y ago

I can’t grasp the meaning of all of your comment, but I think it’s worth pointing out that there’s nothing in the Second Law that prevents life from evolving. We certainly don’t need to invoke multiverses for that. For systems out of thermal equilibrium, such as a planet embedded in a flux of energy from its star, entropy can and does decrease all the time. An air conditioner (heat pump), for example, causes heat to flow opposite to the temperature gradient, pushing heat from your house into the outside, and increasing the temperature difference, thereby decreasing the entropy in your area. But you have to expend energy to do so, hence your electricity bill.

atx42 4y ago
Does reversing time makes it "easier" to get life, than the current direction?
- leephillips 4y ago
  The direction of time is just a convention, and the laws of physics are (still, as far as I know) time-reversible, so this is really irrelevant. You need an energy source to make anything, including life.
  - pfortuny 4y ago
    All except for (precisely) the heat equation, which is not time-reversible (there is a “loss of information” as time passes, very badly and roughly speaking). See https://math.stackexchange.com/questions/628720/why-heat-equation-is-not-time-reversible-time-arrow-in-mathematics https://math.stackexchange.com/questions/628720/why-heat-equ...
    - leephillips 4y ago
      The heat equation is not a law of physics.
- willis936 4y ago
  No one can be sure, but I can effectively time reverse my AC in the winter for efficient heating. :) Metaphysics isn't necessary (or helpful) in figuring out how we got here.
  - atx42 4y ago
    Interesting. I guess it gets back to "first cause" argument. If life is organizational, does it require an organized process to form. Isn't metaphysics the only thing we have to figure out how we got here? Provable or not? Even science agrees that there are true statements that are not provable.
    - leephillips 4y ago
      We don’t need an organized process, intelligence, or intentionality to create order. We just need a flux of energy. We observe order arising without the intervention of an organizational force all the time, all around us. If you, like me, enjoy observing clouds, you may have noticed that they often form regular geometric patterns, often an area of stripes with a definite wavelength. This is a form of convection pattern that you can also observe in your kitchen. Nobody arranged the clouds that way; the order appeared “naturally”.
    - willis936 4y ago
      Math says there are statements that are not provable. Scientific intellect is an entirely empirical set of descriptions of observations. All of its models have finite domains. Speculating outside of determined physical limits is metaphysics: pure fiction. It adds nothing of value and certainly has no bearing on important questions like the origin of life or the universe.
atx42 4y ago
Wouldn't the Big Bang represent a much higher level of entropy than now?
- mensetmanusman 4y ago
  It might be counter intuitive, but no. When you confine systems, the number of states that they can experience goes down quickly, so the Big Bang was an extremely low entropy point. Ever since then the entropy of the universe has been increasing overall. We are about half way to the heat death of the universe from an entropy calculation point of view.
- snowwrestler 4y ago
  If the Big Bang actually happened, just prior to the bang would have been lowest state of entropy ever, as the entire universe existed in one single configuration.
  - zmgsabst 4y ago
    Can you elaborate? The universe has a collective state now, so I’m not seeing how scaling changes that.
    - dekhn 4y ago
      They mean there was only one possible configuration of matter, like a die that could only ever roll to "1". I like to think of entropy as "the universe prefers to have the greatest degrees of freedom available to it and there is a cost to reducing degrees of freedom". For example, to make a perfect crystal you have to cool to absolute zero (and even then there can be residual entropy!). Effectively, entropy = the logarithm of the number of ways a thing can be ordered. If a thing can only be ordered in one way, then its entropy is minimized.
      - zmgsabst 4y ago
        Well, there isn’t matter at that time — and your analogy to dice is exactly my point. That instant could roll to anything from its current position (and according to many worlds, did) — the phase space of that object is huge, far beyond anything we normally see. That single object has all the complexity of every possible universe stacked into an infinitesimal region! We’re in a colder universe than it was at that compacted moment, which I’m having trouble aligning with your explanation about crystals: we’re seeing the cooling towards absolute zero as a superheated droplet spreads out and freezes. Defects forming in the crystallization of iron doesn’t imply the cold, defect laden iron has more entropy than the liquid drop it cooled from.

lrkr097 4y ago

Flying around the Earth super fast won’t literally rewind its state. It will erode current state but not restore old state. An accurate physical theory needs to account for events we can’t recreate influencing present state. That’s hard when objects and forces of influence can be separated by millions of light years at this point. What these physicists are working on though is a math framework that might help simulate old state by being able to account for eons of time and vast distances traversed. So our intuition is as you say it’s just hard to accurately model what things would look like if such a reversing was to happen.

jonsen 4y ago

I think you might enjoy this lecture on the meaning of time: https://m.youtube.com/watch?v=-6rWqJhDv7M https://m.youtube.com/watch?v=-6rWqJhDv7M