12 ms·

The question is why things become more disordered forward in time but not backwards in time, given that the physical laws have time-reversal symmetry. In other

by layer8 2y ago

The question is why things become more disordered forward in time but not backwards in time, given that the physical laws have time-reversal symmetry. In other words, why does the macroscopic world seem to behave differently forward in time than backwards in time (e.g. biological life, or the evolution of stars), while the fundamental equations of physics don’t.

MrMcCall 2y ago
Well, we are a part of a physical system whose laws correspond to mathematical formulas, but aren't formulas, they're existing physical, energetic relationships maintained magically in realtime. And, yeah, it's magic, this universe we live in, my friend. So, while the math is reversable, time is a one-way street by virtue of the construction of the physical universe, which is not reversable. And it's that way because that's how this magnificent universe was instantiated. It may sound trite, but sometimes Occam is bang-on. As well, there are endless unknowns, but there are also unknowables. All matter is >99% empty, not counting field interactions. I don't know how much of it can be understood, but some of it is likely to be unknowable.
s_m_t 2y ago
Let's do a thought experiment. Floating in an otherwise completely empty region of space is a bomb. It explodes. For quite some time after the explosion entropy will be decreasing in that region. I really don't see the mystery here.
- simiones 2y ago
  The point is this: say we see a video of an otherwise completely empty region of space, with two masses orbiting each other for a while, that then fly off in different directions. We reverse the video and see two masses coming towards each other until they get into an orbit. Can we tell which of the two videos was the original and which was the reversed one? The answer is that we can't. However, say we receive a video of a billion billion such masses all starting in a single point, staying more or less still for a few seconds, and then moving out at high speed away from each other. It is obvious that this video is almost certainly playing forward in time, since the reverse, a billion billion balls all coming together to form a single object, is very very unlikely.
  - rrmm 2y ago
    Coming together to form a single object sounds like the work of a gravitational force.

by layer8 2y ago

MrMcCall 2y ago

Well, we are a part of a physical system whose laws correspond to mathematical formulas, but aren't formulas, they're existing physical, energetic relationships maintained magically in realtime. And, yeah, it's magic, this universe we live in, my friend. So, while the math is reversable, time is a one-way street by virtue of the construction of the physical universe, which is not reversable. And it's that way because that's how this magnificent universe was instantiated. It may sound trite, but sometimes Occam is bang-on. As well, there are endless unknowns, but there are also unknowables. All matter is >99% empty, not counting field interactions. I don't know how much of it can be understood, but some of it is likely to be unknowable.

s_m_t 2y ago

Let's do a thought experiment. Floating in an otherwise completely empty region of space is a bomb. It explodes. For quite some time after the explosion entropy will be decreasing in that region. I really don't see the mystery here.

simiones 2y ago
The point is this: say we see a video of an otherwise completely empty region of space, with two masses orbiting each other for a while, that then fly off in different directions. We reverse the video and see two masses coming towards each other until they get into an orbit. Can we tell which of the two videos was the original and which was the reversed one? The answer is that we can't. However, say we receive a video of a billion billion such masses all starting in a single point, staying more or less still for a few seconds, and then moving out at high speed away from each other. It is obvious that this video is almost certainly playing forward in time, since the reverse, a billion billion balls all coming together to form a single object, is very very unlikely.
- rrmm 2y ago
  Coming together to form a single object sounds like the work of a gravitational force.