“It could just as well evolve in the other direction.” If you mean that you could, if you wanted, call the past “the future,” and call the future, “the past,” you can do that if you want: but you will remember things in the direction of lower entropy and expect things in the direction of higher entropy. Which as gjm said is what we mean by talking about an arrow of time. In other way, saying that this can happen “just as well” is like saying that when you flip a coin a thousand times, you can just as easily get a thousand heads as any other sequence. But you will actually get a random looking sequence, and you will actually get increasing entropy, not decreasing entropy.
Your second paragraph is simply incorrect: there is no known asymmetry in the laws of physics that might explain the arrow of time. It is explained (in terms of experience) by the fact that time in one direction is vastly different in entropy from the other. We call the low end “the past,” because we necessarily remember the low-entropy side of time.
If you assume random conditions to the universe, you will not get either low entropy to high, or high to low (which is really the same thing), but high entropy on both sides, and any low entropy situation like conscious experience would be explained as Boltzmann brains.
Your second paragraph is simply incorrect: there is no known asymmetry in the laws of physics that might explain the arrow of time.
On the other hand, CP violation is one of the Sakharov conditions, and it’s not obviously absurd to suspect that the questions “why did the past have so little entropy” and “why does the present have so much more matter than antimatter” might be related to each other.
the questions “why did the past have so little entropy” and “why does the present have so much more matter than antimatter” might be related
Yeah, I wondered (idly—I don’t know enough physics for anything more to be worth while) about that too. I don’t suppose anyone reading this is a physicist who can say whether there’s anything nontrivial likely to be going on here?
His argument seems to be that since CP violation remains a reversible process, it cannot possibly explain why there is less entropy on one side of time than on the other.
[actually, maybe it isn’t that clear—he might just be saying that no one has shown any connection, not that there could not be one]
The bolded text in the article is ‘has absolutely nothing to do with that arrow of time’. This is accurately (if excessively tersely) summarizes the article; no hedging is necessary.
gjm’s response to this is correct.
“It could just as well evolve in the other direction.” If you mean that you could, if you wanted, call the past “the future,” and call the future, “the past,” you can do that if you want: but you will remember things in the direction of lower entropy and expect things in the direction of higher entropy. Which as gjm said is what we mean by talking about an arrow of time. In other way, saying that this can happen “just as well” is like saying that when you flip a coin a thousand times, you can just as easily get a thousand heads as any other sequence. But you will actually get a random looking sequence, and you will actually get increasing entropy, not decreasing entropy.
Your second paragraph is simply incorrect: there is no known asymmetry in the laws of physics that might explain the arrow of time. It is explained (in terms of experience) by the fact that time in one direction is vastly different in entropy from the other. We call the low end “the past,” because we necessarily remember the low-entropy side of time.
If you assume random conditions to the universe, you will not get either low entropy to high, or high to low (which is really the same thing), but high entropy on both sides, and any low entropy situation like conscious experience would be explained as Boltzmann brains.
On the other hand, CP violation is one of the Sakharov conditions, and it’s not obviously absurd to suspect that the questions “why did the past have so little entropy” and “why does the present have so much more matter than antimatter” might be related to each other.
Yeah, I wondered (idly—I don’t know enough physics for anything more to be worth while) about that too. I don’t suppose anyone reading this is a physicist who can say whether there’s anything nontrivial likely to be going on here?
Here is Sean Carroll discussing at least a related question.
His argument seems to be that since CP violation remains a reversible process, it cannot possibly explain why there is less entropy on one side of time than on the other.
[actually, maybe it isn’t that clear—he might just be saying that no one has shown any connection, not that there could not be one]
The bolded text in the article is ‘has absolutely nothing to do with that arrow of time’. This is accurately (if excessively tersely) summarizes the article; no hedging is necessary.
The laws of physics are CPT-invariant, as /u/gjm pointed out; CP symmetry is known to be broken; consequently T symmetry is also broken. The effect has been measured directly: http://physicsworld.com/cws/article/news/2012/nov/21/babar-makes-first-direct-measurement-of-time-reversal-violation.
This is not helpful for explaining the arrow of time, for reasons that Sean Carroll points out in the post I linked.