11 ms·

So, we started with your plausible argument why 2nd law holds, then I point out deficiency in it, then we talk past each other, then you say "we can explain tho

by effie 4y ago

So, we started with your plausible argument why 2nd law holds, then I point out deficiency in it, then we talk past each other, then you say "we can explain those observations in terms of a past that actually existed". In other words, the original argument is not really sufficient, something else is needed. When I push for this to be added and illuminated, you're content with vague remarks on observations of the past. I'm not convinced by your posts at all, there is no convincing argument about illuminating 2nd law (not even in terms of past records) there. Let's agree to disagree.

lisper 4y ago
As you wish. But I think you simply forgot (or maybe chose to ignore) something I said very early on: "The reason entropy was lower in the past is that the universe started out in a low-entropy state. The reason for that is still a mystery."
- effie 4y ago
  > "The reason entropy was lower in the past is that the universe started out in a low-entropy state. The reason for that is still a mystery." And my answer to this is the same. Unsubstantiated assumption that universe started out, and that its entropy at this start was low. In addition, it is not at all clear how this assumption helps us to explain that an isolated system could not have higher entropy in the past long before we observed it.
  - lisper 4y ago
    > Unsubstantiated assumption that universe started out, and... You have elided something very important here. I did not say that the universe started out, full stop. I said it started out in a low-entropy state. It is the "in a low entropy state" part that matters, not the "started out" part. If you don't like the idea that the universe "started out" then you can replace this with "the universe was, at some point in the past (specifically, about 13 billion years ago) in a low-entropy state. We don't know how it got there, or what, if anything, happened before that." And this is not an "unsubstantiated assumption." There is evidence that the universe was in a low-entropy state in the past. In fact, the evidence is overwhelming. Disputing it is akin to disputing that the earth is round. > it is not at all clear how this assumption helps us to explain that an isolated system could not have higher entropy in the past long before we observed it. That's because it isn't true. An isolated system can have a high-entropy past, e.g. https://en.wikipedia.org/wiki/Spin_echo https://en.wikipedia.org/wiki/Spin_echo But any system that we actually observe is not isolated, and that is part of the reason why we never observe decreasing entropy despite the fact that decreasing entropy is not only possible, it actually does occur under the right circumstances. In fact, we can invoke the anthropic principle to explain why we do not observe decreasing entropy: creating a record of the past is not possible in a universe where entropy decreases. There are, by definition, fewer low-entropy states than high-entropy states, and so transitioning from a high entropy state to a low entropy state requires the destruction of information by the pigeonhole principle. And so the reason such universes are not observed is that such universes cannot contain records of the past, and so they cannot contain observers.

by effie 4y ago

lisper 4y ago

As you wish. But I think you simply forgot (or maybe chose to ignore) something I said very early on: "The reason entropy was lower in the past is that the universe started out in a low-entropy state. The reason for that is still a mystery."

effie 4y ago
> "The reason entropy was lower in the past is that the universe started out in a low-entropy state. The reason for that is still a mystery." And my answer to this is the same. Unsubstantiated assumption that universe started out, and that its entropy at this start was low. In addition, it is not at all clear how this assumption helps us to explain that an isolated system could not have higher entropy in the past long before we observed it.
- lisper 4y ago
  > Unsubstantiated assumption that universe started out, and... You have elided something very important here. I did not say that the universe started out, full stop. I said it started out in a low-entropy state. It is the "in a low entropy state" part that matters, not the "started out" part. If you don't like the idea that the universe "started out" then you can replace this with "the universe was, at some point in the past (specifically, about 13 billion years ago) in a low-entropy state. We don't know how it got there, or what, if anything, happened before that." And this is not an "unsubstantiated assumption." There is evidence that the universe was in a low-entropy state in the past. In fact, the evidence is overwhelming. Disputing it is akin to disputing that the earth is round. > it is not at all clear how this assumption helps us to explain that an isolated system could not have higher entropy in the past long before we observed it. That's because it isn't true. An isolated system can have a high-entropy past, e.g. https://en.wikipedia.org/wiki/Spin_echo https://en.wikipedia.org/wiki/Spin_echo But any system that we actually observe is not isolated, and that is part of the reason why we never observe decreasing entropy despite the fact that decreasing entropy is not only possible, it actually does occur under the right circumstances. In fact, we can invoke the anthropic principle to explain why we do not observe decreasing entropy: creating a record of the past is not possible in a universe where entropy decreases. There are, by definition, fewer low-entropy states than high-entropy states, and so transitioning from a high entropy state to a low entropy state requires the destruction of information by the pigeonhole principle. And so the reason such universes are not observed is that such universes cannot contain records of the past, and so they cannot contain observers.