3 ms·

Another way of look at this is time-symmetry. T-symmetry says that the laws of physics look the same even regardless of if you are traveling forwards or backwa

Another way of look at this is time-symmetry. T-symmetry says that the laws of physics look the same even regardless of if you are traveling forwards or backwards in time. Classically speaking, T-symmetry would suggest that A→B→C and A→D→C should be impossible, because the time-reversed universe starting at C can have only state. On the other hand, if you assume T-symmetry, and that it is possible to create superposition, then there must be a case, C → |B> + |D>, where state C goes into a superposition of states B and D. This means that it is no longer a violation of T-symettry for us to have both B → C and D → C. At least from the perspective of someone living in the universe. Under a many worlds interpretation T-symmetry would require you to say that the starting state of both B → C and D → C were actually the superposition all along. However, for someone living in the universe, it would be impossible to tell the difference between the universe being in one of the two states, or a superposition of both. This is another way of saying that T-symmetry implies that all possible histories must have occured. It is worth mentioning, that T-symmetry is believed to be false. However, CPT-symmetry (charge, parity, time) is thought to hold and I think the overall argument above still makes sense under CPT symmetry.

kgwgk 6y ago
> it is no longer a violation of T-symettry for us to have both B → C and D → C But you don’t have A → B → C or A → D → C, you have A → E → C. Where E is a state (that can written in different ways as a combination of other states) Analogy: if a wall is purple → purple → purple, would you say that its color history was purple → red → purple and purple → blue → purple at the same time?
wizzwizz4 6y ago
> This is another way of saying that T-symmetry implies that all possible histories must have occured. Actually, it's the opposite. Consider: • A→{B,C} • B←(A,D} • C←{A,E} That doesn't mean that A, D and E are all possible histories; when you consider B and C separately, D and E appear to be possible histories, but considering both B and C, you find that D and E cancel out,¹ leaving only A. So you can't actually make a claim about how many histories there are. --- ¹: This is horribly misleading. D and E aren't the same thing-I've-been-calling-a-state-but-that's-the-wrong-word-sorry, so their amplitude doesn't get summed, so they can't cancel each other out. My analogy wouldn't work as well if I'd used the same letter, though. (Consider also that I didn't use bra-ket notation. This is an analogy, not rigorous.)
JadeNB 6y ago
> It is worth mentioning, that T-symmetry is believed to be false. I think that T symmetry isn't just believed to be false, but has been ruled out experimentally. (Or maybe I'm confusing these with CP violations https://en.wikipedia.org/wiki/CP_violation https://en.wikipedia.org/wiki/CP_violation, which are simply believed to be T violations?)

kgwgk 6y ago

> it is no longer a violation of T-symettry for us to have both B → C and D → C But you don’t have A → B → C or A → D → C, you have A → E → C. Where E is a state (that can written in different ways as a combination of other states) Analogy: if a wall is purple → purple → purple, would you say that its color history was purple → red → purple and purple → blue → purple at the same time?

wizzwizz4 6y ago

> This is another way of saying that T-symmetry implies that all possible histories must have occured. Actually, it's the opposite. Consider: • A→{B,C} • B←(A,D} • C←{A,E} That doesn't mean that A, D and E are all possible histories; when you consider B and C separately, D and E appear to be possible histories, but considering both B and C, you find that D and E cancel out,¹ leaving only A. So you can't actually make a claim about how many histories there are. --- ¹: This is horribly misleading. D and E aren't the same thing-I've-been-calling-a-state-but-that's-the-wrong-word-sorry, so their amplitude doesn't get summed, so they can't cancel each other out. My analogy wouldn't work as well if I'd used the same letter, though. (Consider also that I didn't use bra-ket notation. This is an analogy, not rigorous.)

JadeNB 6y ago

> It is worth mentioning, that T-symmetry is believed to be false. I think that T symmetry isn't just believed to be false, but has been ruled out experimentally. (Or maybe I'm confusing these with CP violations https://en.wikipedia.org/wiki/CP_violation https://en.wikipedia.org/wiki/CP_violation, which are simply believed to be T violations?)