Quantum Mischief Rewrites the Laws of Cause and Effect
quantamagazine.org
quantamagazine.org
This means that multiple histories contribute to a final state-of-the-universe: If A→B→C and A→D→C are indistinguishable (because C is the same state as C) then they both contribute amplitude – in layperson's terms, both histories are happening in parallel, then combining when they get to the same state.
If you've got states X and Y, and you're doing something like XOR swap, then you can go (X, Y) → (X^Y, Y) → (X^Y, X) → (Y, X) or (X, Y) → (X, X^Y) → (Y, X^Y) → (Y, X). This isn't fundamentally different from the ABC / ADC case; it obviously follows that this would happen too, though I'd never though it of it before.
Very thought-provoking article.
Measuring something doesn't change its behavior, just the range of possibilities.
This would violate unitarity.
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.
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?
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.
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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.)
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, which are simply believed to be T violations?)
https://en.wikipedia.org/wiki/Retrocausality
https://aeon.co/essays/can-retrocausality-solve-the-puzzle-o...
> engines normally need a hot and cold reservoir to work, with a quantum switch they could extract heat from reservoirs of equal temperature — a surprising use suggested a year ago by Oxford theorists.
It seems to me that there will be mutually exclusive states of reality existing in different spatial contexts; it's only at the meeting of light cones that the contradiction is reconciled (the likelihood of superposition mutex events is zero).
There's a fundamental equivalency there in terms of the consequences of the events - you can never outrun the light-cone of the remote event to reach any illegal states.