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Quantum Experiment Confirms Causality Is Fuzzy (physicsworld.com)
"An experiment has confirmed that quantum mechanics allows events to occur with no definite causal order," reports an article shared by long-time Slashdot readers UpnAtom and jd. Researchers at the University of Queensland in Australia believe this could link Einstein's general theory of relativity to quantum mechanics, according to Physics World:
In classical physics -- and everyday life -- there is a strict causal relationship between consecutive events. If a second event (B) happens after a first event (A), for example, then B cannot affect the outcome of A. This relationship, however, breaks down in quantum mechanics because the temporal spread of a particles's wave function can be greater than the separation in time between A and B. This means that the causal order of A and B cannot be always be distinguished by a quantum particle such as a photon.
In their experiment, Romero, Costa and colleagues created a "quantum switch", in which photons can take two paths. One path involves being subjected to operation A before operation B, while in the other path B occurs before A. The order in which the operations are performed is determined by the initial polarization of the photon as it enters the switch.... The team did the experiment using several different types of operation for A and B and in all cases they found that the measured polarization of the output photons was consistent with their being no definite causal order between when A and B was applied. Indeed, the measurements backed indefinite causal order to a whopping statistical significance of 18 -- well beyond the 5 threshold that is considered a discovery in physics.
Science Magazine applauds the experiments for "obliterating our common sense notion of before and after and, potentially, muddying the concept of causality.
In their experiment, Romero, Costa and colleagues created a "quantum switch", in which photons can take two paths. One path involves being subjected to operation A before operation B, while in the other path B occurs before A. The order in which the operations are performed is determined by the initial polarization of the photon as it enters the switch.... The team did the experiment using several different types of operation for A and B and in all cases they found that the measured polarization of the output photons was consistent with their being no definite causal order between when A and B was applied. Indeed, the measurements backed indefinite causal order to a whopping statistical significance of 18 -- well beyond the 5 threshold that is considered a discovery in physics.
Science Magazine applauds the experiments for "obliterating our common sense notion of before and after and, potentially, muddying the concept of causality.
Look up time reversibility and you'll see that it's not that this hasn't been considered (because math would tend to presume you can negative change in time). It notes that at the quantum level "the weak nuclear force is not invariant under T-symmetry alone; if weak interactions are present reversible dynamics are still possible, but only if the operator also reverses the signs of all the charges and the parity of the spatial co-ordinates (C-symmetry and P-symmetry)."
It's one reason some physicists have argued that perhaps anti-particles are merely their like particles going backwards through time and hence exhibiting reverse charge/spacial parity. Waves though are inherently time reversible. But particle-wave duality has everything as a wave, so that's something of an incongruence. There's also the small wiggle that a photon is its own anti-particle, so it if were to simple reverse in time randomly it'd self-annihilate.
My limited understanding of physics is that things like photons don't do that because anything travelling at the speed of light doesn't experience time/change, so the only way it could change directions is if the path it traveled on changed. Everything else that has mass is constantly changing and its those changes that derive its mass (warpage of space) and whatever charge it has. If it were to repeatedly move back/forward through time, then it would repeatedly engage in this conversions and we'd see gravity/charge ripples a lot more frequently than we do or there'd be more to it--the force of each ripple would be smaller (so the total observed would add up to the same) and movement speed would have an inverse effect on the charge (presuming that faster particles don't move more back/forward in time their charge smaller charge would be spread out over a wider area). That's not what observation/math currently shows.
Basically, any idea you come up with has to line up with the math/observations we already have while you introduce new math and/or observations with either the former leading to the latter or the latter requiring the former because the current math doesn't work. As much as we don't comprehend quantum mechanics, we do have math and observation that match and we're not seeing observations that inherently fail the math. This experiment, AFAIK, is really separate because causality is much more a presumption based on the math we have (ie, it's a convenient axiom to describe things). Now, if you could use this experiment to disprove the current math, that'd be interesting/useful (show that if you throw away the axiom and use some new axioms that match this experiment, the math fails).
I'm terrible at even classical physics so please be gentle. But does this experiment show anything other than that if two events are sufficiently close to each other in time, an observer can't determine the order in which the events took place? From the experiment description, it sounds like A didn't actually cause B and B didn't cause A in their experiment. What they showed was that if A had caused B or B had caused A, we wouldn't be able to prove it without designing an experiment with greater time separation.
In classical terms. You got drunk. You broke the table. The two events were simultaneous. We can't know if the events were related or how. Maybe you're always stinking drunk by noon and the table got broken when you tried t swat a fly with your wine bottle.
You can't see ANYTHING from a car, You've got to get out of the goddamned contraption and walk...Edward Abbey
No, they absolutely know the order of events. And the order of events dictates a certain outcome.
But if they place the events close enough together, suddenly the outcome is no longer tied to the correct order of events. In other words, sometimes it looks like they happened in the opposite order, even though that is temporally impossible.