Quantum weirdness: does future affect the past (Introduction)

by David Turell , Monday, July 25, 2016, 02:14 (3044 days ago) @ David Turell
edited by David Turell, Monday, July 25, 2016, 02:34

The long essay says maybe:-http://www.bbc.com/earth/story/20160708-the-past-is-not-set-in-stone-so-we-may-be-able-to-change-it-"Surely we can just say that the future does not affect the past because (duh!) it has not happened yet? Not really, for the question of where time's arrow comes from is more subtle and complicated than it seems.-"What's more, that statement might not even be true. Some scientists and philosophers think the future might indeed affect the past - although we would only find out when the future arrives. And it may be able to due to an emergent property of quantum mechanics.-***-"Those laws of motion make no distinction about the direction of time. If you watched a video of two billiard balls colliding and bouncing away, you would be unable to tell if it was being run forwards or backwards.-"The same time symmetry is found in the equations of quantum mechanics, which govern the behaviour of tiny things like atoms. So where does time's arrow come into the picture?-"There is a long-standing answer to this, which says that the arrow only enters once you start thinking about lots and lots of particles.-***-"In Boltzmann's picture, it takes a while for the arrow of time to find its direction. In the tiny fractions of a second after the partition between the two gases is removed, before any of the molecules have really moved anywhere, there is nothing to show which direction of time is forwards.-"Entropy increases when collisions between atoms even out their energies, as for example when the heat of hot coffee spreads out into the surrounding air. This process, which washes away reservoirs of energy, is called dissipation.-"Until dissipation starts to happen, a process looks much the same backwards or forwards in time. It does not really have a thermodynamic arrow.-"But there is a one-way process in quantum mechanics that happens much faster. It is called decoherence.-***-"Decoherence explains why objects on the everyday scale of coffee cups do not show the wave-like behaviour of quantum objects.-"It arises because quantum particles can be coordinated in their quantum waviness, but if there are lots of them - like the countless atoms in a coffee cup - they rapidly lose any coordination. This means the object they constitute cannot show quantum behaviour.-***-" In other words, decoherence is faster than dissipation - and it seems to only work one way. That means decoherence reveals the arrow of time faster than dissipation.-"This implies that the arrow of time really comes from quantum mechanics, not thermodynamics as Boltzmann thought.-***-"In effect, decoherence comes from the way interactions with atoms, photons and so on in an object's environment carry away information about the object and scatter it around. This is, in fact, a quantum version of entropy.-"In both the classical and the quantum cases, then, time's arrow comes from a loss of information. (my bold)
"This offers a better way to think about time's arrow. It points in the direction in which information is lost and can never be retrieved.-***-"How does the particle "know" that it is going to be detected after passing through the screen, so that when it reaches the slits it "knows" whether to go through both slits or just one? How can the later measurement seem to affect the earlier behaviour?-"This effect is called "retrocausality", and it seems to imply that the arrow of time is not as strictly one-way as it seems. But does it really?-"Most physicists think that retrocausality in these delayed-choice experiments is an illusion created by the counterintuitive nature of quantum mechanics.-***-"According to Ellis, we can regard retrocausality as a kind of fuzziness in the "crystallisation of the present". "Quantum physics appears to allow some degree of influence of the present on the past, as indicated by delayed-choice experiments," he says.-***-"Our perception of time may not have much to do with the actual passage of time.-"What is clear is that the arrow of time, which seems like such a common-sense fact of life, is actually a profoundly tricky concept. The closer we look, the less we can be sure that the arrow is really always one-way."-Comment: And related to a loss of information. Where have we heard of that before? We do look back into the past when we use telescopes to view the universe, but we don't see the future, which we try to predict. Long complex article, needs to be read in full.