THE CHURCH OWES US ALL AN EXPLANATION...:

New pursuit of Schrödinger's cat (Philip Ball, 21st September 2011, Prospect)

The early pioneers of quantum theory quickly discovered that the seemingly innocuous idea that energy is grainy has bizarre implications. Objects can be in many places at once. Particles behave like waves and vice versa. The act of witnessing an event alters it. Perhaps the quantum world is constantly branching into multiple universes.

As long as you just accept these paradoxes, quantum theory works fine. Scientists routinely adopt the approach memorably described by Cornell physicist David Mermin, as "shut up and calculate." They use quantum mechanics to calculate everything from the strength of metal alloys to the shapes of molecules. Routine application of the theory underpins the miniaturisation of electronics, medical MRI imaging and the development of solar cells, to name just a few burgeoning technologies.

Quantum mechanics is one of the most reliable theories in science: its prediction of how light interacts with matter is accurate to the eighth decimal place. But the question of how to interpret the theory--what it tells us about the physical universe--was never resolved by founders such as Niels Bohr, Werner Heisenberg and Erwin Schrödinger. Famously, Einstein himself was unhappy about how quantum theory leaves so much to chance: it pronounces only on the relative probabilities of how the world is arranged, not on how things fundamentally are.

Most physicists accept something like Bohr and Heisenberg's Copenhagen interpretation. This holds that there is no essential reality beyond the quantum description, nothing more fundamental and definite than probabilities. Bohr coined the notion of "complementarity" to express the need to relinquish the expectation of a deeper reality beneath the equations. If you measure a quantum object, you might find it in a particular state. But it makes no sense to ask if it was in that state before you looked. All that can be said is that it had a particular probability of being so. It's not that you don't "know," but rather that the question has no physical meaning. Similarly, Heisenberg's uncertainty principle is not a statement about the limits of what we can know about a quantum particle's position, but places bounds on the whole concept of position.

Einstein attacked this idea in a thought experiment in which two quantum particles were arranged to have interdependent states, whereby if one were aligned in one direction, then the other had to be aligned in the opposite direction. Suppose these particles move many light years apart, and then you measure the state of one of them. Quantum theory insists that this instantly determines the state of the other. Again, it's not that you simply don't know until you measure. It is that the state of the particles is literally undecided until then. But this implies that the effect of the measurement is transmitted instantly, and therefore faster than light, across cosmic distances to the other particle. Surely that's absurd, Einstein argued.

But it isn't.

...for why it didn't burn Galileo at the stake for that nonsense about the heliocentric universe.

Posted by oj at October 1, 2011 4:43 PM
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