30 October 2017

That Fucking Cat.

Most people have heard about Schrödinger's cat. Schrödinger was trying to disprove the so-called Copenhagen interpretation of quantum mechanics - the idea that until a particle is "observed" it is literally in all possible quantum states (i.e. spin, charge, mass, position, velocity, etc) at once. He thought this proposition was absurd so came up with this metaphor of the cat in the box.

We don't know what state the cat is in until we look, so the cat is both alive and dead. Which is clearly absurd where cats are concerned. But it might not be so absurd when we are thinking of, say, the "spin" states of sub-atomic particles. This is still the most popular interpretation of the wave equation, though slightly less than half of all physicists agree with it.

The real problem we get at the popular level is that people don't get that the "observer" is also a metaphor. They think consciousness must somehow be involved. But it isn't. No one ever directly observed anything at the nanometer scale - we are not equipped for it. "Observe" here means physically interact with. The wave function of the particle collapses whenever the particle physically interacts with another particle. And at the particle scale it means any time two matter particles (fermions) exchange a force particle (boson). No consciousness is involved or required.

In terms of the cat metaphor this means that the universe does know whether the cat is alive or dead, because the cat interacts with the matter around it—it has to be in contact with the floor of the box for example. Touching the box collapses the wave function of the cat and the cat is either alive or dead and not in superposition of all states (probably never is).

So the thought experiment does not work unless the cat is suspended in a perfect vacuum, in perfect darkness, isolated from all magnetic fields, and not subject to gravity (even from the box!). In other words it would have to be in another universe which consisted of the cat and only the cat, and which could not communicate with our universe in any way. In which case we'd never know the outcome of the experiment. The uncertainty is either permanent or non-existent.

In other words, in the end it's just a bad metaphor. Schrödinger decisively lost his argument with the Copenhagen crowd. They took this weak attempt to discredit them, and turned it into a powerful positive symbol of their approach. Ouch.

In fact in something on our scale - with (literally) millions of billions of trillions of particles in every gram of matter - the wave function is always collapsed because there is interaction going on all the time. People are always in one particular state at any time (though time flows incessantly).

If humans behaved like particles we would be diffracted every time we walked through a doorway. In other words we would emerge at a random angle every time. (I'm assuming we're sober, eh?).

If there were two doorways close together, as we approached  we would blur out, pass through both doorways simultaneously, interfere with ourselves, come back into focus moving at a random angle. I've staggered through some doorways in my time, but never two at once.

If people were like particles, when you did one forward-roll  (↻360°) you would end up upside-down, and would have to do another one to get right-way up.

If people were like particles we could only know where we are or where we were going, but not both at the same time.

If people were like particles, then having people watch you walk down the road would cause you to swerve.

Quantum doesn't work on our scale. There never was a cat. Consciousness has nothing to do with it. Most of us can safely ignore quantum and behave as if the world is classical because at our scale and in our frame of reference, basically, it is.  Or, to be more accurate, classical theories predict the behaviour of matter on scales of mass, length and energy we can actually experience, to a higher degree of accuracy and precision than we can currently measure.

Which is not to say that on very small scales it is not possible to have quantum effects. It is possible. Gravity is so weak you can ignore it on very small scales. If you cool everything down close to absolute zero, exclude electromagnetic fields, and tweak other conditions, you can see quantum effects up to the scale of some fairly large single molecules such as bucky-balls (C60H60). Beyond this, and much above absolute zero, quantum effects simply disappear.





















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