Experimental metaphysics

Via Pointless and absurd, word of an experiment which has apparently disproved both the Copenhagen and Many-Worlds Interpretations of quantum physics.

The experiment is a variation on Young's double-slit experiment. When you pass a beam of light through a narrow slit, it diffracts (essentially, spreads out); the slit effectively becomes a point-source of waves, like the ripples you get when you stick your finger in water and wiggle it up and down. Young used two slits, side by side, and found that the waves from each interfered with one another. He was able to use this interference pattern to calculate the wavelength of light.

(The first slit in the above setup is used to ensure that the waves impacting on the pair of slits are coherent - all lined up. Nowdays we'd just use a laser)

How is this relevant to quantum mechanics? It's all to do with waves and particles. Young did his experiment because he wanted to prove that light was a wave. However, light also behaves like a particle (your TV remote is proof of this). Quantum physics accepts that it's both ("wave-particle duality"), but according to the complementarity principle (shared by both the Copenhagen and Many Worlds interpretations), "both aspects cannot be revealed in a single experiment". You can see (and measure) light as a wave, or as a particle, but not both at the same time.

The experiment - "Afshar's experiment" - purports to do both. It uses a double-slit apparatus, with a series of wires placed in the minima of the interference pattern. Behind that is a photon detector, placed so as to detect the light from only one slit. Light is passed through the apparatus one photon at a time. According to the complementarity principle, this forces particle-like behaviour. A particle passes through one slit; it follows a discrete path; therefore, it may hit one of the wires and be absorbed. You would therefore expect, over many photons, that the number detected at the detector would be lower than the number emitted, the difference being those that were absorbed by the wires.

This doesn't happen. The number of photons detected is pretty much the same as the number emitted; no photons are absorbed by the wires. What's going on?

The obvious answer is that the wires are where no light is going to go (the minima in the interference pattern), and so of course they don't absorb any. The problem is that we know which slit the photon went through - and when we look at what happens with a single slit, we find that light is indeed absorbed. How can the mere existence of a second slit (which the photon doesn't go through) affect where it goes?

Conversely, we can think of the photon as a wave, which goes through both slits and therefore produces the interference pattern. But then, how do we then see it as having travelled through only one slit?

No matter which way we look at it, it's a problem for complementarity, because we are able to see both wave and particle aspects at once. Which means it's also a problem for the Many-Worlds interpretation, because in their view this means that the world both has and has not split.

No doubt the proponents of the Copenhagen and Many Worlds interpretations will come up with an explanation - that's what paradigms do, after all - but what's really interesting about all this is that they have to at all. Both interpretations are regarded as purely in the realm of metaphysics - after all, what can be more metaphysical than the idea that photons "know" whether they're being looked at as a wave or particle, or that the world continuously splits and we never notice? - but here we have an experiment which purports to disprove them. And this isn't the first time this has happened in quantum physics - the EPR experiments in the 60's, 70's and 80's disproved "hidden variable" theories and showed that actually, some properties really are determined at the instant they are measured (or worse, at the instant the same property of a correlated particle is measured). Quantum physicists just seem to have this bad habit of constructing interpretations which seem on the face of it to be metaphysical, but which turn out in the end to be making testable claims which can be verified or disproved by looking at the world.

The traditional view is that "if it makes testable claims, it's not metaphysics", but what is testable and what is not is a function of both technology and theory. This means that the boundary between physics and metaphysics is not stable. Physics can chip away, bringing things out of the realm of metaphysics and into testable reality. Which means that "experimental metaphysics" isn't the oxymoron it sounds like...