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Quantum Weirdness

Entry 985, on 2009-04-09 at 20:21:58 (Rating 1, Science)

Today a friend called me and wanted to talk about string theory. That was a bit unusual because usually people want to talk about why their computer isn't working, or when I am going to fix a bug in one of my programs, or why the New Zealand cricket team lost its latest game, or what I'm doing in the weekend, but discussions of speculative physics are definitely welcome too as far as I am concerned!

The discussion centered around various odd topics and I talked about how we seem to know so little about cosmology - it seems we know less now than we did 10 years ago. That's not really true of course because 10 years ago we didn't know enough to know how little we knew, if you understand what I mean!

The other major area of weirdness we discussed was quantum physics. I explained how there are some things which just fundamentally don't make sense. Its as if the current theories, no matter how successful they are in predicting outcomes of events in the real world, are just wrong in a most basic way.

One example of this is the double slit experiment. In this classic demonstration of quantum weirdness particles are fired through a narrow gap (or slit) in a wall and onto a screen where they form a single line on that screen. Put two parallel slits in the wall and two lines appear. Seems simple enough.

Try the same thing with waves and the single slit produces a central area which gradually fades on either side, but the two slits produce an interference pattern on the screen. This makes sense because the two waves which emerge from the slits overlap and depending on whether a top and top of a wave or top and bottom of a wave meet will determine whether anything appears on the screen or not.

So that's the behaviour of the macroscopic world - the sort of world we see when we fire marbles through the slits or allow water waves to pass through them for example. What happens when we do this experiment at the quantum level by using electrons or other small particles?

Well that's when things get weird. Electrons produce a particle-like single line when fired through one slit but form a wave-like pattern when fired through two. It seems possible that the electrons are interfering with each other as they pass through the slits but when they are fired one at a time the interference still happens!

Its like the particles change into a wave during travel, and the wave travels through both slits and produces the interference. But only when there are two slits. Does that make sense? I don't think so.

But surely the electron really only travels through one of the slits, so what happens if a detector is set up to measure which slit that is? That's when things get really strange because then the electrons go back to being particles and the interference pattern disappears.

So observing the particles moving somehow changes their behaviour. This really makes no sense and no one seems to really know what's going on there. How would an electron "know" if it was being watched or not? Its really like our whole understanding of particles, waves, and the outcomes of an event are really just easily understood placeholders for what is really happening.

When something so basic is not only poorly understood but just seems so weird that its barely possible to believe then we really do have to wonder whether we really know anything about what happens at the most basic levels in physics. Maybe we will never know what's really going on because its impossible to understand. We might just have to be happy with theories that work without necessarily truly representing reality.

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