Following a lecture by David Wallace from Balliol College, Oxford, on “The World(s) According to Quantum Mechanics”, I thought I’d share the beduzzling theorems and problems it presents.

As a quick introduction, even though light travels in waves (that’s why it can’t go round corners, remember?), it’s also a kind of particle that comes in little chunks, or *quanta*, that we call **photons**. You can’t get half a photon, so the smallest bit of loght you can get is a photon. Pretty simple.

That part makes sense. What surprised the scientists when they were working with these tiny particles, though, is what happens next.

I think the best way to describe this is using Young’s **double slit experiment**, since if you’ve done physics at sixth form or college or its equivalent, you’re probably familiar with it, and if not, you can still get the idea. The basic experiment is where you shine a laser beam through a black screen with two tiny, really close together parallel slits in it, like the picture.

The laser spits into two beams, one through each slit. Because the slits are so thin, they’re close enough in size to the wavelength of light, so each beam diffracts (ie spreads out). The intensity of each beam is greatest in the centre, then it gets lower in a sort of wave pattern as constructive and destructive and occurs when the light waves have to travel slightly different distances to reach the same point. Here is my teacher’s diagram of it:

Where the diffraction patterns of each beam Overlap, superposition occurs (ie the intensities of the two beams add up) and you end up with a kind of stripy version of the single-slit pattern.

So that’s what happens when you’ve got a normal laser beam. But what about when you turn the laser down so low that there’s *only one photon going through*? Surely the photon will have to choose which slit to go through, and since it can only go though one slit, there’s only going to be the single-slit interference pattern, right?

Wrong. If there’s one thing we can learn from quantum mechanics, it’s that **things aren’t always how they should be**. When we only have one photon, it still creates a double-slit interference pattern, as though the photon’s going through *both slits at once*.. But when we put a polaroid filter in front of each slit to “label” the photon with which slit it went through, the interference pattern vanishes. *Spooky*. Even odder, when we put a third filter over both the slits to “scramble” the labels, the interference pattern comes back!

So when we have *interdeterminancy* – that is, nobody knows which way the proton went – it seems like it goes both ways. But when we try to show which way it went, it’s like it’s only gone one way – *as though sometimes there are two photons, and sometimes just one!*

How does this suggest that there is more than one world? Well, quantum scientists think that when there’s a choice between two possibilties on a tiny scale, such as the photon going left or right, *that part of this world splits* into two layers, one where it went left, and one where it went right. Sometimes, if we control the experiment tightly enough, we can get the two worlds to come back together again and interfere with one another. But otherwise, they just drift apart, with the effects and differences getting greater and greater.

If the whole problem with the polaroids gets you, don’t worry about it. I’m going to finish with a quote from the great Richard Feynman on this subject.

“If you think you understand quantum mechanics, you don’t understand quantum mechanics.”

Interesting websites on this topic:

http://www.lifesci.sussex.ac.uk/home/John_Gribbin/quantum.htm

http://news.nationalgeographic.com/news/2014/08/140827-quantum-imaging-cats-undetected-photon-science/

Plus Feynman’s brilliant semi-autobiographical book, “Surely You’re Joking Mr. Feynman!”, which is a good read to start with, but even better if you know a bit of maths and physics.