I’ve been reading Stephen Jay Gould’s The Hedgehog, the Fox and the Magister’s Pox (highly recommended, by the way) so perhaps I’m more aware than usual of crossovers between science and arts, but I think I’ve found a new analogy to use in lectures.
I visited the Hayward Gallery in London to see the Antony Gormley exhibition recently. The centrepiece is called Blind Light best described as a box of fog which 25 people at once are allowed to stumble around in. It’s an incredible feeling, actually; the fog is dense enough that if you stretch out your arms then it’s literally impossible to see your hand in front of your face. Crashing into the edge of the box, which is lit from within, is an extremely disorientating experience, as is watching strangers loom out of the mist.
(Image from Hinke’s photostream).
Clearly there are lights inside the box, but it’s impossible to see them. A photon – a particle of light – leaving the light won’t get very far before it collides with something and scatters off in a completely new direction. If we followed the path of such a photon, we might very well find that it bounces all over the box, maybe even scattering off some of the bodies floundering around inside. Eventually, though, it might happen to find itself near the edge of the box. If it does, and if it then scatters in the right direction then it’ll be out of the fog and free to travel across the room. As observers outside the box, we see the edges glowing as if they are the source of light, as the photons that reach us were last scattered in our direction by the atoms on the outside of the fog.
This is an almost perfect analogy for the early Universe. Before 300,000 years after the Big Bang, the Universe was filled with a fog of fast moving electrons. Light couldn’t travel further than about a centimetre before scattering off these particles. However, as the Universe expanded, it cooled and the electrons lost energy until eventually they were captured by atomic nuclei to form neutral atoms. At this point, light was suddenly freed to travel right across the Universe. We see this light as the cosmic microwave background, a glow which appears to be coming from every point in the sky.
Looking back to the early days of the Universe’s history, we’re like the observers outside the box. The light of the CMB appears to have been emitted from the points where it was scattered in our direction. So just as the edge of Gormley’s box seems to be glowing, so it appears that the Universe of 300,000 years after the Big Bang emitted light which travelled all the way to us today. We see the light as emitted from a surface, like the edge of the box, and we can see all sorts of features in this surface.
All I have to do now is persuade Gormley to build a spherical version of Blind Light with a hollow so that we can climb into the middle. Then I’d have the perfect tool to explain the CMB.