The universe and beyondby Rebecca Priestley
A cosmologist answers questions about the nature of light and how many dimensions there are.
My last Q&A column, in which University of Auckland physicist and cosmologist Professor Richard Easther answered questions about the speed of light and the expansion of the universe, inspired several Listener readers to write in with “questions for Richard”.
Basil McCoward from Napier is puzzled by the dual nature of light, which is known to behave like a particle and a wave: “As I look out my window, rays of light are impinging on my eyes, reflected from sunlight striking buildings, verdure, fences, and animals, to form a clear picture on my retinae. Many more rays are being reflected in every direction. I know that light can behave like a wave, and waves can interfere with one another. “The famous twin-slit experiment seems to prove that even a single photon, passing through one slit, can create interference bands on a screen, appearing to actually interfere with itself. Why and how is it that all those millions of rays or particles do not bang into each other?”
“You are right,” says Easther, “these rays do overlap with each other. Interference patterns form when waves overlap and cancel each other out. On a sunlit day, there will always be spots where all the different light-waves add to zero, just as there are spots on a choppy sea where the water level exactly matches the height it would be on a calm day. “But each cancellation only lasts for a fleeting instant and is thus unseen. One way that you can see this interference is by shining a laser pointer at a painted wall – look closely at the spot of light on the wall, and it will be a pattern of dots called ‘laser speckle’. The light rays from a laser are all in phase, like marching soldiers or a troupe of dancers, so always cancel each other at the same points, leading to a visible pattern. “As for ‘banging into each other’, when you see an interference pattern it’s a wave collision, not a particle collision. Light particles, or photons, don’t interact unless they have an enormous amount of energy – far beyond the range of visible light.”
Andrew Bydder from Hamilton asks: “How many dimensions make up the universe?” “One time and three space, so far as we know. Many theories of fundamental physics – most famously string theory – suggest that space has extra dimensions (beyond the usual up-down, front-to-back and side-toside) that are somehow hidden from us. This might be because they are so tightly curved we cannot probe them, or that our 3D universe is a ‘brane’ – a giant 3D ribbon in a higher dimensional space – and the particles we are made of cannot escape from the brane, so we only see a 3D world. “There is no experimental evidence for these propositions – but the idea that our universe has extra dimensions is remarkably old – it was discussed in detail by European physicists Theodor Kaluza and Oskar Klein in the 1920s.”
Easther watched the recent transit of Venus from the sixth floor of the University of Auckland physics building. But for Easther and his fellow cosmologists, things like planets and stars, no matter how rare the astronomical event, “are mainly foreground, things we have to look past to see what we want to see”. The next big event on the cosmological calendar, says Easther, is in January 2013, when processed data from the Planck Satellite survey of the microwave background radiation will be released, yielding a picture of the universe before the stars and galaxies formed. Cosmologists will be “sitting on the edge of their seats”, he predicts.
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