The Light Fantastic

Before the modern era of technological astronomy, to know anything about the Universe beyond our planet we relied on light. We had to see planets, stars, and so on, to know they existed. Astronomy was based entirely on light.

What then is light? This is a question which occupied the minds of some of the greatest scientists. Many early ideas have been discarded, now we have a good idea of the nature of light. Historically the two rival ideas were that light was either a stream of tiny bullet-like particles or a wave wobbling its way through space.

The light we see (visible light which is not the tautology it sounds) is only part of the electromagnetic spectrum, a great sweep of radiations which also includes radio, infra-red, ultra-violet, x-rays and gamma rays. This is the celebrated Electromagnetic Spectrum. Unlike sound, all electromagnetic radiation can travel through completely empty space at about 300 000 km/s (186 000 miles per second), the fastest speed possible. Any electromagnetic radiation does indeed travel in the form of waves. The obvious question is “waves in what?” and the least-complicated but rather unsatisfying answer is “in itself”. An electromagnetic (EM) wave is a pair of co-joined electric and magnetic waves oscillating together through space. Hold this image in your mind- I’m going to flatly contradict it in a moment or two.

Every EM wave has a frequency (how often per second it wobbles) and a wavelength (how far it travels between wobbles). These are linked; a high frequency EM wave will have a short wavelength while a low frequency wave will have a long wavelength. For example, the waves carrying the data to your computer in a domestic wireless network may be buzzing away more than two billion times per second (2GHz) and have wavelengths about 15 cm (6 inches) long. (Try multiplying two billion by 15cm, you will get a speed in cm/s; turn it into km/s. Does the answer look familiar?) In contrast, a broadcast radio station may transmit the news and music at 96 MHz (i.e. 96 million oscillations per second) in waves 3m or so long. (Try that multiplication thing again.)

The EM spectrum discriminates radiations by their wavelength or frequencies. Let us take a walk through the spectrum. Radio waves are relatively long wavelength (the Extremely Low Frequency signals used to communicate with submerged submarines are 3000-6000 km long!). Shorter wavelength (say 1m to 1 mm) radio waves are termed microwaves (look at the back of your microwave oven; you ought to find the microwave frequency in MHz somewhere), shorter still waves include the infra-red bands and then in the middle of the spectrum we have the familiar ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet) which Isaac Newton showed to make up white light. This is the ‘proper’ light we are all familiar with, most of the Sun’s radiation falls in this comparatively narrow band of wavelengths. Green light (in the centre of the visible band) has a wavelength of about 0.00055mm (about 1/10 the diameter of a red blood cell). Moving beyond violet, we pass into the invisible ultra-violet, as the waves shorten further we enter the realm of X-rays and gamma rays.

By the start of the Twentieth Century it was certain that light was a wave. Many experiments confirmed it. Then some experiments indicated that a beam of light shining on a metal surface could knock electrons out of the metal. This was inexplicable by the wave theory, in fact it suggested strongly that light was actually a stream of tiny particles after all. It was for pointing out this explanation that Einstein was awarded his Nobel Prize rather than his better-known theories of Relativity. The particles were even named photons. So light is either a wave or light is a stream of particles. Which is the right answer? Again the answer, derived from quantum mechanics (a can of worms I have no intention of delving into here) seems an unsatisfying cop-out. Light is both at the same time. Weird though this may seem, something can be a wave and particle at once (although this can only be observed on very small scales). Astronomers are accustomed to this. You will read about, say, a telescope designed to focus gamma ray wavelengths on to a detector which counts the number of photons it picks up.

For most of the last century, astronomy has been scanning the Universe in ever more exotic wavelengths. As a result we have found pulsars, black holes, starforming regions, and Kuiper Belt object and much more. What will we find next?

(This article originally appeared in the March 2008 issue of Astronotes. )

Image Credit: NASA