The 'Light Amplification by Stimulated Emission of Radiation' (that's LASER to you and me) emits electromagnetic radiation such as visible light in a special way. You can think of this radiation as a beam of light particles, called photons. Photons have no mass and travel as fast as is physically possible. It takes light only 8 minutes to reach us from the sun, and it only takes it 2.5 seconds to travel from the Earth to the Moon.

Light can have a very specific `colour' determined by its wavelength and can even be invisible. The wavelength is inversely proportional to the energy of the photons, which means that as the wavelength gets shorter, the energy gets larger. The typical wavelength of visible light is so short that its length is measured in `nanometers' (symbol nm), which is a billionth of a meter!

GEO 600 laser system

GEO 600 laser system. Image credits: Albert Einstein Institute Hannover

Most lamps or other sources of light generate a mix of photons with different wavelength, travelling in different directions. A Laser is a very special type of light source because it can make all the photons play in tune so that they all have exactly the same wavelength and fly exactly in the same direction. This can be useful for many applications, for example, if a very small light beam is needed.

Everyday we use lasers without really thinking about it. You can find lasers all over your house: your DVD player uses a red laser (640 nm), your Blu-Ray player uses a blue-violet coloured laser (405 nm) and your computer uses a laser to read and burn CDs. Ruby lasers (694nm) are used to remove tattoos, other lasers are used to perform eye surgery, to read barcodes, to measure the size of rooms, this list goes on and on.

We like using lasers to make precise measurements: since the beams don't fan out very much over long distances, they are good light sources to use for interferometry. An interferometer takes a beam of light and splits it, then recombines it, and from the interference pattern produced we can measure how much further one beam travelled than the other (see interferometer section). In order to do all these measurements though, we need to make sure that the beams are still beams when they recombine! Try taking the lampshade off a lamp and point it at the wall. What you'll see is a large white spot. Now step back- you can see the spot growing and it's more difficult to make out the edges. If you move far enough away from the wall, you won't be able to see any kind of distinct shape. Repeat this with a laser pointer. Notice how you can move to the far end of the room and the spot size doesn't change very much. There are many reasons why we love lasers for our experiments and this is one of them.

Lasers can provide a very well focused and noise-free light beam, utilising a process called `stimulated emission'. Have a look at the Processing sketch Stimulated Emission to learn how such a laser works:

Gravitational wave detectors typically use a special laser with a wavelength of 1064nm. These are invisible to us, yet can cause permanent eye damage, so make sure you wear your goggles! Laser pointers used in presentations have less than 1 milliwatt of power, a CD/DVD burner uses 100-250 milliwatts and industrial laser cutting requires about 1.5 kilowatts. The power of the main laser beam in a gravitational wave detector needs to lie in a Goldilocks zone: it can't be too low otherwise the intensity will flutter too much, making your sensitivity too low. On the other hand, a power which is too high could force the mirrors in our interferometer out of the right place, and the returning beams would end up in the wrong place and so they wouldn't interfere! No interference means no possibility of gravitational wave detection.