I have just finished my 31-slide monster of a presentation on ‘is the speed of light really constant?’ and thought I’d write a little bit about it. My presentation ends with a brief introduction to the Cherenkov Telescope Array, or CTA. CTA is a project my supervisor Prof. Jim Hinton is working on and the aim is to build – you guessed it – an array of Cherenkov telescopes!
Very high energy cosmic particles (including photons) are very hard to come by. Their intensity per unit area, or flux, is very low and this means that satellites (whose area is less than 1m2) have a very small probability of detecting them. Scientists solved this problem ingeniously: they decided to use the atmosphere as a detector instead. As energetic cosmic particles collide with particles in our atmosphere they induce what is known as a ‘particle cascade’. And this particle cascade can produce something very strange – Cherenkov light.
Cherenkov light is caused by particles travelling faster than light speed (gasp). It is in fact possible to travel faster than the speed of light, just not in a vacuum. Materials have an associated refractive index and it acts to slow a photon. Therefore particles with sufficient energy can actually travel faster than the photons in a refractive medium, though this process does have a consequence; Cherenkov light. This is a cone of light that is produced around the direction of particle motion. In the case of our energetic atmospheric particles, the atmosphere itself has a refractive index and the ensuing Cherenkov light will illuminate an area of the ground.
Cherenkov telescopes reflect this light using a mirror and focus it onto a photomultiplier. The photomultiplier then converts the photon into a current which is amplified and measured. The more energetic the original particle was, the more intense this Cherenkov light will be. Therefore Cherenkov telescopes offer a way of detecting these vastly energetic cosmic particles.
H.E.S.S., VERITAS and MAGIC are current Cherenkov imaging observatories. They have up to four Cherenkov telescopes each in order to better understand the size of the ‘Cherenkov cone’. CTA will top this number quite considerably; it will have 100!
As far as my project is concerned CTA will enable researchers to detect photons with energies greater than can currently be achieved. These photons will open windows into the strange world of quantum gravity and perhaps give physicists the chance to answer that all important question: is the speed of light really constant?
CTA should be working by 2018 and more information can be found about it here.