Cerenkov radiation: particles can travel faster than light, and why that matters
Fionn Milhench
When a charged particle is travelling faster than the phase velocity of light in a particular medium, it interferes with the electric fields of the particles within the medium, producing a blueish glow. This effect is known as Cerenkov radiation. Whilst its first observation is attributed to Marie Curie, the brilliant Polish French physicist responsible for the discovery of both polonium and radium, the discovery of Cerenkov radiation, is rather unsurprisingly (given the name) attributed to the Soviet physicist Pavel Cerenkov ( Cherenkov radiation. n.d.), who observed that when electrons produced by a source of radium entered a bottle of water, the water began to glow an unusual blue colour. While Marie-Curie saw this same blue glow before Cerenkov, it wasn't until Cerenkov explained it that the effect was truly understood.
At first it may appear as if there is an inconsistency between this effect and special relativity, since this effect relies upon a particle travelling faster than light. Einstein's special relativity uses the Lorentz factor (γ) to correct for the difference in the dimensions of an object when it’s travelling at relativistic speeds, when compared with the same object while stationary. The reasons behind the necessity of this correction factor are not immediately intuitive. However, they are formed as a consequence of light travelling at only one speed in a vacuum (c). As a particle's (or any massive object for that matter) speed approaches c, the Lorentz factor becomes exponentially large, and approaches
1
infinity the closer that the speed approaches c. Thus, the particle's mass and other dimensions either approach infinity or zero, depending on how the Lorentz factor is used in their relativistic formulae (for example, relativistic mass is given by 𝑚 ′ = γ 𝑚 0 , so as γ approaches infinity, so too does the mass, and by extension the energy needed to reach higher speeds becomes huge, because kinetic energy is proportional to both mass and the square of velocity), and the reason that, as a particle approaches c, the length contracts is because the formula for relativistic length is defined as 𝐿 ′ = 𝐿 γ 0 . This is the reason that people tell you that travelling at the speed of light (in a vacuum) is impossible, because 'mass becomes infinite'; because the amount of energy needed becomes infinite. The reason that Cerenkov radiation does not violate this principle relies entirely upon the difference between the speed of light in a vacuum, and the speed of light within optical mediums like glass, water, and air. When light enters one of these optical mediums, its behaviour can be treated as a wave (Fermilab, 2019). These optical mediums are made up of atoms, which are each surrounded by a probability field of electrons. As the light wave travels through the optical medium, oscillating
280
Made with FlippingBook interactive PDF creator