There are certain rules to which everything must adhere in the Universe. Any two quanta can interact and energy, momentum, angular momentum, and momentum are conserved. The physics in any system of particles moving forwards in time is similar to that of the physics of the same system reflecting in a mirror with particles exchanged in for antiparticles. There is an absolute limit to the speed of any object. Nothing can ever go faster than the speed light, and nothing that has mass can ever reach that speed.
However, there is a way around the light’s speed. Enter any medium other than a perfect void. Here are the details.
Remember that light, as an electromagnetic wave, is something you must keep in mind. Although it behaves like a particle, it’s much more valuable to consider it an electromagnetic wave. There is nothing that can stop it from traveling through the vacuum of space at the amplitude it would naturally choose. These fields are defined by the wave’s energy, frequency, and wavelength.
Ethan Siegel is an astrophysicist who will help you explore the Universe. The newsletter will be delivered every Saturday to subscribers. All aboard!
If the frequency is constant, it means that the wavelength must change. And since wavelength multiplied by frequency equals speed (frequency multiplied by wavelength), that means that the speed of light must also change as the medium in which you are propagating must change.
The refraction caused by light passing through a prism is a spectacular example of this phenomenon. White light is made up of light that has a constant, broad spectrum of wavelengths. While longer wavelengths like red light have lower frequencies, shorter wavelengths like blue light have higher frequencies. In a vacuum, all wavelengths travel at equal speed. The frequency multiplied times the wavelength equals light speed. The more energetic wavelengths have greater electric and magnetic field strength than the ones with less energy.
The different wavelengths of light react slightly to each other when it is passed through a dispersive media like a prism. The more energy your electric and magnetic field has, the more the light experiences from passing through the medium. While all light is the same frequency, higher-energy light has a shorter wavelength.
Because light travels slower through mediums than vacuums, redder light moves faster than blue light. This results in many optical phenomena like rainbows. The sun’s light breaks down into different wavelengths through water droplets and drops.
Light, regardless of frequency or wavelength, has to travel at one speed in the vacuum. This is the speed that light travels in a vacuum. This is also the speed at which pure radiation (such as gravitational radio) must travel. It also represents the speed at which any massless particle, according to the laws of relativity.
Most particles in the Universe have masses, and they need to follow slightly different rules. Although the speed of light is the ultimate speed limit for mass particles, it is not something you are compelled or able to reach.
Your massive particle will move faster if it has more energy than it needs. However, it must travel slower. The Large Hadron Collider’s most energetic particles, protons, can travel at a speed that is almost equal to light speed in a vacuum. It can travel 299,792,455 meters per second or 99.999999% faster than light.
However, it doesn’t matter how much energy you pump into the particles, only more 9s can be added to the right side of the decimal place. We will never be able to reach the light’s speed limit.
But what occurs if we travel through a medium rather than a vacuum? It turns out that when light travels through a medium its magnetic and electric fields feel the effects. This causes light to travel faster when it enters a medium. When light enters or exits one medium or changes from one medium to the next, it appears as if it is bending. Although light propagates unrestricted in a vacuum, its speed of propagation and wavelength is greatly affected by the properties it passes through.
However, particles suffer a different fate. The behavior of high-energy particles that are traveling through vacuums suddenly becomes entangled in a medium will be very different from that of light.
It will not experience an immediate shift in momentum or energy. First, the electric and magnet forces acting upon it — which can change its momentum over the course of time — are negligible when compared to how much momentum it already has. Instead of bending immediately, as light seems to do, the trajectory changes it experiences can only be made gradually. Particles continue moving in the same way as before they enter a medium.
Second, collisions with other particles are one of the most significant events that can alter a particle’s trajectory in a medium. These scattering events play a vital role in particle physics experiments. The products of collisions allow us to reconstruct the event that took place at the collision point.
However, the most striking fact is that particles moving slower than light inside a vacuum but faster in the medium they enter are actually breaking the speed limits of light. This is the only physical way particles can surpass the speed of light. They cannot exceed the speed of light in a vacuum but they can in a medium. When they do, something amazing happens: Cherenkov radiation is emitted.
Named after Pavel Cherenkov who discovered it, it is one of those physics phenomena that was first noticed experimentally and was never predicted. Cherenkov was examining radioactive samples that had already been prepared. Some of these samples were kept in water. Cherenkov was studying luminescence (where gamma radiations would excite these substances, which would then emit visible lights when de-excited), and the radioactive samples seemed to emit a faint blueish-hued light. This light led him to quickly conclude that it had a preferred direction. This was not a fluorescent phenomenon. It was something entirely different.
Cherenkov radiation is the blue glow seen today in water tanks around nuclear reactors.
Where is this radiation coming from?
If a fast particle is moving through a medium, it will usually be charged. The medium itself has both positive (atomic nuclei), as well as negative (electrons), charges. While the charged particle can collide with one other particle as it travels through the medium, chances are that the collision will be very unlikely. Atoms, which are mostly empty spaces, have a much lower chance of happening over short distances.
Instead, the particle acts on the medium in which it travels. The charged particle causes particles to polarize (where opposite charges attract and like charges repel) in response. The electrons that are being emitted light will return to their original state once the charged particle is gone. They emit blue light in a cone shape. The geometry of this cone is determined by the speed and speed of the light in the medium.
This is a very crucial property in particle Physics since it is this process that permits us to detect the mysterious neutrino. Neutrinos rarely interact directly with the matter. In such times, however, they only transfer their energy to another particle.
It is possible to create a large tank of pure liquid. This liquid doesn’t radioactively emit or decay. It is very resistant to cosmic rays, radioactivity, and other contaminants. We can also line the tank with photomultiplier tubes, tubes that detect one single photon and trigger an electronic cascade, allowing us to identify where, when, or in which way a photon originated.
With quite large the detectors can be used to determine the properties of each neutrino that interacts with a particle within these tanks. Cherenkov radiation, which is produced when light’s speed in the liquid is exceeded by the particle “kicked”, can be used to measure these properties.
Cherenkov’s radiation discovery and its understanding were revolutionary in numerous aspects. But it also had a terrifying application in the initial days of the laboratory particle physics experiments. Although a beam of energy particles has no optical signature as it moves through the air, it emits blue light if the medium it is passing through is faster than it travels in. To confirm that the beam was active, physicists would shut one eye and place their head in the path. This was not necessary, but radiation safety training has made it possible to stop the practice.
Despite all the progress in physics that has been made over the years, the only way to beat that speed is to find a medium that slows that light down. This is the only way speed of the light can be exceeded in a medium. If we do, this blue glow will provide a wealth of data about the interaction that caused it. The Cherenkov glow remains the best option until warp drive or other tachyons becomes a reality.
