In our Universe, there are a few rules that everything must obey. Energy, momentum, and angular momentum are always conserved whenever any two quanta interact. The physics of any system of particles moving forward in time is identical to the physics of that same system reflected in a mirror, with particles exchanged for antiparticles, where the direction of time is reversed. And there’s an ultimate cosmic speed limit that applies to every object: nothing can ever exceed the speed of light, and nothing with mass can ever reach that vaunted speed.
Over the years, people have developed very clever schemes to try to circumvent this last limit. Theoretically, they’ve introduced tachyons as hypothetical particles that could exceed the speed of light, but tachyons are required to have imaginary masses, and do not physically exist. Within General Relativity, sufficiently warped space could create alternative, shortened pathways over what light must traverse, but our physical Universe has no known wormholes. And while quantum entanglement can create “spooky” action at a distance, no information is ever transmitted faster than light.
But there is one way to beat the speed of light: enter any medium other than a perfect vacuum. Here’s the physics of how it works.
Light, you have to remember, is an electromagnetic wave. Sure, it also behaves as a particle, but when we’re talking about its propagation speed, it’s far more useful to think of it not only as a wave, but as a wave of oscillating, in-phase electric and magnetic fields. When it travels through the vacuum of space, there’s nothing to restrict those fields from traveling with the amplitude they’d naturally choose, defined by the wave’s energy, frequency, and wavelength. (Which are all related.)
But when light travels through a medium — that is, any region where electric charges (and possibly electric currents) are present — those electric and magnetic fields encounter some level of resistance to their free propagation. Of all the things that are free to change or remain the same, the property of light that remains constant is its frequency as it moves from vacuum to medium, from a medium into vacuum, or from one medium to another.
If the frequency stays the same, however, that means the wavelength must change, and since frequency multiplied by wavelength equals speed, that means the speed of light must change as the medium you’re propagating through changes.
One spectacular demonstration of this is the refraction of light as it passes through a prism. White light — like sunlight — is made up of light of a continuous, wide variety of wavelengths. Longer wavelengths, like red light, possess smaller frequencies, while shorter wavelengths, like blue light, possess larger frequencies. In a vacuum, all wavelengths travel at the same speed: frequency multiplied by wavelength equals the speed of light. The bluer wavelengths have more energy, and so their electric and magnetic fields are stronger than the redder wavelength light.
When you pass this light through a dispersive medium like a prism, all of the different wavelengths respond slightly differently. The more energy you have in your electric and magnetic fields, the greater the effect they experience from passing through a medium. The frequency of all light remains unchanged, but the wavelength of higher-energy light shortens by a greater amount than lower-energy light.
As a result, even though all light travels slower through a medium than vacuum, redder light slows by a slightly smaller amount than blue light, leading to many fascinating optical phenomena, such as the existence of rainbows as sunlight breaks into different wavelengths as it passes through water drops and droplets.
In the vacuum of space, however, light has no choice — irrespective of its wavelength or frequency — but to travel at one speed and one speed only: the speed of light in a vacuum. This is also the speed that any form of pure radiation, such as gravitational radiation, must travel at, and also the speed, under the laws of relativity, that any massless particle must travel at.
But most particles in the Universe have mass, and as a result, they have to follow slightly different rules. If you have mass, the speed of light in a vacuum is still your ultimate speed limit, but rather than being compelled to travel at that speed, it’s instead a limit that you can never attain; you can only approach it.
The more energy you put into your massive particle, the closer it can move to the speed of light, but it must always travel more slowly. The most energetic particles ever made on Earth, which are protons at the Large Hadron Collider, can travel incredibly close to the speed of light in a vacuum: 299,792,455 meters-per-second, or 99.999999% the speed of light.
No matter how much energy we pump into those particles, we can only add more “9s” to the right of that decimal place, however. We can never reach the speed of light.
Or, more accurately, we can never reach the speed of light in a vacuum. That is, the ultimate cosmic speed limit, of 299,792,458 m/s is unattainable for massive particles, and simultaneously is the speed that all massless particles must travel at.
But what happens, then, if we travel not through a
vacuum, but through a medium instead? As it turns out, when light travels
through a medium, its electric and magnetic fields feel the effects of the
matter that they pass through. This has the effect, when light enters a medium,
of immediately changing the speed at which light travels. This is why, when you
watch light enter or leave a medium, or transition from one medium to another,
it appears to bend. The light, while free to propagate unrestricted in a
vacuum, has its propagation speed and its wavelength depend heavily on the
properties of the medium it travels through.
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