This physics trick allows you to beat the speed of light

In our Universe, there are a few rules that everything must obey. 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 at faster-than-light speeds in the process. However, there is one way to beat the speed of light: enter any medium other than a perfect vacuum.

Here’s the physics of how you can defeat the speed of light. Light is nothing more than an electromagnetic wave, with in-phase oscillating electric and magnetic fields perpendicular to the direction of light’s propagation. The shorter the wavelength, the more energetic the photon, but the more susceptible it is to changes in the speed of light through a medium.

This was known in the aftermath of Maxwell’s equations: by the mid-to-late 1800s. 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, mutually perpendicular 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, in turn, all related to one another.) 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 light’s frequency stays the same, however, that means that its wavelength must change.

Since frequency multiplied by wavelength equals speed, and the wavelength shortens in a medium while the frequency stays the same, that means the speed that light travels at must also change (by slowing down) as the medium it’s propagating through changes. Through the vacuum of space, all light, regardless of wavelength or energy, travels at the same speed: the speed of light in a vacuum. When we observe light from a distant star, we are observing light that has already completed that journey from the source to the observer.