The speed of light, c, is an absolute physical constant. No matter where you are in the Universe, or how fast you’re moving relative to something else, the speed of light in a vacuum is always the same. That’s often taken to imply that nothing can travel faster than light, but things aren’t quite so simple. It turns out there are several ways things can travel faster than light, depending on what you mean by a “thing,” “faster-than-light,” and “travel”.

One way is to note that the immutable speed of light only applies to light in a vacuum. When light travels through a material, its effective speed is reduced. This is often given by an index of refraction, n, where the effective speed of light is c/n. (And n is pretty much always greater than 1.) For example, when light travels through water, its speed is about 0.75c. Because of this, it is possible for particles to “break the light barrier” in a material while still traveling less than c.

The blue glow of Cherenkov radiation. Credit: Matt Howard

For example, in nuclear reactors electrons are emitted with so much energy that they are traveling at nearly the speed of light c. When those electrons travel through the coolant (water) surrounding the reactor they travel faster than light can travel through the water, thus breaking the light barrier. You’re probably familiar with a sonic boom that occurs when a plane travels faster than sound, which is caused by a shock wave of air. A similar effect occurs when an electron breaks the light barrier. The electron causes an optical “shock wave” known as Cherenkov radiation, which gives nuclear reactors their blue glow.

A random path of a photon through the Sun. Credit: ATNF

Another phenomenon that can travel faster than light through a medium is sound waves in a star. In the Sun (as with any star) light is produced in its core through nuclear fusion. Traveling at the speed of light, it should be just a two-to-three second journey to the surface of the Sun. But the Sun’s interior is packed so densely with charged particles that light can’t simply travel in a straight line. On average, a photon in the Sun’s core will travel less than a centimeter before colliding with an ion. It is then scattered in an almost random direction. Imagine a photon trying to leave the Sun, but getting bounced in a random direction every centimeter. This random walk of a photon through the Sun means that it actually takes about 20,000 to 150,000 years for light to travel from the Sun’s core to its surface.

Animation of the l=3, m=0 mode of oscillation (shown greatly exaggerated). Credit: D. B. Guenther.

But sound waves propagate in a different way. They are pressure waves that transfer energy through a material rather than transporting material itself. As a result, they aren’t hampered by the ions in the core. Sound waves can travel through the Sun at thousands of meters per second, and they cause the Sun as a whole to vibrate. The study of these sonic vibrations is known as helioseismology, or in the case of other stars, asteroseismology. By analyzing these sounds we can determine things such as the density and pressure of the Sun’s interior.

While that’s all fine and good, you might argue, neither of these phenomena are actually traveling faster than light. What about something moving faster than the speed of light in a vacuum? It turns out that even that is possible in a way, thanks to general relativity.

Since the 1920s we’ve known that the more distant a galaxy is, the greater its apparent redshift, and thus the faster is appears to be moving away from us. This relation between redshift and distance is known as Hubble’s law. Over time we’ve come to understand that this relation is not due to galaxies racing away like an initial explosion from a single point, but rather it’s due to the fact that space itself is expanding.