Light is the fastest moving thing we know of, but just how fast is it? And does the universe have a speed limit?
A student recently asked me, “When do you think we’ll finally beat the speed of light?”
Like many, he assumed that in a world where we constantly work to make things bigger, better, faster—from our technology to our athletic performances—the speed of light was also something we would eventually be able to best.
Not everything is a record waiting to be broken, though. Sometimes the universe pumps the brakes.
What is the speed of light?
The speed of light, the fastest moving thing we know of, is 299,792,458 meters per second in a vacuum. That’s over 186,000 miles per second.
But physicists didn’t always know light traveled at a finite speed. In fact, physicists once thought light did not have a speed at all but rather traveled instantaneously. Naively, this assumption seems reasonable—I don’t have to wait for the light to reach my eyes once I flip the light switch.
Traveling at over 186,000 miles per second in a vacuum, light is the fastest-moving thing we know of.
In the early 1600s, the Dutch scientist Isaac Beeckman tried to test the assumed instantaneous nature of light by investigating whether observers from different locations saw the flash from a gunpowder explosion sooner or later. His results were inconclusive for reasons we now understand—his terrestrial distances were too small to measure a difference in arrival time.
Around 50 years later, the astronomer Ole Romer noted a difference in the times measured between the eclipses of Jupiter’s moons depending on the position of the Earth. When the Earth was moving away from Jupiter, the downtime between each eclipse—that’s when the moon traverses in front of the gas giant planet from our perspective—was increased by about seven minutes compared to when Earth was moving toward Jupiter. He suggested this difference might be due to a difference in light travel times. Romer measured a value of ~220,000,000 meters per second which, considering his set up, is really pretty good.
In the 1800s, James Clerk Maxwell began to work out the mathematical framework for how electromagnetic radiation, classical optics, and electric circuits work—a framework now known as Maxwell’s Equations. His work revealed that light was an electromagnetic wave and that electromagnetic waves traveled at a consistent speed—the speed of light.
Einstein claimed that space and time may be relative but the speed of light is constant no matter who is making the observations.
Later, in 1905, the constancy of the speed of light became a foundation for Einstein’s Theory of Special Relativity. Einstein claimed that space and time may be relative but the speed of light is constant no matter who is making the observations.
How do we measure the speed of light?
Today, there are more modern ways we can measure the speed of light. One popular grade school experiment even does it with chocolate and a microwave! But a more precise measurement comes from Kerr cell shutters. Essentially lasers are bounced back and forth using mirrors and their travel times are precisely measured using cesium clocks.
The speed of light is not technically constant
Also, note that I have referred a few times now to the speed of light in a vacuum. That’s because, when we discuss the constancy of the speed of light (known simply by the letter “c”), we mean the speed of light in a vacuum. Since interstellar space is very empty and much like a vacuum, astronomers usually don't have to consider the speed of light in different media.
Light moves slower through denser media because more particles get in its way.
However, the speed of light is not constant as it moves from medium to medium. When light enters a denser medium (like from air to glass) the speed and wavelength of the light wave decrease while the frequency stays the same. How much light slows down depends on the new medium's index of refraction, n. The index of refraction is determined by the electric and magnetic properties of the medium. For air, n is 1.0003, so pretty close to 1. For ice, n is 1.31, and for diamond, n is 2.417. The speed of light in a medium with index n is c/n so the higher the n value, the more the speed of light is lowered.
Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed, which causes the particle to vibrate a little, and then the light gets re-emitted. This process causes a time delay in the light's movement so the more particles there are (the more dense the medium), then the more the light will be slowed down.
Can anything travel faster than light speed?
So we’ve talked about how the speed of light can effectively be lowered, but can it ever be raised to “beat” the limit?
Some experiments have shown that light pulses can travel faster than the speed of light, if not the light waves themselves. This happens when you have what is called anomalous dispersion or, effectively, an index of refraction (n) less than 1.
Recent experiments have further shown that particles like Rubidium atoms may be able to exceed the speed of light through a process called quantum tunneling. When quantum tunneling occurs, particles effectively overcome high energy barriers by shortcutting right through them. This may sound obscure, but you can’t tell the story of how the sun shines without quantum tunneling. That means we actually rely on it every day!
The reason particles are able to “tunnel” their way through a barrier has to do with the uncertainties inherent at quantum- (i.e. particle-) sized scales. For example, you can never be precisely sure of a particle’s position and momentum at the same time. So, a tunneling particle will arrive at its destination faster than a photon or light particle that would have to go around said tunnel. Thus, the particle travels faster than light.
Physicists are also testing light’s speed limit in cases without the “trick” of quantum tunneling. When something moves at a speed that approaches the speed of light, we call it “relativistic,” meaning that our normal ways of adding or combining motions don’t apply. We have to apply Einstein’s Theory of Special Relativity. Such relativistic particles are observed and monitored by NASA in space and produced in laboratories here on Earth.
How do particles become relativistic? Electromagnetic fields will cause charged particles to speed up, moving them along in a way somewhat like how gravity will tug on a massive object. An extreme example of this here on Earth is how the particle accelerator known as the Large Hadron Collider manages to speed up particles to 99.99999896% the speed of light.
Collisions of magnetic fields in space that cause those fields to become entangled result in explosive events that send particles flying out at high speeds. Particles can also be accelerated to extremely high speeds when they interact with colliding electromagnetic fields. These interactions happen frequently around the Earth and pose a danger to spacecraft and our satellites. NASA has programs to monitor them to keep astronauts safe.
We didn’t start homing in on the precise definition of the speed of light we use today until the 1950s. Even so, much of our fundamental physics knowledge builds on this value. Our definition of a meter, the conversion of energy into mass and back again (E=mc2!), and, of course, our understanding of the particle nature of light, all depend on this universal speed limit.