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We’ve come a long way from the luminiferous aether. The speed of light, sometimes abbreviated as c, is a universal maximum posted speed that causality will enforce. It’s the greatest speed at which any known substance or object can travel. Even light itself can’t exceed its own speed limit: specifically, 299,792,458 meters per second, 186,000 miles per second, 1,080,000,000 kph, or 671 million mph.
Scientists measure the speed of light using the time-of-flight method. Because the speed of light is constant and finite, a pulse of light will take a finite amount of time to cross a given distance. From there, you can calculate the speed of light by measuring how long it takes a pulse of light emitted at one location to reach an observer or sensor elsewhere. Time-of-flight experiments have confirmed Einstein’s theory of relativity to great precision. However, a prism is a simple experiment that explores the speed of light with no moving parts.
Light and Color
Technically, c denotes the speed of light in a vacuum. Physicists use v for the speed of light through anything else. In a denser medium, such as glass, photons will still travel at a uniform rate—that rate is just slower than in a vacuum. Isaac Newton confirmed this with his prism experiments. For example, if you direct a beam of white light through a triangular prism, the white light will break up into the familiar rainbow spectrum of colors from red to deepest blue.
If the speed of light is constant within a medium, then the difference between the light before the prism and the light after is the length of its path. Traveling through the prism changes the path of photons from our source, with the red side of the rainbow traveling farther than the blue side. The discrepancy in path length through the prism is what breaks out a beam of white light into a rainbow.
This real-life prismatic shard captures and separates the visible light from behind it.
Credit: Aubree Herrick, Unsplash
Rainbows hold other clues to the behavior of light and the speed at which it travels. Sir Frederick William Herschel discovered that if you put a thermometer in the red band of the rainbow cast by that prism, it will read a fractionally lower temperature than one in the yellow, which will be lower than one in the blue. This suggests the red component of visible light from the same beam has less energy than the blue.
Stretching out the same beam of light over a longer distance shifts it toward the redder, lower-energy end of the visible spectrum. The same principle is thought to underpin the redshift observed in sky objects that are very far away.
Lightspeed and the Observable Universe
For a long time, most humans believed that Earth sat dead center in the middle of the universe. But the truth is that we don’t necessarily sit at the center of the universe. We sit at the center of our sphere of causality, sometimes called our Hubble volume or simply the observable universe.
The observable universe is the whole of the space and matter that falls within the cosmological horizon, also known as the Hubble horizon—the farthest distance from Earth where any information created today will ever reach Earth. (A single photon released by spontaneous nuclear decay may be the simplest, lowest-bit-depth packet of information possible.) At the boundary between the observable universe and the great unknown lies the cosmic event horizon, the greatest comoving distance from which any light emitted today can ever reach the observer in the future.
Edwin Hubble and others noticed that light from stars and other sky objects moving toward us tended to be shifted toward the blue, and those moving away shifted toward the red—but far or near, approaching or departing, they all obeyed one principle. Every object in the sky appeared redshifted to a degree proportional to its distance from Earth. The farther away an object is, the farther its light is shifted into the red. In light of the Big Bang hypothesis that everything started out very close together, Hubble reasoned that if the speed of light in a vacuum is constant, then the only thing that explained these observations was that the universe must be expanding—and at a changing rate. Our estimates of the age and size of the universe are still related to one another by way of the speed of light.
Because space itself is expanding, the universe is getting bigger, and everything in it is getting farther away from everything else. However, space is expanding faster than the speed of light. One analogy is a rising loaf of raisin bread. Raisins in the dough that started close together will get farther away from one another as the dough rises and expands. This is how it can be true that the universe is 13.8 billion years old and our observable universe is some 92 billion light-years across.
Traveling Faster, With Light
Special relativity predicts that photons are massless. However, because they have energy (more or less in accordance with their wavelength), photons have momentum. This is possible because of the wavelike aspects of light. Considering light as a wave, the wavefront is traveling at c. Energy in the wave is embodied in the motion of the wave itself.
Quantum physics and Einstein’s relativity discuss light in terms of photons: individual, quantized packets of energy with momentum but no mass. To a particle physicist, a photon is the force-carrying particle of the electromagnetic force. The seeming paradox of massless particles exerting a physical force is also known as radiative pressure, and it’s how solar sails work.
Noticing that comet tails always point away from the Sun, Johannes Kepler proposed the idea of a “heavenly breeze,” which we know today as the solar wind. During the American Civil War, James Clerk Maxwell published his comprehensive theory of electromagnetism showing that light has momentum. To collide is to exchange energy, and when a photon interacts with the matter that makes up a solar sail, it imparts energy—including momentum.
Sadly, Maxwell wouldn’t live to see Pyotr Lebedev successfully demonstrate photon pressure with a torsional balance in 1899. But he and Kepler both would have been thrilled to learn that not only were their ideas proven about the nature of light, but they were also so accurate that just like the sails on a galleon, solar sails could change their directions by adjusting their sail attitudes—and their electrostatic fields.
NASA’s NanoSailD2 steered itself with physical attitude control and electrical fields.
Credit: NASA
Today, we’ve taken our first steps toward deploying such a vehicle. Japanese aerospace agency JAXA launched its successful IKAROS mission in 2010 as a proof of concept for solar sails. Unlike its namesake, the mission was a resounding success. More recently, NASA’s NanoSail-D2 satellite and the Planetary Society’s twin LightSail cubesats have taken up the solar sail baton, with the latter mission successful enough to merit an extension.
Faster-Than-Light Travel
Fan favorite science fiction properties like Star Wars and Star Trek often rest on the idea of traveling faster than the speed of light. Sadly, to the best of our understanding, those speeds are simply not attainable with current materials and methods. The speed of light imposes an absolute minimum travel time, even for trips to nearby stars like Alpha Centauri or Tau Ceti—no ship can get there faster than it will take a photon to get from point A to point B. Tau Ceti is 12 light-years away. Unhindered by any collision, its path unaltered by electromagnetic interference, the shortest time anything—even information—can travel between us and Tau Ceti will be no less than 12 years. In practice, travel time is always longer.
Prepare ship for ludicrous speed!
Credit: Adapted from Spaceballs (1987)
Of course, I can’t breach the concept of faster-than-light travel in our universe without mentioning the Alcubierre Drive. First proposed by physicist Miguel Alcubierre, the Alcubierre drive is a hypothetical spacecraft propulsion system that functions by “warping” spacetime around itself, allowing for faster-than-light travel. Fan trivia: instead of the Star Trek warp drive being inspired by Alcubierre’s ideas, Alcubierre himself told William Shatner via email that Star Trek‘s warp drive directly inspired his 1994 paper. In return, the Star Trek universe features the USS Alcubierre.
Alcubierre argued that instead of accelerating matter faster than the speed of light, perhaps we could warp spacetime between ourselves and our intended target. The laws of physics do not preclude it; instead, they suggest it in the way that a black hole warps spacetime to create its gravity well. But the difference between “does not preclude” and “can actually enable” can be quite vast. Even after refining Alcubierre’s conceptual math and discovering you wouldn’t need more energy than exists in the universe to make the drive work, an Alcubierre drive would still rely on exotic materials like antimatter (which we can create, albeit in minute quantities), and matter with negative mass.
We, uh, don’t know how to craft that yet.
Until such time as we do—or someone discovers a Stargate—faster-than-light travel will unfortunately remain science fiction.