Light exhibits characteristics of both particles and waves, capable of traveling at speeds of approximately 300,000 km per second in a vacuum.
In 1676, while studying the movement of Jupiter’s moon Io, Danish astronomer Ole Romer calculated that light travels at a finite speed. Two years later, based on the data collected by Romer, Dutch scientist Christiaan Huygens became the first to attempt to determine the true speed of light, according to the American Museum of Natural History in New York City.
Illustration of light beams. (Photo: Yuichiro Chino)
Huygens provided a figure of 211,000 km per second. This figure is not accurate by today’s standards, as scientists have measured the speed of light in a vacuum to be approximately 299,792 km per second. However, his calculations still demonstrated that light travels at an astonishing speed.
According to Albert Einstein’s theory of special relativity, light travels so fast that in a vacuum, nothing in the universe can move faster.
“We cannot move through the vacuum of space faster than light,” asserted Jason Cassibry, an associate professor of aerospace engineering at the Propulsion Research Center, University of Alabama.
But what happens when light is not in a vacuum, is this still true?
Technically speaking, the statement “nothing can move faster than the speed of light” is not entirely accurate, at least in non-vacuum environments, according to Claudia de Rham, a theoretical physicist at King’s College London.
Light displays both particle-like and wave-like properties, thus it can be regarded as both a particle (photon) and a wave. This phenomenon is known as wave-particle duality.
If we consider light as a wave, there are several reasons why certain waves can travel faster than white (or colorless) light in a specific medium, according to de Rham. One reason is that when light travels through a medium—such as glass or water—different frequencies or colors of light travel at different speeds.
The most obvious visual example of this occurs with rainbows. Rainbows typically have red wavelengths (longer and faster) at the top and violet wavelengths (shorter and slower) at the bottom, according to the University of Wisconsin-Madison.
Rainbow over Harper Lake in Louisville, Colorado, USA. (Photo: Bambi L. Dingman/dreamstime)
However, when light travels through a vacuum, this is no longer accurate. “All light is a type of electromagnetic wave, and they all travel at the same speed in a vacuum. This means both radio waves and gamma rays travel at the same speed,” explained Rhett Allain, a physics professor at Southeastern Louisiana University.
Therefore, according to de Rham, the only thing that can move faster than light is light itself, but only when not in the vacuum of space. Note that regardless of the medium, light never exceeds the maximum speed of 299,792 km per second.
However, Cassibry points out that there are some considerations when discussing things moving faster than light. “There are parts of the universe that are receding from us at speeds faster than light because spacetime is expanding,” he said.
For example, the Hubble Space Telescope recently detected light that is 12.9 billion years old from the distant star Earendel. However, because the universe is expanding at every point, Earendel is also moving away from Earth. It has been doing so since its formation and is currently 28 billion light-years away from Earth.
In this case, spacetime is expanding, but matter within spacetime still moves within the limits of the speed of light.
Earendel’s position in the image taken by Hubble. (Photo: NASA)
“We can imagine transmitting information at the speed of light across systems beyond our solar system. But transporting humans at the speed of light is impossible because we cannot accelerate ourselves to that extent,” de Rham said.
“Even under ideal conditions, where we could continuously accelerate ourselves at a constant rate—setting aside how we would have technology to allow continuous acceleration—we would never actually reach the speed of light. We could get close, but never hit that threshold,” she added.
Cassibry agrees with this perspective. “Setting aside relativity, if you accelerate at 1G (Earth’s gravity), it would take you a year to reach the speed of light. However, you would never actually achieve that speed because as you begin to approach the speed of light, your mass-energy increases, nearing infinity,” he said.
“One of the few potential shortcuts might be to expand and contract spacetime, effectively pulling the target closer to you. There seems to be no fundamental limit on the rate of expansion or contraction of spacetime, which means we might approach this speed limit someday,” Cassibry added.
Allain also noted that moving faster than light is a distant possibility, but he pointed out that if humans want to explore distant planets, achieving such speeds may not actually be necessary.
He suggests that wormholes could be a solution if humans want to travel quickly. “This method doesn’t actually make us go faster than light, but provides us with shortcuts to certain locations in the universe,” he said.
