It is forbidden to go faster than the speed of light. If there is one idea that’s universally known, even among those of us less versed in physics, it’s that there is a universal speed limit that cannot be broken, because Einstein said so. Of course, this is a great disappointment because it ruins the possibility that we might one day have the technology to holiday near other stars and meet the possible local inhabitants. But is this really the case, and does Einstein deserve the reputation of being the spoilsport who shattered one of the most cherished dreams of science fiction?
The first hint in modern science that light was not instantaneous, but rather had a finite speed, is found in the work of the Italian Giovanni Cassini; however, he had little faith in it. After having proclaimed before the French Academy of Sciences that the abnormalities observed in the times of the eclipses of Jupiter’s moons were due to “the fact that light arrives from the satellites with a delay,” he abandoned this line of thought. Not so his assistant, the Danish astronomer Ole Rømer, who in 1676 achieved the first rough measurement of the speed of light.
The credit for having calculated the speed of light with some precision rests with the American Albert Michelson (19 December 1852 – 9 May 1931), for which he would receive, in 1907, the first Nobel Prize in a science for his country. Michelson began his work in 1877 while studying at the Naval Academy in Annapolis. At that time, scientists believed in the existence of a luminiferous ether that served to support light waves, just as the air propagates sound. Since the Earth must move through this fluid, it was expected that light would have a different speed in the direction of this “ether wind” and in its perpendicular. In his most famous experiment, carried out in 1887 along with Edward Morley, Michelson measured the interference of the phases of light waves in two perpendicular directions. The result was that there were no appreciable differences, thus the ether does not exist and light moves at the same speed in all directions.
This conclusion arrived along with the discovery of the photon: particles do not need a medium through which to move. Michelson’s successive measurements were refined by others until, in 1975, a final value was adopted: 299,792.458 km/s. This value is no longer modified, since in 1983 it was used as a constant to define the new value of the metre in the International System of Units. As for Einstein, what he did was postulate in his special relativity theory that the speed of light is a constant, the “c” coined decades earlier by James Clerk Maxwell, and it is independent of the movement of its source for any observer; in other words, since there is no ether, there is no fixed reference system in the universe.
Einstein did not forbid an object from moving faster than light, but he estimated that accelerating it to such a rate would require infinite energy. However, some scientists say that, in fact, a trip at superluminal speed is not only a logical impossibility, but also that it wouldn’t offer the advantages that we think it would. NASA physicist and science populariser Sten Odenwald explains that the key problem is time dilation, a consequence of special relativity. For the crew of a ship travelling at close to light speed, their clocks run much more slowly than for their relatives back on Earth. When nearing the limit c, from their perspective the trip would be almost instantaneous. If a photon could think, it would sense that its flight from one end of the cosmos to the other was immediate, but to an outside observer it would take the entire age of the universe.
At Warp 37 (a fraction of the speed of light of 0.9999… until there are 37 nines after the decimal point), Odenwald calculates that the trip across the universe would last 0.2 seconds. “The universe will literally have passed in and out of existence in less time than it takes you to take a breath,” he writes. So yes, a traveller could reach the other end of the universe in an instant, but unfortunately the earth would not exist when he returned. As for exceeding the speed of light, it’s easy to understand why this is not possible: a trip cannot be faster than instantaneous, unless the traveller reaches his destination before he left, which is equivalent to travelling to the past, and this would violate a basic principle, that of causality (this is a simplification of a more complex question that is the subject of intense theoretical debate among physicists).
However, the equations can endure almost any contortion, and some physicists have proposed theoretical models of superluminal space travel compatible with special relativity. Perhaps the most famous was devised in 1994 by the Mexican theoretical physicist Miguel Alcubierre, consisting of a local bubble of spacetime that contracts in front of the spacecraft and expands behind it, as if we were dragging an object along a rubber mat. The trick is that it’s the destination that is moved closer; in reality the ship doesn’t travel faster than light. “The object is moved without actually moving; it’s space itself that does the work,” explains Alcubierre to OpenMind. The first direct observation of gravitational waves in 2015 by the LIGO detector confirmed the basis of Alcubierre’s idea, validating the theory. But the physicist acknowledges that systems like this are “almost impossible” because they would require something called negative energy, “and as far as we know that does not exist.”
There are also other seemingly insurmountable obstacles for something like the Alcubierre warp drive to exist in reality. But that doesn’t stop physicists from continuing to develop the theory, and even a NASA advanced propulsion physics lab has been working on it. In 2021, a study developed a general physical model for a type of warp drive, concluding that it “can be constructed based on the physical principles known to humanity today.” But while some media speculated on the possibility of faster-than-light travel, the model not only applies to subluminal velocity, but only eliminates the requirement for negative energy—as does another study on the subject—but all other insurmountable obstacles remain.
Leaving aside objects, another case is the transmission of waves or particles. If it were possible to send instantaneous signals, we could communicate with possible civilizations over cosmic distances. In 2011, an experiment from the international collaboration OPERA claimed to have detected superluminal velocity in some very light particles called neutrinos, but it turned out to be a false alarm; it was an experimental error caused by a fault in a cable and a clock. However, in this case we also find that the equations are quite permissive. Steven Weinstein of the University of Waterloo (Canada) has worked on theoretical models of exotic matter in which disturbances like sound could propagate faster than light. However, he doubts that such a form of matter exists in our universe.
In the theoretical world, there exists a hypothetical particle called a tachyon, which is assigned an imaginary mass and which would always travel faster than light (and possibly backwards in time, according to some physicists). The concept of imaginary mass is important, as it removes the obstacle of the mass of objects increasing towards infinity when they are accelerated to near-luminal velocities. Tachyons, which do not contradict Einsteinian relativity, have long been used in science fiction to propose superluminal travel, but there is not the slightest proof of their existence.
It should be added that when the speed of light is cited as an insurmountable limit in the universe, this refers to light travelling in a vacuum. But the speed of light in a medium—for example, air or water—is lower than in a vacuum. And in this case, there is no theoretical impediment for certain particles to exceed the speed of light in the same medium. A well-known example is Cherenkov radiation, which can be observed in nuclear reactors immersed in water. This phenomenon is due to the fact that the electrons generated move faster than light, whose speed in water is 75% of that in a vacuum. The blue glow produced is an optical equivalent of the sonic boom heard when a vehicle breaks the sound barrier. In 2019 a study proposed that the strange phenomenon whereby certain gamma-ray bursts—the most powerful energetic explosions in the universe—have a pulse structure that appears to be time-reversed can be explained by something like Cherenkov radiation. One case that keeps physicists busy these days is non-local quantum entanglement, the ability of two subatomic particles to synchronise even though they are separated by great distances. If one particle acted upon, the other responds instantaneously. This effect has been corroborated by solid evidence, although it does not convince everyone. Quantum entanglement led to the award of the Nobel Prize in Physics 2022 to Alain Aspect, John F. Clauser and Anton Zeilinger, scientists who pioneered the understanding of this phenomenon that opened the door to quantum computing.
According to physicist and writer John G. Cramer, professor emeritus at the University of Washington, “special relativity forbids faster-than-light communication at a definite speed,” he explains to OpenMind. On the other hand, quantum entanglement operates instantaneously.
But even if quantum entanglement is an exception to the universal speed limit, this does not enable communication: when one of the particles is acted upon, its state is unknown, which prevents it from being manipulated at will to send signals. So-called quantum teleportation exists, but it doesn’t permit the sending of data outside the quantum entanglement system, so it won’t allow instantaneous communication from one side of the galaxy to the other. Cramer acknowledges that “if the current formulation of quantum mechanics is correct, nonlocal signalling is impossible.” But what if it isn’t correct? Cramer doesn’t rule it out. And just as long as physicists don’t abandon this line of research, we can keep on dreaming.