The speed of light is widely known to be the absolute pinnacle of movement. When Albert Einstein first entwined mass and energy in his Theory of Relativity, it basically established the Universe’s speed limit at 299,792 kilometres per second (186,282 miles per second).
According to Einstein, nothing in the Universe that has mass could either match, or move faster than, light.
But that doesn’t mean that nothing can move faster than light. In truth, physicists have discovered a number of phenomena that have the ability to match, and actually beat (in specific respects), the speed of light. And there are several theoretical models that posit specific ways that the speed of light could be surpassed.
NOTE: these things don’t prove General Relativity wrong. But they do help reveal just how complex our universe really is, and they show that very few things in physics can really be boiled down to one simple phrase.
In his presentation on Big Think, physicist Michio Kaku said that “If I have two electrons close together, they can vibrate in unison, according to the quantum theory…if I jiggle one electron, the other electron ‘senses’ this vibration instantly, faster than the speed of light. Einstein thought that this therefore, disproved the quantum theory, since nothing can go faster than light.”
Modern physics rests on the foundational notion that the speed of light is a constant. Einstein established this within his theory of general relativity, first developed in 1906 when he was just 26 years-old. There are several phenomena that travel faster than light, without violating the theory of relativity.
For instance, whereas traveling faster than sound creates a sonic boom, traveling faster than light creates a "luminal boom." Russian scientist Pavel Alekseyevich Cherenkov discovered this in 1934. Cherenkov radiation can be observed in the core of a nuclear reactor. When the core is submerged in water to cool it, electrons move through the water faster than the speed of light, causing a luminal boom.
The Cherenkov radiation is analogous with the more well-known sonic boom effect. If an aircraft moves slower than the sound speed in a medium, the air deflection is smooth around the wings of the aircraft. However if the motion speed exceeds the medium sound velocity, then a sudden pressure change would happen and shock waves propagate away from the aircraft in a cone at the speed of sound.
Maxwell's equations for electromagnetic waves shows the speed of light in vacuum determined by the constants of permeability and permittivity is the well-known parameter of c (3 × 108 m/s). However the wave velocity in a medium can be changed from the wave velocity in free space due to the polarizability of the material both electrically and magnetically.
This so called phase velocity is in fact the velocity at which the phase of photon is propagated within the medium. The ratio of the phase velocity of light in a medium to its velocity in free space is defined as the refractive index n of the material. For most of the materials this is a frequency-dependent parameter and typically a positive number greater than one.
When a charged particle is moving faster than light speed inside a medium, a faint radiation would appear which is called Cherenkov radiation, named after Pavel Alexeevich Cherenkov (1904-1990), who studied this phenomenon experimentally in Lebedev Physical Institute of the Russian Academy of Sciences. It is worth mentioning that before Cherenkov, in 1900, Pierre and Marie Curie had observed a blue glow in their experiments with concentrated radium.
Following Cherenkov's experiments, in 1937 I. M. Frank and I. E. Tamm gave a classical description of the phenomenon based on Maxwell's equations. Due to their joint work on this radiation phenomenon, in 1958 the Physics Nobel Prize was given to Cherenkov, Frank and Tamm.
According to classical physics, a moving charged particle emits electromagnetic waves. In a quantum mechanical picture, when a charged particle moves inside a polarizable medium with molecules, it excites the molecules to the higher levels and excited states. Upon returning back to their ground state, the molecules re-emit some photons in the form of electromagnetic radiation.
According to the Huygens principle, the emitted waves move out spherically at the phase velocity of the medium. If the particle motion is slow, the radiated waves bunch up slightly in the direction of motion, but they do not cross. However if the particle moves faster than the light speed, the emitted waves add up constructively leading to a coherent radiation at angle θ with respect to the particle direction, known as Cherenkov radiation. The signature of the effect is a cone of emission in the direction of particle motion.
On another front, while no particle with mass can travel faster than light, the fabric of space can and does. According to Inflation Theory, immediately after the Big Bang, the universe doubled in size and then doubled again, in less than a trillionth of a trillionth of a second, much faster than the speed of light. More recently, astronomers have discovered that some galaxies, the distant ones anyway, move away from us faster than light speed, supposedly, pushed along by dark energy. The best estimate for the rate of acceleration for the universe is 68 kilometres per second per megaparsec.
Quantum entanglement is another example of a faster-than-light interaction that doesn’t violate Einstein’s theory. When two particles are entangled, one can travel to its partner instantaneously, even if its mate is on the other side of the universe. Einstein called this, "Spooky action at a distance." If we were somehow able to warp or fold space-time, such as with a wormhole, it would allow a spacecraft to pass instantaneously from one side of space to another.
Einstein says that light acts pretty much the same throughout the universe. There’s a problem though. Today, scientists marvel at just how homogenous the universe is. One way we can tell, is by investigating the cosmic microwave background (CMB). This is essentially the light left over from the Big Bang, located in every corner of the universe.
No matter where you examine it, it’s always the same temperature, -454 Fº (-270 Cº). If that’s the case and light travels at a constant speed, how could it have made it from one edge of the universe to the other? To date, scientists have no idea, other than to say, some peculiar conditions must have existed in that early “inflation field.”
The idea of light slowing down over time was first proposed by Professor João Magueijo, from Imperial College London and his colleague, Dr. Niayesh Afshordi, of the Perimeter Institute in Canada. Their paper was submitted to Astrophysics in late 1998 and published shortly thereafter. Unfortunately, the proper instrumentation necessary to investigate the CMB to search for clues supporting it, wasn’t available at the time.
Magueijo and Afshordi eliminated the inflation field altogether. Instead, they argue that the intense heat that existed when the universe was young, ten thousand trillion trillion Cº, allowed particles- including photons (light particles), to move at an infinite speed. Light therefore traveled to every point in the universe, causing a uniformity in the CMB that we can observe today. “We can say what the fluctuations in the early universe would have looked like,” Afshordi told The Guardian, “and these are the fluctuations that grow to form planets, stars, and galaxies.” An experiment the following year lent credence to Magueijo and Afshordi’s theory.
In 1999, Lene Vestergaard Hau at Harvard stunned the world, after she conducted an experiment where she slowed light down to just under 40 mph (64 kph). Hau studies materials at a few degrees above absolute zero. In such an environment, atoms move very slowly. They begin to overlap, turning into what’s known as the Bose-Einstein condensate. Here, the atoms become one big cloud, and behave like one giant atom.
Hau shot two lasers through such a cloud, comprised of sodium atoms 0.008 inches (0.2 mm) wide. The first blast changed the quantum nature of the cloud. This increased the cloud’s refractive index, which slowed the second beam to 38 mph (61 kph). Refraction is when light or radio waves are bent or distorted when passing from one medium into another.
A discovery in 2001 also lent credence to the variable light theory. The eminent astronomer John Webb made an observation while studying quasars in deep space. Quasars are luminescent bodies billions of times as massive as our sun, which are powered by black holes. Its luminosity comes from an accretion disk, made up of gas, enveloping it.
Webb found that one particular quasar when nearing interstellar clouds, absorbed a different type of photon than would’ve been predicted. Only two things could explain this. Either its charge had changed or the speed of light had. In 2002 an Australian team, led by theoretical physicist Paul Davies, found that it couldn’t have changed polarity, as this would’ve violated the Second Law of Thermodynamics.
Another breakthrough study in 2015 further challenged this staple of science. Scottish physicists from Glasgow and Heriot-Watt universities successfully slowed a photon at room temperature, without refraction. They basically built a racetrack for photons. It was made so that two photons raced side-by-side.
One track was unencumbered. The other held a “mask” which resembled a target with a bullseye. In the centre was a passageway so narrow, the photon had to change shape to squeeze through. It slowed that photon down about one micron (micrometre), not a lot, but enough to prove that light doesn’t always travel at a constant speed.
By now, instrumentation had improved to the point where the CMB can be successfully probed. As such, in 2016 João Magueijo and Niayesh Afshordi published another paper, this time in the journal Physical Review D. They are currently measuring different areas of the CMB, and studying the distribution of galaxies, seeking clues to support their claim that light in the universe's earliest moments broke free of its presumed speed limit.
Again, this is a fringe theory. And yet, the implications are astounding. "The whole of physics is predicated on the constancy of the speed of light," Magueijo told Vice’s Motherboard. "So we had to find ways to change the speed of light without wrecking the whole thing." Their calculations should be complete by 2021.
Einstein’s theory says that nothing “with mass” could pass the speed of light, so it’s reasonable to consider that other things that don’t have mass could potentially achieve this feat. One such thing is simply empty space.
Relativity says that objects cannot travel faster than the speed of light through spacetime. It doesn’t, however, have anything to say about spacetime itself. And in fact, spacetime is expanding and pushing matter apart faster than the speed of light; however, matter is not really traveling through spacetime; but the spacetime is pushing it.