Can anything travel faster than the speed of light? To answer this question, let us understand the nature of light. Here are three salient facts about light:

1. First, light can exhibit both the properties of a wave and a particle. For all of the Nineteenth Century, and for the early part of the Twentieth Century, most scientists considered light “a wave,” and most of the experimental data supported that “theory.” However, classical physics could not explain black-body radiation (the emission of light due to an object’s heat). A light bulb is a perfect example of black-body radiation. The wave theory of light failed to describe the energy (frequency) of light emitted from a black body. The energy of light is directly proportional to its frequency. To understand the concept of frequency, consider the number of ocean waves that reach the shore in a given length of time. The number of ocean waves than reach the shore, divided by the length of time you measure them, is their frequency. If we consider the wave nature of light, the higher the frequency, the higher the energy.

In 1900, Max Planck hypothesized that the energy (frequency) of light emitted by the black body, depended on the temperature of the black body. When the black body was heated to a given temperature, it emitted a “quantum” of light (light with a specific frequency). This was the beginning of Quantum Mechanics. Max Planck had intentionally proposed a quantum theory to deal with black-body radiation. To Planck’s dismay, this implied that light was a particle (the quantum of light later became known as the photon in 1925). Planck rejected the particle theory of light, and dismissed his own theory as a limited approximation that did not represent the reality of light. At the time, most of the scientific community agreed with him.

If not for Albert Einstein, the wave theory of light would have prevailed. In 1905, Einstein used Max Planck’s black-body model to solve a scientific problem known as the photoelectric effect. In 1905, the photoelectric effect was one of the great unsolved mysteries of science. First discovered in 1887 by Heinrich Hertz, the photoelectric effect referred to the phenomena that electrons are emitted from metals and non-metallic solids, as well as liquids or gases, when they absorb energy from light. The mystery was that the energy of the ejected electrons did not depend on the intensity of the light, but on its frequency. If a small amount of low-frequency light shines on a metal, the metal ejects a few low-energy electrons. If an intense beam of low-frequency light shines on the same metal, the metal ejects even more electrons. However, although there are more of them, they possess the same low energy. To get high-energy electrons, we need to shine high-frequency light on the metal. Einstein used Max Planck’s black-body model of energy, and postulated that light, at a given frequency, could solely transfer energy to matter in integer (discrete number) multiples of energy. In other words, light transferred energy to matter in discrete packets of energy. The energy of the packet determines the energy of the electron that the metal emits. This revolutionary suggestion of quantized light solved the photoelectric mystery, and won Einstein the Nobel Prize in 1921. You may be surprised to learn that Albert Einstein won the Nobel Prize for his work on quantizing light—and not on his more famous theory of relativity.

2. Second, the speed of light in a vacuum sets the speed limit in the universe. Nothing with a (rest) mass travels faster than light in a vacuum. In addition, this is a constant, independent of the speed of the source emitting the light. This means that the light source can be at rest or moving, and the speed of light will always be the same in a vacuum. This is counterintuitive. If you are in an open-top convertible car speeding down the highway, and your hat flies off, it begins to move at the same speed as the car. It typically will fall behind the car due to wind resistance that slows down its speed. If you are in the same car, and throw a ball ahead of the car, its velocity will be equal to the speed of the car, plus the velocity at which you throw it. For example, if you can throw a ball sixty miles per hour and the car is going sixty miles per hour, the velocity of the ball will be one hundred twenty miles per hour. This is faster than any major league pitcher can throw a fastball. Next, imagine you are in the same car and have a flashlight. Whether the car is speeding down the highway or parked, the speed of light from the flashlight remains constant (if we pretend the car is in a vacuum). The most elegant theory of all time, Einstein’s special theory of relativity, uses this property of light as a fundamental pillar in its formulation.

3. Third, the quanta of light have no rest mass. This last property of light may explain why light in a vacuum sets the upper limit of speed in the universe. According to Einstein’s theory of special relativity, any object with (rest) mass becomes infinitely massive as it approaches the speed of light. By inference we can argue that it would take infinite energy to accelerate a mass to the speed of light.

However, there are other physical entities that have speeds that may equal or even exceed the speed of light. For example, the universe is considered to be expanding faster than the speed of light by numerous cosmologists. Another physical process known as quantum entanglement may also take place at or even faster than the speed of light. Quantum entanglement refers to two particles (photons, for example) which interact and become entangled, such that even when separated the quantum state of one particle will dictate the quantum state of the other particle. For example, if one photon has an angular momentum defined as spin up, the other particle will have an angular momentum of spin down, to conserve spin. If you change the angular momentum of either particle, the other particle appears to instantaneously change, such that they continue to conserve spin. The effects of gravity also appear to propagate at the speed of light. Today, science still questions the nature of gravity. In classic physics, gravity was thought of as an invisible field between two or more masses. However, some physicists speculate the existence of a particle called a graviton, which is a hypothetical elementary particle that mediates the force of gravitation. If gravitons exist, physicists speculate that they travel at the speed of light.

What does all this mean? Basically, it means that light (photons) may not be the only entities that travel at the speed of light in a vacuum.