To the layperson, it may seem that light propagates instantaneously in nature. As soon as you turn a mirror, the sunbeam created by it immediately moves. However, experiments show that light actually has a finite speed, although it is very high.
So what is the speed of light and how is it measured? And what could be faster than the speed of light?
Although the first scientific works on optics date back to antiquity, ancient Greeks and Romans could not determine the speed of light. It is believed that Empedocles, who lived in the 5th century BCE, was the first to express the idea of the finiteness of the speed of light. However, Aristotle and other ancient philosophers disagreed with him, believing that light propagates instantaneously.
Only after two thousand years, Galileo conducted the first experiment in which he tried to test Empedocles’ hypothesis. As a result, he failed to measure the speed of light and concluded that if it is finite, then it is very high.
Only the development of astronomy, associated with measurements of gigantic distances, allowed people to obtain at least approximate estimates of the speed of light. In 1676, Ole Rømer noticed that the timing of the eclipses of Jupiter’s moon Io significantly depends on the distance between Jupiter and Earth (which is not constant due to the planets’ movement at different speeds).
He explained this effect by the finiteness of the speed of light and even managed to conduct calculations, which showed that the speed of light should be 220,000 km/s. Ole was wrong, but not by much. He did not properly document his observation, so the scientific community recognized the finiteness of light only in 1727.
What is the speed of light
Astronomical observations were not particularly precise, so experiments in terrestrial conditions were conducted to measure the speed of light. As early as 1728, Bradley, using the aberration of light effect, obtained a value equal to 308,000 km/s.
In the 19th century, physicists delved more deeply into this topic. Armand Fizeau developed a special “interrupted method” in 1849. Using this method, the Frenchman obtained a figure of 313,000 km/s, although constant improvements to this method allowed Bergstrand in 1950 to achieve a value of 299,793,100 m/s, with measurement error not exceeding 250 m/s.
In parallel, another method of measuring the speed of light was developed, known as the “rotating mirror method.” By 1868, the measurement error using this method was only 500 km/s, and by 1926, it had been reduced to 4 km/s.
New possibilities in measuring the speed of light emerged with the development of lasers. By 1970, the precision of experiments had reached 1 m/s.
Further, physicists encountered a new fundamental limitation. The precision of the so-called “ruler” used to measure the distance traveled by light played a significant role in calculating the experiment’s margin of error. This ruler was limited by the precision of the meter standard. It consisted of a ruler made of a platinum-iridium alloy stored in the French town of Sèvres. Roughly speaking, physicists’ experiments began to exceed the precision of this standard meter.
To bypass this limitation, scientists had to redefine the concept of the meter. In 1983, it was redefined. Now, the meter is defined as the distance traveled by light in 1/299,792,458 of a second. Consequently, this means that the speed of light is exactly 299,792,458 m/s. Therefore, it can be said that the speed of light is a quantity in physics known with absolute precision.
Is the speed of light a constant value?
It is important to note that this value of the speed of light is accurate only when light propagates in empty space, i.e., in a vacuum. From the theory of relativity, it is known that this value is constant and does not depend on the observer’s velocity relative to the light source. Additionally, this speed is the ultimate limit in physics – no object in nature can travel faster than 299,792,458 m/s.
Speed of light in water
The situation changes when considering the propagation of light in a medium, such as water or glass. It turns out that any medium slows down light. For example, in water, its speed decreases to 225,300 km/s, and in diamond, it is only 123,845 km/s.
What is faster than the speed of light?
As a result, situations may arise where particles move faster than light in a medium. A classic example of such motion is Cherenkov radiation. It is created by electrons and other charged particles moving faster than light in a medium.
This effect is most often observed in a liquid-cooled nuclear reactor. The reactor’s active zone is precisely the source of particles overtaking light. Visually, it appears as though the reactor is glowing.
Furthermore, in physics, it is possible for motion to occur at a speed greater than the speed of light in a vacuum! But no material object can achieve this. Examples of such motion include the movement of a pair of scissors’ intersection point and the movement of a sunbeam. The point is that the sunbeam itself is not a material object. It represents a flow of photons emanating from a mirror.
When the mirror is turned, the sunbeam at its new location comprises different photons than those at the previous location. As a result, the photons themselves do not accelerate beyond the speed of light, but the sunbeam formed by them is capable of surpassing light.