# What do we know about the speed of light?

The speed of light in the vacuum is exactly 299,792,458 metres per second, normally rounded to 300,000,000 metres per second. It is a universal constant, symbolised by the letter “c” in the equations.

According to the special relativity of Albert Einstein, the speed of light acts as a speed limit in all the universe: nothing can go faster than light. In addition, the speed of light is a constant (it is always the same), no matter at which speed an observer moves.

To understand it better, think about a person that throws a ball from the window of a train. An observer at the train station would see that, when being thrown, the ball acquires a speed equal to the addition of the speed of the train, plus the speed with which it has been thrown. We now replace the ball with a beam of light. When the person on the train “throws” the beam of light, the observer no longer measures the speed of the train plus the speed of light, but the speed he obtains is exactly the speed of light. Likewise, imagine a spacecraft that travels at a speed of 200,000,000 metres per second, and it is in a race against a beam of light. The spacecraft wouldn’t see how light moves away at about 100,000,000 metres per second from it, but it would observe light move away at about 300,000,000 metres per second (the speed of light).

#### The speed of light to measure distances

As we have seen, the speed of light never changes, making it a perfect way to define any other unit of measurement.

For example, since the 1980s, the definition of the metre is: “the distance that light travels in a vacuum in exactly 1/299,972,458 seconds”. In the same way, all the other distance units can be defined using the speed of light.

Moreover, the distances between stars and galaxies are so great that using units such as the metre or the mile isn’t practical at all. It is for this reason that the light year is used, which is nothing else than the distance travelled by light in one year (almost 9.5 billion kilometres).

#### Is it possible to go faster than light?

As an object approaches the speed of light, its mass increases, for which more energy is constantly needed to continue to accelerate it. When reaching the speed of light, the object’s mass would be infinite, and it would equally require an infinite amount of energy to move it.

So, the speed of light is the speed limit of the universe. Nothing can go faster or as fast as it.

#### Can light slow down?

In a vacuum, light always travels at the same speed. However, when going through any material it can be slowed down. Light changes trajectory when getting into contact with other particles, which makes its speed decrease. The refraction index measures in what amount a material slows down light.

When going through the atmosphere, light travels a little bit slower, as there are relatively few particles. In the water it goes even slower, and, when going through a diamond, light travels at less than half its typical speed: about 124,000,000 metres per second.

Apart from this, light is also influenced by the gravity of massive bodies, which makes the trajectory of photons deviate. This change in trajectory also reduces the speed of light.

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