What is the speed of light?

The recently deceased Stephen Hawking said that one of the ways to travelling to the future could be to build a vehicle capable of “almost” reaching the speed of light. He said “almost” because nothing can exceed the speed of light. This is one of main basic principles of science.

The British physicist proposed the following conundrum: a train is built that travels at 99.99% of the speed of light and inside that train a passenger runs at full speed through the corridors. In theory, this passenger would be exceeding the speed of light, but in practice the inexorable law of nature would take effect.

How? By making time be slower inside the train. It doesn’t matter how long the passenger sprints, time would slow down in order to not pass the speed of light. Time inside the train would pass slower than outside and one week of travel would become 150 years outside.

What is the speed of light?

Beyond the theories that are difficult to prove, the speed of light is one of the indispensable tools for astronomy and physics. Thanks to them, we can know how far we are from other places in the universe. But, how fast does light travel?

The first person to measure it was the English astronomer James Bradley, who determined in 1728 that light moves at 301.000 kilometres per second. The calculation was made by observing the Earth’s travel around the Sun. In 1983, the General Conference on Weights and Measures redefined the metre. Since then, electricity has an exact speed of 299.792,458 kilometres per second.

However, its speed varies as a function of its average. For example, in space, light travels at its maximum speed. However, in air, its speed drops to 299.708 kilometres per second. If it comes into contact with a diamond, it reaches its lowest speed: 123.957 kilometres per second.

How long does it take for light from the sun to reach Earth?

The exact answer is 8 minutes and 19 seconds, given that the sun is nearly 150 million kilometres away from our planet. It may seem like a lot, but in reality, it is nothing compared to the time that it takes photons (the particles that light is made of) to move from the centre of the sun until its surface: 40,000 years.

As they try to emerge, photons do nothing more and nothing less than collide with the atoms in their environment, dispersing them from one side to another. During this age-old exodus, they are capable of travelling 696,000 kilometres until they reach the photosphere in the end.

Einstein and challenging the laws of science

Now is when you ask: why isn’t it possible to travel faster than light?

In truth, the question should be: why can only photons travel at the speed of light? The answer is that acceleration makes the mass of objects increase: the greater it is the lesser the acceleration. However, the mass of the photons is equal to zero and they do not need to accelerate because from the time from they are born, they are already at top speed.

This is where one of the most brilliant scientists in history—Albert Einstein—and his theory of relativity comes into play. Einstein contradicted what we mentioned above, and maintains that some particles called tachyons could surpass the speed of light.