#129 Why is the speed of light 299,792,458 m/s?

I learned this today. There is no reason why the speed of light is 299,792,458 m/s. It just is.

The speed of light is the top speed that matter or energy can travel at. All forms of electromagnetic radiation travel at the speed of light. Radio waves and electricity, for example. Nothing can travel faster than the speed of light and nothing with mass can travel at the speed of light.

Measurements of the speed of light started in the 17th century, but the current specific number has only been known since 1975.

The Ancient Greeks debated whether light was instantaneous, or extremely fast. Empedocles, in the 5^th century BC, was the first person to suggest that the speed of light was finite.

The Greeks believed, and this belief lasted until the 10^th century AD, that we see by sending light out of our eyes. From this viewpoint, light must be instant because it is instantly there when we open our eyes. Distant stars are instantly visible, so the light must have travelled that distance instantly.

In 1021, Alhazen, an Arabian mathematician, published his Book of Optics. Amongst other things, he proposed that we saw by light entering our eyes, rather than coming from our eyes. This made him think that light must have a finite speed.

In 1638, Galileo came up with an experiment to test if light is finite. He had a covered lantern on one hill and put someone on a distant hill to see if there was a delay between uncovering it and the other person seeing the light. I’m not sure how he would have tested that one, but light is so fast that the human eye wouldn’t have seen the delay. He concluded that light was either instantaneous or incredibly fast.

Ole Rømer, in 1676, was the first person to estimate the speed of light. He was observing the eclipses of Jupiter’s moon Io. By observing them in different places on Earth and comparing the times, he hoped to be able to work out the distance between the two points. While he was doing this, he noticed that the times of the eclipses were slightly farther apart when the Earth was far away from Jupiter, and the eclipses grew slowly closer to each other as the Earth moved closer to Jupiter. He realized that the only thing that could account for this would be a finite speed of light. He estimated that it was taking light 22 minutes to cross the diameter of the Earth’s orbit. Christiann Huygens took this estimate, combined it with an estimate for the diameter of the Earth’s orbit and came up with a speed of light of 220,000 km/s. That was the first calculation of the speed of light.

In the 19^th century, scientists worked with time-of-flight measurements. By 1862, the speed of light had been calculated down to 298,000 km/s. By the early 20^th century, calculations were becoming more accurate and in 1975, at the General Conference on Weights and Measures, it was agreed that the speed of light would be 299,792,458 m/s.

So, why is the speed of light 299,792,458 m/s? It turns out that there is no reason. It just is. In our universe, that is what the speed of light is. In other universes, with different laws of physics, the speed of light could be faster or slower, but in our universe, it is what it is.

A better question might be, why can’t anything go faster than light? Two simple reasons are time and energy.

When an object moves faster, time for it slows down. The faster the object moves, the slower time goes, until the object reaches light speed and time goes to zero. The reason it is not possible to go faster than light speed is because there is no time beyond light speed. There is nothing to travel into.

The second reason is to do with mass. The faster an object moves, the more mass it has. In this case, energy and mass are the same thing. The mass keeps increasing until the object cannot move any faster. To accelerate the object to the speed of light would require infinite energy and the object would have infinite mass. This is why nothing with mass can ever reach light speed. Electrons, for example, travel at 90% the speed of light because they have mass. Photons travel at light speed because they have no mass.

It is not possible to go faster than light, but it is possible to move across space in a way that appears to be faster than light. Wormholes, if they exist, could be a way of moving vast distances without ever violating the speed of light law.

So, wondering why the speed of light is the speed it is, is not a useful way of thinking. It is the speed it is because it could be no other. And this is what I learned today.

Photo by Jonas Von Werne from Pexels

Sources

https://www.space.com/speed-of-light-properties-explained.html

https://www.sciencefocus.com/science/why-is-the-speed-of-light-constant/

http://www2.phys.canterbury.ac.nz/~dlw24/c.html

https://aeon.co/essays/why-is-the-speed-of-light-the-speed-of-light

https://www.sciencealert.com/why-is-the-speed-of-light-the-speed-of-light

https://en.wikipedia.org/wiki/Speed_of_light

https://en.wikipedia.org/wiki/Ole_R%C3%B8mer

https://en.wikipedia.org/wiki/Ibn_al-Haytham

https://www.space.com/15830-light-speed.html

https://www.theguardian.com/science/2014/jan/12/einstein-theory-of-relativity-speed-of-light