The Speed of Light

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Podcast Transcript

There is nothing faster in the entire universe than the speed of light. Not only is light the fastest thing, but nothing can ever be faster than light. 

For the longest time, humans didn’t even know that light had a speed, and once they figured out that light wasn’t instantaneous, it took several centuries to determine what that speed was.

Learn more about the speed of light and its implications for physics and engineering on this episode of Everything Everywhere Daily.


The speed of light in a vacuum is exactly 299,792,458 meters per second. 

This is a universal speed limit. Nothing can go faster. It is literally impossible. 

Before I get into the implications of this fact and what it means, I should start with how we know that light even has a speed.

Light is so incredibly fast that, from a human perspective, it seems instantaneous. 

For the longest time, no one was sure if light was instantaneous or just really fast. It was a subject of debate for philosophers.

In the 5th century BC, the Greek philosopher Empedocles believed that light did have a finite speed. Aristotle, on the other hand, said, “light is due to the presence of something, but it is not a movement.”  Hence, he didn’t think it had a speed but was instantaneous. 

The Aristotelian view held sway for centuries. Much of this had to do with the fact that most people thought light emanated from the eye. According to Heron of Alexandria, light had to be instantaneous because when you opened your eyes, the stars could be seen immediately. 

The Aristotelian view was challenged by the Islamic scientist Hassan Ibn al-Haytham in the 11th century. He wrote a treatise called the Book of Optics, which argued for the intromission theory of light, which said that light entered the eye from the outside. This led him to conclude that light must have a finite speed.

These debates continued for centuries with no resolution. Even famed thinkers such as Johannes Kepler and René Descartes thought that light had to be instantaneous. 

In the 17th century, the age of experimentation began, and people began to develop experiments to determine if light indeed had a speed.

One of the first proposals came from Galileo Galilei in 1638. He suggested having two people with covered lanterns standing on hills some distance apart from each other.

One person would remove the cover on their lantern, and the moment the other person saw the light, they, too, would remove the cover on theirs. 

The experiment was finally conducted almost 30 years later by the Accademia del Cimento in Florence. They had two people about one mile apart, but their results were inconclusive. The only thing they could conclude was that if light had a speed, it had to be incredibly fast. 

It wasn’t until 1676 when the Danish astronomer Ole Rømer was finally able to prove conclusively that light had a finite speed. 

He came to this conclusion through observations of the moons of Jupiter, in particular the innermost moon, Io. 

Io regularly passes into Jupiter’s shadow, but from the Earth, we can only see it enter Jupiter’s shadow or leave Jupiters shadow. Which we can see depends on the time of year and the position of the Earth with respect to Jupiter.

Rømer noticed that the time between eclipses wasn’t constant. When the Earth was moving away from Jupiter, the time between eclipses was slightly longer. When it was moving towards Jupiter, it was slightly shorter. 

He figured there were only two explanations for this. Either the orbit of Io was somehow changing based on the location of the Earth, which was highly improbable, or light had a finite speed. 

He calculated that it would take light 22 minutes to travel the diameter of Earth’s orbit. The Dutch astronomer Christiaan Huygens used this estimate with his calculation for the diameter of Earth’s orbit to come up with a value of 220,000,000 meters per second, which is 27% slower than the currently accepted value.

Newton figured out that because Io didn’t change color after an eclipse, all colors of light must travel at the same speed.

By the 19th century, it was known that light was a manifestation of electromagnetism. 

It was then thought that if light was a wave, and electricity and magnetism had fields, then it had to exist in some sort of substance, which was called the ether. 

If that were the case, then light should have different speeds in different directions as the Earth moved around the sun. However, that was not what they found. The speed of light was the exact same in every direction. 

Albert Einstein has several realizations about the speed of light and the extremely strange things which happen at light speed. 

Because light travels at an absolute speed, it does not change its speed relative to an observer.

For example, if you were riding in a train traveling 100 kilometers per hour and you throw a ball forward 50 kilometers per hour to someone outside the train, the ball would seem to be traveling 150 kilometers per hour.

However, light doesn’t work like that. Regardless of your initial position or velocity, light will always go the exact same speed. 

There is actually a lot more he discovered that I’ll leave for a future episode on relativity, but Einstein also determined that it is impossible for anything with mass to travel at the speed of light. To do so would require an infinite amount of energy. This is why protons in a particle accelerator can get to 99.99999% the speed of light, but it can never reach it, in the same way, that temperature can never reach absolute zero.

Also, the closer you get to the speed of light, the more time will dilate. Time will go more slowly the faster someone is moving. 

This has caused all sorts of problems for science fiction writers who need to create some system of interstellar travel that doesn’t involve traveling faster than the speed of light.  

So the Millennium Falcon jumping to light speed….that can’t happen.

Einstein also realized that the speed of light, as a fundamental constant of the universe, was tied up in the relation between matter and energy. The letter used to represent the speed of light in equations is “c,” and is the c in E=mc2.

In the 20th century, measurements of the speed of light became ever more precise. However, it was eventually realized that because the speed of light is an absolute universal constant, it wasn’t something we should even try to measure.

It was something we should use to define our measurements.  So, in 1975 the 15th General Conference on Weights and Measures defined the distance light traveled in one second to be exactly 299,792,458 meters. 


Take one over that, and you get the exact length of one meter.

In our everyday lives, the speed of light doesn’t really factor into things. For all practical purposes, we can treat it as if it were instantaneous. However, there are situations where the speed of light needs to be taken into consideration. 

The most obvious one is space travel. 

The distances involved in anything over than low Earth orbit space flight can make communications awkward or difficult.

The average distance from the Earth to the Moon is approximately 1.28 light seconds.  When mission control would talk to the Apollo astronauts on the Moon, there was always a slight pause because of the time it took for the radio signal to get to the moon and to be sent back. 

Little more than a one-second delay isn’t that big of a deal when having a conversation, but it does become a big deal if you wanted to control a drone remotely from Earth. 

Once you get further than the Moon, then it becomes a big deal. The time it takes to send a radio signal to Mars can vary depending on where the planets are in their orbits. It can be as short as 3.1 minutes or as long as 22.2 minutes. 

If you have ever watched footage of the landing of a Mars rover, you will see a room full of people at mission control at the Jet Propulsion Laboratory waiting to get results on if it managed to land successfully. When they find out, they are actually watching something that might have happened 10 minutes earlier. The delay is due to the speed of light, and this is why all planetary probes and landers have to be fully automated. 

Likewise, if you go out even further, the effect is even more pronounced. Telescopes, like the James Webb, that see the edge of the universe are actually looking into the past.

The distances are so vast that it took the light emitted by distant galaxies billions of years to reach the Earth.  There are parts of the universe that we may never see because they are traveling away from us at a speed such that the light will never catch us. The distant objects are not going faster than the speed of light, but the distance between us is growing at a faster rate.

You don’t have to deal with interplanetary distances to have to take into consideration the speed of light. You can also deal with very short units of time. 

One area where this comes into play is high-frequency stock trading. There is a category of stock traders who make very rapid trades with very small changes in price. For them, differences of fractions of milliseconds can be the difference between making or losing money. 

One company called Spread Networks built a custom fiber optic cable from Chicago to New York in an incredibly straight line just to minimize the time it took signals to get from one city to the other. They invested tens of millions of dollars to reduce the time it took to send a signal via a fiber optic cable from 13.1 milliseconds down to 12.98 milliseconds.

They had people willing to pay for that improved service. 

So far, I’ve mentioned that the speed of light is an absolute universal constant that can’t change, and that is true. However, I also mentioned at the very top of the episode that that was for light in a vacuum.

Light can actually travel slower if it is traveling through some other medium. In fact, when light travels through a fiber optic cable, it goes about ? slower than it does in a vacuum. That is still incredibly fast, but that does become a factor to consider when communicating with the other side of the Earth. 

The company SpaceX has a constellation of low Earth orbit satellites, which provides internet access all over the globe. In the next generation of satellites they will be launching, they will be able to route data between satellites in orbit using lasers. 

Because light travels faster in the vacuum of space than it does in a fiber optic cable, in theory, Starlink should be able to have lower latency across long distances than fiber.

While the route might be a bit longer being in orbit, the difference in the speed of light can more than compensate for it. 

The speed of light is the ultimate speed limit. It determines how fast we can travel and how quickly we can communicate. 

No matter how hard you try, it is the one speed limit you can never violate.