###### By Don Lincoln**, **Fermilab

## The speed of light is very fast. Indeed, it’s the fastest thing in the universe. Numerically, it is about 186,000 miles per second. That’s about fast enough to circumnavigate the Earth, about 7.5 times per second. How do we measure something that fast? Read on more to find out.

### Is the Speed of Light Same for Everyone?

According to the theory of relativity, there are two core principles. The first is that all observers, no matter their speed compared to other people, have an equal right to claim that they are the one, unmoving, center of the universe. That’s not so new and unusual. We’ve accepted that since the days of Galileo.

The second one is much less obvious and that is that the speed of light is the same for everyone. It is less ‘obvious’ because, suppose, one is sitting in a car with a cat in their lap. It can be reasonably said that the cat isn’t moving. On the other hand, someone sitting stationary, watching the car fly by, will say that the cat is moving at something like 60 miles per hour. Thus, it seems that velocities always depend on the frame of reference in which they are measured.

But it’s not true for light and we know this to be true. It’s easier to state the speed of light precisely in the metric system, where it is exactly 299,792,458 meters per second. That level of precision is important for scientists. However, instead of the measurement itself, it might be more interesting to first get a sense of exactly how do we measure this speed of light.

### The Speed of Sound

Understandably, measuring something that fast takes some doing. It’s not like sound, which is slow enough that there can be a perceptible delay or in places like a baseball game, where one can see the distant batter hit the ball and then hear the crack of the bat a split second later. Or when we watch a high-flying plane overhead and hear the sound coming from a different location.

With a speed of 770 miles per hour, or just over 1,000 feet per second, it’s easy to understand how a location determined by hearing and vision would be different. After all, if the speed of sound is 1,000 feet per second and we are looking at a plane that is flying at an altitude of 30,000 feet, we’ll hear the sound coming not from where we see the plane, but from where it was 30 seconds ago.

No, with light, any possible delay in any reasonable situation is imperceptible by human senses. Instead of the full second it takes for sound to travel 1,000 feet, it takes light a millionth of a second. So, how can we measure the speed of light? Well, basically one has to either utilize very fast electronics, or measure very long distances. And the second method is how the speed of light was first measured.

This article comes directly from content in the video seriesThe Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.

### Ole Roemer and the Speed of Light

The first professional attempt to measure the speed of light was made in 1676, by Danish astronomer Ole Roemer. He was making precise measurements of the eclipse of the moon, Io, as it orbited Jupiter, attempting to determine the exact moment at which Io slipped into Jupiter’s shadow. This wasn’t the esoteric exercise it seems to be; it actually had applications for sailors needing to determine their location on the globe.

What Roemer knew was that Io takes 42.5 hours to orbit Jupiter. From that, he could calculate the moment at which the eclipse should occur for every orbit. However, what he found was that his calculation and measurement disagreed. Furthermore, the disagreement changed over time and depended on where Jupiter was in its orbit compared to where Earth was in its. Those two locations were constantly changing, which, therefore, changed the distance between Jupiter and Earth.

### An Incredible Achievement

Using precise measurements and a lot of thinking, Roemer realized that he could explain his observations if light had a fast but finite speed. He even calculated that light would take 22 minutes to travel from one side of the Earth’s orbit to the other and, from that he calculated that the speed of light is 220,000 kilometers per second, which is about 26% lower than the actual speed. But, still, it was an incredible achievement for someone working over three centuries ago.

Today things are different. We have technology on our side. There are not one, but many ways to measure the speed of light. Of course, we can also repeat Roemer’s measurement today and get more accurate results, but now we can make different measurements that more directly measure the speed of light. For instance, we can set two light detectors 1,000 feet apart and then shoot a pulsed laser from one to the other and measure that light took a millionth of a second to travel the distance. Or we could use a single light detector and a distant mirror to do the same thing. A millionth of a second sounds very fast, but even an inexpensive modern oscilloscope can easily measure time durations 1,000 times shorter.

### Common Questions about Measuring the Speed of Light

**Q: What is the speed of light?**

It’s easier to state the speed of light precisely in the metric system, where it is exactly 299,792,458 meters per second.

**Q: Who was the first to measure the speed of light?**

The first professional attempt to measure the speed of light was made in 1676, by Danish astronomer Ole Roemer.

**Q: What were the calculations made by Ole Roemer?**

Ole Roemer calculated that light would take 22 minutes to travel from one side of the Earth’s orbit to the other and, from that he calculated that the speed of light is 220,000 kilometers per second, which is about 26% lower than the actual speed.