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

## Apart from the Michelson-Morley experiment, a different method for showing that the speed of light is the same for all observers exists. It is an undeniable and modern measurement that demonstrates that all observers see the speed of light to be a single value. This measurement involves particle accelerators.

### A Baseball Player and a Particle Accelerator

In order to understand the experiment, let’s first start with a low speed example. Suppose there is a pitcher for a baseball team. If he tries hard, he can throw a baseball about 100 miles per hour. Now let’s put that same pitcher in a fast-moving train, travelling at 100 miles per hour with respect to the track. If we did this and a person standing stationary next to the tracks were to measure the speed of the baseball, they’d see that it was moving at 200 miles per hour. That’s 100 miles per hour from the pitcher’s arm and 100 miles per hour from the train itself. Pretty straightforward.

We can do a similar experiment using a particle accelerator whereby we take an electron and induce it to emit a photon. We can then measure the speed of light by simply setting up two stationary light detectors, a fixed distance apart, and measuring how long it takes for light to go that distance. Dividing the distance by time, we get the velocity and find that it is 300,000 kilometers per second.

### Traveling at the Speed of Light

We use a particle accelerator to shoot the electron down the accelerator at nearly the speed of light. In the most modest modern particle accelerator, we can accelerate electrons to 99.999 999 9% the speed of light. So that is, for all intents and purposes, traveling essentially at the speed of light.

Following our baseball example, if we see an electron travelling at the speed of light and it emits a beam of light that the electron sees traveling at the speed of light, then we should see that light is travelling at twice the speed of light. And, if that’s the case, we should expect to see this super-fast beam of light arrive at the far detector in half the time it takes the electron to travel that distance.

### Light Travels at One Speed for all Observers

But that’s not what we see. The electron and emitted photon travel the proscribed distance in essentially the same amount of time. The photon and electron both travel at the speed of light.

Obviously this makes no sense if the speed of the electron is added to the photon. No sense at all. It only makes sense if the photon is traveling at the speed of light both to the point of view of the electron and to us, who see the electron zooming along at very high speeds.

Furthermore, this experiment gives the same result for any time of day and any time of year. This means that if there were any effects due to traveling through, what we now know to be a non-existent aether, either in the direction of the aether flow, or perpendicular to it, we’d see it. Hence, when we get right down to it, this modern measurement is inarguable proof that light travels at one speed for all observers.

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.

### Frequencies of Light

There is another thing which has to do with the claim that the speed of light is the same for all frequencies of light or wavelengths. Max Planck and Albert Einstein deduced that the energy of a photon of light depended directly on the frequency of the light. Therefore, low frequency light has low energy and high frequency has high energy.

Now, for particles, a low energy particle travels at lower speed than high energy particles. A high energy bullet can do more damage than a low energy bullet of the same weight and caliber. So, it wouldn’t be foolish to imagine that perhaps low energy light might travel at lower speeds than high energy light. We know this isn’t true, and we know it to incredible precision. The way we do is by using an epic cosmic explosion called a gamma ray burst.

### Gamma Ray Burst

Exactly what causes a gamma ray burst is not known. But what we do know is that they are brief emissions of energy that are the brightest things in the cosmos. They can be seen literally from across the universe. Gamma rays can easily be hundreds of billions or even trillions of times higher energy than the lower energy light. It’s a huge difference.

As it happens, we have a variety of astronomical observatories that can see all wavelengths, from gamma rays, x-rays, ultraviolet, visible, infrared, microwaves, radio waves, etc. That means when a gamma ray burst goes off, we can time the arrival of the electromagnetic energy in all wavelengths and frequencies.

### GRB 090510

One gamma ray burst occurred in 2009, with the unimaginative name GRB 090510. It exploded literally a long time ago in a galaxy far, far away—about 7.4 billion years ago. The light travelled for all that time and the different wavelengths all arrived within a few seconds of one another.

Let’s think about what that means—7.4 billion years is about 200 quadrillion seconds. The initial arrival times were all within about two seconds. That means that the various different energy photons, which means frequencies of light, all arrived with a difference of no bigger than one part in 100 quadrillion.

This measurement, combined with the observation that the speed of light is the same to all observers, says something very deep and fundamental about the universe. We don’t know exactly what it is, but it’s clearly telling us something!

### Common Questions about Light and Relativity

**Q: What is the experiment that is conducted using a particle accelerator?**

We can do an experiment using a particle accelerator whereby we take an electron and induce it to emit a photon. We can then measure the speed of light by simply setting up two stationary light detectors, a fixed distance apart, and measuring how long it takes for light to go that distance.

**Q: What deduction did Max Planck and Albert Einstein make?**

Max Planck and Albert Einstein deduced that the energy of a photon of light depended directly on the frequency of the light. Therefore, low frequency light has low energy and high frequency has high energy.

**Q: Are gamma rays high in energy when compared to light?**

Gamma rays can easily be hundreds of billions or even trillions of times higher energy than the lower energy light.