Relativity II: Michelson-Morley Part 2

Hello, and welcome back to MPC! Last week, we started talking about the origins of relativity by discussing the Galilean transform and properties of waves. Today, we will familiarize ourselves with the “spark” that led to the development of modern relativity.

Recall from last week that most waves require a medium to travel through, and scientists of the 19th century assumed that light traveled through a medium known as the luminiferous aether. Although scientists assumed that the luminiferous aether existed, experimental proof for its existence could not be obtained. How could someone detect the existence of something that cannot be felt or seen? Well, Albert Michelson and Edward Morley thought that they had found a way to detect the luminiferous aether with their famous Michelson-Morley experiment.

I will now provide a very simplified explanation of the Michelson-Morley experiment, but one that will suffice for future blog posts on relativity. A bonus blog post will become available in the near future that covers the Michelson-Morely experiment in much greater detail.

To understand how the Michelson-Morley experiment works, let’s picture that you are throwing a rock straight forward. Let’s say that you are throwing the rock at a speed of 10 m/s. Remember that this simply means that the rock will travel 10 meters every second (we will be ignoring gravitational effects). What happens to the speed of the rock if we introduce a 5 m/s wind into this scenario? Of course, the answer depends on how we are throwing the rock: if the rock is being thrown in the same direction as the wind, it will travel at 15 m/s (10 + 5), but if the rock is thrown in the opposite direction of the wind, it will travel at 5 m/s (10 – 5). Note that the aforementioned scenario ignores principles of aerodynamics and is just meant to illustrate a concept.

Rocks are cool, but what do they have to do with the Michelson-Morley experiment and the luminiferous aether? Imagine that the rock we were throwing was a ray of light*. In the previous example, the rock is traveling through the air/wind, meaning that the air/wind can be considered its medium. Since we are claiming that the rock is a ray of light, what can the air/wind be called? The luminiferous aether!

Basically, what Michelson-Morely tried to do was see how the speed of light changed based on the movement of the luminiferous aether and so-called aether winds. For similar reasons that we explained with the rock, Michelson and Morely expected light to travel faster in one direction than in the opposite direction (depending on if the light was traveling with or against the aether wind). What did Michelson-Morely find?

No difference. That is, no matter what direction Michelson-Morley measured the light traveling in, the speed of the light was the same. This result is very important: it suggests that the luminiferous aether does not exist and that light does not travel through a medium like other waves!

Another interesting property of light that was discovered (though not in the Michelson-Morley) was that light always has a constant speed. For example, let’s think about a Galilean transform problem similar to the one that we discussed last week:

I am traveling at 5 m/s and you are remaining stationary (0 m/s). From your perspective, how fast am I goingAnswer: 5 m/s (5 – 0).

I am traveling at 5 m/s and you are traveling at 1 /ms. From your perspective, how fast am I going? Answer: 4 m/s (5 – 1).

As you can hopefully see from this example, if you start moving, my speed (relative to you) will be slower. That makes sense! But what would happen if I were a ray of light:

I am traveling at the speed of light (3 * 10^8 m/s) and you are remaining stationary (0 m/s). From your perspective, how fast am I goingAnswer: speed of light.

I am traveling at the speed of light (3 * 10^8 m/s) and you are traveling at 100 /ms. From your perspective, how fast am I going? Answer: speed of light.

That’s right! No matter how close you get to my speed, I will always be traveling at a constant speed. Not only that, but this also suggests that you can never catch up to me. This is crazy! It suggests that the Galilean transform, something that had been so integral to physics prior to the 20th century, is invalid. What will we do? Perhaps a better question is what did Einstein do! This is what we will talk about next week. See you then!

To learn more:

* At this point, those of you with prior physics knowledge may be getting a little confused: it seems as if we are treating light as both a wave and a particle! As we will discuss in the future, light is actually both a wave and a particle. We will go into much greater detail about the nature of light soon, but, for now, just keep this idea in the back of your head.

(featured image:


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