Relativity II: Michelson-Morley Part 1

Hello, and welcome back to MPC! Last week, we began to dig deeper into relativity by disucssing the meaning of the word relative. Today, we will be exploring the experiment that motivated modern relativity. Let’s get started!

As mentioned in last week’s post, relative quantities, such as speed, can be determined using subtraction. For example, if you are traveling at 5 m/s forward and I am traveling at 10 m/s forward, my speed relative to you would be 5 m/s (10 – 5). Ever since Galilieo proposed this concept in the 17th century, known as the Galilean Transform, it was accepted as true. Why shouldn’t it have been? If you think about it makes sense: for every second, I would be traveling 5 meters farther than you, so, relative to you, I am traveling at 5 m/s. Not only is the Galilean Transformation intuitive — all scientific experiements conform to it.

Well, at least, all of our experiments did conform to it. Everything changed at the turn of the 20th century with the Michelson-Morley experiment.

Let’s think about waves. Waves are just a means of transferring energy. For example, when oceans waves approach you at the shore, energy that was once out in the ocean is traveling towards you. Many phenomena are waves, such as sound, water waves, etc. In all of the aforementioned scenarios, the energy is being transferred through something. For example, sounds waves travel through the particles in the air. Water waves travel through the water particles. We will discuss waves in a more detailed fashion in the future, but for now, just know that what a wave travels through is known as its medium.

Whether or not light is a wave has been a topic of debate amongst physicists for centuries. In the 19th century, Young’s famous double-slit experiment (which we will discuss in the future) demonstrated that light exhibits at least some wave-like properties. For this reason, many physicists of the 1800s believed that, like other waves, light required a medium to travel through. Physicists called this medium the “luminferous aether.”

As we will see in next week’s post, the concept of the lumineferous aether will disprove many physical phenomena that we find intuitive, such as the Galilean Transform. See you then!


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