# Understanding Gravitational Waves in 4 steps

You want to understand Einstein’s gravitational waves in 4 steps? No picture, no equations? Easy. Here it is.

Step #1, Year 1905: Einstein publishes the theory of Special Relativity. It says, among other things, and for reasons too long to explain here, that ‘space’ and ‘time’ must not be considered separatedly (as we always do), but actually are the components of one thing, called the … ‘space-time’. Practically, it means you can’t just make computations with space or time. But you must consider both together. Especially if you have a speed close to the speed of light (which is not infinite). In this new theory, that speed of light has a peculiar role: it is absolute. When you measure your speed relative to the source, no matter where you look at (if you look right into the beam or along it), you always get the same value: the same speed of light. Always. Really, always. You simply can’t go faster than that.

Step #2, Year 1907, Berne: The famous « most thoughtful » idea of Einstein: acceleration is indistinguishable from gravitation. Physicists call this the Equivalence Principle. We all stand in out feet on Earth’s surface. We feel gravitation. In fact, this can be re-stated as feeling a permanent acceleration, since Earth physical surface prevent us to follow a free-fall trajectory in space… Astronauts, assuming they have the eyes closed and forget for a second that they are in space, and there is truly no air, follow in fact a free fall trajectory. They don’t feel the gravitation. They feel no acceleration. Hence gravitation, similary to linear motion, can be « cancelled » in the right reference frame. When you travel in a plane, you don’t feel the motion during all the flight right? But people looking at you see clearly you move. Same thing, for acceleration. Einstein imagined this that a man inside a closed elevator having an acceleration equals to the value of Earth gravitation will feel exactly the same as a man not moving, but on Earth standing on his feet. Gravitation is equal to acceleration. And vice-versa.

Step #3, Year 1911, Prague: Imagine the same man inside that elevator going up with a constant acceleration (hence its speed is constantly increasing). Imagine a small hole in the side of the elevator where a beam of light can enter. Since speed of light is not infinite, and size of the elevator is non zero, it takes some time (pretty small, but still) for the light to enter the elevator and reach the other side. Since the elevator is going up, the point reached by the light is thus lower than the entry point. Hence the trajectory of light inside the accelerated elevator is … curved. But remember that acceleration and gravitation are equivalent?

Well, that is General Relativity, ladies and gentlemen: gravitation is curved space-time, period. And space-time is curved because of the presence of matter. All dynamically, which makes things even more beautiful…

Step #4, 1916, : In particule physics we know that when we shake electrons, waves are created. Electromagnetic waves, the other name for « light ». Einstein predicted that when shaking masses, this should also create waves. Gravitational waves. That was the prediction of the existence of gravitational waves. But Einstein never thought possible to detect such waves. Since one must need an enormous, in fact ultra-giganomous, amount of energy released to produce these waves, and even then, the effect would be ridiculously small.

Guess what LIGO detected? A merging of two 30-solar-masses black holes, releasing as much energy in a few microseconds as all stars in the visible universe, and the effect detected on Earth is of the size of one 10 000th that of a proton!

You can’t beat science in the awesomeness.

If you like this post, tell me about it! I may be to prepare some more. This one in particular is a worked out and personnally twisted transcript of the explanations of  french scientist and educator, Etienne Klein. Here is the video (in french).

## Une réponse sur “Understanding Gravitational Waves in 4 steps”

1. San Shepherd dit :

Nice article. Always good to read something that makes science easily consumed by the average brain… Making science consumable is something I am passionate about.

I’d love to understand the concept of two light beams « hitting » each other sent from opposite directions… From the perspective of each light beam, how do you account for the lack of the other light beam seemingly travelling faster than the speed of light?