Demystifying Special Relativity, Part 1

This is the first part of what will be a multi-part series on Einstein's Special Relativity, which is probably the single most important law for understanding modern Physics and one that is not as complicated as some people think. I'll try to avoid the math here; after all, this is a Physics blog, and Physics is primarily about concepts.
I'm putting a few entries on relativity here because I feel like you need to understand it in order to really understand most of the news entries about modern Physics and Astronomy I will be posting. I know you can read all this information off of Wikipedia and other sources, but I'll try to make it more digestible for non-scientists and you can leave comments if you have any questions.

Galileo: The first principle of Relativity


Galileo's sketch of Jupiter and some of it's moons orbiting it, showing that not every celestial body goes directly around the Earth

Galileo was the first person to propose a theory of relativity. His theory of relativity (sometimes called Galilean Invariance) basically states the following

Newton's laws hold in all inertial reference frames

Let's look at this carefully. Newton's laws are the basic laws of mechanics and kinematics. I'm not going to go in depth into what these laws are but you check them out in more detail here.
An inertial reference frame is one that is moving at a constant velocity. That is, it has a constant speed and a constant direction.
An example of what Galileo's Principle says is that if you are playing table tennis on an inertial train, the game will have to follow the same rules of Physics as a table tennis game played on the ground that the train is moving relative to. This idea of relative motion is really important in both Galilean and Special Relativity. This is because relativity implies that there is no absolute state of motion. If you are on the train traveling at speed v relative to the ground, are you the one who is moving or is the person on the ground you see moving past you at speed v? From your perspective, the guy on the ground is moving at speed v away from you. That's your frame of reference in this situation. From the ground the guy think you are moving at speed v. So, the important point is that there is no absolute motion.
So that is the original principal of relativity. That's it.

Einstein's Great Idea
So what was Einstein's ingenious idea that makes most people think that this theory is so hard? What is the incomprehensible, fantastically amazing, conjecture that Einstein made.

Galilean Relativity said that the laws of motion were the same for all inertial reference frames. Einstein's Special Relativity says that the laws of motion and all other laws of Physics were the same in all reference frames.

That's it. Most of the weird consequences that pop out of Special Relativity come from one law of Electromagnetism. That law is:

Electromagnetic Radiation (including visible light) travels at speed c=299,792,458 m/s=670,616,629.384 miles per hour

Scale model of light traveling from Earth to Moon; the time for light to get there is correct.

The law of special relativity is not so much about things being relative as it is about one thing being constant (the speed of electromagnetic radiation). In order to keep this quantity constant, the static character of space and time has to be given up. In other words, space and time have to change so that the speed of light doesn't change. I will explain how they do this in the next post.

I think that's enough for today. Think about that and I'll continue this ASAP.