Why does special relativity special?

TLDR; it’s only work with the special condition, inertial frame. This article will elaborate on why inertial frame is a thing.

You might have heard about Einstein’s theory of relativity. You might know that there are two versions of the theories; one is special while the other doesn’t. If you’re not familiar with the subject, you might get confused by the adjective. Just the word “relativity” itself is enough to make people confused and scared. Many people find the concept hard to understand and very counter-intuitive. Most people who just have heard about the concept would have questions like, why is there a special one and the general one, and which one is more complicated/simple.

The quick answer to this question is that the special one is easier and the general one is harder. Someone with a background in high-school physics can grasp the fundamental concept of special relativity, but hardly with the general one.

So why’s it special?. The answer to this question is that special relativity can explain physics in inertial frames of reference. It requires “special” condition. In other words, it can describe the phenomena in special cases. What are inertial frames btw? An inertial frame is the frame of reference that measures the position and time of an object while it is moving with a constant velocity, i.e., no net force act upon it. For example, an observer who is moving on the outer space can approximate as an inertia frame. In comparison, an observer who is sitting on a roller coaster can’t say to be an inertia frame.

Notice that we all live in the earth’s gravity field. And that’s the net force acting upon every observer on earth. Why are the benefits of studying something that never really existed? Indeed it does have benefits. The first benefit is that it showed a groundbreaking concept in physics, and the second is that there’s a lot of scenarios that can be approximate to be an inertial frame. The physics before Einstein’s theory of relativity and your high-school physics mainly concern with the inertial frame.

The inertial frame is easier to deal with because it holds the principle of relativity. Unlike the theory of relativity, the principle of relativity states that physics must behave the same independently of the frame of reference.

For those who do not become familiar with this idea, imagine yourself floating in the space. Nothing is surrounding you. You have no chance to tell how fast you’re moving, not even where you are. But if a second person is floating with you, suddenly you can tell if you and the other person how fast you’re moving “relative” to the other person and tell how far you are from the other person.

Let say you see your friend is moving closer to you at the rate of 5 centimeters per second. In the same way, your friend also sees you moving closer to them while they’re stand/float still. From this imaginary scenario, more stuff into space, and it becomes this universe. Velocities, positions are not absolute quantities but relative to those who observed them. Then the principle comes in and dictates that if there’s the correct physics, it should work with both observers since we can’t have one frame to be more truthful than the other. With this in mind, you can see that it is guaranteed to be the same physics just by having a constant velocity.

So these are reasons why the special relativity specials. It only explains physics in an inertial frame of reference. There’s no much scenario to satisfy the condition yet powerful enough to guarantee the same physics across the observers.