Einstein: Special Relativity


Albert Einstein’s name literally sends shivers down my spine.

This is the man who discovered physics as we know it. This is the man who filled in the gaps where even Newton’s laws of motion went wrong and expanded our understanding of the universe. This man was a genius in every right—even if his social skills were somewhat lacking.

By the way…I can’t help but notice this is my first post with actual photographs of the scientist in question, instead of portraits. We’re moving along, people…

So. To the point. Einstein is famous for taking revolutionary and widely accepted laws of physics—those that Newton figured out—and showing where there were some holes in the math. But Einstein wasn’t just an annoying critic.

He took it all a step further…and showed us how physics really works.

He came up with the idea of relativity.

I’ll admit it, guys. Einstein’s relativity is a complicated subject. It took me some time to get it down, and I still don’t completely understand it. I understand being intimidated by its complexity.

Don’t let that stop you.

We’re going to take it a little bit at a time. Over the next few days, I’m going to take you slowly and carefully through the entire theory of relativity. For this post, we’re going to take a look at special relativity.

Imagine that you’re driving in a car.


For the moment, pretend that you’re in the passenger seat so you don’t have to focus on the road. Look out the window and observe the passing landscape. Is it moving, or are you?

Of course you know that you’re the one moving. You’re driving a car, and the Earth is staying still.

Oops…have we forgotten that the Earth is actually moving?

The Earth orbits around the sun at 108,000 kph. That speed is almost unimaginable here on Earth. It takes a whole year to go around, but it covers a whole lot of distance in that time. Just the distance to the sun is 93 million miles.

Does the Earth just stop moving whenever you get in the car and go for a drive?

I honestly hope not, because that kind of sudden stop would splatter all the human beings on the side of Earth opposite the direction of its orbit. They’d get smushed as the surface seemed to rise up to meet them.

So the Earth is always moving. Very fast, in fact. But we can’t tell, and we’re so certain all the time that when we move around, it’s actually standing still.

We say that in a neighborhood, we drive at 15 mph.

15 speed limit.gif

But what does that even mean? Think about it—we human beings follow the Earth in its orbit. If we didn’t, we’d get left behind. So we’re also traveling at 108,000 kph. And yet we say that we drive 15 mph in a neighborhood?

Well, that speed is measured relative to the Earth.

Notice that word. Relative. It’s the key word on the theory of relativity (and in our current case, special relativity). So what exactly does relative mean?

Well, it just means you’re comparing something to something else. You could say that the ocean is a more greenish blue relative to the sky. Or that a certain tree is very tall relative to a medium-sized tree.

And you could say that your speed is a certain number relative to the Earth.

This is especially obvious when you’re in a rocket in deep space.

Imagine that you’re in a rocket, just drifting. You look out your window, and there is nothing to measure your movement against, nothing to tell you if you’re moving at all.

Then another rocket drifts past. You’re certain it’s the one that’s moving.

Or is it?

What if you’re drifting past that rocket?

You have no way to measure your speed except against that other rocket. But the other rocket also has no way to measure its speed except against you. So you’ll both come up with the same number for your velocity. And you’ll both think the other is moving, and you’re not.

That’s the wonder of special relativity. But it doesn’t end there.

I used Earth’s motion around the sun as an example of velocities we don’t notice. But what about the sun’s orbit around the center of the Milky Way Galaxy?


And what about our galaxy’s motion through the universe?

Our galaxy is hurtling towards its neighbor Andromeda at about 300 km/s. So are we, or we’d be left behind. But somehow we still measure our motion around the sun as only 108,000 kph…

(By the way, take a close look at those units. Our galaxy’s velocity looks smaller than that of Earth around the sun, but it’s in kilometers per second, not kilometers per hour.)

Special relativity means amazing things for the universe. Would you believe that the observed mass of an object—meaning, how big it appears to be—actually changes depending on its speed?

That’s right. And the higher its velocity, the greater the object’s mass.

You have probably heard of this relationship, even if you don’t recognize that description of it. It’s the concept behind a certain famous equation, perhaps the most famous equation ever proposed.


We don’t notice this on the level of the solar system’s speeds. But as velocities get closer to the speed of light, the effect is more obvious.

But I have to ask…what’s moving, light or the person observing it?

According to E = mc2, matter can be converted to energy. And because the energy made is equal to mass times the speed of light squared, it’s a huge amount of energy for very tiny amounts of mass.

This is what drives stars. It’s why they burn bright. Without E = mc2, solar systems quite literally would not work. Stars wouldn’t produce massive amounts of energy, and life in the universe would be impossible.

So as you can see, we owe E = mc2 our existence.

These are concepts I haven’t even begun to understand, but they’ve been tested countless times and proven as much as anything in science can be proven. They blow my mind. But we’re not done with the theory of relativity.

Einstein came up with a general theory of relativity, and that’s coming up in my next post.

About Emma

I'm a college student, astronomer, writer, Democrat, global warming activist, hardcore trekkie, daring adventuress, and dreamer. I have four blogs to date. The primary one is Science at Your Doorstep, but I also post to The Sojourns of Star Trek, Common Sense Politics (US), and Toleventures. I guest post to A Momma's View. I write for different reasons. Sometimes because I want to share my process of discovery with others. Sometimes because I think a story is too incredible to keep it to myself. Sometimes just because I want to be heard. Regardless, I hope you enjoy it!
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