Benny Labamba

What is Special Relativity?

I that one reason I reject the Kalam Cosmological Argument is because it requires an A Theory of time, but most physicists reject the A Theory. Why? The first reason is special relativity. So last time, I claimed that . Now I explain Einstein’s theory itself.

One key idea in relativity theory is that of a reference frame. Your reference frame is, basically, everything that shares your state of motion. So if drive past and you measure my speed with radar, and you tell me my speed was 70mph, what you mean is that I was moving 70mph relative to your reference frame. You and the Earth and the radar were all in the same state of motion, and I was moving 70mph relative to your reference frame. But I may have been moving quite a bit faster relative to the reference frame of the Sun, for example.

Or if you’re traveling on a plane at 600mph and you say you’re going to walk down the aisle at 2mph, you don’t mean you’re going to walk at 2mph relative to the Earth. The Earth does not share your state of motion; you’re not in the same reference frame as the Earth at that time. Rather, the things that share your state of motion at that time are the plane, the other seated passengers, the luggage, and so on. So you’re saying you’re going to get out of your seat and walk down the aisle at 2mph relative to the plane, the seated passengers, and the luggage. You’re going to walk down the aisle at 2mph relative to your reference frame.

And as we discussed , the laws of motion are the same for any reference frame in uniform motion. You can play tennis on a planet traveling at 50 million mph (relative to Earth) just as well as you can on Earth.

Let there be light

Here’s another question: Are the laws of electromagnetism () the same for any reference frame in uniform motion?

Maxwell’s equations predict (correctly) that the speed of light is 671 million miles per hour. So an equivalent question is: Is the speed of light 671 million mph for any reference frame in uniform motion?

I’ll tell you what my intuition says. My intuition says that the speed of light depends on the speed of the observer.

Think about it. If you’re standing still and I drive toward you at 70mph and shoot you with a Nerf dart gun, that foam dart is going to hit you a lot faster than if I shoot you with the dart gun while standing next to you. Why? Because when I’m driving at you, your speed (relative to the Nerf dart) is the speed of the dart after it leaves the gun, plus the speed of the car.

Intuitively, it seems to me the same should be true of light. If light normally “fires” at 671 million mph, I would think that means it will come toward me at 671 million mph when sent from a light bulb that is stationary (relative to me). But if a distant star is moving toward me at 300 million mph, then the speed that its light is “fired” toward me should be 300 million mph + 671 million mph, right?

This is true of other kinds of waves. A sound wave travels at 768mph relative to the air it is disturbing, and the shock wave of an earthquake moves at a certain speed (let’s say, 400mph) relative to the rocks and dirt it is disturbing. So if I was in a spaceship and a volcanic moon was coming toward me at 200mph, and one of its volcanos suddenly erupted and sent out a shock wave at 400mph (relative to the moon), then the shock wave would approach me at 200mph + 400mph, or 600mph.

So this is true of electromagnetic waves like light, too, right?

Microwaves on another planet

Before we answer this, let’s first ask: what is it that electromagnetic waves are disturbing? Sound waves are disturbances of air, volcanic waves are disturbances of ground, water waves are disturbances of water. But light seems to travel through empty space.

Well, it must not be empty space, then. There must be a very light, invisible substance pervading all of space that light can be a disturbance of in order to move. Early physicists called this substance the “ether.”

But now, we have a weird difference between the laws of motion and the laws of electromagnetism. The laws of motion are the same for any reference frame in uniform motion. But the laws of electromagnetism are not, because they must always be the same relative to the ether. So if I’m moving 100mph relative to the either, and you’re moving 500mph relative to the ether, then electromagnetic waves are going to work differently for each of us.

But hold on a minute. Remember that you can play tennis on a planet in a distant galaxy that is moving at 80% the speed of light relative to Earth. That’s because the laws of motion work the same for any reference frame in uniform motion. So here’s another question: Can you successfully use a microwave oven on this distant planet?

Your intuition probably says “yes,” and you’d be right. You don’t have to adjust for your speed (relative to Earth) in order for the microwave to work properly. And yet the microwave is operating with electromagnetic waves!

So if you can use a microwave on this distant planet moving at 80% the speed of light relative to Earth, and if you can use a microwave on Earth while moving at 80% the speed of light relative to that distant planet, then the laws of electromagnetism must work the same for any reference frame in uniform motion, just like the laws of motion do.

And that’s all Einstein’s theory of special relativity says. Just like the laws of motion are the same for any reference frame in uniform motion, so too the laws of electromagnetism (and therefore, the speed of light) are the same for any reference frame in uniform motion.

Conflicting intuitions

So my intuition that the speed of light (relative to me) depends on the speed of the observer was wrong. The speed of light is the same for all reference frames in uniform motion. My second intuition – that I could use a microwave oven on a distant planet moving very fast relative to Earth – turned out to be correct. I just hadn’t noticed that these two intuitions were in conflict.

Now one might wonder: If all special relativity says is that I can use a microwave oven on a distant planet moving very fast relative to Earth, then why do people think it’s hard to understand special relativity?

The thing is, once you accept that you can use a microwave oven on that distant planet, then some very odd implications for space and time arise.

More on those, later.

  1. As before, I’m currently following along with on relativity. []

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