Lesson 9: The Theory Of Special Relativity Part I

The Theory of

 Special Relativity

Part I

In 1905 Albert Einstein published four papers while working in a Swiss Patent office.  He was only 26 years old.   On the Electrodynamics of Moving Bodies was the name of his paper on Special Relativity.  Electrodynamics refers to electromagnetism which we discussed in the last lesson.

Einstein asked, “In what frame of reference is electromagnetism valid?”  We learned in lesson 7 that motion is valid in any frame of reference, but what about electromagnetism?  Are the electromagnetic waves the same no matter where you are or how you are moving?

The answer is yes, and that is Special Relativity.

Special Relativity says all laws of physics are the same for all observers in uniform motion.  As we discussed in lesson 7, uniform motion is moving in one direction at a constant speed.  You can do physics anywhere you want and get the same results, including electrodynamics.

This led to some very strange consequences, especially for space and time.

Knowing that electromagnetism was also valid for any observer moving in a uniform frame of reference and that the speed of light was 670,616,529 mph, Einstein forever changed our concept of space and time.

For future reference, the speed of light is also known as c.

Therefore, if electromagnetism is valid for any uniformly moving frame of reference, then c must be measured at 670,616,529 mph by everyone.  Can you see where this is going?  You will.

The speed of light is measured the same for everyone, even if they are moving relative to each other. 

So, what does it mean to be moving relative to each other?  If I am driving my car and I pass you standing on the road, we are moving relative to each other.  Remember, driving in my car; I can just as easily say you are moving relative to me as I am moving relative to you.  If I drive by you at 60 mph, it is just as correct for me to say I saw you moving at 60 mph as it is for you to say you saw me moving at 60 mph.

Let’s say after driving past you, I continue in my car, heading north at 60 mph, and another car driving south passes me, also driving 60 mph.  I will see that car moving at 120 mph (60mph + 60 mph). We add our speeds. When the other car passes you, since you are standing on the road (0 mph), you will see it pass at 60 mph (60 mph + 0  mph).

Now here is where it gets strange.  If instead of a car, I pass a beam of light coming towards me and measure the light, I will measure the light moving at 670,616,529 mph.  Unlike when a car comes toward me, I don’t get a measurement of our speeds added together 670,616,529 mph + 60 mph.

Then when the speed of light passes you, you also measure 670,616,529 mph.


It doesn’t matter your speed; you are always going to measure the speed of light moving at 670,616,529 mph.

Let’s make it even more dramatic.

If you are standing on the road and measure a beam of light coming towards you, the light is going to be coming at you at 670,616,529 mph.  If I am in a spaceship zooming away at half the speed of light, it is still coming at me at 670,616,529 mph.  The light is catching up with me, zooming away from it in the spaceship just as fast as it is catching up with you standing still. 

Think about how crazy that is. 

Here is another example.  If you are playing tag, you are obviously going to catch someone who is standing still before you catch someone who is running.  But light doesn’t work that way.  Light will catch them both equally.


No matter what we are doing, we all get the same value for the speed of light.

Now let’s see what all this crazy light stuff means for space and time.

First, you need to know what an event is.  An event tells you a place and a time.  For example, your birth is an event.  It occurred at a specific place and time.  Just a date, for example, October 18, 1972, is not an event.  You need a place to go along with it.

Look at the image above. It represents a light clock.  A light clock is a box with a light source at one end and a mirror at the other.  A pulse of light leaves the light source, hits and reflects off the mirror, and returns to the source.  This gives us two events, the light leaving and the light returning to the source.  This makes a clock.  Tic going up. Toc coming back.  We want to know how long the time is between the two events.

We are going to start in a frame of reference where the light clock is at rest, not moving.  We know the light goes up and back.  So, we know the distance the light travels is twice the length of the box.

But not let’s say you are in a car with the light clock and you are driving down the road. You see the light clocking moving straight up and down.  You drive past your friend, and your friend sees your car and the light clock moving to the right.  Now because the light clock is moving to the right, your friend sees the light is moving diagonally when it hits the top and diagonally when it returns to the bottom.  Because the path is diagonal, this makes the light path longer.  But remember, light always moves at the same speed.  So, if light is moving at the same speed, it must take more time to take a longer path than a shorter path.  Just like if you are driving 60 miles an hour, it is going to take you longer to drive 60 miles than 30 miles.

This means the time in the two events is different.  Time is shorter for the person driving in the car. Understand this has nothing to do with the clock.  The clock is keeping perfect time.  It is just being seen from two different perspectives.

This is called time dilation.  Time is different in different frames of reference.  Clocks do not keep the same time when moving relative to each other.  Time depends on your point of view.

Impossible? Nope!

What causes time to dilate? 

The speed you are moving.

The faster you move, the slower time moves relative to another person.  This does not mean you feel yourself moving slower. For you, everything is normal.  It is as it should be.  It is all relative.

The reason we don’t really experience time dilation is because we move too slow.  You have to move at speeds close to the speed of light to notice any effect.  Since the speed of light is 670,616,529 mph, and the fastest most of us move in a day, maybe 60 to 70 mph in our car (some of us a little faster), we are nowhere moving close to the speed of light and experiencing time dilation.  Okay.  Have we blown your mind enough today?  Are your dendrites firing like crazy?  What you learned today is nothing compared to our next lesson.  What if we told you time travel is possible?

We know this lesson can get confusing.  

Before tomorrow draw your picture and teach someone this lesson.