Lesson 27: The General Theory Of Relativity

The General Theory Of Relativity

We have discussed a lot about Einstein’s Theory of Special Relativity.  Einstein not only developed Special Relativity but also what is known as The General Theory of Relativity.  Einstein published his work on General Relativity on November 25, 1915.  The General Theory of Relativity has to do with gravity.  We often hear people say when talking about gravity, “What goes up must come down.”  There is actually more to it than that.

You will see there is a very close relationship between gravity and acceleration.

Newton said gravity was a force.  All objects attract each other. The more mass an object has, the more attractive it is. The closer an object is, the more attractive it is.  According to Newton, this is why when we jump, we are pulled back down to the earth. According to Einstein, there is actually more to it than that. 

To explain, let’s say you enter an elevator and head to the top of a tall building. Suddenly the cable snaps, and the elevator begins to fall. Look at the image above.  For you, it will seem as if everything is floating in the elevator. You can pull some change out of your pocket, and you will see the change floating in front of you as if you were in outer space with no gravity. You are taking away gravity when you fall.  It is exactly as if you are in outer space.  Unfortunately, you are not in outer space, and the elevator is going to hit the ground, and the consequences are not going to be good for you, so we are going to stop talking about this before you hit the ground.  You are safe.

Now pretend you are in another elevator. Look at the image above. This time you are safe on earth, flat on the ground. If you pull out a weight and drop it, what happens?  It falls to the floor of the elevator.  Because of gravity, right?

Now let’s say you are in the exact same-looking elevator in a space ship in space with no gravity.  What happens when you release the ball?  It floats. 

But now, let’s have the spaceship accelerate upwards.  If you drop the ball with the spaceship accelerating up, the floor of the elevator will rise to meet the ball.  It will appear exactly the same as if you dropped the ball on earth and gravity pulled it down.  If you are in one of the elevators, unless you are told, you have no way of knowing whether you are on earth or accelerating in space.  Assuming no windows, there is absolutely nothing you can do to tell the difference.  This shows that acceleration and gravity can be considered the same thing.

This led Einstein to conclude that gravity is not real. 


It goes back to relativity. Remember, in Special Relativity, and we said it doesn’t matter where you are there is no special place or state of motion.  It doesn’t matter how you are moving.  If you are in a rocket ship zooming to the moon, the physics is the same for you as it is for me.  Relativity says there is no place in the entire universe that is special.  So if we can put ourselves in a state of motion, as in the free-falling elevator, where we don’t experience gravity, then as Einstein said, gravity cannot be real.  

But it has to be real, right? If I jump, I get pulled back down to the earth.  That is gravity, right?

Einstein said gravity is not a force but actually has to do with geometry and the curvature of space.  I know what you are saying. What in the world is the curvature of space?

Wave your hand back and forth in front of you.  That “empty” space that your hand is moving through can actually be curved.  Very, very hard to picture, right?  Can you picture it at all?  We will try and help you do that and see what that means for gravity.

Look at the image above.  Picture yourself standing on top of a big round ball.  If you could walk around the ball without falling off, you would not be walking in a perfectly straight line. You would be walking in a curved line.

So what causes space to be curved?

Matter and energy cause space to be curved.  Matter moves along this curved space, and this is what we experience as gravity. The more massive the matter, the more the space is curved.  This is why the earth seems to have a bigger gravitational attraction than we do. We don’t curve space hardly at all.  Stick with me; I will help you picture this a lot clearer.

First, let’s really make this interesting. Do you remember the lesson where we said we live in three dimensions of space and one of time?  We live in a four-dimensional universe where time and space are inseparable.   Einstein called this space-time.  So when we say matter and energy cause space to be curved, and we know space and time are inseparable, it then must cause the time to be curved as well.  It curves space-time. What in the world does it mean to curve time?  We will discuss that later.

Look at the image above.  It will help you get a grasp on what it is like for space-time to be curved by the earth.  Again I know it’s hard because we think of space as empty nothingness.

Look at the image above.  Picture a trampoline.  Let’s say the stretchy part of the trampoline is space-time.  Now, if we put a large, heavy ball in the middle of the trampoline, it is going to look like the picture above.  It causes the trampoline to sink in and curve the space around it.  If we were to take a smaller ball and roll it onto the trampoline and the ball enters the area where the trampoline starts to curve in, the little ball is going to find itself moving towards the big ball.  Now, if we couldn’t see the sunken curved trampoline like we can’t see space-time, it is going to look to us as if the large, heavy ball is pulling the little ball towards it.  In actuality, there is no attraction. All the little ball is doing is moving through curved space the only way it can, just like when you were moving on the sphere in the example above.

This is Einstein’s General Theory of Relativity. 

Think about the genius of Einstein. Einstein discovered General Relativity by just using his mind. During his time, there was no experiment he could perform to prove his theory.  But today, we know beyond a shadow of a doubt he was correct. That is because of the invention of The Global Positioning System (GPS).  We all know GPS can pinpoint our exact location.  The thing is, when making calculations, GPS has to take into account the curvature of space-time, or it will not put us in the right location.  And where do the calculations come from?  The math equations Einstein developed in General Relativity.  How connected was Einstein with infinite intelligence? 

Next time we are going to talk about what it means for time to curve and also black holes.  I know you will enjoy the next lesson.