# A Description of Albert Einsteins Theory of Relativity

Albert Einstein first theorized relativity in 1905. Since then it has changed everything we thought we knew and has helped us learn what we know now. There are two types of relativity that we know of, Special Relativity (SR) and General Relativity. I’m mostly going to focus on Special Relativity because it’s my favorite. To help me better explain this I’m going to use a paradox to show this principle. It is called The Pole in the Barn. I will first explain what this paradox is and then explain how it works. A pole-vaulter runs while holding a pole parallel to the ground at very high speeds. In order for this to work we must assume that he can run at near the speed of light. He approaches a barn that is the same length as his pole. The front and back doors are wide open for the runner to pass through.

We know that if he was running at a normal human speed the back of the pole would be just entering the barn while the front of the pole is just exiting. But because of SR the pole will seem to shrink enough to be in a position that, if you had a button that would close both doors simultaneously, the pole would fit fine in between the doors without hitting anything. This is of course from the perspective of the guy inside the barn. SR tells us that if we were this guy in the barn then the pole would appear shorter than the barn. Though relativity tells us something else as well: that all motion is relative. This is shown by this example. Say there was a train. A passenger on the train sees a man walking by him in the direction the train is heading. To him the man is walking slowly past him. But to people on a train station the train is passing, the man would seem to be going faster than the train. In this case the speed of the walking man is relative to those who are observing. It can also work like this. The man sitting on the train could argue that it is not the train that is moving but that they are stationary and the ground is moving past them at that speed.

It would be like watching a person run on a treadmill. Now back to the pole and the barn. From the point of view of the pole-vaulter he could argue that it is not him moving at near light speeds but the barn moving towards him at that speed. Now from his perspective, because of SR, the barn would seem to contract, thus making his pole longer than the barn. Now here is the paradox. From your perspective inside the barn, you are able to shut both doors simultaneously because the pole is shortened. But from the pole-vaulter’s point of view the pole is longer than the barn which means the doors are unable to close simultaneously. So who is right? You or the pole-vaulter? Now I will explain Einstein’s theory to you in two parts known as Einstein’s two postulates of Relativity. First, all motion is relative. This one I have already explained. Second, light moves the same speed no matter how fast its source is moving. This is just like the Doppler Effect but with light waves instead of sound. Also light needs no medium to travel through. These seem innocent but together they are groundbreaking ideas that totally flipped science on its head. To show you this I’d like you to consider two rockets travelling towards each other at near the speed of light.

Now each one could argue that they are stationary and the other is moving towards them. Now, an astronaut in one ship shines a light toward the other ship and, claiming to be stationary measures the speed of the light they are shooting out at the regular speed of a billion kilometers per hour (kph). At the same time the other astronaut can claim that they are stationary and measures the light coming towards them at exactly a billion kph because light travels at the same speed despite how fast its source is moving. This seems paradoxical because they cannot both claim to be stationary because they are moving and yet light still travels the same speed even though they are traveling at near the speed of light themselves. Now the shrinking of distances has been proven through a common high school physics lab. This lab involves measuring the amount of muons that reach the Earth’s surface. Now the muon has a very short life p not nearly long enough to travel the distance between the atmosphere and the Earth despite being so energetic that they travel over 99 percent light speed. But when you set up the detectors, surprisingly almost all muons appear to make it to the Earth’s surface. Now if we look at it from the muon’s perspective it would tell us that it is traveling at near the speed of light or rather the ground is coming up to meet it at near the speed of light and it seems to have plenty of time to reach the Earth. This can only mean one thing: Time must be running more slowly for the muon than it is for us. So consider this: First you and the muon agree on the speed that it is traveling; Second, the muon says its journey takes less time than you think. So to balance the books the muon must also be covering a shorter distance. That is, if the muon is traveling at some speed you both agree on and it is able to cover the distance in less time, then it must see that distance as shorter than you do.

This is known as length contraction. It states that, just as fast-moving objects look shorter than the same objects do when standing still, so distances to be traveled look shorter when viewed from the perspective of the fast-moving objects. Now to put it all together. Remember that you are standing inside the barn watching the pole-vaulter come towards you at high speed. You know that, when stationary, the pole is the same length as the barn. But now it seems to be shrunk to where it can fit with both ends in the barn simultaneously. In fact, there would be a split second where you could close both doors and trap the pole inside. However if we look at it from the pole-vaulter’s perspective it is not him that is moving but the barn that is moving towards him at high speed. Therefore he sees the barn in a squashed state where it would be impossible to close both doors at once. So who is right? In fact, both of you are. The resolution lies in what we mean by simultaneous events.

We said that you could quickly shut both doors to where they’re both shut at the same time and then quickly open the rear door again to let him through. Now, from the runner’s perspective, he would run in and see the rear door close and then open again just in time to let the front end of the pole make it through. Then a short time later the front door would close as the back of the pole enters the barn. From his perspective both doors would close but not at the same time. This phenomenon of the ordering of events looking different to different observers is another principle of Einstein’s Relativity Theory. So, in conclusion, Einstein’s theory of special relativity can be broken down into a few main principles. First, the principle that all motion is relative. Second, that light travels at the same speed independent of the speed of its source. Third, The closer you go to the speed of light the shorter you are from the perspective of an outside source. Fourth, the closer you go to the speed of light the shorter distances seem to you and the slower time moves around you. And fifth, the ordering of events looks different to different observers.