Compared to What?

“Reality is merely an illusion, although a very persistent one”
-- Albert Einstein --

This first part could be considered a continuation of the discussion of Light (see Let There Be Light and The Most Fantastic Scientific Experiment Ever), or as preparation for looking at the Special Theory of Relativity. Take your pick. But regardless Light is such a fascinating phenomena that there always seems to be more to say about it.

Toward the end of the 19th Century scientists thought they pretty well had physics and the laws of the physical universe wrapped up. They had sailed along for almost 200 years on the laws of mechanics developed by Newton, with many other contributions along the way, and had Electricity and Magnetism pretty well finalized with the work of Maxwell, namely Maxwell's equations. In fact, Lord Kelvin, one of the giants of the era said, "There is nothing new to be discovered in physics now, All that remains is more and more precise measurement." Although one of the greats in physics, how wrong he was.

There was one major discovery that threw the world of science, in particular , physics into a state of turmoil. It went topsy-turvy; it drove them crazy; they went gaga. There was chaos, disorder, befuddlement, and pandemonium. Well, you get the idea. What was this discovery? Simply put, that light has the same speed no matter what frame of reference you view it in. Or put another way, all observers will always measure light at same speed. What's the big deal you say. Very big indeed, but this may take some explaining.

First, let's define a frame of reference? If you are traveling in a car and toss a ball up in the air, you would notice that the ball would go up and down as normal. Your frame of reference is the car and everything in the car that is moving at the same speed as you. But someone standing on the sidewalk watching you and the car go by at say 25 m.p.h. would see the ball make an arc as it went up in the air and landed back in your hand. He would perceive the ball traveling at 25 m.p.h. horizontally, as well as moving up and down.

Another example. Suppose you were walking along the sidewalk at 5 m.p.h. Your frame of reference is your body and everything attached to it. Now suppose there is a bus traveling on the street in the same direction as you, at 30 m.p.h. The bus and everything in it is another frame of reference. If you were standing still, you would perceive the bus as moving away at 30 m.p.h. But if you were walking in same direction as the bus, you would perceive it moving away at 25 m.p.h. So the speeds are additive/subtractive depending on direction.


This principle of adding/subtracting velocities runs throughout the physical world and physics as a science. It works for all kinds of phenomena. If sound comes from a train which is traveling toward you at 50 m.p.h. the sound will hit your ear at the speed of sound + 50 m.p.h. If you are in a boxcar on a train traveling at 60 m.p.h., and you throw a ball to someone on the forward end of the boxcar with a speed of 30 m.p.h., an outside observer would measure the speed of the ball as 90 m.p.h. (Though in your frame of reference inside the box car you would just see it moving 30 m.p.h.). This addition of velocities is universal, except for one exception.

Let's look at what would happen if we were in a rocket going 100,000 km/s, about 1/3 the speed of light. Let's say we are moving in the same direction as a light beam. It is moving at 300,000 km/s, the speed of light. We would expect by the law of addition/subtraction of velocities that we would measure the light only traveling at 200,000km/s.


Wrong! It is still the speed of light. No matter how fast we go, or how slow we go, the measured speed of light remains the same. Another example. Say your are in your space ship going 1/2 the speed of light. You decide to turn on the light on the nose of your ship and shine the light out ahead of you. You would expect that an observer off in the distance would measure the speed of the light beam leaving your ship to be 1/2 speed of light (your ship) plus speed of light (light beam) = 1 1/2 speed of light. Nope, it is still measure at exactly the speed of light. What gives?

So you see the physicists conundrum. This phenomena disobeyed all known laws of physics concerning velocities worked in the physical universe. Scientists developed all kinds of experiments to test this unbelievable fact, but no matter how clever they were they could not get around the conclusion that the speed of light, or c as it is symbolized, is always the same for all observers.

This was one of the two premises that Einstein assumed when developing the Special Theory of Relativity. Despite its unpopularity and attempts to explain it away, it was an observable datum that could not be ignored.

The other premise was that all the laws of physics apply equally in inertial frames of reference. We have already talked about frames of reference. Adding the word inertial comes from the concept of inertia, namely that an object that is moving in a set direction will tend to continue in that direction with the same speed. Or an object at rest will tend to continue at rest. Well this is the same with an inertial frame of reference. It is a frame of reference which is at rest and not accelerating, like sitting in your house, or a frame of reference which is moving in the same direction and at the same speed, like driving down the road in your car at a constant speed. If the frame of reference is accelerating, i.e. changing speed or direction, it is not considered inertial. This is what makes the 'Special' Theory of Relativity special. The fact that it only applies to inertial frames of reference not accelerating frames of reference. The 'General' Theory of Relativity deals with accelerating frames of reference. So the second premise says that the laws of physics apply equally well to bouncing a ball on the floor of a moving car as to bouncing the ball on the floor of your house. Identical physics and results; no difference.

So, just based on these two premises Einstein used his mind, imagination, and Gedanken (thought) experiments to carry these propositions to their logical conclusions. Little did he know when he started that he would literally open up a Pandora's Box for physics. We will look at some of these startling conclusions in the future.