Introduction--What Relativity is

What is Special Relativity?

What is General Relativity?

Also see:

The Twin Paradox and Why Relativity Works

What Relativity Is

Relativity is a catch-all phrase for both the theory of special relativity and the theory of general relativity. Albert Einstein is the father of both theories, even though special relativity has it's roots in earlier work.

Contrary to popular belief, the theory of relativity does not say that everything is relative. It does say that the speed of light is constant. Since light is constant, special relativity reasons, things that we once thought were constant, namely length, mass, and time, are not constant.

What do we mean by constant? Think of it this way: in the newtonian model, no velocity was constant. For example, if you see someone riding their bike, you might say that they are going 15 miles per hour in relation to you. However, if you then started jogging along side of the bike, you might say that the bikes velocity is only 5 miles per hour, because you are running 10 miles per hour. Velocities are relative. In relativity theory, this is still for the most part true, with the exception of light. If you are not moving and light is riding a bicycle (for discussion's sake) toward you at the speed of light, you would measure it's speed as the speed of light. However, if you then started jogging along side of the bike, you would still measure it's velocity as the speed of light, even though you are now going 10 miles per hour. In fact, you could be going a million miles per hour in relation to your original position, but you would still measure the speed of light the same.

That's not all relativity tells us. Relativity also tells us that and that gravity is the net effect of the curvature of space-time as a result of mass.

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What is Special Relativity?

Special relativity is the theory published by Einstein in 1905. Specifically, special relativity says that light is constant, and as velocity increases length decreases, mass increases, and time slows down. See Why Relativity Works for a detailed explanation.

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What is General Relativity?

General Relativity is a "generalized" and enhanced version of special relativity. General relativity describes the same odd behavior at high velocities as special relativity, but adds a twist. General relativity throws in gravity.

Einstein realized that there was no difference in the force of gravity and the force of acceleration. For example, someone in a rocket ship without windows cannot tell whether the ship is a rest on Earth or is accelerating through outer-space. The net effect (the person's feet pushing against the floor of the spaceship) is identical. If gravity has characteristics of motion, then strong gravitational feilds must make matter behave similar to high velocities.

The "general" comes from the way the theory explains the phenomena. General relativity can theoretically explain any scenario in space-time.

What is meant by "the curvature of space-time"?  To know that, we must first learn what space-time is.

Just about everything you can see is made out of atoms. Those atoms combine to form molecules, etc. Air is made out of atoms, water is made out of atoms, just about everything around us is made out of atoms. Imagine if we could see down to the atomic level. We could see all the atoms that make up everything. But what is in between the atoms? Nothing?

The answer is space-time. Time is an important part of it, because the theories of relativity tell us that as you move through space you also move through time. Einstein thought of space-time having four dimensions: up-down, east-west, north-south, and a fourth dimension that is time multiplied by i  ( i  is defined as the square root of -1).  Thus, you have all the dimensions of space and one time-like dimension that makes up space-time. How does it curve?

In this illustration, space-time is like a rubber sheet on which a massive ball is placed. The mass of the ball "warps" space-time.

This is a profile of the same illustration.

On the right you can see an illustration of this concept. Imagine space-time is the grid, and the blue sphere is something that is massive, like a star. The mass of the star causes space-time to curve. The greater the curve, the greater the attractive force of gravity. Notice that space-time is more curved closer to the object.

In the lower illustration, imagine that you are walking on the top straight line of the grid. You can measure the strength of the force pulling you closer to the ball by measuring the distance from the top straight line you are standing on to the bottom curved line. As you walk toward the ball, the distance between the line and the curve increases, thus the strength of the force increases.

This, of course, is not a perfect illustration, as it is a 3 dimensional representation of a 4 dimensional concept.

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 History Behind Relativity It's not just Albert. The Mathematics How it works quantitatively. Real Life Applications What is this good for anyway? The Men Behind the Science  Biographies of your favorite scientists. Interactive Illustration - The Twin Paradox Jim and Bob demonstrate the Twin Paradox. Why Relativity Works Jim takes a train trip and Bob disagrees with him. Have a question on this page? Post your questions about relativity--anything on this page.