A Simple Explanation of General Relativity - Part One - Special Relativity

Special relativity was the precursor to general relativity.

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The Speed of Light

Originally people thought the speed of light was infinite. We are too slow to see it move, so it always seems instant. After many “inconclusive” experiments, the first evidence light had a finite speed came from Ole Römer. The problem with prior experiments is that the space the light had to cover was too short for us to measure the difference in time. Ole Römer observed Io, a moon of Jupiter, and found small time differences in the times of its eclipses. These time differences were due to changing distance between Jupiter and Earth. [2]

Scientists also knew light was a form of wave. That combined with the constant speed lead to the conclusion that the universe is permeated by a substance called aether. This aether was supposed to be the medium light travelled through, and would serve as an absolute reference point for space. In experiments designed to measure the speed of Earth relative to the aether, it was found light travelled the same speed in both directions. Many excuses were made defending aether. The most common was how Earth must drag along aether and bringing it to the same speed. Special relativity was the last nail in the coffin for aether. [3]

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Light and Relativity

Special relativity started with one assumption. The speed of light is constant in empty space. It turns out that this assumption rewrites what we know about space and time.

Velocities of objects are usually relative. One example is somebody sitting on a train. Unless you are accelerating, you don’t feel any movement, but it looks like everything outside is moving. When you drop a ball it bounces off the floor and straight back up, with no ground covered horizontally. Anybody watching the train from outside would disagree with that last assumption. They would say there was horizontal ground covered by the ball, because the train is moving. It turns out they are both right, when speaking from their frame of reference. Every object is moving relatively to each other object. Light breaks this rule.

The speed of light is constant in empty space. The person on the train and watching from afar will both agree on the speed of light, wherever it comes from. For this to happen something must give.

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Space and Time

It turns out in this situation time is what gives. As an object’s velocity increases, its speed through time decreases. This is called time dilation. This means something moving quickly relative to you will look like it is going slowly in time. As any object approaches the speed of light its speed through time approaches zero. Since velocity is relative, you won’t actually feel time dilation, while it will look like everything else does.

One example to conceptualise this is a train approaching the speed of light. A beam of light shining towards the front will still go ahead just as fast. You are not moving relative to yourself or the train. Another frame of reference, somebody outside on the station for example, would see the lightbeam go ahead at the same speed. There is one key difference, they see that it is only going a little faster than the train. They will also see everything on the train move slower through time. For every minute that passes on the platform it might look like only a few seconds to anything on the train. This allows it to come full circle, for the same amount of time the distance between objects on the train and the light beam would increase at the speed of light.

The heart of special relativity is that the velocity of a particle moving through space-time is always the speed of light. This is what causes time dilation to occur. There are diagrams that allow us to easily see this.

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Special relativity is not the whole story, we still haven’t added gravity. Gravity turns special relativity into general relativity. I have also omitted some information on frames of reference, because I feel those would be too complex for many readers here.

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