Predicting Gravitational Waves

Einstein's Special Theory of Relativity

In the late 1800’s, many scientists were comfortable with the existing description of the universe. In fact, many of them thought physics research was winding down and that all they had left to do was work out the details. The problem was that many of the “details” were observations or experimental results that couldn’t be explained by current theories. One such “detail” was that experiments always measured the speed of light as 3x10 ⁸ m/s (186,000 miles per second, or mps).

According to everyday experience, it seems that the speed of light should change depending on how fast you are moving. Imagine a car with a “For Sale” sign in the window going 55 mph.

• A person standing still on the side of the road would see a car flying by and the sign would be a blur.
• A person driving next to the car at 55 mph could look out the window and copy down the phone number from the sign.
• A person going down the road in the opposite direction at 55 mph would zoom by the car and probably not even know the sign was there.

The speed we see things moving at depends on the difference between how fast the other person is going and how fast we are going.

• The person standing still sees the car going at 55 mph.
  55 mph (sign) – 0 mph (person) = 55 mph.

• The person in the car next to the sign sees the car going at 0 mph.
  55 mph (sign) – 55 mph (car) = 0 mph.

• The person going the opposite way sees the car going at 110 mph.
  55 mph (sign) - -55 (car) = 110 mph.

Because of this, scientists assumed that if you measure the speed of light in different directions, you should get different speeds since the earth is in orbit around the sun. In 1895 Albert Michelson and Edward Morley performed this experiment and surprisingly, saw no difference in the speed of light for different directions.

Albert Einstein resolved this puzzle in 1905 by suggesting that unlike the speed of a car, the speed of light is the same no matter how fast the observer is moving. In other words, even if you ran very fast you would not be any closer to catching up with a light wave than someone standing still. The first principle of Einstein’s Special Theory of Relativity is that the speed of light is always the same regardless of the motion of the observer or the light source.

Einstein also realized that although people see things differently (for example the speed of the “For Sale” sign), the laws of physics have to be the same for all observers. Someone riding on a train should be able to do the same experiments and get the same results as someone sitting in a classroom. If this were not true, people would get conflicting answers about how nature behaves depending on their motion – but nature does what nature does, it can’t follow different predictions depending on who is observing it. Therefore, the second principle of Einstein’s Special Theory of Relativity is that the laws of physics do not depend on the motion of the observer as long as the observer is not speeding up or slowing down.

Principles of the Special Theory of Relativity

1. The speed of light is always the same regardless of the motion of the observer or the light source.
2. The laws of physics do not depend on the motion of the observer as long as the observer is not speeding up or slowing down.

If these two principles are true, Einstein showed that motion must affect distance and time. Imagine two people watching a beam of light. If one person is standing still, the light will look to her like it is moving at a speed of 186,000 mps. If the other person is in a spaceship traveling 100,000 mps, he still sees the light moving at 186,000 mps. How can this be true? Only if space and time are not absolute.

The traditional views of space and time were that they were two independent, fixed quantities. According to Einstein’s theory, however, an object in motion is shorter than when it is at rest. Similarly, a clock in motion ticks more slowly than a clock at rest. In fact, Einstein concluded that distance and time are more accurately described as one thing – spacetime.