One of Einstein's many theories within Special Relativity is length contraction. When an object is moving close to the speed of light, it will contract.
| Two spaceships are traveling in deep space. The rocket is traveling at a velocity close to the speed of light; the shuttle is at rest relative to the shuttle. | |
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There are synchronized clocks set up at both ends of the rocket which will begin timing when the front sensor passes the sensor on the front of the shuttle. If the rocket measures the time it takes the shuttle to pass from the front to the back of the rocket, they can calculate the length of their ship. |
| As the shuttle passes by the blue sensor at the front of the rocket the clocks begin timing. The rocket clocks say that it took 20 microseconds for the shuttle to pass from the front to the back of the rocket. | |
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In this second situation, the shuttle is measuring the time it takes for the rocket to go by it. There is a single sensor at the front of the shuttle that measures the time it takes for the two sensors on the front and the back of the rocket to pass by. |
| The time recorded by the shuttle is only 18 microseconds. Therefore, the length of the rocket calculated by the shuttle and the length calculated by the rocket would be different. In this case, the rocket's length is shorter when calculated by the shuttle. | |