Albert Einstein's special theory of relativity, published in 1905, predicts counterintuitive effects that occur when objects are traveling close to the speed of light. On what did Einstein base these predictions?
Einstein first assumed that the fundamental laws of physics were the same in all reference frames moving at a constant velocity. Einstein's second assumption was however more significant.
As stated in his original 1905 paper on special relativity, Einstein assumed that the speed of light in a vacuum is a fundamental constant that does not vary as the speed of the light source varies. An immediate consequence of Einstein's assumption is that observers in any reference frame that is not accelerating will measure the same value for the speed of light.
Thought Experiment with Trains as an Analogy
Einstein liked thought experiments involving moving trains; so, to understand the meaning of Einstein's assumption, think about moving trains. Let's say that an observer at rest beside the train track measures (Don't worry about how.) the speed of a train as 100 kilometers per hour.
If a second train on a parallel track is moving at 100 kilometers per hour in the opposite direction (i.e. towards the original train), then an observer on the second train will measure the speed of the original train as 200 kilometers per hour. If the second train is however traveling in the same direction as the original train, then the observer on the second train will measure the speed of the original train as 0 kilometers per hour. For trains and other objects moving much slower than the speed of light the relative speeds add or subtract depending on their directions.
Speed of Light as a Fundamental Constant
Light however works differently. Think about an analogous thought experiment with rockets and light beams. If a light beam is approaching from a distant star, an observer at rest (compared to the star) measuring the speed of the light beam will find that the speed of light is 300,000 kilometers per second (3X10^8 meters per second).
If light worked the same way as trains, an observer in a rocket traveling at 200,000 kilometers per second towards the star would measure the speed of the light beam from the star as 500,000 kilometers per second. If the rocket were traveling away from the star, then the measured value for the speed of the light beam would be 100,000 kilometers per second.
Einstein however realized that light did not work the same way as trains. All three observers, whether staying at rest, moving towards the star, or moving away from the star, will measure the same value, 300,000 kilometers per second, for the speed of the light beam.
By assuming that the speed of light in a vacuum is a fundamental constant; so that, all observers moving at a constant velocity measure the same value for the speed of light, Einstein derived his special theory of relativity.
Some effects of special relativity include:
- Lorentz contraction
- Time dilation
- Mass increases with speed
- The speed of light is the ultimate speed limit
- Mass energy equivalency, E=mc²
These relativistic effects result from the fact that the speed of light in a vacuum is a fundamental constant of the universe.
Further Reading
Einstein, A., "On the Electrodynamics of Moving Bodies" Annalen der Physik, 1905, Translated and reprinted in The Principle of Relativity, Dover, 1952.
Tipler, P.A., Modern Physics, Worth, 1978.
Serway, R.A., Moses, C.J., and Moyer, C.A., Modern Physics, Thomson, 2005.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
