Einstein's Postulates

Einstein’s Postulates

 So where was the ether wind?

The null result of the Michelson-Morley experiment baffled the physics community. Why was the ether wind undetectable? A number of attempts were made to explain this unexpected finding.

It was finally left to a young patent clerk, in 1905, to make the bold suggestion that the Michelson-Morley experiment was actually a vital clue to the very nature of time and space itself. The visionary difference here was the realization that there was no ether at all!

In 1905, while working as a patent clerk, the 26-year-old Albert Einstein published three scientific papers in one issue of the Annalen der Physik. This issue of the physics journal has since become a unique collector’s item because any one of those three papers, alone, would have won Einstein the Nobel Prize in physics.

Ironically, of the three papers, Einstein was awarded the Nobel Prize in 1921 for his paper on the photoelectric effect, and not for the one on the theory of relativity, the theory synonymous with his name. Relativity—Einstein’s main work—was apparently still being disputed in 1921. This was hardly surprising given the bizarre nature of relativity. Effects like time dilation, length contraction, and distortions in space-time, were not exactly your everyday intuitive events. How then did Einstein reason his way to such apparently fantastical scientific conclusions?

In his 1905 paper “On the Electrodynamics of Moving Bodies”, Einstein introduces the two postulates that form the starting point for his theory of relativity. The first of these postulates, which Einstein calls the “Principle of Relativity” gives his whole theory its name.

The Principle of Relativity abolishes the idea of an absolute state of rest. All observers moving at constant velocity relative to each other thus have equal status; no observer can claim to be the “special one” at rest. For example, a man standing on a train station may claim that he is stationary while his friend on the train is moving. His friend, however, may claim, instead, that he is the one actually at rest while the man on the platform is moving. Both are equally correct. According to Einstein, all velocities are relative. Hence, an object’s designated velocity conveys little meaning unless we also know which frame of reference we are viewing this velocity from.

While this first postulate may be revolutionary, it is nonetheless reasonably intuitive, i.e. it feels logical in a “common sense” way.

 Einstein’s next postulate, however, is extremely counter-intuitive, and weird consequences arise from it. Unlike the first postulate, this one defies common sense. The second postulate states that the speed of light is constant relative to all frames of reference.

Why this is so weird? If we observe a flash of light while standing on Earth, we will find it travelling from us at 300,000 km per second. Another observer, travelling in the same direction as this light ray, should then measure its speed, relative to himself, to be slower. He is, after all, chasing after it. Einstein’s second postulate, however, states that this simply does not happen.

Let us look at an analogous situation. If we are in a police car chasing after a getaway car, we would expect the getaway car to move away from us at a slower speed (relative to us). If the police car manages to move at the same speed as the getaway car, the getaway car would not even be moving away from us at all. Its velocity relative to us would then be zero, since we are keeping up with it. This would be common sense.

Now imagine the scenario if the getaway car behaves like Einstein’s light ray. What happens then is this. The getaway car continues to pull away from the police car at the same speed, no matter how fast the police car chases after it. It’s the ideal getaway car. It runs away from us at exactly the same speed no matter how we race after it.

The really strange thing is this. A man standing on the pavement also sees this same getaway car pulling away from him at the same speed as we do in the police car. In other words, the speed of the getaway car relative to the man on the pavement, and relative to us in the police car chasing after it, is exactly the same! For example, if the getaway car is pulling away from the man on the pavement at 100 km per hour, it is also pulling away from us in the police car at 100 km per hour, even though we are tearing after it as fast as we can. How can this be? Surely this defies common sense.

This weird behaviour of the getaway car is, nonetheless, what Einstein is proposing for the light ray. The same light ray travels away from all observers at the same speed of 300,000 km per second, regardless of whether the observer is stationary or moving. The velocity of the observer makes no difference whatsoever. The speed of light is constant relative to all frames of reference. This then is Einstein’s second postulate. It sounds positively crazy!

Nonetheless, this second postulate of relativity has passed the test of time. It has survived a whole century of close scientific scrutiny, and has become a fundamental cornerstone of all modern physics. How do we explain this phenomenon?

Actually, Einstein never gave an explanation for his second postulate. That, in fact, is why it is called a postulate. A postulate is a principle simply assumed to be true, and then used as the basis for further derivations and conclusions. For some reason, Einstein’s second postulate works. But why does it work? And why is nature so strange? That was the question Heisenberg, on his deathbed, was purportedly still puzzling over. Why—Heisenberg was essentially asking—does relativity exist at all ?