Laying Down Einstein's Laws

SciAm Oberservations:

I just got the January 2007 issue of Physics Today, and it has an interesting letter by Richard Kadel of Lawrence Berkeley labs:

Since my undergraduate days, I have been puzzled by the fact that we have Newton's laws of motion but only Einstein's theory of special relativity... It's time to rename it as more than just a theory.

I propose that we, as physicists, define a set of Einstein's laws, just as we have Newton's laws, Coulomb's law, or Faraday's law.

To paraphrase, the laws he proposes are:

• Einstein's First Law. The laws of physics are the same for all observers no matter what their velocity is, as long as they are not accelerating.
• Einstein's Second Law. The speed of light in a vacuum is the same for all observers.
• Einstein's Third Law. The total energy of a body with momentum p and mass m is (m²c⁴ + p²c²)^1/2.

The third law implies that a body at rest (p = 0) has energy mc² and that a massless body such as a photon (m = 0) has energy pc, which in turn implies that light carries momentum. These three laws capture the essence of the special theory of relativity. To rope in the general theory, too, we might add:

• Einstein's Fourth Law. No observer can tell the difference between acceleration and the force of gravity based on local measurements.

The general theory is so rich that it's hard to know what else to include.

It always puzzled me why Newton got "laws" and Einstein only got a "theory".