By Rodney A. Brooks
author of "Fields of Color: The Theory That Escaped Einstein".
The current discovery of gravitational waves at LIGO (Laser Interferometer Gravitational-Wave Observatory) has captured the mind of the public. It will stand as one of the great accomplishments of experimental physics, along with the famous Michelson-Morley experiment of 1887 which it resembles. In fact by comparing these two experiments, you will see that comprehending gravitational waves is not as difficult as you believe.
Contraction. Michaelson and Morley measured the speed of light at different times as the earth moved around its orbit. To their - and everyone's - surprise, the speed turned out to be continuous, independent of the earth's motion. This breakthrough caused great consternation until George FitzGerald and Hendrick Lorentz came up with the only feasible explanation: objects in motion contract. Einstein then showed that this contraction is a consequence of his Principles of Relativity, but without saying why they contract (other than a need to conform to his Principles). In fact Lorentz had previously provided a partial explanation by showing that motion affects the way the electromagnetic field interacts with charges, causing objects to contract. However it wasn't until Quantum Field Theory came along that a full explanation was found. In QFT, at least in Julian Schwinger's model, everything is made of fields, even space itself, and motion affects the way all fields interact.
Waves. Electromagnetic waves, e.g., radio waves, have long been recognized and accepted as a natural phenomenon of fields. Now in QFT gravity is a field and, just as an oscillating electron in an antenna sends out radio waves, so a large mass moving back and forth will send out gravitational waves. But it didn't take QFT to show this. Einstein also believed that gravity is a field that obeys his equations, just as the EM field adheres to the equations of James Maxwell. In fact gravitational waves have been recognized by many physicists, from Einstein on down, who regard gravity as a field.
Curvature. But what about "curvature of space-time", which many people today say is what causes gravity? You may be shocked to learn that's not how Einstein saw it. He believed that the gravitational field causes things, even space itself, to contract, comparable to the way motion causes contraction. In fact Einstein used this analogy to show the similarity between motion-induced and gravity-induced contraction: they both affect the way fields work together. It is this gravity-induced contraction that is sometimes called "curvature".
Evidence. The first uncovering of gravitational waves was done at LIGO, using an apparatus similar to Michelson's and Morley's. In both experiments the time for light to travel along two perpendicular paths was compared, but because the gravitational field is much weaker than the EM field, the distances in the LIGO apparatus are much greater (miles instead of inches). Another difference is that while Michelson, not knowing about motion-induced contraction, anticipated to see a change (and found none), the LIGO staff used the known gravity-induced contraction to see an alteration when a gravitational wave passed through.
Fields of Color: The theory that escaped Einstein explains Quantum Field Theory to a lay audience, without any mathematics. If you want to learn more about gravitational waves or about how QFT resolves the paradoxes of Relativity and Quantum Mechanics, read Chapters 1 and 2, which can be seen free at quantum-field-theory.net.
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