Gravity’s next step

We notice an area of science where theories are so numerous a system has been imposed to keep track.

There’s no question that Einstein’s General Relativity has been an extraordinarily successful theory.  Its predictions concerning gravity have been borne out wherever they’ve been tested, and they’ve been tested in many different ways in the century since it was formulated.  You’d think that most scientists would be satisfied with that, and only a few would be interested in modifying or extending it.

But for all its success, we know that it’s incomplete.  It’s incompatible with Quantum Field Theory, that other wonderful product of twentieth-century physics.  QFT works for three of the four forces of nature (electromagnetism and the strong and weak nuclear forces); GR, for gravity.  No one has yet succeeded in combining them.

Well, perhaps that’s not strictly true.  Stephen Hawking, for example, combined the theories to predict Hawking radiation from black holes, and there are other results from doing quantum fields in curved space.  But a quantum theory of gravity itself still eludes us.

It’s not for lack of trying.  Our astronomer recently read through a concise summary of the main suggested post-GR theories; it was 300 pages long.  With all those possibilities, how is one to keep track?

There is a system.  It’s called the “post-post-Newtonian” formalism, and takes the form of ten numbers that a theory will predict that can be compared with observations.  Newton comes in because almost all the observations we can make are where gravity is relatively weak, so Newtonian predictions are fairly accurate.  Since those are much easier to calculate than in any other theory, we start there, and measure how much and in what direction our other theories disagree.  GR has its own set of PPN numbers; Dicke-Brans-Jordon its set, some of which are different; Horndeski theories likewise.

So a theorist will formulate a new theory, check it for consistency as well as messy infinities and other undesirables, and then calculate its PPN numbers.  On their side the experimenters and observers push the limits of technology to come up with ever more stringent measurements of these numbers.  One can imagine a vast scoreboard, with some new measurement eliminating one class of theory and boosting another, to the roar of the scientific crowd.

But notice that the way of doing science has been utterly transformed.  Though Einstein did not work entirely alone, GR was very much his creation, and perhaps the only viable alternative to Newton’s work at the time.  It was strongly boosted by the 1919 solar eclipse measurement of the deflection of light by the Sun: essentially a single experiment.  In a century, gravity research has gone from artisan work by a few to industrial science by many.  It’s not as romantic a picture as that of the lone genius in the garret, scribbling a calculation that is then passed on to the careful observer at the telescope.  But it is the way science is done nowadays.