GRrrrr part 1.

So they've found gravitational waves. Damn.

This post is not a report about that; I don't understand it well enough to report. This post is the first in a two-part report about my confusion and chagrin.

Let's start with the chagrin; I'd been hopping without hope that gravitational waves would never be found. Like all good scientists should I had been hoping the accepted wisdom was wrong. The tackling of such errors is how science progresses.

But why pin my hopes on gravitational waves in particular? They seem innocent enough as consequence of the central equation in general relativity -- the Einstein equation, which relates gravity to space-time curvature, and that to energy and hence to mass and thence back again to gravity. When you squint at the Einstein equation the right way it turns into perfectly good wave equation.

The doubts creep in when you consider the energetics. Like all waves, they should carry energy away from their sources, and indeed our first observational evidence for gravitational waves was of two block-holes loosing energy as they orbited each-other.

But defining gravitational energy is a tricky and fraught issue -- you can even write down the law of conservation of energy in the form

$$0 = T^{\mu\nu}_{;\nu},$$

which is valid and yet seems to simply ignore gravity. Because of this kind of confusion, Einstein and some others even went through a phase of believing that gravitational waves could not exist, or at least that they could not carry energy away.

Eventual such doubts were put to rest and the consensus emerged that gravitational waves could and did exist, and that they did carry energy away, even if we don't quite know what that means.

But I had been hoping without hope that this consensus was wrong, and that its failure would crack open the puzzle of gravitational energy. As long as no gravitational waves had been detected, I had some slight cause for optimism.

But now all hope is gone.

Comments