The Physicist Who Slayed Gravity’s Ghosts

[Elderberry]

Quanta Magazine 8/18/2020

Claudia de Rham showed how theories of “massive gravity” could potentially get rid of the need for dark energy.

Ever since Albert Einstein’s general theory of relativity recast gravity as curves in space-time, physicists have wondered if his work was the final word. Prodding and tweaking, they’ve tried to modify or even replace Einstein’s gravity, all the while keeping their models within the ever-tighter confines of observation.

Many of these ideas attempt to recast gravity in the language of quantum mechanics, where hypothetical particles called gravitons carry the gravitational force. These gravitons are massless, which means they can, in theory, travel an infinite distance, just as photons (the carriers of the electromagnetic force) let us see to the edges of the universe. 

But there’s another possibility. In the 1930s, Wolfgang Pauli and Markus Fierz proposed a graviton with mass. The idea became more plausible in the 1970s, after physicists discovered that massive particles carry the weak and strong forces. Why couldn’t the force of gravity work in the same way?

But “ghosts” soon plagued these theories of massive gravity.

In our familiar world, in order to move an object you have to give it some energy. But in these massive-gravity models, if you try to move an object you get energy back. This goes against the bedrock principle that energy must be conserved, and it leads to all sorts of problems. “Nothing would prevent the whole universe from going completely crazy because it would cost you less and less energy to become more and more crazy,” said Claudia de Rham, a theoretical physicist at Imperial College London. “We call it a ghost because it’s something that just cannot happen.”

Faced with such an essential flaw, theorists began to lose interest in massive gravity, until de Rham — along with Andrew Tolley and Gregory Gabadadze — published a radical paper in 2011 that overturned the status quo. By adding extra dimensions into models of massive gravity, de Rham and her collaborators were able to avoid the ghosts and rekindle the century-long hope of a mathematically plausible theory.

But why bother with massive gravity at all? De Rham hopes that modifying gravity could resolve one of cosmology’s biggest mysteries: dark energy. Two decades ago, cosmologists discovered that the universe’s expansion appears to be accelerating. Ever since, physicists have puzzled over what might be the cause. A massive graviton — one whose reach doesn’t spread quite all the way across the universe — might be able to explain why the universe acts the way it does.

More: https://www.quantamagazine.org/claudia-de-rham-slayed-gravitys-ghosts-20200818/