We long assumed particles carrying the force of gravity couldn’t have mass. That’s wrong – and it may mean gravity travels at different speeds across the cosmos.
8 July 2020
SOME 384,400 kilometres from Earth, the moon glides silently through space. Actually, we can do better than that: we know exactly where the moon is now, and what path it will take around us, to within a millimetre. To be any more accurate, we would have to consider details of Earth’s mass distribution as subtle as the mass of leaves growing in the spring in one hemisphere as they wither in the other.
The laws of gravity govern the moon’s trajectory, and the precision with which we understand them is thanks to the equations of general relativity, finalised by Albert Einstein just over a century ago. Those same equations apply across the solar system and beyond. Our understanding of gravity is one of the most impressive achievements of our species.
But there is a problem. General relativity doesn’t align with quantum theory, our most successful attempt at understanding reality so far. And the universe is expanding at an accelerating pace that doesn’t tally with gravity’s attractive nature. Our existing picture may allow us to predict the motion of the moon, but when it comes to explaining the wider cosmos, we’re missing something.
My attempts to find out what have led me to ask whether gravity itself has a mass. Physicists have argued about this for decades. Now my colleagues and I have stumbled on an intriguing answer that may lead us to a clearer picture of gravity. If we are right, then this most enigmatic of forces not only does have a mass, but the evidence that proves it is painted across the cosmos.