Of all the fundamental forces of nature, there’s one whose pervasiveness is all too obvious – especially when we get on the bathroom scales: gravity.
Yet for all its familiarity, to physicists it remains by far the most perplexing, refusing to fit their grand theories of the cosmos.
Now a leading theorist thinks he knows why, with a radical new view of gravity that challenges centuries of received wisdom about this strange force.
Ever since Isaac Newton wrote down his famous law describing its action about 350 years ago, the true nature of gravity has caused controversy.
Newton himself came under fire for refusing to say exactly how gravity works. What was this invisible “force-field” that keeps the Earth in orbit around the Sun and pulls apples out of trees?
A century ago, Albert Einstein gave the first detailed description of gravity, linking matter and energy to their effect on the very fabric of space and time.
Put simply, his General Theory of Relativity shows that mass warps space and time in a way that makes other masses appear to accelerate as they approach – as if an attractive force is at work. There isn’t: it’s just a kind of illusion, created by the distortion of space and time.
Einstein showed that his theory not only explained Newton’s law of gravity but predicted other bizarre phenomena like gravitational waves – whose existence was confirmed earlier this year.
Yet Einstein’s view of gravity has proved impossible to reconcile with theories for the other fundamental forces of nature.
From electromagnetism to the strong nuclear force, all are now understood in terms of quantum theory, the laws of the sub-atomic world.
And according to this, nature’s fundamental forces are the result of “exchange particles” flitting from place to place.
Despite decades of effort by Einstein’s successors, their attempts to find a similar explanation for gravity have ended in abject failure.
Now a theorist at the University of Amsterdam is making headlines with what he thinks is the explanation for all the trouble.
According to Professor Erik Verlinde, there’s a simple reason why gravity has resisted all attempts to describe it as a fundamental force. It’s because it isn’t fundamental at all. It’s simply an emergent property of space and time – just as elasticity is an emergent property of rubber.
Claims that smack of arguing “Einstein was wrong” usually get very short shrift from the scientific community but Prof Verlinde is no crank. He has spent years working on so-called superstring theory, widely regarded as the best candidate for unifying gravity with the other fundamental forces.
But about a decade ago, as concern about the lack of success of the superstring approach gained momentum, Prof Verlinde sparked controversy with research hinting that the very nature of gravity had been misunderstood.
It drew on a series of strange discoveries made by theorists including Prof Stephen Hawking at Cambridge University, pointing to a link between gravity and heat.
In the 1970s, Prof Hawking shot to fame for showing that the intense gravity of black holes allows them to create particles literally out of nowhere, which appear as heat streaming out of them.
Theorists have since discovered similarly perplexing connections between gravity and heat, and in 2010 Prof Verlinde combined them with new ideas about the make-up of the universe, showing they led straight to Newton’s 350-year-old law of gravity.
The implication is as simple as it is profound: gravity is no more “fundamental” than heat, and by using the laws of heat – thermodynamics – it’s possible to work out how gravity behaves.
Suddenly the notorious failure of attempts to unify gravity with the other truly fundamental forces no longer seems surprising. Instead, it becomes a huge hint that its true nature has been misunderstood.
Not surprisingly, Prof Verlinde’s radical views have proved deeply controversial. Some theorists think it could be the breakthrough they have sought for decades. Others think it’s just a re-hash of what’s already known. And some think it’s nonsense.
But now Prof Verlinde has gone further, showing that his view of gravity can explain a long-standing cosmic conundrum: the identity of “dark matter”.
Astronomers have long known that galaxies contain far more matter than the bright stars seen through telescopes. But the nature of this “dark matter” remains a mystery.
Calculations show it can’t be simply gas, dust and black holes, prompting speculation that it’s some whole new form of matter.
For years, researchers have been hoping to detect particles of this dark matter, or even create some in the Large Hadron Collider machine near Geneva. So far, they’ve come up empty.
Earlier this month Prof Verlinde made headlines again with another devastatingly simple explanation: dark matter hasn’t been found because it doesn’t exist.
In his latest paper, he points out that all the evidence for the presence of dark matter comes from calculations based on the textbook view of gravity. Change that, and the need for dark matter goes away.
While the details are complex, in essence Prof Verlinde’s explanation exploits the discovery that the universe is filled with a kind of anti-gravity field known as “dark energy”.
When combined with his new view of gravity, this dark energy makes the fabric of space and time more “elastic”. That, in turn, produces an increase in the apparent strength of gravity - one that astronomers have been mistaking for the presence of dark matter.
In his latest research paper, Prof Verlinde has put forward detailed predictions of what astronomers should observe in future observations of galaxies, allowing his new view of gravity to be checked against reality.
If Prof Verlinde is right, then his theory leads to an even starker prediction, however: scientists now looking for dark matter particles will never succeed in their quest. For if such particles did turn up, they’d make a huge re-think of gravity unnecessary.
With so much time and effort invested in the textbook view of gravity, Prof Verlinde can expect a rough ride from his fellow theorists.
Whether his theory will pass muster remains to be seen. But after decades of trying and failing to fathom gravity, the need for fresh thinking has never been more urgent.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham.