Does gravity come in sizes?

Marcus Chown

 

The Milky Way as pictured in an infrared image produced by Nasa's Spitzer Space Telescope. NASA / AP

There is nothing certain in this world, the US founding father Benjamin Franklin once wrote, except death and taxes.

As a scientist, he might have added a third inescapable force: gravity –the unseen hand that keeps our feet on the ground.

Gravity is the universal force. Not only does it stop us getting above ourselves, it keeps Earth orbiting around the sun, our sun swinging around the centre of the Milky Way, the Milky Way in a merry dance around its neighbours and so on upwards.

It is actually the weakest of nature’s four forces, but whereas the other three – electromagnetism and the strong and weak nuclear forces – unleash their full strength only at the scales of atoms and particles, gravity conserves its power to trump all comers in the cosmos at large. Just take any two things that have mass and, whatever their size and wherever they are, they will feel gravity’s grasp in exactly the same way.

Or will they?

Justin Khoury, now of the University of Pennsylvania in Philadelphia, and his colleagues Niayesh Afshordi and Ghazal Geshnizjani of the Perimeter Institute for Theoretical Physics in Waterloo, Canada are not so sure. They have listed a series of observations that cannot readily be explained with a one-size-fits-all gravity.

None of these effects on its own, they stress, necessarily indicates anything amiss. But intriguingly, all of them melt away if you make just one assumption, albeit a controversial one: that how gravity works depends on the scale on which you look at it.

If right, the hunch has truly mind-boggling consequences. According to the theory, this variable gravity would be our first glimpse of spatial dimensions beyond our familiar three – dimensions infinitely large, but which remain forever closed off to us. Dr Khoury acknowledges that it seems wacky. But as long as the observational anomalies are not explained, there is a feeling the idea should not be dismissed out of hand.

“The work is credible, if a little optimistic,” says David Spergel, an astrophysicist based at Princeton University. Intriguingly, the theory makes predictions we can test: so if hidden dimensions are there right under our noses, we should soon have the proof.

Gravity is a familiar, yet deeply perplexing force. Its story is bound up with two of the greatest names in physics, Isaac Newton and Albert Einstein. In 1687, Newton published his universal law of gravitation, embodied the motion of the planets, the flight of a cannonball and the dropping of an apple – all in one succinct formula.

Yet Newton was hard pressed to explain the nature of a force that seem to be transported instantaneously and with unerring accuracy through empty space. It was only in 1915, with Einstein’s general theory of relativity, that a halfway convincing answer was found.

According to general relativity, gravity arises because objects with mass or energy warp space and time around them, causing other objects to fall towards them. Now we can predict gravity’s effects from the smallest scales right up to the scale of the solar system with astounding accuracy.

So if the theory ain’t broke, why try to fix it? The problem is that general relativity is incompatible with the later quantum theories that describe nature’s other three forces. These theories say that forces are mediated by a constant exchange of particles; accordingly, gravity should be transmitted by a quantum particle known as a graviton. General relativity does not allow for such a possibility, so physicists are left seeking a grander framework that will unite gravity and quantum theory into one “theory of everything”.

If you care to look on the very grandest of cosmic scales, there is no shortage of niggling indications that something is not quite right. There’s evidence of dark energy, some kind of invisible “stuff” with repulsive gravity that is the best explanation we have for why the universe’s expansion seems to have begun speeding up in recent aeons. 

Then there is the mystery of “dark flow”, which has emerged from surveys of thousands upon thousands of distant galaxies. Over middling scales of a few hundred million light years, galaxies look as if they are flowing towards a giant central concentration of mass – one so large that it could not possibly have gathered since the big bang.

Finally, then there’s the Lyman-alpha forest. Liberally dabbed across the cosmos are tenuous clouds of hydrogen gas, the building blocks of galaxies. These absorb light, creating a distinctive dip in the spectrum of light penetrating through them known as the Lyman-alpha line. From this forest of spectral lines astronomers can deduce the distribution of hydrogen clouds in space. Like the dark-flowing galaxies, they seem more closely clumped together on middling scales than standard cosmology can explain – again, just as if gravity had once been a stronger force binding them together.

Overall, there’s weaker gravity on one scale; stronger gravity on another. Surely one theory cannot explain both? Remarkably, that is just what Dr Khoury and his colleagues are claiming.

The context of their work is an outgrowth of string theory – the currently favoured route to a theory of everything – known as brane theory, which views our universe as a four-dimensional island or “brane” adrift in a 10-dimensional ocean of space-time. In particular they focused on a set of these theories known as Dvali-Gabadadze-Porrati models after the three theorists at New York University who suggested them. They would be just the ticket for reproducing the gravitational properties of the universe as we see them. They contain hidden dimensions that might nicely explain the weaker gravity seen at the largest scales and the stronger gravity on intermediate scales.

But if brane theories have extra dimensions to the ones we can perceive, why can’t we see them? You and I do not see the extra dimensions because we are made up of ordinary particles of matter that are firmly pinned to the brane, they argue.

Even if we do get proof that these other dimensions exist, we would be a way away from ever entering them. Still, it would be a dumbfounding indication of how even the stuff we are made of deceives us in our perceptions of the universe. Is it going to tie up all the loose ends of the current cosmology? Perhaps, says Jim Peebles of Princeton University. “It is a kludge” – something that by rights should not work, but might just – he says.

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