Does gravity come in sizes?
The Milky Way as pictured
in an infrared image produced by Nasa's Spitzer Space
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
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
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
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.