Was Newton right and Einstein wrong? It seems that unzipping the
fabric of spacetime and harking back to 19th-century notions of time
could lead to a theory of quantum gravity.
Physicists have struggled to marry quantum mechanics with gravity
for decades. In contrast, the other forces of nature have obediently
fallen into line. For instance, the electromagnetic force can be
described quantum-mechanically by the motion of photons. Try and work
out the gravitational force between two objects in terms of a quantum
graviton, however, and you quickly run into trouble—the answer to every
calculation is infinity. But now Petr Hořava, a physicist at the
University of California, Berkeley, thinks he understands the problem.
It’s all, he says, a matter of time.
More specifically, the problem is the way that time is tied up with space in Einstein’s theory of gravity: general relativity.
Einstein famously overturned the Newtonian notion that time is
absolute—steadily ticking away in the background. Instead he argued
that time is another dimension, woven together with space to form a
malleable fabric that is distorted by matter. The snag is that in
quantum mechanics, time retains its Newtonian aloofness, providing the
stage against which matter dances but never being affected by its
presence. These two conceptions of time don’t gel.
The solution, Hořava says, is to snip threads that bind time to
space at very high energies, such as those found in the early universe
where quantum gravity rules. “I’m going back to Newton’s idea that time
and space are not equivalent,” Hořava says. At low energies, general
relativity emerges from this underlying framework, and the fabric of
spacetime restitches, he explains.
Hořava likens this emergence to the way some exotic substances
change phase. For instance, at low temperatures liquid helium’s
properties change dramatically, becoming a “superfluid” that can
overcome friction. In fact, he has co-opted the mathematics of exotic
phase transitions to build his theory of gravity. So far it seems to be
working: the infinities that plague other theories of quantum gravity
have been tamed, and the theory spits out a well-behaved graviton. It
also seems to match with computer simulations of quantum gravity.
Hořava’s theory has been generating excitement since he proposed it
in January, and physicists met to discuss it at a meeting in November
at the Perimeter Institute for Theoretical Physics in Waterloo,
Ontario. In particular, physicists have been checking if the model
correctly describes the universe we see today. General relativity
scored a knockout blow when Einstein predicted the motion of Mercury
with greater accuracy than Newton’s theory of gravity could.
Can Hořřava gravity claim the same success? The first tentative
answers coming in say “yes.” Francisco Lobo, now at the University of
Lisbon, and his colleagues have found a good match with the movement of
Others have made even bolder claims for Hořava gravity, especially
when it comes to explaining cosmic conundrums such as the singularity
of the big bang, where the laws of physics break down. If Hořava
gravity is true, argues cosmologist Robert Brandenberger of McGill
University in a paper published in the August Physical Review D,
then the universe didn’t bang—it bounced. “A universe filled with
matter will contract down to a small—but finite—size and then bounce
out again, giving us the expanding cosmos we see today,” he says.
Brandenberger’s calculations show that ripples produced by the bounce
match those already detected by satellites measuring the cosmic
microwave background, and he is now looking for signatures that could
distinguish the bounce from the big bang scenario.
Hořava gravity may also create the “illusion of dark matter,” says cosmologist Shinji Mukohyama of Tokyo University. In the September Physical Review D,
he explains that in certain circumstances Hořava’s graviton fluctuates
as it interacts with normal matter, making gravity pull a bit more
strongly than expected in general relativity. The effect could make
galaxies appear to contain more matter than can be seen. If that’s not
enough, cosmologist Mu-In Park of Chonbuk National University in South
Korea believes that Hořava gravity may also be behind the accelerated
expansion of the universe, currently attributed to a mysterious dark energy.
One of the leading explanations for its origin is that empty space
contains some intrinsic energy that pushes the universe outward. This
intrinsic energy cannot be accounted for by general relativity but pops
naturally out of the equations of Hořava gravity, according to Park.