Splitting Time from Space
Zeeya Merali
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 planets.
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.
HoYava’s theory, however, is far from perfect. Diego Blas, a quantum gravity
researcher at the Swiss Federal Institute of Technology (EPFL) in Lausanne has
found a “hidden sickness” in the theory when double-checking calculations for
the solar system. Most physicists examined ideal cases, assuming, for instance,
that Earth and the sun are spheres, Blas explains: “We checked the more
realistic case, where the sun is almost a sphere, but not quite.” General
relativity pretty much gives the same answer in both the scenarios. But in
HoYava gravity, the realistic case gives a wildly different result.
Along with Sergei M. Sibiryakov, also at EPFL, and Oriol Pujolas of CERN near
Geneva, Blas has reformulated HoYava gravity to bring it back into line with
general relativity. Sibiryakov presented the group’s model in September at a
meeting in Talloires, France.
HoYava welcomes the modifications. “When I proposed this, I didn’t claim I had
the final theory,” he says. “I want other people to examine it and improve it.”
Gia Dvali, a quantum gravity expert at CERN, remains cautious. A few years ago
he tried a similar trick, breaking apart space and time in an attempt to
explain dark energy. But he abandoned his model because it allowed information
to be communicated faster than the speed of light.
“My intuition is that any such models will have unwanted side effects,” Dvali
thinks. “But if they find a version that doesn’t, then that theory must be
taken very seriously.”
Note: This article was originally printed with the title, "Splitting Time from
Space."
--- On Sat, 1/2/10, TurquoiseB <no_re...@yahoogroups.com> wrote:
Subject: [FairfieldLife] Dog Years
Date: Saturday, January 2, 2010, 8:26 AM
Time is relative, not static. Different species on
this planet have different perceptions of time, and
how quickly it seems to be flowing past.
Nothing reminds one of this more than having dogs.
Conventional wisdom says that there is such at thing
as "dog years," and that for every human year a dog
experiences, it's as if he or she is experiencing
six of our years.
This turns out not to be true. Different dog breeds
have vastly different life expectancies, depending
on size and breed. My dogs statistically have a pro-
jected lifespan that has them aging five times faster
than humans age.
But does that affect their *perception* of time? Not
as far as I can tell. One of my dogs...let's call him
Paris...lives in the same time continuum I do. Wake
him up suddenly, and he's instantly awake, ready for
anything. Take him on a walk and watch him get totally
lost in some obsession like a passing cat, and it's
*gone* from his mind thirty paces onwards.
My other dog is...not like this. When asked to describe
him, I often refer friends to the movie "Zoolander."
Those of you who have seen the film, do you remember
how Ben Stiller as Derek Zoolander specialized in
"striking poses" for the camera that *looked* as if
he was pondering existence deeply, but in reality was
just striking a pose? That's Pippin, the smaller and
more infuriating of my dogs.
Infuriating occasionally because he insists on striking
these poses while we're out on walks, and expecting me
and Paris to react to his pose-striking as if he actually
*was* pondering existence deeply.
Paris can decide where to pee or poop in a heatrbeat.
"Need to poop? Why not here?" seems to be his credo, as
it is mine (within normal social boundaries, that is).
By comparison, Pippin is working on a completely different
level of time, and the perception thereof. The concept of
"pooping" first seems to hit him as a vague concept, one
that has very little relationship to Here And Now. But he
has to stop dead in his tracks and ponder (or seem to ponder)
the concept anyway. He's often upset when, after a few
moments of him stopping dead in his tracks and producing
nothing for all that pondering but a great photo-op, I
drag him onwards.
Repeat ad frustratium. It can take Pippin ten minutes from
the onset of the *concept* of pooping to attain actual
pooping. If the concept of "dog years" were accurate, that
would mean that it takes him almost an hour (human time)
longer to associate the *concept* of pooping with actual
pooping than it does me, or Paris.
I often catch the same looks of "What is *taking* you so
long?" on Paris' face that I feel on mine. This may mean
that both of us are lesser evolved than Pippin, and are
just missing the "fine points" of having to sniff every
square centimeter of terrain in a ten-block area before
deciding that one of them "deserves" to be pooped on.
Or it could indicate that Paris and I live on a more
realistic, here-and-now time frame, and that heavy
decisions such as where to take a dump don't necessarily
have to take all day.
