Another GUT Candidate

[djolds1]

Hořava Gravity.

Splitting Time from Space—New Quantum Theory Topples Einstein's Spacetime
Buzz about a quantum gravity theory that sends space and time back to their Newtonian roots

By 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.

Hořava’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 Hořava 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 Hořava gravity to bring it back into line with general relativity. Sibiryakov presented the group’s model in September at a meeting in Talloires, France.

Hořava 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."

[TDPerk]

"'My intuition is that any such models will have unwanted side effects,'"

I suppose he meant unwanted in the sense of unobserved or unobservable?

[Stoney3K]

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.

That doesn't neccessarily imply the physics behind it were wrong, as the article said, on low energies, the space-time continuum restitches, it's only at the quantum level these effects occur.

This may be consistent with entanglement effects which *do* operate faster than the speed of light (as far as we can percieve), but as yet we are unable to convey useful information through entanglement.

Also, the matter of time always running in one direction, while we are free to operate in any other dimension (aside from the >4 postulated by other theories), has always been a great head-breaker among physicists and this might provide an explanation for it.

I guess I'll need to hand over those TARDIS keys to CERN.

[rcain]

...he {HoYava} explains that within this regime, space stretches only a third as quickly as time. "The three spatial dimensions effectively mimic just one normal relativistic dimension," he says, making it look as though two dimensions have vanished.

http://www.scientificamerican.com/artic ... e-evidence

...'the big bounce'... guess i could live with that. though it seems he's got a way to go before he convincingly 'topples Einstien' as the article title suggests.

[kunkmiester]

Unexplained stuff is fine as long as there's a way to test them eventually.

How would this go with the flyby anomaly, Heim theory, Mach effect, and other such things?

[TallDave]

I read that.

I was interested until they said nonspherical bodies ruined it.

Bleah.

[kcdodd]

The solution, Hořava says, is to snip threads that bind time to space at very high energies

This I find interesting, as high energies is exactly where you need special relativity. I guess he is saying that low energy, as in every day stuff, and very high energy space-time is irrelevant. This does seem to indicate ftl is possible at high enough energy. LHC might be able to push this simple fact and see if they can break c. Somehow I doubt it though. By the same token why not simply say quantum mechanics is irrelevant at high energy and we avoid the problem all together.

[rcain]

...I was interested until they said nonspherical bodies ruined it.Bleah.

likewise.

but then again, could just be an 'artifact' of the maths. its only supposed to be represntational after all. think trig functions, fractals.

as someone said, would be bludy nice if someone could actually test it.

[kunkmiester]

Well, I think there's three tests for stuff like this:

Predict observed activities--we have all sorts of measurements of motion for everything from falling feathers to space probes. We need to wait and see if the revised equations can account for non-spherical bodies and stuff on this step.

Explain unexplained phenomenon--flyby anomaly, quantum gravity, and other things that aren't solved, would have to be solved, or at least most of them. I think the flyby anomaly is the most likely, considering that it's a gravitational theory.

Other predictions--a new set of rules would probably predict new phenomena, like the FTL information thing. Some of these should test out.

How much of these you need to have a useful theory I don't know(physics starts next semester), but I think it's a good start.

[Giorgio]

"'My intuition is that any such models will have unwanted side effects,'"

I suppose he meant unwanted in the sense of unobserved or unobservable?

No, more like in the sense that some of the mathematical solutions gives a meaningless result.

I have been following Hořava gravity theory from some time, and it does not convince me at all.
I suggest for the one interested to read Luboš Motl blog about Hořava (and a lot of other interesting stuff as well).

http://motls.blogspot.com