Posted on 11/25/2009 12:25:53 AM PST by Daffynition
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.”
bump
I LOVE reading articles like this, even though I don’t understand a word of it.
Einstein is stupid
Cut to the chase!
Does this mean we can expect FTL travel in our lifetimes?
But does it explain why the climate models don’t predict global cooling?
I am reading this and honestly am thinking this is too simple, I can’t believe they overlooked this before.
It seems an obvious thing to test.
Maybe the problem was, group think prevented someone from presenting it because they were afraid of retribution from other scientists?
Link to the math: Solutions to Horava Gravity - H. Lu, Jianwei Mei, C.N. Pope
"........ Petr Horava introduced an intriguing idea - that one of the physical principles at the heart of general relativity might be violated. The principle is called Lorentz symmetry (or Lorentz invariance) and it is the principles that physics is the same in any reference frame. The Lorentz violation would only happen at very small scales, of course, which is why it's never been observed to be violated, but if Horava's theory is correct then a theory which doesn't include Lorentz invariance at small scales might still give rise to Lorentz invariance at large scales. The great benefit of this theory, if it can be fully worked out, is that it would be much easier to introduce quantum mechanics into the theory. (General relativity and quantum mechanics are not currently able to work together easily in the same theoretical frameworks.)
A team out of Texas A&M University have now investigated how Horava's theory would affect generic solutions of general relativity, such as those that are spherically symmetrical. Another physicist, Horatiu Nastase, has taken this work and believes that, as it currently exists, the scales are off and Lorentz invariance would only be replicated on scales larger than the observable universe. "Modifications of the detailed balance action, within Hořava theory, can cure this problem," explains Nastase. He also points out, "It is still not known if the quantum Hořava theory makes sense, and more work in that direction is needed."
Other alternative gravity theories, such as those discussed in John Moffat's Reinventing Gravity: A Physicist Goes Beyond Einstein, have been bouncing around in some form or another pretty much since before Einstein even completed general relativity in the first place. The difference between these other theories and the Horava theory, according to Nastase, is that "Hořava['s] theory presents the tantalizing possibility that we have a well-defined quantum theory at short distances, without the need for additional fields."
Tell your husband.
At this point in time i am convinced beyond all reasonable doubt that the one and only reason I am still alive is because i have remained undetected. Trapped In this house, in this place.
HAHAHA! I love the Stephen King cameo!
lulz...proxychecker...
daffy...
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.