"The hardest thing in the world to understand is the income tax."
-- Albert Einstein
Posted on 03/07/2010 2:11:48 PM PST by LibWhacker
Cutting the threads of the spacetime fabric and reinstating the aether could lead to a theory of quantum gravity.
If there’s one thing Einstein taught us, it’s that time is relative. But physicist Petr Hořava is challenging this notion and tearing through the fabric of spacetime in his quest for a theory of quantum gravity. His work may also resurrect another entity that Einstein had seemingly buried—the aether.
Physicists have spent decades searching for a way to reconcile the seemingly incongruous twin foundations of modern physics: quantum theory, which deals with the infinitesimally small, and Einstein’s theory of gravity, general relativity, which deals with the vast cosmos. This effort has led to a dazzling array of candidate theories—including superstring theory, loop quantum gravity, and doubly special relativity—but none have succeeded in unambiguously bridging the quantum-gravity divide. The problem: When you try to do the math to work out the strength of forces on the quantum-gravitational scale, your calculations return a maddening proliferation of infinite answers that have no physical meaning.
Now Hořava, at the University of California, Berkeley, claims to have found a solution that is both simple and—in physics terms, at least—sacrilegious. To make the two theories gel, he argues, you need to throw out Einstein’s tenet that time is always relative, never absolute.
Hořava’s controversial idea is based on the fact that the description of space and time in the quantum and relativistic worlds are in conflict. Quantum theory harks back to the Newtonian concept that time is absolute—an impassive backdrop against which events take place. In contrast, general relativity tells us that space and time are fundamentally intertwined; two events can only be marked relative to one another, and not relative to an absolute background clock. Einstein’s subjective notion of time is well accepted and is the hallmark of Lorentz invariance, the property that lies at the heart of general relativity.
"Lorentz invariance is not actually fundamental to a theory of quantum gravity," says Hořava. "But the problem so far has been that many cosmologists are wedded to the concept."
Good Gravitons
By restoring the absolute nature of time at very high energies, such as those in the early universe where quantum gravity would be important, Hořava can treat variations in space and time differently. The upshot of this is that in your calculations at very short distances you do not get such dramatic spatial variations as you do in general relativity, taming the infinities that frustrate other candidate theories of quantum gravity. This makes it possible to describe gravity on the quantum level using a well-behaved graviton—the hypothesized quantum particle thought to mediate gravity, just as the photon mediates the electromagnetic force (arxiv.org/abs/0901.3775).
So far Hořava’s potential resolution of a decades-long physics stalemate has been creating a buzz. Last year, five of the top ten cited academic papers in high energy physics dealt in some form with Hořava’s model.
"The existence of an absolute time might ensure that the usual framework of quantum mechanics can survive even the most exotic regimes of quantum gravity," says physicist Ted Jacobson at the University of Maryland, College Park.
Surprisingly, Hořava’s trick is fairly commonplace in the laboratory. Condensed matter scientists looking at complex real-world systems, such as superconductors at low temperatures, have been using the idea that space and time are not on the same footing for years. Cosmologists do not usually take the lead from their condensed-matter cousins because of "sociological barriers," but the groups should look to each other for inspiration more often, says Hořava. He borrowed ideas from condensed matter models when developing his theory of quantum gravity. "In some condensed matter systems, relativistic behaviour and Lorentz invariance only emerge at lower energies," he says.
But while condensed matter physicists have shown that their models can recover relativistic behaviour as required at low energies, the big question is whether Hořava gravity can successfully morph back into the classical theory of relativity, in a way that agrees with all observations. In principle, general relativity should emerge at lower energies and larger distances. In other words: Look at a patch of the universe with infinitely powerful glasses and you would see that time and space are distinct from one another. Zoom out and the picture blurs, restoring Einstein’s more familiar spacetime fabric.
Knife-Edge
There is some support that this emergence does indeed happen from computer simulations of quantum gravity carried out by Jan Ambjørn of the Niels Bohr Institute at the University of Copenhagen and his colleagues. Ambjørn’s simulations showed that at short distances, the familiar four-dimensional spacetime of our macroscopic universe seems to shrink to just two dimensions—one space and one time. Hořava believes that his theory can explain how those spatial dimensions disappeared.
According to Hořava, this vanishing point marks the knife-edge at which general relativity breaks down and his theory of gravity comes into play. As the fabric of spacetime rips, space and time start to stretch at different rates. The stronger constraints on short distance spatial variations mean that space now stretches only a third as quickly as time, effectively reducing the familiar three spatial dimensions into just one.
Since Hořava first proposed his theory in 2009, other researchers have used it to answer important cosmic questions about the nature of the Big Bang, dark matter and dark energy. Jacobson, however, feels there is much work still to be done before the theory can be widely accepted. "Hořava’s paper triggered a feeding frenzy, but most workers outside that frenzy remained wisely sceptical," he says.
Gustavo Niz at the University of Nottingham, UK, notes that physicists have found that in its original form, Hořava theory has plenty of "pathologies" and does not recover general relativity. "However, the idea behind the model is encouraging and scientists have ideas on how to cure all these secondary problems," he says.
Among those attempting to fix the original model are Diego Blas and Sergei Sibiryakov at the Swiss Federal Institute of Technology (EPFL) in Lausanne, and Oriol Pujolas at CERN near Geneva. Their work has revealed a flaw in the model: Minor variations in the initial conditions used in calculations in Hořava gravity can give dramatically different results (arxiv.org/abs/0909.3525). The culprit is a unique and unstable "breathing mode" in which space can locally expand or contract, wreaking havoc with your answers. To address this problem, they’ve modified Hořava’s initial proposal, making it harder for this breathing mode to develop. They have dubbed their formulation "extended Hořava gravity."
“In my view, the extended version of Hořava gravity is the only currently viable approach and needs to be extensively analysed,” says Jacobson.
Einstein’s Aether?
Jacobson’s own current research, funded by FQXi, examines the short distance structure of space and the quantum vacuum as space expands. He is also now looking at connections between Hořava gravity and an earlier modification of relativity, dubbed "Einstein-aether theory" that he had proposed a decade back.
Nineteenth-century physicists believed light waves must move through an "aether"—a medium that permeates all of space, allowing light to propagate just as sound waves move through air. However, a series experiments by Michelson and Morley failed to find any evidence that Earth moves through an aether. Einstein’s theory of relativity was the final nail in the aether’s coffin, because it explained that light moves through a vacuum.
Jacobson does not believe that the nineteenth-century aether exists. However, within Einstein-aether theory—in contrast to general relativity—there is a preferred time that can be used as an absolute reference to mark events against. It is as if spacetime were filled with a fluid—an aether—which defines a "rest frame" at each event.
Like Horava’s theory, Einstein-aether theory breaks Lorentz invariance and may lead to a viable mechanism for producing gravitons. To get from the general Einstein-aether theory to extended Horava gravity, you simply assume that the aether rest frame arises from an absolute time.
Jacobson has shown that some of the tests proposed to confirm or rule out Einstein-aether theory over the years could also falsify Hořava gravity (http://arxiv.org/abs/1001.4823). "The list of potential experimental signatures includes everything gravitational: from modified orbits to gravitational radiation—there is a new type of gravity wave in Hořava theory from the breathing mode—to the structure of neutron stars and black holes, and perhaps even more exotic stuff," says Jacobson.
For now though, Hořava remains modest, and is glad that others are examining his work. "My papers present the basic idea but don’t present a full theory yet," he says. "It is still unclear which of the possible different trajectories is best."
"Stop telling God what to do." Fermi.
L
"Stop telling God what to do." Fermi.
Have a friend, calls me up
Says, "Hello" and then hangs up
He must have read my mind
These are the days of a different paradigm
Maybe once, even twice
He said "God does not play dice."
Yet if He's everywhere
He's in casinos with aces to spare.
-Tears for Fears
God's Mistake
Anybody notice a difference between the attitudes of the scientists doing this controversial work toward having their work questioned, versus those working in another controversial area of some current interest?
Same here. I wonder though if “old” light may be tired, but fresh “new light” has plenty of get up and go. And does “old light” get a boost if you shoot it full of “new light.”
And, the Big Bang thing has always bothered me. It’s like science has to keep adding new things to support it, like dark matter, dark energy, and mass transit.
parsy
OK. Ether makes me sleepy, anyway....
parsy
Unless you get into all that “sums over” stuff, that guy whose name started with a D?, said, then the little thingies traveled all possible paths including into the future at C+ and back.
Oh, what’s his name?
BTW, here is a cool site. I have the book somewhere. And the one by Machu Pichu, whatever, where he goes over all the quantum theory and string theory stuff. I get about 2 sentences into some of that stuff, and I have to go back and re-read.
http://bigbangneverhappened.org/
parsy, who also don’t believe life is an accident
But spacetime or the vacuum is "something". It has properties that can be measured. Permeability and permittivity for instance.
"The hardest thing in the world to understand is the income tax."
-- Albert Einstein
But is it nothing? I thought M & M were trying to find a cross current or something, to discover which way the ether was blowing. And, would not a bullet fired in a vacuum go as at the same speed in any direction?
parsy, who is like getting this expansive feeling in his head, dude, like whoa......
Same here, particle physics ans astrophysics are based on conjecture upon more and more conjecture . It’s like links in a chain, only the weakest one has to break to make all the others useless. If one of these links break I guess they just say “never mind”.
True. And the Big Bang may be right, but its like they keep finding new ways to explain why the measurements don’t work. After a while, you have to kind of wonder is this all done just to keep from saying, “We don’t know how it all got started...”
parsy, who figures there is something beyond our senses——
Thanks for the ping!
Relative to WHAT?*
*Unless you're into that absolute frame stuff
Gravitons??
There are at least three elephant-in-the-living-room kinds of problems with what Einstein's ideas about gravity. One is that Einstein claimed that information could not be transmitted faster than C in the universe while we know that gravity or at least the FORCE of gravity propagates instantaneously to within our ability to measure. Two is that there is no way to start with Einstein's description of gravity as some sort of a four-dimensional differential geometry thing, and believe it could have ever changed much near the surface of our own planet; nonetheless it is an easy demonstration that it has, and that the super animals of past ages would be crushed by their own weight in our present world and could not live here. Three is that likewise, there is no way to believe the Podkletnov experiment ever could have worked in an Einsteinian world, or that the ESA could have reproduced that experiment under controlled conditions in 06, but we know that they did.
The "Big Bang(TM)" idea is going away.
It was never based on anything more than a misinterpretation of redshift data and Halton Arp has basically provided the counter examples which destroy it.
“Unless you get into all that “sums over” stuff, that guy whose name started with a D?, said, then the little thingies traveled all possible paths including into the future at C+ and back.
Oh, what’s his name?”
Richard Feynman
I was in naval nuclear power for nine years, and even though reactor principles was my best subject gravitrons were not really something we worried about on a day to day basis.
I try to read threads like this every once in a while to see if my brain is still capable of scientific thought.
The answer appears to be ‘not really’.
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