Posted on 08/09/2009 12:08:19 PM PDT by LibWhacker
Are cosmic rays revealing the quantum nature of spacetime? Could theories of (not) everything help solve the puzzle of quantum gravity? The architect of doubly special relativity thinks so.
In his youth, there were two things that regularly competed for Giovanni Amelino-Camelia’s attention: his favorite soccer team, Napoli, and "anything that came close to being scientific." And since Napoli was struggling in the Italian soccer league in the summer of 1978, Amelino-Camelia found himself watching a series of programs on special relativity instead of soccer. "That was really the point of no return for me," he remembers. "Although I was 13-years old, nothing could have happened after that to keep me away from fundamental physics," he says. "It was lucky for me that those television shows were broadcast in a year when Napoli did very poorly!"
Lucky for him, and, in many ways, lucky for us, because almost 30 years later Amelino-Camelia, at La Sapienza University in Rome, Italy, is dedicated to pursuing answers to foundational questions. Armed with a $65,000 grant from FQXi, he is currently making a run at redefining the way physicists attack perhaps the most vexing and elusive puzzle of all, the problem of quantum gravity. Many lament that experimental tests of quantum gravity lie way beyond reach. However, Amelino-Camelia believes that cosmic rays may already be revealing clues about the frothiness of the fabric of the universe, and he is using these hints to probe the quantum nature of spacetime—one step at a time.
Physicists have been struggling to find an overarching theory that unites quantum mechanics with gravity for decades. Amelino-Camelia, by contrast, is taking a different approach. He isn’t searching for a single, all-encompassing theory of everything, such as promised by string theory—which suggests that elementary particles are made of tiny, vibrating strings—or the more recent loop quantum gravity—which posits that at the smallest scales spacetime is a woven fabric of quantum threads. Rather, he explains, he is searching for theories of not everything.
That doesn’t mean that Amelino-Camelia thinks that searches for a theory of everything should be abandoned. In fact, the majority of the simple models exploring portions of the quantum gravity problem that he works on were inspired by results obtained in the development of loop quantum gravity.
"Even assuming my concerns are correct—and only time will tell—research looking for a full solution of the quantum gravity problem would be very valuable because it provides guidance for the development of meaningful theories of not everything," says Amelino-Camelia. "But if all we did was research on the more ambitious theory-of-everything level, then inevitably we would be wasting some opportunities for valuable insight within the reach of the not everything approach."
Doubly Special Relativity
Okay, we now have a decent understanding of what Amelino-Camelia is not looking for. But what exactly is he looking for? The simplest example of a theory of not everything is one that can describe a "quantum spacetime." Both general relativity and quantum mechanics are formulated using the intuitive concept of a classical spacetime, he explains. But many arguments—including some based on theory-of-everything studies—suggest the correct microscopic description of spacetime should be based on a nonclassical geometry, that is, on a quantum spacetime.
The concept gels well with a theory that Amelino-Camelia introduced in 2000, which he dubs "doubly special relativity." While Einstein’s special relativity tells us that there is a maximum speed limit for light, Amelino-Camelia’s doubly special relativity posits that there is also a minimum length—the Planck length—below which space cannot contract. Spacetime, in this picture, is not continuous but can be thought of as a froth, made up of roiling pieces, or Planck-scale grains, that are just one hundred billion billionth the size of an atomic nucleus. This grainy spacetime is said to be quantized.
Instead of trying to fit this notion of a quantum spacetime straight into a larger quantum-gravity model, Amelino-Camelia argues that it will be fruitful to first fully examine its implications. Taking smaller bites out of the quantum gravity puzzle, in this way, gives physicists manageable chunks that they can chew on more easily.
Crucially, this opens up the possibility of examining each portion thoroughly and subjecting each portion to experimental scrutiny, explains Amelino-Camelia. Some aspects of the puzzle can be tested using today’s technology, while others will hopefully be testable in the coming years, as new technologies are developed.
Probing Spacetime
For example, different types of quantum spacetime could affect particles in slightly different ways. In most scenarios, these tiny effects would be tough to detect. But one place where the effects might show up is in the behavior of some high-energy particles, called cosmic rays, and bursts of high-energy radiation, known as gamma rays. Cosmic rays and gamma rays travel huge distances across the universe, and over the course of their long journey the tiny effects of spacetime quantization would have had a chance to accumulate to an observable level, Amelino-Camelia explains.
Cosmic rays have already helped to rule out some of the simplest quantum pictures of spacetime. Those models predicted that the maximum possible energy of cosmic rays, known as the GZK cutoff, would be significantly higher than currently thought. However, data recently gathered by the Pierre Auger cosmic-ray observatory does not support this. "In order to get to the level of probing more promising pictures, we still have some way to go," says Amelino-Camelia. "Although it is something we will realistically start doing within a few years."
In addition, some quantum-spacetime models predict a particular relationship between the speed of photons in gamma rays and their energy. There have already been tantalizing hints of these effects. In 2007, for instance, the MAGIC gamma-ray telescope collaboration based on La Palma in the Canary Islands announced that they had measured a 4-minute time difference between the arrival of high and low-energy gamma rays released at the same time in a flare from the Markarian 501 galaxy, some half a billion light years away (Physics Letters B, 668, 253-257, 2008). Standard theories suggest that the photons should have arrived simultaneously.
Along with Lee Smolin at the Perimeter Institute in Waterloo, Ontario, Amelino-Camelia has begun analyzing new gamma-ray data from NASA’s Fermi Telescope, launched in June 2008 (see image above right). The new data show similar delays in the arrival times of photons, which they believe will help physicists discriminate between these models. In this way, theories of not everything are falsifiable—giving them the edge over more ambitious theory-of-everything candidates.
John Stachel, a physicist at Boston University, Massachusetts, admires this drive to produce a falsifiable theory. "Amelino-Camelia grew impatient with the exclusively theoretical nature of most work on quantum gravity," he says. "The smallness of most predicted effects threatened to make the field more an area of speculative scholasticism than a part of science."
But it’s not just about being able to scrutinize the theory using experiments and observations. Ultimately for Amelino-Camelia, theories of not everything offer a greater sense of wonder than an all-encompassing theory of everything ever could. "I do not see so much beauty in the picture of having a theory of everything," he says. "It seems to me it is much more appealing to find partial answers, answers that close the door on a few open issues but actually open the door on many more puzzling issues."
If you had, I would have said the answer was:
Two - one to climb the giraffe and the other to fill the bathtub with brightly colored machine tools.
You were caught in a Schrodinger’s Cats 22 situation.
parsy, whose wave function has collapsed
I’d been swimming in a luxurious, long, deep pool in the San Fernando Valley at. 83 degrees by a thermometer in the water while 100+ in the air.
Sitting in the Sun, feet dangling in the water, I could see the light’s refracted patterns (Snell’s law) from it’s surface along the bottom of the pool.
I’d seen video documentary mock-ups to show how existence “looks” at or near the Planck Scale. Yet here and now was an example even clearer.
Envisioning part of the pool bottom as a slice of “quantum space”, filaments fleeted into - and out of - existence, each having been created by that gone before. And out of them bits of brightness (again, reflections of the Sun along the surface, cast on the pool bottom and sides) would move along the length of each segment. Intersecting with their ends such “strings”- quickly dissipating here, and reappearing there - danced on with each such interaction.
Moreover, there would enter waves of interference - by admittedly moving my legs, in fact simulating energy from outside the system - that would yield to the “strings” an even greater sense of overall excitation.
In a stretch, this might serve as a representation of the “Uncertainty Principle” where at the quantum level, “virtual” entities come into and go out of existence, yet presumably sum to an “actualization” of reality, which however even billions of years hence, may as likely decay and zap back into the void from whence they came.
And I’d bet too that this could all be worked out mathematically.But, as I said, it’s merely a representation of such conditions...which is all that can ever be achieved at the Planck Scale anyway.
So, as this lazy day’s imaginings gave rise to other waxings, back into the water I went, even in that second of submergence, back to childhood again.
But these guys interest me. They are all about thinking outside the box, and the academic box is as stubbornly orthodox and self-protective as the Communist Party USA.
Great site, too. Great thought experiments, based on better data, like a four minute separation in the arrival of cosmic rays of different energies from a galactic flare at extra-galactic scale distance.
I wonder if it's occurred to anyone that gravity, unlike electro-magnetism, propagates instantly; that it is an echo of super-position left over from the earliest states of energy. That would rock the conference, wouldn't it?
And would this be the means by which physically separated entangled particles are able to "communicate" instantaneously?
You found it. Check with the Nobel people. If they can give Al Gore a prize I’m sure they’d consider that. :)
The earth is about 45 minutes away from Jupiter. Wouldn't we be being tugged by Jupiter from where Jupiter was 45 minutes ago, rather than where Jupiter is now?
Is this effect so small that it is still unmeasureable or has someone already looked into it?
Just wondering.
To observe things in our solar system we use light waves. Any observation of gravity would travel to us at the same rate. By the time we see it, it’s happened essentially. Indirect stellar measurements with much larger masses make it easier. If you want to test say, the concept of a battery for instance. It’s much easier to confirm a car battery’s got juice as opposed to a watch battery. It’s a matter of scale. Binary stars decaying in orbit with respect to each other are much easier to measure as the forces are larger and the decay rate itself is dependent on gravity.
Just thought I should add. The models used to calculate the speed of gravity through gravitational dampening with respect to binary pulsars, I don’t agree with personally. It’s the best we’ve got at this point. Like you implied earlier. Theorists need more people who think outside the box. The boxes we’re in are leading to dead ends at this point.
So long as its not that dang box with the cat and flask of poison in it, I guess we're OK.
>>Camelia’s doubly special relativity posits that
>>there is also a minimum length—the Planck
>>length—below which space cannot contract.
But if spatial density is variant, then so too are units of distance - including the Planck Length.
I’m sticking with Occam’s Razor and assuming the gravitational lensing observed and associated with hypothetical dark matter is due, not to “dark matter”, but to variation in spatial density.
If space is to energy as energy is to space, then it is reasonable to assume there are areas of space where the energy density is not sufficient to manifest the strong forces required for the formation of matter - but still strong enough to manifest the weak force of gravity, observable as gravitational lensing.
>>But when he got to the other side Bohr was there also.
Probably.
You were in the pool? Okay, a little jealous...
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