Finally, a MAGIC test for string theory?

reposted from New Scientist

BEING 4 minutes late doesn't usually cause physicists around the world to fizz with excitement - but it's a different matter if the latecomer is a photon, and its tardiness could indicate a breakdown of relativity on cosmic scales. What's more,

this delay could provide us with our first hints of quantum gravity at work, and thus be a unique way of testing string theory.

Last month, 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.

According to Einstein's theory of special relativity, both sets of photons should have arrived simultaneously, and the team is controversially claiming that the discrepancy is due to the first detected effects of quantum gravity.

Theories of quantum gravity - which attempts to shoehorn gravity into quantum mechanics - predict that space-time fluctuates rapidly on so-called Planck scales of about 10^-35 metres. These fluctuations could have slowed down high-energy gamma-rays, causing MAGIC's observed time delay

, says team member Nick Mavromatos at King's College London. Mavromatos and his colleagues have developed

a model for quantum gravity that is based on an unconventional version of string theory, which they say predicts the 4-minute delay exactly. Alternative quantum gravity models based on standard versions of string theory can't explain the effect

, he says.

Not surprisingly, the announcement prompted a burst of high-energy activity on physics blogs, as arguments raged over whether or not the results are real. "If true, it would be a Nobel prize-winning discovery," says Subir Sarkar, an astrophysicist at the University of Oxford.

For the two best-understood string-theory models,the news is not good. "If the result is correct it would be earth-shaking," says string theorist Joe Polchinski at the University of California in Santa Barbara. "If MAGIC is right then [conventional] string theory is wrong."

Amid the excitement, there is still a big question mark hanging over MAGIC's time delay. The collaboration

cannot rule out the straightforward explanation that the high-energy and low-energy gamma-ray photons were emitted at different times at the source

, admits team member Dimitri Nanopoulos at Texas A&M University, College Station. But he stands by his team's interpretation.

"Nature would be playing a dirty trick on us, if it was doing something strange at the source that created the exact time delay that our theory predicts,"

he says. The team plan to analyse more gamma-ray flares to check whether they also display the same effect.

Even if the time delay does turn out to have another explanation, the result is still important, says Sarkar. "Theories of quantum gravity are so mathematical, people think you can only get at the right one by looking for the most aesthetically pleasing theory," he says. "Now MAGIC has confirmed that gamma rays can probe the scales where quantum gravity might kick in, and that in itself is exciting."

Like Polchinski, string theorist Leonard Susskind at Stanford University in Palo Alto, California, believes that ultimately the time delay will be explained simply. "Most of the time the results go away," says Susskind. "But once in a very great while they stick."

From issue 2620 of New Scientist magazine, 08 September 2007, page 17

The Elegant Universe - by Brian Greene

reposted from: http://www.pbs.org/wgbh/nova/elegant/program_d.html

Look at the 3 hour programme hosted by Brian Greene.

New particle accelerator could rule out string theory

• 22:04 01 February 2007
• NewScientist.com news service
• David Shiga

String theory could be ruled out by experiments at the Large Hadron Collider (LHC), a particle accelerator scheduled to open by the end of 2007, a new study says. The finding offers a new approach for testing this potential "theory of everything", a goal that has so far proven elusive.

reposted from: New Scientist
my highlights / emphasis / edits

According to string theory, particles like electrons and photons are actually tiny, vibrating strings. The beauty of the theory is that it accounts for all of the known forces – including gravity, which the standard model of physics does not. But its critics have complained that there is essentially no way to test it.

Strong evidence for string theory could come from the observation of short-lived, mini black holes at the LHC (see Watching God play dice: The Large Hadron Collider). But the chance of them appearing is extremely small, so a failure to see them would not be a death blow for the theory.

In 2006, string theorist Allan Adams of MIT in Cambridge, US, and others offered a more promising check. They showed that some particle collisions could reveal whether certain fundamental assumptions underlying string theory are wrong.

Now, another team has shown that the energies needed to reveal such effects are achievable at the LHC, which is being built in Geneva, Switzerland. The team was led by Jacques Distler of the University of Texas in Austin, US.

High energies

One of string theory's assumptions comes from Einstein's theory of relativity – that the speed of light is the same for all observers, a principle called Lorentz invariance.

This principle – and three others underlying string theory – determine how strongly particles called W bosons, which transmit the weak nuclear force, interact.

If these interactions are below the strength calculated by Distler's team, it would signal that one of the assumptions built into string theory is incorrect and that therefore string theory itself is wrong, the researchers say.

"They did a very important thing," Adams told New Scientist.

Quantised space

If string theory does seem to be ruled out, physicists will have to find another theory of everything that can explain the LHC observations. "If we see these violations, people will start working very feverishly on some sort of alternative that will produce these violations," Distler told New Scientist.

That alternative may turn out to be a theory called loop quantum gravity, which posits that space itself is quantised into tiny chunks. Some physicists argue that loop quantum gravity does not satisfy Lorentz invariance. "So that's one possible direction people might go," Distler says.

Although the test could in principle rule out string theory if violations are found, both Distler and Adams suspect that the results will turn out to respect the four assumptions, leaving string theory as a viable candidate for the theory of everything.

Quantum World - Learn more about a weird world in our comprehensive special report.

Journal reference: Physical Review Letters (in press)

Physics Will Not Achieve a Theory of Everything

Reposted from: http://edge.org/q2007/q07_14.html
my highlights in blue

FRANK WILCZEK
Physicist, MIT; Recipient, 2004 Nobel Prize in Physics; Author, Fantastic Realities

Physics Will Not Achieve a Theory of Everything

I'm optimistic that physics will not achieve a Theory of Everything.

That might seem an odd thing to be optimistic about. Many of my colleagues in physics are inspired by the prospect of achieving a Theory of Everything. Some even claim that they've already got it. (Acknowledging, to be sure, that perhaps a few i's remain to be dotted or a few t's to be crossed.) My advice, dear colleagues: Be careful what you wish for. If you reflect for a moment on what the words actually mean, a Theory of Everything may not appear so attractive. It would imply that the world could no longer surprise us, and had no more to teach us.

I don't buy it. I'm optimistic that the world will continue to surprise us in fascinating and fundamental ways.

Simply writing down the laws or equations is a long way from being able to anticipate their consequences. Few physicists—and no sober ones—seriously expect future work in fundamental physics to exhaust, for example, neuroscience.

A less literal reading of "Theory of Everything" is closer to what physicists who use it mean by it. It's supposed to be a theory, not really of everything, but of "everything fundamental". And here "fundamental" is also being used in an unusual, technical sense. A more precise word here might be "basic" or "irreducible". That is, the physicists' Theory of Everything is supposed to provide all the laws that can't be derived logically, even in principle, from other laws. The structure of DNA surely emerges—in principle—from the equations of the standard model, and I strongly suspect that the possibility of Mind does too. So those phenomena, while they are vastly important and clearly fundamental in the usual sense, aren't fundamental in the technical sense, and elucidating them is not part of a Theory of Everything.

I think we're about to enter a new Golden Age in fundamental physics. The Large Hadron Collider (LHC), which should begin to operate at CERN, near Geneva, starting in summer 2007, will probe the behavior of matter at energies higher than ever accessed before. There is no consensus about what we'll find there. I'm still fond of a calculation that Savas Dimopoulos, Stuart Raby and I did in 1981. We found—speaking roughly—that we could unify the description of fundamental interactions (gauge unification) only within an expanded version of relativity, which includes transformations of spin (supersymmetry). To make that dual unification we had to bring in new particles, which were too heavy to be observed at the time, but ought to be coming into range at the LHC. If they do exist we'll have a new world of phenomena to discover and explore. The astronomical riddle of dark matter could well be found there. Several competing ideas are in play, as well. The point is that whatever happens, experimenters will be making fundamental discoveries that take us by surprise. That would be impossible, if we had a Theory of Everything in the sense just described—that is, of everything fundamental.

In recent months a different, much weaker notion of what a "Theory of Everything" might accomplish has gained ground, largely inspired by developments in string theory. In this concept, the Theory provides a unique set of equations, but those equations that have many solutions, which are realized in different parts of the Universe. One speaks instead of a multiverse, composed of many domains, each forming a universe in itself, each with its own distinctive laws. Now even the fundamental—i.e., basic, irreducible—laws are beyond the power of the Theory to supply, since they vary from universe to universe. At this point the contrast between the grandeur of the words "Theory of Everything" and the meager information delivered becomes grotesque.

The glamour of the quest for a Theory of Everything, or a Final Theory, harks back Einstein's long quest for his version, a Unified Field Theory. Lest we forget, that quest was fruitless. During his great creative period, Einstein produced marvelous theories of particular things: Brownian motion, the photoelectric effect, the electrodynamics of moving bodies, the equality of inertial and gravitational mass. I take inspiration from the early Einstein, the creative opportunist who consulted Nature, rather than the later "all-or-nothing" romantic who tried (and failed) to dictate to Her. I'm optimistic that She'll continue to surprise me, and my successors, for a long time.

The Return of the Discipline of Experiment Will Transform Our Knowledge of Fundamental Physics

Reposted from: http://edge.org/q2007/q07_8.html
my highlights in blue

LEE SMOLIN
Physicist, Perimeter Institute; Author, The Trouble With Physics

The Return of the Discipline of Experiment Will Transform Our Knowledge of Fundamental Physics

In science as in politics it seems that Eldredge and Gould's metaphor of punctuated equilibrium holds. When progress happens, it happens fast and the whole culture vibrates with the excitement of it. We have had a bit too much equilibrium lately, of disappointed expectations following as a natural consequence of unwisely reduced ambitions. But I am optimistic that the next decades will see breakthroughs in key problems on which we now seem stuck. In physics, new experiments including the LHC, AUGER, GLAST, PLANCK, LIGO and others are likely to transform our knowledge of fundamental physics, and end the long period when theory sought to progress without the discipline of experiment. Very likely we will be surprised and humbled by what is seen, but this will be followed by rapid progress as new ideas are quickly invented to explain the surprising data.

How can I be optimistic without knowing what direction science will take? This is exactly the point. There are two kinds of optimism, the optimism of people who think they know the future and the optimism of people who believe the future will be more interesting and, if always imperfect, more wonderful than they can imagine. I am of the second kind. The first kind sometimes comes along with a belief that time and change are illusions, and that the world is evolving towards an eternal timeless state of perfection. This is the optimism of religious fundamentalists and orthodox Marxists, and one sees it reflected also in the cosmologies in which our evolving universe is just a transient fluctuation in an otherwise permanent state of thermal equilibrium. The opposite kind of optimism lies behind the evolutionary theorists who believe the world is so intricate that the simplest mechanism that could predict the future of life and the cosmos is the universe itself. If we are the first kind of optimist we seek to transcend the complexities of life to discover something eternal behind it, something like the imagined view of God. If we are the second, we seek to live and think within the swirl of life; we aim for comprehension and wisdom but have no illusions of transcendence or control.

The Future Of String Theory

Reposted from: http://edge.org/q2007/q07_7.html
my highlights in blue

GINO SEGRE
Physicist, University of Pennsylvania; Author: Faust In Copenhagen: A Struggle for the Soul of Physics

The Future Of String Theory

I am optimistic about the future of our thinking regarding string theory and the early universe. Until fairly recently I did not feel this way since string theory seemed to be a community unto itself, albeit a very talented one. Controversy has created an important dialogue and strife has erupted. I think this is all to the good. The basis for the disagreement goes back 30 years.

A unified understanding or so-called "theory of everything" has long been sought. The standard model that emerged in the 1970s provided a very significant step forward but left undetermined some 20 parameters: the values of the six quark and six lepton masses, various couplings etc. Initially it was hoped that string theory, aside from a unification of forces with quantum gravity, would determine the values of these parameters. That dream has not been realized.

A very significant group of theoretical physicists has now abandoned the dream. Pointing out that even string theory supports the view that an essentially infinite number of possibilities can be realized for a universe, the so-called landscape, they maintain that we live in one of these choices, the universe where the 20 or so parameters are fixed to be the values we observe. Other universes, with other values of the parameters, are continuously emerging and dying and still others live by our side. However we are limited in the possibility of observations and measurements to our own universe so that, in a deep sense, the 20 parameters that determine our world are completely arbitrary. We would not exist if they were not what they are, but there is no further understanding of their values.

A second group maintains that abandoning the dream that set elementary particle physics on its course a century ago, that of determining the forces and parameters of the sub-atomic world, is both premature and intellectually wrong. They maintain this is not science.

There is an intermediate position that, understandably, has not been embraced vigorously by either side. Perhaps very few of the 20 or so parameters, some of the mass scales, correspond to the universe we live in, but the others are set by string theory or some future theory we have not yet discovered. This could happen if e.g. the quark and lepton masses are calculable numbers that multiply a mass given by the particular universe we happen to live in. In this case both sides would be right. The numbers would be set by the theory and the mass scale by the choice of universe. I find the notion intriguing, but it may also be that both sides are wrong and some other stunning synthesis will emerge.

So why am I optimistic? Because I believe that controversy, with clearly drawn out opposing positions, galvanizes both sides to refine their opinions, creates excitement in the field for the participants, stimulates new ideas, attracts new thinkers to the fray and finally because it provides the public at large with an entrée into the world of science at the highest level, exhibiting for them heated arguments between great minds differing on questions vital to them. What could be more exciting?