Archive for March, 2008

A new class of photon gun

Monday, March 31st, 2008

Photon gun

Photons are easy to produce, at least en masse. But making them one at a time in a controlled fashion is much harder. Until recently the only trick physicists had for this was to reduce the brightness of a beam until it contained only one photon at a time, on average.

Of course, the “on average” clause allows for any manner of multiphoton sin: the photons may be produced in pairs or bunches or not at all. That’s a serious problem for techniques such as quantum cryptography because any extra photons are equivalent to a leak of information that an eavesdropper can use to crack the code.

Enter the photon gun brigade, a growing band of physicists attempting to build devices that can fire identical, single photons on demand. The single photon part of this problem has been cracked by a number of groups who have built guns that work by inducing atoms, ions, molecules or quantum dots to fluoresce in a cavity.

What has turned out to be more difficult, but just as important, is making each of these single photons identical. What tends to happen is that the cavites vibrate in various ways and this subtly distorts the wavelength of the photons they produce. The result is that each photon is very slightly different, which spoils their ability to interfere and become entangled with each other.

Here’s the breakthrough: Andrew Shields, at Toshiba’s research labs in Cambridge, UK, plus a few pals from a nearby university, say they can create the photons using a quantum dot that they zap with two precisely timed voltage pulses. The first pulse injects charge carriers into the diode while the second suddenly shifts the quantum dot’s emission characteristics so that it can emit light. When that happens, the dot emits a photon of a specific energy.

But the key is to use such short voltage pulses that the photon doesn’t have time to be influenced by its surroundings. And that means the quantum dot always emits photons of precisely the same wavelength.

So it looks as if Shields and co have a neat new trick up their sleeves, albeit one that has to be performed at 4K. But it’s also an entirely new mechanism for producing photons of this kind so improvements are gonna be forthcoming. How long before we see a commercial version?

Ref: arxiv.org/abs/0803.3700: Indistinguishable Photons from a Diode

In case ya missed ‘em…

Sunday, March 30th, 2008

…this week’s peaches from the physics arXiv blog:

The wound ballistics question

How Hawking radiation may explain dark energy

Rubik’s cube proof cut to 25 moves

The curious case of the disappearing physicist

Questioning the Big Bang

Bang ‘n’ bucks

Saturday, March 29th, 2008

Other highlights from the physics arXiv:

Predicting Synthetic Rescues in Metabolic Networks

Dangerous Implications of a Minimum Length in Quantum Gravity

Punctuated Chirality

Quantum Pattern Recognition With Liquid State NMR

The STARFLAG Handbook on Collective Animal Behaviour

Questioning the Big Bang

Friday, March 28th, 2008


The Big Bang dominates current thinking in cosmology. But the experimental evidence that backs it up is surprisingly thin. In fact there are only two pieces of evidence: the galactic redshift and the cosmic background radiation.

The Big Bang explains these observations but only by introducing problems of their own. So are there any alternative hypotheses that do a better job? Robert Soberman and Maurice Dubin have developed an idea that they say better explains the observations. They also make some testable predictions which should be able to tell which theory is right.

The main problem with the Big Bang theory is that the cosmic background radiation does not have the characteristics you’d expect to see from a Big Bang-type event. For a start, the radiation curve has the distinct whiff of a black body about it, something that can only be produced by ordinary matter radiating at a specific temperature (according to quantum mechanics, anyway). Most theorist do not imagine the Big Bang like this.  Next, the cosmic background seems to cover the sky smoothly in all directions, unlike the matter we see which is clumped into galaxies. Even the microvariations discovered by satellites such as COBE and WMAP bear no relation to the distribution of visible matter.

So what is Soberman and Dubin’s alternative? They hypothesize that interstellar space is filled with tiny clumps of hydrogen and helium atoms called cosmoids (short for cosmic meteroids). The pair have calculated that cosmoids ought to radiate at 2.735K which is exactly the temperature of the cosmic microwave background and this explains the blackbody curve (they say these cosmoids could be easily created and tested in the lab). This  radiation need only be produced by a locally smooth distribution of cosmoids for it to look the same in all directions to us.

The cosmoid idea also explains the galactic redshift. Soberman and Dubin say that cosmoids absorbing and re-emitting light from distant galaxies should redshift the light albeit in a way that is subtely different from a doppler redshift generated by an expanding universe. That subtle difference shuld be relatively easy to spot with a few observations, they say.

The pair add that evidence that cosmoids exist has already been found by experiments onboard the Pioneer and Helios spacecraft.

Oh, and as a by product, the cosmoids make up the missing mass that astronomers call dark matter.

A cracking idea! I’m looking forward to seeing how the cosmologists dismantle it.

Ref: arxiv.org/abs/0803.3604: Was There A Big Bang?

The curious case of the disappearing physicist

Thursday, March 27th, 2008

If you work in particle physics, cosmology or condensed matter, you’ll probably be familiar with the name Majorana, as in Majorana fermions and Majorana neutrinos.

But Ettore Majorana is famous for another reason. As one of the leading lights of theoretical physics in the 1930s, he made important contributions to nuclear, atomic and molecular physics as well as quantum electrodynamics and relativity. But on 26 March 1938, his career was cut short when he disappeared in mysterious circumstances in Naples and was never heard from again.

“On Friday March 25, 1938 Majorana went to the Institute of Physics and handed over the lecture notes and some other papers to one of his students. After that, he returned to his hotel and, after writing farewell letters to his family and to the director of the Institute of Physics, Carrelli, apparently embarked on a ship to Palermo. He reached his destination the following morning, where he lodged for a short time in the Grand Hotel Sole. It was there that he wrote a telegram and a letter to Carrelli pointing out a change of mind about his decisions. On Saturday evening Majorana embarked on a ship from Palermo to Naples. From here onwards, no other reliable information about him are available. “

So writes Salvatore Esposito from the University of Naple in Italy in a retrospective of Majorana’s life and work, posted on the arXiv on the 70th anniversary of his disappearance.

Nobody knows what happened to Marjorana but there is no shortage of theories including a retreat in a monastery, a flight to a foreign country and most likely suicide for which there is some circumstantial evidence. We will almost certainly never know for sure.

Good topic for a Hollywood movie, I’d say.

Ref: arxiv.org/abs/0803.3602: Ettore Majorana and his Heritage Seventy Years Later

Rubik’s cube proof cut to 25 moves

Wednesday, March 26th, 2008

s-cube

Last year, a couple of fellas at Northeastern University with a bit of spare time on their hands proved that any configuration of a Rubik’s cube could be solved in a maximum of 26 moves.

Now Tomas Rokicki, a Stanford-trained mathematician, has gone one better. He’s shown that there are no configurations that can be solved in 26 moves, thereby lowering the limit to 25.

Rokicki’s proof is a neat piece of computer science. He’s used the symmetry of the cube to study transformations of the cube in sets, rather than as individual moves. This allows him to separate the “cube space” into 2 billion sets each containing 20 billion elements. He then shows that a large number of these sets are essentially equivalent to other sets and so can be ignored.

Even then, to crunch through the remaining sets, he needed a workstation with 8GB of memory and around 1500 hours of time on a Q6600 CPU running at 1.6GHz.

But Rokicki isn’t finished there. He is already number-crunching his way to a new bound of 24 moves, a task he thinks will take several CPU months. And presumably after that, 23 beckons.

Where is this likely to finish? A number of configurations are known that can be solved in 20 moves but it’s also known that there are no configurations that can be solved in 21 moves.

So 20 looks like a good number to aim at although that will still be an upper limit. No news yet on whether 20 might also be the lower limit, which would give the answer a satisfying symmetry.

What this problem is crying out for is a kindly set theorist who can prove exactly what the upper and lower limits should be without recourse to a few years of CPU time (although it may take a few years of brain time). Any takers?

Ref: arxiv.org/abs/0803.3435: Twenty-Five Moves Suffice for Rubik’s Cube

How Hawking radiation may explain dark energy

Tuesday, March 25th, 2008

Dark Hawking Energy

In 1993, the Dutch Nobel prize-winning physicist Gerard t’Hooft suggested that all the information in a region of space can be represented as a hologram, an idea that implies that the laws of physics that govern our universe are somehow encoded on its (higher dimensional) boundary.

This idea, known as the holographic principle, has a certain elegance and so has received widespread attention from some theorists although nobody knowns whether it is a true description of the universe or not.

If it is true, Jae-Weon Lee from the Korea Institute for Advanced Study in Seoul and some pals, say that this boundary should emit Hawking radiation.

Hawking originally dreamt up this radiation idea to describe a process that might occur near the event horizon of a black hole. When pairs of virtual particles pop into existence (as they do all over the universe), they normally annihilate each other and disappear again. But near a black hole, one of these particles can cross the event horizon while the other makes its escape and this gives the impression that the black hole is emiting radiation.

Lee’s team say a similar thing may happen at the holographic boundary and that the energy this creates might be responsible for making the expansion of the universe accelerate.

They also explain why this radiation does not interact with ordinary matter and so is not seen in other ways: it’s wavelength, being universe-sized, is too long.

Seems as good an explanation as any other at this stage, after all the competition is ideas like quintessence, k-essence and quintoms.

Two things though: if this radiation exerts a force, why would it act to accelerate the expansion of the universe and not decelerate it? Lee and co are not convincing on this point. And, I wonder whether t’Hooft, who has some exotic ideas of his own about quantum determinism, would say that this kind of cosmic Hawking radiation is not compatible with the holographic principle and therefore bunkum.

Ref: arxiv.org/abs/0803.1987: Is Dark Energy from Cosmic Hawking radiation?

The wound ballistics question

Monday, March 24th, 2008

Wound ballistics

The gloves are off in the world of wound ballistics. The question is: how do handgun bullets do their damage?

According to Martin Fackler, a retired colonel and battlefield surgeon in the US Army Medical Corp, the main cause of injury is along or close to the wound channel, the path the bullet takes through the body. This directly damages tissue and causes loss of blood.

But husband and wife team, Amy and Michael Courtney, from the Ballistics Testing Group at West Point, New York, say there is another process at work. When a bullet enters the body it creates a pressure wave that radiates and reflects through the body. This wave can do damage in areas that are physically distant from the wound channel, a process known as hydrostatic shock.

The Courtneys point to a number of lines of research in pigs and dogs that show strong evidence that a thigh wound can generate significant pressures in the brain.

The Courtneys have written their paper to take on people such as Fackler who they say dispute the idea of hydrostatic shock. They say Fackler’s argument in particular is based on the idea that ultrasonic kidney stone treatment creates pressure waves which produce spatially limited damage.

(The Courtneys also end one of their papers with this: “Get good training, practice regularly, learn to use cover, and pray that you will never have a lethal force encounter armed only with a handgun,” achieving what must be an arXiv first in combining the notions of prayer and lethal force in the same sentence.)

From my limited knowledge of this debate, we’re witnessing a spat over magnitude. Nobody disputes the possibility that hydrostatic shock occurs if the bullet energies are high enough, just whether it occurs with handgun bullets.

There are some gruesome studies from WWII that note the entry and exit wounds of dead soldiers and attempt to correlate this with the cause of death. My understanding is that hydrostatic shock is well established in these cases although these were not generally caused by handguns.

So what to make of this work? My take is that this is an ordinary academic spat in an extraordinary and emotive field of research, one that’s mere existence raises all kinds of ethical issues.

We’re talking the science of killing people and the questions that fascinate me, I guess, are not so much to do with  hydrostatc shock but with what drives and motivates ordinary men and women scientists in this area.

Ref:

arxiv.org/abs/0803.3053 : The Ballistic Pressure Wave Theory of Handgun Bullet Incapacitation

arxiv.org/abs/0803.3051 : Scientific Evidence for “Hydrostatic Shock”

In case ya missed ‘em…

Sunday, March 23rd, 2008

…the sweetmeats from this week’s physics arXiv blog:

Single photons bounced off orbiting satellite

Future brightens for quantum imaging

New type of pulsating star discovered

Do galactic cosmic rays influence global warming? 

Entanglement beats gravitational test

Photons ‘n’ fermions

Saturday, March 22nd, 2008

The best of the rest from the physics arXiv this week:

An Introduction to the Dark Energy Problem

Designing Potentials by Sculpturing Wires

Emergent Gravity and Dark Energy

Is the CMB Cold Spot a Gate to Extra Dimensions?

Evacuation Dynamics: Empirical Results, Modeling and Applications

Coherent Meta-materials and the Lasing Spaser

Translation of Leonhard Euler’s: General Principles of the Motion of Fluids