Have three little photons broken theoretical physics?
An artist’s impression of a gamma ray burst. ESO / A. Roquette
They've just arrived after epic journey to Earth, and offer surprising tale about structure of universe
By Natalie Wolchover updated 8/31/2012 3:24:19 PM ET
Seven billion years ago, three cosmic travelers set out together on an epic journey to Earth. They just arrived, and the trio has a surprising tale to tell about the structure of the universe. Their story could overturn decades of work by theoretical physicists.
But first, an introduction: Scientists have long wondered about the nature of space and time. Albert Einstein envisioned the two concepts as an interwoven fabric that extends smoothly and continuously throughout the universe, warping under the weight of the matter it contains. The smoothness of this stretchy "space-time" fabric means that no matter how closely one inspects it, no underlying structure emerges. The fabric is completely pure even at infinitesimal scales.
Modern "theories of everything" try to reconcile Einstein's big picture view of the universe, built of space-time, with the small-scale picture of the universe described by quantum mechanics. Most of these theories, collectively called "quantum gravity," posit that space-time must not be smooth after all, but must instead be comprised of discrete, invisibly small building blocks — sort of like 3-D pixels, or what scientists have dubbed a "foam."
But real or not, such space-time pixels seemed to be permanently out of human reach. For reasons having to do with the uncertainty that exists in the locations of particles, theories suggest the pixels should measure the size of the "Planck length," or about a billionth of a billionth of the diameter of an electron. With the key evidence for quantum gravity buried at such an inaccessible scale, physicists were at a loss for how to confirm or refute their ideas.
Then, a paper published 15 years ago in the journal Nature proposed an ingenious method of detecting space-time pixels. Giovanni Amelino-Camelia, a theoretical physicist at Sapienza University in Rome, and colleagues said the building blocks of space-time could be discovered indirectly by observing the way light of different colors disperses as it travels through the pixels on its journey across the universe, just as light spreads into its component wavelengths when it passes through the crystalline structure of a prism.
As long as one is sure all the photons, or particles of light, left their source at exactly the same time, measuring how much photons of different wavelengths spread out during their commute to Earth would reveal the presence, and size, of the pixels they passed through.
Such studies hadn't been feasible, until now.
"Very few of us were suggesting that the structure of space-time could be detected, and now 15 years later facts are proving us right," Amelino-Camelia told Life's Little Mysteries.
Burst of light
Seven billion years ago, 7 billion light-years away, a gamma-ray burst sent a blitz of photons tearing into space. Some of them headed for Earth.
Gamma-ray bursts occur when an extremely massive, rotating star collapses in on itself, unleashing in less than a minute as much energy as our sun will radiate in its entire 10-billion-year lifetime. These shockwaves of gamma rays and other energetic photons are the brightest events in the universe. When gamma ray bursts have occurred in the Milky Way galaxy, scientists speculate that they might have altered Earth's climate and induced mass extinctions. Thankfully, the bursts are so rare that they typically occur a safe distance away — far enough that only a light mist of photons reaches our planet. NASA's Fermi Gamma-ray Space Telescope [ http://www.space.com/13838-nasa-gamma-ray-targets-blazars-fermi.html ] was launched into orbit in 2008 to scan the skies for these mists of shockwaves past.
Robert Nemiroff, an astrophysicist at Michigan Technological University, and colleagues recently took a look at data from a gamma-ray burst detected by the Fermi telescope in May 2009.
"Originally we were looking for something else, but were struck when two of the highest energy photons from this detected gamma-ray burst appeared within a single millisecond," Nemiroff told Life's Little Mysteries. When the physicists looked at the data more closely, they found a third gamma ray photon within a millisecond of the other two.
Computer models showed it was very unlikely that the photons would have been emitted by different gamma ray bursts, or the same burst at different times. Consequently, "it seemed very likely to us that these three photons traveled across much of the universe together without dispersing," Nemiroff said. Despite having slightly different energies (and thus, different wavelengths), the three photons stayed in extremely close company for the duration of their marathon trek to Earth.
Many things — e.g. stars, interstellar dust — could have dispersed the photons. "But nothing that we know can undisperse gamma-ray photons," Nemiroff said. "So we then conclude that these photons were not dispersed. So if they were not dispersed, then the universe left them alone. So if the universe was made of Planck-scale quantum foam, according to some theories, it would not have left these photons alone. So those types of Planck-scale quantum foams don't exist."
In other words, the photons' near-simultaneous arrival indicates that space-time is smooth as Einstein suggested, rather than pixilated as modern theories require — at least down to slightly below the scale of the Planck length, a smaller scale than has ever been probed previously. The finding "comes close to proving (that space-time is smooth) for some range of parameters," Nemiroff said.
The finding [ http://prl.aps.org/abstract/PRL/v108/i23/e231103 ], published in June in the journal Physical Review Letters, threatens to set theoretical physicists back several decades by scrapping a whole class of theories that attempt to reconcile Einstein's theory with quantum mechanics. But not everyone is ready to jettison quantum gravity.
Other effects?
"The analysis Nemiroff et al. are reporting is very nice and a striking confirmation that these studies of Planck-scale structure of space-time can be done, as some of us suggested long ago," said Amelino-Camelia, an originator of the idea that gamma rays could reveal the building blocks of space-time. "But the claim that their analysis is proving that space-time is 'smooth with Planck-scale accuracy' is rather naive."
To prove that Planck-scale pixels don't exist, the researchers would have to rule out the possibility that the pixels dispersed the photons in ways that don't depend in a straightforward way on the photons' wavelengths, he said. The pixels could exert more subtle "quadratic" influences, for example, or could have an effect called birefringence that depends on the polarization of the light particles. Nemiroff and his colleagues would have to rule out those and other possibilities. To prove the photon trio wasn't a fluke, the results would then require independent confirmation; a second set of simultaneous gamma-ray photons with properties similar to the first must be observed.
If all this is accomplished, Amelino-Camelia said, "at least for some approaches to the quantum-gravity problem, it will indeed be a case of going back to the drawing board."
Recently people around the world have been exposed to an unfamiliar scientific term, due to the observation of a new elementary particle by physicists at CERN. The term is "boson," and the particle, the Higgs boson, completes a chapter in the modern sub-atomic description of nature. The word surely sounds odd to non-expert ears, with its echoes of a certain American TV clown, and no obvious Greek or Latin root. What is the origin of this term, and why is it worth specifying that the Higgs is not just any old supersmall particle, but a member of the particular family of fundamental physical objects grouped together as bosons? In fact this terminology goes back almost 90 years, to the dawn of the modern understanding of the atom. It stems from one of the most fortuitous episodes in the modern history of science, and involves intimately the most famous physicist of all time, Albert Einstein.
In June of 1924 Albert Einstein had become not just the best-known scientist of his era, but one of the most recognized names on the planet. The demands on his time and attention had grown exponentially due to the publicity associated with his now experimentally confirmed General Theory of Relativity. Thus when an unknown 30-year old Indian physicist, Satyendranath Bose, sent him an unsolicited manuscript to read, the chances that it would end up anywhere but the circular file were very low. "Respected Sir, I have ventured to send you the accompanying article for your perusal and opinion," the letter began, and after explaining the scientific goal of the paper, it closed with an astonishing request, that Einstein translate the English manuscript into German for publication. "Though a complete stranger, I do not feel any hesitation in making such a request. Because we are all your pupils..." Despite the long odds, in this case Bose hit the scientific lottery. At that time Einstein was deeply involved in the struggle to understand how atoms and light behaved, a 20-year quest that had begun in 1905 when had dared to suggest that light, which had been "proven" to be an electromagnetic wave during the 19th century, consisted of localized particles, which we now call photons. Bose's paper was on this topic and Einstein read it carefully, decided that it "signifies an important advance," and translated it for publication in a top German physics journal. This began a chain of events that ultimately enshrined Bose's name in the modern theory of nature.
Bose had tried to solve a longstanding problem in describing thermal radiation (the electromagnetic energy emitted by any hot object) using Einstein's photon concept. The fundamental law determining how much energy there is in thermal radiation had been found by Max Planck twenty-four years earlier, but up to that point all attempts to deduce this law from the "photon gas" picture, using thermodynamic principles had failed. Somehow Bose, in a terse document of less than two journal pages, had succeeded. But how had he done it?
The key was to count the number of states of motion that a photon can take on, when confined to a certain volume; this would determine the "entropy" of the gas, from which the Planck Law followed. However, in counting the photon states Bose had, apparently unknowingly, counted them differently from all previous physicists, including Einstein. When his new approach gave the right answer (Planck's Law), he simply wrote up the calculation, without any detailed discussion, and sent it to Einstein. Somehow, Einstein intuited that this new counting method was not simply an error by an inexperienced researcher, but represented a correct guess about the bizarre properties of the unobservable atomic domain.
How could something as mundane as an atomic accounting method actually change our view of nature? Well, as any gambler knows, the laws of statistics are also laws of nature. The reason that when we flip two coins we find a heads and a tails half the time (on average) is that the coin is equally likely to land on either side. Moreover there are two ways to get a heads and a tails (coin 1 = heads, coin 2 = tails; coin 1 = tails, coin 2 = heads) and only one way to get either of the other results. But what if we had two really identical coins, and instead of flipping them in the open we jiggled them around in a closed box, and then opened it for each trial? In this case we would not know, when we found a heads and a tails, whether it came from one or the other of the two ways. Would this change the probability that we get a heads and a tails? Absolutely not. These probabilities stem from the fact that each coin is a distinct object with independent properties. But Bose's accounting had essentially denied that this was true of micro-particles like photons.
Bose's reasoning assumes that photons are not like macroscopic coins, and that it makes no sense to ask whether photon 1 is in state 1 and photon 2 is in state 2, or vice-versa. These two states do not separately exist and hence there is only one such configuration of two photons. If we think of photons as "quantum coins," the probability of flipping two of them and getting a tails and a heads is only one third, not one half (and correspondingly the probability of heads-heads or tails-tails is now increased to one third). Note, and here's the mind-bending part, this is not because photons (or atoms) are small and we can't tell which photon is in which state. Unlike macroscopic coins, the quantum coins exist in a single fuzzy combination of heads-tails + tails-heads. While all of this was implicit in Bose's reasoning, he much later admitted that he "had no idea that what I had done was really novel."
Einstein however quickly grasped the enormous implications of this change of viewpoint. By December of 1924 he had understood the meaning of Bose's new statistics and applied them to a conventional gas consisting of atoms. He discovered that at ultra-low temperatures atoms can form a new state of matter, called a Bose-Einstein condensate, which eventually was observed in Nobel prize winning experiments in 1995. Within the next few years, Werner Heisenberg, Erwin Schrodinger and others found the basic equations describing atoms and light, the theory now known as quantum mechanics. It turned out that in addition to particles that obey Bose statistics, now called bosons, there is another category of particles, called fermions, after the physicist Enrico Fermi. These particles are indistinguishable in the Einstein-Bose sense, but also cannot share the same state with each other. In the coin analogy, the states head-heads and tails-tails can't occur. Protons and electrons are fermions, whereas bosons are the force-carrying particles in nature, the Higgs being the newest member of the club. All these force-carriers carry the name of a physicist whose elevation into the physics pantheon hung on the slimmest of chances, that the great man, Einstein, would rescue his groundbreaking paper from obscurity.
A. Douglas Stone is Carl Morse Professor and Chair of Applied Physics at Yale University. His forthcoming book from Princeton University Press is titled, "Einstein and the Quantum: The Quest of the Valiant Swabian."
2012 Dirac Lecture: The Accelerating Universe - presented by Professor Brian P. Schmidt
Published on Aug 10, 2012
The UNSW Science 2012 Dirac Lecture was presented by Nobel Laureate Professor Brian P Schmidt. The Accelerating Universe: In 1998 two teams traced back the expansion of the universe over billions of years and discovered that it was accelerating, a startling discovery that suggests that more than 70% of the cosmos is contained in a previously unknown form of matter, called Dark Energy. The 2011 Nobel Laureate for Physics, Brian Schmidt, leader of the High-Redshift Supernova Search Team, describes this discovery and explains how astronomers have used observations to trace our universe's history back more than 13 billion years, leading them to ponder the ultimate fate of the cosmos.
.. not sure if it's the one on tv now, i've learned one thing though .. he just said "we use 'dark' for something we really don't understand" .. hmm, a thought just while typing that was it could apply to the lack of understanding of racists, too .. course those woans of 'man' do understand that .. yup, woans do know racists exist in their own tiny, diminished and 'fixed' universe .. sod, so easy it is to slip from light to dark .. oh, he just finished .. me too .. grin ..
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