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Magnetic fields in gamma-ray burst jets

Saturday, January 30, 2010

Gamma-ray bursts (GRBs) are the most dramatic short-lived violent events observed in the universe. They are often described as releasing a quantity of energy, in less than a minute, that is at least as much as a star like the Sun releases in its entire 10 billion year lifetime. Since the first detection of a gamma-ray burst in 1967, the central question has been to determine the nature of the process or processes that can release so much energy so quickly.

We've discussed gamma-ray burst several times before, such as here, here, and here.

The defining characteristic property of a GRB is a rapid, highly energetic burst of gamma rays, lasting only a few seconds. Most of the GRB energy is released in that event. But beyond that, GRBs exhibit a bewildering diversity of characteristics – reflecting a diversity in the conditions that can produce a GRB.

The most noticeable difference observed in GRBs is that some events are over very quickly – within 1 or 2 seconds – while others include an "afterglow" of radiation less energetic than gamma rays, lasting as long as minutes in some cases. In a few instances there are events that have some properties of both "short" and "long" types of GRB.

Gamma rays cannot penetrate the Earth's atmosphere, so the initial phase of any GRB is detectable only from a satellite-based instrument. This is rather limiting in terms of the types of observations that can be made. For example, satellites lacked instruments that could record a spectrum of a GRB event in lower-energy electromagnetic radiation. Without a spectrum, astronomers cannot measure redshift, and hence the distance of the event.

The development and deployment within the last 10 years of systems that could use satellite detection of GRBs to activate automated ground-based telescopes have dramatically improved this situation. It's now possible to collect much more detailed data, so that astronomers have been able to learn a lot more about GRBs – in many cases.

Even so, many short GRB events are over in just a few seconds, and therefore much less is yet known about short GRBs. The best current guess is that such events are caused by the merger of a pair of binary neutron stars.

The research to be described here, therefore, concerns long GRBs, where it is relatively easy to study the characteristics of the lower-energy electromagnetic radiation, which makes up the afterglow for several minutes or even tens of minutes after the initial burst.

A general consensus has emerged that gamma-ray burst progenitors are certain types of supernovae. Not just any type, either, because the progenitor must be capable of releasing the amount of energy actually observed in a GRB. This rules out Type Ia supernovae, which result from a thermonuclear explosion of matter that has accreted onto the surface of a white dwarf star.

Type Ia supernovae are important in cosmology, because they all have roughly the same intrinsic brightness. This makes it possible to determine the approximate distance of a Type Ia supernova event, just from the observed brightness. By comparing this distance with the redshift of the supernova it is possible to determine how rapidly the universe has expanded in the past. This, in turn, is what made it possible to conclude, in 1997, that the expansion of the universe is accelerating.

However, the energy released by a Type Ia supernova is far too small to account for a GRB. Instead, a different supernova mechanism, known as "core collapse" is needed. Classification of supernovae is a little confusing, since it was originally done on the basis of spectral characteristics. Type II supernovae have a particular line in their spectrum due to hydrogen, while Type I supernovae do not.

Type II supernovae result from the collapse of very massive stars at the end of their lives, when they can no longer support their own weight by the pressure of fusion occuring in their constituent matter. But it turns out that some supernovae lacking the hydrogen spectral line are too energetic to result from the same mechanism as that of Type Ia supernovae. These types are known as Type Ib and Type Ic, and they also result from core collapses of massive stars (that have already burned off all their hydrogen).

So there are significant differences even among core collapse supernovae, resulting from such factors as the total mass of the progenitor star and the original composition of the star, among other things. Such differences can account for some of the differences observed if such supernovae are responsible for GRBs.

But by no means all core collapse supernovae produce a GRB. Theoretical considerations dictate that several other factors must also be present. For one thing, the supernova must result in the formation of a black hole that is massive enough to support a large accretion disk of matter orbiting around it. A neutron star, which is the alternative remnant of a supernova, just isn't massive enough. To get a sufficiently massive black hole, the progenitor star must be at least 40 Solar masses.

High mass alone, however, is not enough. The progenitor star must also be rotating rapidly enough that the angular momentum of the system is large enough to cause most of the matter and energy from the supernova explosion to be focused into a jet of angular width at most about 20 degrees. This concentrates most of the energy of the explosion into a narrow beam, so that the energy emitted in our direction matches what we actually observe. If the beam were not so narrow, the energy would not appear to be the magnitude that we observe.

There are additional factors that affect the varying characteristics of GRBs that we observe. In particular, the distribution of matter in the interstellar medium surrounding the supernova is important. It is the collision between the jets and this matter that determines the intensity and duration of the afterglow we observe for some time after the original burst.

And there's more. The jets of matter and energy from a GRB event may well be powered by the energy of the original explosion. But that's not the only possibility. Suppose there are strong magnetic fields surrounding the progenitor star. Then there will also be a considerable amount of energy in the magnetic flux, and this can also supply power to the jets.

Strong magnetic fields would have another consequence as well. The jets consist partly of electrons moving at relativistic speeds (very close to the speed of light). These electrons will follow a spiral path around lines of magnetic flux. This creates a type of electromagnetic radiation known as synchrotron radiation. If present, this radiation would make up part of the afterglow we can observe.

How would we know if synchrotron radiation, and hence magnetic fields, are present? That's simple – the radiation would be partly polarized, provided that the magnetic fields are orderly and not all tangled up.

And this is precisely what recent research has observed in the case of one particular GRB event (GRB 090102), which was detected January 2, 2009. Specifically, a polarization of 10±1% was observed at optical wavelengths. This degree of polarization is quite rare in astrophysical events, and it strongly suggests the presence of large-scale magnetic fields associated with GRB 090102. These fields should contribute substantially to the observable energy of the GRB.


Ten per cent polarized optical emission from GRB 090102
The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRBs) remains unclear. In a baryon-dominated jet only weak, tangled fields generated in situ through shocks would be present. In an alternative model, jets are threaded with large-scale magnetic fields that originate at the central engine and that accelerate and collimate the material. To distinguish between the models the degree of polarization in early-time emission must be measured; however, previous claims of gamma-ray polarization have been controversial. Here we report that the early optical emission from GRB 090102 was polarized at 10 ± 1 per cent, indicating the presence of large-scale fields originating in the expanding fireball. If the degree of polarization and its position angle were variable on timescales shorter than our 60-second exposure, then the peak polarization may have been larger than ten per cent.

Steele, I., Mundell, C., Smith, R., Kobayashi, S., & Guidorzi, C. (2009). Ten per cent polarized optical emission from GRB 090102 Nature, 462 (7274), 767-769 DOI: 10.1038/nature08590

Further reading:

Magnetic Power Revealed in Gamma-Ray Burst Jet (12/9/09)

Huge Cosmic Explosions Fueled by Magnetism (12/9/09)

Gamma-ray bursts: Magnetism in a cosmic blast (12/10/09)

In the News this month: the role of magnetic fields in GRBs (1/3/10)

Selected readings 1/30/10

Interesting reading and news items.

These items are also bookmarked at my Diigo account.

Narcolepsy research triggers myriad brain studies
Research over the past decade has shown that narcolepsy is caused by the loss of a type of brain cell that produces orexin. Scientists have found that the chemical also helps determine when we are asleep and awake and plays a role in regulating appetite and addiction. [Boston.com, 11/30/09]

H.M. recollected
These brains, normal and with various pathologies, will be preserved on thousands of slides that, in turn, are converted into extraordinarily high-resolution digital images freely available online. Researchers around the world will be able to use the material to conduct investigations ranging from parsing basic cognitive functions or the physical effects of diseases like Alzheimer’s to more abstract inquiries such as how memories are created and changed and the organic nature of consciousness. [SignOnSanDiego.com, 11/30/09]

A Lost European Culture, Pulled From Obscurity
New research, archaeologists and historians say, has broadened understanding of this long overlooked culture, which seemed to have approached the threshold of “civilization” status. Writing had yet to be invented, and so no one knows what the people called themselves. To some scholars, the people and the region are simply Old Europe. [NYTimes.com, 11/30/09]

The Psychology of Power
Joris Lammers at Tilburg University, in the Netherlands, and Adam Galinsky at Northwestern University, in Illinois, have conducted a series of experiments which attempted to elicit states of powerfulness and powerlessness in the minds of volunteers. Having done so, as they report in Psychological Science, they tested those volunteers’ moral pliability. Lord Acton, they found, was right. [The Economist, 1/21/10]

New-found galaxies may be farthest back in time and space yet
By pushing the refurbished Hubble Space Telescope to its very limits as a cosmic time machine, astronomers have identified three galaxies that may hail from an era only a few hundred million years after the Big Bang. The faint galaxies may be the most distant starlit bodies known, each lying some 13.2 billion light-years from Earth. [ScienceNews, 1/3/10]

7 Tipping Points That Could Transform Earth
When the Intergovernmental Panel on Climate Change issue its last report in 2007, environmental tipping points were a footnote. A troubling footnote, to be sure, but the science was relatively new and unsettled. Straightforward global warming was enough to worry about. But when the IPCC meets in 2014, tipping points — or tipping elements, in academic vernacular — will get much more attention. Scientists still disagree about which planetary systems are extra-sensitive to climate shifts, but the possibility can’t be ignored. [Wired.com, 12/23/09]

Cancer genomes sequenced
Scientists have charted the most complete cancer genomes to date, according to two studies published in Nature this week, providing a catalog of some 90% of all the somatic mutations in melanoma and a type of lung cancer, as well as a starting point for identifying potentially causal mutations common to these types of cancer. [The Scientist, 12/16/09]

Pluripotency process unveiled
Scientists have identified a key component of cellular reprogramming that may aid in more efficiently creating induced pluripotent stem (iPS) cells, according to a study published online in Nature today (December 21). [The Scientist, 12/21/09]

Glial cells aid memory formation
Neurons need non-electrical brain cells known as astrocytes to establish synaptic memory, according to study published this week in Nature. The findings challenge the long-standing belief that this process involves only the activity of the neurons themselves, and bring glial cells onto the center stage in the study of brain activity. [The Scientist, 1/13/10]

Geeky Math Equation Creates Beautiful 3-D World
The quest by a group of math geeks to create a three-dimensional analogue for the mesmerizing Mandelbrot fractal has ended in success. They call it the Mandelbulb. The 3-D renderings were generated by applying an iterative algorithm to a sphere. [Wired.com, 12/9/09]

Hungry Amoebas Spawn Biggest Viruses Ever
Made from a hodgepodge of genetic bits and pieces, the newly discovered Marseillevirus is the world’s largest virus. But fame is fleeting: It’s almost sure to be supplanted by another, even bigger virus. What’s really special about Marseillevirus is where it comes from. Like other giant viruses, it was found inside amoebas — lowly, single-celled organisms that devour anything they can absorb. Their voracious appetites make them incubators of genetic remixing among their prey, and may hint at processes that spawned complex life. [Wired.com, 12/8/09]

Rethinking artificial intelligence: Researchers hope to produce 'co-processors' for the human mind
The field of artificial-intelligence research (AI), founded more than 50 years ago, seems to many researchers to have spent much of that time wandering in the wilderness, swapping hugely ambitious goals for a relatively modest set of actual accomplishments. Now, some of the pioneers of the field, joined by later generations of thinkers, are gearing up for a massive 'do-over' of the whole idea. [Physorg.com, 12/7/09]

Cosmic rays hunted down: Physicists are closing in on the origin of cosmic rays
A thin rain of charged particles continually bombards our atmosphere from outer space. The mysterious particles were first detected 100 years ago but until 10 years ago when a new type of telescope began to come online physicists weren't sure where the "cosmic rays" came from or how they were generated. They suspected the particles were accelerated by supernova shockwaves. [Physorg.com, 12/7/09]

Creativity in mathematics
Mathematicians have always felt a strong creative aspect in their subject, but only in recent years has the flowering of connections between mathematics and the arts made this aspect apparent to the general public. The collection of three articles in the Notices, together with Atiyah's short introductory piece, explore some of the various ways in which art and beauty appear in mathematics. [Physorg.com, 12/8/09]

XMM-Newton Celebrates Decade of Discovery
ESA's XMM-Newton X-ray observatory is celebrating its 10th anniversary. During its decade of operation, this remarkable space observatory has supplied new data for every aspect of astronomy. From our cosmic backyard to the further reaches of the Universe, XMM-Newton has changed the way we think of space. [ScienceDaily, 12/10/09]

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Selected readings 1/24/10

Sunday, January 24, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.

Full Speed Ahead
Physical forces acting in and around cells are fast—and making waves in the world of molecular biology. [The Scientist, 12/1/09]

24 Questions for Elementary Physics
One of the motivating ideas that was mentioned more than once was the famous list of important problems proposed by David Hilbert in 1900. These were Hilbert’s personal idea of what math problems were important but solvable over the next 100 years, and his ideas turned out to be relatively influential within twentieth-century mathematics. Our conference, 110 years later and in physics rather than math, was encouraged to think along similarly grandiose lines. [Cosmic Variance, 1/15/10]

Superior Super Earths
Super Earths are named for their size, but these planets - which range from about 2 to 10 Earth masses - could be superior to the Earth when it comes to sustaining life. They could also provide an answer to the ‘Fermi Paradox’: Why haven’t we been visited by aliens? [Physorg.com, 11/30/09]

Super Earths May Be Superior at Fostering Life
Rocky planets found so far are actually more massive than our own. Dimitar Sasselov, professor of astronomy at Harvard University, coined the term "Super-Earths" to reflect their mass rather than any superior qualities. But Sasselov says that these planets – which range from about 2 to 10 Earth masses – could be superior to the Earth when it comes to sustaining life. [Space.com, 12/1/09]

Believers' inferences about God's beliefs are uniquely egocentric
Religious people tend to use their own beliefs as a guide in thinking about what God believes, but are less constrained when reasoning about other people's beliefs, according to new study published in the Nov. 30 early edition of the Proceedings of the National Academy of Sciences. [Physorg.com, 11/30/09]

What Is the Meaning of 'One' Plant or Animal?
High cooperation and low conflict between components, from the genetic level on up, give a living thing its "organismality," whether that thing is an animal, a plant, a bacteria -- or a colony. [ScienceDaily, 12/2/09]

Double Sunsets May be Common, But Twin-Star Setups Still Mysterious
The Earth may orbit around a single star, but most stars like our sun are binaries — two stars orbiting each other as a pair. In fact there are many three-star triple systems, even going up perhaps as high as seven-star — or septuplet — systems. Although astronomers once thought these systems might not easily support planets, worlds with multiple sunsets might actually prove common. [Space.com, 1/18/10]

European space missions given cost warning
Europe's scientists have presented the six dream space missions they would like to fly before 2020. The concepts ranged from a quest to map the "dark" components shaping the cosmos, to a plan to find far-off planets that most resemble Earth. [BBC News, 12/2/09]

Quantum computer simulates hydrogen molecule just right
Groups at Harvard and the University of Queensland in Brisbane, Australia, have designed and built a computer that hews closely to these specs. It is a quantum computer, as Feynman forecast. And it is the first quantum computer to simulate and calculate the behavior of an atomic, quantum system. [Science News, 1/22/10]

A tale of two qubits: how quantum computers work
Quantum information is the physics of knowledge. To be more specific, the field of quantum information studies the implications that quantum mechanics has on the fundamental nature of information. By studying this relationship between quantum theory and information, it is possible to design a new type of computer—a quantum computer. [Nobel Intent, 1/18/10]

Supernova winds blow galaxies into shape
New computer simulations show that winds generated by supernovas, which are the explosions of massive stars, can push stars out from the center of a dwarf galaxy. This simulation of supernova winds redistributes both ordinary matter and invisible dark matter in a way that almost perfectly matches observations of the way matter is distributed in actual dwarf galaxies. [Science News, 1/13/10]

Fermi’s excellent adventure
Since its launch in June 2008, the Fermi Gamma-ray Space Telescope has shed light on some of the brightest, most explosive events in the universe and opened tantalizing windows into dark matter and the nature of space-time. [Symmetry, 12/1/09]

The Maverick Bacterium
Whether it’s powering through the cytoplasm leaving a trail of polymerized actin, activating an arsenal of virulence factors through changes in RNA structure, or storing the code for RNA transcripts on the wrong side of DNA, Listeria makes up its own rules for survival. [The Scientist, 1/1/10]

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Cosmic "Dig" Reveals Vestiges of the Milky Way's Building Blocks

Monday, January 18, 2010

Cosmic "Dig" Reveals Vestiges of the Milky Way's Building Blocks (11/25/09)
Peering through the thick dust clouds of our galaxy's central parts (the "bulge") with an amazing amount of detail, a team of astronomers has revealed an unusual mix of stars in the stellar grouping known as Terzan 5. Never observed anywhere in the bulge before, this peculiar cocktail of stars suggests that Terzan 5 is in fact one of the bulge's primordial building blocks, most likely the relic of a dwarf galaxy that merged with the Milky Way during its very early days.

Terzan 5 – click for 1280×1297 image

More: here, here, here

Selected readings 1/15/10

Friday, January 15, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.

Crashing the size barrier
Over the past 50 years, the most common method of increasing the energy of a particle accelerator has been to increase its size. Yet that tactic is reaching a breaking point. While even higher energies are needed to answer many of science's most pressing questions—such as the origin of mass and the identity of dark matter—simply scaling up the current technology is becoming prohibitively expensive. Scientists need less costly, more efficient means of accelerating particles to ever-greater energies. [Symmetry, 10/1/09]

Are Black Holes the Architects of the Universe?
Black holes are finally winning some respect. After long regarding them as agents of destruction or dismissing them as mere by-products of galaxies and stars, scientists are recalibrating their thinking. Now it seems that black holes debuted in a constructive role and appeared unexpectedly soon after the Big Bang. “Several years ago, nobody imagined that there were such monsters in the early universe,” says Penn State astrophysicist Yuexing Li. “Now we see that black holes were essential in creating the universe’s modern structure.” [Discover, 1/4/10]

Genome advances promise personalized medical treatment
Six years after scientists finished decoding the human genome -- the genetic instruction book for life -- they're starting to take their new knowledge from the research laboratory to the doctor's office and the patient's bedside. ... Researchers are seeking ways to tailor treatments to individuals -- they call it "personalized medicine" -- in order to improve patient outcomes and to lower costs in the overburdened U.S. health care system. [Physorg.com, 11/18/09]

Hunting for Planets in the Dark
In Europe, the Euclid mission is a proposed space telescope for characterizing dark energy, but some believe that it might be more attractive to funding agencies if it included an exoplanet survey. [Physorg.com, 11/19/09]

Dark Energy Search Could Aid Planet Hunters
The search for dark energy might help in the search for life in the universe. That's because planet hunting through a technique called microlensing requires a similar sort of instrument as a dark energy mission. [Space.com, 11/19/09]

Recipes for planet formation
Observations of extrasolar planets are shaping our ideas about how planetary systems form and evolve. Michael R Meyer describes what's cooking elsewhere in our galaxy – and beyond. [Physicsworld.com, 11/2/09]

The Americanization of Mental Illness
We have for many years been busily engaged in a grand project of Americanizing the world’s understanding of mental health and illness. We may indeed be far along in homogenizing the way the world goes mad. This unnerving possibility springs from recent research by a loose group of anthropologists and cross-cultural psychiatrists. [New York Times, 1/8/10]

What Life Leaves Behind
The search for life beyond our pale blue dot is fraught with dashed hopes. Will the chemical and mineral fingerprints of Earthly organisms apply on other worlds? [Seed, 11/9/09]

3 Questions: Sara Seager on searching for Earth-like planets
MIT planetary scientist Sara Seager has been studying exoplanets — planets circling stars other than the sun — for many years. The first such planet was discovered just 15 years ago, and now more than 400 others are known. This week, a paper co-authored by Seager and NASA scientist Drake Deming in the journal Nature reviews what we know about exoplanets so far, what we can expect to learn about them in the next decade or so, and the chances for finding a twin of our own planet. She has also just published an online book to answer questions about exoplanets and the lessons they hold. [Physorg.com, 11/23/09]

Quest for the Holy Grail: Sara Seager Seeks to Complete a Revolution
Sara Seager is fascinated by stories of explorers visiting uncharted places. From her groundbreaking work on the detection of exoplanet atmospheres to her innovative theories about life on other worlds, Seager has been a pioneer in the vast and unknown world of exoplanets. Now, like an astronomical Indiana Jones, she's on a quest after the field's holy grail - another Earth-like planet. [NASA, 10/6/08]

Two-qubit quantum system used to model the hydrogen molecule
Even though quantum computers are still in their crawling phase, computer scientists continue to push their limits. Recently, a group of scientists used a two-qubit quantum system to model the energies of a hydrogen molecule and found that using an iterative algorithm to calculate each digit of the phase shift gave very accurate results. Their system, while not directly extensible, has the potential to help map the energies of more complex molecules and could result in significant time and power savings compared to classical computers. [Arstechnica.com, 1/13/10]

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The Crab Nebula: A Cosmic Icon

Wednesday, January 13, 2010

The Crab Nebula: A Cosmic Icon
A star's spectacular death in the constellation Taurus was observed on Earth as the supernova of 1054 A.D. Now, almost a thousand years later, a super dense object -- called a neutron star -- left behind by the explosion is seen spewing out a blizzard of high-energy particles into the expanding debris field known as the Crab Nebula. X-ray data from Chandra provide significant clues to the workings of this mighty cosmic "generator," which is producing energy at the rate of 100,000 suns.

NGC 1952 – click for 831×792 image

More: here, here

Space is very fine-grained

Monday, January 4, 2010

It would take you a lot longer to hike a significant distance over very hilly terrain than it would over a completely flat plain. For much the same reason, it would take light longer to cover the same distance depending whether the space through which it moves does or doesn't have large "hills".

But what does it mean for space to contain "hills"? And how large do "hills" need to be to make a difference?

Consider the second question first. There's no natural place on Earth that is perfectly flat, of course. So if you look closely enough, there are always "hills" of some size to cross when you're on a hike. But if the height of those hills is a lot less than the length of your stride, they will make little difference. In the same way, if the irregularities in the texture of space are much smaller than the wavelength of light, they won't make much difference either.

Physicists aren't even sure that space does contain "irregularities" at very short distances. No measurement yet made has given any evidence of irregularities. But all theories of quantum gravity – none of which is yet considered satisfactory – predict that irregularities must exist at a sufficiently small scale.

No attempted quantum theories of gravity are satisfactory yet, because physicists have not been able, with a mathematically consistent theory, to reconcile general relativity and quantum mechanics at very small scales. Nature, however, is somehow able to do the trick. So it seems quite plausible that space does have irregularities at sufficiently small scales. The only real question is "how small?"

Recent research now says, "smaller than can presently be detected."

And how would one go about detecting the smallest irregularities? With light whose wavelength is about the same size, of course. Which would mean photons with the highest energy one can find. So far, photons with the highest energy physicists know of are not indicating any quantum irregularities in space.

We're talking about photons with energies far higher than can be produced in laboratories. The most energetic photons correspond to the part of the electromagnetic spectrum known as gamma rays. By convention, physicists regard gamma rays as electromagnetic radiation with a wavelength of less than 10-11 m (10 picometers). That corresponds to an energy of 105 eV (eV = electron-volt). (Recall that photon energy is related to frequency by the relation E=ℎ&nu, where ℎ is Planck's constant and ν is the photon frequency. Since wavelength λ is inversely related to ν so is energy; i. e. E ∝ 1/λ.)

Gamma rays are produced naturally in some types of atomic decay, and such gamma rays have energies up to 107 eV. That's not terribly energetic in the great scheme of things. The Large Hadron Collider will be able to accelerate protons up to energies around 1013 eV.

Since photons have no electric charge, unlike protons, they can't be accelerated the way protons can. Much more energetic gamma rays can be produced in particle-antiparticle annihilations, but such high-energy photons are rather rare, even in cosmic events like gamma-ray bursts, which may produce gamma-rays with energies of 3×1010 eV or more (3.34×1010 eV is the largest yet observed). That's 3×105 times as energetic as the least energetic gamma rays and so corresponds to wavelengths ~.3×10-16 m.

Fortunately, even with gamma rays in this energy range, much smaller irregularities in space can be detected if the gamma ray photons travel a very large distance. Suppose, for instance, that a gamma-ray photon were detectably slowed down only by 1 part in a million, a factor of 10-6, over a distance of 100,000 light-years, about the diameter of a large spiral galaxy.

But really cosmic distances at which we can observe gamma-ray bursts are about 1010 light-years, or 105 times the diameter of a galaxy. Since a light-year is about 1016 m, we're talking scales like 1026 m. On that scale, the possible slow-down of a gamma-ray photon could be very noticeable – roughly 1 part in 10.

The result is that if we can detect how much a gamma-ray photon is slowed down over a distances around 1010 light-years, we could be able to probe energies much higher than those possessed by a gamma-ray photon itself, and therefore distance scales much smaller than a very energetic gamma-ray photon wavelength of about 10-17 m.

Detecting small differences in the velocities of photons of different wavelengths is made much easier when both photons travel 1010 light-years over a time of (naturally) 1010 years. Gamma-ray bursts, fortunately, produce gamma rays over a range of energies that differ by several times 105 from smallest to largest.

To be more concrete, suppose you have a gamma-ray burst at a redshift of z~.9. That corresponds to a distance of ~7×109 light-years and a travel time of ~7×109 years. The wavelength of photons traveling that distance is stretched by a factor of 1.9 (i. e. 1+z), but there's still a factor of several times 105 between the wavelenghts of the most and least energetic gamma-ray photons, so they are readily distinguishable. If the difference in arrival time of gamma-ray photons is 1 second, that is only one part in 2×1017 (i. e. 7×109 years times about 3×107 seconds/year ≅ 2×1017 seconds).

It's reasonable to suppose that the slowdown of the least energetic gamma-ray photons due to irregularities in space is negligible. Suppose we could relate the slow-down of the most energetic photons to the actual size of spatial irregularities. For instance, suppose a spatial irregularity of 1 part in 1017 of the photon's wavelength caused a slow-down that was also 1 part in 1017 of the photon's velocity. That would cause a 2 second delay in photon arrival times – which is very readily detectable.

Recall that energetic gamma-ray photons from gamma-ray bursts have wavelengths of .3×10-16 m or less. Thus we might expect to be able to detect spatial irregularities as small as ~.3×10-33 m = 3×10-34 m = 3×10-32 cm.

In fact, it is now possible to measure arrival times of photons from gamma-ray bursts to within mere hundredths of a second. So we can probe length scales around 10-33 cm. Interestingly enough, this is very close to the Planck length scale lPlanck ≈ 1.62×10-33 cm.

That's just a back-of-the-envelope calculation based on hypothetical data. But we don't need to be hypothetical, because photons from the gamma-ray burst GRB 090510, which was observed on May 10, 2009, were measured very precisely. The most energetic photon observed had an energy of ~3.1×1010 eV (31 GeV), and it showed up precisely .829 seconds after the very first photons from the burst were detected. Further, spectroscopic observations of the afterglow from this burst showed a redshift of z very close to .9, as in our hypothetical example.

Since we don't have a reliable quantum gravity theory, we don't know exactly how much of a slowdown very high-energy, short-wavelength photons should experience. However, several theories predict that, to first order approximation, if vph is the effective average velocity of the photon, then its ratio to c, the speed of light, should satisfy |vph/c - 1| ≈ Eph/(MQGc2). One can think of MQG as the "mass" that the photon would have in the quantum gravity theory so that the photon energy is c2 times the mass.

Given that notation, then if you have two photons that differ in energy by ΔE, the difference in arrival times should be Δt ≈ (|ΔE|/(MQGc2))D/c, where D is the distance traveled. In this relationship, all quantities except for MQG are directly measured, which implies a value of MQG.

The most sensible unit in which to measure MQG is in terms of the Planck mass, MPlanck ≅ 2.17644×10-5 g, which is quite a lot for small things, being about the mass of 20 million bacteria.

Most quantum gravity theories predict MQG ≤ MPlanck, so that the ratio MQG/MPlanck ≤ 1. Surprisingly, however, measurements made of GRB 090510 imply that this ratio is actually no smaller than 1.2, and could be quite a bit larger – as much as 100 or so.

There's a great deal of uncertainty in the estimate of Δt, the difference in arrival times between the 31 GeV photon and lower energy photons that were emitted at the same instant. That's because there is no way to know how long after the start of the GRB event the 31 GeV photon was emitted. Since it was observed .829 seconds after the very first photons, a large part of that delay could actually be due to emission of the 31 GeV photon at any point up to .829 seconds after the start. That would make the actual Δt much smaller, and MQG much larger.

Only the most conservative assumption, with the 31 GeV photon emitted as early as possible, gives MQG/MPlanck ≈ 1.2. More realistic assumptions would make the ratio 100 or more.

In any case, all of these estimates "strongly disfavor" the simplest theories of quantum gravity, in the words of the research paper describing the observations. Otherwise said, spacetime at the smallest scale must apparently be much less bumpy than most theories predict.

Here's the research paper and abstract:

A limit on the variation of the speed of light arising from quantum gravity effects (11/19/09)
A cornerstone of Einstein's special relativity is Lorentz invariance—the postulate that all observers measure exactly the same speed of light in vacuum, independent of photon-energy. While special relativity assumes that there is no fundamental length-scale associated with such invariance, there is a fundamental scale (the Planck scale, lPlanck ≈ 1.6×10-33 cm or EPlanck = MPlanckc2 ≈ 1.22×1019 GeV), at which quantum effects are expected to strongly affect the nature of space–time. There is great interest in the (not yet validated) idea that Lorentz invariance might break near the Planck scale. A key test of such violation of Lorentz invariance is a possible variation of photon speed with energy. Even a tiny variation in photon speed, when accumulated over cosmological light-travel times, may be revealed by observing sharp features in gamma-ray burst (GRB) light-curves. Here we report the detection of emission up to ~31 GeV from the distant and short GRB 090510. We find no evidence for the violation of Lorentz invariance, and place a lower limit of 1.2EPlanck on the scale of a linear energy dependence (or an inverse wavelength dependence), subject to reasonable assumptions about the emission (equivalently we have an upper limit of lPlanck/1.2 on the length scale of the effect). Our results disfavour quantum-gravity theories in which the quantum nature of space–time on a very small scale linearly alters the speed of light.

This post was chosen as an Editor's Selection for ResearchBlogging.org
Abdo, A., Ackermann, M., Ajello, M., Asano, K., Atwood, W., Axelsson, M., Baldini, L., Ballet, J., Barbiellini, G., Baring, M., Bastieri, D., Bechtol, K., Bellazzini, R., Berenji, B., Bhat, P., Bissaldi, E., Bloom, E., Bonamente, E., Bonnell, J., Borgland, A., Bouvier, A., Bregeon, J., Brez, A., Briggs, M., Brigida, M., Bruel, P., Burgess, J., Burnett, T., Caliandro, G., Cameron, R., Caraveo, P., Casandjian, J., Cecchi, C., Çelik, �., Chaplin, V., Charles, E., Cheung, C., Chiang, J., Ciprini, S., Claus, R., Cohen-Tanugi, J., Cominsky, L., Connaughton, V., Conrad, J., Cutini, S., Dermer, C., de Angelis, A., de Palma, F., Digel, S., Dingus, B., do Couto e Silva, E., Drell, P., Dubois, R., Dumora, D., Farnier, C., Favuzzi, C., Fegan, S., Finke, J., Fishman, G., Focke, W., Foschini, L., Fukazawa, Y., Funk, S., Fusco, P., Gargano, F., Gasparrini, D., Gehrels, N., Germani, S., Gibby, L., Giebels, B., Giglietto, N., Giordano, F., Glanzman, T., Godfrey, G., Granot, J., Greiner, J., Grenier, I., Grondin, M., Grove, J., Grupe, D., Guillemot, L., Guiriec, S., Hanabata, Y., Harding, A., Hayashida, M., Hays, E., Hoversten, E., Hughes, R., Jóhannesson, G., Johnson, A., Johnson, R., Johnson, W., Kamae, T., Katagiri, H., Kataoka, J., Kawai, N., Kerr, M., Kippen, R., Knödlseder, J., Kocevski, D., Kouveliotou, C., Kuehn, F., Kuss, M., Lande, J., Latronico, L., Lemoine-Goumard, M., Longo, F., Loparco, F., Lott, B., Lovellette, M., Lubrano, P., Madejski, G., Makeev, A., Mazziotta, M., McBreen, S., McEnery, J., McGlynn, S., Mészáros, P., Meurer, C., Michelson, P., Mitthumsiri, W., Mizuno, T., Moiseev, A., Monte, C., Monzani, M., Moretti, E., Morselli, A., Moskalenko, I., Murgia, S., Nakamori, T., Nolan, P., Norris, J., Nuss, E., Ohno, M., Ohsugi, T., Omodei, N., Orlando, E., Ormes, J., Ozaki, M., Paciesas, W., Paneque, D., Panetta, J., Parent, D., Pelassa, V., Pepe, M., Pesce-Rollins, M., Petrosian, V., Piron, F., Porter, T., Preece, R., Rainò, S., Ramirez-Ruiz, E., Rando, R., Razzano, M., Razzaque, S., Reimer, A., Reimer, O., Reposeur, T., Ritz, S., Rochester, L., Rodriguez, A., Roth, M., Ryde, F., Sadrozinski, H., Sanchez, D., Sander, A., Saz Parkinson, P., Scargle, J., Schalk, T., Sgrò, C., Siskind, E., Smith, D., Smith, P., Spandre, G., Spinelli, P., Stamatikos, M., Stecker, F., Strickman, M., Suson, D., Tajima, H., Takahashi, H., Takahashi, T., Tanaka, T., Thayer, J., Thayer, J., Thompson, D., Tibaldo, L., Toma, K., Torres, D., Tosti, G., Troja, E., Uchiyama, Y., Uehara, T., Usher, T., van der Horst, A., Vasileiou, V., Vilchez, N., Vitale, V., von Kienlin, A., Waite, A., Wang, P., Wilson-Hodge, C., Winer, B., Wood, K., Wu, X., Yamazaki, R., Ylinen, T., & Ziegler, M. (2009). A limit on the variation of the speed of light arising from quantum gravity effects Nature, 462 (7271), 331-334 DOI: 10.1038/nature08574

Further reading:

Gamma-ray Observations Shrink Known Grain Size Of Spacetime (10/28/09) – Science News

Fermi Telescope Caps First Year With Glimpse of Space-Time (10/28/09) – NASA/JPL

7.3 Billion Years Later, Einstein’s Theory Prevails (10/28/09) – New York Times

Gamma-ray burst restricts ways to beat Einstein’s relativity (10/28/09) – Symmetry Magazine

Special relativity passes key test (10/28/09) – Physics World

Gamma-ray photon race ends in dead heat; Einstein wins this round (10/28/09) – Physorg.com

An intergalactic race in space and time (10/28/09) – Nature

Astrophysics: Burst of support for relativity (11/19/09) – Nature

Space-Time Observations Find Einstein Still Rules (10/28/09) – Space.com

Quantum gravity theories wiped out by a gamma ray burst (10/28/09) – Ars Technica

A Gamma Ray Race Through the Fabric of Space-Time Proves Einstein Right (10/29/09) – Discover

Nature, NYT report the demise of Lorentz-violating theories (10/29/09) – The Reference Frame

Einstein Still Rules, Says Fermi Telescope Team (10/28/09) – Universe Today

Selected readings 1/3/10

Sunday, January 3, 2010

Interesting reading and news items.

These items are also bookmarked at my Diigo account.

Is the DNA in our cells always the same?
This challenges one of my fundamental assumptions in biology: that of all somatic cells sharing the same genome. In an article entitled BAK1 Gene Variation and Abdominal Aortic Aneurysms the authors show that the BAK1 gene, associated with apoptosis, exists in multiple variants in our bodies. Specifically the authors found differences between the gene found in the blood cells and other tissues. [Cancerevo, 12/23/09]

Cancer genomes
Nature published online the papers describing two new cancer genomes, bringing the total number of human cancer genome sequences published to five. Pleasance et al sequenced a malignant melanoma and a lung cancer cell line, comparing them to the genomes of healthy cells from the same individuals. [Cotch.net, 12/22/09]

Sex, Violence and The Male Warrior Hypothesis
Throughout the history of human civilization, wars have a common feature of being practiced primarily by males. This group aggression by males is a persistent trait of human behavior, seen across different continents among civilizations that have developed independent of each other. [Brain Blogger, 12/21/09]

How to Find Signs of Life on Mars
Certain environments on Earth that host life are very similar to places on Mars and other terrestrial planets, scientists have found. So if life can exist here, why not there? Nora Noffke is a geobiologist at Old Dominion University in Norfolk, Virginia. ... Noffke and her colleague Sherry Cady of Portland State University in Oregon recently wrote an article... detailing how the melding of geology and biology can teach us about the environments most likely to host extraterrestrial life. [Space.com, 12/17/09]

Dark Horse Challenges Dark Matter to Explain Missing Matter
One of the greatest mysteries of astronomy is the problem of the missing mass: All of the matter scientists can see in the universe accounts for only a small percent of the observed gravity. Astronomers often invoke the concept of dark matter to explain this discrepancy, but some researchers say the problem is really our understanding of gravity. These scientists tout an idea called MOND - Modified Theory of Newtonian Dynamics - to explain why the universe seems to behave as if there's much more matter in it than we think. ... Though no one has yet proven or disproven either dark matter or MOND, supporters of the latter are in the minority. And MOND may be becoming even more of a long shot. [Space.com, 11/5/09]

How astronomers fill in uncharted areas of the universe
Astronomers are filling in the blank spaces on their 3-D map of our universe thanks to their ability to sense almost every conceivable form of electromagnetic radiation. Those blanks include remote regions of space and time when the first stars formed and when young galaxies began to group themselves into gravitationally bound clusters. [CSMonitor.com, 11/2/09]

After Setbacks, Small Successes for Gene Therapy
Not long ago, gene therapy seemed troubled by insurmountable difficulties. After decades of hype and dashed hopes, many who once embraced the idea of correcting genetic disorders by giving people new genes all but gave up the idea. But scientists say gene therapy may be on the edge of a resurgence. There were three recent, though small, successes. [New York Times, 11/5/09]

Q&A: Gene therapy turnaround
Judging by the stream of studies in the last few months, it seems the field of gene therapy is beginning to replace its troubled history with the beginnings of a promising future. ... With the flurry of recent successes, Mark Kay, director of the Human Gene Therapy program at Stanford University School of Medicine and one of the founders of the American Society of Gene and Cell Therapy, believes that "the mood in the field is pretty positive." [The Scientist, 11/12/09]

A line on string theory
A Harvard theoretical physicist has discussed with scientists at the Large Hadron Collider in Switzerland the possibility that they may discover a theorized "stau" particle, with a lifetime of a minute or so, that could provide the first experimental confirmation of string theory.[Physorg.com, 11/12/09]

Explained: RNA interference
In recent years, biologists have discovered a myriad of other molecules that fine-tune this process, including several types of RNA (ribonucleic acid). Through a naturally occurring phenomenon known as RNA interference, short strands of RNA can selectively intercept and destroy messenger RNA before it delivers its instructions. [Physorg.com, 11/12/09]

Disappearing Before Dawn
Gene expression studies are lending support to a new hypothesis for why everyone sleeps: to prune the strength or number of synapses. [The Scientist, 4/1/09]

Black Holes: Powerhouses of the Universe
Black holes, so named because even light cannot escape their gravitational grasp, can only be sensed through their tug on other matter. While black holes themselves are invisible, the regions around them are reigned by powerful magnetic and gravitational forces that create some of the most luminous radiation ever seen. [Space.com, 11/9/09]

Unlocking the mysteries of speech
Animals may use sounds to communicate but talking is uniquely human. Yet despite decades of research scientists still haven't unlocked the secrets of speech. So why do we talk? [BBC News, 11/10/09]

The Mind Is a Mirror
Congenitally blind subjects showed mirror network activation in response to action sounds in the same brain areas that were active in response to both visual and auditory stimuli in sighted individuals. The authors conclude that the human mirror system can develop without visual input and is able to process information about actions that comes from other sensory modalities, as well. [Scientific American, 11/10/09]

Titanic Thirty Meter Telescope Will See Deep Space More Clearly
Four hundred years after Galileo’s telescope revolutionized humanity’s view of the universe, a gigantic telescope is in the works that could take us to a new, deeper level of understanding. The enormous Thirty Meter Telescope, with a primary mirror the size of a blue whale, is part of a new generation of super powerful ground-based telescopes. Scheduled for completion in 2018, it will have nine times the collecting power of the Keck telescopes and 12 times the resolution of the Hubble Space Telescope. [Wired, 11/16/09]

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