Dark matter at the Fermi Symposium

November 5, 2009 | 8:52 pm

A prime motivator for building the Fermi Gamma-ray Space Telescope was to search for signals of dark matter. After one year of data collection, scientists have learned a lot from Fermi and are improving their models of the universe, which will eventually provide road maps to this elusive phenomenon. But as far as actually finding definite signals of the stuff: not yet.

Simona Murgia of the Kavli Institute for Particle Astrophysics and Cosmology at SLAC gave a presentation at this week’s Fermi Symposium about the many ways Fermi is searching for dark matter, but reported that its initial explorations haven’t turned up any smoking guns.

But astrophysicists aren’t deterred. What was perhaps most promising was that some of the items that Murgia listed as needing improvement in order to advance the dark matter search were already being presented as new results by other scientists. Other speakers reported finding potential new sources of the diffuse gamma ray background or untangling the many gamma ray sources at the galactic center, where we expect to find a dense concentration of dark matter. The work of scientists in various sub-areas is clearly linked by a common need to understand our gamma-ray universe.

Murgia also made an announcement in relation to the dark matter search by Fermi’s predecessor EGRET (Energetic Gamma Ray Experiment Telescope). EGRET detected an excess of gamma rays in the GeV range during its nine years of observation. It was theorized that this detection might be evidence of some new phenomenon, possibly dark matter.

To determine if there is an excess of gamma rays, you first need to define what is normal. Scientists can use models to approximate the number of gamma rays we should see coming from individual point sources in the galaxy and the rest of the universe, and add them all up. When researchers subtracted that total from the EGRET data, they found that there were more gamma rays than they had accounted for. That immediately suggests that either an as-yet-unidentified source, possibly dark matter, is emitting gamma rays, or more likely that the model needs to be refined in any number of ways. Any claim of an excess relies heavily on the strength of the model, which relies on an ever-growing understanding of astrophysics and a growing library of observations.

Murgia announced that Fermi has not found the same excess reported by EGRET. This does not necessarily negate the EGRET result, but brings into question the models on which it was based. The final answer may lie somewhere in between.

“You make a model that includes dark matter, and if your observation doesn’t match your model then sometimes you really have to rethink your parameters,” said Murgia. “Maybe the right one is one we haven’t even thought of yet.”

The meeting did see a little commotion over the dark matter search. Last week, a paper by researchers outside the Fermi collaboration who analyzed the telescope’s publicly available data  reported finding an excess of gamma rays between us and the galactic center. In their paper the group steered clear of claiming that this was a signal of dark matter; but after the symposium talk by Neal Weiner of New York University, the suggestion hung thick in the air (their paper also stirred up rumors of dark matter on the blogosphere). At the end of his talk, when the moderator asked if there were any questions, an audience member immediately went to the microphone and said the excess could be explained another way – namely inverse Compton scattering (which is when energized electrons strike photons and boost their energies) created by a cosmic bubble very near our solar system. Weiner responded quite pleasantly, “Yes, and we’re looking forward to discussing that.”

The person who spoke up at the end of Weiner’s talk was referring to work  by Jean Marc Casandjian and Isabel Grenier of CEA Saclay, which was presented in a poster. Casandjian and Grenier, representing the Fermi collaboration, say the excess gamma rays fit the already known shape of the LOOP 1 cosmic bubble. LOOP 1 might be the remnants of an exploded supernova or other massive event that left a radiating bubble about 100 parsecs from the sun (we are surrounded by a similar bubble called the local bubble). LOOP 1 lies directly in our line of sight to the galactic center, and radiation from the bubble interferes with our view of the busy galactic plane. Casandjian hopes the analysis he is doing will allow researchers to subtract the effects of LOOP 1 and get a much clearer view of the Milky Way and the galactic center. There is more work to be done on their analysis before it is ready for publication.

In response to the notion that the gamma-ray excess could be something other than LOOP 1, Casandjian says other analyses did not take the time to extract the detailed structure of the gamma-ray emissions, but instead saw them as a general haze. From the detailed map that he and Grenier constructed, Casandjian said he believes the emissions are a very good fit to the known structure of LOOP 1, which can also be seen in radio frequencies.  Still, he acknowledged that he could not claim to understand everything going on with the gamma ray excess.

Calla Cofield

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NOvA neutrino detector gets full construction approval

November 4, 2009 | 4:38 pm

Construction workers prepare to pour concrete for the loading dock at the future site of the NOvA neutrino detector on Oct. 20 in Ash River, Minnesota.

NOvA experiment collaborators have more to celebrate this holiday season.

The neutrino experiment recently received Critical Decision-3b approval from the US Department of Energy. The decision signifies approval for the start of full construction. DOE had approved long-lead procurements and limited construction activities for the NOvA experiment when they granted CD-3a approval in October 2008. Fermilab now can continue construction of buildings on the Fermilab site and in Ash River, Minnesota, as well as complete the Fermilab accelerator upgrades and start neutrino detector fabrication.

“This is a big deal,” said NOvA project manager John Cooper. “This means we can move forward on the full project scope within the constraints of approved funding by Congress.”

Scientists will use the NOvA experiment to analyze the mysterious behavior of neutrinos and look for muon neutrinos oscillating to electron neutrinos. Ultimately, scientists hope to understand whether neutrinos contributed to the imbalance between matter and antimatter that enables our matter-dominated universe (including ourselves) to exist.

A construction worker placing concrete forms for the NOvA service facility.

The experiment involves 180 scientists from some 28 institutions who have worked hard to get the experiment ready for full construction.

“So many people have worked so long and hard to reach this point within the NOvA collaboration and the project management—it is a great feeling to have this level of support and approval from DOE,” said Pepin Carolan, DOE NOvA project director. “The team is very highly motivated to move full steam ahead to get the work done safely, on schedule, and within budget, and then to get on with the science.”

Carolan also pointed out that the project’s success was made possible by the partnership between Fermilab and the University of Minnesota. A cooperative research agreement between the US Department of Energy and the University of Minnesota supports the construction of a NOvA facility.

The American Recovery and Reinvestment Act provided a total of $55 million (to Fermilab and the University of Minnesota) toward completion of the NOvA project. This includes funding supporting purchase of key high-tech components and commodities for the detector from US companies, allowing these firms to retain and hire workers. It also includes the funding for the University of Minnesota’s contract to construct the detector hall in Ash River, Minn., awarded in May 2009.

Read articles about work for NOvA funded by the ARRA.

Rhianna Wisniewski

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Hunting blazars with VERITAS and the Fermi Large Area Telescope

November 4, 2009 | 4:11 am

From VERITAS scientist Wystan Benbow: "Here is the VERITAS sky catalog in Galactic Coordinates (i.e. those with the Galactic Plane at the equator). The sky catalog is all objects which VERITAS has seen (& released publicly). The different colors of dots represent different kinds of astrophysical objects. The blue regions are those best visible to VERITAS. Almost all of the marked objects have also been detected by Fermi LAT." Courtesy of VERITAS.

Two spots in the night sky look a bit brighter this week, thanks to collaborative efforts between the Large Area Telescope, which is the primary instrument aboard the Fermi Gamma Ray Space Telescope, and VERITAS, or the Very Energetic Radiation Imaging Telescope Array System, an array of ground-based telescopes in Amado, Arizona. The two collaborations announced last week the discovery of one new blazar and one likely blazar; if the second is confirmed these will be the third and fourth blazars that the observatories have collaborated to find.

Blazars are a type of active galactic nucleus (AGN),  the very violent systems that lie at the center of some distant galaxies. The systems each include a massive black hole, some of which are thought to be billions of times more massive than the sun. Blazars are distinct from other AGN because they emit two jets of energetic gas out their poles, which point toward Earth, making them distinguishable by observatories like VERITAS.  These violent and energetic systems radiate radio waves or X-rays, but they are identified by their emission of very-high-energy gamma rays.

The first new discovery appeared as a bright spot in the publicly available Fermi LAT data, and VERITAS scientists decided to investigate. They confirmed that it radiated in the TeV range, and that it aligns with an AGN, and hope to soon confirm if it is indeed a blazar. VERITAS made an announcement on October 25. The second high-energy source was jointly spotted in the sky by VERITAS and LAT, who communicated with each other to identify it as a blazar. The results were announced in a joint astrophysics telegram on October 29.

VERITAS is a targeted telescope array that looks for light in the TeV range–that means light with energy even higher than the gamma rays being searched for by Fermi (Fermi searches between 10 and 300 GeV, where 300 GeV is equal to 0.3 TeV). These extremely high-energy photons commonly come from very energetic, violent sources in the universe, such as blazars. VERITAS hopes to glean information about these objects, which could also provide answers to larger questions about our universe.

The trouble for VERITAS is knowing where in the sky to look for these TeV gamma ray sources (the trade off is a tremendous increase in sensitivity, making the two observatories complimentary). VERITAS can only look at a section of the sky about as wide as seven full moons. Without being able to scan the entire sky very quickly, VERITAS looks to observations made by radio and X-ray telescopes, and makes a list of objects that they think might radiate in the TeV range. Upon investigation, there are hits and misses: some of the objects on the list do emanate the high-energy gamma rays, and some don’t.

LAT offers VERITAS a tremendous new source list of potential high-energy gamma ray sources. Because the energy range of Fermi is much closer to that of VERITAS than radio or X-ray telescopes, LAT should reveal more potential TeV sources than radio or X-ray telescopes alone, and save VERITAS time and effort by reducing the number of sources they investigate that turn out to be misses.

“The degree to which LAT can help us can’t be overstated,” said University of Delaware Professor of Physics and Astronomy and VERITAS researcher Jamie Holder, after his talk at the Fermi symposium. In his talk, one of his slides read, “The LAT is changing the way we do TeV astronomy!”

And VERITAS can help Fermi, too.

In the first three months of Fermi data collection, VERITAS identified the blazar RGB J0710+591. They alerted LAT, which searched its data and found that the blazar radiated lower energy gammas as well.

Shortly after, and still within the LAT’s first three months of data collection, LAT saw a bright spot in the sky and alerted VERITAS, which confirmed that the spot radiated in the TeV gamma ray range. This blazar, PKS-1424+24, was the first Very High Energy Gamma Ray Source that was identified as a result of the LAT data.

As Fermi continues to collect data, and its resolution increases, the likelihood that VERITAS will identify a bright source first will decline. At the same time, there is a rare type of blazar that, at this point in its data collection, LAT does not see. Over time, the rare blazars could appear in the LAT data, and together the two observatories hope to learn more about these powerful objects.

Calla Cofield

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This week’s Fermi Symposium

November 3, 2009 | 2:58 pm

The Fermi Gamma Ray Space Telescope is holding its second collaboration symposium in Washington DC this week. The symposium began Monday morning (see our post about the announcement of some new pulsars) and stretches until noon on Thursday.

The symposium features talks by both Fermi collaboration members and outside scientists who are using  Fermi data, on subjects including pulsars, dark matter, supernova remnants, active galactic nuclei, gamma ray bursts, magnetars, blazars and many more.

The Fermi telescope’s main instrument, the Large Area Telescope, was assembled at SLAC National Accelerator Laboratory and launched by NASA in June 2008. The Fermi collaboration consists of scientists from all over the world, and the Fermi data is made publicly available, so non-collaboration members can analyze  it,  as well.

Calla Cofield

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Fermi Telescope finds more new pulsars

November 2, 2009 | 3:26 pm

Clouds of charged particles move along the pulsar's magnetic field lines (blue) and create a lighthouse-like beam of gamma rays (purple) in this illustration. Credit: NASA

Today at the 2009 Fermi Symposium in Washington DC, postdoctoral researcher Lucas Guillemot of the Max Planck Institute reported that the Fermi Gamma-ray Space Telescope has detected eight more pulsars that had not been seen in other wavelengths of light, bringing the total of these gamma-ray-only objects to 24.  It also bagged one new gamma ray millisecond pulsar.  However, Guillemot said these results are preliminary and have not been submitted for publication yet.

Pulsars are the remnants of supernovas – stars that run out of fuel, expand and then collapse into masses heavier than the sun but with an average radius of only 10 km. Pulsars emit powerful jets of radiation, and as they rotate the radiation sweeps across the cosmos like the beam from a light house.  From Earth, it looks as though the neutron star’s light is pulsing on and off.

The vast majority of pulsars emit radio waves; that’s how most of them were detected. But in blind searches of the sky, Fermi has found previously unknown pulsars that appear to radiate only gamma rays. It may be that these pulsars emit  in radio wavelengths, too, but their radio beams are simply not visible from Earth; follow-up observations should reveal if they radiate in other frequencies, including radio.

Fermi’s discovery of a new gamma ray millisecond pulsar, or MSP, brings the telescope’s tally up to nine. The pulse of light from an MSP has a period of milliseconds, meaning the neutron star is rotating hundreds of times per second and appears to blink rapidly.  The gamma ray MSPs studied by Fermi had  already been identified by radio telescopes; Fermi was the first gamma ray telescope to identify MSPs that radiate gamma rays.

Including these nine new pulsars, Fermi has identified 55 pulsars that radiate in the gamma range, in just one year of data collection. This can be compared to the 5 new pulsars discovered over nine years by Fermi’s predecessor EGRET (Energetic Gamma Ray Experiment Telescope).  Guillemot said in an interview after his presentation that the great benefit of finding so many pulsars is being able to look for trends and similar characteristics among them.

“If you have less than ten detected pulsars, they’re all black sheep, they’re all different from each other,” he said. “When you have lots, you can start to categorize them and draw trends; and that is how you understand how things work.”

Fermi has posted a “catalogue” of its confirmed identified pulsars on the arXiv: 16 gamma-ray-only pulsars (not including the 8 new ones), 22 radio loud pulsars (already identified by radio telescopes), and 8 gamma ray MSPs (not including the newest one).

Calla Cofield

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Imagine Science Film Festival: Documentary Shorts

November 2, 2009 | 8:08 am

When I left the Imagine Science Film Festival Documentary Shorts screening, it was almost impossible to wipe the grin off my face. All of the films were gorgeous and creative; nearly all of them used science in ways I’d never seen or thought of. The variety of storytelling methods was tremendous; the film festival truly displayed how science in film is not limited to educational documentary, but instead offers a playground of new possibilities. The fact that these films are out there and I might never have seen them otherwise is the only tragedy. We need more festivals like this. In the meantime, you can watch many of these shorts online by following the links below.

A few months ago I came across Magnetic Movie, from Semiconductor films, floating around the internet, and was so happy to see it at the festival. It ended up winning the highest prize that the festival had to offer, the Nature Scientific Merit Award. Very much earned. I wish all the films were available, but am especially happy that this one is free to watch:

Magnetic Movie from Semiconductor on Vimeo.

What’s the Matter at CERN, Or: How I Learned to Stop Worrying and Love the LHC (which you can watch here) takes a stab at dispelling the accusations that the LHC will create a black hole and destroy the world. It worked best to convey how the scientists at CERN were certainly shocked by the claims, mostly because they got so much attention, yet were so outlandish. One scientist even says that if the founders of these claims really believe such a thing, he respects them for bringing them forward. On the other hand, he points out with a shrug, he knows the claims are wrong. Like the scientists, the filmmaker is almost giggling at the fringe groups that have taken these claims and run: the funniest moment came from the inclusion of some homemade videos, found on Youtube, depicting a black hole originating at CERN and swallowing the Earth.

The second film about the LHC, Big Bang Day, captured the excitement and buzz that filled CERN on the day the LHC sent proton beams around the ring in both directions. It includes footage of the camera crews that arrived and the scientists who stopped working to assist with all the visitors. But the best part of Big Bang Day were the quiet, yet powerful words of graduate student Adam, who is featured as the film’s centerpiece. Adam talks openly about how sometimes the routine of work makes him forget what an exciting facility CERN is. But he also conveys the thrill and privileged of working at CERN as the LHC is finally starting, and what it feels like to reach the end of your training, to become an expert in your field, and realize that if you want answers to your questions, you have to find them yourself. This film is just one in a series (reported on by symmetry earlier this year) called Colliding Particles: Hunting the Higgs, by director Mike Paterson. If you’re at all interested in the LHC, these are really a treat. There are currently five in all, with new episodes still appearing.

A definite crowd favorite was Hairytale (watch it here), the story of former hair dresser Ronn Thompson, who now collects hair clippings and makes a fiber-glass-like building material out of them. This film was materials physics in disguise. The director pushed the conservation angle–Thompson was motivated by a desire to reduce human waste products, so he now rescues many tons of hair from landfills every year. He hopes to create a substitute for fiber-glass, which takes a tremendous amount of energy to make and is dangerous once it’s disposed of. The incredibly strong properties of hair have long been known by biologists, but Thompson displays just how far mother nature’s design can go. While a fiberglass brick cracks under about 65 tons of pressure, the brick of hair holds up to 87–stronger, softer, and more environmentally friendly than fiberglass. Once the audience got over the “ew” factor (all the hair was kind of gross) and saw that the material is hardly recognizable as hair once it’s finished, people were whispering, “wow!”

Another crowd favorite was the most amateur of the whole festival–the actors, at least, were astronomy students rather than film students. But the creators put so much heart and genuine silliness into their product, that people were laughing out loud and cheered at the end. The Agony and Ecstacy of Planet X (free to watch) was the only mockumentary in the bunch, and featured staged interviews and a made-up historical film reel that followed Clyde Tombaugh during his discovery of Pluto. It ended with a pie fight.

Also of interesting note was Decoding Alan Turing, a film less about Alan Turing’s life, and more about how his life is remembered now. Books, films, statues, art projects, and rock songs have been dedicated to the man who broke German codes in WWII, laid the foundation for modern computers, and did it all in the face of potential imprisonment for his homosexuality. The poetic connections in Turing’s life make him one of the most fascinating people in 20th century mathematics, or even just in the 20th century.

The night also included the short film Babbage about Charles Babbage–the man who invented the first computer and then failed to build it (see a trailer here). Things wrapped up the night was the visually savory Planes Lapse; only two minutes long with no words.

That’s all from the Imagine Science Film Festival! Keep your eyes out for science films coming to your area–most of these films won’t have wide release but are appearing at multiple film festivals.

Calla Cofield

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Quantum Quest, an animated adventure

October 29, 2009 | 6:53 am

On Wednesday October 20, 2009, the Imagine Science Film Festival hosted the world premier of Quantum Quest: A Cassini Space Odyssey, an animated tale about a photon named Dave, who lives in the sun and is sent on a quest to save the telescope Cassini. Quantum Quest has all the ingredients for a good kids movie: funny and lovable characters throughout, a battle of good and evil, and a take-home message about believing in yourself. But more than anything, the film boasts quite possibly the most up-to-date and astounding animations of the solar system available. Astronomy lovers will weep.

Dave is a photon living in the core of the sun and mostly living a safe, normal life. He and the other photons look like humans, except that they’re glowing yellow and have odd body tattoos and tails down to their feet. And, of course, they talk. Living in the sun with the photons are neutrinos (also human-shaped but green, and with Tweety-bird-like heads) and protons (upright-walking lizards). Dave’s world is shaken up when he is given the task of carrying a vital message to Cassini in order to prevent its destruction by the armies of The Void and his right-hand man Captain Fear. His minions of dark matter threaten to gobble up anything living or light, and they hate knowledge–hence their desire to destroy Cassini.

While the film is cute, it’s no Pixar triumph, and packs the plot into very few words and very little time. Still, its characters got to me and I laughed at the jokes. Plus, it did a neat job of working some real science into a story of this kind (for example, the neutrino Reina can escape from the enemy because none of the fundamental forces affect her much). It did boast an unbelievable cast, which included two Captain Kirks (William Shatner and Chris Pine) and two Darth Vaders (James Earl Jones and Hayden Christensen).  It also included Jason Alexander, Sandra Oh, Abigail Breslin, Amanda Peet, Mark Hamill, Samuel L. Jackson, and others. Oh yeah, and Neil Armstrong.

But the film’s greatest achievement isn’t its story line or the acting–it’s the animations of the solar system that will captivate audiences both young and old. The film will have a major release in IMAX theaters in February 2010, where it will be shown in 3D (the admittedly unfinished draft we saw was only in 2D). Quantum Quest will probably appear mostly in science museums, and it represents a leap forward in movies of this kind. The imagery of the inner planets, the rings of Saturn, and the Kuiper belt near the edge of the solar system are stunning, and extremely accurate. Based on information from a variety of NASA missions, the film actually identifies on screen which mission provided information about which solar system body. Where was this movie when I was 12? I can easily see this production giving a big boost to kids’ curiosities about space.

At a panel discussion after the film, the writer and co-director Harry Kloor discussed how he got into science through science fiction. Kloor holds PhDs in both physics and chemistry, but has also written for Star Trek: Voyager. Someone asked how he decided when to use very accurate science in the film, and when to use science fiction; the audience member gave the example that Dave the photon and Reina the neutrino could hear each other talk in space. Kloor reminded the viewer that it was a leap to have the photon and neutrino talk at all, and in cases where the story was clearly fictionalized they didn’t worry about scientific details. But, he says, whenever science is presented as science, it’s totally accurate. Kloor said kids seem to know where the science and the science fiction split–it’s adults who can’t seem to separate them.  And perhaps Kloor is onto something in terms of combining science and fiction, by making it very clear when the film is discussing science and when it isn’t.

While I couldn’t quite figure out why (and was too happy to care) Kloor brought with him to the panel discussion four-time space shuttle astronaut Dan Barry. Barry has logged over 700 hours in space, with 25 hours in space walks working on the international space station. Over six feet tall and thin as a rail, Barry was a giant on the stage next to Kloor as they discussed science and entertainment. Barry discussed how he got into science through his desire to fly (“I was the kind of kid who jumped off everything…”), which eventually led him to a career as an engineer. In the Q&A, an audience member asked Berry about his experience in space, at which point he recounted, with awe and grace, the first time he looking down at the Earth on a space walk. He had made sure NASA bought him a new visor so he could see the whole thing clearly. His description of the colors, shapes, and gradients of the earth, compared to the pure blackness of space, had the whole audience silent. “You just can’t capture it with a camera,” he said. “You really have to go see it yourself.”

Quantum Quest is planned for wide release in February 2010.

Calla Cofield

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Gamma-ray burst restricts ways to beat Einstein’s relativity

October 28, 2009 | 1:00 pm

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect, eliminating some approaches to a new theory of gravity. Click for an animation that shows the delay scientists had expected to observe. Credit: NASA/Sonoma State University/Aurore Simonnet

Sometimes a single photon can tell us a lot. Especially when it comes barreling toward us with extremely high energy from a gamma-ray burst billions of light-years away. If you catch just the right photon in just the right way, it might even tell you something about the fundamental structure of space-time and provide guidance toward unifying gravity and quantum physics.

In a paper first published on the arXiv on August 13, and then published in Nature today, the Fermi Gamma-ray Space Telescope team presents a reconstruction of the gamma-ray burst GRB090510, observed on May 10, 2009. The burst included a 31 GeV photon, one of the highest-energy photons ever observed from a gamma-ray burst. Although no grand sweeping statements can be made, this observation does place some limits on the kinds of theories physicists can develop about the nature of space-time.

This burst and photon have already received a lot of attention over the past few months and will get a lot more in the coming days, probably with a series of arguments and claims about how this kills or doesn’t kill various theories of quantum gravity. Based on what has already happened on the Web, the discussion is likely to become pretty rancorous with a small re-inflammation of the string wars on the way. But behind all the shouting, there is a lot of interesting science going on here and the real potential to increase understanding of astrophysics and provide some guidance for the future of quantum gravity theories.

What quantum gravity says about space-time

To set the scene, we need to know a little about quantum gravity. At this time, there is no complete theory of quantum gravity, but there are various frameworks for trying to understand the problem. String theory and loop quantum gravity are probably the best known of these, but there are many other attempts to build quantum physics and gravity into one theory.

Without going into details of specific frameworks, many quantum gravity frameworks adopt the idea that space-time itself might be quantized, or made up of some kind of fundamental unit or grainy structure, down on the level of billionths of billionths of the size of an atom’s nucleus. If that is indeed the case, there are many reasons to think that light moving through that quantized space-time might “feel” the quanta as it travels. One consequence could be that light of different energies would travel at different speeds.

The biggest problem is that no frameworks of quantum gravity really make concrete predictions of anything much. Instead, much of this research is phenomenology, a realm that sits somewhere in between fundamental theory and experimental observation. Phenomenological theories are designed to match experimental observation, and are often inspired by ideas from fundamental theory, but generally can’t be derived purely from fundamentals.

For example, some quantum gravity frameworks suggest that space-time is quantized. This could show up as the speed of light depending on the energy of the photons. But if it does, then the speed of light should vary with energy in some generic way that can described by an equation. In particular, it seems reasonable that the speed of light should be modified by a small correction that depends on the ratio of the energy of the light to some other energy scale. Quantum gravity frameworks suggest that the appropriate energy scale is called the Planck energy. And because energy scales and distance scales are related in these frameworks, that energy corresponds to a length called the Planck length, which is about the scale on which space starts to look like it is made up of little chunks rather than a continuum. The Planck length is about 10^-35 meters—a proton is about 100 billion billion times larger in diameter.

That equation that relates speed of light to energy can’t yet be deduced strictly from the fundamental quantum gravity ideas but these phenomenological equations should have the right structure and properties. Importantly, the equation can be tested against observation. If scientists could observe light of different energies traveling over long distances, they might be able to notice any differences in speed and see if the equation matches.

If that phenomenological equation does matches observation, it gives theorists a hint that their underlying theories could be on the right track and that the details should predict the kind of universe that equation describes. If not, then theorists had better look elsewhere.

Light from gamma-ray bursts

Physicists realized a decade or so ago that if light of different energies travels at different speeds, then the current generation of gamma-ray space telescopes might be able to observe those effects. What would be needed is to observe light coming from a compact source in a short burst a long way from us. By being compact and from a short burst, physicists could be confident that the light is all starting from the same place at the same time. By being a long way off, the light has enough of a chance to separate out, with the highest energy light being delayed slightly relative to the lower energy light. It might seem strange that it is the highest-energy light that is delayed, but a hand-waving argument for this is that the highest energy photons are more likely to interact with the graininess of space-time and so feel its effects more and are slowed most.

GRB090510 is precisely the kind of burst that physicists were hoping for to make this kind of measurement and test, and the Fermi telescope managed to collect good clean data to analyze.

Of course, nothing in astrophysics is as simple as it might sound, and the situation here is quite messy. For starters, nobody knows precisely what happens in a gamma-ray burst. Without knowing the mechanism of the burst, they can’t pin down precisely where and when the gamma-ray photons come from. That means physicists can’t just compare the arrival times as they depend on energy and calculate a speed. Instead, physicists can place some limits on how much quantum gravity effects could influence the speed working under a set of assumptions about the astrophysics involved. In particular, they can look at their phenomenological equation and see what the measurements say about the energy scale of quantum gravity.

The energy scale of quantum gravity

At what scale should quantum gravity become important? We mentioned earlier that the Planck energy and Planck length might be the reasonable scales. One key reason to choose these scales is that they occur naturally as scales defined by the other various constants of nature such as the speed of light and the strength of gravity. Indeed, one way to look at this is to say that any other choice of energy or length scale would require justification. That is precisely the position particle physicists find themselves in as they try to understand why particles have the masses they have and why various phenomena seem to occur at particular energies. Particle physics doesn’t seem to live in the simplest possible world, so to solve those issues physicists need to invoke ideas like symmetry breaking and other concepts more complicated than what the simplest possible universe would require.

Conceptual arguments and choosing the simplest path forward implies that the appropriate scale to observe effects seems to be this Planck scale, and it is often represented by the Planck mass in the equations. (The Planck energy, length, and mass are all related to each by multiplying or dividing by constants of nature.)

If the dependence of speed on energy has the simplest form possible, then the time delay between high- and low-energy photons depends on the energy difference between them and an appropriate mass scale for quantum gravity. So by measuring the time delay and energy difference, physicists can determine limits on this mass scale and see whether it matches up with the expected Planck mass.

What the measurement says about the mass scale of quantum gravity

When the Fermi team did the calculations, using the most conservative estimates for how astrophysics plays into this, they determined that the mass scale must be at least 1.2 times the Planck mass, and by using reasonable but less conservative assumptions, they derived lower limits on the mass scale of up to 100 times the Planck mass. One way to interpret this is to say that there is no variation of the speed of light coming from any quantum gravity effects at less than 1.2 times the Planck mass. And given that some quantum gravity frameworks predict that effects should be showing up at that point, perhaps those models are simply wrong, and there is no changing speed of light.

There are, however, quite a few caveats. The limit on the mass scale is only true if the quantum gravity effects show up in the simplest possible phenomenology where the time difference is proportional to the energy difference scaled by the quantum gravity mass. Some models suggest that the time difference might be proportional to the square of the energy difference scaled by the quantum gravity mass. That would be a much smaller time difference and not observable in this kind of experiment.

Additionally, other quantum gravity models could still have quantized space-time but wouldn’t show an energy-dependent speed of light in this form. Instead, speed might depend on the polarization of the light (called birefringence, like the optical property of calcite crystals which create two images when you look through them). There are other options floating around as well.

To be fair to the claim, though, ruling out the simplest dependence of speed on energy at the expected Planck scale is a significant constraint on future theories of quantum gravity.

The future of quantum gravity theories

In one sense, this result doesn’t change a whole lot. None of the quantum gravity frameworks were really predicting concrete results so it is not really an explicit test of quantum gravity, nor does it rule out any particular frameworks. It does, however, provide guidance to theorists about what kinds of theories might be viable as they develop their ideas further, and that is pretty important to the field. Any experimental constraints are good constraints, especially as string theory is now in a position of predicting almost anything and this result limits some of the versions of string-like theory. Meanwhile, loop quantum gravity had a class of ideas that have now been ruled out.

What is particularly clear is that the technique of observing gamma-ray bursts is powerful for placing real observational limits on the kinds of predictions that quantum gravity can get away with. The Fermi Gamma-ray Space Telescope likely to see many more bursts that can even more tightly constrain quantum gravity theories and some theorists have suggested that combining the observations of gamma-ray bursts with neutrino observations, it might be possible to develop tighter constraints or even measurements of how much light speed changes with energy if the speed of light does indeed depend on energy.

Read more

Fermi Telescope Caps First Year With Glimpse of Space-Time — NASA press release, October 28, 2009
Constraining Modified Dispersion Relations with Gamma Ray Bursts — BackReaction blog, June 26, 2009
Prospects for constraining quantum gravity dispersion with near term observations — Preprint by Giovanni Amelino-Camelia and Lee Smolin, June 23, 2009

Update: Some other news coverage that has appeared since this was published

An intergalactic race in space and time — Nature (news section)
Quantum gravity theories wiped out by a gamma ray burst — ars technica (looks like it got the embargo time wrong so it came out early)
Special relativity passes key test — Physics World

NASA’s Fermi Gamma Ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

David Harris

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America’s accelerator future

October 27, 2009 | 10:27 pm

Director of the DOE Office of Science William Brinkman addresses the Accelerators for America's Future symposium in Washington, DC.

The next big thing in particle accelerators may not be so big, and it might not have anything to do with research into the subatomic secrets of the universe. Instead it could offer a better way to slice silicon into chips, treat cancer, stop terrorist attacks, tap new sources of energy, reduce the world’s growing burden of nuclear waste or turn air pollutants into fertilizer.

More than 400 people are in Washington, DC, this week to draw up a list of possibilities for the Office of High Energy Physics in the DOE’s Office of Science, which builds and operates America’s major research accelerators and funds research on accelerator technology. Called “Accelerators for America’s Future,” it kicked off Monday with an all-day symposium and continued Tuesday and today with invitation-only working groups focusing on industrial applications and production, national security, energy and the environment, medicine and biology, and discovery science. They’ll report their findings later.

What the Office of Science hopes to get from all this is a sense of what these various accelerator users need, both now and in the future; the major cost, technical and policy barriers they face; which areas of accelerator R&D hold the most promise; and how to bridge what one speaker called “the valley of death” between basic research and deploying a new technology, according to Dennis Kovar, associate director of the Office of Science for High Energy Physics. (You can find slides from his talk and other presentations here. )

Most of the public buzz about accelerators these days is focused on the Large Hadron Collider, the big underground ring beneath the Swiss-French border that will bring particles into collision with seven times more energy than any machine before. Although it will ramp up much more slowly than expected due to an accident last year that did considerable damage to its magnets, crews there injected particles into the collider last Friday and Saturday and are on track to restart in November. And the accomplishments of other research accelerators are well known, from revealing the constituents of the atom and the forces that hold them together to creating more than 25 new chemical elements, investigating high-temperature superconductors, and creating conditions in the laboratory that have not existed since shortly after the big bang.

But behind the scenes, smaller and more modest accelerators have been cutting big swaths through the lives of ordinary Americans.

For instance, “The argument’s been made that accelerators have saved more lives than any other biomedical device,” with an estimated 10,000 of them being used to treat cancer, Tom Katsouleas of Duke University told the audience.

More than 18,000 industrial accelerators have been built over the past half-century and most of them are still in use, according to a commentary by Robert W. Hamm in the Oct 09 issue of symmetry; they sterilize medical supplies, analyze materials, toughen the rubber in tires, play a key role in manufacturing the semiconductor chips at the hearts of electronic devices, and even create shink-wrap, among many other things.

Meanwhile, work at synchrotron lightsources–accelerator rings that produce bright beams of X-rays–has illuminated the structures of the rhinovirus that causes colds and 50,000 of the proteins that carry out critical functions in every living thing; how nerve cells function and insects breathe; and, after a 30-year-struggle, the structure of the ribosome, an exceeding complex snarl of molecules within our cells that builds proteins based on instructions encoded in DNA. That last discovery earned the Nobel Prize in Chemistry for three biologists this year, and in fact lightsources have become all-purpose tools for research in a number of fields.

None of this would have been possible without advances in accelerator technology that went hand-in-hand with basic research, said Maury Tigner of Cornell University, who is leading the working group focused on discovery science: “This is really a case of science driving technology driving science driving technology, which is the way that most of these sciences go forward.” He likened accelerators to modern ships of discovery: “They take us where we cannot go unaided, enable us to see what we cannot see unaided.”

In today’s economic climate, however, it’s especially challenging to make the case that the technology and the basic science are worth supporting, not only for the discoveries they enable but for the role they play in driving innovation and keeping America competitive.

That challenge came into sharp focus earlier this month at a hearing of the Subcommittee on Energy and the Environment of the House Science and Technology Committee; the American Institute of Physics’ FYI bulletin summarizes the main points here. Chairman Brian Baird of Washington, who is considered a strong supporter of science, said US tax dollars allocated to research facilities such as the Large Hadron Collider and other “big gizmos” could have been used to address pressing societal needs, and asked scientists how they could justify those expenditures.

“All of us in this room need to help answer those questions,” Fred Dylla, executive director and CEO of the American Institute of Physics, told the symposium on Monday.

William Brinkman, director of the DOE’s Office of Science, said new approaches are needed to bring down the cost of accelerators and create new paths to discovery.

“I believe we’re pushing hard on the limits of conventional accelerators today,” he said. With the proposed International Linear Collider estimated to cost on the order of $20 billion, “It’s starting to get to the point where the scientific community can’t afford these things.”

Brinkman cited several promising approaches that DOE-funded researchers are investigating, including a muon-muon collider, superconducting radiofrequency cavities for propelling particles along, and plasma wakefield acceleration, which has been shown to accelerate electrons to high energies in very short distances. (See “Crashing the size barrier” in the Oct 09 symmetry.) Although plasma wakefield acceleration is still a decade away from practical use, several speakers mentioned it as a promising development that could greatly decrease the cost and size of future accelerators for research, medicine and other applications.

Monday’s keynote talk was given by Norman Augustine, the retired chairman CEO of Lockheed Martin Corp. and chairman of the committee that produced the report Rising Above the Gathering Storm: Energizing and Employing America for a Brighter Economic Future for the National Academies of Science.

While the DOE is now on track to double its budget after decades of relatively flat funding for physical sciences, he said, much of this year’s increase came in the form of one-shot stimulus funding. “That implies, of course, that we’re going to fall off a cliff in about a year–the asteroid is going to hit–if we don’t make the case” that the physical sciences are critical for research in all fields, Augustine said.

With industry cutting back on basic research and universities facing draconian budget cuts, he said, it’s more important than ever for the government to fund university research and maintain federal laboratories that can deal with large-scale problems, perform high-risk research, build large facilities, plan for the long term and foster research that cuts across disciplines.

“If science is the keystone to the quality of life in the future, that’s a message we need to convey,” Augustine said. “I think it’s important to point out how broadly that impact is felt. People take for granted their iPods, their GPS, their laptops. Most don’t realize that it was people years ago, working in the field of quantum mechanics, that made all this possible.”

Glennda Chui

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Beam is back in the LHC

October 26, 2009 | 11:29 am

The first ion beam entering point 2 of the LHC, just before the ALICE detector, on 23 October 2009. Courtesy CERN.

CERN reports that beams of protons and lead ions were injected into the Large Hadron Collider this weekend. The beams made a partial tour of the LHC in both directions before being dumped. This marks the first time in more than a year that particles have entered the LHC, and the first time ever that lead ions traveled through part of the LHC.

On Friday, protons and lead ions traveled clockwise through the LHC, passing through the ALICE detector before being dumped. On Saturday, protons traveled counterclockwise through the LHCb detector. These injection tests allow the scientists and engineers working on the LHC to check that the various sectors are prepared for the particle beam and that the beam is stable. Rama Calaga of Brookhaven National Laboratory was among the scientists monitoring the tests. Calaga noted that these tests were “a spectacular success and there were no surprises.”

The CERN news item also has a photo of the first beam of lead ions entering the LHC.

-Daisy Yuhas

Symmetry Intern

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