High-powered jets beaming at us from distant galaxies may hint at the existence of an exotic particle, an Italian team suggested recently on arXiv.org. The study is highly speculative — a fact the team is quick to point out — but the proposed particle’s characteristics match those put forward by other researchers to explain another astrophysical problem.

So why is it so speculative? Because the particle requires an extension to standard physics. But before you shake your head and click away from this page, consider that pushing the boundaries of run-of-the-mill physics is the bread and butter of scientific advancement. And in this case, there may soon be a straightforward way to test whether the exotic particle actually exists.


This highly stylized illustration shows the heart of an active galaxy, where some of the matter falling into the supermassive black hole is funneled out as jets of particles traveling near the speed of light. For active galaxies classified as blazars, one of these jets beams right toward Earth.

NASA / Goddard Space Flight Center Conceptual Image Lab

Jets of superhot, ionized gas shoot out from some galaxies’ cores, funneled out along magnetic field lines by the galaxies’ central supermassive black holes. These active galaxies can have a variety of orientations from Earth’s point of view, and when the jet points straight at us the object is called a blazar. Blazars are incredibly luminous, especially in gamma rays, the most energetic form of photons. But because these galaxies are so far away many of the higher-energy gamma rays won’t survive the journey to us: en route they’ll collide with lower-energy photons between us and them, thereby making the blazars look fainter and less energetic than they really are.

What astronomers have found is that, even at the minimal level of intergalactic interference, we’re still seeing blazars’ most energetic gamma rays too well. This problem led a trio of Italian physicists to suggest in 2007 that photons from a blazar’s jet are fickle: they oscillate between being photons and an as-yet-undetected particle, called an axion-like particle.

Axions (and axion-like particles, or ALPs) are hypothetical, low-mass particles that interact rarely with matter. Part of their definition is that they can turn into photons (and vice versa) when in a strong magnetic field. ALPs are attractive because they may help explain a perplexing discrepancy between our understandings of two fundamental physical forces, the strong and the weak.

ALPs are also attractive because, unlike gamma rays, they can pass pretty much unmolested through a sea of lower-energy photons. If photons could oscillate between being photons and ALPs as they encounter magnetic fields along their journey between blazar and Earth, the observed blazar power could increase by a factor of 10.

So far, there’s no experimental evidence for ALPs. Blazars could provide that long-sought evidence. Unlike the 2007 scenario (elaborated last year in Physical Review D), the new study looks at what happens to photons behind the curtain of the blazar’s innermost sanctum. The jet’s base should lie inside the so-called broad-line region (BLR), an area of hot, rapidly moving clouds emitting lots of optical and ultraviolet radiation. These photons suffuse the region inside the BLR just as a room with lamps along the walls is filled with light, explains study coauthor Fabrizio Tavecchio (Astronomical Observatory of Brera, Italy). Impeded by these photons, gamma rays have trouble escaping. But there are plenty of high-energy gamma rays seen, more than expected if this interference is such a problem.


The well-known elliptical galaxy M87 has a jet pointing somewhat toward us, although not as straight-on as a blazar. The jet extends several thousand light-years.

NASA and The Hubble Heritage Team (STScI / AURA)

To add insult to injury, observations of the blazar PKS 1222+216 with the Cherenkov MAGIC telescopes in the Canary Islands caught the blazar’s gamma-ray brightness doubling in a mere 10 minutes. To be so quickly variable, the emitting region must be compact. Tavecchio and his collaborators suggested in 2011 that the gamma rays might actually originate outside the BLR, avoiding the whole interference conundrum, but because the source is small that would put it a couple of light-years from the black hole, and there’s no evidence to support that.

On the other hand, if the gamma rays could oscillate between being photons and ALPs, it could explain everything. Interacting with the magnetic field inside the blazar could switch the photon to an ALP, allowing it to traverse the BLR unimpeded; when the particle hit another magnetic field later on — perhaps that of the Milky Way — it could flip back, allowing us to observe it.

I admit, it sounds farfetched. Yet ALPs in various forms are actually a common prediction of expansions to the standard model of particle physics. What’s more, the particular particle characteristics supported by Tavecchio and his colleagues to explain how gamma rays make it from one side of the BLR to the other are similar to those put forth to explain the larger blazar-to-Earth problem.

Naturally, researchers are responding with caution. “It’s possible that these observations point to the existence of an axion-like particle,” says Clare Burrage (University of Nottingham, England). “But they may also just be telling us that blazar physics is more complicated than we thought.”

Still, says Alessandro de Angelis (University of Udine and the National Institute of Nuclear Physics, Italy), who coauthored the 2007 paper and its successor, the fact that the particle’s properties match those suggested by his team makes the new study “fascinating.” Various non-exotic explanations exist for the extra high-energy gamma rays, but they’re all rather unsatisfactory, he explains. And fickle photons aren’t a new idea: axion-like particles are just the current flavor in favor. These things could be out there, he says.

The neat part of the ALP suggestion is that researchers may be able to verify the particle’s existence in a few years, perhaps with the hoped-for Cherenkov Telescope Array or planned upgrades to the Deutsches Elektronen-Synchrotron ALPS experiment in Germany. This experiment shoots laser photons through a strong magnetic field at a wall to see if some photons pop up on the other side — where they have no right to be, according to standard physics.

“I always like it when people present models that are straightforwardly testable, as this one is,” says Katherine Mack (University of Cambridge, England). “If you're really looking for evidence of new physics these days, astronomical observations are where you’re most likely to find it, unless you have your own personal LHC.”

It’s worth noting that this particular brand of ALP could not explain dark matter, Mack says. It just wouldn’t contribute enough to the universe’s mass to be a large fraction of the missing matter.


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Bruce Mayfield

March 9, 2012 at 1:21 pm

Nice article, Camille. Particle oscillation was what solved the "missing" neutrino problem. Is this a simalar process? Such astronomical observations are much more economical too, seeing aa how personal LHC's are so hard to come by.

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Mario Motta

March 9, 2012 at 3:35 pm

Hi Camille, great article, I very much enjoyed the reading of it. Such a scenario would in fact solve many issues with blazers, very innovative, and maybe correct. Thanks, keep up with those kind of articles,
Mario Motta

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John Anderson

March 10, 2012 at 8:26 pm

There is a lot more to this article than meets the eye. First is the fact that photons don’t scatter off photons because the photon doesn’t carry charge. Gamma rays are just high energy photons. The statement “Impeded by these photons, gamma rays have trouble escaping” implies a rare quantum electrodynamics effect. Photons can “collide” via interaction with a vacuum fluctuation and create real particles. This was first detected in 1995 at SLAC.

Second, if a particle can oscillate into another, it implies that it has mass. This argument led to the prediction that neutrinos have mass. Ergo, the implication is that photons have mass if they can oscillate into ALPs. So photons have mass now? Are they ‘off-shell’ relativistically?

If quantum field theory requires extensions, which seems to be the assertion, then anything I have learned is obsolete. I can only watch science dumbfounded. (No snide comments.)

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Mark Gradwell

March 11, 2012 at 11:00 am

There could possibly be a whole new set of physical laws out in and beyond our universe. Just think if black holes actually lead out to other universes or dimensions then there will no doubt be new ALPs and lots of other hypothetical phenomena waiting to be discovered. Could there even be such a thing as "anti time" or even a time particle. Could all that mass unaccounted for in the universe actually be physical time particles. It makes you think doesn't it.

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