ONE WEEK in 1997, a mouse of a galaxy between the shoulders of Hercules
turned into a monster. “It was rather inconspicuous before,” says Heinz
Völk, who watched the action from the island of La Palma. “But all of a
sudden it became the strongest source we’ve ever seen.”
This outburst from the galaxy Markarian 501 has left scientists in a
quandary. The most energetic photons in the blast had trillions of times the
energy of a visible photon, and according to the laws of physics as we
understand them, they should never have made the vast journey from that galaxy
to Earth. They should have been snuffed out by the sea of infrared radiation
that fills space.
So what’s going on? Some physicists think there’s something weird happening
inside Markarian 501: bunches of photons are ganging up into exotic globs called
Bose-Einstein condensates. Others say there’ll be an everyday explanation once
they’ve mulled over the facts and figures a little more. But some scientists
think that Markarian 501 is telling us something momentous. It might, they say,
go down in history as the key to a 21st-century revolution, a theory that at
last marries quantum mechanics with Einstein’s theory of gravity. Then this
galaxy would be a gateway to a hidden realm of nature where space and time are
radically transformed.
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Markarian 501 is no newcomer to astronomers. It appears on photos of the
constellation Hercules dating back at least a century, and gets its name from
Beniamin Markarian, a Georgian astronomer at the Byurakan Astrophysical
Observatory in Armenia who started to compile a catalogue of hundreds of bluish
galaxies in the 1960s.
Numbers 501 and 421 in his catalogue are special. At around 300 million light
years away, they are the two closest examples to Earth of a rare and strange
type of astronomical object known as a blazar. Like other kinds of active
galaxy, such as quasars and radio galaxies, blazars are thought to be powered by
a central black hole which feeds on the gas, dust and stars that whirl around it
in a hot disc. Above and below the hole, two jets of energetic protons and
electrons shoot millions of light years into space.
Blazars are capricious, flaring up and dimming again within just a few days.
Astronomers think that this is because of their orientation. We see an active
galaxy as a blazar if one of its jets is pointing towards us, as though we’re
looking down the barrel of a gun. The jet sends out a narrow beam of radiation
whose brightness can change rapidly as it shifts slightly or its supply of
material from the black hole changes.
This special alignment also means we are assailed with ferociously energetic
radiation. In 1992, the orbiting Compton Gamma Ray Observatory picked up
high-energy gamma rays from Markarian 421. Astrophysicists believe they are
cooked up in the jet by superfast particles. As electrons and protons spiral
around the jet’s strong magnetic fields, they emit powerful radiation. They
could also be colliding with ordinary photons, boosting them to ultra-high
energies.
But it wasn’t until March 1997 that astronomers saw what a blazar can do when
it really flexes its muscles. They watched astonished as Markarian 501 flared up
from one of the puniest gamma-ray sources in the sky to upstage even the Crab
Nebula, the debris of an exploded star in our own galactic backyard, which is
the brightest steady gamma source in the sky. The outburst lasted several
months, and at its peak Markarian 501 was ten times as bright as the Crab,
despite being 50 000 times farther away. “The distance difference is just
mind-blowing,” says Völk, a director at the Max Planck Institute for
Nuclear Physics in Heidelberg.
Air shower
Völk is a spokesman for an experiment called HEGRA (High Energy Gamma
Ray Astronomy), which kept its eye on Markarian 501’s storm from La Palma, in
the Canary Islands. When a high-energy gamma ray hits the upper atmosphere,
it sparks an “air shower”—a spreading cascade of superfast subatomic
particles. These emit light, and because they move faster than the speed of
light in air their emissions pile up into blue flashes known as Cerenkov
radiation—just as sound waves from supersonic aircraft pile up into a
sonic boom.
During 501’s outburst, HEGRA’s six big mirrors saw astoundingly bright blue
flashes. These indicated that some of 501’s gamma rays had energies of up to 22
teraelectronvolts (Astronomy and Astrophysics, vol 349, p 11). This is
trillions of times as much as a photon of visible light, which has an energy
between 1 and 3 electronvolts.
What is hard to explain is why the gamma rays made it to Earth. When a
high-energy gamma ray and an infrared photon collide, they have enough energy to
mutate into an electron and a positron. So the gamma rays should be gradually
mopped up by the sea of far-infrared photons that fills space, emitted by
forming stars and hot dust.
How far the gamma rays get depends on how many far-infrared photons are out
there. In the past two years, several teams of scientists have taken old images
from NASA’s Cosmic Background Explorer (COBE) satellite and the European Space
Agency’s Infrared Space Observatory (ISO) and used some novel mathematical
tricks to cancel out the infrared from our own Solar System and Galaxy. The
results show that the far-infrared background is so bright that gamma rays with
energies of more than 10 teraelectronvolts should never reach the Earth from as
far away as Markarian 501. So why did we see them?
Perhaps the gamma rays are colluding against us, says Peter Biermann, an
astrophysicist at the Max Planck Institute for Radio Astronomy in Bonn. He and
his colleagues suggest that several gamma rays from Markarian 501 might merge
into a Bose-Einstein condensate—a densely packed globule of lower-energy
photons that have exactly the same positions.
This should happen to light from a super-efficient laser—one far more
efficient than any yet built on Earth. Nature does build lasers: in many active
galaxies, X-rays make clouds of water vapour emit microwave laser light. The
Universe is dotted with billions of these microwave lasers, or “masers”. But is
a natural, super-efficient, ultra-high energy laser plausible? Biermann claims
that it could conceivably happen when a group of excited atoms in a blazar’s jet
all stimulate each other to emit light at the same time (Astrophysical
Journal Letters, vol 524, p 91).
Say the blazar fired out a Bose-Einstein condensate of 20 identical gamma
rays with energies of 1 teraelectronvolt each. Because these photons have
relatively low energy, they would be unimpeded by the far-infrared background.
Arriving together in the Earth’s atmosphere, they would dump 20
teraelectronvolts of energy at the same point in the atmosphere, just like a
single 20-teraelectronvolt gamma ray.
Scientists are now taking another look at HEGRA’s observations to test this
idea. They’re looking for a subtle difference between the air showers a
high-energy photon would trigger in the atmosphere and those produced by a ball
of less energetic ones. Though they both have the same total energy, a lone
high-energy photon would create a narrower, more chaotic air shower. “It’s like
if you have a very heavy truck in a accident in the highway—there’s an
incredible scatter.” says Biermann. “But 20 teeny trucks might do next to
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Natural gamma-ray lasers may sound like an outlandish explanation, but
another possibility would be far more momentous. Giovanni Amelino-Camelia, a
physicist at the University of Rome, believes that Markarian 501’s gamma rays
might be subject to an entirely new kind of physics that rules the high-energy
world. For decades, physicists have been trying to marry quantum theory with
general relativity, Einstein’s theory of gravity. Most of their fledgling
theories of quantum gravity predict that on tiny scales, approaching 10-35
metres, our picture of smooth space and time falls apart, giving way to a
seething froth of quantum gravity fluctuations dubbed space-time foam.
If so, odd things start to happen. As photon energies get higher, the speed
of light might start to drop off by a tiny amount, because the very short
wavelength would mean that the light started to “feel” the bumpiness of
space-time. “A very rough analogy is that if you roll a soccer ball across a
table with lots of tiny ridges, it will travel at roughly the same speed it
would have done if there were no ridges,” says Amelino-Camelia. “But if you roll
a tiny little ball, its path will be strongly altered by all the little valleys
in the table.”
Feeling the bumps in space would not only slow very high-energy photons, it
would help them avoid infrared photons. Raymond Protheroe of the University of
Adelaide in South Australia and Hinrich Meyer of Wuppertal University in Germany
calculate that provided quantum gravity does indeed kick in at a scale of 10-35
metres, this could give 20-teraelectronvolt photons just the edge they need to
ignore the far-infrared background and make it from Markarian 501 to Earth.
What’s most compelling, Amelino-Camelia says, is that this could also explain
another cosmic conundrum. Protons with giant energies of more than 1020
electronvolts are occasionally detected hitting our atmosphere. For years,
astrophysicists have wondered why. The only known sources that could produce
such energy are distant active galaxies, which means that these protons should
also be eaten up by background radiation—this time the relic microwave
radiation of the big bang (New Scientist, 7 December 1996, p 38).
According to calculations announced last month by Amelino-Camelia and Tsvi
Piran of the Hebrew University in Jerusalem, the same roughness of space-time
needed to explain the gamma rays from Markarian 501 also solves the cosmic ray
problem. “The remarkable point is that you have these two problems at very
different energy scales and contexts,” says Amelino-Camelia. “Yet with the same
equations, we can explain both.”
Amelino-Camelia admits there’s a lot of guesswork going on. But for the first
time, he says, nature might be throwing us some solid clues to quantum gravity.
“And even if the correct explanation is different, we are finally obtaining data
that are relevant to our understanding of the small-scale structure of
space-time,” says Amelino-Camelia. “This really is a turning point.”
To find out if space-time is truly fuzzy in this way, Amelino-Camelia thinks
astronomers should look to gamma-ray bursts. These are bright bursts of gamma
rays that appear unpredictably anywhere in the sky and come from distant,
mysterious sources. If astronomers could catch a very distant and bright burst,
the highest-energy photons may lag slightly behind (Nature, vol 393, p
763). He says present-day detectors aren’t sharp enough to pick up the tiny
timing differences necessary. “But on the space station and other orbiting
observatories, we’ll acquire this level of sophistication over the next few
.”
If Amelino-Camelia’s speculations hold true, they could lead to a change in
our concept of time as radical as that brought about by relativity at the
beginning of the 20th century. “Special relativity said that time is not
absolute. That was the breakthrough for mankind at that time,” he says.
Quantum gravity implies that time comes in discrete pieces. What’s more, like
Schrödinger’s proverbial cat, which is neither dead nor alive until we
choose to look at it, time would exist as a jumble of different possible values.
“The concept of ‘now’ becomes just a rough approximation,” says
Amelino-Camelia.
Not everyone agrees that Markarian 501’s message is this radical. “That’s a
rather outré possibility,” says Sheldon Glashow of Boston University. He
believes that other experiments have made such large departures from smooth
space-time look extremely unlikely, and thinks there’s probably a simpler answer
to the puzzle—perhaps that we’ve overestimated the distance to the blazar.
If it is closer than we think, energetic gamma rays could make the journey to
Earth despite the infrared background.
Erasing the Milky Way
Biermann agrees that something mundane could turn out to be the key. “My gut
feeling is that the solution is something simple that we’re just not seeing,” he
says. Along with Völk, he’d put his money on the latest far-infrared
measurements being wrong. “The measurement of the far-infrared background is
notoriously difficult,” says Biermann. He thinks that improved tricks for
erasing the Milky Way from COBE and ISO images to work out the strength of the
far-infrared background might show it to be weaker than we now think.
One way to resolve this would be to look at more distant blazars. A whole new
generation of gamma-ray telescopes is due to get under way. Scientists from
Germany, France and Italy are building a telescope array in Namibia called HESS
(High Energy Stereoscopic System). The array, with up to 16 telescopes, will be
10 times as sensitive as HEGRA, and could pick up blazars 30 times as distant as
Markarian 501.
The first four HESS telescopes should start operating next year, along with a
German telescope called MAGIC (Major Atmospheric Gamma Imaging Cerenkov
Telescope). MAGIC, at the HEGRA site on La Palma, will gather Cerenkov light
using a mirror 17 metres across. Further down the line, there are plans for an
American telescope called VERITAS (Very Energetic Radiation Imaging Telescope
Array System). Sited at the foot of Mount Hopkins in Arizona, this array will
have seven mirrors, each 10.4 metres across, and start operating sometime in
2004 or later.
If the new telescopes find that Markarian 501’s even more distant cousins are
relentlessly pelting us with 20-teraelectronvolt gamma rays, ordinary physics
will be hard pushed to explain why. “This would deepen the suspicion that
something dramatically new is happening,” says Meyer. Whatever happens, the
performance of the Universe’s most histrionic galaxies will be under the
spotlight for years to come.
