Article: Hints of a breakdown of relativity theory?

On Aug 29, 11:43 pm, "Robert Karl Stonjek"
wrote:
Hints of a breakdown of relativity theory?
The MAGIC gamma-ray telescope team has just released an eye-popping preprint
(following up earlier work) describing a search for an observational hint of
quantum gravity. What they've seen is that higher-energy gamma rays from an
extragalactic flare arrive later than lower-energy ones. Is this because
they travel through space a little bit slower, contrary to one of the
postulates underlying Einstein's special theory of relativity -- namely,
that radiation travels through the vacuum at the same speed no matter what?

The team studied two gamma-ray flares in mid-2005 from the black hole at the
heart of the galaxy Markarian 501. They compared gammas in two energy
ranges, from 1.2 to 10 tera-electron-volts (TeV) and from 0.25 to 0.6 TeV.
The first group arrived on Earth four minutes later than the second. One
team member, physicist John Ellis of CERN, says: "The significance of the
time lag is above 95%, and the magnitude of the effect is beyond the
sensitivity of previous experiments."
...

It is notable that the higher energy rays appear to travel slower. In
theoretical discussions of tachyons, the higher energy particles
actually travel slower though above light speed than lower energy
tachyons, which is the reverse of the case with slower than light
particles. It would be interesting to find out if even higher energy
gamma rays appear to travel even slower.
In an earlier report, the higher energy gamma rays from Markarian 501
were also regarded as being anomalous because they were expected to be
absorbed by cosmic background radiation which wasn't observed.
Breaking of Lorentz invariance was one of the explanations proposed
for this also:


Newsgroups: sci.astro, sci.physics, sci.physics.relativity
From: Robert Clark
Date: 2000/11/17
Subject: Superluminal speeds as an explanation of cosmic ray
anomalies.

The post below originally from New Scientist of 23rd September 2000
discusses the anomalous detection of high-energy gamma rays from
Markarian 501. One proposed solution is that this may be due to
Lorentz invariance breaking at ultra-high energies:

PLANCK SCALE DEFORMATION OF LORENTZ SYMMETRY AS A SOLUTION TO THE
UHECR AND THE TEV GAMMA PARADOXES.
http://xxx.lanl.gov/abs/astro-ph/0008107

This paper and others investigating breaking of Lorentz invariance at
ultra-high energies all note that this would require a preferred
frame,
that is, implicitly, violations of causality are not required.

--
_______________________________

"Sig file on vacation."
- Bob Clark
_______________________________

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Forum: sci.astro
Subject: Outburst from a galaxy leaves scientists in a quandry
(Forwarded)
Date: 09/22/2000
Author: Andrew Yee

New Scientist
http://www.newscientist.com

UK Contact:
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EMBARGOED FOR RELEASE: September 20, 2000, 14:00 EDT US

Outburst from a galaxy leaves scientists in a quandry
By Hazel Muir

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
Vslk, 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.

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 Vslk, a director at the Max Planck Institute for Nuclear Physics
in Heidelberg.

Air shower

Vslk 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 nothing."

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 10**20 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 years."

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 Schrsdinger'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 Vslk, 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.

###

Author: Hazel Muir, New Scientist

New Scientist issue: 23rd September 2000

PLEASE MENTION NEW SCIENTIST AS THE SOURCE OF THIS STORY AND, IF
PUBLISHING ONLINE, PLEASE CARRY A HYPERLINK TO:
http://www.newscientist.com

--
Andrew Yee

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