Re: Cosmic explosion, but no gravitational waves


"Sam Wormley" <swormley1@xxxxxxxxx> wrote in message news:kxUkj.309166$Fc.80673@xxxxxxxxxxxx
Peter Webb wrote:

"Sam Wormley" <swormley1@xxxxxxxxx> wrote in message news:noDjj.303426$Fc.262494@xxxxxxxxxxxx
Cosmic explosion, but no gravitational waves
http://physicsworld.com/cws/article/news/32461


Physicists searching for gravitational waves with the LIGO detector
in the US have released their first major scientific result. But
instead of heralding the much-anticipated first direct detection of
these tiny ripples in space-time, the team announced that
gravitational waves did not appear to emanate from the source of a
gamma-ray burst detected last year. The LIGO team has used this
apparent absence of gravitational waves to gain further insight into
the origins of the dramatic astrophysical events that produce intense
bursts of gamma rays.

"I wish that the first major announcement were a detection of
gravitational waves, but this is not the primary goal of our
field," Kip Thorne of Caltech told physicsworld.com. Thorne, who
is a long-time member of the LIGO team, also said: "As I see it, that
goal is to open up the gravitational wave window onto the universe so
that we can explore poorly understood processes. The LIGO
non-observation is in that spirit."

Disturbances in space-time

Gravitational waves are predicted by Einstein\u2019s general theory
of relativity, in which gravity arises from the curvature of
space-time. The waves are oscillations of space-time that are
produced when a mass accelerates. However, despite strong indirect
evidence for their existence -- in particular from measurements
of the rate at which neutron stars in binary systems lose energy and
spiral towards one other (a result that earned Russell Hulse and Joe
Taylor the 1993 Nobel Prize for Physics) -- there is no direct
proof. This is partly because their amplitude is so small, with even
the most violent astrophysical events disturbing space-time by less
than one part in 10^22.

LIGO (the Laser Interferometer Gravitational-wave Observatory) is the
largest of several facilities designed to detect such disturbances.
It comprises two giant interferometers, one located at Hanford,
Washington state, and the other at Livingston in Louisiana. By
bouncing a laser off mirrors located at the ends of two 4::km-long
arms at right angles to one another, any changes in the relative
lengths of the arms caused by the passage of a gravity wave would
produce a characteristic interference pattern.

Crucially, LIGO's Hanford interferometer was in "science mode" on
February 1st last year, when several space telescopes registered a
short burst of gamma rays in the direction of the nearby Andromeda
galaxy.

First glimpsed 40 years ago, gamma-ray bursts (GRBs) are among the
most energetic and mysterious events in the universe. They come in
two broad types: "long", lasting between 2::s and a few minutes; and
"short", lasting from a few milliseconds to 2::s. In 2003 researchers
successfully traced the former to supernovae, but astrophysicists are
only beginning to understand the origins of short GRBs.

Colliding black holes

The leading candidate for the majority of short GRBs is the merger of
two ultra-dense objects such as neutron stars or black holes -
events that should also produce a burst of gravitational waves.
However, at a conference on GRBs held in Santa Fe last November, the
LIGO team announced that its interferometers had detected no such
signature at the time when "GRB070201" went off.

"We know that coalescing binary have to produce gravitational waves,"
says Jim Hough of Glasgow University , who is principle investigator
for the UK of the GEO600 gravitational wave detector based in
Hannover, Germany. "Therefore, either the source was not a coalescing
binary or there is some exotic situation where the gravitational
waves disappear into another dimension. The latter seems unlikely,
but would be very exciting of course!"

Other causes for the event, such as a "soft gamma ray repeater" (SGR)
or a binary merger from much further away, are now the most likely
contenders. However, Stan Woosley of the University of California at
Santa Cruz -- who was one of the first to link long-lived GRBs with
supernovae -- points out that the merger of neutron stars is excluded
only to the 90% level, which is not as tight as astrophysicists would
like. " If the event was indeed in Andromeda, it was likely a SGR.
The likelihood of two neutron stars merging in this nearby galaxy
while we happen to be watching is perhaps one in a million years ,"
he says. "However, the result is a technological tour de force which
illustrates the potential of co-ordinated gravity wave and gamma-ray
observations."

The result has recently been accepted for publication in the
Astrophysical Journal.

Has LIGO found any gravitational waves at all? Or could this be a billion dollar version of Michelson Morley?



You should go to the LIGO Pages and read the published results. BTW
null results are extremely important in science... the Michelson Morley
was one of many.


I did. I couldn't find any references to actually finding gravity waves - just stuff about improving detector accuracy etc. Have they actually found any?

And I am aware that null results are important in physics, which is why I specifically compared LIGO to MM. However, having failed to find ether drift, physicists didn't propose spending billions of dollars building more accurate versions of the MM. Null results give you only one "bit" of information. If gravity waves are detected, it would give us far more usable information than getting a null result, and in this sense a null result is the least valuable outcome. One has to wonder if its worth spending billions to produce an instrument that produces no observational data - the fact that it doesn't produce data is interesting, but I doubt that it is worth the money being spent.

If LIGO produces no observational data, will it continue to be operated, essentially repeating the same experiment and same outcomes day after day - like a continuously running version of MM? Or, if no gravitational waves are detected, will they decommission LIGO and use the money tied up in operation to run experiments that actually do provide usable experimental data?

So, has LIGO detected any gravity waves? Is there any experimental evidence that these exist at all? Could we be pissing billions of dollars away building a device to measure something that doesn't actually exist?




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