Re: einstein@home

Curt <curt2@xxxxxxxxxxxx> wrote:

> I recently signed up to that 'Einstein@home' project; I don't know how
> many of you have heard about it. For those who don't know, it's a large
> volunteer network of computers analysing data from various gravity wave
> detectors worldwide.

> Has there been any definitive evidence of gravitational waves thus far?

No, though to some extent it depends on what you count as a detection.
According to general relativity, a binary star system should lose energy
to gravitational radiation at a calculable rate, and this should show up
as a gradual orbital decay. This has been observed in three separate
binary pulsar systems, with a decay rate exactly equal to the prediction.
(Hulse and Taylor won the 1993 Nobel Prize in Physics for the first such
discovery.) This gives some very strong indirect evidence of gravitational
radiation -- any alternative explanation would have to explain not only
why the observed orbits are decaying, but why the rate, and the change in
the rate over time, exactly matches the predictions of general relativity
-- but it's not direct detection.

> If not, why are they so difficult to detect? Does it have something to do
> with gravity being the weakest of all forces?

Yes, exactly. The weakness hits us twice. First, gravitational radiation
is hard to produce. The Solar System radiates, for example, mainly because
of Jupiter's orbit around the Sun, but at only about 5000 Watts. To get a
large amount of radiation, you need very large masses -- a pair of neutron
stars, for instance, or a neutron star and a black hole -- moving at very
high speeds. But unless we are very lucky, such sources are going to be
far away. So we lose a lot of the power by the usual inverse square law;
not much of the radiation reaches us here.

Second, when we try to *detect* gravitational radiation, we are again faced
with the weakness of the interaction. Even a relatively strong gravitational
wave will affect a detector only weakly, and an enormous amount of noise has
be be eliminated or accounted for. (At LIGO, they have worried about such
effects as gravitational fields of tumbleweeds blown against the building
and of people walking within 10 meters of the mirrors.)

There's a third, slightly more technical, issue related to conservation laws.
As you may know, any kind of wave may be described as a sum of "multipoles"
-- a spherical monopole component, for example, and higher multipoles with
more complicated shapes. For both gravity and electromagnetism, conservation
(of energy and charge) imply that there can be no monopole radiation. For
electromagnetism, the next order, dipole radiation, is allowed. For gravity,
though, conservation of momentum prohibits dipole radiation, and the first
order that is allowed is quadrupole radiation. Quadrupole radiation is
typically smaller than dipole radiation; for a source with a typical velocity
of v, quadrupole radiation is suppressed relative to dipole radiation by a
factor of order (v/c)^2. Quadrupole radiation also requires a less symmetric
source. A spherical, or even axially symmetric, supernova will not produce
gravitational radiation; the explosion has to be significantly nonsymmetric.

Steve Carlip
.

Relevant Pages

  • Re: Welcome to Dipole Gravity blog
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  • Re: Is LIGO just observing viscosity of the vacuum?
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    (sci.physics.research)
  • Re: einstein@home
    ... helps me visualise it) ie it has two "poles", whereas gravity does not (or ... If I were to be hit by a massively powerful gravitational wave ... > to gravitational radiation at a calculable rate, ... > order that is allowed is quadrupole radiation. ...
    (sci.physics.relativity)
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  • Re: An explanation for the increasing expansion
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