Re: einstein@home

Hi there,

thank you very much for the reply; you've answered all my questions. I've
been thinking about getting that book by Kip thorne, I think i definitely
will do so now. To be honest the bit about "multipoles" confuses me a
little; I'm currently doing AS-level physics in the UK, and have not come
across these terms so I've just done a few web searches. Most of the sources
actually don't explain with anything less than high level maths; don't get
me wrong, I love maths, but this stuff is way over my head!

As a guess, does it have something to do with the fact that electromagnetism
fluctuates from positive to negative, north to south (very classical, but it
helps me visualise it) ie it has two "poles", whereas gravity does not (or
does it?). If I were to be hit by a massively powerful gravitational wave
(absurd, but bear with me) what would I experience? A 'push' or a pull? What
is the wavelength of the gravity waves you are trying to detect? Sorry if
most of these questions seem naive, but you've got me genuinely interested.

Thanks for your time,

Curt

<carlip-nospam@xxxxxxxxxxxxxxxxxxx> wrote in message
news:d2ulmd$nfr$1@xxxxxxxxxxxxxxxxxxxxxx
> 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

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