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Old August 16th 04, 08:05 AM
Androcles
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"dkomo" wrote in message
news | I found an old PBS documentary on VHS from 1991 called _The Astronomers_
| at the local public library. One of the programs in the series was
| "Waves of the Future" about gravitational waves. In the program Kip
| Thorne was shown making a bet with one of his collaborators on gravity
| wave theory that these waves would positively be detected by 2000.
|
| I found this both humorous and a touch sad. The program described some
| of the early planning for LIGO (Laser Interferometer Gravitational Wave
| Observatory). Curious, I went to the LIGO web site to see what was
| going on. I found nothing of substance there -- just a lot of slick PR.
|
| So my question is, what are the prospects that gravity waves will be
| detected anytime soon?

Why not consider the biggest "wave" or pulse in our galaxy imaginable?
That would correspond to ALL the mass of a star entirely disappearing, as
could just conceivably happen if the star were converted to entirely to
energy
in one enormous supernova. We'd see a brilliant flash of EM radiation,
brighter than daylight, but the entire gravity would be gone. This would
correspond to a negative going pulse radiating outward, which for the sake
of argument we'll allow to be at velocity c (although we have no evidence of
that), and ask if we could detect it. Next, we ask if this cataclysm were to
occur at the nearest star (other than the sun, of course), since that would
provide the strongest negative pulse obtainable to any instrument capable of
detecting it. So... how much gravity do we at present detect from Proxima
Centauri that would disappear?
In reality, even a supernova leaves a considerable remnant, so at best we
could expect 1/10th of a full pulse.
There is a gravity 'wave' we can detect. As our planet turns upon its axis,
it passes through the lunar gravity and causes the tide to rise and fall,
which is clearly detectable as a sinusoidal wave. Take the moon away, and
this would vanish. Also, a lesser tide is produced by our own sun, and this
is superimposed on the oceans. We have spring and neap tides depending on
the relative positions of the moon and sun with respect to the earth. Take
the sun away and this too would vanish, but so would we.
The highest tides we know of are in the Bay of Fundy,
"Every 12 hours 25 minutes 100 billion tonnes of water surges out of the NE
Atlantic and slams into a 70 mile wide funnel between Nova Scotia and New
Brunswick. When it reaches this point, 100 miles in, the channel narrows and
divides into two narrow prongs. The bottom one, Minas Channel, is only 10-15
miles wide. All that lunar-driven water has nowhere to go but up. Near
Wolfville in the Minas Channel high tide is about 16 metres above low tide,
the highest on earth." - http://www.tvnature.com/obh/bald-eagle.htm

and that channel tells we could artificially duplicate our own miniature Bay
of Fundy anywhere in the world, passing the water through a narrow tube and
measure it's velocity and volume as the Earth turns with considerable
precision, since we can measure the height of the water in the tube with
great precision using the wavelength of light.
So... how much does the tide rise and fall as a result of Proxima Centauri
being nearby? I'll leave you to do the trigonometry of Earth, Moon, Sun and
Proxima Centauri and the numbers, you'll have more luck detecting the pull
of Jupiter, Mars, Venus and Saturn, and you'll be looking for the effect of
something far less than the miniscule tug of Phobos or Deimos.
I'm going to say LIGO was doomed to failure at the outset, and it will not
detect a darned thing. That is not the same as saying there is nothing to
detect, just that the effect is so miniscule it cannot be measured. The
inverse square law totally precludes any possiblity of detecting anything
with a source of gravity as far away as Proxima Centauri.

So the answer to your question is... zilch.
Androcles.



Is LIGO still having technical problems or what?
| It is now 2004, after all. Other detection labs are being built around
| the world. Are these labs going to have any better luck?
|
| Also, what are people's opinions about gravity waves? Is it possible
| that these are a scientific dead end like the decay of the proton turned
| out to be? If gravity waves are never detected, what are the
| implications for the general theory of relativity?
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