Re: gravitational-wave sources
- From: tessel@xxxxxx
- Date: Mon, 23 Jan 2006 19:55:47 +0000 (UTC)
Joe Fischer proposed a "rail gun" experiment for artificially producing
gravitational radiation which he hoped could be detected by LIGO.
Jonathan Thornburg pointed out that a quick computation using the well
known quadrupole radiation formula shows that any such rail gun
experiment would produce gravitational radiation much too weak to be
measurable by the LIGO detector, even at "close range". OTH, he wrote,
>> this would
>
>> (b) produce large *Newtonian* gravitational effects which would
>> interfere with LIGO's operation, and
John Bell commented:
> I presume, if you are answering in the context of the original
> question, that we wish to shield from *Newtonian* gravitational effects
> because they appear to travel at infinite speed as do their Einsteinian
> equivalent.
As I understand it, JT's points were these:
1. Fischer's rail gun experiment -would- produce gravitational
radiation, according to gtr, but this would be much too weak to detect
with current interferometric gravitational wave detectors even at close
range, even -if- all other effects could somehow be eliminated--- which
they almost certainly could not be!,
2. in particular, a rail gun would very likely produce ground vibration
sufficient to disrupt LIGO (recall that vibration from nearby
tumbleweeds blowing around, vibration from railway trains fifty miles
distant, etc., has in fact interferred with these instruments),
3. in gtr (and in Newtonian gravitation), moving mass around changes the
gravitational potential, which is almost always a much stronger effect
than gravitational radiation (in gtr), since such nonradiative
components of the field typically scale like 1/r^3 rather than 1/r,
where r is distance to the source (IIRC, such nonradiative time varying
fields have in fact interferred with various other gravitational
experiments, such as precision measurements of G).
Before Joe asks, quadrupole radiation formula estimates suggest that any
radiation we could in principle produce near Earth, short of blowing up
the Moon as was frequently suggested by the late A. Abian, would not be
detectable by the proposed space-based interferometric detector, LISA,
even though LISA would of course not be subject to interference from
ground vibrations!
> You appear to have missed the point here. The sensitivity of a
> gravitational wave detector does not depend on the strength of the
> source. It depends solely on the resultant strength of the signal at
> the location of the detector. Consequently, although Joe Fischer's
> suggestion has serious defects in practice, it does not appear to have
> such defects in principle.
I think JT knows this. And I doubt that anyone here would be other than
delighted if someone cooked up a -workable- scheme for producing
measureable gravitational radiation in a laboratory!
The thing is, our current understanding suggests that the only effective
way to produce undirected strong radiation is to move huge amounts of
mass very rapidly, and pretty much the only thing which can do that is a
black hole or other highly compact object. No doubt everyone can
appreciate that a black hole with substantial mass would be hard to
produce on Earth, and hard to manage even if it could be created
artificially.
OTH, it has been pointed out that certain axisymmetric pp wave solutions
answer to the name of "thunderbolts" (a notion proposed by Penrose). I
have studied explicit examples in some detail, and there is no question
that gtr says they would propagate just fine, and they would be rather
hard not to notice -if- they could be created. Even better, they would
have some quite distinctive physical effects such as violent type N
tidal acceleration of test masses. From the point of view of
theoretical and experimental physics, it would indeed be interesting to
discuss how to artificially generate narrow beams of extremely intense
gravitational radiation, if other considerations did not interfere.
> Consequently, I suggest that it makes perfect sense to test the theory
> more elegantly (and economically) by placing a viable gravitational wave
> source and compatible gravitational wave detector as close together as
> possible, provided that we:
>
> (a) include means to isolate from the effects of vibration
> (b) include means to isolate from atmospheric acoustic coupling
> (c) include means to isolate from the detection of *Newtonian*
> gravitational effects and their Einsteinian equivalent.
Indeed yes.
> This is not mere idle speculation, as we performed such an experiment
> over 20 years ago whilst the relevant applied technology was under
> British State Secrecy Classification.
Yet another argument for migration of science to the nonhuman realm.
Humans are too unclean to do science efficiently.
In any event, without seeing the details which you are not at liberty to
share, naturally we cannot comment on whether we agree that the specific
scheme you are familiar with is truly practical. I'll just say that
experience with LIGO (and other experiments) has shown that even
experienced investigators often find it surprisingly difficult to shield
sensitive equipment from ground vibration, sound waves, and other
effects of nonfundamental (not to say uninteresting!) physics. So I'd
tend not to take anyone's word for anything, while generally supporting
giving any promising idea a fair trial.
For example, I have suggested exploring the idea of a suitable
space-based experiment using LISA. Here we wouldn't need to worry about
vibration, but of course we'd still need to experimentally separate the
radiation part of the field from time variations in the nonradiation
part of the field (i.e. terms additional to any type N components in an
adapted NP tetrad, if you like). And we'd probably need to worry about
many other unwanted physical effects, such as effects of any EM or
thermal radiation produced as byproducts of the experiment. I think the
biggest problem is still producing radiation in sufficient quantities,
rather than any decay of the radiative field with distance from the
source.
> The irony of that situation is that the US government has subsequently
> committed millions of dollars in taxpayers money to investigate a far
> less efficient test of the theory.
As you no doubt know, this same government has invested far more in
missile defense, manned spaceflight, and many other
scientifically/technically dubious schemes, which to my mind can be
fairly described as "clearly wasteful". OTH, I expect that
LIGO/VIRGO/GEO will amply prove their value once gravitational wave
astronomy (I hope!) becomes a practical reality. In the long run I
doubt that it will turn out that these programs have been wasteful or
even remarkably inefficient.
If we just want to -directly- verify the -existence- and properties of
gravtational radiation, I tend to agree that there might be better ways,
but as I see it, the ultimate aim of projects like LIGO and LISA is to
study -naturally occuring- gravitational radiation, not as a test of
gravitation theories, but as a new window on the universe. For a test
of gravitation theories, I tend to agree that an experiment in which the
source is under the experimenters control is in principle preferable!
The problem, it seems, is to devise appropriate experiments using only
publically available information, and then to get such experiments
funded.
The example of RSA cryptosystems (allegedly an open source rediscovery
of a previously but secretly known scheme) may be inspiring to those not
involved with big government and willing to have a think...
"T. Essel"
.
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