What's up with gravity wave detection?

(Dave Houseman) wrote in message . com...
Sure, the planets would have a big gravity wave signal, in the same
way that jupiter appears brighter than P Centauri in the night sky.


How could they? Gravitational waves result from a changing quadrupole
moment. Planets simply orbiting the sun won't do it!


But surely LIGO can tell what direction a wave is coming from - that's
the whole point, isn't it?

Definitely, and that's one reason why they have detectors in more than
one location.

Dear Igor:

"Igor" wrote in message
m...
(Dave Houseman) wrote in message
. com...
Sure, the planets would have a big gravity wave signal, in the same
way that jupiter appears brighter than P Centauri in the night sky.


How could they? Gravitational waves result from a changing quadrupole
moment. Planets simply orbiting the sun won't do it!

Not planets orbiting *our* Sun, anyway. It would need to be something
happening near an angle of about 45 deg to the plane of the detector.

David A. Smith

"Joseph Lazio" wrote in message
...
| "A" == Androcles writes:
|
| A "Dave Houseman" wrote in message
| A om...
| Sure, the planets would have a big gravity wave signal, in the same
| way that jupiter appears brighter than P Centauri in the night sky.
|
| A No comparison whatosever. Jupiter is reflecting sunlight. You
| A cannot observe any planets in orbit about Proxima Centauri. You are
| A comparing soldier ants to eleph ants.
|
| Actually, the planets do not produce a gravitational wave signature
| (or if they do, it is incredibly small).
|
| But surely LIGO can tell what direction a wave is coming from -
| that's the whole point, isn't it? Dave
|
| A Can it? I don't think so. As dkomo said, all he's seen is PR hype,
| A not results.
|
| In principle it can, crudely. There are two of them. So by timing
| when the signal gets to one versus when it gets to the other, one gets
| a rough sense of the direction from which the signal came.


That assumes there is a wave to detect. None so far.
As I pointed out originally, the greatest pulse detectable would be negative
going and the total annihilation of a star. If Proxima Centauri vanished
overnight, what effect would be detectable, gravitationally, upon our solar
system ?
I submit the answer to be : none at all that was detectable. The entire
solar system as a group might change its path with respect to the galactic
centre, but we would not be aware of it. I fail to see how we could expect a
pulsar 250 times further away could have greater signal
Androcles

Igor wrote:
(Dave Houseman) wrote in message . com...
Sure, the planets would have a big gravity wave signal, in the same
way that jupiter appears brighter than P Centauri in the night sky.

How could they? Gravitational waves result from a changing quadrupole
moment. Planets simply orbiting the sun won't do it!

Sure they do, as long as you consider the sun+planet system.

But this is really a near field situation, and can be computed using the
Schwarzschild metric (i.e. ignoring the [constant] earth and considering
only sun or moon, not both at once). If you compute the effect of the
moon's or the sun's gravitation on the LIGO apparatus, you find that the
signal is larger than their sensitivity, _IF_ you ignore the frequency.
That is, the 1/(24*3600) Hz signal is larger than their sensitivity at
100-3000 Hz. But because of their dampers (to remove micro-seismic
noise), they have no sensitivity at 1/(24*3600) Hz at all. That is, of
course, right where the micro-seismic noise is the worst (induced by
both solar and lunar gravitation, and diurnal solar heating).


Tom Roberts

Eric Flesch wrote:
On Mon, 16 Aug 2004 21:00:24 +0000 (UTC),
wrote:
In sci.astro Eric Flesch wrote:
On Sun, 15 Aug 2004 23:58:58 -0800, Eric Gisse wrote:
No, EM radiation gravitates too.
No, that's just an error in the practice of current models -- which
themselves don't require such gravitation

Certainly they do. In general relativity, the right-hand side of the
field equations must include the stress-energy tensor for all matter.
Otherwise, the vanishing covariant divergence of the left-hand side is
inconsistent.

You're missing the point.

No. You're changing the context. The original discussion was about GR
and gravity wave detectors, and Steve Carlip answered in that context.


The stress-energy tensor is a classical
description which assumes continuous motion.

Yes.


But QED shows that the
photon path is the summation of all possible paths (diffraction
gratings are an application of this) and the delayed-choice experiment
shows explicitly that the travelling photon cannot be pinpointed to
any particular location in its presumed path(s).

The context did not include QED until you mentioned it right here.


The point is that
the "travelling photon" can be modelled only by a quantum description,
and the classical stress-energy tensor does not apply.

That is a different point.

BUT -- we know that for reasonably-large systems (e.g. 1 mm or so),
that classical wave optics is an EXCELLENT approximation to QED, and
that applies here. The last time I looked, LIGO was much larger than 1mm
(:-)).


More generally, electromagnetic radiation observably responds to
gravitational fields. If it does not itself gravitate, you get an
immediate violation of basic Newtonian physics.

No, the equivalence principle shows that relative acceleration is
achieved by just one of the two bodies accelerating -- they do not
both need to. Travel on the null geodesic is the default -- a
non-gravitating body would not choose a different path.

That's not at all what he said or implied. In fact, in GR we consider
"non-gravitating bodies" all the time; we call them "test particles".


Let's keep in firm view that observation is far more real than theory,
especially theory which is mis-applied.

Good idea. Keep it in mind.


Tom Roberts

"A" == Androcles writes:

A As I pointed out originally, the greatest pulse detectable would be
A negative going and the total annihilation of a star.

It wouldn't be "negative" (though I'm not entirely sure I know what
you mean by that) and there would be a signal only if the explosion
were asymmetric. Supernovae are expected to produce a gravitational
wave signature. Pulsar velocities are good evidence that supernovae
are asymmetric (even if we don't understand the details).

A If Proxima Centauri vanished overnight, what effect would be
A detectable, gravitationally, upon our solar system ? I submit the
A answer to be : none at all that was detectable. The entire solar
A system as a group might change its path with respect to the
A galactic centre, but we would not be aware of it. I fail to see how
A we could expect a pulsar 250 times further away could have greater
A signal

Of course, Proxima Centauri hasn't vanished nor is it likely to do.
Moreover, there's not been a naked-eye supernova in the Milky Way in
something like 400 years and probably not a single one in the past 50
years. Thus, this is a prediction that's a bit hard to test.

I don't think the current emission levels from PSR B1913+16 or PSR
J0737-3039 would be detectable. However, in a few million years, as
they continue to in-spiral, the gravitational wave emission level is
expected to increase. Obviously, we won't be around to see that, but
there may be other systems in the nearby Universe that would be in the
last stages of in-spiral.

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

On Wed, 18 Aug 2004 00:43:43 GMT, "Androcles"
wrote:


"Joseph Lazio" wrote in message
...
| "A" == Androcles writes:
|
| A "Dave Houseman" wrote in message
| A om...
| Sure, the planets would have a big gravity wave signal, in the same
| way that jupiter appears brighter than P Centauri in the night sky.
|
| A No comparison whatosever. Jupiter is reflecting sunlight. You
| A cannot observe any planets in orbit about Proxima Centauri. You are
| A comparing soldier ants to eleph ants.
|
| Actually, the planets do not produce a gravitational wave signature
| (or if they do, it is incredibly small).
|
| But surely LIGO can tell what direction a wave is coming from -
| that's the whole point, isn't it? Dave
|
| A Can it? I don't think so. As dkomo said, all he's seen is PR hype,
| A not results.
|
| In principle it can, crudely. There are two of them. So by timing
| when the signal gets to one versus when it gets to the other, one gets
| a rough sense of the direction from which the signal came.


That assumes there is a wave to detect. None so far.

Indirect evidence exists for gravitational radiation. Hulse and Taylor
got a nobel for this.

http://www.nobel.se/physics/laureate...on-speech.html

Would be pretty interesting if gravitational waves did not exist even
though energy is being lost by a means which happens to fit
gravitational radiation.


As I pointed out originally, the greatest pulse detectable would be negative
going and the total annihilation of a star. If Proxima Centauri vanished
overnight, what effect would be detectable, gravitationally, upon our solar
system ?

Reform your question in a way that makes sense. Proxima Centauri
cannot just vanish. Perhaps convert it to photons, or neutrinos mabey,
but it cannot just vanish.

I submit the answer to be : none at all that was detectable. The entire
solar system as a group might change its path with respect to the galactic
centre, but we would not be aware of it. I fail to see how we could expect a
pulsar 250 times further away could have greater signal

Let see your calculations! For once back up your statements!

How much energy would be released in the mode you choose to destroy
Proxima Centauri with? How much of it would be transformed to
gravitational radiation? I hope im not asking questions that are too
difficult for you to justify in a way that doesn't involve handwaving.



Androcles

"Joseph Lazio" wrote in message
...
| "A" == Androcles writes:
|
| A As I pointed out originally, the greatest pulse detectable would be
| A negative going and the total annihilation of a star.
|
| It wouldn't be "negative" (though I'm not entirely sure I know what
| you mean by that) and there would be a signal only if the explosion
| were asymmetric. Supernovae are expected to produce a gravitational
| wave signature. Pulsar velocities are good evidence that supernovae
| are asymmetric (even if we don't understand the details).
|
| A If Proxima Centauri vanished overnight, what effect would be
| A detectable, gravitationally, upon our solar system ? I submit the
| A answer to be : none at all that was detectable. The entire solar
| A system as a group might change its path with respect to the
| A galactic centre, but we would not be aware of it. I fail to see how
| A we could expect a pulsar 250 times further away could have greater
| A signal
|
| Of course, Proxima Centauri hasn't vanished nor is it likely to do.
| Moreover, there's not been a naked-eye supernova in the Milky Way in
| something like 400 years and probably not a single one in the past 50
| years. Thus, this is a prediction that's a bit hard to test.
|
| I don't think the current emission levels from PSR B1913+16 or PSR
| J0737-3039 would be detectable. However, in a few million years, as
| they continue to in-spiral, the gravitational wave emission level is
| expected to increase. Obviously, we won't be around to see that, but
| there may be other systems in the nearby Universe that would be in the
| last stages of in-spiral.

Thus, this is a prediction that's a bit hard to test.:-)
Androcles





| --
| Lt. Lazio, HTML police | e-mail:
| No means no, stop rape. | http://patriot.net/%7Ejlazio/
| sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

"eric gisse" wrote in message
...
| On Wed, 18 Aug 2004 00:43:43 GMT, "Androcles"
| wrote:
|
|
| "Joseph Lazio" wrote in message
| ...
| | "A" == Androcles writes:
| |
| | A "Dave Houseman" wrote in message
| | A om...
| | Sure, the planets would have a big gravity wave signal, in the same
| | way that jupiter appears brighter than P Centauri in the night sky.
| |
| | A No comparison whatosever. Jupiter is reflecting sunlight. You
| | A cannot observe any planets in orbit about Proxima Centauri. You are
| | A comparing soldier ants to eleph ants.
| |
| | Actually, the planets do not produce a gravitational wave signature
| | (or if they do, it is incredibly small).
| |
| | But surely LIGO can tell what direction a wave is coming from -
| | that's the whole point, isn't it? Dave
| |
| | A Can it? I don't think so. As dkomo said, all he's seen is PR hype,
| | A not results.
| |
| | In principle it can, crudely. There are two of them. So by timing
| | when the signal gets to one versus when it gets to the other, one gets
| | a rough sense of the direction from which the signal came.
|
|
| That assumes there is a wave to detect. None so far.
|
| Indirect evidence exists for gravitational radiation. Hulse and Taylor
| got a nobel for this.


Not fair... Nobody gives me a nobel for the incubation of bright green
flying elephants eggs.




| http://www.nobel.se/physics/laureate...on-speech.html
|
| Would be pretty interesting if gravitational waves did not exist even
| though energy is being lost by a means which happens to fit
| gravitational radiation.
|
|
| As I pointed out originally, the greatest pulse detectable would be
negative
| going and the total annihilation of a star. If Proxima Centauri vanished
| overnight, what effect would be detectable, gravitationally, upon our
solar
| system ?
|
| Reform your question in a way that makes sense. Proxima Centauri
| cannot just vanish. Perhaps convert it to photons, or neutrinos mabey,
| but it cannot just vanish.

Had you been following, you would have realized I suggested a complete
conversion of all mass to radiation. I'm not about to repeat myself every
time for your nit-picking.
|
| I submit the answer to be : none at all that was detectable. The entire
| solar system as a group might change its path with respect to the
galactic
| centre, but we would not be aware of it. I fail to see how we could
expect a
| pulsar 250 times further away could have greater signal
|
| Let see your calculations! For once back up your statements!

Go away, idiot. If you can't assess quantities without working numbers you
don't belong in any form of science.

|
| How much energy would be released in the mode you choose to destroy
| Proxima Centauri with?

Irrelevant. I was conducting a thought experiment on the maximum imaginable
negative going gravity pulse being detectable and you are responding to a
follow up.
Androcles.

Joseph Lazio wrote:

"d" == dkomo writes:


d I found an old PBS documentary on VHS from 1991 called _The
d Astronomers_ at the local public library. One of the programs in
d the series was "Waves of the Future" about gravitational waves. In
d the program Kip Thorne was shown making a bet with one of his
d collaborators on gravity wave theory that these waves would
d positively be detected by 2000.

d I found this both humorous and a touch sad. The program described
d some of the early planning for LIGO (Laser Interferometer
d Gravitational Wave Observatory). Curious, I went to the LIGO web
d site to see what was going on. I found nothing of substance there
d -- just a lot of slick PR.

You don't define "slick PR" but I see links for "Observational
Results," "Publications," "Research Bulletin Boards," "LIGO Email
Archives," .... At least some of these strike me as more than PR.


I did take some quick looks at these links, but spending hours poring
over technical papers and jargon encrusted research bulletin boards just
to get an answer to a simple question isn't what I had in mind. And
that simple question is "what's the status of LIGO?"

Of course, you've hit upon a problem not unique to LIGO. If the Web
site isn't slick and flashy, then it's derided as boring and useless.
If it is slick and flashy, then it's derided as just PR.


Actually, whoever created the web site did not do a good job in
organizing it. There's a big gap between the highly specialized,
somewhat disorganized technical material posted there and the high
school level explanations of gravitational waves and their detection.

What I would have liked to see is a one or two sentence status summary
in a LIGO FAQ like this:

"Currently we're still trying to get the bugs out of the equipment."

or

"LIGO is fully operational now. We expect to detect a gravitational
wave any day now."

or

"We are running smoothly and looking for gravitational waves 24 hours a
day, 7 days a week. Only minor glitch is that when someone flushes the
toilet down the hall, it produces a spurious gravity wave reading."

d So my question is, what are the prospects that gravity waves will
d be detected anytime soon? Is LIGO still having technical problems
d or what? It is now 2004, after all. Other detection labs are being
d built around the world. Are these labs going to have any better
d luck?

There are a host of other gravitational wave laboratories around the
world. The LIGO site has links to VIRGO, GEO600, TAMA300, ACIGA,
LISA, and IGEC.

As for prospects of detection, that's probably a good question to post
on sci.astro.research.

I may do that.

In part that depends upon how kind Nature is
to us. I think if a supernova went off in our part of the Galaxy
tomorrow, LIGO would (or should!) detect it.

Surely when they built LIGO they weren't depending on a supernova going
off in order for LIGO to detect its first gravitational wave. They must
be expecting to detect waves from existing gravitational sources.

Right now, of course, it
has not detected anything, so the best that one can do is say that the
density of fill in your favorite gravitational wave source is no
larger than some upper limit.

Some will, of course, decry this as adjusting the theory. However,
getting a handle on potential gravitational wave sources is tough.
For instance, just last year the binary pulsar PSR J0737-3019 was
discovered. It's (probably) less than 2000 light years away from us
and yet had gone undetected for the first 35 years of pulsar
searching. Binary neutron star systems are thought to be a prime
candidate for gravitational wave sources as they spiral together.
Finding a binary neutron star system so close to us immediately
boosted the expected signal strength for gravitational waves from
binary neutron star systems.


Then is binary pulsar PSR J0737-3019 one of the sources LIGO should be
detecting right now?


d Also, what are people's opinions about gravity waves? Is it
d possible that these are a scientific dead end like the decay of the
d proton turned out to be? If gravity waves are never detected, what
d are the implications for the general theory of relativity?

What about PSR B1913+16? This is a binary neutron star system in
which one of the neutron stars is seen as a pulsar. The orbit is
decaying at a rate exactly consistent with that predicted by general
relativity, if gravitational waves are carrying away energy. In a few
years (if not sooner), the orbital decay of PSR J0737-3019 should also
be measurable. (PSR J0737-3019 is a much tighter system, a 2.4-hr
orbit, as compared to PSR B1913+16, an 8-hr orbit, so the rate of
orbital decay should be higher.)

The decay of the orbit of PSR B1913+16 is only an indirect
measurement, of course, but it is a powerful indicator that
gravitational waves do exist.