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Guest

Posted: Sat Dec 02, 2006 10:23 am

What makes the Hulse-Taylor binary pulsar special--why shouldn't ANY
matter emit gravitational waves? Is there any difference?

"[examples of] systems which emit gravitational waves are binary star
systems, where the two stars in the binary are white dwarfs, neutron
stars, or black holes." --Wikipedia

-----------------------------------------------------------------
Related: will gravitons ever be detected, and why/why not? I know
they're very weak, I know they're very small... but what else is there
to know about them?

Gerry Quinn

Posted: Mon Dec 04, 2006 6:01 pm

Guest

In article <1164951297.350025.217830@n67g2000cwd.googlegroups.com>,
dharrington@southern.edu says...

Quote:

What's special is that its two component stars are massive enough and
close enough together that it emits enough gravitational waves to lose
a detectable portion of its energy over a timescale of years. This
results in the stars coming closer together, which we can detect by a
reduction in the orbital period.

The decrease matches the loss in energy predicted by general
relativity.

More info with figures:
< http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm >

Other systems (such as the Earth-Moon system) will also lose energy by
gravitational radiation, but the loss is typically so small that it
would be undetectable even if it were not swamped by other effects.
The Earth and Moon, for example, are actually getting further apart as
the rotational energy of the Earth gets converted into rotational
energy of the entire system.

Hulse-Taylor must consist of two objects so compact and dense that
their orbits are not affected to a great degree by such non-
gravitational interactions, even though they are very close together.

There have been other systems similar to Hulse-Taylor discovered, but I
don't know if the orbital period decrease has been tracked for long
enogh to give consistent data.

- Gerry Quinn

Gerry Quinn

Posted: Mon Dec 04, 2006 6:01 pm

Guest

In article <1164951297.350025.217830@n67g2000cwd.googlegroups.com>,
dharrington@southern.edu says...

Quote:

Related: will gravitons ever be detected, and why/why not? I know
they're very weak, I know they're very small... but what else is there
to know about them?

Gravitons from the Hulse-Taylor pulsar have a frequency of several
hours. Since E=hv, that means their energy will be very low; so low
that there seems no practical system that could detect them. We only
detect individual photons at frequencies similar to that of light, i.e.
about 10^18 times higher than that of gravitons radiated by Hulse-
Taylor. We can't practically detect individual photons even from radio
waves etc.

So, can we make a source of high energy gravitons? Not easily, because
you need to make objects with no electric charge, even internally,
oscillate very fast. [If they have electric charge, they will
overwhelmingly emit light rather than gravitational radiation, because
the electromagnetic interaction is so much stronger.]

Even putting smallish black holes in close orbit isn't quite good
enough.

Add to this the fact that the weakness of gravitational interactions
means that neither your source nor your detector will be very
efficient, and detecting individual gravitons seems a hopeless task.

- Gerry Quinn

Jonathan Thornburg -- rem

Posted: Tue Dec 05, 2006 11:39 am

Guest

In article <1164951297.350025.217830@n67g2000cwd.googlegroups.com> you wrote:

Quote:

What makes the Hulse-Taylor binary pulsar special--why shouldn't ANY
matter emit gravitational waves?

Yes, any matter (or energy) which *accelerates* in a
*non-spherically-symmetric* manner emits gravitational waves (GWs).
(Spherically-symmetric motion doesn't emit GWs.)

Quote:

Is there any difference?

Only that for the Hulse-Taylor binary pulsar (PSR B1913+16) our accuracy
of measuring the orbital motion is sufficiently good, and the GW emission
is strong enough, that (given careful data analysis) we can clearly see
the GW effects on the orbital motion. It's that combination which is
(so far) unique to this system.

[I emphasize that we have not *directly* detected the GWs emitted by
the 1913+16 system. Rather, we have detected the change in the pulsar
orbit induced by those GWs.]

There are large efforts under way to undo that "uniqueness", by
(a) observing other binary-pulsar systems, and
(b) building and operating new detectors which can *directly* detect
GW signals from other systems.

Note that for (b), current GW detectors (the LIGO, GEO, Virgo, and TAMA
ground-based laser interferometers) won't be able to directly detect the
1913+16 GWs, because these GW's frequency is way too low: The 1913+16
system has an orbital period of about 8 hours, so its GWs have a
fundamental frequency of 1 cycle per 4 hours (plus lots of higher
harmonics), i.e. a frequency around 1e-4 Hz. In contrast, ground-based
interferometers have peak sensitivity between 100 and 500 Hz.
Likely GW signals in that frequency band include supernova explosions
and compact-object (black hole and/or neutron star) binary coalescences.

The space-based LISA mission planned for around 2015-2020 (if NASA
ever decides to fund science again instead of astronauts-on-the-moon)
will have peak sensitivity between 1e-3 and 1e-2 Hz. Likely GW signals
in *that* frequency band include somewhat closer binary pulsars, binary
white dwarf binaries, and supermassive BH binary coalescences.

ciao,

--
-- "Jonathan Thornburg -- remove -animal to reply" <jthorn@aei.mpg-zebra.de>
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam

Guest

Posted: Tue Dec 05, 2006 11:39 am

dharrington@southern.edu wrote:

Quote:

What makes the Hulse-Taylor binary pulsar special--why shouldn't ANY
matter emit gravitational waves? Is there any difference?

Well, why should ANY matter emit gravitational waves? Look at it from
the point of view of Newton. Have two masses rotating about a center
of mass. A mass in the distance in the plane of the rotation will feel
a non-constant gravitational force. The further from the masses, the
less this variation will be, so that at cosmic distances this effect is
negligible.

The solutions of General Relativity predict that this variation will be
greater than Newton would predict in certain cases, so much greater
that detecting it at cosmic distances seems possible.

Perhaps you have these gravitational waves confused with
electromagnetic waves. As far as anyone knows there is no connection
between the two concepts. Though many have tried, and may succeed one
day.

Quote:

"[examples of] systems which emit gravitational waves are binary star
systems, where the two stars in the binary are white dwarfs, neutron
stars, or black holes." --Wikipedia

-----------------------------------------------------------------
Related: will gravitons ever be detected, and why/why not? I know
they're very weak, I know they're very small... but what else is there
to know about them?

That's the Nobel-prize-winning question. Maybe they "exist," maybe
they don't. No one has been able to get the math to work out. Yet.

Jim O'Reilly

Posted: Tue Dec 05, 2006 11:39 am

Guest

In article <1164951297.350025.217830@n67g2000cwd.googlegroups.com>,
dharrington@southern.edu writes

Quote:

--
Jim O'Reilly

Guest

Posted: Wed Dec 06, 2006 11:18 am

In article <1164951297.350025.217830@n67g2000cwd.googlegroups.com>,
<dharrington@southern.edu> wrote:

Quote:

Related: will gravitons ever be detected, and why/why not? I know
they're very weak, I know they're very small... but what else is there
to know about them?

Here's a nice, provocative article that claims that it's impossible even in
principle to detect a single graviton:

"Can Gravitons Be Detected?", Tony Rothman, Stephen Boughn,
Foundations of Physics, vol. 36, No. 12, 1801-1825 (2006),
http://arxiv.org/abs/gr-qc/0601043

Here's the abstract:

Freeman Dyson has questioned whether any conceivable experiment in
the real universe can detect a single graviton. If not, is it
meaningful to talk about gravitons as physical entities? We attempt
to answer Dyson's question and find it is possible concoct an
idealized thought experiment capable of detecting one graviton;
however, when anything remotely resembling realistic physics is
taken into account, detection becomes impossible, indicating that
Dyson's conjecture is very likely true. We also point out several
mistakes in the literature dealing with graviton detection and
production.

I'd be quite intereted to know what people think of the argument
in this article.

-Ted

--
[E-mail me at name@domain.edu, as opposed to name@machine.domain.edu.]

Gerry Quinn

Posted: Fri Dec 08, 2006 10:42 am

Guest

In article <1165210523.600568.31970@j72g2000cwa.googlegroups.com>,
frisbieinstein@yahoo.com says...

Quote:

dharrington@southern.edu wrote:
What makes the Hulse-Taylor binary pulsar special--why shouldn't ANY
matter emit gravitational waves? Is there any difference?

Well, why should ANY matter emit gravitational waves? Look at it from
the point of view of Newton. Have two masses rotating about a center
of mass. A mass in the distance in the plane of the rotation will feel
a non-constant gravitational force. The further from the masses, the
less this variation will be, so that at cosmic distances this effect is
negligible.

The solutions of General Relativity predict that this variation will be
greater than Newton would predict in certain cases, so much greater
that detecting it at cosmic distances seems possible.

The main difference, as far as gravitational radiation is concerned, is
that Newton's theory has gravitational changes propagating at infinite
speed, whereas in any relativistic theory they will be limited to
lightspeed.

Quote:

Perhaps you have these gravitational waves confused with
electromagnetic waves. As far as anyone knows there is no connection
between the two concepts. Though many have tried, and may succeed one
day.

The connection is this: changes in the electromagnetic field also
propagate at lightspeed.

The theory of electrostatics is precisely the e/m equivalent of
Newtonian gravity. It works very well for electrically charged objects
moving at low speeds, just as Newtonian gravity works very well for
massive objects moving at low speeds.

Of course with e/m the magnetic and radiative effects that are observed
when charged objects move or oscillate very rapidly are at least as
obvious as the electrostatic effects, though it was a long time before
scientists made the connections between them. With gravity you have to
look hard for the equivalent effects.

- Gerry Quinn

Igor Khavkine

Posted: Sun Dec 10, 2006 10:54 am

Guest

ebunn@lfa221051.richmond.edu wrote:

Quote:

In article <1164951297.350025.217830@n67g2000cwd.googlegroups.com>,
dharrington@southern.edu> wrote:

Related: will gravitons ever be detected, and why/why not? I know
they're very weak, I know they're very small... but what else is there
to know about them?

Here's a nice, provocative article that claims that it's impossible even in
principle to detect a single graviton:

"Can Gravitons Be Detected?", Tony Rothman, Stephen Boughn,
Foundations of Physics, vol. 36, No. 12, 1801-1825 (2006),
http://arxiv.org/abs/gr-qc/0601043

Here's the abstract:

Freeman Dyson has questioned whether any conceivable experiment in
the real universe can detect a single graviton. If not, is it
meaningful to talk about gravitons as physical entities? We attempt
to answer Dyson's question and find it is possible concoct an
idealized thought experiment capable of detecting one graviton;
however, when anything remotely resembling realistic physics is
taken into account, detection becomes impossible, indicating that
Dyson's conjecture is very likely true. We also point out several
mistakes in the literature dealing with graviton detection and
production.

I'd be quite intereted to know what people think of the argument
in this article.

I think the crux of this argument rests on what they mean by "graviton
detection". After quickly reading at the article, it seems that they
are looking for a "smoking gun" type of experiment: something that
demonstrates the quantization of energy stored in gravitational
excitations. For the photon, they consider the photoelectric effect,
for instance, to be such an experiment. They then propose different
experiments that fit this criterion. In my understanding, they are
looking for ionization of hydrogen through graviton absorption. Their
estimates then show that such an experiment is impossible, since a
reasonable size detector won't detect a single such event in more than
the lifetime of the universe.

As far as it goes, their arguments seem valid. However, I think the bar
they set is quite high. We know that gravity is extremely weak comapred
to EM forces, so asking for a strikingly visible gravitational effect
in a system where the energy scales are set by EM interactions is
somewhat unreasonable.

I can think of one context that they haven't considered: cosmic
graviton background radiation. Presumably, early enough in the history
of the universe, gravitational radiation was in thermal equilibrium
with other matter and settled into a Planck spectrum. If we can measure
this frequency spectrum (which doesn't require "quantum jumps" like
those the authors of the above paper were seeking) and compare it to
the Planck curve, this would also be evidence of gravitons.

Last, even if we can never detect gravitons, in the sense used by the
cited paper, we still have a reason to quantize gravity itself.
Gravitons are not expected to be main quantum effect in very strong
fields, which is where the quantization really matters.

Igor

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