A laymen's guide Gravity, Gravity Waves, LIGO, and how it all works

S@NL - Marleen
S@NL - Marleen
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> For example, this page

Message 5238 in response to message 5234

> For example, this page about pulsars on
> Wikipedia says that the discovery of PSR B1913+16, confirmed general
> relativity and proved the existence of gravitational waves. I'm
> not sure that's true.
I think this refers to the slow decrease in the rate the two pulsars are circling each other. This effect is in accordance with general relativity. And according to this theory, the only way the pulsars can do that is by sending out gravitational waves.
I think the gravitational waves are "proved" within the context of general relativity. But those waves have not been detected independently by eg LIGO or other searches. As long as we haven't done that, we haven't really proved the existence of gravitational waves.

S@NL - Marleen
S@NL - Marleen
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> quantum mechanics predicts

Message 5239 in response to message 5236

> quantum mechanics predicts that over vast periods of time all massive objects
> eventually loose some of their mass in the form of gravitational radiation...
Hmm, last time I checked quantum mechanics and gravitation/relativity did not go well together... it's one of the big questions of physics how to get them into one theory.

> LIGO and GEO are not looking for "gravitons" but rather are
> set to detect these periodic pulses as minute distortions in spacetime.
LOL, if they were looking for "gravitons" they would detect a very nice strong gravitational field, that of earth itself...

> So what we are looking for, as our computers do Fourier transforms to correct
> for the Doppler effect, is a tiny hiccup in the more or less uniform expansion
> rate of the universe. Is that somewhere close to the truth?
I think the Doppler effect is corrected first and that the way in which you do it determines which point in the sky you are "looking" at. THEN you do the Fourier transform, which is a way of finding periodic signals in (seemingly) random noise.
LIGO is looking for distortions in spacetime. The hope is that a distortion changes the length of one of the arms of LIGO in a different way from the other arm, and this is detected by the laser (simple explanation of an interferometer).
I think LIGO can detect a "hiccup", but it would be very hard to prove that such a single pulse is due to a gravitational wave and not to something else, like an earthquake. That is probably why they are looking for periodic signals, they are a lot easier to pick out of the noise and it is also easier to prove they are not generated by something on earth or in the solar system.

As for my background, I have a master's degree in physics, but after that I got work in ICT so it's been a while...

ghstwolf
ghstwolf
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I've got a newbie-esque

I've got a newbie-esque questions.

It's kinda hypothetical, with the 2 dimentional detectors we are using (or at least thats how I would charactorize it), how many waves would we miss in 3 dimentional space? Or does something in the system acount for this?

Maybe I'm way off base on this. I don't know enough about the physics to even know if it is a valid concern. However it is a question that is burning in my mind. Thanks in advance


StarCharter
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> I've got a newbie-esque

Message 5241 in response to message 5240

> I've got a newbie-esque questions.
>
> It's kinda hypothetical, with the 2 dimentional detectors we are using (or at
> least thats how I would charactorize it), how many waves would we miss in 3
> dimentional space? Or does something in the system acount for this

I'm not sure I'm following this. A gravitational wave would disturb the detector in 3 dimensions. No matter how the wave affected the laser beam, either right/left, up/down, or front/back, an event would be triggered and this survey would detect it if it were peroidic.
Did I answer your question?

There are two secrets to life: 1) Don't tell everything you know...

StarCharter
StarCharter
Joined: 19 Feb 05
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> > For example, this >

Message 5242 in response to message 5238

> > For example, this
> page about pulsars on
> > Wikipedia says that the discovery of PSR B1913+16, confirmed general
> > relativity and proved the existence of gravitational waves.
> I'm
> > not sure that's true.
> I think this refers to the slow decrease in the rate the two pulsars are
> circling each other. This effect is in accordance with general relativity. And
> according to this theory, the only way the pulsars can do that is by sending
> out gravitational waves.
> I think the gravitational waves are "proved" within the context of general
> relativity. But those waves have not been detected independently by eg
> LIGO or other searches. As long as we haven't done that, we haven't really
> proved the existence of gravitational waves.

I was pretty sure what they were talking about. I think my problem with the statement was a semantic issue. While I agree that the best explanation of the slowing of the period may be the shedding of energy by gravitational waves, it isn't the only one. It may not even be the simplest.
I decided to go back and read the information from The Royal Swedish Academy of Sciences (the folks who gve Hulse and Taylor the Nobel Prize in 1993 for studying PSR B1913+16). It is located HERE for those who want to read it. It says:

"The good agreement between the observed value and the theoretically calculated value of the orbital path can be seen as an indirect proof of the existence of gravitational waves. We will probably have to wait until next century for a direct demonstration of their existence. Many long-term projects have been started for making direct observations of gravitational waves impinging upon the earth. The radiation emitted by the binary pulsar is too weak to be observed on the earth with existing techniques. However, perhaps the violent perturbations of matter that take place when the two astronomical bodies in a binary star (or a binary pulsar) approach each other so closely that they fall into each other may give rise to gravitational waves that could be observed here. It is also hoped to be able to observe many other violent events in the universe. Gravitational wave astronomy is the latest, as yet unproven, branch of observational astronomy, where neutrino astronomy is the most direct predecessor. Gravitational wave astronomy would then be the first observational technique for which the basic principle was first tested in an astrophysical context. All earlier observational techniques in astronomy have been based on physical phenomena which first became known in a terrestrial connection."

So there you have it. If the Royal Swedish Academy of Sciences says it's a proof, it's a proof and I'll sit down and shut up.

There are two secrets to life: 1) Don't tell everything you know...

ghstwolf
ghstwolf
Joined: 9 Feb 05
Posts: 24
Credit: 59,103
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> > I've got a newbie-esque

Message 5243 in response to message 5241

> > I've got a newbie-esque questions.
> >
> > It's kinda hypothetical, with the 2 dimentional detectors we are using
> (or at
> > least thats how I would charactorize it), how many waves would we miss in
> 3
> > dimentional space? Or does something in the system acount for this
>
> I'm not sure I'm following this. A gravitational wave would disturb the
> detector in 3 dimensions. No matter how the wave affected the laser beam,
> either right/left, up/down, or front/back, an event would be triggered and
> this survey would detect it if it were peroidic.
> Did I answer your question?
>

Ok, I guess I see it now. If the beam bent up/down (inside the tube) it would change it's period. I was thinking it had a simple answer, but I was having a visualization block last night when I asked. Thanks for pointing me in the right direction.


Mo
Mo
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Another newbie-esque

Another newbie-esque question.

If gravity waves were detected, how could one know that it was the result of a pulsar?


Look here, brother, who you jiving with that cosmic debris?--Frank Zappa.

StarCharter
StarCharter
Joined: 19 Feb 05
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Credit: 641,079
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> Another newbie-esque

Message 5245 in response to message 5244

> Another newbie-esque question.
>
> If gravity waves were detected, how could one know that it was the result of a
> pulsar?

Our computers are doing Fourier transforms. According to Ben Owen, "A Fourier transform is a way of looking at a time series as a sum of sine waves at different frequencies. Pulsar signals should be nearly sinusoidal after the Doppler shifts are taken out, so Fourier transforms pick them up pretty easily. Fourier transforms are numerically pretty efficient, but there are an awful lot of them to do in an all-sky search. That is why Einstein@Home is doing this particular search, and not for example the searches for known pulsars which can be done pretty quickly on a single computer." A fourier transform is an integral transform that re-expresses a function in terms of sinusoidal basis functions, i.e. as a sum or integral of sinusoidal functions multiplied by some coefficients ("amplitudes"). It's really good about re-expresssing a peroidic episode.

The key here is peroidic. Lots of things probably generate gravitational waves. There only a very few that could generate peroidic gravitational waves. The most likely that we will be able to do a followup detection of is a pulsar in a binary relationship with another massive body, like another neutron star.

Of course, it could something else that we haven't seen or can imagine. But as Damon Runyon said, "The race doesn't always go to the swift, nor the fight to the strong...but that's the way to bet".

There are two secrets to life: 1) Don't tell everything you know...

debugas
debugas
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> The key here is

Message 5246 in response to message 5245

> The key here is periodic.

In other words we are looking for a signature of special periodic form which is very characteristic to pulsars and not any other objects.

It's like picking up one particular person's voice in a croud.

Or imagine that lot of different people write a single book by putting short phrases into it but each person in its own language.
Then when english man takes the book he will pick up english phrases only, if russian man takes the book he will pick up russian phrases.
If you look for chinese phrases you will know in advance that they were written by someone who speaks chinese :)

cIclops
cIclops
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There is an excellent, easy

There is an excellent, easy to follow webcast lecture archived here. It's given by Dr. Barry Barish, director of LIGO at Caltech, well worth the 60 minutes.

For a more advanced background try Kip Thorne's full course on Gravitational Waves in the form of a series of downloadable webcast lectures and slides - archived here.

--
searching for gravitational waves since 2005

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