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

barkster
barkster
Joined: 3 Apr 05
Posts: 71
Credit: 447,475
RAC: 0

Sounds like you have it

Message 5258 in response to message 5257

Sounds like you have it right. A signal from a specfic direction in space will have a fixed difference in the arrival time a each of the three observatories. In the type of analsys being done here such time differences are commonly phase shifts. By adjusting for these differences (phase shifting) it is possible to 'focus' on the specfic direction in question.

Yes... but it was explained to me that the "phase shifting" that E@H is doing is just multiple iterations of changing the variables that would define the predicted phase shift measured by LIGO from each measured point in the sky (each place you see the cross hair on the screensaver represent another set of variables for "doppler shift") as it calculates and looks for a result in the data that will match a predicted result based on that Doppler. It was also explained that each LIGO itself is omni-directional, and does not used "phased array" engineering to determine direction of the gravity wave source. It measures the stretching or shrinking of each arm (the presence of a gravity wave, or what we think a gravity wave is) by way of constructive and destructive interference of the laser beams from each arm.

See the How can they "aim" a LIGO? thread.

One site alone can't determine direction of origin, however, the three existing sites combined in synchronous operation could constitute an array and use time difference and/or phase difference, or perhaps frequency difference of arrival techniques to compute a azimuth and elevation to the source.... close enough to point a telescope in the genereal direction, at least.

There are also a couple of video lectures (1 to 2 hours long) posted at the LIGO site that explain the LIGO operating principles in detail and also mention the construction of at least 3 more gravity wave detectors to add more sites to the "array".

And if you really want to blow steam out your ears, now people are talking about the existence of INVERSE Doppler....

"No, I'm not a scientist... but I did stay at a Holiday Inn Express."

MarkF
MarkF
Joined: 12 Apr 05
Posts: 393
Credit: 1,516,715
RAC: 0

Sounds like you have it

Message 5259 in response to message 5258

Sounds like you have it right. A signal from a specfic direction in space will have a fixed difference in the arrival time a each of the three observatories. In the type of analsys being done here such time differences are commonly phase shifts. By adjusting for these differences (phase shifting) it is possible to 'focus' on the specfic direction in question.

Yes... but it was explained to me that the "phase shifting" that E@H is doing is just multiple iterations of changing the variables that would define the predicted phase shift measured by LIGO from each measured point in the sky (each place you see the cross hair on the screensaver represent another set of variables for "doppler shift") as it calculates and looks for a result in the data that will match a predicted result based on that Doppler. It was also explained that each LIGO itself is omni-directional, and does not used "phased array" engineering to determine direction of the gravity wave source. It measures the stretching or shrinking of each arm (the presence of a gravity wave, or what we think a gravity wave is) by way of constructive and destructive interference of the laser beams from each arm.

I agree that the phase shifting is done in the analsys stage not at the observing stage. In addition to the time differences there also velocity differences that must be compenstated for in the analsys stage by remove the doppler effect.

See the How can they "aim" a LIGO? thread.

One site alone can't determine direction of origin, however, the three existing sites combined in synchronous operation could constitute an array and use time difference and/or phase difference, or perhaps frequency difference of arrival techniques to compute a azimuth and elevation to the source.... close enough to point a telescope in the genereal direction, at least.

There are also a couple of video lectures (1 to 2 hours long) posted at the LIGO site that explain the LIGO operating principles in detail and also mention the construction of at least 3 more gravity wave detectors to add more sites to the "array".

And if you really want to blow steam out your ears, now people are talking about the existence of INVERSE Doppler....

michael
michael
Joined: 23 May 05
Posts: 16
Credit: 463
RAC: 0

hmm, i never tryed a forum

hmm, i never tryed a forum before, im more used to IM on online games, any explanitions on how it works?

and one: hows my avatar? and two: what are signatures?

gravywavy
gravywavy
Joined: 22 Jan 05
Posts: 392
Credit: 68,962
RAC: 0

hmm, i never tryed a forum

Message 5261 in response to message 5260

hmm, i never tryed a forum before, im more used to IM on online games, any explanitions on how it works?

and one: hows my avatar? and two: what are signatures?

Your avatar is a truly stellar performance Michael ;-) Welcome to the forums.

Forums work differently to live chat in that people dip back to the threads weeks or months later to read the stuff again, or sometimes even to re-start a thread. That is why it is helpful to try to keep discussions on the point. Here, the point is how LIGO, etc, all works: a better place to explain how to use forums would be in the getting started board.

I've taken the liberty of quoting your post, and replying to it over on that board, and you can click here to see it

~~gravywavy

Ben Owen
Ben Owen
Joined: 21 Dec 04
Posts: 117
Credit: 34,513,092
RAC: 20,834

Folks, Those of you

Folks,

Those of you wondering about the direction sensitivity of the instrument might want to check out some of my previous posts. Actually, the last time this question came up seems to be here, which was way back when we only had a single board with a lot of traffic.

Basically, for periodic sources the directionality is not from comparing arrival times at multiple detectors, or phase shifts which is more or less equivalent. It comes from watching the time variation of the Doppler shift of a signal as the Earth rotates, which means even a single detector is extremely sensitive to the sky location. If you look at the names of the current work units you'll see they're all coming from Hanford (H1) and none from Livingston. For the next search we'll use Livingston data, but not combining it with Hanford to get the direction of a candidate signal - that will already be well nailed down by tracking the Doppler shifts.

The trick of combining different detectors to get the direction is used for short-lived signals (like colliding black holes), but E@H is not looking for those at the moment because periodic sources are where the computational demands are. However, even without the directionality issue, multiple detectors are useful for building confidence in a detection - it's harder for two detectors to glitch simultaneously than one.

Although there was an interesting issue that came up recently and caused a lot of consternation for a while. Turns out both detectors were using certain electronics racks from the same manufacturer that were producing periodic signals at the same frequency.... But no Doppler shifts as the Earth moved, which gave away that it was not an astrophysical signal.

Hope this helps,
Ben

barkster
barkster
Joined: 3 Apr 05
Posts: 71
Credit: 447,475
RAC: 0

Basically, for periodic

Message 5263 in response to message 5262

Basically, for periodic sources the directionality is not from comparing arrival times at multiple detectors, or phase shifts which is more or less equivalent. It comes from watching the time variation of the Doppler shift of a signal as the Earth rotates, which means even a single detector is extremely sensitive to the sky location.Ben,

I'm sure in this case I may share some of the blame for miscommunicating the facts.... and I'm certainly not questioning your superior knowledge and experience on the subject... but wasn't the directionality derived from the pre-computed Doppler and matched filtering done within the E@H computations of the sensor data?... and not from the nature or design of the LIGO sensor itself?

Ref How can they "aim" a LIGO?

That is to say that "even a single detector is extremely sensitive to the sky location" is by virtue of the computation and not the sensor itself, right?

I'll crawl back under my perfectly engineered rock, now.

Glenn

"No, I'm not a scientist... but I did stay at a Holiday Inn Express."

Ben Owen
Ben Owen
Joined: 21 Dec 04
Posts: 117
Credit: 34,513,092
RAC: 20,834

Barkster: Yes, for pulsars

Message 5264 in response to message 5263

Barkster:

Yes, for pulsars the directionality comes almost entirely from the computation. The computation essentially says "If there was a source at this sky location with this frequency, the frequency would drift like this due to the changing Doppler shift. Let's filter the data assuming that time-changing Doppler shift ... nope, not much here; time to move the orange bullseye to the next sky location."

Ben

Tom Awtry
Tom Awtry
Joined: 18 Jul 05
Posts: 100
Credit: 520,861
RAC: 0

RE: I've seen more than one

Quote:

I've seen more than one post asking about how this project works. There's a decent source of information for lay folks posted at The American Museum of Natural History that discusses Newton's ideas, Einstein's Theories, Graity and what it's all about. There are some decent videos, a virtual tour of LIGO, and a nifty simulator that lets you fire the laser and search for your very own gravity wave. Click HERE if you're interested.


Finally after numerous hours of reading threads, along with most of their associated mentioned links, I came to the quoted posting, which for a layman in physics, explains in understandable detail how LIGO functions along with what the expected achievements may bring.

Chipper Q
Chipper Q
Joined: 20 Feb 05
Posts: 1,540
Credit: 708,571
RAC: 0

RE: RE: I've seen more

Message 5266 in response to message 5265

Quote:
Quote:
I've seen more than one post asking about how this project works. There's a decent source of information for lay folks posted at The American Museum of Natural History that discusses Newton's ideas, Einstein's Theories, Graity and what it's all about. There are some decent videos, a virtual tour of LIGO, and a nifty simulator that lets you fire the laser and search for your very own gravity wave. Click HERE if you're interested.

Finally after numerous hours of reading threads, along with most of their associated mentioned links, I came to the quoted posting, which for a layman in physics, explains in understandable detail how LIGO functions along with what the expected achievements may bring.


Hi, Tom, Welcome aboard -

The physicists and mathematicians probably thought their job was tough – how much harder it is for us laypersons! (For when one desires truth, the bliss of ignorance becomes frustrating disability.)

BTW, nice sig. What do you figure that quote means? One of my favorite quotes (also from Einstein) is “the most incomprehensible thing about the universe is that it is comprehensible.”

Tom Awtry
Tom Awtry
Joined: 18 Jul 05
Posts: 100
Credit: 520,861
RAC: 0

RE: RE: RE: I've seen

Message 5267 in response to message 5266

Quote:
Quote:
Quote:
I've seen more than one post asking about how this project works. There's a decent source of information for lay folks posted at The American Museum of Natural History that discusses Newton's ideas, Einstein's Theories, Graity and what it's all about. There are some decent videos, a virtual tour of LIGO, and a nifty simulator that lets you fire the laser and search for your very own gravity wave. Click HERE if you're interested.

Finally after numerous hours of reading threads, along with most of their associated mentioned links, I came to the quoted posting, which for a layman in physics, explains in understandable detail how LIGO functions along with what the expected achievements may bring.

Hi, Tom, Welcome aboard -

The physicists and mathematicians probably thought their job was tough – how much harder it is for us laypersons! (For when one desires truth, the bliss of ignorance becomes frustrating disability.)

BTW, nice sig. What do you figure that quote means? One of my favorite quotes (also from Einstein) is “the most incomprehensible thing about the universe is that it is comprehensible.”

Chipper – Difficult for me to even prophesize when it comes to understanding an intellect like Einstein! The only thing that I can guess at, without knowing the timeframe Einstein communicated these words is; in reference to Isaac Newton’s theory of gravity and how Einstein altered the way mathematicians conceived Newton’s findings without question or spirit of adventure to carry Newton’s past work to the next step of logical progression.

Also, Einstein uses the pronoun “they” throughout his statement, it would be interesting to learn who exactly “they” are!

Your favorite quote is also an interesting play on words, or is there a double meaning that we’re not able to adequately pickup on, without knowing the man personally; and your absolutely correct about these geniuses feeling their vocations were trying, for me the toughest choice I usually have during the day is what I’m going to feel the dog tonight for dinner!

Thanks for you kind and generous welcome.

Take Care,
Tom

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.