APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Comments and questions about the APOD on the main view screen.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by neufer » Thu Feb 11, 2016 9:03 pm

Craig Willford wrote:
If power output from the conversion of 3 solar masses to gravitational waves put out, for that fractional second, 50 times that of the whole universe, perhaps quite a bit of energy in the universe is tied up in gravitational waves still rippling.
If quite a bit of energy in the universe were tied up in gravitational waves
then we would have easily detected gravitational waves before 2015.
Craig Willford wrote:
Of course that energy gets absorbed (else LIGO would not have been able to detect it). How much did this event in September heat up the Earth? I presume it is indetectably small.
An insignificant proportion of the ~1079 gravitons released will ever be absorbed.
Craig Willford wrote:
How much did this event in September heat up the Earth? I presume it is indetectably small.
It is way, way too weak and at the wrong frequency.

The Earth has vibrational normal modes with periods on the order of an hour.

Now, if the 4 million solar mass black hole at the center of the Milky Way turns out to be 2 closely orbiting 2 million solar mass black holes (with a period on the order of an hour) then they might generate enough strong low frequency gravitational waves to excite detectable Earth vibrational normal modes.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Qev » Thu Feb 11, 2016 9:06 pm

Chris Peterson wrote:Something like this could happen very nearby and not pose any risk from the gravitational radiation. I'm not sure if a black hole merger also produces enough electromagnetic radiation to be dangerous for some distance, but I think that would be a larger concern than the gravitational waves.
I was just about to ask about this, haha... so, how close would one need to be to an event like this for the gravitational radiation to show... deleterious effects on matter? I assume it would basically transfer energy in the form of heat?

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Ann » Thu Feb 11, 2016 11:08 pm

Chris Peterson wrote:
Boomer12k wrote:LONG LIVE ALBERT!!! Hope he gets a Nobel Prize....He certainly deserves it.
He already did, although not for relativity. And since the Prize only goes to people who are alive, he can't get another.

The next prize in physics should go to Thorne, Drever, and Weiss. Anything else would devalue the Physics prize as much as the nomination of Trump for the Peace Prize devalued it.
I sure hope they get their Nobel Prize!

As a Swede though, I note that public service science reporting in Sweden (where public service TV and radio are extremely influential) almost scoffs at astronomy and astrophysics. That is seen as "useless" science, since it doesn't influence our lives here on Earth. Listen to the daily science report on Swedish public service radio, and they can't give you enough news about possible future breakthroughs in the fields of DNA mapping and altering, new viruses and other scary diseases, new scary bacterial resistance to antibiotics, new findings on why exercise is good for us, new findings on why sugar is bad for us, and all kinds of news of how we may cure cancer and diabetes twenty years from now. Oh, and will it be possible to really create dinosaurs from dinosaur DNA found in fossilized mosquitoes, just like in Jurassic Park? Let's talk to Professor So-and-So... Astronomy? Yes, there is a full Moon tonight, why don't you go outside and look at it? And goodness me, look at that Hubble Deep Field. Professor, what do we humans feel when we look at an image like that one?

The Nobel Prize committee for physics are part of that Swedish sphere of thinking about science, but they aren't as disdainful of astronomy as public service radio when they consider possible winners of the prize for physics. Still, they are not as enthusiastic about astronomy and astrophysics as science reporters and science magazines typically are in the United States. Even so, I sure, sure hope that Thorne, Drever, and Weiss will get the physics prize, and I think they will, too. But it will not happen this year, and not next year, either. Probably not five years from now, either. The Nobel Prize committee are known for waiting a long time before they award the prize to the person(s) behind a breakthrough discovery.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Ann » Thu Feb 11, 2016 11:25 pm

APOD Robot wrote:
ringdown
Love that word! :D
Cardiff University wrote:
The merger will result in a highly deformed single black hole which rids itself of its deformity by emitting gravitational radiation that is characteristic of the mass and spin of the final black hole. This is called the quasi-normal mode or the ring down signal.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Nitpicker » Fri Feb 12, 2016 12:08 am

Is it correct to assume that the gravitational "pull" from a black hole is so great, that no electro-magnetic, nor gravitational radiation can escape its threshold (or event horizon thingy)?

If so, then the radiation detected in this APOD, must have been produced from mass that was in the vicinity of the black hole(s), that was either about to be consumed, or was never to make it into the black hole(s). Correct?

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by neufer » Fri Feb 12, 2016 12:53 am

Nitpicker wrote:
Is it correct to assume that the gravitational "pull" from a black hole is so great, that no electro-magnetic, nor gravitational radiation can escape its threshold (or event horizon thingy)?

If so, then the radiation detected in this APOD, must have been produced from mass that was in the vicinity of the black hole(s), that was either about to be consumed, or was never to make it into the black hole(s). Correct?
Trying to think anything out along those sort of lines leads to madness :!:
(E.g., the gravitational fields themselves can be thought of as a source of gravitational waves.)

The only truly important things are that:
  • 1) the total energy of the system must decrease
    2) and the total entropy must increase.
    --------------------------------------------------------------
    Energy before = 36 + 29 = 65
    Energy after = 62


    Entropy before = 362 + 292 = 46.232
    Entropy after = 622
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Nitpicker » Fri Feb 12, 2016 1:05 am

Hmmm. I was more thinking that the gravitational field is what is being detected, and the small fluctuations in the field are the waves in the detected signal, with a signature indicative of the merger.

So, I guess that answers my own question. I'd say that by virtue of the fact that a black hole has a gravitational field around it, related to its internal mass, that gravitational radiation can escape the event horizon, but EM radiation cannot. Until someone says I'm completely wrong, I'm sticking with this, as it makes sense to my feeble mind.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Nitpicker » Fri Feb 12, 2016 1:09 am

I'm sure it is explained somewhere in the links, but do the LIGO facilities need to be aimed and focused in a particular portion of the sky, like a regular telescope? I imagine they must.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Qev » Fri Feb 12, 2016 1:23 am

Nitpicker wrote:I'm sure it is explained somewhere in the links, but do the LIGO facilities need to be aimed and focused in a particular portion of the sky, like a regular telescope? I imagine they must.
No, they're entirely stationary installations (well, as stationary as anything situated on a spinning rock whizzing through space can be). Gravitational waves basically brake for nobody, so they'll pass right through the Earth or what-have-you. They can determine the approximate direction toward a source by the time each detector receives the signal.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Chris Peterson » Fri Feb 12, 2016 1:26 am

Nitpicker wrote:I'm sure it is explained somewhere in the links, but do the LIGO facilities need to be aimed and focused in a particular portion of the sky, like a regular telescope? I imagine they must.
No. The waves pass right through the Earth. They are detected whichever way they come from. Their sky is above and below.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Nitpicker » Fri Feb 12, 2016 1:30 am

Qev wrote:No, they're entirely stationary installations (well, as stationary as anything situated on a spinning rock whizzing through space can be). Gravitational waves basically brake for nobody, so they'll pass right through the Earth or what-have-you. They can determine the approximate direction toward a source by the time each detector receives the signal.
I see. Thank you. So, I suppose you effectively point/focus by adjusting the lag times on all the detectors, when you combine all their signals together.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by daddyo » Fri Feb 12, 2016 5:40 am

Nitpicker wrote:I see. Thank you. So, I suppose you effectively point/focus by adjusting the lag times on all the detectors, when you combine all their signals together.
I bet you're right, two detectors should define the cone where the merger occurred on, three should form a direction, and three way out in space might give you a point in space.
Last edited by daddyo on Fri Feb 12, 2016 6:03 am, edited 1 time in total.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by MarkBour » Fri Feb 12, 2016 5:55 am

Fantastic news event in Astronomy! Totally worth APOD "holding the presses" for a bit, in spite of the odd chatter it produced in the forum. Thanks for posting it, and thanks for the warm-up posting 4 days earlier about LIGO.

While it would not detract at all from the historicity of these observations, I'd just like to ask something from the perspective of not jumping to too many conclusions. The title for the APOD is nicely conservative: "LIGO Detects Gravitational Waves (2016 Feb 11)" .

bystander pointed to some source publications, though, which go further in their titles. e.g.: Observation of Gravitational Waves from a Binary Black Hole Merger - B.P. Abbott et al

I did not have the good fortune to catch the live announcement and press conference.

Let's say one was a skeptic on some parts of the theory of black holes. And then saw this detection. I believe one should conclude that the observations corroborate a detection of gravitational waves such as would be consistent with a merger of two objects, of the approximate mass sizes mentioned (29 and 36 sols), producing an object of mass 62 sols. And I respect the headline of the cited paper saying "Black Hole" in that the current most-accepted theory would assume that these objects were black holes. But, back to the skepticism: Is there anything in the observations that corroborates that the objects were indeed black holes?
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Nitpicker » Fri Feb 12, 2016 7:15 am

daddyo wrote:
Nitpicker wrote:I see. Thank you. So, I suppose you effectively point/focus by adjusting the lag times on all the detectors, when you combine all their signals together.
I bet you're right, two detectors should define the cone where the merger occurred on, three should form a direction, and three way out in space might give you a point in space.
Presumably, the "1.3 billion years ago" gives us a point in space from three detectors on the surface of the Earth. I was simply assuming that either of the two sites on their own, had at least 3 detectors far enough apart to give a direction to the source. But then I read:
By looking at the time of arrival of the signals—the detector in Livingston recorded the event 7 milliseconds before the detector in Hanford—scientists can say that the source was located in the Southern Hemisphere.
I've no idea how they can estimate the "1.3 billion years ago", either.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by alter-ego » Fri Feb 12, 2016 7:44 am

MarkBour wrote: ... But, back to the skepticism: Is there anything in the observations that corroborates that the objects were indeed black holes?
The paper rationalizes that conclusion using the observed data (p.3). At coalescence, the orbital frequency = 75Hz (1/2 the GW frequency). The objects were very close and compact to reach this frequency, and the total mass is extracted from the data, so although 2 neutron stars could get close enough without contact (≈350km separation estimated at merger), but there masses are too low. A neutron star + BH would have yet a different signature: The deduced "chirp" mass would have a much larger total mass and the merge frequency would be much lower that 75Hz. Therefore, as deduced by GR, 2 black holes merging is the only option that fits the data.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by alter-ego » Fri Feb 12, 2016 8:40 am

Nitpicker wrote: ... I've no idea how they can estimate the "1.3 billion years ago", either.
GW detection may present us with a new standard "candle" method. The GW frequencies gives us red-shifted masses, while the GW amplitude contains the distance information. Through GR, GW data yields both masses and distance which are conceptually self consistent, but the distance and mass errors appear coupled. For this case, the uncertainty in the luminosity distance is ±40%.
Estimating parameters of binary black holes from gravitational-wave observations of their inspiral, merger and ringdown wrote: ...From signals producing signal-to-noise ratio 5 in at least two detectors, we estimate the posterior distributions of the binary parameters using the Bayesian parameter estimation code LALInference. The GW signals will be redshifted due to the cosmological expansion and we measure only the “redshifted” masses. By assuming a cosmology, it is possible to estimate the gravitational masses by inferring the redshift
from the measured posterior of the luminosity distance. We find that the measurement of the gravitational masses will be in general dominated by the error in measuring the luminosity distance.

... The GWs, produced purely by the motion of the black holes in the binary and well described by the GR,
travel to the detector completely unaffected by the intervening matter. By comparing the observational data with theoretical
templates of the expected signals (as computed by GR) it is possible to extract the parameters of the binary (such as the masses
and spin angular momenta of the black holes, sky-location and luminosity distance to the binary, etc.).
Black Hole Collisions wrote: ...Just as optical radiation and radio waves, the luminosity of gravitational radiation falls off in inverse proportion to the square of the distance from the source. This makes binary black hole inspirals standard sirens: if we know what the masses of the two black holes are then we can infer the distance to the source by measuring its apparent luminosity. We can precisely measure the masses because the rate at which the frequency and amplitude of an inspiral increases depends only on the masses.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by hamilton1 » Fri Feb 12, 2016 12:07 pm

Chris Peterson wrote:
Boomer12k wrote:LONG LIVE ALBERT!!! Hope he gets a Nobel Prize....He certainly deserves it.

The next prize in physics should go to Thorne, Drever, and Weiss. Anything else would devalue the Physics prize as much as the nomination of Trump for the Peace Prize devalued it.
Ever since it was given to three (!) left-wing American politicians, the peace prize has become a laughing stock.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by hamilton1 » Fri Feb 12, 2016 12:08 pm

Ann wrote: The Nobel Prize committee are known for waiting a long time before they award the prize to the person(s) behind a breakthrough discovery.
Peter Higgs might disagree.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Chris Peterson » Fri Feb 12, 2016 3:05 pm

Ann wrote:The Nobel Prize committee are known for waiting a long time before they award the prize to the person(s) behind a breakthrough discovery.
Historically, yes. Lately the time between work and award has been quite a bit shorter.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Chris Peterson » Fri Feb 12, 2016 3:06 pm

hamilton1 wrote:
Chris Peterson wrote:
Boomer12k wrote:LONG LIVE ALBERT!!! Hope he gets a Nobel Prize....He certainly deserves it.

The next prize in physics should go to Thorne, Drever, and Weiss. Anything else would devalue the Physics prize as much as the nomination of Trump for the Peace Prize devalued it.
Ever since it was given to three (!) left-wing American politicians, the peace prize has become a laughing stock.
We don't really have left-wing politicians in the U.S.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Chris Peterson » Fri Feb 12, 2016 3:17 pm

MarkBour wrote:Let's say one was a skeptic on some parts of the theory of black holes. And then saw this detection. I believe one should conclude that the observations corroborate a detection of gravitational waves such as would be consistent with a merger of two objects, of the approximate mass sizes mentioned (29 and 36 sols), producing an object of mass 62 sols. And I respect the headline of the cited paper saying "Black Hole" in that the current most-accepted theory would assume that these objects were black holes. But, back to the skepticism: Is there anything in the observations that corroborates that the objects were indeed black holes?

Here's what the analysis looked like: they used the math of GR to build a numerical simulation, solving the field equations. By varying different parameters (such as the mass of the presumed black holes) they eventually produced a simulated result that looks virtually identical to the signal recorded at LIGO.

In other words, if there's no such thing as a "black hole", the combination of simulation and observation points to there instead being something that is the same size and mass of black hole and which acts exactly like theory predicts a black hole should behave (including all the physics of coalescence). At this point, claiming that what we observe and interpret as black holes are really something else is kind of like making the claim that what we observe as stars aren't really stars, but rather, plasma bodies which are fusing elements and radiating large amounts of energy!

One of the important conclusions of the discovery is the first direct observation of black holes. Which is a reasonable view.
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Dave#5

Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Dave#5 » Fri Feb 12, 2016 4:09 pm

What kind of observational baseline would be required to accurately triangulate the source? The 3 terrestrial (2 US, 1 future EU) would seem too close together for anything other than vague directionality. As long as we're dreaming, what kind of observational method(s) would allow for imaging gravitational waves?

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by Chris Peterson » Fri Feb 12, 2016 4:36 pm

Dave#5 wrote:What kind of observational baseline would be required to accurately triangulate the source? The 3 terrestrial (2 US, 1 future EU) would seem too close together for anything other than vague directionality. As long as we're dreaming, what kind of observational method(s) would allow for imaging gravitational waves?
Even with the current two detectors the source was localized to a fairly small region of the sky. With three it would be pinpoint (and there will be three detectors running in the near future). No achievable baseline is wide enough to directly recover distance, but fortunately there are other ways to get that.

Three detectors would also allow "imaging" in the same way that three radio telescopes do. In fact, even with the LIGO data it is possible to produce a sort of low resolution image.
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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by daddyo » Fri Feb 12, 2016 5:06 pm

Chris Peterson wrote:Three detectors would also allow "imaging" in the same way that three radio telescopes do. In fact, even with the LIGO data it is possible to produce a sort of low resolution image.
I'm thinking that radio telescopes can image because of the wide angular span of their objects of interest. But since possibly anything that can create detectable gravity waves would always be angularly single points in the sky, there would never be anything to image, other than locate that point, unfortunately.

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Re: APOD: LIGO Detects Gravitational Waves (2016 Feb 11)

Post by neufer » Fri Feb 12, 2016 5:07 pm

Chris Peterson wrote:
Dave#5 wrote:
What kind of observational baseline would be required to accurately triangulate the source? The 3 terrestrial (2 US, 1 future EU) would seem too close together for anything other than vague directionality. As long as we're dreaming, what kind of observational method(s) would allow for imaging gravitational waves?
Even with the current two detectors the source was localized to a fairly small region of the sky. With three it would be pinpoint (and there will be three detectors running in the near future). No achievable baseline is wide enough to directly recover distance, but fortunately there are other ways to get that.

Three detectors would also allow "imaging" in the same way that three radio telescopes do. In fact, even with the LIGO data it is possible to produce a sort of low resolution image.
Remember that we are only talking about point sources here so "localization" is probably a more apropos term than "imaging" (unless one is mapping all the events over an extended period of time).
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