1. From Very High Energy Gamma-Ray Astronomy, by T.C. Weekes:

The Earth's atmosphere effectively blocks all electromagnetic radiation of energies greater than 10 eV. The total vertical thickness of the atmosphere above seal level is 1030 g cm^-2 and since the radiation length is 37.1 g cm^-2, this amounts to more than 28 radiation lengths. This is equivalent in blocking power to a 1 m thickness of lead. This is true up to the energy of the highest known cosmic rays (some of which may be gamma rays).
It goes on to say:

However, there is a 'gamma-ray window' from about 100 GeV to 50 TeV where it has been possible to successfully pursue gamma-ray observations of cosmic sources using ground-based instruments. It is a fortunate coincidence in nature that while the gamma ray itself may not survive, the secondary products of its interaction with the atmosphere do survive and can be detected with the simple detectors described here.

2. @Staticman:

Please explain to me: Why would a very faint GRB (which occured 0.5 seconds after the aLIGO signal) be able to produce said signal, whereas all of the much brighter GRBs that have occurred since then seem to have not done anything?

3. Originally Posted by Don Alexander
May I refer to: http://arxiv.org/abs/1602.07352 Ok, but that seems to be addressing the unlikelihood of an accretion or as magnetic field scenario as causal to a grb possibility.
Just for fun, I tried to determine the entropy difference between a 65 solar mass BH and a 62 solar mass BH. The difference amounts to about 5.5E63 ergs/K. [Uses the black hole entropy of Sbh = kA/(4L^2); k~ Boltzman, A~ area of EH, L is Planck length] I have no idea what this means since entropy was something that faded in and out of my grasp during the old days I studied it, which has long past. But there is a chance it may be significant in this huge astronomical event.

Of course, the mass difference is due to the energy being pulsed away in the gravity wave, but there is more to the entropy story. It turns out that the entropy of the separate black holes actually doubles just prior to the pulse, based simply on spherical surface areas of a 65 solar mass EH vs. the two separate mass surface areas. What does that mean? I'm glad you didn't take up my sundae offer, though I would love to see the entropy question addressed by someone who has some idea what implications come from these entropy changes.

Also, should the GWs themselves not be a strongly entropic signal? While weakly interacting, you have distortions of space-time moving out at the speed of light.
Great question? In perfectly empty space, can we say that the wave is reversible with no heat loss? Add all those raisins and the tidal stress alone by the wave will generate some "friction" (heat), but this is energy from the wave and not the main "splash" component of the event, which is separate from the wave.

This is exciting stuff regardless of the outcome. I'll be more surprised if there is no splash but a perfect nice little pure 3 solar mass energy pulse that is more pure than a proton. [/hyperbole]
Last edited by George; 2016-Feb-26 at 10:07 PM.

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Originally Posted by Don Alexander
@Staticman:

Please explain to me: Why would a very faint GRB (which occured 0.5 seconds after the aLIGO signal) be able to produce said signal, whereas all of the much brighter GRBs that have occurred since then seem to have not done anything?
I was under the impression that they were still investigating that almost coincident GRB so I don't really have enough information on that to be certain about its exact timing or its faintness, maybe you do? On the other hand, those other brighter GRBs, it would be important to know whether they are long or short bursts, and also have access to the signals detected by aLIGO after the september 14th one(I think it was you who said in another thread that there was an embargo by aLIGO of data related to certain GRBs that should appear on a public site). So there is too much information missing for me to say anything other than there could be many variables influencing wheter those other GRBs could end up producing a signal, like direction and angle with respect to the mirrors,etc.
And again I am of course not claiming that the aLIGO detection was caused by a GRB, I'm just saying that any effort should be undertaken to discard based on scientific certainty this possibility or others.
Last edited by Staticman; 2016-Feb-26 at 10:40 PM.

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Originally Posted by Amber Robot
From Very High Energy Gamma-Ray Astronomy, by T.C. Weekes:

It goes on to say:
Thanks, I have the reference by Weekes.
But I stand by my assertion that the atmosphere by itself is not an absolute block to gamma-rays, the proof of this is that it is possible to detect terrestrial gamma-rays from satellites. Surely the distance is much shorter than the sources of GRBs but it shows that the atmosphere is not an absolute shield.

6. If you read a standard reference on the interaction of gamma rays with the Earth's atmosphere -- Weekes is one example, as is this paper by Diehl (http://www2011.mpe.mpg.de/~rod/gamma-ray_processes.ps) -- you'll see the term "optical depth." Suppose that the optical depth of a medium is represented by the letter 't'. Then as radiation passes through that medium, the amount which penetrates the medium is given by

(amount penetrating to optical depth t) / (original amount) = exp(-t)

I've written "exp(-t)" to represent the negative exponential function; it is often written as "e" with a superscript to denote the power.

Suppose that a cloud of gas has an optical depth of t = 3 to some incoming radiation. That means that the fraction of the radiation which goes through the cloud is just exp(-3) = 0.0498, or about 5 percent.

If the cloud has optical depth t = 10, then the fraction passing through the cloud will be exp(-10) = 0.00004.

The optical depth of Earth's atmosphere to gamma rays depends on the energy of the gamma rays, but Diehl gives a rough estimate of about t ~ 100. That implies that the fraction of gamma rays from space which reach the ground is exp(-100) = 10^(-44). That's ... really small. That implies that one would need to release 10^(44) gamma rays on one side of the atmosphere for a single gamma ray to reach the other side.

(Digression: a megaton of gamma rays is only about 10^(28) photons, so this quick calculation seems at first to suggest that satellites in orbit should not be able to detect nuclear bomb blasts near the Earth's surface. Since satellites _do_ detect nuclear blasts, I must assume that the initial flash of gamma rays is able to modify the atmosphere in such a way as to render it somewhat transparent to gamma rays emitted a fraction of a second later. Interesting)

The bottom line is that only a teeny, tiny fraction of the gamma rays from a celestial object will pass through the atmosphere and reach the ground. Such a tiny fraction that no known and detected sources produce enough to be measured from the ground directly. Sure, if a supernova were to explode very close to the Sun, we might detect it ... but not GRBs from the far reaches of the universe.

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Originally Posted by StupendousMan
If you read a standard reference on the interaction of gamma rays with the Earth's atmosphere -- Weekes is one example, as is this paper by Diehl (http://www2011.mpe.mpg.de/~rod/gamma-ray_processes.ps) -- you'll see the term "optical depth." Suppose that the optical depth of a medium is represented by the letter 't'. Then as radiation passes through that medium, the amount which penetrates the medium is given by

(amount penetrating to optical depth t) / (original amount) = exp(-t)

I've written "exp(-t)" to represent the negative exponential function; it is often written as "e" with a superscript to denote the power.

Suppose that a cloud of gas has an optical depth of t = 3 to some incoming radiation. That means that the fraction of the radiation which goes through the cloud is just exp(-3) = 0.0498, or about 5 percent.

If the cloud has optical depth t = 10, then the fraction passing through the cloud will be exp(-10) = 0.00004.

The optical depth of Earth's atmosphere to gamma rays depends on the energy of the gamma rays, but Diehl gives a rough estimate of about t ~ 100. That implies that the fraction of gamma rays from space which reach the ground is exp(-100) = 10^(-44). That's ... really small. That implies that one would need to release 10^(44) gamma rays on one side of the atmosphere for a single gamma ray to reach the other side.

(Digression: a megaton of gamma rays is only about 10^(28) photons, so this quick calculation seems at first to suggest that satellites in orbit should not be able to detect nuclear bomb blasts near the Earth's surface. Since satellites _do_ detect nuclear blasts, I must assume that the initial flash of gamma rays is able to modify the atmosphere in such a way as to render it somewhat transparent to gamma rays emitted a fraction of a second later. Interesting)

The bottom line is that only a teeny, tiny fraction of the gamma rays from a celestial object will pass through the atmosphere and reach the ground. Such a tiny fraction that no known and detected sources produce enough to be measured from the ground directly. Sure, if a supernova were to explode very close to the Sun, we might detect it ... but not GRBs from the far reaches of the universe.
The general idea that the atmosphere is an excelent shield to gamma rays has always been clear to me. It is all a question of getting the details right. I was not able to find where in the pages you link the author gives a figure of 100 for the optical depth. He mentions depth of 1000g/cm2 for gamma rays in 10km thick atmosphere. How do you obtain t about 100 from g/cm2?

8. From the article:

"The penetration depth for gamma-rays corresponds to a few games of material per cm^2."

Call this depth 'd', and approximate it to be about d=10 g cm^(-2).

Continuing from the next sentence of the article:

"For a characteristic thickness of the Earth's atmosphere of 10 km, and a typical density of air of 1 mg cm^(-3) this amounts to 1000 g cm^(-2)".

Call that thickness D. Then the optical depth of the atmosphere is of order t = D / d = 100.

9. I thought observation satellites cheated and looked for the characteristic double flash of a nuclear explosion.

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The prime minister of India has announced that India will build a Laser Interferometer Gravitational-Wave Observatory (LIGO).

http://timesofindia.indiatimes.com/h...w/51177962.cms

The laboratory will be third of its kind in the world after Hanford in Washington and Livingston in Louisiana, both in the US.

"Recently the Gravitational Waves have been discovered by the scientific community of the world, which is indeed a major achievement. We should be proud of the fact that Indian scientists were also part of it. Keeping this in mind, we have taken a decision to open a LIGO (Laser Interferometer Gravitational-Wave Observatory) in India," said Modi.

11. Originally Posted by selvaarchi
The prime minister of India has announced that India will build a Laser Interferometer Gravitational-Wave Observatory (LIGO).
The more the merrier... and the better able to locate the source of the gravitational waves.

12. Here is something that seems to support, somewhat, the "splash" idea, and it gives some credence to the grb (and neutrino production and annihilation).

There is a problem when black holes merge, namely the entropy will, in this case, essentially double. But such an increase is not possible, apparently. They show that an energy dump ("splash"?) must take place: "...if the entropy of the new black hole is to be the same as the sum of the two initial black holes, energy must be shaken off as they coalesce. The lower bound for this is... ~ 0.59M". M is the sum of the two masses. About 0.05M seems to have been in the form of the gravity wave, perhaps more if propagation has irreversible experiences. The remainder is a great deal of energy.

They state that neutrinos and antineutrinos form during the event but that they annihilate each other to produce.... grbs. There is quite a lot of energy that seems unaccounted for, though perhaps an increase of entropy for mergers is permissible, but probably not double their original sum.

Interesting, ain't it?

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Originally Posted by Cougar
The more the merrier...
Be nice to see one on Mars. It'd not only improve source localization, but allow for elimination of earthly and solar noise.
Sadly, NASA doesn't have the spare cash laying around.

14. Originally Posted by Squink
Be nice to see one on Mars. It'd not only improve source localization, but allow for elimination of earthly and solar noise.
Sadly, NASA doesn't have the spare cash laying around.
It would probably be easier to build one on the Moon, and/or a nearby asteroid, so you don't need to work so hard to evacuate the line of sight to the reflectors.
This could be a job for specialized robots 30 years from now.

15. Originally Posted by George
Here is something that seems to support, somewhat, the "splash" idea, and it gives some credence to the grb (and neutrino production and annihilation).
Although that paper concludes:

Conclusions: Under some modest assumptions we have concluded that either mergers involving black holes that generate larger black holes are rare, or they must capture nearly all the entropy generated in the process.

Such mergers are apparently not so rare, therefore....

16. Originally Posted by Cougar
Although that paper concludes:

Conclusions: Under some modest assumptions we have concluded that either mergers involving black holes that generate larger black holes are rare, or they must capture nearly all the entropy generated in the process.
It looks like a bit of a conundrum, which makes it all the more interesting. The problem seems to be an easy math one: combine two black holes and you will increase the EH radius, but the surface increases with the square of the radius so, if entropy goes with the surface area in every case, the entropy will always increase more than their sum. Combine two of equal mass, for instance, and this will give you twice their original EH radius and 4x the entropy of one, or double their sum!

For a SMBH merger, they note that the luminosity would do serious damage to the galaxy; estimating a solar luminosity out to 3Mpc! So the entropy story is a dandy, one way or another. The GW pulse, which lowers the mass of resulting bh, may or may not be accompanied by some additional energy pulse more associated with the entropy story, as they suggested earlier in the paper, but perhaps the energy release is limited to the outer region in the zone where the EH shrinks from the GW pulse. The real story is bound to be really cool, no doubt.
Last edited by George; 2016-Feb-29 at 03:07 PM.

Recall Hawking's great one-liner, "There are no black holes." Article here. He envisioned an "apparent horizon", separate or in lieu of the EH, as an alternative to the "firewall" view where the EH is so hot it would turn someone falling in to a crispy critter. This argued that black holes have temperature and are subject, apparently, to the second law. As the apparent horizon shrinks, more energy is released, shrinking the hole further. What happens when you suddenly kick-off 5% of your mass in a GW?? Would the energy that is just below this surface (prior to the pulse), as suggested, be thermalized (ie high entropy?) and likely producing (as a result of the pulse) a Planck distribution-like pulse? Has anyone mentioned whether or not there was any optical flash from this region, or do we have any imager capable of such a quick catch?
Last edited by George; 2016-Feb-29 at 06:13 PM.

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Originally Posted by StupendousMan
From the article:

"The penetration depth for gamma-rays corresponds to a few games of material per cm^2."

Call this depth 'd', and approximate it to be about d=10 g cm^(-2).

Continuing from the next sentence of the article:

"For a characteristic thickness of the Earth's atmosphere of 10 km, and a typical density of air of 1 mg cm^(-3) this amounts to 1000 g cm^(-2)".

Call that thickness D. Then the optical depth of the atmosphere is of order t = D / d = 100.
Those quantities in g/cm2 I don't know what they exactly represent, if they are equivalent to radiant fluxes(W) or intensities(W/m2) their quotient should be e^-t, not t itself. But again I understand that it is extremely difficult for a gamma-ray to cross the atmosphere and if it did it would have probably lost much energy. But my general point(Iwrote another post in a different thread on this but centered on Khz frequencies that got no replies) is that we should be totally certain we can discard an electromagnetic cause of the signal detectd by aLIGO(after all, all the wave signals at light speed detected in the history of physics until september 14th last year were electromagnetic in origin, extraordinary claims require extraordinary proof). And we know the interferometers are sensitive to EM waves(you can read it on the technichal papers).
I guess from the point of view of plausibility a source like a terrestrial gamma flash(TGF) is also something to consider. It could even have been what the Fermi telescope detected, exactly what caused it is still being intensely debated(Integral telescope didn't detect any GRB for instance).

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A space-based observatory planned by ESA would be able to detect ripples with much lower frequencies than would be possible on Earth, bringing into view a greater variety of astronomical events, including mergers between supermassive black holes.

This proposal by ESA might see Chinese and American scientist working with the Europeans on the project. But regulatory hurdles may hinder proposed partnerships with the United States and China.

http://www.nature.com/news/us-and-ch...ission-1.19848

In February, researchers working on the US-based Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that they had detected ripples in space-time that had been produced by the merger of two black holes. The space-based observatory planned by ESA would be able to detect ripples with much lower frequencies than would be possible on Earth, bringing into view a greater variety of astronomical events, including mergers between supermassive black holes.

Such a detector is widely seen as “the best thing you could do in gravitational waves”, says Robin Stebbins, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. After a mission to test crucial technologies for the observatory proved successful, the ESA advisory team last month concluded that not only are the agency’s plans feasible, but also that the launch could even be brought forward, from 2034 to 2029.

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"China university to build simulated gravitational wave observatory"

http://www.xinhuanet.com/english/201..._137357976.htm

Sun Yat-sen University in south China's Guangdong Province announced Monday that it will build a ground simulation system for space-based gravitational wave observation.

The system is expected to provide a complete simulation environment and new research methods for China's research on space-based gravitational wave observation, the university said.

It will be built on the university's campus in the metropolis of Shenzhen, which borders Hong Kong, with an investment of more than 1 billion yuan (146.6 million U.S. dollars).

The ground simulation system is part of the gravitational wave research project "Tianqin" launched by Sun Yat-sen University in 2015.

21. https://arxiv.org/abs/1801.04268

Cosmological Backgrounds of Gravitational Waves

Chiara Caprini, Daniel G. Figueroa
(Submitted on 12 Jan 2018 (v1), last revised 5 Feb 2018 (this version, v2))

Gravitational waves (GWs) have a great potential to probe cosmology. We review early universe sources that can lead to cosmological backgrounds of GWs. We begin by presenting definitions of GWs in flat space-time and in a cosmological setting, and discussing the reasons why GW backgrounds from the early universe are of a stochastic nature. We recap current observational constraints on stochastic backgrounds, and discuss some of the characteristics of present and future GW detectors including advanced LIGO, advanced Virgo, the Einstein Telescope, KAGRA, LISA. We then review in detail early universe GW generation mechanisms proposed in the literature, as well as the properties of the GW backgrounds they give rise to. We classify the backgrounds in five categories: GWs from quantum vacuum fluctuations during standard slow-roll inflation, GWs from processes that operate within extensions of the standard inflationary paradigm, GWs from post-inflationary preheating and related non-perturbative phenomena, GWs from first order phase transitions (related or not to the electroweak symmetry breaking), and GWs from topological defects, in particular from cosmic strings. The phenomenology of early universe processes that can generate a stochastic background of GWs is extremely rich, and some backgrounds are within the reach of near-future GW detectors. A future detection of any of these backgrounds will provide crucial information on the underlying high energy theory describing the early universe, probing energy scales well beyond the reach of particle accelerators.

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