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Gorn
2016-Feb-16, 01:24 AM
Hello. Apparently these waves that were detected were very 'small' or weak by the time they got to Earth. Over time these waves I think I heard lose energy and or size as they travel across the Universe.

How energetic would these waves be within a few astronomical units of the Black Holes themselves? I would like to know because apparently space time itself changes size when these waves pass..which means in
the near vicinity of these things 1 meter of spacetime might be no distance at all.

Thanks for your responses
Bye
G

DaveC426913
2016-Feb-16, 01:42 AM
I believe that gravity waves fall off as the square of the distance, and here at 1.3Gly away, they are on the order of .0001 proton diameters. You should be able to work out the gravitational flux at any point closer.

Back of napkin calculations suggest to me that, at a distance of one light year, distortion detected by a strategically-placed LIGO would be on the order of 10 cm over its length (which I think is 4,000m).

To wit:

We are measuring distances of 1 /10,000th (10^-4) of a proton (10^-15), resulting in a distortion of 10^-19 over 4000m.

One light year is about 1 billionth of the distance to us. So the force that close would be a billion squared, or 10^18.

10^-18 divided by 10^-19 equals 10^-1.

WaxRubiks
2016-Feb-16, 01:51 AM
posted on another thread:


But just for fun let’s look at how close we could get. Before the merger the two black holes have a diameter of about 212 and 170 kilometers respectively. After the merger the final black hole has a diameter of about 365 kilometers. If we were really close to the black holes, the tidal forces (https://briankoberlein.com/2014/01/08/best-face-forward/) alone would kill us, so let’s assume we’re at least 10,000 kilometers from ground zero. At that distance the shift caused by the gravitational waves would be about one part in a thousand. If you were floating in space you would likely feel that, since a person would experience a shift of a millimeter or two. Would it hurt, or possibly harm you? That’s hard to say. It would really depend on how resilient humans are to gravitational wave distortion, and we don’t have any experimental data on that. If I were to guess I’d say as long as your space suit held up you’d be fine.http://www.forbes.com/sites/briankoberlein/2016/02/13/could-gravitational-waves-ever-be-strong-enough-to-feel/#ac39ffb4aac3

DaveC426913
2016-Feb-16, 02:04 AM
posted on another thread:

http://www.forbes.com/sites/briankoberlein/2016/02/13/could-gravitational-waves-ever-be-strong-enough-to-feel/#ac39ffb4aac3

[/B]

I'm not sure if a "shift" of a millimeter or two is an adequate description. The change in g would happen at light speed.

I wonder if it is tantamount to a shock wave.

I have a feeling it's enough to turn you into an expanding cloud of atoms.

WaxRubiks
2016-Feb-16, 02:09 AM
I'm not sure if a "shift" of a millimeter or two is an adequate description. The change in g would happen at light speed.

I wonder if it is tantamount to a shock wave.

I have a feeling it's enough to turn you into an expanding cloud of atoms.

yes, I wouldn't be surprised myself.

DaveC426913
2016-Feb-16, 02:18 AM
I'm trying to figure this out. By the principle of equivalence, you would go from 0 to an incredible acceleration instantly, except it would happen to every atom in your body virtually simultaneously.

I was comparing it to having every atom shifted simultaneously back and forth by one millimeter, between 100 and 10,000 times per second.

Like being in a microwave - it excites every atom simultaneously.

What power of microwave radiation would be required to induce a 1 millimeter amplitude vibration in a hunk of meat?

kzb
2016-Feb-16, 12:23 PM
It's going to depend on the wavelength surely?

If the wavelength is many times longer than your body would you feel anything at all?

If the frequency is 10,000 Hz and gravitational waves propagate at c, the wavelength is about 30km.

ngc3314
2016-Feb-16, 03:11 PM
I believe that gravity waves fall off as the square of the distance, and here at 1.3Gly away, they are on the order of .0001 proton diameters.

Something surprising, that I learned only from hints in some of the LIGO coverage, is that gravitational-wave amplitude falls off only as the inverse of the distance (sample reference (https://books.google.com/books?id=Gi9nBAAAQBAJ&pg=PA560&lpg=PA560&dq=gravitational+radiation+inverse+falloff+distanc e&source=bl&ots=m5ni3azNR0&sig=m775YcvlDy9RgEX-f5PxIObtPQc&hl=en&sa=X&ved=0ahUKEwiV8_uHxvzKAhXINiYKHUzjBMEQ6AEILTAD#v=on epage&q=gravitational%20radiation%20inverse%20falloff%20 distance&f=false), as well as Wikipedia (https://en.wikipedia.org/wiki/Gravitational_wave)). This helps explain why the first (strongest?) source detected was already so distant. Unlike EM radiation, in this case we can measure the amplitude rather than its square.

ShinAce
2016-Feb-16, 06:32 PM
Correct. Here the amplitude is used in exactly the same way you would talk about the amplitude of an electric field, or the voltage amplitude. The intensity of the gravitational wave, like any intensity, is proportional to the amplitude2. The amount of power a gravitational wave carries falls off as inverse square. Luckily, we can measure its amplitude, and our ability to detect far away sources falls as 1/r .

selvaarchi
2016-Feb-19, 10:10 AM
There are more of these Gravitational Waves detectors planned around the world. Two of them are in Asia. Both the Chinese (http://www.shanghaidaily.com/article/article_xinhua.aspx?id=320280) and Indian (http://spaceref.com/news/viewpr.html?utm_content=api&pid=47965&utm_medium=srs.gs-twitter&utm_campaign=&utm_source=t.co) governments have given their approval.

Strange
2016-Feb-19, 09:02 PM
Another comment on this from Amber Stuver:

How far away do you have to be from this kind of black hole merger to live to tell the tale?
Stuver: For the black hole binary we detected with gravitational waves, they produced a maximum change in the length of our 4 km (~2.5 mi) long arms [of] 1x10-18 meters (that is 1/1000 the diameter of a proton). We also estimate that these black holes were 1.3 billion light-years away.
Now assume that we are 2 m (~6.5 ft) tall and floating outside the black holes at a distance equal to the Earth’s distance to the Sun. I estimate that you would feel alternately squished and stretched by about 165 nm (your height changes by more than this through the course of the day due to your vertebrae compressing while you are upright). This is more than survivable.
http://gizmodo.com/your-questions-about-gravitational-waves-answered-1758269933

Although the "speed of light" issue is an interesting complication.

George
2016-Feb-24, 12:26 AM
Let me be the fly in the ointment. Think speghettification near the event, not something barely noticeable. Linearity should be fine, but not close to the event. This is speculative, but look at what we have:

1) A three solar mass energy blast! Anything description of serenity seems ludicrous. It may be serene, but if so, that would be shocking news, and extremely non-intuitive.
2) Entropy is greatly increased in an instant. [Surprisingly, the surface area of the event horizon is proportional to the black holes total entropy. The EH area shrinks with 3 solar mass loss, thus entropy is reduced associated with the b.h. This entropy change should, I think, manifest itself in the surrounding environment.]. This is not something small scale. [Added: I assume the greatest entropy (surface area) is that of the odd shaped EH envelope formed by the binary, thus making the entropy issue even more pronounced.]
3) If confirmed, a gamma ray burst followed from this region with a ~ O.4 sec. delay. Serenity is the wrong word for describing these events. The delay seems to make some sense because the g.r.b. observed would have had the b.h. lensing effect and time dilation (and redshift?) applied to it.
4) The shrinkage of the EH might have released some very energetic particles and photons from within this zone.
5) This event had two masses going nuts, so wave propagation near this event should, perhaps, produce interference regions where the amplitudes slope might cause speghettification all by itself at certain points nearby?
6) How often do serenity claims go south? Besides most binary orbits, is there serenity? The cosmological constant tried to force serenity upon a serene Static Universe. Look at the size of cat that brought in the house!
7) Did I mention the energy release was ~ 3 solar masses? Super novae aren't that serene, really, though their energy release is diiferent.

Are we not being a little naive in guessing that the boat would not rock when fishing at these holes. I know this is speculative, but that's where this gets fun, and this one is extra fun. I think it is a serious topic, so much so that no gravity pun will be used...this time.

Cougar
2016-Feb-24, 01:52 PM
Think speghettification near the event....

You wouldn't be spaghettified. We need a new term! You'd be stretched one way and then the other, as shown in the animations. Then the large waves generated from the final merger would be past you, and you'd be back to your original configuration. What effect this would have on your individual atoms - and survival - I don't know. I do know it happens very quickly.


2) Entropy is greatly increased in an instant. [Surprisingly, the surface area of the event horizon is proportional to the black holes total entropy. The EH area shrinks with 3 solar mass loss, thus entropy is reduced associated with the b.h. This entropy change should, I think, manifest itself in the surrounding environment.]. This is not something small scale.

Yes, folks much smarter than me have figured that the surface area of the event horizon is proportional to the black hole's total entropy. The actual meaning of this is not exactly clear, at least to me. Offhandedly, I'm thinking this 3 solar mass measure of entropy loss is likely transferred and manifested in the gravitational waves. (?) Where/how else could it go? There could be a scientific paper here waiting to be written.


4) The shrinkage of the EH might have released some very energetic particles and photons from within this zone.

I don't see how. I imagine the gamma rays are coming from the two accretion disks slamming together.


7) Did I mention the energy release was ~ 3 solar masses?

Haha, I think you said that somewhere. All that energy goes into the gravitational waves. But it drops off with distance. Prior to merger, these black holes are going to be orbiting each other. You wouldn't want to be too close to this action in the first place.

Squink
2016-Feb-24, 04:10 PM
Wiggling of atoms has something or other to do with temperature (http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html).
Being in a rapidly fluctuating gravity field might cook you.

George
2016-Feb-24, 04:16 PM
You wouldn't be spaghettified. We need a new term! You'd be stretched one way and then the other, as shown in the animations. Then the large waves generated from the final merger would be past you, and you'd be back to your original configuration. What effect this would have on your individual atoms - and survival - I don't know. I do know it happens very quickly

It depends on the slope ( gravity gradient) of the 3 solar mass wave. This can't be gentle, at least not on my mind. Amplitude and wavelength are important here. What is the gradient of such a wave at, say, 10,000 km from the pulse origin?


Yes, folks much smarter than me have figured that the surface area of the event horizon is proportional to the black hole's total entropy. The actual meaning of this is not exactly clear, at least to me. Offhandedly, I'm thinking this 3 solar mass measure of entropy loss is likely transferred and manifested in the gravitational waves. (?) I am guessing not. Drop a ball bearing in calm water and the energy is transferred into the wave. The splash, however, represents the entropy increase to the universe as it is irreversible.

So, is there no "splash" that would not cause havoc to our boat, ignoring the uber Wiemea wave?


I don't see how. I imagine the gamma rays are coming from the two accretion disks slamming together.
Inside a fixed EV, nothing escapes. Retract the prison wall and out some will go, I think.



Haha, I think you said that somewhere. All that energy goes into the gravitational waves. But it drops off with distance. Prior to merger, these black holes are going to be orbiting each other. You wouldn't want to be too close to this action in the first place. Ok, but the bigger the wave, the bigger the splash, perhaps!

George
2016-Feb-24, 04:31 PM
Wiggling of atoms has something or other to do with temperature (http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html).
Being in a rapidly fluctuating gravity field might cook you. Splash!! :). [Added: This is not the event splash, however, but something that happens along the wayve.]

Also, the tidal stress near the EV -- for non-supermassive black holes-- is enough for speghettificaton, but how much more so when a ginormous gravity wave is added?!

Both effects might cause any matter to produce a GRB, even with the gravity redshift. Not that I know, admittedly.

Don Alexander
2016-Feb-25, 02:31 AM
A colleague of mine forwarded this. All credit goes to Keith F. Lynch http://keithlynch.net/

>> This surprises me, the equivalent of 3 solar masses radiated away in less than a second from 96 million miles away and we wouldn't notice?

> It's not just the energy, but rather the effect it produces on whatever it interacts with (or not).

Right. I just ran some numbers, and I came to the remarkable conclusion that the total solar power output (4E+26 watts) could harmlessly pass through you if it was in the form of gravitational waves rather than heat and light.

I used the formula c^3 h^2 f^2 pi / (8 G) to convert strain to flux. G is the gravitational constant, h is the strain, c is the speed of light, and f is the frequency in Hz (250 in this case). The peak strain of the recent event was 1E-21, so I get apeak flux of 10 milliwatts per square meter.

No wonder they always list the sensitivity of LIGO in terms of strain rather than in terms of watts per square meter. The latter doesn't sound nearly as impressive! Indeed, if the event had given off light rather than gravitation waves, it would have been not only bright enough to see from here, but bright enough to read by!

As a sanity check, I divided the reported peak power output of the event, 3.6E49 watts, i.e. 200 solar masses per second annihilated, by the area of a sphere 1.3 billion light years in radius. I get about 20 milliwatts per square meter. What accounts for the factor of two discrepancy? Probably polarization. LIGO, if I understand correctly, is sensitive to only one of the two polarizations.

("Only" 3 solar masses were annihilated, because the event lasted less than a second.)

Let's get closer to the event and see what happens. I hope you're reading this with a fixed font.

Distance flux (W/m^2) strain N

1.3E25 m (1.3E9 ly) 1E-2 1E-21 4E33
1.3E22 m (1.3E6 ly) 1E+4 1E-18 4E39
1.3E19 m (1.3E3 ly) 1E+10 1E-15 4E45
1.3E16 m (1.3 ly) 1E+16 1E-12 4E51
1.3E13 m (66 AU) 1E+22 1E-9 4E57
1.3E10 m (8 M miles) 1E+28 1E-6 4E63

The last column is the number of gravitons per square meter per second. I get that by multiplying the flux by the frequency and dividing by Plank's constant.

In each case, I assume you're floating in space, in a good spacesuit, facing toward the event.

I assume that a strain of one part in a million isn't going to hurt you, especially if it's front-to-back rather than head-to-toes. Note that that last distance is much less than 1 AU. 1E+28 watts per square meter -- your cross-sectional area is probably roughly one square meter -- means 25 times the sun's total power output is going through you. I wonder what it would feel like.

Of course I'm also assuming it was a "clean" event, i.e. nothing but gravitational waves was given off. If it consisted of nothing but two black holes, that's pretty much certain. But if there was other stuff in the area, all bets are off. Indeed, there was a weak gamma ray burst half a second after the event, which may or may not be a coincidence. We don't know the direction of either the event or the gamma ray burst, except very roughly.

> Supernovae radiate a huge amount of energy in neutrinos, but these hardly affect anything else.

Neutrinos aren't nearly as stealthy as gravitons. According to Randall Munroe, a typical supernova will emit 1E57 neutrinos, and they will be lethal at about 2 AU. During the peak tenth of a second of the event at the closest distance I list, 100,000 times as many gravitons will harmlessly pass through you as the *total* number of neutrinos given off by a supernova!

chornedsnorkack
2016-Feb-25, 05:34 PM
250 Hz sound has wavelength in room temperature air of 130 cm, in water of 6 m, and in flesh and bone slightly more.
How much is a 170 cm man stretched or shrunk, head to toe, by sound of 250 Hz and 100 dB?
By sound of 250 Hz and 0 dB?

George
2016-Feb-25, 05:38 PM
I used the formula c^3 h^2 f^2 pi / (8 G) to convert strain to flux. G is the gravitational constant, h is the strain, c is the speed of light, and f is the frequency in Hz (250 in this case). The peak strain of the recent event was 1E-21, so I get a peak flux of 10 milliwatts per square meter.

No wonder they always list the sensitivity of LIGO in terms of strain rather than in terms of watts per square meter. The latter doesn't sound nearly as impressive! Indeed, if the event had given off light rather than gravitation waves, it would have been not only bright enough to see from here, but bright enough to read by! If it were visible light energy, seen from Earth, this would be about a 20 mag. reduction when compared to the Sun –- 1E8 brightness difference using roughly 1 kw m^-2 for the Sun’s illuminance here. So it would be roughly equivalent to a -7 mag. star, which is much brighter than Venus, but dimmer than a crescent Moon. Very easily seen, but I doubt I could read by it, and I don’t read that quick. :)



I assume that a strain of one part in a million isn't going to hurt you, especially if it's front-to-back rather than head-to-toes. Note that that last distance is much less than 1 AU. 1E+28 watts per square meter -- your cross-sectional area is probably roughly one square meter -- means 25 times the sun's total power output is going through you. I wonder what it would feel like. That makes sense, but what about at the 10,000 km distance used by another? This would make it 1 part in a thousand over 100 times in about 1/3 of a second. I would guess that this would at least be very loud to the ear drum and would be at an audible frequency. Here (https://www.youtube.com/watch?v=rqRDla4zIZs) is a Youtube (w/commercial) of what it might sound like. Would it burst the ear drum?


Of course I'm also assuming it was a "clean" event, i.e. nothing but gravitational waves was given off. If it consisted of nothing but two black holes, that's pretty much certain. But if there was other stuff in the area, all bets are off. Indeed, there was a weak gamma ray burst half a second after the event, which may or may not be a coincidence. I can’t imagine something of this magnitude being squeaky “clean”. There is still the question, at least for me, as to the impact from the entropy change. There must be a “splash” in there somewhere, which would help explain a grb. Can this event cause the “foam” to bust lose? How cool would that be?

Don Alexander
2016-Feb-26, 12:25 AM
Why should it not be "clean"? Systems with accretion discs need donor stars in orbit. Furthermore, it seems it has been shown that two merging black holes will eject any surrounding matter before coalescence.

Personally, I strongly doubt the connection to that GRB is real.

StupendousMan
2016-Feb-26, 01:14 AM
Personally, I strongly doubt the connection to that GRB is real.

+1

Staticman
2016-Feb-26, 11:18 AM
Personally, I strongly doubt the connection to that GRB is real.

Have you considered the possibility of the gamma-ray photons from the GRB producing a signal in the interferometer(shedding its momentum to the mirrors and inducing the chirp)?

antoniseb
2016-Feb-26, 11:50 AM
Have you considered the possibility of the gamma-ray photons from the GRB producing a signal in the interferometer(shedding its momentum to the mirrors and inducing the chirp)?
If that were possible, why hasn't ALIGO (& LIGO before it) seen this sort of thing from the many much more intense GRBs? As to it being possible, those gamma-ray photons don't make it through the atmosphere, so they can't get to a place where they could do what you suggest, and there aren't enough of them.

George
2016-Feb-26, 02:44 PM
Why should it not be "clean"? Systems with accretion discs need donor stars in orbit. Furthermore, it seems it has been shown that two merging black holes will eject any surrounding matter before coalescence. Maybe, but were pulsars supposed to have planets? I certainly am not qualified to say the modeling is oversimplified, especially given the likely sophistication needed to make such a model, but I would enjoy hearing about how the entropy change takes place as that, for me at least, is an interesting puzzle. If the surface area of the E.H. does represent the total entropy, and this surface area is greatly reduced with the merger, then how does the off-setting entropy increase manifest itself? If my refrigerator suddenly was (were?) to blast freeze everything inside, it would sure heat-up the kitchen!


Personally, I strongly doubt the connection to that GRB is real.21357 ???

Don Alexander
2016-Feb-26, 05:24 PM
May I refer to: http://arxiv.org/abs/1602.07352

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.

Staticman
2016-Feb-26, 08:17 PM
If that were possible, why hasn't ALIGO (& LIGO before it) seen this sort of thing from the many much more intense GRBs?
The progress in terms of sensitivity from Ligo to Advanced Ligo has been remarkable, so it is reasonable to think that if detection of high energy EM waves is sensitivity dependent it wouldn't have been possible in the first decade of observation to detect them. On the other hand in the remote case the signal detected on september was actually from a GRB it was only the first few days of the engineering run, so it didn't have much time and there aren't so many GRBs in a few days coming the Earth's way. According to rumors there have been more signals detected after that first one in this first run.


As to it being possible, those gamma-ray photons don't make it through the atmosphere, so they can't get to a place where they could do what you suggest,
and there aren't enough of them.Surely, most gamma rays are absorbed by the atmosphere but the barrier is not perfect so you'll concede some photons may reach the surface, then a model of just what intensity of radiation is enough to produce the vibration of the mirrors that would be necessary to produce a signal of the form detected.

Please note that I'm suggesting this putative source of signals as a very remote possibility but one that is neverthelees worth considering to avoid later chagrin and discredit of science. If that remote possibility ever materialized we would all know in time and it would still be an amazing engineering feat in terms of sensitivity of an instrument.

My internet conversations with aLIGO workers lead me to think they haven't considered this possibility(maybe rightly so if there are really no theoretical grounds on which to consider it possible but I would like to hear some solid science ). At the very least there are no sensors in the interferometer dedicated to this particular source of noise.

antoniseb
2016-Feb-26, 08:25 PM
T... most gamma rays are absorbed by the atmosphere but the barrier is not perfect so you'll concede some photons may reach the surface...
No. They don't reach the surface, not even close, not even the highest mountaintops.

Staticman
2016-Feb-26, 08:48 PM
No. They don't reach the surface, not even close, not even the highest mountaintops.
In science it is not so often that one can be so categorical, can you direct me to some reference?
What would you consider the lowest frequency at wich EM waves can reach the surface?

antoniseb
2016-Feb-26, 08:55 PM
What would you consider the lowest frequency at wich EM waves can reach the surface?
Some long wave radio band, but I'm sure that isn't what you're asking about. You probably meant what is the highest frequency (shortest wavelength). That would be somewhere in the ultraviolet range that we recommend people avoid so they don't damage their skin.

Staticman
2016-Feb-26, 09:02 PM
Some long wave radio band, but I'm sure that isn't what you're asking about. You probably meant what is the highest frequency (shortest wavelength). That would be somewhere in the ultraviolet range that we recommend people avoid so they don't damage their skin.

Oops, yeah that's what I mean. But I take that range to be the one in wich significant amounts of radiation can reach us, enough to be potentially harmful. But there must be some threshold in higher frequencies. One thing is that we may find it impossible to detect and another that there isn't none.

Amber Robot
2016-Feb-26, 09:11 PM
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.

Don Alexander
2016-Feb-26, 09:53 PM
@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?

George
2016-Feb-26, 09:57 PM
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]

Staticman
2016-Feb-26, 10:30 PM
@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.

Staticman
2016-Feb-26, 10:39 PM
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.

StupendousMan
2016-Feb-27, 03:49 AM
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.

Staticman
2016-Feb-27, 08:00 PM
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?

StupendousMan
2016-Feb-27, 08:55 PM
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.

Solfe
2016-Feb-28, 05:08 AM
I thought observation satellites cheated and looked for the characteristic double flash of a nuclear explosion.

selvaarchi
2016-Feb-28, 11:14 AM
The prime minister of India has announced that India will build a Laser Interferometer Gravitational-Wave Observatory (LIGO).

http://timesofindia.indiatimes.com/home/science/India-to-establish-lab-to-study-gravitational-waves-Narendra-Modi/articleshow/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.:clap:

"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.

Cougar
2016-Feb-28, 12:47 PM
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.

George
2016-Feb-28, 06:31 PM
Here (http://cds.cern.ch/record/549082/files/0204081.pdf) 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?

Squink
2016-Feb-28, 06:35 PM
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.

antoniseb
2016-Feb-29, 12:51 PM
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.

Cougar
2016-Feb-29, 02:00 PM
Here (http://cds.cern.ch/record/549082/files/0204081.pdf) 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....

George
2016-Feb-29, 03:02 PM
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.

George
2016-Feb-29, 06:10 PM
Some additional fodder...

Recall Hawking's great one-liner, "There are no black holes." Article here (http://www.nature.com/news/stephen-hawking-there-are-no-black-holes-1.14583?wafflebotCursorId=1390650685363202:0:0). 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?

Staticman
2016-Feb-29, 07:25 PM
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).

selvaarchi
2016-May-04, 10:04 PM
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-china-eye-up-european-gravitational-wave-mission-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.

selvaarchi
2018-Aug-02, 06:59 AM
"China university to build simulated gravitational wave observatory"

http://www.xinhuanet.com/english/2018-07/31/c_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.

Roger E. Moore
2018-Aug-08, 05:28 PM
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.