Graduate Rumors of Gravitational Wave Inspiral at Advanced LIGO | Sept 2015 Launch

[bcrowell]

It probably kills most any approache that suggests the QG effects diverge from classical GR outside the horizon.

Interesting point. My understanding is that the people saying this kind of thing were working within semiclassical gravity, and they had to renormalize the results of their calculations. It seems to me that once you give yourself the power to arbitrarily renormalize away any effect you feel like getting rid of, you immunize your theory against this kind of straightforward falsification by observation. If they had previously predicted some big effect at or outside the event horizon, now they can probably just say, "Oh, we'll make that go away by subtracting out a certain term from our equations."

 

[PAllen]

Interesting point. My understanding is that the people saying this kind of thing were working within semiclassical gravity, and they had to renormalize the results of their calculations. It seems to me that once you give yourself the power to arbitrarily renormalize away any effect you feel like getting rid of, you immunize your theory against this kind of straightforward falsification by observation. If they had previously predicted some big effect at or outside the event horizon, now they can probably just say, "Oh, we'll make that go away by subtracting out a certain term from our equations."

Yes, but if the claim to distinction of such approach was what it said outside the horizon, it loses that. Unless it has some other point of interest, even its authors might not bother with it anymore.

 

[OrionX76]

Curious if there are other highly sensitive experiments that see an effect from this event. (Such as gama ray detectors or dark matter searches) It would be interesting to take the know arrival time and look for blips in the data...
Or is this effect so weak as to make this search pointless?

 

[marcus]

Dennis Overbye in NYT, 12 Feb:

http://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html

 

[.Scott]

The event radiated a total of 3 solar masses in gravitational waves.

What portion of the universe's mass is in the form of gravitational waves?

 

[bcrowell]

What portion of the universe's mass is in the form of gravitational waves?

Almost none, and we don't know why. A maximum-entropy big bang, which is the most overwhelmingly likely possibility, would have had its gravitational degrees of freedom equilibrated with all the other degrees of freedom, so primordial gravitational waves would have been extremely strong. Instead, we got a big bang that was low in entropy, mainly because of the almost complete lack of gravitational waves.

 

[mfb]

That's why I said background ("and used this background data to estimate how often stronger signals occur [by background fluctuations]").
The event rate is poorly constrained with a single signal, of course, but we'll know more in a year or once more events are available (whatever happens faster).

Let's see if LISA gets more funding now. The science case certainly got stronger.

What portion of the universe's mass is in the form of gravitational waves?

Cosmic energy inventory
About 30 part in a billion, with a large uncertainty (~factor 3).

 

[Buzz Bloom]

I noticed some odd inconsistencies between among several reports. (Bolding mine.)

From http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave
Observatory simultaneously observed a transient gravitational-wave signal​

From https://en.wikipedia.org/wiki/LIGO#Advanced_LIGO

On September 18, 2015, Advanced LIGO began its first formal science observations at about four times the sensitivity of the initial LIGO interferometers.
​

From https://www.ligo.caltech.edu/news/ligo20150918

On, Friday, September 18th 2015, the first official 'observing run' (O1) of LIGO's advanced detectors in Hanford WA and Livingston LA quietly began when the clock struck 8 a.m.​

I looked on the internet for earlier dates than September 18th for when the advanced LIGO began its search for gravitational waves, but that was the earliest date I could find. It seems that the discovery took place four days before the "official" search began.

 

[bcrowell]

It seems that the discovery took place four days before the search began.

This was discussed at the press conference. They were doing an engineering run, which wasn't supposed to be a physics run. However, the apparatus was functioning as it needed to be in order to detect a real signal, and that happened to be when they got this signal.

 

[gentsagree]

Ahah, nice interpretation of the picture.

Yes, they have claimed today they have indeed observed gravitational waves from one event 1.3 billion light years away. The wave form they recorded matches the theoretical prediction, with increasing amplitude and frequency up to a peak, after which it settles down to a more stable configuration. This is indeed what is expected in the event of two black holes orbiting each other at ever decreasing distance, until they merge.

I have a question, which was partially discussed in other places on the web, for anyone to comment on: can gravitational waves be red-shifted?

It seems they can, as they travel against a gravitational potential, from a region of high gravitational field (low potential) to one with lower gravitational field (higher potential). In particular, as electromagnetic radiation, the rate at which time passes as they travel through the potential increases, thus their frequency decreases.

However, as they travel, they also distort time, so I am wondering if in any case these two effects influence each other.

Does anyone have any insight on this?

Thanks!

 

[mfb]

I have a question, which was partially discussed in other places on the web, for anyone to comment on: can gravitational waves be red-shifted?

They are redshifted in the same way light is. The distance gives a redshift of ~9%. The effect of galactic gravitational potentials is negligible.

 

[Buzz Bloom]

the apparatus was functioning as it needed to be in order to detect a real signal, and that happened to be when they got this signal.

Hi bcrowell:

Thanks for the explanation. I did not get a chance to see the press conference.

Of course we don't know how frequently LIGO will find more gravitational waves in the future, but it seems like wonderful luck they had LIGO on the air at that time. The kind of event LIGO detected may well be quite rare, and it LIGO had had its engineering run a day later, it might have been maybe years before any detection occurred. On the other hand, perhaps LIGO will find more BH pair crashes near daily and be overwhelmed with the need to verify so much data.

Regards,
Buzz

 

[Buzz Bloom]

I have a question about the location of the source galaxy.
From http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102

With only two detectors the source position is primarily determined by the relative arrival Time and localized to an area of approximately 600 deg² (90% credible region).
z = 0.09^+0.03_-0.04.​

If my math is OK, this means that the source can be located with 90% confidence to a region of the sky with an angular diameter of about 14 deg. Combining that with the z value, how many galaxies are candidates for the source?

 

[DiracPool]

What was the eventual fallout from the BICEP2 experiment, btw. Was that totally debunked?

 

[mfb]

I have a question about the location of the source galaxy.
From http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.116.061102

With only two detectors the source position is primarily determined by the relative arrival Time and localized to an area of approximately 600 deg² (90% credible region).
z = 0.09^+0.03_-0.04.​

If my math is OK, this means that the source can be located with 90% confidence to a region of the sky with an angular diameter of about 14 deg. Combining that with the z value, how many galaxies are candidates for the source?

Too many. ~500 million light years uncertainty for the distance, and the distance of ~1.3 billion light years gives ~400 million light years for 14 degrees. So roughly a volume of (400Mly)^3. Tens of millions of galaxies I guess.

What was the eventual fallout from the BICEP2 experiment, btw. Was that totally debunked?

The updated measurement sets an upper limit that excludes the previous value. Gravitational waves could still be there in a sizeable amount, but BICEP2 didn't see them.

 

[fresh_42]

Too many. ~500 million light years uncertainty for the distance, and the distance of ~1.3 billion light years gives ~400 million light years for 14 degrees. So roughly a volume of (400Mly)^3. Tens of millions of galaxies I guess.

But how could they know then the masses of the merging BH and that it was a merger?

 

[PAllen]

But how could they know then the masses of the merging BH and that it was a merger?

From analyzing the waveform of the signal.

 

[Tphysics]

I have a question about how they were able to detect the gravitational wave. They say they have two different sites approx. 4000 km away from one another. If the size of the wave is one tenth of a electron-mass how did both sites detect the wave came through?

 

[Vanadium 50]

If the size of the wave is one tenth of a electron-mass

I don't know what that means, but the wavelength varies from 10,000 miles to about 1000 miles.

 

[Tphysics]

I don't know what that means, but the wavelength varies from 10,000 miles to about 1000 miles.

So the wavelength is so large that the wave might encompass half of earth? So there was a chance per say that only one site could detect the wave because the two sites were thousands of miles apart?

 

[Ibix]

So the wavelength is so large that the wave might encompass half of earth? So there was a chance per say that only one site could detect the wave because the two sites were thousands of miles apart?

No. Think of ripples on a pond. The black holes are the stone dropped into the water; the LIGO detectors are two rocks sticking out of the water some distance away. The wavelength is the spacing between one ripple and the one following it and the strength of the signal is the height of the wave. Whatever the wavelength is, and however tiny the height of the wave is, the wave washes over both rocks.

Gravitational waves are different from water waves in a number of ways, but they will always pass through both detectors.

 

[Tphysics]

No. Think of ripples on a pond. The black holes are the stone dropped into the water; the LIGO detectors are two rocks sticking out of the water some distance away. The wavelength is the spacing between one ripple and the one following it and the strength of the signal is the height of the wave. Whatever the wavelength is, and however tiny the height of the wave is, the wave washes over both rocks.

Gravitational waves are different from water waves in a number of ways, but they will always pass through both detectors.

I guess I was sort of thinking about them as a particle instead of a wave. But duh this makes total sense. Shows how sensitive these machines were.

 

[Vanadium 50]

So there was a chance per say that only one site could detect the wave because the two sites were thousands of miles apart?

Why would you think that? The wavelength for FM radio is about 10 feet, yet people can tune it in all around the city.

 

[sanman]

Once again, I'd like to ask about Gravitational Waves and Dark Matter - would Dark Matter potentially be detectable/identifiable through Gravitational Waves? What sort of criteria would the signal have to meet in order to indicate Dark Matter?

 

[PAllen]

Gravitational lensing is already a major piece of evidence for dark matter. I don't really see any way GW would help unless there were a model of a large mass of dark matter with changing acceleration - totally unlikely. Matter (dark or not) between us and a GW source is invisible to the GW. Short answer: GW will not help with dark matter, but GR has helped a lot via lensing.

 

[sanman]

Gravitational lensing is already a major piece of evidence for dark matter. I don't really see any way GW would help unless there were a model of a large mass of dark matter with changing acceleration - totally unlikely. Matter (dark or not) between us and a GW source is invisible to the GW. Short answer: GW will not help with dark matter, but GR has helped a lot via lensing.

Hmm, so Gravitational Wave observation can only be useful for observing violent cataclysmic phenomena in space? What about for example the rotation of galaxies, for which Dark Matter has been hypothesized as an explanation on why such galaxies don't fly apart? If we could observe some waves from rotating galaxies, this might tell us how much mass/matter is in those galaxies, which we could then cross-reference against visible light observations. Gravitational Waves should at least afford us proper mass measurements of galaxies and other large entities.

I'm even wondering if SETI could use Gravitational Wave observation to look for signs of intelligent life. Perhaps some advanced civilizations use Gravitational Waves for communication, since they pass through everything instead of being absorbed. Perhaps any exotic "FTL" propulsion would likewise generate some telltale Gravitational Wave signature. Perhaps large artificial constructs like Dyson Spheres could also exhibit peculiar characteristics.

What I really see an opportunity for is the further development of Atom Interferometry (and even molecular interferometry) for better detection of Gravitational Waves. They might be able to afford a sensitivity that goes far beyond LIGO. Combine that with "Big Data" analytics and you might be able to parse out very detailed signals on much smaller astrophysical phenomena. Maybe this too could lead to Dark Matter detection.

 

[bcrowell]

If we could observe some waves from rotating galaxies, this might tell us how much mass/matter is in those galaxies, which we could then cross-reference against visible light observations.

This isn't going to work. The rate of radiation of gravitational waves is proportional to the frequency raised to the 6th power. The period of rotation of a galaxy is on the order of 10^8 years, so the frequency is extremely small.

 

[sanman]

Would it at least be possible to artificially generate Gravitational Waves - very tiny ones, obviously - and detect them using a detector like LIGO or like an Atom Interferometer? Even though Man-made Gravitational Waves would be far, far smaller than those from astrophysical phenomena like Black Holes, at least any attempts to measure them would be done at distances far, far smaller than the distant Black Holes that LIGO has been getting signals from. Wouldn't the far lower distance offset the fact of the smaller amplitudes, to make detection feasible? I was just imagining that if Gravitational Waves could be harnessed for communication purposes, then there'd be no need for satellite relays and such. We could have direct communication between Earth and Mars even when both are on opposite sides of the Sun.

 

[PAllen]

Would it at least be possible to artificially generate Gravitational Waves - very tiny ones, obviously - and detect them using a detector like LIGO or like an Atom Interferometer? Even though Man-made Gravitational Waves would be far, far smaller than those from astrophysical phenomena like Black Holes, at least any attempts to measure them would be done at distances far, far smaller than the distant Black Holes that LIGO has been getting signals from. Wouldn't the far lower distance offset the fact of the smaller amplitudes, to make detection feasible? I was just imagining that if Gravitational Waves could be harnessed for communication purposes, then there'd be no need for satellite relays and such. We could have direct communication between Earth and Mars even when both are on opposite sides of the Sun.

I have never heard of a small detector (e.g. atom interferometer) being able to detect GW. Do you have any source for this? So far as I know, the bigger the detector, the better for GW.

There is no currently conceivable method to detect gravitational waves that can be produced locally. Note that the total power output of GW from all sources in the solar system is estimated to be enough to power a few light bulbs - distributed over the volume of the solar system.

You seem to have the syndrome "if you have a hammer everything looks like a nail". This detection is fantastic, and more will come, but GW are not some magic tool that solves a broad range of problems.

 

[sanman]

I have never heard of a small detector (e.g. atom interferometer) being able to detect GW. Do you have any source for this? So far as I know, the bigger the detector, the better for GW.

There is no currently conceivable method to detect gravitational waves that can be produced locally. Note that the total power output of GW from all sources in the solar system is estimated to be enough to power a few light bulbs - distributed over the volume of the solar system.

You seem to have the syndrome "if you have a hammer everything looks like a nail". This detection is fantastic, and more will come, but GW are not some magic tool that solves a broad range of problems.

Since the atom is more massive than the photon, it has a much lower DeBroglie wavelength, thus allowing much greater precision in interferometry. Photons were turned into coherent light long ago with the invention of the laser. The more recent invention of the BEC has led to the "atom laser" and atom interferometry.





The atom interferometer can be a much more compact device than the huge LIGO, and could be launched into space (perhaps to a LaGrange Point). It can have far greater precision/sensitivity, and would also be much less expensive to build.