LIGO Continues To Make Waves in Gravitational Wave Detection Physics

American Physical Society ^ | June 1, 2017 | Rachel Gaal

[Lonesome in Massachussets]

The Laser Interferometer Gravitational-wave Observatory (LIGO) continues to make waves within the physics community. On June 1, they published their third gravitational wave detection from an observation in early January 2017. This new detection may help vet previously established theories of how these massive mergers behave, and has already provided a new round of observational data to push forward this new kind of astronomy.

The confirmed results, published in Physical Review Letters, tell a story of two black holes that coalesced 3 billion light-years from Earth. So far, this is the farthest source from which gravitational waves have been detected (the first and second detections were located 1.3 and 1.4 billion light-years away, respectively). The black hole formed by the merger has a mass about 49 times that of our sun. This fills in a gap between the masses of the two merged black holes detected previously by LIGO, with solar masses of 62 and 21 times that of our sun.

[Lonesome in Massachussets]

Hadn't heard much about this. I was worried that the initial detection might have been a flash in the pan. It now appears that they are starting to collect statistics.

[IronJack]

Do gravitational waves travel at the speed of light?

[Lonesome in Massachussets]

Yes, gravitational waves travel at the speed of light. At least according to Einstein’s theory of General Relativity. It would be very difficult to test directly. When we had Maxwell’s Equation predicting the speed of light based on measurable parameters of free space and means of direct measurement of the speed of light (stellar aberration, moons of Jupiter, spinning mirrors, and laser interferometry all give similar results) act to reinforce confidence in Maxwell’s predictions.

It would be nice to have a way of measuring the speed of gravity. I believe that the precession of the perihelion of Mercury is confirmation of the speed of gravity, but my command of General Relativity is nowhere near good enough to really understand it.

[InterceptPoint]

Does anyone know how they are measuring the distance to the event, the number of light-years?

It seems to me that they need to know that value in order to calculate the masses of the source of the gravitational wave but the means by which they accomplish the measurement is less than obvious.

[Lonesome in Massachussets]

Short answer: We measure phase history and amplitude of the two signals. Details:

https://physics.stackexchange.com/questions/235579/how-were-the-solar-masses-and-distance-of-the-gw150914-merger-event-calculated-f

[sportutegrl]

If you like to party, the Cal Tech LIGO at Hanford will be having a post eclipse party on August 21, at 6:00 pm. There will be walking tours of the miles long laser. At 6:00 pm, Professor Weiss will give a lecture on Einstein and eclipse. At 7:30 ANOTHER lecture! I am so there. Tickets are free. Let’s geek out.

[Lonesome in Massachussets]

I have a date in Illinois that afternoon :(

[InterceptPoint]

Much thanks for your response.

While the explanation is a bit over my head it is refreshing to learn that the question I posed was an importantant one, that it had obviously been addressed by the LIGO folks and that the methods they developed are well documented in the technical literature.

Once again, thanks for your response.

[Lonesome in Massachussets]

My question has always been, how does the LIGO receiver take energy from the incident gravitational wave? I understand how a radio receiver matched in impedance absorbs energy from a radio wave, but how does the LIGO absorb energy? What is the equivalent of impedance matching?

[IronJack]

If that's the case, then if the sun suddenly vanished, the earth would continue to orbit as usual for about 8.3 minutes, until the absence of gravity was felt 93 million miles from the event.

I thought Einstein suggested that gravity wasn't a wave at all, but a distortion in space-time. But my understanding of General Relativity is probably worse than yours.

[Lonesome in Massachussets]

General relativity does not accommodate the possibility that the sun would just disappear. The sun’s gravitational field is tied to the sun’s mass. If the sun “just disappeared” I suppose that you would have ripples in time-space and the effects would take about that long to reach earth. But remember, if the sun “just disappeared” it would take that long for it to go dark, starting at the center and propagating out to the edges. It takes light from the edge of the sun about 2.3 seconds to reach earth than light from the middle, as seen from earth.

[InterceptPoint]

How does the LIGO absorb energy?
++++
I’m guessing. My background is microwave engineering so I understand amplitude detection and phase measurement.

But light is just very, very high frequency electromagnetic energy propagating through space at the speed of light. Same as microwave energy. So the same principles apply.

In the microwave case you would add an adjustable phase shift element to one side of interferometer and then form the sum and difference of the signals from the two legs. The you just adjust the variable phase element to null the difference and maximize the sum and use a detector to make an amplitude measurement. Phase information is obtained from the reading of the variable phase element.

The above works fine for a single sinusoid in the microwave world. The gravity wave detection problem is much more complex since you are looking for a relatively low frequency perturbation of a light wave bouncing around in the LIGO system. And that perturbation has a finite bandwidth. It is not, I assume, a simple sinusoid.

But the basic principle for detection still must apply. You are using detectors that measure the magnitude of LIGO lasers signal amplitudes in both arms and, I’m betting, lots of computing power to add in the equivalent of my microwave phase shifter across the frequency range of the gravitational wave.

Or something like that.

[Garth Tater]

It now appears that they are starting to collect statistics.

or maybe not:

A CRITICAL ANALYSIS OF LIGO'S RECENT DETECTION OF GRAVITATIONAL WAVES CAUSED BY MERGING BLACK HOLES - Stephen J. Crothers

Take away from the article (emphases mine):

"Signal GW150914 was extracted from a database containing tens of thousands of numerically determined waveforms generated on the false assumptions of the existence of black holes and gravitational waves. A 'generic' signal cGW was initially reported by LIGO , after which powerful computers extracted GW150914 from the waveform data base for a best fit element."


“The initial detection was made by low-latency searches for generic gravitational-wave transients [41] and was reported within three minutes of data acquisition [43]. Subsequently, matched-filter analyses that use relativistic models of compact binary waveforms [44] recovered GW150914 as the most significant event from each detector for the observations reported here." Abbott et al. [1]

" With such powerful computing resources and so many degrees of freedom it is possible to best fit just about any LIGO instability with an element of its numerically determined waveform database. This is indeed the outcome for the LIGO-Virgo Collaborations, as they have managed to best fit a numerically determined waveform for and to entities that not only do not exist, but are not even consistent with General Relativity itself. This amplifies the futility of applying numerical and perturbation methods to ill-posed problems."

"There are no known Einstein field equations for two or more masses and hence no known solutions thereto. There is no existence theorem by which it can even be asserted that Einstein's field equations contain latent capability for describing configurations of two or more masses [4,23,24,47]. General Relativity cannot account for the simple experimental fact that two fixed suspended masses approach one another upon release. It is for precisely these reasons that all the Big Bang models treat the universe as a single mass, an ideal indivisible fluid of uniform macroscopic density and pressure that permeates the entire universe."

[SuperLuminal]

Aha! Just as I thought...

Now, lets get back to detecting the ether...

[Garth Tater]

Been there, done that.

We had two bags of grass, seventy-five pellets of mescaline, five sheets of high-powered blotter acid, a salt shaker half-full of cocaine and a whole galaxy of multicolored uppers, downers, screamers, laughers... also a quart of tequila, a quart of rum, a case of Budweiser, a pint of raw ether, and two dozen amyls. But the only thing that worried me was the ether. There is nothing more irresponsible and depraved than a man in the depths of an ether binge, and I knew we would be getting into that rotten stuff sooner or later.

     Hunter S. Thompson - Fear and Loathing in Las Vegas