LIGO Observation of Gravitational Waves: Questions Answered

[Andrew Washington]

I am doing a term paper on G. Waves and I have a couple of questions about them.
- How do we know that the G. Waves detected by LIGO on September 14, 2015, come from 1.5 billion light years?
- How is Einstein's Theory related to them?

 

[Nugatory]

What do you already know about Einstein's theory, and what have you already read about the LIGO observation?

 

[Andrew Washington]

Thanks for your quick reply,
I know that Einstein predicted the existence of G. Waves in 1916 and that in his theory space and time are connected. Plus I know that G. Waves are ripples created in the frabric of space-time.
About LIGO I read how they were able to detect G. Waves; What they used to detect them; How G. Waves are produced; The G. Waves spectrum.

 

[Ibix]

Regarding the distance to the source - if you knew how powerful the source was if you were sat right on top of it, could you work out the distance? Is there anything about the signal that would let you work out how powerful the source was?

 

[Andrew Washington]

Well, I know that the G. Waves LIGO detected did not have a strong magnitude, because the detection lasted for 0.2 seconds.
Plus, if I were sitting on the source and if I knew how powerful the source is...I think I could work out the distance.

 

[Ibix]

Sorry - got interrupted and that wasn't clearly phrased. If you know how powerful the emission event was, and you know how much power you picked up, can you figure out how far away you are?

Do you know what affects the power output?

 

[Andrew Washington]

Well, I do not know. What affects the power output?

 

[pervect]

Well, I do not know. What affects the power output?

The power output can be derived from knowing the total energy of the black holes before the merger, the total energy of the black holes after the merger, and the time it takes for the merger to occur.

The total energy of the black holes before the merger can be taken as the sum of the masses of the black holes multiplied by c^2. The total energy of the single black hole after the merger can be taken as the mass of the resultant black hole times c^2. The difference in energies must be radiated away by the merger, the total radiated energy is the radiated power multiplied by the time of the merger.

The time it takes for the merger to occur is measurable directly by observing the gravitational wave signal.

So given initial and final masses of the black hole pair, and the final mass of the black hole pair, one can estimate the power output. There are some other issues, though, which relate to the angular distribution of the radiated power - it's symmetric around the axis of rotation, but not necessarily spherically symmetric - and the relative orientation of the Ligo receivers to the gravitational wave, which affects how efficient Ligo is at receiving the radiated power.

The initial and final masses of the black hole pair are reported in the Ligo papers, but you probably want to ask the question - how were these masses determined? The first step for answering all the questions is to track down the original Ligo paper(s) themselves, and read them. I believe the september 14 observation was the first observed gravitational wave, which is good - you only need to track down and read the papers on the original observation. It will perhaps be useful to track down other papers and commentary, but finding the original source of the Ligo team would be an excellent first step. Have you done this?