# Can we make any implications about the internal structure of black holes from the 'chirp' that LIGO observed?

Are there any implications about a black hole's internal structure we can make based on the chirp issued? For example that a black hole does in fact contain or does not contain a singularity rather than a super-dense form of matter/energy that we can't otherwise discover anything about due to the event horizon and information paradox?

• I don't think you can even prove that black holes exist with LIGO experiment results, so I don't think you can say much about their structure from such results. – rmhleo Feb 21 '16 at 9:04
• and this is a duplicate of 20 questions just for the last 2 weeks. – Fabrice NEYRET Feb 21 '16 at 9:44
• I did have a look and saw similar questions, but nothing that asked about the implications from the data rather than currently understood theories. – Alex Feb 21 '16 at 10:26
• @rmhleo yes you can. The chirp matched the results of merging black holes to 5$\sigma$. If that didn't prove again the existence of BHs, I don't know what would. – Kyle Kanos Feb 21 '16 at 11:55
• @rmhleo The LIGO event is consistent with the gravitational inspiral of two compact bodies. Parameters of the signal give the masses of the bodies and the distance between their centers of mass when the signal switches from inspiral to ringdown. That distance is consistent with black holes and not with other theorized objects. Leaving aside the dicey word "prove" the signal does constrain our understanding of the world. – dmckee Feb 21 '16 at 16:14

Are there any implications about a black hole's internal structure we can make based on the chirp issued?

No. The chirp data is consistent with the waves generated by the spacetime outside of two merging black holes. Therefore if there are two balck holes, we'd get that data. Similarly if there were two compact and time dilated bodies such that the spacetime outside them were very very similar to the spacetime outside a black hole, then we'd get the same data (no data is perfect, so if the compact and time dilated body had a spacetime outside that was similar enough we'd get literally the same data).

For example that a black hole does in fact contain or does not contain a singularity rather than a super-dense form of matter/energy that we can't otherwise discover anything about due to the event horizon and information paradox?

Waves don't come from inside the horizon, so we don't learn anything about the inside of the horizon. We don't even learn whether there is an inside to the horizon, let alone what, if anything, might be going on inside. We learn about the outside.

We learn that the outside is similar to the outside of a balck hole. So the outsides of compact bodies are very similar (or possibly identical) to the outsides of black holes. And produce waves like the outsides of black holes produce.

• Ah thanks - it hadn't occurred to me that the waves would be generated outside of the event horizon - so still no violation of the information paradox. This seems to answer my question, thank you. – Alex Feb 21 '16 at 18:08
• @Alex, but be absolutely clear, there are no other possible compact objects that have masses 30 times that of the Sun. – Rob Jeffries Feb 21 '16 at 19:08
• @RobJeffries An eternal black hole of 30 solar masses, and a compact time dilated remnant of a larger star are technically different objects. The latter exists and the spacetime outside it is very very similar to the spacetime outside the former. And when someone talks about the latter, that's what they mean when they say astrophysical black hole, even when they model it with the former. They are different. But the differences are often unimportant. – Timaeus Feb 21 '16 at 19:12
• I'm misunderstanding something. The two objects have 30 solar masses and an orbital period that shrinks to around 1 second before they merge. What other astrophysical objects but black holes can they be? – Rob Jeffries Feb 21 '16 at 19:34
• Ok, your comment to Kyle explains. They're black holes by another name. – Rob Jeffries Feb 21 '16 at 19:37

Gravitational waves are produced outside the black hole event horizon.

Background One of the ways LIGO conducts their search for gravitational waves is by using templates, so they match the known waveforms coming from binary black hole (or other compact object) systems with what is expected from theory.

The story After they made the initial estimates for the parameters, they ran a supercomputer simulation (which shows pretty well where the gravitational waves are generated) to verify the results.

What they can theoretically infer:

• Lower limit for the mass of the graviton
• Mass of the binary black holes
• Spin
• Future observations: Velocity of gravitational waves (speed of light, fingers crossed)
• There are other things they can infer but I guess these are the main ones for now? – Otto Feb 22 '16 at 2:23
• about the velocity of gravitational waves, as best as i can tell (a mere electrical engineer that, at one time, knew how to design antenna array) that this latest LIGO result can only put a top limit on the wavespeed of gravitational waves at about 140% $c$. when they infer the possible direction in the sky where the source came from, using the 7 ms delay between WA and LA, that was assuming the wavespeed was $c$. – robert bristow-johnson Feb 22 '16 at 4:34
• @robertbristow-johnson Thanks, you're right. I added the velocity there since that's one of their goals, even if it was not accomplished by this detection. – Otto Feb 22 '16 at 5:16
• i believe that, in order to nail the coffin shut on any other wavespeed than $c$, they will need a third LIGO detector somewhere in the world. or somehow integrate these two LIGO detectors with the VIRGO detector in Italy. i think then they will be able to unambiguously infer the wavespeed and the angle of incidence of the wavefront. with two detectors i think they can derive one or the other, but not both independently. – robert bristow-johnson Feb 22 '16 at 5:48
• Ah right, I do not think they will infer it from the angle, the angular resolution even with three detectors I suppose is really bad? A couple of ways to get around that is to use EM and/or neutrino signals to detect known events and compare. Anyway, I didn't ask about the details but I heard the point about measuring speed of gravitational waves from a presentation given by one of the people working in LIGO. – Otto Feb 22 '16 at 7:29