Are black holes actually holes?

[weirdoguy]

perhaps you didn't look at the Staff Directory

Perhaps you didn't look at the PF rules concerning acceptable sources.

 

[mfb]

jbriggs444 , perhaps you didn't look at the Staff Directory
https://www.nsf.gov/staff/staff_list.jsp?orgId=496&subDiv=y&org=NSF&from_org=

What is your point?
Popular science descriptions are always simplified, can often be misleading and they are never a good source for actual science, no matter who writes them.

 

[FactChecker]

On the lighter side:
The first "photo" of a black hole was recently (April, 2019) taken. Here is an interesting TEDx talk by Katie Bouman on the problems with taking a photo. And here is a cute (IMHO) photo of her reaction to first seeing the "photo".

 

[PAllen]

Funny that just today I was wondering about how to address the intuition that a BH has a density when it really doesn't (at least per GR) - it is mostly vacuum, and suffered a topology change making volume not meaningful, and you can basically get almost any volume you want depending on how you slice it. I came up with the following:

For a collapsed body, the state you currently see from the outside is of the body epsilon before the surface passed into the horizon. This will always be true, with ever shrinking epsilon (and a very generous definition of 'see'). This past state you currently see does have a meaningful volume and density (it is coordinate dependent, but there are defensible 'reasonable coordinates'). However it is not simply using the Schwarzschild radius in a Euclidean geometry sphere formula - the interior geometry is not at all flat, and is given by some complex Ricci curvature along with Weyl curvature. However, to rough order of magnitude, the simple approach would work ok, giving you a sense of how low the density of a supermassive BH would have been in the past, if it formed all at once. Of course, supermassive BH do not form all at once, so a low density state just before horizon crossing has never existed for them.

In this limited past (or hypothetical past for the supermassive case) sense you can give some meaning to density intuitions. You are answering: if this event horizon I detect evidence of, were the result of a monolithic collapse leading to that horizon state (assuming truth of no hair theorem), then the density just before horizon crossing would be such and such.

 

[mfb]

Do we need to make it so complicated? Make a very large box around the black hole. Consider how much volume it would have without the black hole, consider how much accessible volume it has with the black hole. Subtract. It gives a volume of the order of the Schwarzschild radius cubed. Not sure how to define the accessible volume for rotating black holes, but it gives at most a numerical prefactor.

 

[PAllen]

Do we need to make it so complicated? Make a very large box around the black hole. Consider how much volume it would have without the black hole, consider how much accessible volume it has with the black hole. Subtract. It gives a volume of the order of the Schwarzschild radius cubed. Not sure how to define the accessible volume for rotating black holes, but it gives at most a numerical prefactor.

The problem I have with this is that the volume of box with the BH, treated with spacelike slices, can be infinite for perfectly reasonable slices. So you are really doing infinite - infinite equals some finite value.

Maybe my argument seems complicated, but it sidesteps this problem by going to the past, before the interior topology change.

 

[Dragrath]

The problem I have with this is that the volume of box with the BH, treated with spacelike slices, can be infinite for perfectly reasonable slices. So you are really doing infinite - infinite equals some finite value.

Maybe my argument seems complicated, but it sidesteps this problem by going to the past, before the interior topology change.

Honestly I think sidesteping the sigularity is missing the point as the emergence of the singularity within the event horizon is considered one of the reasons we "know" GR can't be the final say as it is generally accepted that the emergence of infinities are evidence of the break down of a theory. Perhaps more importantly because no observations can emerge from within a black hole's event horizon any interpretation of what, if anything, exists inside a black hole is inheretly outside the scope of science as it can not be falsified. With the observations of M87*'s photon sphere, the distance at which light orbits the black hole and thus the closest we could ever hope to see light to a black hole, and gravitational waves produced by two black holes merging we can say that fudamentally unless indicated by some new theory of everything these observations rule out anything not observationally indistinguishable from a black hole. The question of if a Black hole is actually a hole is so far outside the fields of science.

 

[Drakkith]

The question of if a Black hole is actually a hole is so far outside the fields of science.

Science may not be able to answer every detail of what a black hole is, but we can certainly put many constraints on it, and I believe that we can confidently say that a black hole is not a 'hole' in the normal sense of the word.

 

[Dragrath]

Science may not be able to answer every detail of what a black hole is, but we can certainly put many constraints on it, and I believe that we can confidently say that a black hole is not a 'hole' in the normal sense of the word.

True in the familiar 3D sense at the very minimum so I guess I can agree with that but the main point I had wanted to convey was the limitations of science as much of the conversation seemed focused on the interior of a black hole. We have lots of untested hypothesizes out there and I am leery about supporting any of them acknowledging the lack of observational tests. As there is a long history of wild theorizing disconnected from observations.