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Comment by claytongulick | Hacker News Reader

Can someone who really knows this stuff clarify my understanding?

I've been given to understand that black holes remain a physics theory that has not been directly observed or proven.

I thought that we had a fair amount of observational evidence that suggests their existence, but no "smoking gun" proof of it, and that these types of "observed a black hole" articles are a little misleading - that in reality, what was observed was much indirect, interpreted data that is consistent with black hole theory, but could also be something else.

Am I off base here?

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The theoretical prediction of black holes dates back to 1916, when Karl Schwarzschild proposed a solution to Einstein's field equations that were part of his general theory of relativity. General relativity is arguably the most robust theory in physics - that being said, general relativity introduces some tension with the other most robust theory in physics, namely quantum mechanics and the Standard Model. These two theories are largely accepted to be accurate and correct, and since they predict different phenomena at different length scales, most scientists have no trouble subscribing to both.

With respect to "direct observation" of a black hole, astronomers at the Event Horizon Telescope successfully reconstructed pictures of a black hole using light in the visible spectrum. But to be fair, this comes after decades of overwhelming experimental evidence of black holes + GR. The EHT result probably received the media attention it did because it is easily interpretable as nice lil JPEG, rather than a 12-page paper with dense plots and LaTeX.

See https://en.wikipedia.org/wiki/Tests_of_general_relativity

I'm familiar with the indirect evidence, and there's a lot of it.

This article [1] seems to affirm my understanding that this is still all indirect evidence until LISA becomes operational.

[1] https://www.science.org/content/article/after-decades-effort...

As far as I know, the existence of black holes is accepted by basically every astrophysicist.

There are several smoking guns:

https://en.wikipedia.org/wiki/Black_hole#Observational_evide...

I’ve read something like: there are bunch of observations of extremely dense masses in various suroundings. Talented people can find explanations other than black holes in each case. But that feels adhoc, and black hole fits quite nicely for whole class of observations. Current consensus is that b holes exist.

> Current consensus is that b holes exist.

I don't think it works like that?

My read from looking at the literature is that the feeling is more like "black holes are a mathematical theory that have not been falsified by observational evidence, and for which we have some compelling, but inconclusive data, reconstructed from intensely noisy and poor resolution sources. Seems likely, but more data needed."

If you're sufficiently cynical, you can describe almost all astronomy results that way. Lots of alternate explanations for the observations have been proposed, and fail to match the full evidence.

Just how much evidence are you going to demand of phenomena hundreds of lightyears away before you say, "yeah, that's probably what it is"? Anyway, do be sure to read the whole list of observations linked above by csours, as there are quite a few.

Scientist recently got a good picture of one

See, this is my beef with the reporting on this stuff and the source of my question.

The "photo" you're referring to was constructed by a super computer massaging enormous amounts of data to fit a model and constructing an image from weighted averages of thousands of data images across the spectrum from an enormously complex array of detectors in an unfathomably noisy environment [1].

It was presented by the media as a "photo", but it seems to me like it's more similar to the "image" we get from reconstruction of wifi signals in a room to determine interior object's shape.

Or maybe a better example would be the "photos" of atoms, which are not photos at all but a reconstruction of miniscule electromagnetic resistance readings from a STM whisker driven by a piezoelectric crystal.

I think we're supposed to get the ability to have better "photos" with the activation of LISA in 2037 [2] but until then, claiming that we have photos of black holes seems a bit misleading (not by you, I mean the science reporting).

[1] https://eventhorizontelescope.org/blog/astronomers-reveal-fi...

[2] https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...

LISA is a scaled-up version of LIGO, and won't produce "photos" any more than LIGO does. Of course, LIGO has provided extremely good evidence that black holes exist (most of what it detects are the gravitational waves from two black holes merging), but if you don't buy that, then you won't buy LISA's observations, either.

> but if you don't buy that, then you won't buy LISA's observations, either.

Because science is about "buying" it?

Yes, in the real world. Even if it wasn't, you've made this conversation about you personally "buying" it, so yeah.

> you've made this conversation about you personally "buying" it, so yeah

When?

> It was presented by the media as a "photo", but it seems to me like it's more similar to the "image" we get from reconstruction of wifi signals

Are you suggesting that emitting or reflecting visible radiation is required before we can prove something exists in objective reality? Your own point about Wifi signals proves you wrong - we can detect wifi in other wavelengths and thus learn a great deal about it. Just like we can with black holes.

There is no distinction between "images" and "photos" - they're both electromagnetic radiation converted to a visual medium. There's nothing unique about radiation in the 380 to 700 nanometers wavelength range that makes it somehow more scientifically valid for remote sensing.

Did you read the methodology that was used to come up with the "photo"?

It's a far cry from just shifting the spectrum a bit to the visible range.

Any digital or analog photo is a far cry from our biological signal processing.

Yes, their methodogy used lots of signal manipulation to produce the photo. Just like an RGB digital camera does. Just like analog film. Just like our rods, cones and visual cortex. Which one is the "correct" way to process the signal?

What you're claiming is that "photos" have privileged scientific status over other types of sensors/processing hardware. Someone else might claim that only biological sensors have privileged status. But neither claim is scientific since it cannot be verified.

In lieu of a testable hypothesis, we have to conclude that all frequencies of electromagnetic radiation and all mathematically-valid signal processing techniques are fair game for empirical observation.

I think you are confusing Black Holes with Hawking Radiation . The later's status is basically exactly as you describe.

Einstein didn't think black holes existed (1916) and it took until 1971 with Cygnus X-1 for serious consideration, which was the smoking gun. 55 years of being considered probably a math fluke and then observational evidence.

There's been more than one picture of one

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