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To this day I believe black holes remain a mostly theoretical phenomena. Which also happens to deal with infinity-mathematics in the singularity, something we d

To this day I believe black holes remain a mostly theoretical phenomena. Which also happens to deal with infinity-mathematics in the singularity, something we don't see in reality otherwise. Yes, observations have been made to their advantage, through LIGO and more recently the image from 2022 of a black hole made by the Event Horizon Telescope. But I would stress that the ""image"" is much more complex than simply a photo from really good ""telescope"", it's a image made of combined data from radio telescopes all around the world. There is enough weird hoops that they need jump through that the observation can reasonably be doubted in my opinion, which isn't to say that what they've attempted is awesome and inspiring. Please correct me if you have other information.

empath75 2y ago
They've been detected with gravity waves, radio telescopes and optical telescopes and explain a wide variety of phenomena in the center of galaxies. What evidence would you require to confirm that they exist?
- pdonis 2y ago
  The objects we have detected look like classical GR black holes, yes. The problem is that, theoretically, there are other possible objects that look like classical GR black holes from the outside for a very, very long time, but still aren't classical GR black holes. We have no direct way of telling from observation whether the objects we have detected really are classical GR black holes, or whether they are one of the other possibilities.
pdonis 2y ago
> black holes remain a mostly theoretical phenomena The fact that there are compact objects in our universe that, from the outside, look like the black holes that GR predicts, is not "theoretical", it's observation. The theoretical question is whether these objects really are classical GR black holes "all the way down", so to speak, or whether there is some new physics that comes into play deep inside them (which many physicists think will involve some form of quantum gravity) that changes things. Even if the latter is the case, the objects might still look like classical GR black holes from the outside for a very, very long time, on the order of the Hawking evaporation time (10^67 years for a one solar mass hole, and the time goes up as the cube of the mass). So for all practical purposes, treating them as classical GR black holes works fine now and for the foreseeable future.
cedws 2y ago
The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected?
- privong 2y ago
  > The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected? The images of the EHT data were made using several independent techniques performed by different sub-teams who largely worked independently (e.g., [0] for M87). Most of the imaging techniques used are not machine learning approaches, but are rather standard imaging approaches, some of which have have been used for decades in radio astronomy (e.g., CLEAN). The fact that the images made from these different techniques look similar is what lends support to the idea that the "photo" is a reasonable representation. Sure, some images have been created using machine learning approaches, but other images have been done using well vetted approaches that are not machine learning. But if one doesn't want to trust the imaging techniques, the size of the black hole shadow can be estimated directly from the radio telescope measurements. These are the "visibilities" (essentially measurements of components of the Fourier Transform of the distribution of radio emission on the sky) and the amplitudes of the visibilities (e.g., figure 2 of [1]) indicates that the brightness distribution of the source is approximately a thin ring, with the ring diameter setting the baseline length at which the amplitudes have their minimum. So the basic result about the black hole shadow can be made from the visibilities themselves, without relying on the imaging. But the images of course make it easier to see some of the complexity (e.g., that the ring around M87 is not axisymmetric). [0] https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...4E/abstract https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...4E/abstra... [1] https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...1E/abstract https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...1E/abstra...

empath75 2y ago

They've been detected with gravity waves, radio telescopes and optical telescopes and explain a wide variety of phenomena in the center of galaxies. What evidence would you require to confirm that they exist?

pdonis 2y ago
The objects we have detected look like classical GR black holes, yes. The problem is that, theoretically, there are other possible objects that look like classical GR black holes from the outside for a very, very long time, but still aren't classical GR black holes. We have no direct way of telling from observation whether the objects we have detected really are classical GR black holes, or whether they are one of the other possibilities.

pdonis 2y ago

> black holes remain a mostly theoretical phenomena The fact that there are compact objects in our universe that, from the outside, look like the black holes that GR predicts, is not "theoretical", it's observation. The theoretical question is whether these objects really are classical GR black holes "all the way down", so to speak, or whether there is some new physics that comes into play deep inside them (which many physicists think will involve some form of quantum gravity) that changes things. Even if the latter is the case, the objects might still look like classical GR black holes from the outside for a very, very long time, on the order of the Hawking evaporation time (10^67 years for a one solar mass hole, and the time goes up as the cube of the mass). So for all practical purposes, treating them as classical GR black holes works fine now and for the foreseeable future.

cedws 2y ago

The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected?

privong 2y ago
> The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected? The images of the EHT data were made using several independent techniques performed by different sub-teams who largely worked independently (e.g., [0] for M87). Most of the imaging techniques used are not machine learning approaches, but are rather standard imaging approaches, some of which have have been used for decades in radio astronomy (e.g., CLEAN). The fact that the images made from these different techniques look similar is what lends support to the idea that the "photo" is a reasonable representation. Sure, some images have been created using machine learning approaches, but other images have been done using well vetted approaches that are not machine learning. But if one doesn't want to trust the imaging techniques, the size of the black hole shadow can be estimated directly from the radio telescope measurements. These are the "visibilities" (essentially measurements of components of the Fourier Transform of the distribution of radio emission on the sky) and the amplitudes of the visibilities (e.g., figure 2 of [1]) indicates that the brightness distribution of the source is approximately a thin ring, with the ring diameter setting the baseline length at which the amplitudes have their minimum. So the basic result about the black hole shadow can be made from the visibilities themselves, without relying on the imaging. But the images of course make it easier to see some of the complexity (e.g., that the ring around M87 is not axisymmetric). [0] https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...4E/abstract https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...4E/abstra... [1] https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...1E/abstract https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...1E/abstra...