In 2018-2019, I spent a couple of months worth of my free time writing open-source software to simulate what an observer sees as they’re falling into a black hole. It was a lot of fun. I learned some physics I hadn’t known before. (I have a PhD in physics, but my training was in experimental nuclear physics, not general relativity.) When the software was done, I used it to create a video animation, with some very basic text explaining what was going on:
At that time, there were quite a few other pop-sci videos on youtube on the same topic, but although some of them had much more elaborate production values in terms of human entertainment (breathless narration, animated human figures in spacesuits), the science was uniformly just awful.
Recently (November 2022), I took another look around on youtube, and I found that although there is still a lot of awful dreck on the topic, the situation has improved a lot. If you want to learn correct science instead of misconceptions, and at the same time see something with good production values and a good explanation of the physics, you have some good options. Here are my reviews of several of these videos, in order from best to worst.
2021, 4 million views
This is a very nice and scientifically accurate video, much better than most of what one finds on youtube for this topic. The quality of the explanation for laypeople is excellent. It refrains from overblown “gee whiz” stuff, and it accurately describes the current state of knowledge – both what is known and what is uncertain.
My only scientific complaint has to do with the question of whether an observer falling into a black hole sees the entire future evolution of the outside universe before they die. The answer is complicated and uncertain as of 2022, but the video presents it as if the possibility was just a naive popular misconception based on a lack of understanding of the basic physics. It’s always been pretty clear that the see-forever idea does not apply to a non-spinning black hole, but real-life black holes are all spinning, and for them the theoretical situation is not really clear as of 2022. Physicists may want to read the more detailed discussion of this at the end of this article.
2021, 18 thousand views
Strangely enough, youtube’s search algorithms decided to show me this video as its top-ranked choice. I don’t know if that was because of some kind of paid advertising placement. It’s not the best of the lot scientifically, it doesn’t have that many views, and the production values are not that great. They seem to have copied snippets out of a lot of videos by other people, with only minimal credit to them.
Most of the video is fairly accurate, but there are some mistakes.
The visuals mix realistic optical simulations with two-dimensional metaphors.
“…a region of spacetime so dense that even light cannot escape it.” The interior of an astrophysical black hole is hard vacuum.
“Recent gravitational-wave observations have shown that not all black holes are alike. Instead, the universe is littered with a vast zoo of different types of black holes, and a trip to each of them would be an entirely different experience.” The no-hair theorems tell us that this is not true, and nothing in gravitational wave observations has changed this picture. From context, it seems like they mean just that black holes come in radically different sizes, but that was known long before gravitational wave astronomy got going.
“So yes, you would be as dead hundreds of miles even before you hit the singularity [of a solar-mass black hole].” This is a reasonable numerical estimate, but the narration makes it sound as if this is inside the horizon, when in fact it’s outside the horizon.
The discussion of time dilation, Doppler shifts, and the spacelike nature of the singularity are all pretty good. (We don’t actually know whether an astrophysical black hole has a spacelike singularity or a timelike one.)
“It might provide a route to a parallel universe or an opening to the throat of a wormhole.” This is not accurate for a black hole formed by gravitational collapse.
2021, 13 million views
There’s a lot of fun in this video, and a certain amount of correct science. The metaphorical description of the broad structure of general relativity at the beginning was, for example, very nice. But the depiction of the science is a mixed bag, and a very large percentage of it is wrong. Some of it is just imprecise or wrong on technical points, which is not so bad in a video that is aiming for the feel of an LSD trip. But some of it is just grossly incorrect and shows both a lack of expertise and a failure to do careful research from reliable sources.
In the following, I’m interpreting everything up to 7:26 as a description of a non-spinning black hole, because that’s what the video tells us retrospectively at that point. The simplest model of this type of black hole is called a Schwarzschild black hole.
There is a major mistake at 4:30: “Meanwhile, from your perspective the void of the black hole rises up to meet you, as light from fewer directions can reach you. The blackness envelopes you until your only view of the universe you left is a tiny spot of light.” This is completely wrong. Actually, an observer falling into a non-spinning black hole never sees less than 50% of their field of view filled with a view of the outside universe.
4:47 - “Here, inside the event horizon, space and time are so horribly broken that real time travel is possible, so it’s probably a good thing that nothing gets out. If anything could escape it could create all sorts of time travel paradoxes and issues that break the universe.” Also totally wrong. This is written as if a non-spinning black hole contains closed timelike curves (CTCs). That’s just not the case, nor is it currently believed that astrophysical (spinning) black holes contain CTCs. The CTCs that exist in the Kerr spacetime are not something that is believed to exist in black holes formed by astrophysical collapse.
7:00 - A non-spinning black hole’s singularity is described as “… a single point with all the matter that has ever crossed the event horizon, all crushed to a point infinitely small. [There,] curvature becomes infinite, and density becomes infinite.” No, a non-spinning black hole’s singularity is not a crushing singularity, it’s a spaghettifying one. (In technical language, the singularity of a Schwarzschild black hole is not a strong curvature singularity, it’s only one where the Einstein tensor is singular.) It’s also wrong to describe this type of singularity as a point in space. Actually it has spatial extent, in the sense that different observers can fall in and hit different points on it. (In technical terms, their past light cones can become separated from each other, which is the definition of a spacelike singularity.)
There are various other less awful mistakes.
There is a lot of fake quantification of velocities of infalling objects: “…their insides implode, at nearly a quarter of the speed of light;” “infalling matter flies in at half the speed of light;” “spinning 90% of the speed of light.” This is all wrong because general relativity doesn’t allow velocities to be defined at all in these contexts. GR only lets velocities be defined locally and relative to something else. It doesn’t define velocities of distant objects or velocities without relation to another observer or frame of reference.
The cartoon description of a gravity well as a kind of tunnel is really unfortunate in my opinion. It’s at best a metaphor, and it plays into people’s misconception of a black hole as if it were a two-dimensional disc.
“The stronger the gravity, the slower time passes.” This is possibly not so horrible in a popular-level cartoon, but it’s wrong. Gravitational time dilation is related to the gravitational potential, not the gravitational field. It also isn’t defined inside the horizon and can’t be unambiguously separated from kinematic time dilation.
My understanding of the current state of knowledge on the see-forever issue is the following. In models that have a bifurcate Cauchy horizon, the see-forever thing is actually what happens if you pass through the horizon that is topologically connected to the boundary at timelike infinity. Most likely you’re destroyed before that by energetic processes at a mass inflation horizon, and therefore can’t actually see the universe’s whole future history, but this is not known for sure, and the see-forever idea is not just a silly misconception based on neglecting the Doppler effect, as claimed by the ScienceClic narration. For an early presentation of this idea, see Penrose, Battelle Rencontres: 1967 Lectures in Mathematics and Physics, p. 222. Roger Penrose is not just some naive person who doesn’t understand basic physics!
This is one of the reasons why numerical simulations of the interior are so hard: you can’t simulate the interior without simulating the entire future evolution of the exterior.
It’s also possible that Penrose’s diverging blueshift is not actually present in black holes that are formed by gravitational collapse, but based on my limited understanding of the currently literature, that is probably an outmoded attitude. Many people, including me, used to feel very comfortable with the idea that all black hole interiors had a Penrose diagram qualitatively like that of region II of a Schwarzschild black hole. For this type of causal diagram, it’s immediately obvious that there is no see-forever going on. The other, crazy stuff, we fervently hoped, was only an artifact of the unphysical features of the Kerr spacetime, which after all was never thought to be correct in detail as a model of an astrophysical black hole. A fairly popular idea was that the diverging blueshifts were actually signs of a kind of blueshift instability, and that this instability would cause the inner horizon never to form in the first place for collapse from general initial conditions. But that just doesn’t seem to be how things have turned out. Recent work by Dafermos and coworkers seems to make it more plausible that in classical GR there really is a Cauchy horizon inside a spinning black hole formed by gravitational collapse. That’s way more scary and threatening than a tame little spacelike singularity!
Ben Crowell, 2022 Nov. 25