During NASA’s Black Hole Week I saw a lot of social media posts, press releases, videos etc. that were not really correct.
One big issue with science communication about black holes is that while it has gotten good at dispelling the trivial myths (like “black holes suck everything into them and so you should be afraid Sgr A* will kill us all”) it has perpetuated other myths that require more detailed knowledge of general relativity and astronomy to debunk.
I thought it would be interesting to go over some of these misconceptions…
Seems like a good idea to reblog this post in case some astronomy social media managers say stuff freezes when it falls into a black hole again.
As far as the actual content of the original post, some things I would like to add:
A new misconception:
Myth: Hawking radiation comes from the event horizon of the black hole.
Reality: Hawking radiation comes from all the space-time around the black hole.
Hawking radiation is a very weak radiation all black holes are predicted to emit, and shrink down to nothing in the process. It is too weak and slow for us to detect for black holes of astrophysical size (i.e. those with the mass of stars) but if really tiny black holes exist, they could evaporate due to the radiation on timescales short enough for us to observe.
A common description of Hawking radiation in popularizations is that of matter and antimatter pairs of particles forming at the event horizon of the black hole, and one particle being sucked into the black hole and the other flying off into space as radiation. From this, one would expect the radiation to come from the event horizon, or very slightly above it.
In fact, Hawking radiation comes from a large region around the black hole, and it has the same wavelength as the size of the black hole itself. It’s like the black hole is surrounded in a diffuse bath of radiation that cannot be localized to it. This is because the particle pair description of Hawking radiation, while intuitive…is not how it actually is modeled or calculated in physics.
The actual process is too complicated for me to do calculations with (I’m a lowly astronomy & astrophysics PhD student, not a string theorist) but in general, space-time is filled with things called quantum fields. These are essentially more complex versions of stuff like the magnetic and electric fields (”classical fields”) that are more familiar. Their oscillations can appear to us as particles. When a black hole is present, it alters the possible ways the quantum fields can oscillate, like the presence of a hole in a drumskin changes the sounds the drumskin makes when you beat it. We see the new modes of oscillation as new particles.
…this is of course just an analogy, but it at least gives some idea of the physics involved.
Other notes:
-The field is leaning more and more to the idea that AGN feedback plays a major role in the quenching of star formation, at least for large spiral galaxies and the giant elliptical galaxies. That is, some form of “black holes kill galaxies” is looking more plausible as the years go by. Some form of AGN feedback appears to be necessary to get models of galaxy formation to work.
-Supermassive black holes may show jets up to higher luminosities (and so likely higher accretion rates) for their size than stellar black holes do. So the idea that quasars without visible jets lack them entirely is definitely not something that is proven at this time.
-Literally anything that has to do with what you would actually observe when you fall into a realistic astrophysical black hole that is rotating and accreting is full of disagreements between scientists. Other than that “you would die.” Remember that. “You would die.”
Black holes are hard to find. Like, really hard to find. They are objects with such strong gravity that light can’t escape them, so we have to rely on clues from their surroundings to find them.
When a star weighing more than 20 times the Sun runs out of fuel, it collapses into a black hole. Scientists estimate that there are tens of millions of these black holes dotted around the Milky Way, but so far we’ve only identified a few dozen. Most of those are found with a star, each circling around the other. Another name for this kind of pair is a binary system.That’s because under the right circumstances material from the star can interact with the black hole, revealing its presence.
If the star and black hole orbit close enough, the black hole can pull material off of its stellar companion! As the material swirls toward the black hole, it forms a flat ring called an accretion disk. The disk gets very hot and can flare, causing bright bursts of light.
V404 Cygni, depicted above, is a binary system where a star slightly smaller than the Sun orbits a black hole 10 times its mass in just 6.5 days. The black hole distorts the shape of the star and pulls material from its surface. In 2015, V404 Cygni came out of a 25-year slumber, erupting in X-rays that were initially detected by our Swift satellite. In fact, V404 Cygni erupts every couple of decades, perhaps driven by a build-up of material in the outer parts of the accretion disk that eventually rush in.
In other cases, the black hole’s companion is a giant star with a strong stellar wind. This is like our Sun’s solar wind, but even more powerful. As material rushes out from the companion star, some of it is captured by the black hole’s gravity, forming an accretion disk.
A famous example of a black hole powered by the wind of its companion is Cygnus X-1. In fact, it was the first object to be widely accepted as a black hole! Recent observations estimate that the black hole’s mass could be as much as 20 times that of our Sun. And its stellar companion is no slouch, either. It weighs in at about 40 times the Sun.
We know our galaxy is peppered with black holes of many sizes with an array of stellar partners, but we’ve only found a small fraction of them so far. Scientists will keep studying the skies to add to our black hole menagerie.
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