> According to Albert Einstein’s general theory of relativity, the behavior of a black hole depends on two numbers: how heavy it is, and how fast it is rotating. And that’s it. Black holes are said to have “no hair” — no features that distinguish them from their fellows with the same mass and spin.
As far as I know, there's a third property that black holes have - electric charge. Would a sufficiently strong electric charge between two black holes be detectable, whether they both have the same charge, or opposing charges?
I suppose based on the article, the effects would only take places once the black holes got within 40km of each other...
Black holes can have charge but a charged black hole would attract ions of the opposite charge strongly and would quickly neutralize. So it would be quite difficult to get a non-negligible charge on a black hole.
So some very advanced and bored civilization could take black holes and shoot gigantic numbers of protons into it, and get a very positively charged black hole.
If you pump in enough charge that the electrical repulsion is stronger than the gravity attraction, you can then store them safely next to each other, for when you might need one.
The question of hairy black holes is intimately connected to the greatest puzzle in modern physics: How can general relativity be merged with quantum theory?
Consider the situation where an object crosses a black hole’s point of no return, called the event horizon. According to general relativity, all outsiders will see is how the swallowed object contributes to the two numbers that describe the black hole: how much mass the object adds, and how much faster or slower it makes the black hole rotate.
The idea that anything within the event horizon should be treated as an opaque black box -- could it be reinterpreted as saying that any property which has dependence on spatial or temporal distribution within becomes an unknowable quantity? If so, can it be tied somehow to Quantum Mechanic’s idea of the removal of degrees of freedom by observation, since now certain quantities are not unobserved but rather unobservable in principle? I am asking this from a naive laymen’s point of view, so I may be conflating entirely unrelated ideas.
No. The "no hair theorem" has a condition that says "after a while", but people ignore it because it's a short time. So intermediately after the object entered the black hole will have bump and will emit gravitational waves and after a while it will be a perfect sphere (if it's not rotating, or a similar round elongated shape if rotating).
As far as I can understand, it totally unrelated to Quantum Mechanics.
For one, observers inside the event horizon can still observe each other. From the point of view of an observer falling into a black hole, nothing unusual happens when they cross the event horizon. They still observe stuff falling in with them in the same way as before (assuming a large enough black hole so that the local curvature is negligible).
If Hawking radiation is real, it might also expose information from inside the black hole, possibly solving the information paradox. In that sense, the inside of a black hole would still be “observable” from the outside in the QM sense. But since we don’t know how quantum gravity works, that is an open question.
How can that be true? Inside the event horizon, there is only one direction: down. If light cannot escape, how can it happily bounce around, reflect on something, then reach your eyes? How could signals from your feet (say) even reach your brain, assuming a feet-first entry into the black hole?
When you cross the event horizon, you’re falling at close to the speed of light, relative to the event horizon. But due to relativity, light bouncing off the falling objects still propagates at light speed in all directions relative to the falling objects (including the falling observer). That light can’t move outwards, due to being inside the event horizon, and accelerates towards the singularity, but the observer accelerates to the singularity faster, relative to light being reflected outwards. The falling rest frame accelerates towards the singularity, but within that accelerating rest frame, light behaves normally between objects in that frame (assuming a large enough black hole so that the differences in radial distance between the objects is negligible).
Consider two cars driving on a highway at the same speed. If one of the cars decelerates, it appears to be moving backwards relative to the other car, while still continuing to move forward relative to the road. It’s similar for light and an observer falling towards the singularity. They both fall towards the singularity, but light being reflected backwards will fall a bit slower, thus appearing to move backwards relative to the observer, even though the light still moves forward toward the singularity.
I think the short answer is that no one knows, because a proper answer would need an understanding of how black holes and quantum theory go together, which is not currently known.
I think what is relevant here, is that there is a separation between the inside and the outside of the black hole. Since there is no communication between the two, on the inside it could be anything. And its not just the matter and space, even the laws of physics might be completely different. If it weren't so, then there had to be a "super-space" that communicates how ordinary space behaves on both the outside and the inside the black hole.
This reminds me of something I mull over; Greek style philosophy seems so smart, rational and correct until you actually observe the world. Imo the perfect example is elementary particles - the Greeks (mostly) were convinced that it's impossible for something physical to be elementary (cannot be further split) and logically it is hard to understand why can't we continue to split the particle?! but hey, we can't.
Physical beings which cannot be distinguished from one another would have made their heads spin...
To be fair to the Greeks, thay also came up with the idea of elementary particles. I think it's Democritus who's credited with the idea of "atoms".
It's been ages since I've read about him in any depth but I remember something about him having an idea that because of the indefinite divisibility of length then the elementary particles would have to have a diameter smaller than any given length, i.e. be infinitesimal, so they had quite sophisticated speculations
Aren't quarks and, probably, electrons point-like particles? In that sense the fundamental subatomic particles aren't "physical" in the sense the Greeks would have thought of them, e.g. having a non-0 diameter, and in that way modern physics sort of vindicates their deductive reasoning. Their logic wasn't wrong, per se, it was their starting assumptions that were flawed (or at least incomplete), which is acceptable as far as deductive reasoning goes.
No. Because the GR/QM conflict comes up in the other direction that way. GR predicts infinities for zero-radius particles. Indeed, so too does classical electromagnetism. In any case, "quanta" does not really mean the same as the classical notion of point particles.
An interesting consequence of having "no hair" on them is that information on everything that falls in them is lost forever, which leads us to an information loss paradox. Not nice.
You do not need a black hole for the information loss. Just shout a flashlight into empty space. Any information encoded in photons will be lost from the point of view of the sender.
And then the classical notions like information or entropy are not really compatible with General Relativity. Richard Tolman almost 100 years ago proposed interesting extension to the classical thermodynamics that is compatible and can potentially explain apparent paradox of information loss, but it is not known if that extension matches reality.
> Any information encoded in photons will be lost from the point of view of the sender.
The crucial difference is "from the point of view of the sender". The black hole information paradox is that the information is lost from any possible receiver (even any possible ensemble of receivers surrounding the black hole). It's not the same thing
>The information is not lost from the point of view of receivers inside the black hole horizon.
Information is lost beyond the event horizon. There is no such thing as "point of view of receivers inside that black hole", everything collapses into a single point or some dimensionless thing or whatever. That's why it is called a singularity.
You might want to consider the article more carefully, especially the section "Short Hair, Long Hair". The predictions of string theory are in no way falsified by the analysis conducted by these researchers.
Yet another clickbait title by the quantamagazine. I'm afraid these "science popularization" techniques only make it more confusing for the general public. Just say the exact, scientific word and cut this bs.
This is the first time I heard quanta magazine being accused of click bait. They show no ads and don't sell anything. What would be their motivation for using click bait?
Click bait used to mean things like "five reasons you always get sick - number three will SHOCK you" or "doctors hate this one weird trick!".
The title of this article isn't click bait at all. Black holes having hair or not is almost the technical expression, as evidenced by the "no hair" theorem. Physicists are quirky like that sometimes.
Yeah. The photo of a hairy astrophysicist claiming that if black holes have hair, it is shorter than 40 km seems like a petty way to spark a public interest.
If they finds "casual" names for scientific principles annoying, they're going to just LOVE the names for genes. hERG activity is a bad thing that gets drugs taken off the market (cardiac arrhythmia). hERG means "human ether-a-go-go-related gene". Scientists are human, and obviously playful at times.
It's not a recent thing. Check out quark names and their associated properties.
> According to Albert Einstein’s general theory of relativity, the behavior of a black hole depends on two numbers: how heavy it is, and how fast it is rotating. And that’s it. Black holes are said to have “no hair” — no features that distinguish them from their fellows with the same mass and spin.
As far as I know, there's a third property that black holes have - electric charge. Would a sufficiently strong electric charge between two black holes be detectable, whether they both have the same charge, or opposing charges?
I suppose based on the article, the effects would only take places once the black holes got within 40km of each other...
Black holes can have charge but a charged black hole would attract ions of the opposite charge strongly and would quickly neutralize. So it would be quite difficult to get a non-negligible charge on a black hole.
So some very advanced and bored civilization could take black holes and shoot gigantic numbers of protons into it, and get a very positively charged black hole.
If you pump in enough charge that the electrical repulsion is stronger than the gravity attraction, you can then store them safely next to each other, for when you might need one.
Cool. You would of course end up negatively charged yourself and therefore quite strongly attracted to the black hole.
So I would recommend moving your civilization to a Dyson sphere around the black hole before aiming your cluster of LHCs at it.
It also allows you to move the black hole around, somewhat freely.
It may be a very useful thing if it turns out that we can make small ones.
> move the black hole around, somewhat freely [...] small ones
At the risk of spoiling the mystery of a 50-year-old short story, this happens in the The Borderland of Sol by Larry Niven.
I immediately thought of that story as I was reading the comment you replied to :P
If you enjoy that sort of thing, also read Killing Vector by Charles Sheffield.
Flinging one at an enemy civilization could be a clean way to resolve conflicts!
"Aside from permanently rending the very fabric of space and time itself, there was very little collateral damage."
It has been proposed and is often studied in different contexts, but it seems unlikely that we will be able to measure it any time soon.
I think you might just need a comb and some pieces of paper
Don't forget color charge:
https://arxiv.org/abs/2310.16877
Judging by what the article says they can't be different
> In theory, there’s a third defining property: electric charge. But real, astrophysical black holes have negligible net charge.
No. The "no hair theorem" has a condition that says "after a while", but people ignore it because it's a short time. So intermediately after the object entered the black hole will have bump and will emit gravitational waves and after a while it will be a perfect sphere (if it's not rotating, or a similar round elongated shape if rotating).
As far as I can understand, it totally unrelated to Quantum Mechanics.
For one, observers inside the event horizon can still observe each other. From the point of view of an observer falling into a black hole, nothing unusual happens when they cross the event horizon. They still observe stuff falling in with them in the same way as before (assuming a large enough black hole so that the local curvature is negligible).
If Hawking radiation is real, it might also expose information from inside the black hole, possibly solving the information paradox. In that sense, the inside of a black hole would still be “observable” from the outside in the QM sense. But since we don’t know how quantum gravity works, that is an open question.
How can that be true? Inside the event horizon, there is only one direction: down. If light cannot escape, how can it happily bounce around, reflect on something, then reach your eyes? How could signals from your feet (say) even reach your brain, assuming a feet-first entry into the black hole?
When you cross the event horizon, you’re falling at close to the speed of light, relative to the event horizon. But due to relativity, light bouncing off the falling objects still propagates at light speed in all directions relative to the falling objects (including the falling observer). That light can’t move outwards, due to being inside the event horizon, and accelerates towards the singularity, but the observer accelerates to the singularity faster, relative to light being reflected outwards. The falling rest frame accelerates towards the singularity, but within that accelerating rest frame, light behaves normally between objects in that frame (assuming a large enough black hole so that the differences in radial distance between the objects is negligible).
Consider two cars driving on a highway at the same speed. If one of the cars decelerates, it appears to be moving backwards relative to the other car, while still continuing to move forward relative to the road. It’s similar for light and an observer falling towards the singularity. They both fall towards the singularity, but light being reflected backwards will fall a bit slower, thus appearing to move backwards relative to the observer, even though the light still moves forward toward the singularity.
Very well explained. Thank you
I think the short answer is that no one knows, because a proper answer would need an understanding of how black holes and quantum theory go together, which is not currently known.
The longer answer (if you want to get in to the extensive literature) is that your question seems to relate to the black hole information paradox https://en.wikipedia.org/wiki/Black_hole_information_paradox
I think what is relevant here, is that there is a separation between the inside and the outside of the black hole. Since there is no communication between the two, on the inside it could be anything. And its not just the matter and space, even the laws of physics might be completely different. If it weren't so, then there had to be a "super-space" that communicates how ordinary space behaves on both the outside and the inside the black hole.
Not exactly. Black holes are not lopsided, for instance. There isn't anything on "one side" of them, once the matter has crossed the event horizon.
Rotating black holes are pretty well modeled as a spinning ring: https://en.wikipedia.org/wiki/Ring_singularity
Black hole goes into a barbershop
Barber goes "You're kidding me, right?"
Black hole goes into a barbershop.
Barber goes, "You're kiddiinnngggg mmmmmmeeeeeeeeee, riiiiiiiiiiiiiiiiiiiiiiiiiiiiiii...."
This reminds me of something I mull over; Greek style philosophy seems so smart, rational and correct until you actually observe the world. Imo the perfect example is elementary particles - the Greeks (mostly) were convinced that it's impossible for something physical to be elementary (cannot be further split) and logically it is hard to understand why can't we continue to split the particle?! but hey, we can't.
Physical beings which cannot be distinguished from one another would have made their heads spin...
To be fair to the Greeks, thay also came up with the idea of elementary particles. I think it's Democritus who's credited with the idea of "atoms".
It's been ages since I've read about him in any depth but I remember something about him having an idea that because of the indefinite divisibility of length then the elementary particles would have to have a diameter smaller than any given length, i.e. be infinitesimal, so they had quite sophisticated speculations
Aren't quarks and, probably, electrons point-like particles? In that sense the fundamental subatomic particles aren't "physical" in the sense the Greeks would have thought of them, e.g. having a non-0 diameter, and in that way modern physics sort of vindicates their deductive reasoning. Their logic wasn't wrong, per se, it was their starting assumptions that were flawed (or at least incomplete), which is acceptable as far as deductive reasoning goes.
No. Because the GR/QM conflict comes up in the other direction that way. GR predicts infinities for zero-radius particles. Indeed, so too does classical electromagnetism. In any case, "quanta" does not really mean the same as the classical notion of point particles.
* https://youtube.com/watch?v=zPZrDbPAuKs
Indeed. That's rotational frame dragging for you. (-:
An interesting consequence of having "no hair" on them is that information on everything that falls in them is lost forever, which leads us to an information loss paradox. Not nice.
You do not need a black hole for the information loss. Just shout a flashlight into empty space. Any information encoded in photons will be lost from the point of view of the sender.
And then the classical notions like information or entropy are not really compatible with General Relativity. Richard Tolman almost 100 years ago proposed interesting extension to the classical thermodynamics that is compatible and can potentially explain apparent paradox of information loss, but it is not known if that extension matches reality.
> Any information encoded in photons will be lost from the point of view of the sender.
The crucial difference is "from the point of view of the sender". The black hole information paradox is that the information is lost from any possible receiver (even any possible ensemble of receivers surrounding the black hole). It's not the same thing
The information is not lost from the point of view of receivers inside the black hole horizon.
Which again points out that the classical notion of information just does not work in a universe with multiple causally disconnected regions.
What happens when the black hole evaporates then, and there is no more horizon? That's the black hole information paradox
Single point of ~infinite charge?
>The information is not lost from the point of view of receivers inside the black hole horizon.
Information is lost beyond the event horizon. There is no such thing as "point of view of receivers inside that black hole", everything collapses into a single point or some dimensionless thing or whatever. That's why it is called a singularity.
Another blow against string theory.
You might want to consider the article more carefully, especially the section "Short Hair, Long Hair". The predictions of string theory are in no way falsified by the analysis conducted by these researchers.
Yet another clickbait title by the quantamagazine. I'm afraid these "science popularization" techniques only make it more confusing for the general public. Just say the exact, scientific word and cut this bs.
This is the first time I heard quanta magazine being accused of click bait. They show no ads and don't sell anything. What would be their motivation for using click bait?
Click bait used to mean things like "five reasons you always get sick - number three will SHOCK you" or "doctors hate this one weird trick!".
The title of this article isn't click bait at all. Black holes having hair or not is almost the technical expression, as evidenced by the "no hair" theorem. Physicists are quirky like that sometimes.
Yeah. The photo of a hairy astrophysicist claiming that if black holes have hair, it is shorter than 40 km seems like a petty way to spark a public interest.
If they finds "casual" names for scientific principles annoying, they're going to just LOVE the names for genes. hERG activity is a bad thing that gets drugs taken off the market (cardiac arrhythmia). hERG means "human ether-a-go-go-related gene". Scientists are human, and obviously playful at times.
It's not a recent thing. Check out quark names and their associated properties.
A few examples:
https://www.wikipedia.org/wiki/List_of_unusual_biological_na...
https://en.wikipedia.org/wiki/List_of_chemical_compounds_wit...
No-hair theorem is just the name
https://en.m.wikipedia.org/wiki/No-hair_theorem