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Do black holes have a maximum electric charge? · Daniel and Kelly’s Extraordinary Universe

0:08
Speaker 1: Hey, or Hey, I've got a physics dad joke for you.
0:11
Speaker 2: Well, is that a joke that's only funny to physicist dads.
0:16
Speaker 1: Let's find out why do physicists complain about the price
0:19
Speaker 1: of protons.
0:20
Speaker 2: I don't know what.
0:21
Speaker 1: They think they've been overcharged.
0:27
Speaker 2: I think I paid too much for that joke.
0:29
Speaker 1: What do you mean it was free?
0:30
Speaker 2: Exactly? I am positive I was overcharged.
0:34
Speaker 1: Oh, that's such a negative reaction.
0:51
Speaker 2: I am hory Ham, a cartoonist and the creator of
0:53
Speaker 2: PhD Commics. Hi.
0:54
Speaker 1: I'm Daniel. I'm a particle physicist and a professor at
0:57
Speaker 1: UC Irvine, and I'm always trying to find the in
1:00
Speaker 1: this crazy cosmos.
1:02
Speaker 2: What does that mean is? Do you think the universe
1:03
Speaker 2: is funny?
1:05
Speaker 1: The universe is kind of hilarious, and you can respond
1:08
Speaker 1: to it either by being freaked out, like wow, this
1:11
Speaker 1: place is insane, or you can laugh at it and
1:13
Speaker 1: say like wow, somebody out there has a sense of
1:16
Speaker 1: humor when they put this thing together.
1:17
Speaker 2: Oh, do you think the universe appreciates being left at
1:20
Speaker 2: what if it's not doing it on purpose?
1:23
Speaker 1: I like to think we're laughing along with the universe.
1:25
Speaker 1: You know, we're not mocking it. We're appreciating the cleverness
1:29
Speaker 1: of its structure, which sometimes you know elicits a chuckle.
1:32
Speaker 2: Yeah, But is it funny like funny haha, or is
1:34
Speaker 2: it funny like.
1:37
Speaker 1: I think it's a big mixture of the two. It's
1:39
Speaker 1: sometimes pretty weird and that makes me laugh.
1:42
Speaker 2: But anyways, welcome to our podcast, Daniel and Jorge Explain
1:45
Speaker 2: the Universe, a production of iHeartRadio, in which we.
1:48
Speaker 1: Try our best to find the sense in the universe.
1:51
Speaker 1: We look out there at all the particles buzzing and
1:53
Speaker 1: frothing and doing all sorts of bizarre things and try
1:57
Speaker 1: to weave it together into a story. Something in our
1:59
Speaker 1: minds explains why electrons zig this way and why protons
2:03
Speaker 1: zig that way, and how they all come together to
2:05
Speaker 1: make this incredible, delicious, hilarious universe that we appreciate.
2:10
Speaker 2: That's right, because it is a pretty curiously funny universe,
2:13
Speaker 2: full of amazing mysteries and contradictions sometimes and paradoxes, and
2:17
Speaker 2: here in the podcast we try to find the punchline
2:19
Speaker 2: to it all, just to deliver some lass and maybe
2:23
Speaker 2: also some mind blowing moments.
2:25
Speaker 1: Sometimes what you need is humor to crack open a
2:28
Speaker 1: deep mystery of the universe. We'd love to joke our
2:32
Speaker 1: way into the hearts of black holes and understand what
2:35
Speaker 1: is the singularly most funny joke in the universe.
2:39
Speaker 2: Well, my job is definitely to find jokes and write jokes.
2:42
Speaker 2: But are you saying that your job is also a joke?
2:44
Speaker 1: No, you got to have a straight man on the podcast.
2:46
Speaker 2: Right, Which in a physics podcast, which one is the
2:48
Speaker 2: straight man?
2:50
Speaker 1: We're both straight men in one way and jokesters in
2:53
Speaker 1: another way.
2:54
Speaker 2: We're both crooked. Is that what you're saying.
2:57
Speaker 1: Look, I'm just trying to keep everybody out there awake.
2:59
Speaker 1: I've been getting all these emails from people saying they
3:01
Speaker 1: love to fall asleep to the podcast. And as much
3:04
Speaker 1: as I'm glad that people are finding some way to
3:06
Speaker 1: appreciate it, I hope they're at least staying awake to
3:09
Speaker 1: the end.
3:10
Speaker 2: We're more of a meditation podcast. Is that where we are?
3:13
Speaker 1: I'm trying to transition us from meditation to humor.
3:17
Speaker 2: I think that's a big business. Actually, like podcasts that
3:19
Speaker 2: helps you fall asleep. I heard like a really good
3:22
Speaker 2: book to fall asleep. This is some kind of math
3:24
Speaker 2: book by Stanley Tushi or Tony Shlou one of those actors.
3:29
Speaker 2: They just read this math book really slowly and people
3:32
Speaker 2: found it and they found it a great way to
3:34
Speaker 2: fall asleep.
3:35
Speaker 1: Great physics ASMR. Well, maybe when they sleep, one of
3:39
Speaker 1: our listeners will get a bolt of inspiration, an idea
3:43
Speaker 1: helps us crack open some of the mysteries of the universe.
3:47
Speaker 1: Or maybe that's too charged an idea.
3:49
Speaker 2: I wonder if people are listening to us just as
3:50
Speaker 2: they are falling asleep, if we have a chance now
3:53
Speaker 2: to like implant ideas into their subconscious.
3:57
Speaker 1: Welcome to podcast inception right.
4:00
Speaker 2: Send money to Jorge tomorrow. You will wake up refreshed
4:05
Speaker 2: and feeling alive and feeling like you and to Venmo
4:07
Speaker 2: me a few thousand dollars.
4:09
Speaker 1: Maybe you're in a dream right now and this podcast
4:12
Speaker 1: isn't really happening.
4:13
Speaker 2: Hopefully won't turn into a nightmare.
4:15
Speaker 1: Hopefully you'll get a torrent to cash from the listeners.
4:18
Speaker 2: But speaking of dark topics and scary nightmares, let's talk
4:21
Speaker 2: about black holes. Black Holes are notoriously I guess humorous
4:25
Speaker 2: in the sense that they are big mysteries and they
4:28
Speaker 2: seem to be full of contradictions inside them.
4:30
Speaker 1: That's right, like dad jokes, black holes are some of
4:32
Speaker 1: the most extreme things in the universe. Instead of being
4:36
Speaker 1: the funniest things in the universe, they are the most massive,
4:39
Speaker 1: the most dense, the most gravitationally powerful things in the universe.
4:43
Speaker 2: Would you say they're the holy grail of dad jokes?
4:47
Speaker 2: Or does that make too light over the gravity of
4:49
Speaker 2: the situation.
4:50
Speaker 1: I think that was a massively successful joke.
4:52
Speaker 2: Well, at least it doesn't suck us. I guess that's
4:54
Speaker 2: all black holes do.
4:55
Speaker 1: Jokes aside, black holes are the most massively interesting things
4:59
Speaker 1: in the ar because we think they have within them
5:02
Speaker 1: some of the answers to the deepest questions in modern physics.
5:06
Speaker 1: What is a theory of quantum gravity? What happens when
5:09
Speaker 1: you take tiny particles and squeeze them all together so
5:12
Speaker 1: that they are gravitationally powerful? And so people are always
5:15
Speaker 1: trying to figure out a way to crack open a
5:17
Speaker 1: black hole to somehow peek inside and get a snapshot
5:21
Speaker 1: of those secrets of the universe.
5:23
Speaker 2: Yeah, because it seems like black holes can eat pretty
5:25
Speaker 2: much anything, right, any kind of matter, any kind of light,
5:28
Speaker 2: any kind of energy. It can absorb even dark matter.
5:31
Speaker 1: Right, black holes can certainly eat dark matter, that's right.
5:34
Speaker 1: And because everything out there in the universe has energy
5:38
Speaker 1: and energy means gravity, everything in the universe is attracted
5:42
Speaker 1: to a black hole. But there are of course always
5:46
Speaker 1: some caveats to that. If black holes have electric charges,
5:50
Speaker 1: you might be able to turn them from attractive into repulsive,
5:53
Speaker 1: and you might also weirdly be able to crack them open.
5:58
Speaker 2: Yeah, that's another interesting thing that black holes can eat
6:00
Speaker 2: is they can eat electric charge. Right, Like, if you
6:03
Speaker 2: throw an electron into a black hole or a bunch
6:05
Speaker 2: of electrons, the black hole is going to gain a
6:08
Speaker 2: negative charge.
6:09
Speaker 1: That's right, because charge is conserved in the universe. It
6:11
Speaker 1: doesn't just go away. So in the same way that
6:14
Speaker 1: when you add energy to a black hole, it gains
6:17
Speaker 1: in mass proportional to that energy, when you add a
6:20
Speaker 1: charged particle to a black hole, it gains its charge.
6:24
Speaker 2: Do we actually know that, Daniel, Or is that just
6:26
Speaker 2: from the theory that the theory says that if you
6:28
Speaker 2: throw an electron in the black hole, now is a
6:31
Speaker 2: little bit more negatively charged or does charge maybe I
6:34
Speaker 2: don't know, decompose inside of a black hole and turn
6:37
Speaker 2: into energy.
6:38
Speaker 1: That's definitely just from the theory. We're not even one
6:40
Speaker 1: hundred percent sure that black holes exist in the universe.
6:43
Speaker 1: We have a bunch of candidates that seem like black holes,
6:46
Speaker 1: but of course there's a whole bevy now of proposals
6:48
Speaker 1: for what they might be instead of black holes. But
6:51
Speaker 1: we think that charge is conserved in our universe, and
6:54
Speaker 1: so even if those particles transform into some other state,
6:58
Speaker 1: the electric charge is made. But again that's just theoretical.
7:02
Speaker 1: But that electric charge does create some very strange conditions
7:06
Speaker 1: inside and outside the black hole.
7:09
Speaker 2: And I guess the question you can ask is how
7:11
Speaker 2: many electrons can you throw into a black hole? And
7:14
Speaker 2: is there possibly a maximum number of them that you
7:17
Speaker 2: can throw in?
7:17
Speaker 1: That's right, because as black holes get more and more charged,
7:21
Speaker 1: they get weirder and weirder, and so if you can
7:24
Speaker 1: add an infinite amount of charge to a black hole,
7:26
Speaker 1: you might even be able to crack it open.
7:28
Speaker 2: So to be on the podcast, we'll be asking the
7:30
Speaker 2: question do black holes have a maximum electric charge? Now,
7:40
Speaker 2: this is like how much they charge for people to
7:42
Speaker 2: go inside or what?
7:44
Speaker 1: Yeah, I think we get a podcast discount though.
7:46
Speaker 2: Oh yeah, two for one, two podcast hosts for.
7:50
Speaker 1: One because we've been featuring black holes on the podcast
7:54
Speaker 1: so many times. We're basically like friends of the black Hole.
7:57
Speaker 1: By now we're on the sort of special VIP list.
8:00
Speaker 2: We're like the mater shells for a black hole, the
8:03
Speaker 2: black hole industry.
8:04
Speaker 1: That's right, we're paid under the table by big black hole.
8:06
Speaker 2: The problem is they can never pay us because nothing
8:08
Speaker 2: can escape a black hole. You have to go in
8:10
Speaker 2: there to get your money.
8:11
Speaker 1: They tell us it's in there, but we haven't actually
8:13
Speaker 1: verified that.
8:14
Speaker 2: Well, this is kind of a weird question. Can a
8:16
Speaker 2: black hole have a maximum electric charge? Which is weird
8:19
Speaker 2: to think about because it doesn't seem like a hole
8:22
Speaker 2: can have a maximum. A black hole can have a
8:24
Speaker 2: maximum of anything, right, Like, black holes can gobble mass
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Speaker 2: and energy almost as much as it wants, as far
8:30
Speaker 2: as we know, right.
8:31
Speaker 1: Yeah, there's no theoretical limit to the mass that black
8:33
Speaker 1: hole can have. There's some practical limits there because they
8:36
Speaker 1: have to actually absorb the stuff and has to fall
8:38
Speaker 1: in at the right angle, and sometimes black holes can
8:41
Speaker 1: create conditions that repel the stuff around them. So practically speaking,
8:45
Speaker 1: there does seem to be a limit to how big
8:47
Speaker 1: black holes have gotten in our universe. But Theoretically, there's
8:50
Speaker 1: no limit to how big a black hole can get.
8:52
Speaker 1: It can be billions of solar masses. It could be
8:55
Speaker 1: trillions of solar masses. It could be quadrillions of solar masses.
8:58
Speaker 1: It can keep inventing metric pre fixes, and black holes
9:01
Speaker 1: could theoretically grow to be that large. But that's not
9:04
Speaker 1: the same for a black holes spin or for a
9:06
Speaker 1: black hole's charge. As these black holes get more and
9:09
Speaker 1: more charged, that they get weird and they start to
9:12
Speaker 1: border on breaking the rules.
9:14
Speaker 2: Of general relativity, and then they get charged with the
9:16
Speaker 2: crime of breaking the rules. That's right, and they go
9:18
Speaker 2: to physics jail, But they're tracked with physicist dads for
9:23
Speaker 2: the rest of their existence, telling them bad dad jokes.
9:27
Speaker 1: That's a terrible punishment. Absolutely, yeah, No, The punishment for
9:30
Speaker 1: going to physics jail is that you puff out of existence. Right.
9:33
Speaker 1: Going to physics jails means you've broken a law of
9:36
Speaker 1: the universe and therefore you no longer exist.
9:38
Speaker 2: It seems kind of harsh.
9:39
Speaker 1: Hey, everybody knows the rules. These are just the consequences
9:42
Speaker 1: of your actions, man, But.
9:43
Speaker 2: Don't you need to know the law before? I guess
9:46
Speaker 2: you can break the law? Without knowing it and still
9:48
Speaker 2: be guilty. It seems a little unfair.
9:50
Speaker 1: Well, maybe people should listen to this podcast. We'll educate
9:53
Speaker 1: them about what the laws of physics are so they
9:55
Speaker 1: can avoid breaking the rules and go into physics jail.
9:58
Speaker 2: No, no, then we're we're live, Daniel. They might sue
10:01
Speaker 2: us for, you know, not telling them the full story.
10:04
Speaker 1: Well, anybody who's broken the laws no longer exists, so
10:07
Speaker 1: they can't sue us.
10:08
Speaker 2: Oh yeah, that's true too.
10:10
Speaker 1: Well.
10:10
Speaker 2: As usually, we were wondering how many people out there
10:12
Speaker 2: had thought about this question of whether black holes can
10:14
Speaker 2: have a maximum electric charge.
10:17
Speaker 1: So thanks very much to all of you who volunteer
10:19
Speaker 1: to answer these questions. We really appreciate it. And if
10:21
Speaker 1: you would like to hear your voice on the podcast
10:23
Speaker 1: in the future, please don't be shy. Write to me
10:26
Speaker 1: two questions at Danielandjorge dot com and I will set
10:30
Speaker 1: you up.
10:30
Speaker 2: So think about it for a second. Do you think
10:32
Speaker 2: black holes can have a maximum electric charge? Here's what
10:35
Speaker 2: people have to say.
10:36
Speaker 3: I do not think that a black hole would have
10:39
Speaker 3: a maximum electrical charge. I think it would all be
10:42
Speaker 3: dependent on how big it is, so I would think
10:46
Speaker 3: that the bigger the black hole is, the more the higher,
10:49
Speaker 3: like the maximum electrical charge would be.
10:52
Speaker 4: I would have thought that a black hole would remain
10:56
Speaker 4: roughly neutral or probably no theoretical limit to the charge
11:03
Speaker 4: it could have. I do wonder if a high charge
11:06
Speaker 4: black hole could discharge with some sort of lightning. Perhaps.
11:12
Speaker 2: If black holes can grow indefinitely, then maybe they can
11:16
Speaker 2: grow their electric charge indefinitely as well. All Right, a
11:21
Speaker 2: pretty interesting range of answers. Some people said maybe, some
11:24
Speaker 2: people said probably not, and some people said it depends.
11:29
Speaker 1: I feel like these are all leaning towards the direction
11:32
Speaker 1: that black holes could have an arbitrary charge.
11:35
Speaker 2: M Yeah, because I guess that makes sense, right, Like,
11:37
Speaker 2: if it can eat an infinite amount of mass, why
11:40
Speaker 2: can it have an infinite amount of charge.
11:42
Speaker 1: Well, as we'll see, mass in charge are different, right,
11:45
Speaker 1: Mass tends to accumulate more mass and create more attractive gravity.
11:49
Speaker 1: It's not the same thing with electric charge, right. Electric
11:52
Speaker 1: charge can do things that mass can't do, like it
11:55
Speaker 1: can repel other charges. So it gets to be quite complicated.
11:59
Speaker 2: Well, they gat to it, Daniel. Let's recap what a
12:02
Speaker 2: black hole is again.
12:03
Speaker 1: The basic idea of a black hole and what we
12:05
Speaker 1: need to understand for today's episode is that it's a
12:08
Speaker 1: region of space where particles can never escape. So we
12:12
Speaker 1: talk about this thing called the event horizon, behind which
12:15
Speaker 1: things are hidden forever. What the event horizon really means
12:19
Speaker 1: is that there's a region of space that if a
12:20
Speaker 1: particle enters it, it will never leave. Like if you
12:23
Speaker 1: fast forward to the infinite future of the universe, that
12:26
Speaker 1: particle never escapes the event horizon. It's not like literally
12:30
Speaker 1: a physical barrier in that sense. It's just a region
12:33
Speaker 1: you draw in space and say, by the way we've
12:35
Speaker 1: done the calculation, anything that enters here never leaves. And
12:39
Speaker 1: the reason is that space time is curved so much
12:43
Speaker 1: by the mass of the center that the particles never escape.
12:46
Speaker 1: And it's important to understand that the event horizon is
12:48
Speaker 1: not something you can ever exactly say where it is
12:51
Speaker 1: right now. In order to really know where the event
12:54
Speaker 1: horizon is, you have to kind of do that calculation
12:57
Speaker 1: out to the future infinity and say, oh, these particles
13:00
Speaker 1: never escape, but those particles do escape.
13:03
Speaker 2: But you can take a picture of a black hole,
13:05
Speaker 2: wouldn't that kind of tell you the size of it
13:08
Speaker 2: and to what extent the black part of the black
13:11
Speaker 2: hole is.
13:12
Speaker 1: Yeah, you can take a picture of a black hole,
13:13
Speaker 1: and we've done it. We have a couple of really
13:15
Speaker 1: cool images, and you can see a ring of gas
13:17
Speaker 1: around it, and then at the center you see an
13:19
Speaker 1: area that isn't emitting any light. What I'm saying is that,
13:22
Speaker 1: mathematically speaking, or most technically in general relativity, the region
13:26
Speaker 1: around the black hole that we call the event horizon
13:28
Speaker 1: is something you can only ever determine, sort of like
13:31
Speaker 1: in the infinite future of the universe. Like, even if
13:34
Speaker 1: you're looking at the black hole and you see no
13:36
Speaker 1: light emitted from it, that doesn't mean that there isn't
13:39
Speaker 1: like some path for a photon to like graze just
13:42
Speaker 1: within it and eventually make it out. You sort of
13:45
Speaker 1: like can't call it the event horizon until all those
13:48
Speaker 1: particles have had their chance to escape and you can
13:50
Speaker 1: say which ones made it and which ones can't.
13:53
Speaker 2: All right, So then it's a region of space where
13:55
Speaker 2: nothing can get out, not even light. But there's some
13:57
Speaker 2: debate about what's exactly at the center of a black
14:00
Speaker 2: hole and whether or not it's possible for something to
14:03
Speaker 2: be at the middle, or maybe it's fuzzier than people think.
14:07
Speaker 1: Exactly in general relativity, we have this picture of the
14:10
Speaker 1: interior of a black hole is being very very curved
14:12
Speaker 1: in space time, which means everything falls towards the center.
14:15
Speaker 1: In fact, space time is so distorted that, like the
14:18
Speaker 1: timelike coordinate for these things, the future path of everything
14:22
Speaker 1: is just to head towards the center of the black hole,
14:25
Speaker 1: and that creates sort of a runaway condition where it
14:27
Speaker 1: gets more curved, and then things fall further in, which
14:30
Speaker 1: makes it more curved, and so things fall in no
14:33
Speaker 1: stopping it, and so eventually it just all coalesces into
14:36
Speaker 1: the densest possible point. In general relativity, we call this
14:39
Speaker 1: a singularity, and nobody knows if a singularity is real
14:42
Speaker 1: or if there are really singularities inside black holes. I
14:46
Speaker 1: think if you pulled a bunch of physicists, something like
14:48
Speaker 1: ninety nine percent of them would say singularities are not real,
14:51
Speaker 1: that this picture from general relativity is sort of like
14:54
Speaker 1: an indication that the theory has broken down, not like
14:57
Speaker 1: an actual physical prediction the singularities.
15:01
Speaker 2: Are there, right, But I guess it maybe depends on
15:03
Speaker 2: how you define a black hole, like is the black
15:05
Speaker 2: hole the region of space within the event horizon, or
15:09
Speaker 2: do you only call a black hole the singularity in
15:11
Speaker 2: the middle.
15:12
Speaker 1: Well, we can never observe anything within it. So I
15:14
Speaker 1: think the operational definition of the black hole is something
15:17
Speaker 1: that creates an event horizon, and then there's a question
15:20
Speaker 1: of what's inside it, what has created that event horizon,
15:23
Speaker 1: and general relativity says, actually, any equal distribution of mass
15:26
Speaker 1: within the event horizon would cause a black hole. It's
15:29
Speaker 1: just that general relativity predicts that distribution of mass will
15:32
Speaker 1: collapse into a singularity. Other theories predict that it won't
15:35
Speaker 1: predict that. You have other sorts of weird states, like
15:38
Speaker 1: quantum mechanics says you can't have a singularity. It's too
15:41
Speaker 1: much information concentrated at one point. You know the location,
15:44
Speaker 1: you know the energy. It's just it violates the basic
15:47
Speaker 1: laws of quantum mechanics, and so various theories of quantum
15:50
Speaker 1: gravity predict like weird fuzzy blobs at the hearts of
15:53
Speaker 1: black holes. But the truth is we don't know, and
15:55
Speaker 1: we'd love to know, because that would help us build
15:58
Speaker 1: a deeper theory of the unit. It would help us
16:01
Speaker 1: understand how to unify quantum mechanics and general relativity. But frustratingly,
16:06
Speaker 1: those answers are all hidden within the black hole.
16:09
Speaker 2: Right, Like inside of the black hole, everything could be
16:11
Speaker 2: bunched up in the middle of the black hole, or
16:13
Speaker 2: maybe things are more evenly spread out, or maybe it's
16:16
Speaker 2: in some kind of fun, interesting flowery pattern of stuff
16:20
Speaker 2: inside of the black hole.
16:21
Speaker 1: Right. Yeah, we have theories of like fuzzballs from string theory.
16:25
Speaker 1: We have theories of gravitationally slowed down collapsing stars. We
16:29
Speaker 1: have theories where a black hole will eventually turn into
16:32
Speaker 1: a white hole. There's many, many theories for what might
16:34
Speaker 1: be happening inside them. The reality of what's happening inside
16:37
Speaker 1: there is probably something even weird or something that no
16:41
Speaker 1: existing human physicist has imagined. If we could crack these
16:45
Speaker 1: things open and see what was inside, probably we would
16:48
Speaker 1: all be shocked. It'd be one of those delicious moments
16:50
Speaker 1: of science when we're like, what is going on? That's impossible,
16:55
Speaker 1: And then of course we'd be scrambling to come up
16:57
Speaker 1: with explanations. But those explanations would give us the clue
17:00
Speaker 1: is to like, what's really happening. What space really is
17:04
Speaker 1: deep down is space a bunch of quantum things that
17:06
Speaker 1: have woven themselves together to make the universe or is
17:10
Speaker 1: something else going on?
17:12
Speaker 2: Yeah, and I guess the problem is that, you know,
17:13
Speaker 2: you can't study what's inside of a black hole because
17:16
Speaker 2: if you ever get inside of a black hole, you're
17:18
Speaker 2: never getting out. Or even if you can send a
17:21
Speaker 2: robot or something, the robot can send a signal because nothing,
17:25
Speaker 2: not even radio ways or light can escape the black hole. Right,
17:28
Speaker 2: that's the main problem.
17:29
Speaker 1: That is the main problem. But fortunately black holes are
17:32
Speaker 1: more complex than this picture. What we've talked about so
17:35
Speaker 1: far is basically just a dot of mass. It's not spinning,
17:38
Speaker 1: it doesn't have electric charge, it's just mass. And so
17:42
Speaker 1: in general relativity you call this a short style black hole.
17:44
Speaker 1: It's like the simplest possible model of a black hole.
17:47
Speaker 1: But we don't think that any of the black holes
17:49
Speaker 1: actually out there in the universe are like this. We
17:51
Speaker 1: think it's more likely that black holes have a little
17:53
Speaker 1: bit of charge and maybe even have a lot of
17:56
Speaker 1: spin that can really change what happens to a part
17:59
Speaker 1: of that comes near the black hole in sort of
18:02
Speaker 1: surprising ways, especially if the black hole ends up with
18:05
Speaker 1: a lot of charge.
18:07
Speaker 2: Interesting. So, usually people think of black holes as just
18:09
Speaker 2: absorbing mass, but maybe when it absorbs other things like
18:13
Speaker 2: charge or the spin of things. Maybe the picture changes,
18:16
Speaker 2: and so let's get into what those changes could be
18:18
Speaker 2: and whether or not divide. Let us open a black hole,
18:22
Speaker 2: but first, let's take a quick break. Or are we're
18:37
Speaker 2: talking about black holes and whether or not they have
18:39
Speaker 2: a maximum electric charge. I wonder if black holes have
18:43
Speaker 2: like a little battery meter to tell you when they're
18:45
Speaker 2: fully charged, or maybe they do, but you just can't
18:48
Speaker 2: see it because it's a black hole.
18:49
Speaker 1: I think if you overcharge a black hole, either you
18:53
Speaker 1: or the black hole or both might have to go
18:55
Speaker 1: to physics jail.
18:56
Speaker 2: Wait, why would the why would the black hole have
18:59
Speaker 2: to go to jail too? I mean, wouldn't it be
19:01
Speaker 2: the victim in this case because it was overcharged?
19:04
Speaker 1: You're right, it's totally not fair, but it would be
19:06
Speaker 1: the one breaking the rules. General relativity tells us that
19:10
Speaker 1: if you put too much charge in a black hole, weird, surprising,
19:14
Speaker 1: nonsense things might happen.
19:15
Speaker 2: Mm. But I guess the question is who's going to
19:18
Speaker 2: put the black hole in handcuffs? Is there physics police
19:23
Speaker 2: that is gravity proof?
19:24
Speaker 1: If there's physics police out there, I hope they're not
19:26
Speaker 1: policing my physics. Dad jokes, because then I am.
19:28
Speaker 2: In trouble, you'll get multiple life sentences. I'll do more physics.
19:34
Speaker 1: I hope to get off relatively lightly.
19:36
Speaker 2: Well, we know that if a black hole is a
19:38
Speaker 2: regional's face or nothing can escape, not even light, and
19:41
Speaker 2: so it's really hard to look inside of them. But
19:43
Speaker 2: only if you consider mask going into a black hole.
19:45
Speaker 2: If you consider other things going into a black hole,
19:48
Speaker 2: the picture changes, maybe to the point where you can
19:50
Speaker 2: open a black hole. So let's talk about some of
19:52
Speaker 2: these other things that can go in. One of them
19:54
Speaker 2: was a spin that's right. When we calculate the radius
19:58
Speaker 2: of the event horizon, our prediction for the region of
20:02
Speaker 2: space where if you enter it you're trapped forever. That
20:05
Speaker 2: calculation is pretty simple if the black hole.
20:07
Speaker 1: Only has mass, right, if it's just a dot of
20:10
Speaker 1: stuff and it's not spinning, so that's the short styled
20:12
Speaker 1: black hole. But if the stuff is spinning, and the
20:14
Speaker 1: calculation is a little bit more complicated, we call this
20:18
Speaker 1: a curb black hole KERR, and it has a different
20:21
Speaker 1: event horizon, like the radius of the event horizon of
20:25
Speaker 1: a spinning black hole is different from the radius of
20:28
Speaker 1: the event horizon of a black hole with the same mass,
20:31
Speaker 1: but that's not spinning. And you might ask like, well,
20:34
Speaker 1: how can black hole spin? What can you do to
20:36
Speaker 1: spin a black hole? Well, you know, angular momentum in
20:39
Speaker 1: the universe is conserved, and so if something hits the
20:42
Speaker 1: black hole with angular momentum, the black hole is going
20:45
Speaker 1: to have angular momentum. The same way if your friend
20:47
Speaker 1: pushes you, but they don't push you straight on, they
20:49
Speaker 1: push you like on your shoulder, it could spin you around.
20:53
Speaker 1: If an electron falls into the black hole but doesn't
20:55
Speaker 1: fall straight in, if it falls like a little bit
20:57
Speaker 1: to the side, then it's going to push the black
20:59
Speaker 1: hole a little bit and get it spinning.
21:01
Speaker 2: Interesting, that's a weird picture to think of a black
21:04
Speaker 2: hole spinning, Like can a hole spin? What would it
21:06
Speaker 2: mean for a hole to spin? Does that mean that
21:08
Speaker 2: the event horizon is spinning or just that the things
21:11
Speaker 2: inside of the black hole have kind of a general
21:14
Speaker 2: spin to it.
21:15
Speaker 1: Yeah, the event horizon is not itself a physical thing, right,
21:18
Speaker 1: it's a distance from the center, So in that sense,
21:20
Speaker 1: that part doesn't spin, but the mass inside the black
21:23
Speaker 1: hole is spinning. Now, a singularity a dot in space
21:27
Speaker 1: that has no extent that can't spin. Right, A true
21:31
Speaker 1: singularity with no volume can't have any spin because spin
21:34
Speaker 1: requires some length. So inside a black hole that is spinning,
21:39
Speaker 1: we don't think there is a singularity. General relativity predicts
21:42
Speaker 1: something else. It predicts a ringularity, a little ring of
21:47
Speaker 1: singularities that's spinning. So that's what's sort of like keeping
21:50
Speaker 1: the spin alive. That the event horizon you can still
21:53
Speaker 1: calculate at some radius, and the space around the black
21:56
Speaker 1: hole now gets weird. You get frame dragging effects. So
21:59
Speaker 1: we've talked about a few times on the podcast. So
22:01
Speaker 1: in Einsteiny and gravity, it matters whether something is spinning
22:05
Speaker 1: or not. You have a different gravitational effect.
22:08
Speaker 2: So if your black hole is spinning, it doesn't have
22:10
Speaker 2: a singularity in the middle like a point you're saying,
22:14
Speaker 2: it has like a weird.
22:16
Speaker 1: Ring, Yeah, a ringularity.
22:18
Speaker 2: So then does the event horizon also change shape or
22:20
Speaker 2: is it still a sphere.
22:22
Speaker 1: The event horizon itself is still a sphere, but now
22:24
Speaker 1: you have a region around the event horizon called the
22:27
Speaker 1: ergosphere where things get weird. We talked to once in
22:30
Speaker 1: a podcast about whether you can extract energy from black holes,
22:33
Speaker 1: and in this ergosphere, which is sort of a doughnut
22:35
Speaker 1: shaped region around the black hole. The black hole will
22:38
Speaker 1: make things spin in a weird way, and you can
22:41
Speaker 1: use that to extract energy from the black hole. So
22:44
Speaker 1: a spinning black hole has sort of two different regions.
22:46
Speaker 1: Is this spherical event horizon and then there's this doughnut
22:49
Speaker 1: shaped ergosphere around it.
22:51
Speaker 2: Where space time is curved differently or is it all
22:54
Speaker 2: still curved like a sphere? Like what does this ergo
22:57
Speaker 2: sphere do?
22:58
Speaker 1: So space time is different inside the ergosphere and not
23:01
Speaker 1: inside the ergosphere, and it's different in this sort of
23:04
Speaker 1: a complicated way. It's always, of course pointing towards the
23:07
Speaker 1: black hole. If you're in the ergosphere, you're going to
23:09
Speaker 1: feel a gravitational pull towards the black hole. But if
23:11
Speaker 1: you're in the ergosphere, you also feel other effects because
23:14
Speaker 1: space time has this frame dragging thing happening to it,
23:17
Speaker 1: which is sort of counterintuitive because we think of gravity
23:19
Speaker 1: as like just attractive it just pulls you towards stuff.
23:22
Speaker 1: But this frame dragging effect can actually also create a spin.
23:26
Speaker 1: Remember that we've measured this frame dragging effect using gravity
23:29
Speaker 1: Probe B. This satellite that orbits the Earth with super
23:33
Speaker 1: hyper precise gyroscopes that measure these kinds of spins. So
23:37
Speaker 1: if you're within the ergosphere or near any spinning object,
23:41
Speaker 1: Einstein says, you're going to get a little bit of
23:43
Speaker 1: a twist as well as a pole.
23:45
Speaker 2: Interesting. Now, let's talk about charge, because you said you
23:48
Speaker 2: can also change the properties of a black hole by
23:50
Speaker 2: throwing in charge. And this happens when you throw in
23:53
Speaker 2: charge objects into the black hole, like electrons. If you
23:56
Speaker 2: throw an electron in the black hole is going to
23:58
Speaker 2: gain a negative charge.
24:00
Speaker 1: Yeah, so you take a black hole and you toss
24:02
Speaker 1: in an electron, it'll happily gobble it up. But that
24:05
Speaker 1: charge doesn't just go away. Right, the electron had an
24:08
Speaker 1: electric field, and just before it fell into the black hole,
24:11
Speaker 1: there was an electric field there. Now that electron has
24:14
Speaker 1: gone inside the black hole, there's still an electric field there. Right,
24:18
Speaker 1: it's sort of frozen in time from the last moment
24:21
Speaker 1: just before the electron crossed the event horizon. No more
24:24
Speaker 1: information can update it. Like what the electron does within
24:27
Speaker 1: the event horizon can't change the electric field. And so
24:30
Speaker 1: there's an electric field there, and you can ask like, well,
24:32
Speaker 1: where does that come from? Well, the whole black hole
24:35
Speaker 1: now has an electric charge.
24:37
Speaker 2: Like from a distance, if you were to see a
24:40
Speaker 2: black hole up ahead of you, you would suddenly deteg
24:43
Speaker 2: that it's a little bit more negatively charged, right Like
24:45
Speaker 2: if you had a magnet, it would be a little
24:47
Speaker 2: bit more attracted to it. Perhaps if you had a
24:49
Speaker 2: positive charge, you'd feel more of a pull towards the
24:52
Speaker 2: black hole exactly.
24:54
Speaker 1: And the super weird thing is, think about what happens
24:57
Speaker 1: to the next electron. So you have like a bag
24:59
Speaker 1: full of electric as you throw one in, Now your
25:01
Speaker 1: black hole has a negative charge. Now electron number two,
25:04
Speaker 1: what happens to it, Well, it feels a gravitational attraction
25:07
Speaker 1: from the mass of the black hole, but now it
25:09
Speaker 1: feels an electrostatic repulsion from the charge of the black hole.
25:14
Speaker 1: The black hole is negatively charged, so just like the
25:16
Speaker 1: electron that fell in, it's going to repel electron number two.
25:19
Speaker 1: So electron number two feels an attraction towards a black
25:22
Speaker 1: hole and a repulsion as well.
25:25
Speaker 2: Right. Interesting, It's sort of like if you are collecting
25:28
Speaker 2: electrons in your pocket. At some point it'd be harder
25:31
Speaker 2: to throw another electron in because you have so much
25:33
Speaker 2: a negative charge in your pocket. That adding another electrical
25:37
Speaker 2: negative charge would be hard to.
25:39
Speaker 1: Do exactly, And this is why I mentioned earlier like
25:41
Speaker 1: mass and charge are different. Mass just attracts itself. The
25:45
Speaker 1: more mass you have, the more mass you're attracted to,
25:47
Speaker 1: the more it pulls on other masses. But charge is different. Right,
25:50
Speaker 1: if something accumulates a lot of negative electric charge, it
25:53
Speaker 1: will then repel other things with negative electric charge, and
25:56
Speaker 1: that has a really important consequence for where the event
26:00
Speaker 1: horizon is. If you try to solve all equations and calculate, like, well,
26:03
Speaker 1: where is the event horizon for a black hole with
26:06
Speaker 1: or without an electric charge? You discover that a black
26:08
Speaker 1: hole with an electric charge has a smaller event horizon.
26:12
Speaker 2: Well, it's only pushing away other negative electric charges. But
26:15
Speaker 2: like for if you're a positron or a positively charged particle,
26:19
Speaker 2: now your really negative black hole is more attractive. So
26:24
Speaker 2: it wouldn't the event horizon grow for a positively charged particle.
26:28
Speaker 1: Well not exactly. There are two things going on here
26:30
Speaker 1: at once. First, the event horizon radius does depend on
26:35
Speaker 1: the charge of the black hole. Right, as the charge
26:38
Speaker 1: of the black hole increases, the radius shrinks, and we're
26:42
Speaker 1: tempted to say that that's because of the electromagnetic force
26:46
Speaker 1: from the charge now working against gravity, right, gravity pulling
26:49
Speaker 1: things in and the electromagnetic force pushing things away. But
26:53
Speaker 1: that's not actually what's happening when it comes to the
26:55
Speaker 1: event horizon. If it were, it would mean that the
26:58
Speaker 1: event horizon would be different for different charges of particles. Positive,
27:03
Speaker 1: neutral or negative particles would see different event horizons. But
27:06
Speaker 1: it isn't. It shrinks the event horizon for all particles,
27:09
Speaker 1: regardless of charge. And unfortunately there's not a very simple
27:14
Speaker 1: intuitive way to understand that. I actually talk to a
27:17
Speaker 1: bunch of GR experts, and the best I can do
27:19
Speaker 1: is to tell you that the size of the black
27:21
Speaker 1: hole is affected by the electromagnetic energy that's inside the
27:26
Speaker 1: event horizon. But it's not in a simple way. Remember
27:29
Speaker 1: that GR is based on the stress energy tensor, and
27:32
Speaker 1: it's not just that more energy means more curvature. It's
27:35
Speaker 1: quite complicated, and in the end, the greater the charge,
27:38
Speaker 1: the smaller the event horizon. But the second thing is
27:41
Speaker 1: that the black hole charge does push or pull on
27:44
Speaker 1: particles outside the event horizon, and that does depend on
27:48
Speaker 1: the charge of the particle. So a negatively charged black
27:51
Speaker 1: hole can resist negative particles from ever crossing over the
27:55
Speaker 1: event horizon, and it can accelerate positively charged particles towards it.
28:00
Speaker 1: The event horizon is the same for all of them,
28:02
Speaker 1: but the black hole charge can prevent some particles from
28:05
Speaker 1: crossing it, or it can assist them. What I think
28:08
Speaker 1: is super fascinating is that the event horizon is shrinking
28:11
Speaker 1: as the charge grows. So you toss in an electron
28:14
Speaker 1: and the event horizon shrinks, and then if you cram
28:17
Speaker 1: another electron in there, the event horizon shrinks further, and
28:21
Speaker 1: so you can keep going. You keep going, and there's
28:23
Speaker 1: a point where you have so much charge that essentially
28:27
Speaker 1: you erase the event horizon.
28:29
Speaker 2: WHOA, Okay, Now I think you're imagining a scenario where
28:32
Speaker 2: you only put electrons, Like you make a black hole
28:35
Speaker 2: out of electrons, and you only put more electrons in it,
28:39
Speaker 2: and then you ask, what does that black hole look
28:41
Speaker 2: like to another electron? So, now, like, if you do
28:44
Speaker 2: the math, does it work out that it does erase
28:47
Speaker 2: the black hole to an electron? Because I imagine if
28:49
Speaker 2: there's a singularity in the middle of the black hole,
28:52
Speaker 2: then you sort of can't beat a singularity, can you.
28:55
Speaker 1: That's exactly the question, right, And so people have been
28:58
Speaker 1: doing these kinds of calculations, and according to general relativity,
29:02
Speaker 1: there's a maximum charge that a black hole can have,
29:06
Speaker 1: and if you manage somehow to give it more charge
29:09
Speaker 1: than that, then effectively there will be no event horizon.
29:13
Speaker 1: You've removed the event horizon. You revealed the singularity. Like
29:18
Speaker 1: you sit down and do the calculations. The radius of
29:21
Speaker 1: the event horizon depends on the mass enclosed in the
29:23
Speaker 1: black hole and on the charge of it. And if
29:26
Speaker 1: the charge gets above a certain number relative to the mass,
29:29
Speaker 1: then there's no solution. Right. As the charge grows, the
29:33
Speaker 1: radis event horizon shrinks and shrinks and shrinks and shrinks,
29:35
Speaker 1: until eventually it's at zero. And that's what we call
29:38
Speaker 1: the maximum charge the extremal black hole. And if you
29:41
Speaker 1: somehow added another electron to that, nobody knows what would happen, Right,
29:46
Speaker 1: would you get a naked singularity? Would we all go
29:49
Speaker 1: to physics jail? Nobody knows. Let's find out.
29:52
Speaker 2: Well, I guess I imagine the math is complicated because
29:55
Speaker 2: an electron also has mass. Right, So if you throw
29:58
Speaker 2: an electron into a black hole. Well, you are increasing
30:01
Speaker 2: the mass of the black hole, making it more of
30:03
Speaker 2: a whole, but you're also making it more repellent to
30:06
Speaker 2: other electrons. And you're saying that the way it works
30:09
Speaker 2: out is that electrons maybe have more charge than they
30:12
Speaker 2: have mass, and so therefore if you make a black
30:14
Speaker 2: hole out of just electrons, it is going to repel
30:16
Speaker 2: other electrons.
30:17
Speaker 1: Yeah, there's sort of two parts of the question. One
30:19
Speaker 1: is like, assume that somehow you can get enough electrons
30:22
Speaker 1: in there, what would happen? And the answer to that
30:25
Speaker 1: question is nobody knows. Right, nobody knows what would happen.
30:28
Speaker 1: It predicts basically a naked singularity. And there's been a
30:31
Speaker 1: lot of discussion in theoretical physics for a long time
30:34
Speaker 1: about like, hmm, this seems weird. Would the universe actually
30:38
Speaker 1: allow this or would something stop it? And Roger Penrose
30:42
Speaker 1: has this idea called the cosmic censorship hypothesis that says
30:46
Speaker 1: something is going to intervene. There must be a reason
30:48
Speaker 1: why this is impossible, because you can't have a singularity
30:51
Speaker 1: just out in space. Naked singularity is like an anathema
30:55
Speaker 1: to particle physicists or to theoretical physicists, so they imagine
30:58
Speaker 1: something must prevent from happening. And then the other question,
31:01
Speaker 1: the second question, or the other part of it, is
31:04
Speaker 1: could you actually ever create this situation? Is there something
31:08
Speaker 1: preventing us? What could be preventing us from getting enough
31:11
Speaker 1: electric charges altogether, all at once?
31:14
Speaker 2: But again, I feel like this is only if you're
31:17
Speaker 2: an electron. The black hole disappears only if you're an electron.
31:20
Speaker 2: But like, light still can't escape this all electron black hole,
31:24
Speaker 2: right because light is neutral, Right, Like light doesn't carry
31:28
Speaker 2: you have a lot of negative charge in it.
31:30
Speaker 1: The black hole charge only affects charged particles on the
31:33
Speaker 1: outside of the event horizon, so it prevents more electrons
31:37
Speaker 1: from falling in, but it doesn't prevent photons or protons
31:41
Speaker 1: or other positively charged particles from falling in. But remember
31:44
Speaker 1: that the event horizon is the event horizon, and the
31:47
Speaker 1: radius of the event horizon is the same for all
31:50
Speaker 1: particles charged or neutral. Having a charge on the black
31:53
Speaker 1: hole shrinks the event horizon for all particles.
31:56
Speaker 2: But if you were I think maybe the point is
31:59
Speaker 2: that if something has a negative electric charge, it could
32:01
Speaker 2: maybe escape what we see as the black hole.
32:04
Speaker 1: I think the more accurate way to say it is
32:06
Speaker 1: that outside the event horizon, electrons would be repelled by
32:10
Speaker 1: the charge, but photons could still fall in once they
32:13
Speaker 1: pass the event horizon. It doesn't matter what the charge was,
32:16
Speaker 1: and all particles of any charge see the same event horizon.
32:20
Speaker 2: And protons would see us super duper extra huge event horizon.
32:24
Speaker 1: Right, Not exactly. It's true that outside of the event
32:28
Speaker 1: horizon of a negatively charged black hole, protons would feel
32:32
Speaker 1: a strong attraction from the electromagnetic force and get pulled
32:36
Speaker 1: in towards the event horizon. So at some distance outside
32:40
Speaker 1: the event horizon, electrons are repelled and protons are attracted
32:44
Speaker 1: towards it. So it sort of effectively sweeps up more
32:46
Speaker 1: protons at larger radii than electrons. But the event horizon
32:50
Speaker 1: isn't actually different. The point of no return is the
32:53
Speaker 1: same for the two particles outside the event horizon. It's
32:56
Speaker 1: still possible for those protons to avoid falling in if
33:00
Speaker 1: if someone comes along and zaps them with energy to
33:02
Speaker 1: push them back away from the event horizon. But there's
33:05
Speaker 1: another outcome which is super weird, which is that black
33:08
Speaker 1: holes which are charged can repel each other, like in
33:11
Speaker 1: our universe. We see black holes eat each other, they
33:14
Speaker 1: attract each other, they gobble each other up, they form
33:16
Speaker 1: a super black hole. But if you had two negatively
33:19
Speaker 1: charged black holes, they would actually repel each other right
33:22
Speaker 1: the same way two electrons do, and if it had
33:25
Speaker 1: enough charge, they would avoid falling in together. You might
33:29
Speaker 1: even get them into like a stable configuration where these
33:32
Speaker 1: two forces are in balance and they could orbit each other.
33:35
Speaker 2: Now what I said earlier, is that true. Like if
33:37
Speaker 2: we see a black hole, it looks like it has
33:40
Speaker 2: an event horizon. It has a certain radius to the
33:42
Speaker 2: black sphere, but if it happens to have a lot
33:44
Speaker 2: of negative charge, that means that something could escape what
33:47
Speaker 2: we see as the neutral event horizon.
33:50
Speaker 1: Yeah, exactly. Remember the event horizon not a physical barrier.
33:53
Speaker 1: It's just like the limit where we say nothing that
33:55
Speaker 1: passes this will ever escape. But it's just determined by
33:58
Speaker 1: whether particles do or do not escape. So it's interesting
34:02
Speaker 1: because the event horizon is at the same place for
34:04
Speaker 1: all of the particles, electrons, photons, protons. If they pass it,
34:08
Speaker 1: there's no coming out. But electrons, photons, and protons are
34:12
Speaker 1: more or less likely to fall into that event horizon
34:15
Speaker 1: because of their charge. Technically, that doesn't change the location
34:19
Speaker 1: of the event horizon, but it does mean the fate
34:22
Speaker 1: of particles depends on their charge. There are some places
34:25
Speaker 1: near a negatively charged black hole where electrons cannot go
34:30
Speaker 1: because of their negative charge. They'll get repelled, but protons can.
34:33
Speaker 1: They are attracted, so protons will fall in and eventually
34:37
Speaker 1: get past the event horizon, but electrons will never reach
34:40
Speaker 1: the event horizon.
34:41
Speaker 2: I feel like we've just kind of destroyed the definition
34:44
Speaker 2: of the holes a little bit because we've usually talked
34:47
Speaker 2: about black holes as saying, you know, region of space
34:49
Speaker 2: where nothing, not even light can escape. But it sounds
34:51
Speaker 2: like it sort of depends on the charge of the
34:53
Speaker 2: black hole.
34:53
Speaker 1: Yeah, it does depend on the charge of the black hole.
34:56
Speaker 1: And in practice this isn't that big a deal because
34:58
Speaker 1: we think that most black holes don't have a lot
35:01
Speaker 1: of charge. I mean, things out there in the universe
35:04
Speaker 1: tend to be mostly neutral, like stars are mostly neutral,
35:08
Speaker 1: the Earth is mostly neutral, and that's because the electromagnetic
35:11
Speaker 1: force is pretty strong. So if a black hole somehow
35:14
Speaker 1: ends up with a significant charge, it's going to end
35:17
Speaker 1: up very powerfully attracting the opposite charge, like it will
35:20
Speaker 1: pull very hard on protons flying nearby and suck them
35:24
Speaker 1: up and end up being neutralized. So the electromagnetic force,
35:28
Speaker 1: because it has two charges and is very powerful, tends
35:31
Speaker 1: to mostly neutralize stuff. So we don't think this is
35:33
Speaker 1: something that happens very often in the universe, very very
35:35
Speaker 1: charged black holes.
35:37
Speaker 2: Yeah, I guess that was kind of the earlier point
35:39
Speaker 2: that you were trying to make, which is that maybe
35:41
Speaker 2: it's impossible to make these super duper charged black holes.
35:44
Speaker 2: And so let's get into the scenarios that you would
35:47
Speaker 2: need to make a repellent black hole, a super negative
35:51
Speaker 2: black hole, and what would it mean about the laws
35:54
Speaker 2: of the universe. So let's dig into that. But first
35:56
Speaker 2: let's take another quick break. All right, we're talking about
36:11
Speaker 2: black holes and whether or not you can make one
36:15
Speaker 2: disappear in a way by overcharging it, by putting so
36:19
Speaker 2: much electric negative charge into it that it actually starts
36:23
Speaker 2: to repel other negative charges, and so to an electron,
36:27
Speaker 2: the black hole just kind of disappears. Now, theoretically, you're
36:30
Speaker 2: saying that this is sort of possible, Like if I
36:32
Speaker 2: can construct a black hole with enough negative charge, it
36:35
Speaker 2: would sort of disappear. There wouldn't be a black hole.
36:37
Speaker 1: Theoretically, yeah, theoretically according to general relativity, which again we
36:41
Speaker 1: know is probably wrong about what's going on inside a
36:44
Speaker 1: black hole. But if you follow this theory, it says
36:48
Speaker 1: that if you did somehow manage to add enough charge
36:51
Speaker 1: to a singularity, then the event horizon would shrink down
36:54
Speaker 1: to zero. You essentially would reveal the singularity to the universe.
36:58
Speaker 1: You have a naked singularity.
37:00
Speaker 2: Well, meaning like I'm neutrally charged overall, all of my
37:04
Speaker 2: particles are canceling each other out, So like this black
37:07
Speaker 2: hole would still be deadly to me. I would still
37:10
Speaker 2: die going into it. But maybe if I was made
37:12
Speaker 2: up of only electrons, I could get out.
37:14
Speaker 1: If you were made of electrons, you would be repelled
37:17
Speaker 1: by it, so you'd have less chance to fall in again.
37:20
Speaker 1: Depending on the charge of the black hole, and if
37:23
Speaker 1: the black hole is charged, it would have a smaller
37:25
Speaker 1: event horizon, so you could get closer and still escape,
37:29
Speaker 1: but once you cross the event horizon, you aren't coming out.
37:31
Speaker 1: I still would not recommend a trip to a highly
37:34
Speaker 1: charged black hole, even if you could shoot electrons at
37:36
Speaker 1: it and have them survive. But this is a real
37:38
Speaker 1: opportunity for physics to try to figure out, like what happens.
37:41
Speaker 1: Does this mean that something weird would actually happen in
37:44
Speaker 1: the universe, or is there some reason why this is
37:47
Speaker 1: not possible, something like preventing this from occurring. I remember
37:51
Speaker 1: there's been lots of times in theoretical physics when they
37:54
Speaker 1: predicted something really weird they thought, well, that certainly can't happen.
37:57
Speaker 1: There must be something preventing it. Like black holes, which
38:00
Speaker 1: for a long time people imagine or a ridiculous prediction
38:03
Speaker 1: of general relativity in the universe must somehow arrange for
38:07
Speaker 1: them to never exist, but then we think we see them.
38:10
Speaker 1: So the question for super charged black holes and like
38:13
Speaker 1: naked singularities is like, is this something which could actually happen,
38:17
Speaker 1: you could really do this and see it, or is
38:19
Speaker 1: there some reason why it's impossible to set this thing
38:22
Speaker 1: up this way?
38:23
Speaker 2: Yeah, I think what you're saying is that we probably
38:25
Speaker 2: don't see a lot of super negatively charged black holes
38:27
Speaker 2: out in the universe. And so there's two possibilities. One
38:30
Speaker 2: is that there's something theoretical, some kind of fundamental law
38:34
Speaker 2: in the universe that prevents that from happening. And there's
38:36
Speaker 2: also maybe more practical limitations or barriers making this, Like
38:42
Speaker 2: you just can't practically make a black hole like this
38:44
Speaker 2: because you can't aggregate enough negatively charged stuff together practically speaking,
38:50
Speaker 2: from what we have in the universe exactly.
38:52
Speaker 1: And it's fascinating if the practical limitation is sort of
38:55
Speaker 1: what saves us from the principled problem, I would say, like, wow,
38:58
Speaker 1: this would be a crazy we'd all go to physics
39:01
Speaker 1: jail if this happened. And then it turns out, well,
39:03
Speaker 1: you can't actually build that kind of thing, Like there's
39:06
Speaker 1: no theoretical reason why it can't exist, but you couldn't
39:09
Speaker 1: ever assemble it. That's really fascinating the insight into like
39:12
Speaker 1: how the universe protects itself from going to physics jail.
39:15
Speaker 1: And so there's been a bunch of papers over the
39:17
Speaker 1: last twenty years trying to do this calculation. Like heuristically,
39:20
Speaker 1: you can imagine, all right, as I tossed electrons in,
39:23
Speaker 1: the thing gets more negatively charged and that makes it
39:26
Speaker 1: harder to toss in the next electron. Right, But people
39:29
Speaker 1: have sat down and actually done this calculation, which turns
39:32
Speaker 1: out to be quite complex, and papers over the last
39:34
Speaker 1: few years have concluded that the self force from the
39:38
Speaker 1: black hole. As the black hole gets more and more
39:40
Speaker 1: negatively charged, it will repel those electrons and it will
39:44
Speaker 1: prevent more charge from falling in before it hits this
39:48
Speaker 1: sort of maximum before the event horizon decreases to zero.
39:52
Speaker 2: Right, that's the practical. Although, what happens if you shoot
39:55
Speaker 2: an electron with like super fast, Like if I shoot
39:59
Speaker 2: an electron in towards the center of the black hole
40:02
Speaker 2: with enough energy, want it overcome the electric repulsion and
40:05
Speaker 2: get close enough to the black hole where it gets
40:08
Speaker 2: sucked in by gravity.
40:09
Speaker 1: Yeah, I love that your temptation is to like try
40:11
Speaker 1: to break the rules and like build an electron gun
40:13
Speaker 1: and shoot it at a black hole just to see
40:15
Speaker 1: what happens. That's exactly what leads us down the road
40:19
Speaker 1: do great discoveries. And so you know, the way to
40:22
Speaker 1: think about it is that anything that manages to fall
40:24
Speaker 1: into the black hole is basically going to have more
40:27
Speaker 1: mass than charge. Like, anything that falls in, it's going
40:30
Speaker 1: to have some mass, and so it's going to make
40:31
Speaker 1: the black hole bigger, which is going to allow it
40:34
Speaker 1: to have more charge without sort of like going over
40:36
Speaker 1: this threshold. And so basically anything that manages to fall
40:39
Speaker 1: in has to have a certain mass to charge ratio.
40:42
Speaker 1: And as soon as you get above the place where
40:44
Speaker 1: electrons can no longer fall in, then you're asking like, well,
40:46
Speaker 1: couldn't you force them in? Couldn't you like build an
40:49
Speaker 1: electron gun that makes them go super duper fast and
40:51
Speaker 1: forces them in? And effectively, what you're doing there is
40:54
Speaker 1: you're adding more energy to the electrons, and when the
40:56
Speaker 1: black hole eats that electron, now it gets even more
40:59
Speaker 1: energy and that adds to its mass, and so that
41:02
Speaker 1: keeps it from going over the limit. So if you
41:05
Speaker 1: shoot super high energy electrons at it, that energy keeps
41:09
Speaker 1: it from going over the threshold.
41:10
Speaker 2: Keeps it from going over the threshold for its size.
41:14
Speaker 2: But if I grow the black hole, then it can
41:16
Speaker 2: have more negative charge, right.
41:18
Speaker 1: Yeah, exactly, So anything you can squeeze into a charge
41:21
Speaker 1: black hole has to grow its mass more than its
41:25
Speaker 1: charge effectively in order to keep it from violating this limit.
41:28
Speaker 1: And practically it turns out you can't actually shoot anything
41:31
Speaker 1: to a black hole to break that limit. Anything you
41:34
Speaker 1: can actually get into the black hole would have to
41:36
Speaker 1: have more mass than charge or more energy than charge.
41:40
Speaker 2: Equivalently, So it sort of sounds like you're saying that
41:42
Speaker 2: I can shoot more electrons into a super negatively charge
41:46
Speaker 2: black hole. I just have to give it enough energy
41:48
Speaker 2: and then it'll get sucked in exactly.
41:49
Speaker 1: So technically there's no maximum charge to a black hole.
41:53
Speaker 1: There's a maximum ratio. Like for a given mass, there's
41:57
Speaker 1: a maximum charge. If you make it more massive, then yeah,
42:00
Speaker 1: can have more charge, but you can't have more energy
42:02
Speaker 1: in the charge effectively than in the mass. So there's
42:05
Speaker 1: no like absolute number that says black holes can't be
42:08
Speaker 1: more charged than this, But there is a ratio of
42:11
Speaker 1: the charge to mass for a black hole.
42:13
Speaker 2: So it's kind of a complicated maximum charge than an
42:17
Speaker 2: electron a black hole can have, Like I think you're saying, like,
42:20
Speaker 2: for a certain size of black hole, you can't just
42:23
Speaker 2: throw any electron in. It has to be an electron
42:25
Speaker 2: that has more mass or more velocity or energy than
42:28
Speaker 2: it has charge. So really there's no limit to the
42:32
Speaker 2: maximum charge of a black hole. It just starts to
42:35
Speaker 2: limit the kinds of electrons you can throw in.
42:38
Speaker 1: Yeah, things which would violate this ratio, Things that have
42:41
Speaker 1: like a lot of charge but almost no mass, would
42:44
Speaker 1: not end up in the black hole because they would
42:47
Speaker 1: be repelled by the black hole's charge. So things that
42:50
Speaker 1: would put this black hole in physics jail can't actually
42:53
Speaker 1: fall into the black hole.
42:54
Speaker 2: But to some kinds of electrons, to like slow moving electrons.
42:59
Speaker 2: I think you're saying that the black hole does kind
43:01
Speaker 2: of disappear and for them, the singularity is sort of exposed.
43:04
Speaker 1: No, slow moving electrons would be repelled by this thing, right,
43:07
Speaker 1: They wouldn't have enough energy essentially to get in there.
43:10
Speaker 2: Well, that's what I mean, Like the event horizon disappears
43:13
Speaker 2: for them, right.
43:13
Speaker 1: They would never even be able to enter that region
43:15
Speaker 1: of space.
43:16
Speaker 2: Yeah, But I think what you're saying is that there's
43:19
Speaker 2: maybe like an electron out there for a certain black
43:21
Speaker 2: hole with a lot of negative charge that could get
43:24
Speaker 2: into the regular event horizon, go right up to the
43:27
Speaker 2: singularity and then come back out because it has just
43:30
Speaker 2: enough more electric charge and mass for it to be
43:34
Speaker 2: able to get out again.
43:35
Speaker 1: Yes, for an extremal black hole, but then that electron
43:38
Speaker 1: would be forced out. It wouldn't be able to join
43:40
Speaker 1: the singularity and reveal the Singularity to the next electron.
43:44
Speaker 1: Like that event horizon still has to be non zero.
43:46
Speaker 1: There's no way to get that last electron into the
43:49
Speaker 1: singularity to make the event horizon actually have zero redius
43:53
Speaker 1: for the next one m.
43:55
Speaker 2: So let's maybe dig into what you mean by physics
43:58
Speaker 2: to jail and breaking the law. Why exposing a black
44:02
Speaker 2: hole's singularity break the laws of physics?
44:04
Speaker 1: Yeah, it's a great question. We don't really know what
44:06
Speaker 1: that means. General relativity has no solution there. It's like
44:08
Speaker 1: you try to solve the equations and you get the
44:10
Speaker 1: square root of a negative number. So now what's the
44:12
Speaker 1: radius of the event horizon. Well, we don't really know, right.
44:16
Speaker 1: General relativity basically breaks down there. It doesn't know what
44:18
Speaker 1: to predict. It can't even predict things as nonsensical as
44:21
Speaker 1: a singularity. There's just really no prediction there. And so
44:25
Speaker 1: we don't really know what would happen according to general
44:28
Speaker 1: relativity in that regime. And that's sort of the bigger
44:31
Speaker 1: story that general relativity isn't predicting singularities orringularities or post
44:36
Speaker 1: extremal black holes. It's really breaking down here. It's saying
44:40
Speaker 1: it can't make any predictions. The sign of an infinity
44:43
Speaker 1: or you know, like square roots of negative numbers in
44:46
Speaker 1: the theory is telling you that you've gone beyond the
44:48
Speaker 1: limits of that theory, that it's an approximate theory, and
44:51
Speaker 1: you've pushed it into a territory where it does not belong,
44:54
Speaker 1: where it cannot make any reasonable predictions.
44:57
Speaker 2: Hmmm, well it would break I guess our theory of it.
45:00
Speaker 2: But that's also assuming the universe follows our theories, Like
45:03
Speaker 2: maybe it could do. It would just break our math.
45:05
Speaker 1: Yeah, it would break our math. Certainly, something would happen, right,
45:08
Speaker 1: the universe probably wouldn't disappear in a puff of logic.
45:12
Speaker 1: If you did somehow force an electron inside an already
45:15
Speaker 1: extremo black hole, something would happen, and whatever that would
45:18
Speaker 1: be would be a great clue to tell us, like
45:21
Speaker 1: what's really going on. It would force the universe in
45:24
Speaker 1: some sense to reveal what its true laws are, you know,
45:27
Speaker 1: And that's what experimental physics is all about. It's like
45:30
Speaker 1: forcing the universe in a corner where it has to
45:33
Speaker 1: give up some secrets, or it has to tell us
45:36
Speaker 1: what happens in this situation, which gives us some answer.
45:39
Speaker 1: Maybe it would reveal some weird naked singularity and we'd
45:42
Speaker 1: learn how to modify general relativity, or maybe you would explode,
45:45
Speaker 1: or maybe you would turn into a wormhole, or we
45:47
Speaker 1: just don't really know. But whatever happened in that situation
45:50
Speaker 1: would be a huge clue towards solving quantum gravity, to
45:54
Speaker 1: understanding what happens when things get very very small and
45:58
Speaker 1: very very gravitational.
46:00
Speaker 2: All right, well, it sounds like the answer to our
46:01
Speaker 2: main question then is do black holes have a maximum
46:04
Speaker 2: electric charge? And it sounds like the answer is sort
46:08
Speaker 2: of like it has a maximum electric charge for certain
46:12
Speaker 2: other electric charges, but not others. It doesn't have a
46:15
Speaker 2: maximum electric charge in terms of how much charge any
46:19
Speaker 2: black hole can be because you can have maybe infinite
46:21
Speaker 2: electrical charge if you have an infinitely large black hole.
46:24
Speaker 1: That's right. If you want to charge up your black hole,
46:27
Speaker 1: you also got to make it massive at the same time.
46:30
Speaker 1: That's the recipe for charging up your black hole. You
46:32
Speaker 1: can't just take your black hole and add charge without
46:34
Speaker 1: adding mass. That would break the rules of the universe.
46:37
Speaker 2: Well, it would break the rules of the practical universe.
46:40
Speaker 2: But theoretically, you know, if if I was you know,
46:43
Speaker 2: if I had a cart blong on doing anything I
46:46
Speaker 2: wanted in the universe, could I create a universe with
46:48
Speaker 2: suddenly a super duper extra charge black hole for its
46:52
Speaker 2: mass to appear.
46:53
Speaker 1: Yeah, I think there's no theoretical reason why that arrangement
46:56
Speaker 1: cannot exist. There's just no theoretical way to build it.
47:00
Speaker 1: Starting from a universe and creating that configuration seems to
47:04
Speaker 1: be impossible. Though the actual configuration itself isn't technically impossible,
47:10
Speaker 1: right to put enough electrons in the same place where
47:12
Speaker 1: you lelate that condition. But practically speaking, we don't know
47:16
Speaker 1: of any way to build it. But if you were
47:17
Speaker 1: somehow able to do it, we would break the laws
47:19
Speaker 1: of our universe as we understand it, but we'd learn
47:22
Speaker 1: more about the true laws of the actual universe.
47:25
Speaker 2: Well, you wouldn't break the laws of the universe. You
47:27
Speaker 2: would just kind of break the practical laws of the universe.
47:30
Speaker 2: It'd be unexplained, Yes, it'd be unexplained. Like if you
47:33
Speaker 2: came across one of these things in space, you'd have
47:35
Speaker 2: no way of explaining how it got there.
47:38
Speaker 1: And that's important, right. We look out in the universe,
47:40
Speaker 1: we see stuff, We see atoms out there and all
47:42
Speaker 1: sorts of different elements, and we want to explain, like
47:44
Speaker 1: how do we get to this state? You start from
47:46
Speaker 1: the Big Bang, How is it possible to build all
47:48
Speaker 1: these heavy metals, and we have theories for how that happens.
47:51
Speaker 1: Everything that's out there in the universe, we look for
47:53
Speaker 1: an explanation for how it was made, how it was assembled,
47:56
Speaker 1: and so this would be a big.
47:57
Speaker 2: Mystery, unless, of course, it was just a bunch of
48:00
Speaker 2: this is Dad's telling negative jokes that I'm imploded into
48:04
Speaker 2: a super overcharged black hole.
48:06
Speaker 1: I hope they all collapse in shame.
48:08
Speaker 2: All right, Well, we hope you enjoyed that. Thanks for
48:11
Speaker 2: joining us, See you next time.
48:21
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain
48:24
Speaker 1: the Universe is a production of iHeartRadio. For more podcasts
48:28
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48:33
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