Questions about Black Holes - Sixty Symbols

0:00 Can anything really cross the event horizon of a black hole?

0:04 Yes.

0:05 Yeah, of course.

0:06 Because we see stuff going into black holes.

0:08 Yes.

0:08 Well, no, we don't see them going in, right?

0:11 We because as they're head we see them heading in.

0:14 I think this might be the essence of the question.

0:15 We see them heading into the black hole.

0:17 But if if it's a to if they were emitting light as they

0:20 they went in, we'd see that light getting stretched and stretched and stretched.

0:24 So maybe start in yellow and begin to go

0:26 to red and get darker and darker and darker

0:28 and eventually the wavelength as far as we

0:31 on earth far away from the black hole perceive it.

0:34 It sort of freezes and I think this is the essence of the question.

0:38 We we never actually see it cross over.

0:40 But if you are if you're the person that is

0:43 heading towards that event horizon actually you see nothing.

0:46 You don't feel anything.

0:47 That's the really weird thing about event horizon especially

0:50 of a big black star which is very low curvature.

0:53 As you pass over the event horizon,

0:55 there isn't a little there should be a little signal saying you

0:58 are like you're passing one county to another or one country to another.

1:01 You now entering the black hole that you will pass through straight through

1:05 it and you won't feel it until you actually sort of quite a way

1:08 into the black hole and then you'll begin to feel oh my word there's

1:11 something down there singularity that's beginning

1:14 to pull me and then you'll get stretched.

1:16 But yeah, things fall in.

1:17 We know black holes grow in size and we have

1:20 super massive black holes right in the core of a galaxy.

1:22 So this famous spaghettification that we all hear about where you get stretched

1:27 if you fall into a black hole that happens after you've passed the event.

1:31 That's right.

1:31 Yeah.

1:31 Yeah.

1:32 You for for really big black holes, super massive black holes,

1:35 it's quite a long I think quite a long way in.

1:37 It might not be a long time,

1:39 but quite a long way in compared to the where the event horizon is before

1:43 the the the the tidal forces which that singularity

1:47 begins to draw on you really begin to kick in.

1:50 I think when you first pass in you think what's what's the big question

1:53 going on here assuming you've known that you've

1:55 passed the horizon you don't you don't

1:57 feel anything in particular as you go past I think this question I think

2:01 it kind of the problem with it it kind of assumes like a a universal

2:06 time like there's a big clock in the sky I know it I know

2:10 it tries to sort of not assume that but I think at the back

2:13 of its mind it is and I think what you've got to remember is

2:17 when you're an observer falling into the black

2:19 hole you have your own clock, right?

2:22 And that's what's important.

2:23 And you do pass through the black hole.

2:26 It is indeed true that the universe, you know,

2:28 as a distance will will you'll see the whole history of the universe.

2:31 And and likewise, somebody watching you fall in will see um you know,

2:36 never actually sees you fall in because of the the the the way

2:39 in which time slows down between the two observers,

2:41 the difference at which the rate at which time ticks for the two observers.

2:44 So Brady, if I if I watch you fall into a black hole,

2:48 um what will happen is that I will see your clock

2:52 on your watch slow down and slow down and slow

2:55 down until eventually it feels like you're sort of suspended

2:58 in time on the on the edge of the black hole.

3:01 So you'll just see my last desparing look as I fall away forever.

3:05 Exactly.

3:06 But I never actually see you fall in.

3:07 But what do you experience?

3:08 You actually do.

3:09 You don't see anything weird, right?

3:11 If you're just looking forward at the black hole,

3:12 your your you know your clock is just ticking at a normal rate.

3:15 You go through the event horizon

3:17 and until you start to approach the singularity, nothing weird happens, right?

3:21 So there's not now if you look if you look backwards,

3:23 you would as you approach the horizon, as I see you, what would I say?

3:27 Yeah, you would you would see the whole history of the universe.

3:29 That is true.

3:30 You would see me die, right?

3:31 You'd see, you know, generations pass and so on and so forth.

3:35 The whole history, right?

3:36 Now it becomes problematic when you start

3:38 trying to resolve this on really really small

3:40 time scales because then you don't really know

3:42 what the physics is on those time scales.

3:44 But you know when you coarse grain over it I think there's there's no problem

3:47 with this and it's indeed it's just a an effect of the different rates of time.

3:52 And I think that for you you have your time.

3:55 You're going to cross that event horizon.

3:57 No big deal.

3:58 you do cross it, but I have

4:00 a different perspective because I'm because I'm a different

4:04 observer and I'm moving at a different rate

4:06 in a different point in the gravitational field.

4:08 And so I see I see your time slow down relative to me,

4:12 but it's a purely relative effect.

4:16 Will we ever develop powerful enough

4:18 particle accelerators to create mini black holes?

4:23 Okay, of course.

4:24 back in 2008 this was the concern right when they turned

4:30 the LHC on at 13 at the time I think it was 8

4:34 TV was the that we would collide particles together protons together

4:39 at such a high energy that they would create a mini black hole

4:42 that then would eat up the world and I remember our girls

4:45 who were at school um at junior school having the teachers having

4:50 to say you might have heard about this this isn't going to happen

4:53 and Actually CERN had a a put together a group of physicists.

4:58 I remember Michallangelo Mangano my friend and who's at CERN headed it

5:03 all where they produced a document explaining that you're not going to do

5:06 this and on in the minuscule chance that you did you'd create black

5:10 holes which are so small that thanks

5:12 to Hawking evaporation they'd immediately decay.

5:14 We're not likely to be able to build colliders that are

5:17 going to get much more powerful than we have right now.

5:20 They're talking about 100 I mean this is 13 TV that the 100

5:26 km one will be will take us up by factors of 2 three

5:30 the same properties of black holes will still be there if you were

5:33 to find them then they'll decay almost

5:36 immediately so they're not going to cause us any problem of course there are

5:39 some quite interesting theories where in principle

5:42 you could these higher dimensional theories where

5:44 you could form them and that's what

5:46 generated the interest in the first place but the fact we've not seen

5:49 any at the T at the LHC means those theories are now tightly constrained.

5:54 So they're not likely to produce any.

5:57 Don't panic.

5:57 You say don't panic.

5:59 Okay.

6:00 Will we ever develop powerful enough

6:02 particle accelerators to create many black holes?

6:06 Uh I think it could happen.

6:07 I I think it's not impossible.

6:09 Um it's certainly true that you know we that we could be

6:15 at the at the edge of of detectability on on on many black holes

6:19 that colliders that it requires certain things to be true of nature and it

6:23 requires that you know that we that we live on some higher dimensional

6:28 on on one of these sort of surfaces these four dimensional surfaces

6:31 in some higher dimensional space time

6:33 and that we're tricked into thinking the scale

6:35 of quantum gravity is a bit lower than we than we already that we

6:39 currently think it is and that makes it easier to produce these black holes.

6:43 So that that that could be the case.

6:45 If those things are true and those are

6:47 really you know interesting sort of theories

6:51 um then yeah I mean we we could well be on the edge of of detection.

6:55 So you say 100 TV collider which is not impossible could produce one of these.

7:00 So yeah I would say yes but it's nothing to be worried about.

7:04 It's nothing to be worried about.

7:05 Brady we've done a video on this.

7:06 So, we did a video a long time ago.

7:08 We've done a video on everything, Tony.

7:09 Yeah, that's true.

7:10 Yeah.

7:11 No, I would say not.

7:12 No, I mean, look, so so the black holes that I mean,

7:14 including the question, right?

7:15 It says they're mini black holes.

7:17 So, a mini black hole, small black hole.

7:19 That means the weird thing about black holes

7:20 is they're not really as black as they seem.

7:23 They give off a radiation called Hawking radiation.

7:26 It's when you apply quantum mechanics to a black hole.

7:28 But the weird thing is the smaller a black hole is, the hotter it is.

7:32 So, it's actually it's not actually the big black holes are really cool.

7:34 barely give off any Hawking radiation.

7:36 It's the small ones that give off a lot.

7:38 So if you're at a collider, you're necessarily going to be producing small black

7:42 holes because the process is a small process, right?

7:46 And so they are going to be small black holes.

7:48 Those are really really hot.

7:49 A really really hot black hole is going

7:50 to evaporate very very quickly through Hawking radiation.

7:54 It'll do so in a really distinctive way.

7:56 It'll light up the the detector

7:58 in the collider sort of equally in all directions

8:01 and in a completely democratic way as well

8:03 in the sense that it'll produce all types of particle, right?

8:06 It won't just produce one particular type.

8:07 It be very democratic about it.

8:09 And you start to see that then you're like,

8:11 yeah, I reckon that was a black hole.

8:13 That sounds awesome.

8:14 Yeah, it is awesome.

8:15 It's well awesome.

8:15 I love the idea of the black holes today actually.

8:18 What happens when an atom straddles the event

8:21 horizon of a super massive black hole?

8:28 What happens if it straddles it?

8:30 I'm my guess is it falls in unless you've

8:33 unless you have given it a kick which allows it

8:38 you know if it's moving close to the event horizon

8:40 but but is somehow sort of just going past it.

8:44 It could get away but I think if it's straddling it then the gravitational

8:49 pull of the event horizon is just going to force it to to come in.

8:52 It's not it's not going to do anything special on that.

8:54 I guess.

8:55 Yeah, because I guess where that question is coming from is

8:57 we all know that you can't pass back over an event horizon,

9:00 but if but if the atom's half in and half out, you know, it's going to go in.

9:04 I think I think it's going to go in.

9:06 Okay.

9:08 In fact, you I mean there are you you

9:10 can demonstrate that there are there's a set of trajectories

9:14 that you have to be on in order to sort

9:16 of skirt the event horizon and get away from it.

9:19 It's got to be coming in with a certain velocity in order to do that.

9:23 Oh, there was a second part to the question.

9:25 Sorry.

9:25 The second part was, does it become instantly ionized even when

9:29 the nucleus is on the outside of the event

9:31 horizon because the nucleus can't feel the effect

9:33 of electrons on the inside of the event horizon.

9:36 Right?

9:37 So I my my gut feeling is it won't be because I think the gravitational

9:41 field I mean it can only be ionized here by the gravity by pull of gravity.

9:44 There's no other electromagnetic forces interacting with it.

9:49 So, and the and on a super massive black holes, it's very weak.

9:53 It's very weak.

9:53 You you barely feel it.

9:55 So, I don't think it will ionize it.

9:57 What happens when an atom straddles the event

10:00 horizon of a super massive black hole?

10:02 Does it become instantly ionized even when

10:05 the nucleus is on the outside of the event

10:07 horizon because the nucleus can't feel the effect

10:10 of electrons on the inside of the event horizon?

10:14 I I think it's going to get stretched.

10:16 I mean you have to that's a difficult question to answer because you have

10:19 to it's an extended object right you want to treat it as an extended object.

10:23 So we have to define well by whose reference frame are we going to discuss

10:27 it because if I okay let me kind of spin the question around a little bit.

10:34 You're about to fall into a black hole, Brady.

10:37 Right.

10:38 Um, and you're the black hole horizon's just there, right?

10:42 It's just in front of you.

10:43 Okay.

10:44 So, you reach out.

10:46 Okay.

10:46 What do you see?

10:48 Right.

10:49 Do you just see your hand there?

10:50 Like, no.

10:51 Your hand is going to stre there's going to be a dilation

10:54 effect which is going to stretch your hand out all around the horizon.

10:58 It's like everything just gets stretched.

11:00 You won't see your hand like a hand.

11:02 you see this stretched out weird hand thing, right?

11:06 It's completely weird because, you know,

11:08 so so when I when you talk about this particle straddling the horizon,

11:13 it's not like you've got a part you you and you're looking at it, right?

11:17 You're not going to see a little bit of particle and just gets cut off.

11:20 You're going to see some weird distortion thing going

11:22 on and the part part of the particle is closest to the horizon

11:25 and is going to be stretched all around it and then

11:27 of course that effect is decreases as you move away.

11:30 So, it's it's really weird, but that's what I'm seeing with my eyes.

11:34 That's not what's actually happening to my hand.

11:36 So, what's actually happening to the atom?

11:38 Does the I mean, I guess they're saying,

11:40 does the atom get destroyed crossing an event horizon because

11:44 part of it has to cross before the other part?

11:46 Well, I guess I wouldn't say it's destroyed.

11:51 No.

11:52 Um because if it's far from the um if it's a if it's a large black hole,

11:58 let's say it's a large black hole.

11:59 Okay.

12:00 Then um there's nothing unusual about

12:03 the gravitational field at the end horizon.

12:05 Absolutely nothing.

12:06 It's just a gentle gravitational field.

12:08 There'll be not there's nothing like the gravitational forces

12:11 to rip rip that atom apart or anything like that.

12:13 It's just an opt.

12:14 It's just like a a thing about the sort of time scales and and the length

12:17 contraction and time dilation which is perfectly

12:20 fine that that you would see, right?

12:23 But in terms of the part, it's not being ripped apart or anything like that.

12:26 Absolutely not.

12:27 Are we inside a black hole now?

12:29 And if so, how would we test for that?

12:33 I'd have thought we weren't.

12:36 And um cuz you told me if we pass over an event horizon, we might not know it.

12:43 Yeah.

12:43 I think passing over event horizon, we might not know it.

12:46 Could we not be in like some mega black hole?

12:48 Yeah.

12:49 Um there there are papers that I have read which do do say this.

12:55 Yeah.

12:56 So it's so so huge that we are a small

13:00 island within it and so we're not yet feeling the singularity.

13:04 I don't know.

13:05 I don't know how to test.

13:07 I mean if if we could if we could somehow get some feel

13:11 for our overall flow compared to the the space in which we live.

13:16 I mean that we we are flowing in in terms of we we are on earth if

13:22 you we're moving in a given direction within

13:26 um the universe and people say that's because there's

13:30 potentially some very large mass structures there

13:33 that that are called great attractor in fact that's that's

13:37 causing us to to flow but no one ever

13:39 thinks of that as being flowing a black hole.

13:43 I guess if we really were in a black hole,

13:45 we would be heading towards a singularity and we would

13:47 begin to see our slowly but surely things get elongated.

13:52 Um, that might be a test if you could test it all the way

13:55 around everything and see if gradually over

13:59 many millennia that things begin to get elongated.

14:03 That might be a a test.

14:05 Yeah.

14:06 Otherwise, I don't know.

14:07 Are we inside a black hole?

14:09 And how would you test it?

14:12 So, I don't think we can know.

14:14 I think it's possible that if we're in a giant giant truly ginormous black hole,

14:18 which is larger than the size of our cosmic horizon, we have no way of knowing.

14:23 And I don't think we've got any way of testing it either.

14:25 So, doesn't that mean there's some kickass

14:27 singularity somewhere if the event horizon's that big?

14:32 Right.

14:33 I guess it would.

14:39 Um, which sounds a bit alarming, but I would Yeah.

14:42 Okay.

14:42 Now I'm bothered.

14:44 Uh, because I don't want that to be I don't

14:47 want to be it to be naked relative to us, right?

14:49 That sounds bad.

14:50 Although it's a time that would be What do you mean naked relative to us?

14:55 So I the idea of a naked singularity is is a bad one, right?

14:58 I mean, you can't make any predictions if you're exposed to a naked singularity.

15:01 But of course, that's that depends on the nature of the singularity.

15:04 If the singularity is a singularity in time, it's an end of time.

15:07 which actually depending on the nature of the black hole it could well be.

15:11 So if we're talking about um yeah swatch black hole

15:15 it's an end of time so it's a future singularity.

15:18 Um so it's not like it's just lurking there.

15:20 It's lurking in the future not not not um in space.

15:24 This sounds freaky.

15:25 I still don't know what naked means but so naked singularity.

15:28 So, so, so you want the this is why

15:30 people talk about horizons or one of the reasons

15:31 people like to talk about horizons because the idea

15:33 is is that if there's a black hole, let's let's not talk about us living in one.

15:36 Now, let's just imagine there's a black hole.

15:38 So, a black hole, yeah, indeed it contains a singularity or some okay,

15:44 we might not really be a singularity.

15:45 In fact, it's probably not really a singularity.

15:47 It's probably something resolving it.

15:48 But, let's there's some weird place inside that black hole.

15:52 If if that singularity is shielded by an event horizon,

15:56 then we're kind of the physics outside

15:58 of the event horizon is protected from it.

16:00 We can't be influenced by it.

16:02 If it's not, if we have a naked singularity, so we don't have an event horizon.

16:06 So we or we're inside the event horizon.

16:09 Well, again, then it depends.

16:10 It might just be in the future then if it's rather than spatial.

16:13 But but let's think about let's imagine

16:14 we're exposed to a spatial singularity, right?

16:17 It's a singularity in space and we're exposed to it.

16:20 there's nothing protecting us from it,

16:22 then we really have no control physics- wise of what

16:25 is coming going in or out of that singularity.

16:27 It's it's a divergent point in in space, right?

16:31 It's completely so we have so so we can't

16:33 say anything about the evolution of the universe, right?

16:36 Because because anything could come out of that guy.

16:39 There could be elephants coming out of it.

16:41 There could be Donald Trump coming out of it.

16:42 There could be anything, right?

16:43 We we we don't know because we cannot control the equations there.

16:47 So you lose all control of the physics the minute you have a naked singularity.

16:51 So they're bad, right?

16:53 We're talking about one in the future.

16:54 It's a kind of, you know, that's that's different.

16:56 But a spatial singularity is just sitting there.

16:59 No, no, no.

16:59 That's we need we need a horizon protecting us from it.

17:02 Otherwise, we're screwed.

17:03 We can't do physics.

17:04 So if we're living inside a black hole, all physics could be wrong.

17:12 No, because I would think the kind of thing you're

17:13 imagining is probably a future event or a future singularity.

17:17 So, I'm not sure that's necessarily problematic.

17:20 Um, in the same way, you know,

17:23 the disruption it would cause, the expense it would cause.

17:26 And they did a lot of research.

17:28 You know, it wasn't that the the ring which uh was built,

17:32 you know, has just opened in 2009, was was built devised in 2008.

17:37 Back in the 1980s, people began to talk about it.

17:40 We're building all this sort of this great big kit and you know we're

17:43 we're making these um these protons sort

17:46 of giving them the energy of a mosquito.