# The Naked Scientists Forum

### Author Topic: What happens at the boundary of a black hole?  (Read 2053 times)

#### thedoc

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##### What happens at the boundary of a black hole?
« on: 28/09/2015 02:50:01 »
Dr Toby Fagan  asked the Naked Scientists:

I've only recently discovered your programme on BBC's iPlayer, but I'm now an avid listener even if I am in County Durham at the moment!

I have a question about time dilation and black holes. I've got my head round most of the weirdness that happens with general and special relativity, with one exception. I'm sure I've heard this at least twice now, once I think from the nouveau majeste that is Dr Brian Cox, and on a downloaded lecture covering relativity and particle physics.

The bit I don't get is that they both seem to be saying that, to an outside observer, nothing can ever actually fall into a black hole since time slows so massively that time stops at the event horizon.

I get that a fixed light speed means time is variable, slowing as you approach the speed of light, and that something similar happens to distance (the lecture on relativity from Audible used an illustration along the lines of the twin paradox, one twin gets on board a space ship bound for a star 10 light years away, and their ship is capable of 0.8c as max velocity; for the twin who stays on earth, the ship is gone on a round trip lasting (10/0.8)x2 = 12.5 x 2 = 25 yrs; for the twin on the spaceship however he gives an experienced time of 7 yrs each way (no, I'm not working through the entire Lorentz transformation!), which then implies that to the twin on the spaceship they only travelled 5.6 light years each way); I get that gravity has a similar effect, such that time runs slower in a gravity well; I gather this has even been proven using atomic clocks that have been up in orbit around the Earth for a while.

Now, what I don't get, is what is supposed to happen when you approach a black hole.

What my understanding about it says, is that if you take the twins again, (one of whom possibly has a terminal illness or is feeling acutely suicidal, as they choose to fly a spaceship straight into a black hole, or maybe they just want to get into the record books, from the point of view of the twin who is outside the black hole, watching the dive, they will see the ship approaching the black hole at, presumably, some crushing rate of acceleration from the local gravity, and that the twin outside will see their twin's ship disappear as it crosses the event horizon; this will take place in an appropriately small amount of time which is dependent solely on the ship's starting distance from the black hole, and the initial speed of the ship and it's acceleration due to gravity as it approaches the black hole, in a more or less mechanistic manner.

For the twin who is actually falling *into* the black hole, all the relativistic stuff comes into play, and they experience a massive time dilation effect, so that for 1 second of their time, several minutes/hours/days/months/years could potentially pass for the outside observer; however this seems to then lead to the conclusion that, if time were to actually stop at the event horizon, the twin who is diving into the black hole never enters the black hole, since time has stopped, and to the external universe their ship will always be sitting just on boundary of the event horizon.

To my mind, this is patently nuts.

Surely regardless of the time dilation experienced by the one flying into the black hole, from the external view, they will approach the black hole at an accelerating rate until they disappear inside it; if this wasn't true, then how can Hawking quantum black holes radiate energy (if no time passes at the event horizon, how can anything be emitted?), and how can things like quasars exist since they are thought to be supermassive black holes giving off radiation as matter is sucked in; if time freezes and nothing ever enters the event horizon, how can quasars be sending out blasts of radio waves and other EM radiation? I would have thought the conclusion would end up being that the twin's final approach to the black hole is happening so fast that even though time slows nearly to zero, the actual amount of time that is needed to enter the black hole also becomes so tiny, that to the external observer, they disappear on cue...

I don't know if I've explained myself very well here, but I'm hoping you understand my question! I realise it may not be all that topical, but then again you did recently talk about making wormholes from exotic matter

Yours,

Toby
What do you think?
« Last Edit: 28/09/2015 02:50:01 by _system »

#### Bill S

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##### Re: What happens at the boundary of a black hole?
« Reply #1 on: 28/09/2015 21:05:08 »
Hi Toby, welcome to TNS.

I'm no scientist, so nothing I say has any authority.

Have you considered that if time can be thought of as stopping at the EH, it is only in the F of R of a distant observer, and that in the F of R of the in-falling astronaut time will appear to pass normally with no observable time dilation.

#### dlorde

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##### Re: What happens at the boundary of a black hole?
« Reply #2 on: 28/09/2015 21:16:11 »
Have you considered that if time can be thought of as stopping at the EH, it is only in the F of R of a distant observer, and that in the F of R of the in-falling astronaut time will appear to pass normally with no observable time dilation.
Yes, AIUI this is the consensus view.

The twin observing at a safe distance will see the in-falling twin appear to slow and fade into the infra red. The image would fade out of the visible spectrum before the EH proper was reached, so they wouldn't appear to hang there forever as some popular accounts would have it. The in-falling twin would only notice tidal forces becoming ever stronger, for a sufficiently large BH, crossing the EH would be not be noticeable, and visually, it would appear to retreat ahead. There have been debates about a 'firewall' idea that would have the EH destroy anything entering it, but last I heard, it had been shown likely to be incorrect.
« Last Edit: 28/09/2015 21:22:25 by dlorde »

#### evan_au

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##### Re: What happens at the boundary of a black hole?
« Reply #3 on: 28/09/2015 22:03:23 »
Quote from: Dr Toby Fagan
how can things like quasars exist since they are thought to be supermassive black holes giving off radiation as matter is sucked in?
As you say, Quasars and Active Galactic Nuclei are thought to originate from the Accretion Disk swirling around a black hole.

This is heated to enormous temperatures through friction. It is thought to create intense magnetic fields which spews some percentage of the infalling matter out at the poles of the black hole, at a significant fraction of the speed of light.

So the light of the quasar originates from just outside the event horizon, rather from inside the event horizon.

#### jeffreyH

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##### Re: What happens at the boundary of a black hole?
« Reply #4 on: 28/09/2015 22:17:30 »
So then what is the origin of the magnetic field?

#### Spike the Hedgehog

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##### Re: What happens at the boundary of a black hole?
« Reply #5 on: 29/09/2015 09:59:59 »
Most depictions of a quasar show gas and matter in a spiral around the black hole; I'm guessing heat through friction and possibly increasing density or conversion of some of the gravitational potential energy into heat will turn gases into plasma, and charged plasma ions spiraling around the black hole will then create a magnetic field since any moving charge will create a magnetic field...

I can't remember if this is the left hand or right hand rule, but the magnetic field would be perpendicular to the plane of the spiralling matter I think, and I think this then makes the ejected energy from the effective 'poles' of the black hole?

#### evan_au

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##### Re: What happens at the boundary of a black hole?
« Reply #6 on: 29/09/2015 11:26:00 »
Quote from: jeffreyH
So then what is the origin of the magnetic field (in an accretion disk)
The generation of jets from an accretion disk has so far resisted detailed analysis, and there are several theories.

It is not even certain if the jets are emitted as a continuous stream, or as a series of "burps".

Analysis of accretion disks involves magnetohydrodynamics, an area that is quite complex to model. It is even harder than the Navier-Stokes equations, which are recognized as worthy of a Millennium Mathematics Prize.

#### jeffreyH

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##### Re: What happens at the boundary of a black hole?
« Reply #7 on: 29/09/2015 20:00:52 »
The crux of the problem is the composition of the accretion disk. It it clumps of matter or gas? Can a magnetic field generated by the disk extend to the poles of the black hole. Also what happens if the field crosses the event horizon. If it is considered that virtual photons are force carriers for the field then this may generate magnetic monopoles as those virtual photons crossing the horizon are trapped and cannot be part of the normal circulation of a magnetic field. This will also limit the range of parts of the magnetic field.

EDIT: Of course there may be a prohibition on monopoles which would force the field to circulate outside the horizon and distort the field. This would intensify the strength of the field outside the horizon and may be the cause of the jets as the field circulates over the polar regions.
« Last Edit: 29/09/2015 20:05:21 by jeffreyH »

#### jeffreyH

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##### Re: What happens at the boundary of a black hole?
« Reply #8 on: 30/09/2015 21:42:39 »
As an addition to the above, since the combined magnetic fields generated by the accretion disk would all intersect and cross at the poles this may well be the reason for the jets. In fact they may not intersect at al, and there may be curls in the fields at the poles.

#### The Naked Scientists Forum

##### Re: What happens at the boundary of a black hole?
« Reply #8 on: 30/09/2015 21:42:39 »