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Topic 194713

Yet Another (few) Question(s) About Black Holes...

So... We've got a little bit of a hang up about black hole infinities and a presumed 'singularity' and the breakdown of all known physics as we might know it...

However...

What if...

Black holes maintain a small but finite physical radius?

OK, so we cannot directly detect that due to the event horizon, but what evidence do we have that a mathematical singularity is actually formed?

Would not gravity itself suffer a limit at some ultimate density due to the nature of the hypothesized Higgs boson?

Can gravity act within an infinitely short range to continue the collapse?

Would the Plank limit limit further collapse?

Would not angular momentum come into play to cause a phenomenal speed of rotation and generate impossible sheer forces upon collapse? Would not the subsequent heating boil everything into radiation that would then limit the collapse?

And is not a singularity an impossibility by the mere fact that time will be progressively slowed for the object such that the collapse can never reach the point of ever being a singularity within the lifetime of the universe?

Also, a black hole exhibits a gravitational field. Do black holes also emanate a magnetic field or electrostatic field?

And by how much does time slow at the event horizon?

Regards,
Martin

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YAQABH

Quote:

Yet Another (few) Question(s) About Black Holes...

So... We've got a little bit of a hang up about black hole infinities and a presumed 'singularity' and the breakdown of all known physics as we might know it...

However...

What if...

... Would not the subsequent heating boil everything into radiation that would then limit the collapse?

... never reach the point of ever being a singularity within the lifetime of the universe?


That looks like matter converted into an awful lot of photons (energy) all stacked up in one place (as only photons could be)...

And do not photons live forever?...

Bosons anyone?...

Or a subatomic zoo but on the scale of a beachball?

Possibly an alternative view?

Regards,
Martin

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Mike Hewson
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RE: Yet Another (few)

Quote:

Yet Another (few) Question(s) About Black Holes...

So... We've got a little bit of a hang up about black hole infinities and a presumed 'singularity' and the breakdown of all known physics as we might know it...

However...

What if...

Black holes maintain a small but finite physical radius?

OK, so we cannot directly detect that due to the event horizon, but what evidence do we have that a mathematical singularity is actually formed?

Would not gravity itself suffer a limit at some ultimate density due to the nature of the hypothesized Higgs boson?

Can gravity act within an infinitely short range to continue the collapse?

Would the Plank limit limit further collapse?

Would not angular momentum come into play to cause a phenomenal speed of rotation and generate impossible sheer forces upon collapse? Would not the subsequent heating boil everything into radiation that would then limit the collapse?

And is not a singularity an impossibility by the mere fact that time will be progressively slowed for the object such that the collapse can never reach the point of ever being a singularity within the lifetime of the universe?

Also, a black hole exhibits a gravitational field. Do black holes also emanate a magnetic field or electrostatic field?

And by how much does time slow at the event horizon?


Black holes are sort of the Ultimate Catch-22. Both physically and mentally. :-) :-)

One needs to distinguish between what is modelled ( choose your variant ) vs What We Really Know To Be True ( not a whole lot ).

The infinity/singularity comes from the modelling - even Newtonian gravity has infinite strength at zero radius. See John Michell who was arguably the first to broach the topic of the objects we would now label as black holes. You'd also find this zero radius business with Coulomb's Law too - but this has been averted by various trickeries involved with Quantum Electrodynamics ( Feynman/Schwinger/Tomonaga Nobel 1965 ). As gravity has not as yet been quantised ( due to GR non-linearity or self energy - gravity attracts gravity ) then that type of QED escape from singularity hasn't yet been achieved.

If, quite sensibly, the central area was a non-zero radius thus not of an infinite density we could perhaps feel better about it. But the only way you would know is if you went in and had a look. We who stayed at home couldn't because (a) we're not you and (b) the event horizon prevents you telling us! So in a very real ( at least classical ) sense when you go through the horizon you may as well have departed this universe from our point of view.

So the key seems to be the horizon passage. There are two stories here : the one you experience ( silly chap you are diving into the hole! ) and the one we experience ( us distant slackers watching what happens to you! ). I place you as the brave traveller because you volunteered the question! :-)

Martin's Personal View ( provided the hole is massive enough so that when you pass the event horizon the gravity gradient doesn't rip you apart ) : nothing particularly is felt as you descend. If you try to you can't get out. If you look up at the spectators then they will appear to speed up. Eventually you will hit the centre which will be bad news regardless of singularity or not. The radius at which the event horizon occurs is proportional to the overall mass of the hole. The larger the radius the gentler the rate of change ( gradient ) of the force at that radius. So a few tens of solar masses BH will rip you up possibly even before you get to the event horizon compared to a multi-mega solar mass BH. Here on Earth we have little discernible difference b/w the acceleration due to gravity over the distance from our head to our toes. So if we skydive the risk is not related to that. :-)

[ also further out from the event horizon radius is the 'static limit' within which stable orbits - not spiralling inwards - can only be maintained through some application of extra energy. Thrust from rocket say. ]

Martin's Spectators : We see you approach some apparent bounding surface with your movements ( or any other time related activity ) gradually slow to a perceptible stop. And you fade too because any photon's you emit are progressively red-shifted. We never see you cross the event horizon boundary, and assuming certain predictions about the evolution of black holes over cosmic timelines, we never will ( but see Leonard Susskind about this ).

Now angular momentum is a curiosity. In reality probably all actual black holes have this, it being unlikely that collapse from a larger/diffuse object would not have some nett rotation about the centre of mass. Theory indicates that the central singularity would be shaped like a donut/torus - the type with a hole in the middle like a vehicle tyre - the axis of rotation goes perpendicularly through the centre of the hole. Some hypotheses state that passing through the event horizon from say either 'North' or 'South' of the torus and thus plunging from one side to another via the gap in the donut could actually be a route to 'some other universe' ( see Kruskal embedding diagrams ). You still won't be coming back to the original one though!! [ at least not so as to upset cause and effect ]

Black holes certainly can have electromagnetic properties - an overall nett charge say - but like most extended objects in the universe usually something drifts in to neutralise that.

Regarding the lifetime of the universe it may be true that eventually the BH's evaporate as per Hawking radiation ( quantum effects or 'blurring' of the exact radius of the event horizon ). In which case all that fell in should be re-radiated out come that day. I think 10^(+60) years is seen as a reasonable estimate. We are only at ~ 14 billion years now, so that's 10^(+10) years. Note these are exponents.

As for Higgs/Planck/short-range : pick your model. They abound ... :-)

Cheers, Mike.

( edit ) Also one book I read talked of a second horizon wrapped within the traditional first - in the case of fast rotating BH's. But by that stage I was well and truly boggled out. So I don't know what that means. :-)

( edit ) The 'easiest' book I've found on this topic is this one. Note the nominative determinism ..... :-)

( edit ) More generally it is worth mentioning that you don't really get absolutes in this topic. Relativity does imply that The Universe gives us different stories depending on the point of view of a given situation. Even involving what we would a priori think of describing the same thing ..... the saving grace is simply whether a specific viewpoint gives rise to any contradiction within that viewpoint. In the above cases we will never/ever get to compare notes, so disparate descriptions can hold.

( edit ) And Roger Penrose has some diagrams too .....

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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RE: RE: Yet Another (few)

Message 96455 in response to message 96454

Quote:
Quote:
Yet Another (few) Question(s) About Black Holes...
.....


Thanks for the good discussion.

Trying out one bit at a time...

So black holes can exhibit electrostatic and magnetic forces beyond their event horizon?...

Obviously gravity is felt beyond the event horizon...

So is there an assumption that the "particles" that mediate/communicate those forces are 'massless'?

Cheers,
Martin

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Mike Hewson
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RE: RE: RE: Yet Another

Message 96456 in response to message 96455

Quote:
Quote:
Quote:
Yet Another (few) Question(s) About Black Holes...
.....

Thanks for the good discussion.

Trying out one bit at a time...

So black holes can exhibit electrostatic and magnetic forces beyond their event horizon?...

Obviously gravity is felt beyond the event horizon...

So is there an assumption that the "particles" that mediate/communicate those forces are 'massless'?


Ah, this is the weird(er) bit. To a certain extent everyday words fail as we don't have anything similiar on a human scale. So my description is flawed and approximate. I am beginning to appreciate that the best language for this is mathematics. But I'm finding tensor arithmetic rather hard ..... :-)

You're thinking in God's Eye terms. An overall model/visualisation of the total situation. Fine. But the predictions, who/what feels and does this/that, have to be in terms of some particular viewpoint. Sooo ....

.... from outside of the black hole horizon everything seems to pile upon the surface that it represents. The horizon is not a real object but an apparent limit. Everything just goes on down to it and freezes in place. So the charge/angular-momentum/whatever is felt by outsiders as existing on that surface. The gravity - and here is the crucial point - is not an extra thing on top of this geometric behaviour, it is the geometric behaviour! We have already accounted for gravity when describing the horizon and it's nature. So specifically we don't talk of gravity from within the horizon reaching out and grabbing stuff on the other side. You could say that the mass in the black hole creates the horizon, and it is said horizon that we interact with from afar.

As for the electromagnetic stuff reaching over the horizon from the inside - from an outsider's position it never went in. You have to include time in the description.

Now you might want to think of the following : sit around well away from the hole for a while. Chuck stuff in. Watch it gradually stack up, slowed down and fading, on the horizon surface. Then go down and approach the horizon yourself. If you look back up from where you came from ( I'll be there waving at you, say ) things appear to speed up. In a sense you are travelling into the future of those who remained afar. Now look down to the horizon below you. The stuff that was stacked up before now speeds up - your clock speed is approaching theirs - and things begin to move again ( a little ) and brighten up a bit ( less red-shifted ).

You could even perceive an ordering, meaning that stuff that you threw down earlier will appear to be deeper in the well than stuff you put in later on. So if you marked your litter as 1, 2, 3, 4 .... etc in sequence as you had been throwing it down beforehand when up with me, you will now 'catch up with it' and see it in order ( from the nearest to you to furthermost away ) as .... 4, 3, 2, 1. You will approach the most recently thrown in stuff before you could overtake the earlier dross. Or put another way, we don't lose cause and effect, or re-arrange timelines. Plus the lower numbered items, being 'further down' are running progressively slower than the later numbered items.

[ I'm assuming I originally threw each piece of the garbage in with the same velocity. So there'd be no reason for them to pass by one another. Freefall only and with no rockets or stuff like that. 'Timewarp' is a bit of a misleading phrase. I just like to remember the idea as it really is, clocks compared from different positions can in general disagree in 'how fast they run'. It's tricky to get the right language ..... ]

Don't go over the horizon though. Come back up again to sit with me. I'll be rather older than you may expect though. From your personal time passage I have aged somewhat, and I'll think you're a whippersnapper. For me, your time slowed when you went down and back. For you my time sped up while you were away. If we were born as identical twins, we will now ( after this difference in who toured where ) agree upon which of us is now the older or the younger.

Had you gone over the edge you would follow the stuff you threw in to whatever is at the centre. But as we no longer communicate there are no mutual observations to reconcile :

- I saw you hit the horizon, splat, and stay there. The universe and me in it grows old and dies, or whatever, without you changing ( much ).

- You saw me and the rest of the universe speed up to a ridiculously fast rate. Plus blue-shifted for that matter. You think you've gone into the universe's infinite future. But your personal time is finite, even if you were otherwise immortal on biological grounds. The end is the core of the black hole .....

Well photons are massless. The mediators of the strong and weak nuclear forces aren't ( all that I've mentioned above will be similiar for many phenomena, like radioactive decay etc ). In any case with gravity, the classical field as it is represented in GR has not yet been quantised. So I guess we can't as yet talk of gravitons confidently - we don't know how to characterise them in a consistent and useful way. As I've alluded to above, gravity is unique among the forces - it acts on absolutely everything and is the background upon which other forces behave. Roughly speaking if charge A wants to know how far way charge B is, it has to ask gravity - which is the geometry of the spacetime that the charges are sitting in!

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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