Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: neilep on 10/11/2007 23:07:48
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If the forces of a black hole is gravity in bedlam then what is keeping the black hole together in the first place ?...why does it not just swallow itself into nothingness....just like that ?
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In one way that is precisely what it does, but it leaves behind a gravitational field like the grin on the cheshire cat the fact that some other things might occasionally fall into the hole is just an aside and quite difficult
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'nothingness'?
The description of a black hole, out of which stuff can't escape. That's pretty well nothing. Certainly nothing to see. Yours is a fair description.
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THANK YOU BOTH very much indeed.
but...don't black holes have width ?...you get big black holes, supermassive black holes and then small black holes yes ?...all with definitive edges ?
.....I guess I just don't grasp the concepts like you.... (understatement of the millenium)....I'm thinking that if it has an edge then there must be a force creating that edge......but if the gravity is so strong then why does it not just swallow itself and and just wooosh out of existence !! ?
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A black hole doesn't have an edge. It is a point mass but it appears to have an edge. This edge is in fact the Schwarzschild radius, or, the "point of no-return!" Ha, this is the distance from the black hole (which is a point) that the escape velocity is greater than the speed of light. Thus, nothing can escape because nothing can travel faster than light. This makes it look like there is a "black hole" in space but really there is only a super-dense point.
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A black hole doesn't have an edge. It is a point mass but it appears to have an edge. This edge is in fact the Schwarzschild radius, or, the "point of no-return!" Ha, this is the distance from the black hole (which is a point) that the escape velocity is greater than the speed of light. Thus, nothing can escape because nothing can travel faster than light. This makes it look like there is a "black hole" in space but really there is only a super-dense point.
THANK YOU THANK YOU very much Mr Andrew !
Presumably Schwarzschild radius is defined by the specific mass of the Black Hole yes ..and is therefore it's border is variable dependent on how big the black hole is ?
I'm getting there !...thank you all for you valuable insights.
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If light isnt fast enough to make it out of a black hole, why is gravity, which also moves at the speed of light?
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The brain13 has a point. String theorists has posited the existence of gravitons. How do the gravitons get out?
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Here Here....That is a great point by thebrain13 !
Perhaps Gravity does not affect itself ?
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I'm kind of unsure about the nature of gravitons but I've always explained it to myself as if gravity is a curve in spacetime analogous to the curve a heavy metal ball might make in a mattress then the curve produced is a constant and so doesn't need to be mediated by anything and just is. So in that way the gravity of a black hole can in effect "escape" the pull.
But is seems that one can't include gravitons in the explanation as Mr Andrew says:
"gravitons and the warping of spacetime it necessary to recognize that gravitons are, by necessity, incompatible with the general theory of relativity"
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Quote from: Mr Andrew on 11/11/2007 03:40:46
A black hole doesn't have an edge. It is a point mass but it appears to have an edge. This edge is in fact the Schwarzschild radius, or, the "point of no-return!" Ha, this is the distance from the black hole (which is a point) that the escape velocity is greater than the speed of light. Thus, nothing can escape because nothing can travel faster than light. This makes it look like there is a "black hole" in space but really there is only a super-dense point.
I have heard this line of thought before and used to think it true when calculating escape velocities of objects at college.There is a bit of a problem with it though. light or the photons that compose it don't have mass. So if the mass of the photon is zero then F = Gm1m2/r2 m1 = 0 and so the force of attraction is also zero and the photon along with any other light escapes the black hole.
Now we know from plenty of evidence that light does not escape from black holes but as a mechanism for explaining black holes I can't see how this argument works unless you assign mass to light.
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Doesn't Hawkins say something about an Electron and a Positron forming near the event horizon of a black hole. The electon escapes and the positron, with its negative gravity, is drawn into the black hole.
On another tack, what is at the middle of a black hole?
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the positron, with its negative gravity
AFAIK the positron doesn't have negative mass - just negative charge, so it wouldn't have 'negative gravity'.
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Am I right in assuming "dark matter" is basically black holes + all visible matter obscured from our line-of-sight? I've always wondered about that term :)
p.s. Hi guys! 1st post :P
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In reply to blueprint dark matter is that which neither emits nor absorbs e/m radiation but is detectable only by its gravitational effect - holding spiral galaxies together etc.
The fovourite candidate at the moment is the hypothetical non-baryonic neutralino - a weakly interacting particle that is massive enough to cause gravity, emits no radiation and would not collapse with a galaxy's disc; just what is required for a dark matter halo!
Of course, nobody has seen one but there are experiments in the Antarctic ice (AMANDA)and the Boulby Mine site - both based on scintillation detection.
I've often wondered - if neutralinos are detected, what certainty have we that they are the stuff of dark matter?
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A black hole's event horizon is at a different distance from its centre depending on how far you are away from it. The Schwarzchild radius is the distance of the event horizon from the centre only when viewed from infinity (or, more practically, a long way off). As you get closer to a black hole this horizon moves inwards and eventually you would pass the original Schwarchild horizon (from which you could never escape back to your original distance away) without noticing very much - only providing the black hole was big enough. If the black hole is not very big, then the gravitational tidal forces would tear you apart before you got too close, but this is just a practical detail:-)
To go back to the original question, the black hole cannot swallow itself to nothingness. It is never nothing. There is always a gravitational field and, if it had a net charge, an electromagnetic field too. You could also have rotating black holes with angular momentum.
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Doesn't Hawkins say something about an Electron and a Positron forming near the event horizon of a black hole. The electon escapes and the positron, with its negative gravity, is drawn into the black hole.
On another tack, what is at the middle of a black hole?
Afraid didn't study Hawkins at college but I remember a bit about Anderson and the discovery of the positron in 1933. A high energy cosmic ray when incident on a lead plate in a strong magnetic field and viewed through a cloud chamber left a vapour trail indicating a particle equal in mass to the electron but with positive charge subsequently called the positron. nice pic below
<A HREF="http://www.hep.man.ac.uk/babarph/babarphysics/positron.html">
and I remember that positrons and electron annihilate each other giving an intense burst of radiation but I can't see how this constrains light within the black hole to remain trapped.
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the black hole... (whole reason I joined this site in the first place)
as far as I can tell, the photon is the light particle, which acts in waves. I believe this is possible because like space vs time, matter and energy are the same thing. it could be theorized that like space/time, matter and energy become warped with increased velocity. according to this, energy travelling at high velocitys would begin turning into matter, thus allowing a black hole, with its gravitational pull, to affect said photons.
about the black hole swallowing itself into nothingness, it is fundamentally impossible. according to the laws of conservation of mass/energy, mass and energy can neither be created or destroyed. and since matter and energy are like two states of the same thing, the only possible way I see this would require all the matter to be converted into energy. surely there would be universal catastrophe if this were true, since a single black hole is usually around 100 solar masses (maybe way more, cant remember).
I once learned somehwere that with all the "mattergy" in the universe, the universal system would collapse if you removed a single dime's worth of matter from the equation
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Instead of looking at how gravity 'pulls' on a photon, perhaps a better way to look at why they can't escape a BH is to think about how the presence of matter changes the shape of spacetime, introducing slopes, or gradients in it so that it isn't linear. Movement requires both space and time and the presence of matter compressed to a point in a BH creates a slope so steep in spacetime that it exceeds 'c'. Gravity isn't constrained by the steepness of the slope because it is the slope itself.
Umm... I think that's right:)