BBC Focus Magazine

[MrIsaksson]

Hi,
I am not claiming to understand black holes but one area where I am really struggling conceptually is how blackholes can grow when there is an event horizon...

If I understand it correctly (and I am sure someone is going to tell me I am not) is the event horizon the point of a black hole where the gravity is so strong that even time stops.

So how can a black hole grow if materia cant get passed the event horizon?
As materia falls towards the black hole time will slow down further and further until it reaches the event horizon where it will completely stop, so how does it grow? As all materia should line up at the event horizon but never actually reaching the black hole?

Can anyone give me the blackholes-for-dummies guide?

Regards
Jon

[The Beige Avenger]

how is the event horizon's position determined in the first place?

The location of the event horizon is defined by the Schwarzchild radius, this is proportional to mass and therefore as mass enters, the radius expands.

What goes on inside a black hole is unknown... You're correct in the respect that gravity becomes "infinite" beyond the event horizon... but time becomes infinite only for the travelling object if it approaches the speed of light... the observer outside of the influence of the black hole's gravity will see the thing get sucked in rather quickly.

Just like a photon that travels experiences "no" "time", so too will the falling body if sufficiently accelerated... certainly once passed the event horizon... but what goes on inside????

[KingPhillip]

The event horizon is the point from which light, travelling at light-speed, cannot escape from the object exerting its tremendous gravitational force, because light can't go any faster.

Objects outside this event horizon travelling at lower speeds would have their own event horizons a lot farther out.

As this "rolling stone" continues to gather moss (sic), the event horizon is extended farther out.

I surmise what happens within a "black hole' is the gravitational force overwhelms the other three, nuclear, weak and electro-magnetic, such that protons and neutrons no longer exist. I would also extend it to the quarks, which would lose their charge and spin properties. If quarks are made of smaller constituents (?), then these constituents are what's left, compressed and heated to such degree we've no science to describe, as yet, beyond what we know as solids, liquids, gases and plasmas.

[M Paul Lloyd]

I think KingPhillip has a point there, under the extreme coditions created gravity could gain the upper hand.
On a similar tack I have often wondered what would happen if two Black Holes collided?
I can't imagine them dancing around each others event horizons but never actually merging, surely not?

[MikeG]

On a slightly different note here, I was wondering something else about black holes. Since a black hole is a collapsed star, and it is essentially a collection of super dense matter at its core, then it's not really a hole, is it? If we could enter the hole without being torn apart by gravity, shouldn't we find the matter resting at the core of the black hole. The reason I mention this, is because it puzzles me when I read that it could be a doorway to another dimension . Basically all that's happening, is a massive object is acting like a magnet, and pulling in anything within range (essentially increasing its mass and power), and distorting space time around it. But at the end of the day, it's not really a hole, there is someone there.

[KingPhillip]

It's akin to looking through a keyhole into an unlit occupied bedroom. Something is going on, but there is no light escaping with information.

[Shadowwolf]

Regards event horizons, tis only the one that light cannae scape from that matters, others may technically be there but we can see them and it's all fairly mundane there. Beyond the horizon that matters is where the fun begins.

Regards doorways 'n' such, well it is a result of a collapsed star but I'm not sure that what lies beyond the horizon could be thought of as merely super-dense matter. It is also best not to be too fixated on the popular name, because much like the label 'Big Bang', 'black hole' - as I understand it - can be a bit misleading as the hole part is referring to the region in space from whence light is not escaping and not a literal hole.

As for two of em colliding, well galaxies do merge so something must happen, bound to be energetic. Read somewhere that one outcome would be one of the central black holes being flung out into the void at a substantial rate. An asteroid may be the least of our worries, imagine one of those things passing through never mind impact.

[M Paul Lloyd]

I'm pretty sure that the name 'black hole' was though up by someone (sorry can't recall the name, somebody 'Weller' possibly?) who didn't actually think they could exist to try and emphasise their apparent absurdity?
Likewise with the 'big bang'.
But I could be wrong.

[Nails]

Bravo MPL, you should be a physics teacher.

Objects whose gravity field is too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was not fully appreciated for another four decades. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black holes of stellar mass are expected to form when massive stars collapse in a supernova at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may be formed.

http://en.wikipedia.org/wiki/Black_hole

Totally wierd.
But not as wierd as white holes.....

http://en.wikipedia.org/wiki/White_holes

[M Paul Lloyd]

Bravo MPL, you should be a physics teacher.

As in, a bit vague and dithering you mean?