Black hole questions

[huggiebear]

I've got a couple of blackhole questions that i can't seem to find a dummies answer for

1 - If nothing including light can escape a blackhole then why do quasers escape from it?

2 - There must be a distance from the black hole where gravity gets weak enough for light to escape. So why can't we see a a round disc with light around it something that looks like a total eclipse?

[kenmyers]

1. I do not believe quasars do escape from black holes at all.

2. The point at which light doesnt get pulled down is the same point you will see space as it should look again. Without light distortions. It would be invisible as it will bend light around it filling the gap like a cloak.

The black hole is a bad terminology. It is not even a hole. It is a supermassive solid object, like a sun, but harder than hard and under extreme compression.

Charged particles escape in jets of plasma but these are probably orbiting close to but not within the black hole.

[acey]

1 - If nothing including light can escape a blackhole then why do quasers escape from it?

A quasar is a galaxy containing an active black hole; matter accelerates towards the hole, releasing energy, and this is why quasars are so bright.

Why doesn't the whole galaxy disappear into the black hole? Because black holes, from a distance, are like any other gravitating object. Earth orbits the sun without falling in; if the sun were replaced by a black hole of equal mass then Earth would continue to orbit it. But if you could somehow make the Earth stop dead in its tracks (or even just disrupt its orbit sufficiently) then it would fall towards the central gravitating body, be it sun or black hole.

In a quasar, there's plenty of gas and dust around the hole that cannot maintain stable orbit, so falls in. Eventually this supply runs out and the galaxy becomes "normal", like ours.

2 - There must be a distance from the black hole where gravity gets weak enough for light to escape. So why can't we see a a round disc with light around it something that looks like a total eclipse?

The force of gravity, either in Newtonian theory or general relativity, is believed capable of extending over infinite distance (there's certainly no evidence to suggest otherwise). Light can freely escape from any point outside the balck hole's event horizon, which is effectively its "edge".

A black hole could eclipse a bright object by passing in front of it. But the strong gravitational field of the black hole distorts light passing round it: this can result in "gravitational lensing".

[Buccaneer]

1. The radiation is emitted before the matter reaches the extremely large black hole, so it is not actually escaping from the black hole. This is why it can escape. As far as is known no information can escape from a black hole.

2. Black holes are very difficult to see as they are extremely small despite their large mass (think how difficult it is to see planets in our own galaxy then imagine something smaller and darker). The only way they are detected is by the behaviour of the celestial objects orbiting them. The images you may have seen of black holes are not real but graphical impressions of what they may look like.

[George Jones]

So why can't we see a a round disc with light around it something that looks like a total eclipse?

We should be able to do this in a few years. I highly recommend the article "Portrait of a Black Hole",

https://www.cfa.harvard.edu/~loeb/sciam2.pdf,

from the December 2009 issue of Scientific American. The February 2012 issue of Sky & Telescope has a more recent but less detailed article on this, "Einstein's Shadow".

This is very exciting, because it will give observational tests of strong-field general relativity.

[acey]

Black holes are very difficult to see as they are extremely small despite their large mass.

Black holes formed from collapsing stars are indeed small, with a Schwarzschild radius of a couple of miles. But the supermassive black hole at the centre of our galaxy is believed to have a radius of around 6 light hours (considerably bigger than the orbit of Pluto), and the one at the centre of M87 (one of the largest known) is reported as four times the size of Neptune's orbit (i.e. about 16 light hours).

http://www.environmentalgraffiti.com/sciencetech/supermassive-black-hole-milky-way/6671

The Solar System Swallower - ScienceNOW