The Black Holes-Thread

[Sigurd]

• It is a very very big star, at least three times a big as the sun, that collided because the thermic pressure inside ceased.
• The mass gets compressed to such a density, that the sun would end up with a diameter of 3 km only.
• And because of its gigantic density, the escape-velocity (sorry a direct translation) is bigger than the velocity of the light. Its gravitation is so gigantic that it catches every light falling upon it, so that we actually cannot see it.
• What leads us to the conclusion, that a black hole is indeed not black, but invisible.
• Its mass is so big that it disrupts the space/time continuum and thus, there is no time at all inside it. Why?
• And because there is no time inside, there is no perception, and no information could ever reach us from the inside.
• Dying from a black hole means you just get attracted by that corpus, and because of its incredibly gravitation, you get immediately squashed to the size of nano-pieces on its surface.

I think Stephen Hawking revised this point and recently too. Apparently black holes never completely destroys what falls in but continue to emit radiation.
To quote him:

The Black Hole only appears to form but later opens up and releases information about what fell in, so we can be sure of the past and we can predict the future.

From the top of my head, I think there was a theory that spiral galaxies such as the milky way we live in, rotates around a black hole.

[PBI]

From the top of my head, I think there was a theory that spiral galaxies such as the milky way we live in, rotates around a black hole.

More than a theory from what I've heard, I'm pretty sure they observed some of the stars in Sagittarius interacting with an apparently empty patch of space, implying the presence of a black hole.

Ah, here is a link to the UCLA group working on it. Apparently the object in question is 3 million times the mass of the sun, they're really not kidding about the "supermassive" part are they?

What is the Schwarzschild radius? And why is there no time inside?

The Schwarzschild radius is the distance from the center of a black hole at which gravity becomes so strong that not even light can escape. It's the same thing as an event horizon, if you happen to hear that term used.

I never understood the concept of "not even light can escape the gravitational force". If light has no mass, how can gravity have any effect on it?

This is the key principal underpinning General Relativity; the point is that massive objects do not attract each other so much as simply distort space-time, and it is this distortion that changes the way things move compared to how they would in undistorted space-time. This change in the way things move is what gives rise to the appearance of gravitational attraction. Because it is a distortion in space time rather than simply an attractive force between masses, things with no mass such as light are also affected by it; in fact this was one of the key observations in favour of General Relativity, the so-called gravitational lensing of light around massive objects such as stars.

It's real mind-melting stuff, I'd be lying if I said I understood it fully, but that hopefully gives a general gist.

As for the question about there being no time inside a black hole, I'm not sure about that, but I believe there is supposed to be no time on the surface of a black hole (i.e. the event horizon) due to gravitational time dilation. Any object falling into the black hole will appear to slow down as approaches the event horizon, until it appears to be frozen in time. This is one of the fundamental reasons why we could never send a probe into a black hole to see what's inside it; from our point of view, it would never actually reach the black hole. The other reason of course being, it would be ripped apart into its constituent atoms by the tidal forces.

I'm not sure that last paragraph makes any sense, I'll check my GR notes at work tomorrow to make sure I've got it right.

[Viking]

Gravitational lensing is an interesting pheomena; for example the Einstein Cross which, is the light of a distant quasar bent such that it is duplicated four times around a relatively nearby galaxy: