The Black Holes-Thread

[Centurio Nixalsverdrus]

Today I learned from a short video with famous German tv-physicist Prof. Harald Lesch that a black hole is indeed not a hole, but a body / corpus actually!

• 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.

So if I understood that right, it is not black, and not a hole at all. Why is it called a black hole then, instead of invisible corpus? And please tell me if I understood that correctly, or what I got wrong. This is very complicated for a formula-hater like me.

So why is there no time inside it? and,

Could small black holes, like the ones possibly created in CERN, kill me? For example, if such a mini black hole would be flying around invisibly, and it would be accidentally flying through me, would I die? Would I get sucked in?

I know that some of my questions might be stupid, but I really don't know it better.

[Megas Methuselah]

Thx for the info. I look forward to getting a response.

[PBI]

That's actually a pretty good summary of what a black hole is. As for the name, I guess the discoverer probably just thought "black hole" to be a catchier name than "invisible corpus". Your questions certainly aren't stupid, although I struggle to imagine what would count as a stupid question on such a bizarre topic as black holes.

The trickiest question you gave is this:

So why is there no time inside it?

It's not at all obvious and deserves a decent answer, so I will sleep on it and try to come up with a sensible answer tomorrow. On your second question though, the question of the danger of micro-black holes, I can hopefully provide some insight and reassurance.

The first important fact to note about a black hole, is that it has exactly the same mass as the matter it was created from. If the sun were to turn into a black hole tomorrow, we would not fall into it; we would continue to orbit just as we do now since its mass would be the same. Similarly, if you were to crush, say, a teacup into a black hole, it would not immediately suck in everything around it; it would still exert the same gravitational pull as, well, a teacup (i.e. not very much at all).

Secondly, much as you dislike formulae, I'm afraid I must introduce one at this point. It is the formula describing the Schwarzschild radius, i.e. the distance from the singularity at which nothing, not even light, can escape:

rs = 2Gm / c^2

The only important parameters here are the mass of the black hole, m, and the Schwarzschild radius itself, rs, the other stuff is just constants.

By my calculations, the largest black hole that the LHC could produce in a single collision would have a mass of roughly 10^-21 kg and a Schwarzschild radius of roughly 10^-48 metres*. For comparison, an atom has a radius of roughly 10^-10 metres, so such a black hole would be vanishingly small even compared to an atom.

The gravitational pull exerted by such a black hole a metre away would be roughly 10^-31 Newtons per kilogram. For comparison, the gravitational pull exerted by a teacup a metre away is roughly 10^-10 Newtons per kilogram, that's 1,000,000,000,000,000,000,000 times stronger than the effect of the black hole.

So in answer to you question of what would happen if such a black hole were to pass through you, it is probable you would not even notice, since it would be so vanishingly small, and would be unable to attract any of the atoms in your body into itself.


*If anyone's interested, I present my back-of-the-envelope calculation in the spoilers below. It would be good if someone could check it, since it is 1am here so I am not exactly at my mental peak; there is a fair chance I have made an error somewhere.

Spoiler Alert, click show to read: 

The maximum energy of lead ions in the relativistic heavy ion collisions: Roughly 1000TeV. Assuming that all of this energy is transformed into a black hole (the very worst case scenario, and extremely unlikely), we have an energy of 1000TeV which is about 10^-4 Joules, coverted entirely into the mass of the black hole. Applying Einstein's E=mc^2 to work out the actual mass in terms of kilograms, we have:
m= 10^-4/(9*10^16) ~ 10^-21 kg

Inserting this value into the equation for the Schwarzschild radius we have:

rs= 10^-10* 10^-21 / 9*10^16 ~ 10^-48 m

To get the gravitational pull per kilogram, we apply Newton's law of Gravitation:
F= Gm1m2/r^2
= 10^-10*10-21 *m2
= 10^-31 Newtons per kilogram

[Jolt]

Let's not forget that if you get sucked into a Black Hole, you will feel the might of the "Spaghettification Effect". Let's just say that according to what scientists know, that isn't a very good effect to experience.

EDIT: Bleh, why not simply say what might happen if you enter into it. Giving an example, imagining you enter the Black Hole with your feet first, the differential gravity would accelerate your feet faster than it would accelerate your head, and this effect would be enough to tear you apart further from the event horizon than where the time dilation effects are comparably important.

Basically put, you probably die.

[||Lz3||]

yep that's basically a "black hole"... a big mass compacted in a tiny space , such that not even the light can scape the gravitational force...


on other news:
it's nice to have a physic here in the org

I for myself always liked physics...

I even wanted to study them , but... I don't like giving classes <.< that's the main reason I tend to refuse scientific studies , cause I don't like teaching >.>

[a completely inoffensive name]

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?

[Lemur]

I think it has to do with that freaky wave-particle duality.

[Ferret]

That's actually a pretty good summary of what a black hole is...........

Are you a world leading physicist or something? You seem really clever

[PBI]

Nowhere near world leading I'm afraid, but I am studying for a PhD in Particle Physics. So, right at the bottom of the ladder in terms my physics career.

[||Lz3||]

Nowhere near world leading I'm afraid, but I am studying for a PhD in Particle Physics. So, right at the bottom of the ladder in terms my physics career.

in wich university are you studying? if I may ask

[PBI]

You may, and the answer is Swansea.

[Viking]

[*]It is a very very big star, at least three times a big as the sun, that collided because the thermic pressure inside ceased.

*collapsed

It should be noted that size in this respect is measured in mass. It should also be noted that stars with three solar masses are not going to collapse as black holes; they'll end their lives as planetary nebulae. The mass you refers to is what's left after the supernova explosion. If the mass is not sufficient to form a black hole, a neutron star will be what's left after the supernova (SN) instead.
If you two neutron stars collide, you might also end up with a black hole.

There are also at least two types of black holes: stellar black holes and supermassive black holes. The former forms in the gravitational collapse of a star (the way you described) while the other is much more massive and resides at the center of most galaxies (I'm not sure how they form).

[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: