Black Holes

[Normandy6644]

Ok, we seem to have several people on the board who either have beef with the currently accepted theory of black holes or beef with their existence period (mmmm....beef :wink: ). So I figured I'd set up a thread for a discussion about them (mainly to keep the FAQ clean). So, where's the beef? :lol:

[Tensor]

Ok, we seem to have several people on the board who either have beef with the currently accepted theory of black holes or beef with their existence period (mmmm....beef :wink: ). So I figured I'd set up a thread for a discussion about them (mainly to keep the FAQ clean). So, where's the beef? :lol:

I don't have a beef with black holes. Of course that's because I ate all the beef. :wink:

[dvb]

I can't see a black hole, therefore they do not exist! :P

I'm kidding. Wormholes exist too. Just look at the one in DS9.

[TravisM]

Black holes are one of the best theoretical predictions that we now believe actually exist. Imagine: Thinking this up as one of the solutions for the collapse of a star. Intense! =D>

[Sam5]

Black holes are one of the best theoretical predictions that we now believe actually exist. Imagine: Thinking this up as one of the solutions for the collapse of a star. Intense! =D>

They were first proposed by Rev. John Mitchell in 1783.

“If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity.”

SOURCE (http://216.239.53.104/search?q=cache:Vgs1CbR8MsIJ:www.astronomyedinburgh .org/publications/journals/39/blackholes.html+rev.+john+mitchell+black+holes&hl= en&ie=UTF-8)

[Normandy6644]

Black holes are one of the best theoretical predictions that we now believe actually exist. Imagine: Thinking this up as one of the solutions for the collapse of a star. Intense! =D>

They were first proposed by Rev. John Mitchell in 1783.

“If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity.”

SOURCE (http://216.239.53.104/search?q=cache:Vgs1CbR8MsIJ:www.astronomyedinburgh .org/publications/journals/39/blackholes.html+rev.+john+mitchell+black+holes&hl= en&ie=UTF-8)

True, but he had no real basis for his work. It was just a bunch of (really good :wink: ) guesses. I'm not even sure where he got his numbers. I suppose it doesn't much matter though.

[Tobin Dax]

So, where's the beef? :lol:

If I play "Hoedown" as I approach the event horizon of a BH... ah, forget it. ;)

[edit to make the sentence make (a little) more sense]

[Sam5]

Black holes are one of the best theoretical predictions that we now believe actually exist. Imagine: Thinking this up as one of the solutions for the collapse of a star. Intense! =D>

They were first proposed by Rev. John Mitchell in 1783.

“If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity.”

SOURCE (http://216.239.53.104/search?q=cache:Vgs1CbR8MsIJ:www.astronomyedinburgh .org/publications/journals/39/blackholes.html+rev.+john+mitchell+black+holes&hl= en&ie=UTF-8)

True, but he had no real basis for his work. It was just a bunch of (really good :wink: ) guesses. I'm not even sure where he got his numbers. I suppose it doesn't much matter though.


It was Newton who first proposed that light would bend when it passed the sun, due to the gravitational effect. You can find it in his “Optics”, 1704 edition.

Here is some stuff from 1847 from a French book. I got it from my copy of Humboldt’s “Cosmos”, first published in the 1850s:


“...also Laplace according to Moigno Repertoire d' Optique
moderne, 1847, t. i. p. 72. " Selon la theorie
de l'emission on croit pouvoir demontrer que sj 16
diametre d'une etoile fixe serait 250fois plus grand que eelui
du soleil, sa densite restant la meme, l'attraction eX03rCee a sa
surface detruirait la quantite de mouvement, de la molecule
lumineuse emise, de sorte qu'elle serait invisible a de grandes
distances.' , [translation:]"It seems demonstrable by the theory of emission that if the diameter of a fixed star be 250 times greater than
that of the sun – its density remaining the same – the attraction
exercised on the surface would destroy the amount of motion
emitted from the luminous molecule; so that it would be in-
visible at great distances."

One big problem these early guys had was no motor controlled telescope mounts, low res images, and no spectrometers, plus they didn’t have a very good outline of the atomic theory or the theories of electrodynamics yet.

[Pi Man]

My favorite thread on BH's:

http://www.badastronomy.com/phpBB/viewtopic.php?t=1142

... even though the author was banned...

[Taibak]

Black holes are one of the best theoretical predictions that we now believe actually exist. Imagine: Thinking this up as one of the solutions for the collapse of a star. Intense! =D>

They were first proposed by Rev. John Mitchell in 1783.

“If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity.”

SOURCE (http://216.239.53.104/search?q=cache:Vgs1CbR8MsIJ:www.astronomyedinburgh .org/publications/journals/39/blackholes.html+rev.+john+mitchell+black+holes&hl= en&ie=UTF-8)

True, but he had no real basis for his work. It was just a bunch of (really good :wink: ) guesses. I'm not even sure where he got his numbers. I suppose it doesn't much matter though.

It's a direct consequence of escape velocity, actually. Classical escape velocity is given by:

v = (2GM/r)^1/2

Plug in r = 2GM/c^2 and you get v = c. Coincidentally, 2GM/c^2 is the Schwarzschild radius....

I'm not sure if you'd consider that sufficient basis for his work or not. I mean, he had no more observational evidence than the theorists who realised that black holes are a consequence of GR. What I would agree with is that Mitchell's theory isn't a great model of black holes, since the classical escape velocity equation implies that photons are slowed as they leave the object, stop, and fall back in.

Taibak

[Normandy6644]

Black holes are one of the best theoretical predictions that we now believe actually exist. Imagine: Thinking this up as one of the solutions for the collapse of a star. Intense! =D>

They were first proposed by Rev. John Mitchell in 1783.

“If the semi-diameter of a sphere of the same density as the Sun in the proportion of five hundred to one, and by supposing light to be attracted by the same force in proportion to its [mass] with other bodies, all light emitted from such a body would be made to return towards it, by its own proper gravity.”

SOURCE (http://216.239.53.104/search?q=cache:Vgs1CbR8MsIJ:www.astronomyedinburgh .org/publications/journals/39/blackholes.html+rev.+john+mitchell+black+holes&hl= en&ie=UTF-8)

True, but he had no real basis for his work. It was just a bunch of (really good :wink: ) guesses. I'm not even sure where he got his numbers. I suppose it doesn't much matter though.

It's a direct consequence of escape velocity, actually. Classical escape velocity is given by:

v = (2GM/r)^1/2

Plug in r = 2GM/c^2 and you get v = c. Coincidentally, 2GM/c^2 is the Schwarzschild radius....

I'm not sure if you'd consider that sufficient basis for his work or not. I mean, he had no more observational evidence than the theorists who realised that black holes are a consequence of GR. What I would agree with is that Mitchell's theory isn't a great model of black holes, since the classical escape velocity equation implies that photons are slowed as they leave the object, stop, and fall back in.

Taibak

I agree with that. My point was that the entire theory of black holes and Newtonian gravity came out of Einstein's equations, and rather than being "forced" so to speak from observations. Without GR you don't have a very good theory behind black holes.