A question on black holes

[Derek Smith]

I've just been telling my grandkids about space and bigness. I showed them bits of the excellent BBC2 programme on the Voyager mission, a real treat for me, perhaps less so for them. I tried to get over some of the exciting things about it, which is easy enough to do with images from NASA. I mentioned strange and wonderful phenomena including black holes and I said, Hawking notwithstanding, that nothing comes out of it. I confused myself though.

I can't use the excuse that the kids asked me this, but I wondered why it is forever lost to us. I understand escape velocity and all that. In fact, it's pretty simple once it isn't revolutionary. But:

I thought energy and matter convert into one another, E=MC2 and all that. So what if the 'stuff' inside a singularity was converted into energy: would it still be massive enough to maintain the event horizon?

All rather esoteric I know, but it is irritating me. After all, how massive is energy?

The Voyager programme is on BBC iPlayer. It's nostalgic rather than revealing. However, it is magic for those of us who followed it at the time. The gaps between the planets came over with the gaps between the fly-bys. Exciting times.

https://www.bbc.co.uk/iplayer/episode/b09gvnty/sto...

[Eric Mc]

It's mass that maintains the existence of the black hole (and its event horizon).

[Simpo Two]

Derek Smith said:

I thought energy and matter convert into one another, E=MC2 and all that. So what if the 'stuff' inside a singularity was converted into energy: would it still be massive enough to maintain the event horizon?

There'd be a fking great bang and the event horizon would be blown to kingdom come.

But even a hydrogen bomb converts only a very small amount of mass to energy.

[Beati Dogu]

Less than 1% for fission weapons apparently. Hydrogen bombs are about 1.5%

But when only 3 grams of uranium converting to energy is enough to destroy Hiroshima, the poor efficiency isn't such an issue.

[Derek Smith]

I wasn't particularly considering a bomb, only whether it is possible if, for some strange reason, the mass was converted into energy. I might not know much about sciency things, even hair shampoo gets complicated for me, but if, as some suggest, the soup that mills about in the bottom of the gravity well doesn't follow the laws in our universe, why shouldn't/couldn't it change mass into energy. I mean, electricity doesn't weigh all that much.

[Simpo Two]

Beati Dogu said:

Less than 1% for fission weapons apparently. Hydrogen bombs are about 1.5%

But when only 3 grams of uranium converting to energy is enough to destroy Hiroshima, the poor efficiency isn't such an issue.

I remember my chemistry teacher (who was a big fan of black holes) saying that if you could convert a matchstick into energy it would lift Mount Everest by a foot.

Anyone fancy doing the math?!

[FarmyardPants]

Simpo Two said:

I remember my chemistry teacher (who was a big fan of black holes) saying that if you could convert a matchstick into energy it would lift Mount Everest by a foot.

Anyone fancy doing the math?!

The big unknown in that equation is the mass of everest, but taking the number from this: https://www.econworks.co/uploads/9/6/1/6/96165748/...

Energy contained in match = m(match)c^2
Energy required to lift Everest h metres = mgh = m(Everest)*g*h

Equate the two:

m(match) * c^2 = m(Everest) * g * h

Assuming roughly g = 10

h = ( M(match) * C^2 ) / ( M(Everest) * 10 )

M(match) ~= 0.0005 Kg (half a gramme - this is also a guess)
M(Everest) ~= 1.4e12 Kg
C^2 ~= 9e16 m2/s2

Hence h = 3.2m or roughly ten feet.

But my match mass estimate might be off.