# Black holes, White Holes and Baby Universes:

Discussion in 'Astronomy, Exobiology, & Cosmology' started by paddoboy, Aug 13, 2013.

1. ### brucepValued Senior Member

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The photon sphere is at 3M and the last stable circular orbit for the Schwarzschild geometry is r=6M. All orbits inside 6M require boosts to maintain the orbit to avoid falling into the hole. It's referred to as a 'knife edge orbit'.

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No I'm pretty sure I am right on both counts......The photon sphere is as I said, at 1.5 Schwarzchild radius, and the last stable orbit for the Schwarzchild geometry is 3 Schwarzchild radius.

see......
http://en.wikipedia.org/wiki/Photon_sphere
A photon sphere is a spherical region of space where gravity is strong enough that photons are forced to travel in orbits. The radius of the photon sphere, which is also the lower bound for any stable orbit, is:

r = \frac{3GM}{c^{2}}

which is one and one-half times the Schwarzschild radius.

also.....
3 Schwarzschild radii marks the radius of the innermost stable orbit. Outside this radius circular orbits are stable, whereas within it circular orbits are unstable.
Astronomers argue that, if a black hole is accreting, then the inner edge of the accretion disk probably lies at the innermost stable orbit. At that radius, gas peels off from the accretion disk orbiting the black hole, and falls headlong into the black hole.

When we talk about BHs, unless specifically stated, as far as I am concerned we talk of the simplest BH, being the Schwarzchild metric.

5. ### wegsMatter & Pixie DustValued Senior Member

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a 30 year black hole. lol :roflmao:

Okay...Here is something funny I ran across...

If you wanted to turn the earth into a black hole you would have to compress it all down to about the size of a large mosquito

Science can be fun(ny)

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...Not quite!
From memory, we would need to squeeze the mass of the SUN into a sphere around 5kms in diameter....Do mosquitos grow this big???

The Earth would be about 3 cms....Now mosquitos this big I can vouch for in Fiji!!!

8. ### wegsMatter & Pixie DustValued Senior Member

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the article only specified ''large.''

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Apologies wegs, I misread your post, and have now corrected it.....You are close to the mark!!!

10. ### brucepValued Senior Member

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This is the mass of the earth expressed in geometric unit's.
M_earth=.00444m. One solar mass = 1477m [for comparison].

So the Earth Schwarzschild radius is

r=2M=2(.00444m)=.00888m [the earth as a spherically symmetric non rotating black hole]. The size of a large mosquito. LOL.

An Earth sized black hole would be a case where dTau_ouch occurs outside the event horizon.

11. ### brucepValued Senior Member

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Fiji? Ever hear of cloudbreak?

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Surfing spot near the Island of Tavarua just off Nadi....

13. ### wegsMatter & Pixie DustValued Senior Member

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:roflmao:

Really? This stuff is so much fun to play with!

14. ### brucepValued Senior Member

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4,098
So, is that your home or was it a visit? I always wanted to try to surf that break. On a long board so I must have had a bit of a death wish. I'm 68 now and had to give up surfing 12 years ago because I messed up my neck in a car accident. Like losing your best friend.

Actually I wouldn't really make a nuisance of myself by trying to surf such a break on a long board. I wouldn't be making any new friends.

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Well, let's say my second home

...My wife is Fijian, and generally pay a visit to the Islands at least once a year...Only a short 4 hour hop from Australia's East coast.
She comes from Rakiraki, around the north side of Viti Levu.

16. ### brucepValued Senior Member

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3M is 1.5 x 2M. The stable orbit at 3M is for light. The limit for objects with mass is r=6M. So whoever wrote that is wrong.

It's to much work to show this in the language of mathematics so I'll just write down a part of the first set of queries I did in my favorite text. Exploring Black Holes by Edwin Taylor and John Wheeler.

Chapter 4 Exercises. Part D of exercise 2 'Radii of Circular Orbits Around a Black Hole'.

"D. Show that there are no circular orbits of any kind for angular momentum given by L/m<M(12)^1/2. Show that for the minimum angular momentum L/m=M(12)^1/2, the radius of the circular orbit is r=6M. This is the stable circular orbit of smallest radius."

17. ### brucepValued Senior Member

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Wonderful. I really like polynesian folks. My mom was Australian. Never been there either. I should have.

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More Melanesian then Polynesian.
What part of Australia does your Mum come from?

things to do...be back much later!!!

19. ### brucepValued Senior Member

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Thanks for clarifying that for me. My mom was born in Hobart Tasmania. My dad met Mum in Melbourne during the War. He was on leave from New Guinea and the Army Air Force. New Guinea is another place I would like to visit. Jamaica. Yeah.

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An old thread of mine....I still would like to hear what others have to say about the predicted outcome by myself, taking into account of course some speculative/mainstream cosmological ideas about the existence of WHs..

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All human beings also have the capacity to be the next Bozo the Clown.
I've crossed swords with at least two today.