Expert: Jayendra Upadhye - 9/10/2009

Question
QUESTION: Hi, I asked a question about black holes a bit agao and I just would like to clear something up.  I've read the laws of physics break down at the black hole's singularity.  What laws of physics break down?  Time and space are bending and slowing, so what laws are being violated?  Also, I understand that mass is a controling factor, therefore, if a black hole were to meet a star with a smaller mass the black hole would consume the star, but, I want to clarrify; if a object, say a galaxy had a larger mass than a black hole and met a black hole, would the galaxy therefore consume the black hole, and, what would that entail?  THank you for answering these issues, Marc

ANSWER: Hi Marc,
The beauty of physics and maths is that we can break down the larger issues into the smaller ones.
Then look at the soultions of the smaller ones and let those guide us.

case 1:-
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Breaking down of the laws of physics at the singularity.
Let alone the singularity, we dont even know what happens inside the event horizon which is a fairly large spherical volume of 3d space, that marks the swarzchild radius of the singularity.
Consider it as a black box.
we "dont know" what happens inside the box, but we do know that the weak force gravity extends from all the way in its center, to the farthest corners of the universe, and that it does follows the inverse square law.
The laws of physics break down because space is impossibly warped there in the vicinity of the singularity.
All objects in space occupy volume if they have mass. This basic law is violated at the singularity.
A black hole HAS mass, but NO VOLUME. IT is a dimensionless entity, a point, with no length, breadth nor hieght.
in such a place where no physical dimensions apply, which other law can hold?
THAT is the argument.
Gravity remains as it is a warp in the very fabric of space-time, caused by the mass.
If the black hole is a bullet breaching 4d space time, its gravity is the bullet mark!

case 2:-
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Here is how we break down and understand things one piece at a time.
1 - A black hole has a finite mass.
2 - As long as this mass is constant, the inverse square relation ship holds.
3 - So provided you have sufficient distance from the event horizon, you would be experiencing just the normal pull of gravity as from the sun or any planet.
Only in the accretion disk very near to the event horizon, is the rate of change of gravitational intensity great enough to cause individual particles of a body to accelerate away from each other with such a large differential that the body experiences "tides" and is shredded by that differential across its very physical dimensions.
The tidal locking of the moon is caused by a similar but weak tide caused by the earth. And the solar and lunar tides we see in the oceans are so natural for us.
The tides of the black hole are titanic to say the least.
But if the sun were to be a black hole tomorrow, it would have no ill effect on the earth, as the sun's mass would continue to be the same.

Extending this small piece of knowledge, A black hole would consume a star only as lomh as the star's surface was so close to it that the tides lifted matter off the star and onto the hole.
Cygnus X-1 was such a candisate. It is believed that a black hole is orbiting a red giant star and siphoning away matter into an accretion disk.
Galaxies gobbling black holes...why not, a galaxy has a mass a 100,000,000,000 solar masses!
Black holes a million solar masses routinely reside in the hubs of spiral galaxies. Ours has one known as sagittarius A. It is 4.31 million solar masses!
And yet the galaxy and us have survived unhurt for the last 4.5 billion years.
And the galaxy is older than the sun!
refer:- http://en.wikipedia.org/wiki/Sagittarius_A*
So there you have your answer.
regards
Jayen

---------- FOLLOW-UP ----------

QUESTION: Hi and thank you for your response, my only follow up would be, if a galaxy with a larger mass than a black hole would eventually pull a black hole towards it, the galaxy would "consume" the black hole, but what would that mean?  Would the black hole cease to exist?  What would such a act entail?  Thank you again for your response?

Answer
Hi,
Marc I thought I was unambiguous.
Black holes are just as dangerous as stars are to planets that go too near.
Conversely, when black holes in turn are consumed by galaxies (to put it your way, and to assume extragalactic black holes exist in isolation in the first place, for them to be gobbled up later), all that happens is the black hole, depending on its velocity goes into orbit around the galactic core.
It is treated just as any other star by the galaxy.
The million solar mass black hole at the galactic center (and in most galactic centers of spiral galaxies) is a primordial black hole, a leftover from the early days of the galaxy, when the globular clusters (building blocks from the galactic halo) were merging and gravitationally interacting to form the spiral milky way galaxy.

Primordial black holes form during the initial phases when due to a large density of stars in the galactic hub, many aging and young stars alike, merge into one huge object.
This soon collapses into a singularity and more and more stars add to its mass, until a fairly large clearing emerges and the black hole stops growing or slows down new accretion.

At some point of time, it reaches a stasis.
near by stars continue doing high speed circumambulations out of "respect"! :)

That's all there is to it.
The galaxy though "dense" is actually incredibly sparse.
So much so that when the Andromeda spiral will collide with us 5 billion years from now, The leading edges of both will release copious amounts of x rays as their interstellar gases hit each other at 100s of km/sec.
The stars will form long bridges long before actual merging, but no stars will actually collide, except may be a very small percentage.

The shock waves in the leading edges will accelerate star formation in those areas.

But that is all.

In oanother few billion years, we will have a huge elliptical galaxy, rich in old red stars, a new addition to many such in the universe.

regards
Jayen