h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

According to today's New Scientist, there's a theory that reasonably large black holes created very early in the life of the universe may have acted as gravitational seeds to enable galaxy formation to occur as rapidly as it did.

Neat.

Nobody seems to have mentioned the pretty important formula for gravitational force (as proposed by Newton), though:

Force = (Gravitational Constant x Mass One x Mass Two) / (Distance^2)

It is this, I believe, which gives black holes their pulling power; the fact that they're so small (lots of mass in a very small space) means that when you get close to them you're close to *all* of the mass in them, whereas when you get close to the earth you're actually many thousands of miles from most of the mass. Hence we don't get squished from just standing on the earth.

Apparently, if the earth were the size of a wallnut, it would in fact have its own event horizon. That being also the size of a wallnut.

26199 (hoping that *some* of this is right)

You got most of that right, the fact that the gravitational force is opposite the actual size of the whole (i.e. the smaller the whole is the greater forces there are to tear you apart), but if I'm not mistaken, it all the mass of the Earth were sqeezed into a walnut's size, then the Event Horizon would actually be where the Earth's perimeter now stands, (I think anyways). Because despite the fact that the mass has been squeezed infinetly small, the amount of mass is actually remaining the same, thus the amount of the curvature of space time (to the horizon) remains the same as it is. Its this same reason that if teh sun were to collapse into a black hole, the orbit of the planets would not change any.

There are some innacuracies in that statement too.

If the sun were to collapse into a black hole, the orbits of the planets would almost certainly be affected. Before you get a black hole, you first have to have a massive supernova, or at least a really big boom.

And the event horizon of the earth, if it were turned into a black hole, would be about the size of an American Silver Dollar. The practical gravitational reach would stay about the same. That is, a space shuttle orbiting at the same distance above the Earth now and the Earth as a black hole would experience roughly the same gravity. It still has the same mass, after all. It's only when you get closer that things start to change.

This is true, the part about the orbits was strictly gravitationally speaking. (Curvature in space time), althought the resultin nova would send gravitational wave that would in change the orbits.
Guess I had forgotten about that part

Has anyone tried working out the Schwarzchild radius for a black hole the mass of the Earth?

I believe I saw it worked out somewhere, but I cannot at this moment remember where, ill see if I can locate it though

There you go again, Jeff, tryin' ta figure out the meaning of life 'n' everything. Didcha forget, it's 42.

The Schwarschild radius of a mass is the radius to which the actual body must contract for the gravitational field to have created a black hole. Worked out for the Sun this is about 1cm. The reason? There is an associated minimum mass needed to create a black hole which is approximately three solar masses, so the Sun would never create a black hole. Infact it's not even massive enough to supernova in the first place.
So there you go!

Exactly right

The Force due to gravity is calculated with the distance between the relatice Centres of mass - which would not change.

However, if you were standing on the earth when this happened you would fall down and be squished.