Would we always know when we reached a black hole's event horizon by noticing its high density? Explain.?

Answer 1

The best way to know that you've moved inside the event horizon of a black hole is by finding that you can no longer communicate with the rest of the universe. For instance, if you were in a probe launched by a nearby mother ship and passed inside the event horizon, all radio contact with the mother ship would be totally lost because nothing can escape from inside the event horizon.

Answer 2

Black holes may come in all sizes; one the mass of the Sun is a tad over a kilometer in diameter, while the black holes at the hearts of some spiral galaxies are millions of times larger.

If there are a hundred billion stars in our Galaxy, and there are 100 billion Galaxies in the Universe, then the combined mass of all of these - accompanied by any 'dark matter' or unforeseen energy that adds mass-inertia to our Universe, than the radius of the black hole that would contain such a mass would be... several billion light years.

Since one cannot tell from outside what the contents of a black hole is, for all we know, we are already inside one.

How's your densitometer working?

Answer 3

Density? The event horizon is a mathematical boundary. There is nothing physical there.

Answer 4

No. An "event horizon" is an interesting concept, and if you want to put some thought to it, try these links:

http://en.wikipedia.org/wiki/Event_horizon

http://archive.ncsa.uiuc.edu/Cyberia/NumRel/BlackHoleAnat.html

http://www.astronomical.org/astbook/blkhole.html