# Risk from black holes

1. Nov 1, 2007

### hammertime

So I read in Yahoo! that a new black hole (the largest ever) had been discovered 1.8 million light years away. It got me thinking.

Do black holes pose a threat to the planet earth?

Do rogue black holes pose a threat to the planet earth?

Thanks.

2. Nov 1, 2007

### mathman

Any body exerting a sufficently large gravitational effect to disturb the solar system would pose a threat to the earth. It could be a large star, neutron star, or black hole. Fortunately there doesn't seem to be any nearby and since the solar system is around 4.5 billion yrs old, I don't think anything is likely to happen soon.

3. Nov 1, 2007

### Staff: Mentor

It would be unlikely that we'd find random huge masses of any kind hurtling through the galaxy. Think about it this way - they used to be stars, right? So do we have any stars nearby that look like they might hit us soon?

4. Nov 1, 2007

### Chris Hillman

Currently as far as we know there are no significant black holes in our vicinity nor do we expect there to be any during the expected lifetime of the solar system. ("Significant" and "in our vicinity" meaning close enough/massive enough to gravitationally disrupt the solar system. Before you ask, no microholes are known, although physicists hope to create them in the lab, but these would be far too small to harm anyone on Earth.)

Define "rogue".

It is possible that "hammertime" is refering to the possibility that the merger of two black holes might produce sufficient gravitational radiation in a sufficiently asymmetric fashion as to "kick" the resulting single hole in some direction with a sizable proper velocity. However, such holes haven't been spotted yet (that's a surprise, actually) but in any case it was never expected that such an event would be likely to send a sizable hole near the solar system in the next few billion years.

Last edited: Nov 1, 2007
5. Nov 1, 2007

### pixel01

Blank holes are just stars if you are beyond the event horizon. So the risk is just the same. If the earth were so near to a star, you would die because of the heat before anything else happens, so a star even is more dangerous than a black hole. There are stars about 80 times heavier than the sun out there by the way.

6. Nov 1, 2007

### Chris Hillman

Careful!

I'd put it like this (in fact, in past posts I often have put it like this): a black hole of mass m (interesting slip of the keys, pixel!) behaves just like any object of mass m, except that it is in effect a much more compact object than anything except a neutron star. In all cases, tidal forces associated with a typical object scale like $m/r^3$, so the reason why the gravitational fields near the surface of a neutron star, or near the event horizon of a black hole is that in these cases, r is not much larger than m. So a stellar mass black hole will cause the same tidal disruption at several AU as an ordinary star would.

In the case of hypothetical miniblack holes, these would be so small that it would very hard to cram matter into the horizon, so hard that they would not have much effect before evaporating.

Last edited: Nov 1, 2007