February 17, 2006
Black Holes do not suck!
I decided to visit the Exploration of the Universe Division web page this afternoon, just to see what current stories were coming out of the high-energy astronomy world.
Much to my horror, this is what greeted my eyes:
Why is Galaxy Girl upset over that? Afterall, it's "well-known" that black holes are, indeed, giant vacuum cleaners, right?
WRONG
In fact, this is the very misconception of black holes that plagues those of us who interact with the general public. There is a widespread impression that a black hole will suck up anything in its vicinity. The truth of the matter is that black holes don't suck any more than anything else in the Universe. In fact, you have to get relatively close to a black hole to see the weird effects that everyone likes to dwell on.
One example I like to use to show that black holes don't suck is to imagine that our Sun suddenly and non-violently turns into a black hole. (Rest assured, this won't happen – our Sun is destined to die in a much less exciting way, quietly fading away into a white dwarf.) The Earth's orbit would not change*. Most people find this surprising. But the truth of the matter is that the Earth's orbit depends only on the mass of the Sun, and a black hole with the mass of the Sun has the same gravitational pull as the Sun itself.
That's not to say that weird things don't happen around black holes, but you have to be fairly close to the event horizon to start noticing them. Further out, a black hole behaves no differently, gravitationally speaking, than any other object with the same mass.
Hence, black holes do not suck.
* Of course, if the Sun suddenly turned into a black hole, we would notice it – we would no longer have the Sun's radiation that warms the Earth, and we'd pretty much be screwed. However, the point is that the orbit of the Earth would not change.
Posted in Soapbox by Barb at February 17, 2006 1:22 PMThink I'd have to disagree with you with our Sun going BH and the mass remaining the same. I don't think our oribit would remain the same.
You forgot to take into consideration the impact of the mass of the Sun (1391000 kilometers) and the "dent" in the space-time "fabric" vs. that same mass now at a tiny fraction - thus distorting the space-time "fabric" totally differently, and thus our orbit.
Posted by: at February 20, 2006 3:41 PMYou need to be careful what you're talking about there, Anonymous. The Sun's mass is 2e30 kilograms, and we've assumed in the above problem that it doesn't change.
I think the point you're trying to make is that the radius of the Sun changes. Let's see what it changes to. The schwarzschild radius is defined by R_s = 2GM/c^2. For the Sun, then, R_s is 3 km. Its normal radius is 7e5 km, so indeed, the black-hole-sun takes up much less space.
However, the Earth is 1.5e8 km from the Sun. At the exact moment that the Sun changes into a black hole, in my above scenario, then, the Earth is 5e7 times the Schwarzschild radius away from the Sun. Any weird stuff that occurs near a black hole, is going to happen within a few times the Schwarzschild radius -- the Earth is too far away to get sucked in or even see any of the cool effects of strong gravity.
Posted by: Barb at March 9, 2006 6:23 PM