Two Black HolesScience brain teasers require understanding of the physical or biological world and the laws that govern it.
Aliens are placing you near one of two black holes in outer space. They give you a choice of being placed near a large black hole or a small black hole. Keeping in mind that when you go close to a black hole you must worry about large gravitational tidal forces produced by a black hole, which do you prefer, if you wish to escape?
AnswerThe large black hole. Tidal forces (near the event horizon) are smaller for large black holes than for small holes, because more massive black holes actually have a lower density, and the exterior spacetime is less curved. If you were approaching a 10-solar-masses black hole with a radius of 30 kilometers, you would be killed long before you reached the horizon at an altitude of 400 kilometers. However, you could reach the horizon of a 1,000-solar-masses black hole. The tidal forces at the horizon of this gigantic black hole would be weaker than those produced by Earth, which are already impossible for us to feel. Of course, once you crossed the horizon, you would eventually be pulled into the singularity and then be torn to pieces by the finite tidal forces. This final fate awaits you no matter what the mass of the black hole.
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