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A Myth about Black Holes

  1. Mar 22, 2008 #1
    Black holes irresistibly suck things in. That is a common misconception in science fiction. In fact, a spherical black hole of mass M attracts exterior mass no more strongly than a spherical star of mass M. Their exterior spacetimes are the same Schwarzschild geometry. But there is a sense in which it is more difficult to escape from close to a black hole than from a Newtonian center of acctraction of the same mass. Imagine using the thrust of a rocket to hover at a constant Schwarzschild coordinate radius R outside a spherical black hole of mass M. How much thrust would the rocket of mass m need to exert? Would it be infinitely larger as the radius R approaches 2M?
    Last edited: Mar 23, 2008
  2. jcsd
  3. Mar 23, 2008 #2

    George Jones

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    Yes, the thrust required approaches infinity as the event horizon is approached. See https://www.physicsforums.com/showpost.php?p=1633296&postcount=5".
    Last edited by a moderator: Apr 23, 2017
  4. Mar 23, 2008 #3
    That's true, but the black hole is _deceptively_ massive. If you were approaching a red giant, because it was so large and so hot you'd know not to get so close that you'd be sucked in. If you were approaching a black hole, gravity would be felt much further away from the surface of the hole than it would from the surface of a star, and also there'd be no heat radiating from it, so you might not even know it was there until it was too late.
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