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Starski asked in Science & MathematicsPhysics · 1 decade ago

Can matter fall into a black hole?

For any outside observer, all black holes evaporate in a finite amount of time. But time stops at the event horizon, so wouldn't the hole evaporate before anything could fall in?

3 Answers

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  • Alex
    Lv 7
    1 decade ago
    Favorite Answer

    As predicted by general relativity, the presence of a large mass deforms spacetime in such a way that the paths particles take bend towards the mass. At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.

    To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole. Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it. At the same time, all processes on this object slow down causing emitted light to appear redder and dimmer, an effect known as gravitational redshift. Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.

    On the other hand, an observer falling into a black hole does not notice any of these effects as he crosses the event horizon. According to his own clock, he crosses the event horizon after a finite time, although he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.

    As for evaporation...

    In 1974, Stephen Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation. He got this result by applying quantum field theory in a static black hole background. The result of his calculations is that a black hole should emit particles in a perfect black body spectrum. This effect has become known as Hawking radiation.

    Since Hawking's result, many others have verified the effect through various methods. If his theory of black hole radiation is correct then black holes are expected to emit a thermal spectrum of radiation, and thereby lose mass, because according to the theory of relativity mass is just highly condensed energy (E = mc2). Black holes will shrink and evaporate over time. The temperature of this spectrum (Hawking temperature) is proportional to the surface gravity of the black hole, which for a Schwarzschild black hole is inversely proportional to the mass. Large black holes, therefore, emit less radiation than small black holes.

    A stellar black hole of 5 solar masses has a Hawking temperature of about 12 nanokelvins. This is far less than the 2.7 K produced by the cosmic microwave background. Stellar mass (and larger) black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and will thus grow instead of shrink. To have a Hawking temperature larger than 2.7 K (and be able to evaporate) a black hole needs to be lighter than the Moon (and therefore a diameter of less than a tenth of a millimeter).

    On the other hand if a black hole is very small, the radiation effects are expected to become very strong. Even a black hole that is heavy compared to a human would evaporate in an instant. A black hole the weight of a car (~10−24 m) would only take a nanosecond to evaporate, during which time it would briefly have a luminosity more than 200 times that of the sun. Lighter black holes are expected to evaporate even faster, for example a black hole of mass 1 TeV/c2 would take less than 10−88 seconds to evaporate completely. Of course, for such a small black hole quantum gravitation effects are expected to play an important role and could even – although current developments in quantum gravity do not indicate so – hypothetically make such a small black hole stable.

  • Lola F
    Lv 7
    1 decade ago

    Time doesn't stop at the event horizon. That doesn't even make sense; time never stops for any observer anywhere, because time is the interval between ticks of a light-clock. Since the local speed of light is c for all observers, including observers falling into a black hole, all observers experience the passing of time at the same rate. Falling bodies cross the event horizon and impact the singularity in finite (and small) proper time. The fact that distant observers cannot observe this is immaterial.

  • 5 years ago

    it does not attain the Black hollow from our inertial physique, yet in all risk reason the black hollow to loose mass in form of power on the form horizon, a extensive time later in our view. The effect of anti-rely on a black hollow is expected interior the hypothesis of the Hawking radiation, even in spite of the undeniable fact that it continues to be untested. time-venerated is purely, that conservation of power additionally applies to Black holes - purely besides as different regulations of thermodynamics.

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