Black Hole: Test Your Understanding
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How does the horizon of a black hole act as a one-way membrane?
Particles and light can go inward through the horizon toward the interior of the black hole, but nothing
can travel outward to escape the black hole.
How were black holes first detected?
Black holes were first detected by studying binary x-ray sources.
Critical Thinking: Why might Hawking radiation never be detectable?
It is unknown whether black holes exist that are small enough to evaporate due to Hawking radiation.
The smallest black holes might still receive enough ambient radiation from stars and the cosmic micro-
wave background to offset their losses from Hawking radiation, and thus remain stable in perpetuity.
