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Physics/Black bodies and Black holes


How can a black body can emit radiation.. if it has the tendency to absorb everything..? same question with black holes.. with infinite gravity.. how can a black hole can emit radiation(Hawking radiation)if even light can't escape(you can say that radiation also as a form of light which is not visible or something)?

A little background on terminology.

Anybody who has walked barefoot on a sunny day can tell you that black asphalt is hotter than white cement. In other words, dark objects absorb more of our Sun's light energy than do light objects. Scientists in the 1800s studied this simple fact a little more rigorously, and concluded -- both in experiment and theory -- that dark objects at a specific temperature also radiate more energy than white objects at the same temperature. They also noted that the spectrum of light coming from a heated object did not depend on the chemistry of that object, but only on its temperature. This latter fact led scientists to conclude that light coming from a heated object is due to a fundamental property of matter -- and thus worthy of study.

In order to do theoretical work on this phenomena, Gustav Kirchoff asked the question of what would happen if there was a material that absorbed 100% of the light energy coming upon it. In other words, a perfect "black body" (his words). In examining the phenomena of light coming from heated objects, the name stuck.

The question, "How can a black body emit radiation?" is akin to the question, "How can a bank loan out money?" Yes, a black body DOES absorb radiation, and a bank DOES take in money. But, just like a bank can take some of the money it takes in, and then loan some of it out; a black body can take some of ITS energy and emit it out as light.

> how can a black hole can emit ... Hawking radiation ?
Prior to the work of Stephen Hawking, scientists did not think light could come from a black hole. Hawking was able to show these objects DO emit light, due to quantum mechanical effects near the event horizon. Think of it as a momentary instance of the uncertainty principle, in which, for a VERY short time, light can escape from a black hole.

If our Universe operated without general relativity, then black holes would not exist. If our Universe operated without quantum effects, but only with general relativity, then Hawking Radiation would not exist. But our Universe operates under BOTH of these frameworks, and thus uncertainty effects near a black hole result in this emission (well, in theory, anyway -- Hawking Radiation has never been observed, let alone measured).


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Have been fascinated by physical laws ever since I learned, at age seven, that magnets work under water. My study continued through college and has not ceased even after I retired.

B.A. in Physics (with honors) from University of California at Berkeley.M.A. in Physics (with honors) from University of Texas Austin.

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