Why does blackbody radiation exist?

  • #1
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It seems it is predicted by both classical and quantum physics. Why is it predicted in the first place? Is it purely a mathematical reasoning, or is there some qualitative reason why it must exist?

In other words, why would some philosopher/physicist hundreds of years ago say to himself, "I think every object emits electromagnetic energy in accordance with its temperature."?
 

Answers and Replies

  • #2
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I believe Black body radiation is a purely quantum phenomenon and cannot be explained through classical physics. It is important because of its universal nature. The intensity profile is completely Independent of the source as it comes about because of the distribution of photons in equilibrium.
 
  • #3
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It seems it is predicted by both classical and quantum physics. Why is it predicted in the first place? Is it purely a mathematical reasoning, or is there some qualitative reason why it must exist?

In other words, why would some philosopher/physicist hundreds of years ago say to himself, "I think every object emits electromagnetic energy in accordance with its temperature."?
One way to see it is through equipartition theorem stating that every degree of freedom (DoF) in thermodynamic equilibrium takes up kT/2 energy on average. If you start with finite number of matter DoF possessing all available energy (i.e. an object at temperature T) coupled to an "array" of infinite number of DoF (i.e. electromagnetic field), final state is that energy would be distributed equal, in a sense; EM field would "suck out" all energy or, if you will, body will radiate out all internal energy. Of course, that would happen either way, but classical physics suggest this energy transfer would be instantaneous - any thermal radiation would have infinite power. Quantum mechanics provides correct explanation of the way matter couples to EM field, more precisely, details how is the interaction suppressed at very high frequencies (Planck hypothesis) thus mitigating unphysical behavior of the system seen by classical approach mentioned above (ultraviolet catastrophe).
 

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