Physicists in Israel have shown that radiation from an analogue black hole has a thermal spectrum. This allows them to assign the “black hole” a temperature, providing indirect experimental evidence for the existence of Hawking radiation.
The popular conception of a black hole is a remorseless cosmic vacuum cleaner that sucks everything in and lets nothing – not even light – escape. When quantum mechanics is added to the mix, however, things get more complicated. In 1974, Stephen Hawking showed theoretically that pairs of photons are created at a black hole’s event horizon – one with positive energy and one with negative energy. The negative energy photon is drawn into the black hole, whereas the positive energy photon is emitted into space. This provides a steady energy flux from the black hole.
The theoretical implications of this are revolutionary, because it suggests that black holes have temperatures. Temperature is defined in statistical mechanics by the average energy in a system that has a large number of degrees of freedom (such as a gas comprising many molecules). In Einstein’s general theory of relativity, however, a black hole is defined purely by its mass, charge and spin. Assigning a black hole a temperature, therefore, requires either giving it additional degrees of freedom or redefining the concept of temperature itself.
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