Hawking Radiation Content from the guide to life, the universe and everything

Hawking Radiation

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Hawking radiation is a theoretical phenomenon first proposed by (and named after) the famous astrophysicist, Dr Stephen Hawking.

What is Hawking Radiation?

According to certain principles of quantum mechanics, the probability of any event occurring is always greater than zero. One of the stranger consequences of this idea is that what we think of as 'empty' space isn't really empty at all; it's filled with 'virtual particles', bits of matter and energy that are almost, but not quite, real. Despite being unreal, virtual particles play a vital role in the descriptions of how the universe works on the quantum scale; they're necessary to explain how photons and electrons interact, for example.

Under normal conditions, virtual particles rarely have any noticeable effects. In certain unusual environments, such as the intense gravitational fields generated by black holes, they can 'borrow' energy from their surroundings and temporarily become real. When virtual particles manifest themselves, they must always do so in pairs of particles and anti-particles, which cancel each other out and release their energy back into the void. However, it is possible for the particles to materialise just on the edge of the event horizon, the boundary that separates the 'inside' and the 'outside' of the black hole. When this occurs, one particle is sometimes consumed by the black hole while the other escapes. The escaping particle carries away a tiny fragment of the black hole's mass, the extra energy that allowed it to become real. Over long periods of time, the black hole will eventually evaporate, losing all of its energy to escaping particles. These particles are the Hawking radiation.

An unusual aspect of Hawking radiation is that it may be a proof of the 'arrow of time'. According to classical physics, the universe is time-reversible; anything that happens in the universe could just as easily happen 'backwards' as 'forwards'. For instance, it's possible in theory to reconstruct the content of a burnt newspaper by examining the pattern of the ashes, combustion products, and electromagnetic radiation produced by the fire. However, it's clear that time only flows 'forwards', a phenomenon that has been referred to as 'time's arrow'. Scientists have never been able to understand why this is so, despite many attempts at explanation. It has been suggested that the decay of certain subatomic particles can only happen in one direction, although this has never been proven. It is also thought that the aspect of uncertainty in some interpretations of quantum mechanics (specifically, the idea that a quantum event has a random outcome) might explain why time doesn't run backwards. However, the pattern of Hawking radiation emitted by a black hole bears no relationship to the pattern of matter and energy that the black hole consumed; the 'information' contained in the things that fall into the hole has essentially been erased, making it impossible to reconstruct the nature of anything that went in. This interpretation is being hotly debated in the world of quantum astrophysics at the moment, as it predicts the first example of a non-quantum violation of the laws of causality.

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