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Olber's Paradox is a well-known paradox in cosmology.

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Although known as Olber's paradox this problem was first stated by Jean Philippe Leys de Cheseaux of Lausanne in 1744. "Why is the sky dark? If the number of stars is infinite, a stellar disk should cover every patch of sky." De Cheseaux claimed that a slight loss of light would solve the problem.

However, this is not true, because from conservation of energy the light absorbed by one object would cause it to heat up until it was emitting light at the same rate as it was absorbing it. It is this that makes the sky bright during the day as sunlight is scattered by air molecules or water drops.

In 1826 Heinrich Wilhelm Mathäus Olbers, the discoverer of the minor planets Pallas and Vesta, reformulated the paradox; "Why is the sky dark at night? The intensity of light reduces with the square of the distance from the observer. If the distribution of stars is uniform in space, then the number of stars at a particular distance from the observer should be proportional to the surface area of a sphre whose radius is that distance.

At each radius therefore the amount of light should be both proportional to the radius squared and inversely proportional to the radius squared. These two effects will cancel and so every shell should add the same amount of light. In an infinite universe the sky would be infinitely bright."

So de Cheseaux thought the sky would be as bright as the sun, whereas Olbers argued that it would be even brighter, and yet as everyone knows the sky is dark at night.

This simple fact firmly roots cosmology in the observable sciences, and takes it out of the realm of pure theory. At least one of the assumptions that the universe is static, uniform and infinite in extent must be wrong. Olbers' solution was that the stars had not all been shining in the past but that something had caused them to switch on. What that was he did not know.

This is not the only paradox to affect cosmology.

Paradoxes in time Travel: -The Novikov self-consistency principle and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes - there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalized they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.

Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel histories would form a branching tree symbolizing all possible outcomes of any interaction.

Since all possibilities exist, any paradoxes can be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction. However, in actuality, physicists believe that such interaction or interference between these histories is not possible.

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