A brief history of time 4

 

Chapter 4.

The uncertainty principle

·         The scientific determinism of French Marquis de la Place remained the standard assumption of science from the early 19^th to the early 20^th century.

One of the first indications that this belief would have to be abandoned came when two English scientists suggested that a hot object, such as a star, must radiate energy at an infinite rate.

In order to avoid this ridiculous result, the German scientist Max Planck suggested in 1900 that light, X-rays and other waves could not be emitted at an arbitrary rate, but only in certain packets that he called quanta.

Moreover, each quantum had a certain amount of energy that was greater the higher the frequency of the waves. So at a high enough frequency the emission of a single quantum would require more energy than was available. Thus the radiation at high frequencies would be reduced, and so the rate at which the body lost energy would be finite.

·         In 1926 the German scientist Werner Heisenberg formulated his famous uncertainty principle.

·         In order to predict the future position and velocity of a particle, one has to be able to measure its present position and velocity accurately.

 Heisenberg found that one can never be exactly sure of both the position and the velocity of a particle; the more accurately one knows the one, the less accurately one can know the other.

 So this put an end to Laplace’s theory. One cannot predict future events exactly if one cannot even measure the present state of the universe precisely.

 Βy cutting out all the features of a theory that cannot be observed Heisenberg, Erwin Schrödinger and Paul Dirac were led in the 1920s to reformulate mechanics into a new theory called quantum mechanics based on the uncertainty principle.

In general, quantum mechanics does not predict a single definite result for an observation. Instead, it predicts a number of different possible outcomes and tells us how likely each of these is.

 Quantum mechanics therefore introduces an unavoidable element of unpredictability or randomness into space.

 Albert Einstein never accepted that the universe was governed by chance, despite the important role he had played in the development of these ideas. His feelings were summed up in his famous statement “God does not play dice”.

 The only areas of physical sciences that quantum theory has not yet been incorporated are gravity and the large-scale structure of the universe.

 Although light is made up of waves, Planck’s quantum hypothesis tells us that in some ways it behaves as if it were composed of particles: it can be emitted or absorbed only in packets, or quanta.

 Equally, Heisenberg’s uncertainty principle implies that particles behave in some respects like waves: they do not have a definite position but are “smeared out” with a certain probability distribution.

  Einstein’s general theory of relativity seems to govern the large scale structure of the universe.

However, in strong gravitational fields such as black holes and the big bang the effects of quantum mechanics should be important. But we still do not have a complete consistent theory that unifies general relativity and quantum mechanics, but we do know a number of the features it should have.