What is Quantum Mechanics?

This is a review of chapter four in A Brief History of Time: The Uncertainty Principle. By the way, if you haven’t read chapters 1-3, you should read them in order first.

Newton’s success with gravity and other theories let the Frenchman, Marquis de Laplace around the early 19^th century to argue that the universe was made in a way such that if we knew the position and velocity of every object at one time then we could predict the future. It is understandable in the case of predicting a planet’s rotation, but predicting human behavior, that’s hard to believe. In early 20^th century, two British scientists, Sir James Jeans and Lord Rayleigh said that a hot body must radiate infinite amount of energy. To avoid this ridiculous result, German scientist Max Planck suggested in 1900 that waves were emitted in packets called quanta and making each quanta required a certain amount of energy that is higher than the frequency of the wave making the energy emitted being finite. This suggestion lead to a bigger discovery. In case you forgot, light is a wave. This suggestion about quantum made a German scientist, Werner Heisenberg to say that the longer the wavelength of an object, the easier it is to tell it’s velocity and harder to tell the position and vice-versa with a high wavelength. Because of this, Laplace’s argument might be true, but will never be used since it impossible to tell and objects’ position and velocity at the same time. This became known as the Uncertainty Principle. If you multiply the uncertainty in the position of a particle times the uncertainty of the velocity of the particle multiplied by the mass of the particle your product will always be less than a certain amount known as the Plank’s constant.

Because of the Uncertainty Principle, a new theory called Quantum Mechanics, led by Werner Heisenberg, Erwin Schrödinger, and Paul Dirac, sprang up in the 1920’s. This theory stated that objects did not have specific positions and velocity but rather a quantum state, in which they had a combination of both. In general, Quantum Mechanics didn’t predict a single observation rather the probability of that observation. This received a lot of criticism, especially from Einstein but most scientists agreed with it. Quantum Mechanics tells that things can’t be particles or waves, but it is the observation that is a particle or wave. A result of this idea is that when two wave crests and two wave troughs occur at the same time, the wave is reinforced and in phase. If the crests happen at the same time as a trough then both waves cancel each other out and the wave is out of phase.

A good example of this is the two slit experiment. This experiment is shown in the picture above. Say you have a partition with two narrow slits. There is a light source shining at it. Behind the slit is a screen. On the slit, the light creates a pattern of light and dark fringes. How is that possible? When the light passes through the slit the distance of the screen for each photon traveling through the partition is different. Because of that when the light reaches the screen, some of the light will be in phase, and some will be out of phase. The light out of phase will cancel each other out making the dark pattern and the light in phase will reinforce each other and create the light fringe. This results in the pattern of light and dark fringes. Now the remarkable thing about this experiment is that if you replace the source of light by a source of particles which have a definite speed, like an electron, you’ll get the same light and dark fringe pattern. If the partition had one slit, you would have a uniformly amount of electrons on the screen. This whole experiment is proves that waves can interfere with each other.

Before Quantum Mechanics is that the electrons should lose energy and spiral into the nucleus. That means all matter should collapse into a very high density. That can’t be true because it has not happened to all matter! A partial solution was discovered by Danish scientist Niels Bohr in 1913. He said that electrons could only orbit from certain distances form the nucleus. The problem with this is that it was seemed very reasonless. This was fixed in Quantum Mechanics because it said that the electron would travel like a wave.

Quantum Mechanics has helped in technology, creating circuits, building computers and televisions, chemistry, and even biology. General Relativity and Quantum Mechanics have not fully been combined, but will have to work together with other forces of nature to create a single unified theory of the universe.