3 The Quantum World

3.1 The early days

On 19 October 1900, Max Planck, a 42 year old physicist, presented to a meeting of the German Physical Society a ground-breaking solution to a long-standing problem: Why does the colour of radiation from any glowing body change from red to orange and ultimately to blue as its temperature increases? Planck based his answer on the assumption that radiation (like matter) comes in discrete quantitiesof energy, the so-called “quanta", the Latin for amount.
max planck
 
 Figure 1: Max Planck

The quantization of energy proved to be a revolutionary and fundamental rule of nature, with very strange conclusions contradicting what common sense or classical Newtonian mechanics imposed: things change because you “look” at them, an action can precede its cause (Compton effect), travelling faster than light might be possible (EPR), objects have “random” behaviour, etc.

How come that an innocuous statement about energy can determine such an alarming behaviour as the uncertainty principle (one can never measure anything as accurately as one would like, to put it in a rather misleading way)?

Here is a simple example suggested by Feynman. Consider a mirror which reflects a light, say in a proportion of 95%: 5% of light passes through and it is absorbed or lost while 95% bounces off the mirror’s surface.

If light is seen as a continuous stream of energy then we don’t have any difficulty in explaining the above phenomenon. However, if light is a stream of indivisible quanta (photons), then we get a problem: each photon is either reflected or absorbed in its entirety, but one cannot accept that 5% of a photon goes into one direction and the remainder of 95% goes into a different direction! Consequently, one is led to the conclusion that out of 20 photons, 19 bounce off the mirror’s surface and just one photon goes in a different direction. OK, this is not difficult to accept, but who decides which photon goes astray? This is the crucial question!

Quantum theory claims that we cannot know the answer to the above question: what happens to any individual photon is completely unpredictable! We can only talk about chance, about probability: a photon will have a 95% chance of bouncing off and 5% chance of being absorbed/transmitted. One cannot say more. The unpredictability/randomness is part of the game, it’s innate! In opposition with classical physics, in quantum theory one can only describe a quantum “state" of a photon in terms of its probabilities and probabilities themselves change depending upon what one plans to do with the photon.


From the time of original discoverers to the present day people expressed wonder and disbelief:
Quantum mechanics is very impressive. But an inner voice tells me that it is not the real thing. The theory produces a great deal but hardly brings us closer to the secrets of the old one. _A. Einstein.
…we always have had (secret, secret, close the doors!) …a great deal of difficulty in understanding the world view that quantum mechanics represents. At least I do, because I’m an old enough man that I haven’t got to the point that this stuff is obvious to me. Okay, I still get nervous with it. _R. Feynman.
But, to put it bluntly, [quantum mechanics] gives you the wrong answer. R. Penrose.
Still, quantum theory is one of the most successful scientific theories: it makes unbelievable good predictions which were tested to an unprecedented degree of accuracy and underpins modern technology, from supermarket laser scanners to all goodies of microelectronics, contributing – according to Tegmark and Wheeler [133] – to an estimated 30% of the U.S. gross national product.



Planck felt very uncomfortable himself with the idea that quantization is a law of nature. In this connection, he is notorious for saying that “new scientific theories supplant previous ones not because people change their minds, but simply because old people die."