# Are there actually photons

A storyRemarks on this

Imagine you have two friends - let's call them Alice and Bob - who live in two different places. You send each of the two a box in which you have previously placed one white and one black ball. They bring the boxes to the post office, address them, and two days later, at an agreed time, Alice and Bob open their boxes and each take out a ball, blindfolded. Then the boxes are thrown away, because the rule of the game is not to look into them.

The whole process is repeated every week, because it has a special story with him: for some reason Alice and Bob are always drawing balls same Colour! All of your friends are puzzling as to how this can be done: Once Alice and Bob both pull a white ball, another time they both pull a black ball out of the box. But the colors of the balls are never different - Alice and Bob swear that they did not meet in secret and that they would not use any other trick!

The behavior told here actually exists! Not with boxes and balls, but with photons (light particles) or electrons, for example. It's called Entanglement. It works like this with photons: First two photons are generated (according to a certain process, the details of which are not of interest here), which fly away from each other. A horizontally aligned polarization filter is placed in the path of each photon. In principle, every photon can be transmitted through its polarization filter or absorbed by it. Both are possible, and sometimes one thing happens, sometimes the other. But both photons always do the same thing: they are both let through or both are absorbed.

One of your friends - Niels - developed the theory that the two boxes together form an inseparable unit. Before When they are opened, the colors of the balls drawn are not fixed - they are in a fundamental way indefinite. Therefore, only a probability statement can be made in advance.

Albert contradicts this view, who has a completely different solution to offer. In his opinion, there is only one in each of the two boxes that Alice and Bob receive only one Bullet. Some stranger has to open the boxes at the post office the night before they are delivered and take out two balls of the same color! Since nobody - apart from the culprit - knows which balls are now in the box, it cannot be predicted which color Alice and Bob will draw - the only thing that is certain is that they will be balls of the same color. The vaguenessthat Niels insists is just one in Albert's eyes ignorance.

Since it is one of the irrefutable rules of the game not to look into the caskets, it is unfortunately not possible to check whether Albert is right.

According to the so-called Copenhagen interpretation of quantum theory, of which Niels Bohr was the main proponent in the 1930s, the two photons form an inseparable unit. Before when they hit their polarizing filters, it is fundamentally indefinitewhether they are transmitted or absorbed.

Albert Einstein (together with his colleagues Podolsky and Rosen) contradicted this view in 1935. Einstein was of the opinion that unknown quantities (the so-called local hidden variables) that determine the behavior of the photons in advance, i.e. that the quantum theory is incomplete and the vagueness just one ignorance is.

For thirty years it was unclear whether Einstein could be right.

Some time later, another acquaintance - John - has an idea. It is too complicated to be told here because it has to do with mathematics and statistics. John writes some formulas on a piece of paper and with their help proves that it is possible to resolve the controversy between Niels and Albert! Alice and Bob have to do something related to the balls and boxes, but without breaking the rules of the game.

When an inspection is carried out according to John's plans, it becomes clear that there are in every box two Bullets when they are delivered. Nobody tampered with the caskets.

In the 1960s, Irish physicist John Bell had an idea. It is related to the fact that the two polarization filters that are placed in the path of the photons can be rotated as desired and, strictly speaking, represent the measurement of a different quantity in each position. John Bell wrote down an inequality (called the Bell's inequality), which is based on a statistical evaluation of the photon behavior when the two polarization filters are rotated independently of each other in all possible directions. If Einstein were right, then this inequality would have to hold. (Admittedly, the bullet story is a bit too simple on this point!)

Alain Aspect succeeded in experimentally verifying the situation in 1982 (and has since been perfected, including by Anton Zeilinger in 1998). All results are clear: Bell's inequality is violated. There are no local hidden variables!

So it looks like the two balls are coordinating their behavior in some puzzling way, and that is only when Alice and Bob open the box. However, this behavior could never be used to send messages! When Alice has taken her ball out of the box, she knows which ball Bob is holding in his hand at the same time, but no communication can be established on it.

So it looks at first as if the two photons are coordinating their behavior with each other in some puzzling way only at the time of measurement. However, according to the theory of relativity, "instant" information transfer is not possible and has never been observed: Entanglement cannot be used to send messages faster than light! In this regard, quantum theory is in perfect harmony with relativity.

Since then, you and your acquaintances have been discussing the phenomenon of miracle balls from time to time, and whether they might not but use some trick ...

The discussion about the interpretation of quantum theory has never stopped. Even today, theories with hidden variables (which then not local must be) developed ...