The quantum description of the universe is very different than the classically observed one, or our perceptions in everyday life. This new way of looking at nature has many consequences, both philosophically and practically. The modern technological development of the second half of the last century may be a very good example of the consequences of the discovery of the quantum world. Now we have a bunch of gadgets from cellular phones to long-lasting batteries, from engineered drugs to space missions, from pocket size computers to nanotechnology, a wide range of end-products of the quantum world. Certainly, these will not be the only changes in our life; quantum sciences will eventually affect the way we look at life.
One of the most dramatic potential changes in thought may arise from the discovery of the quantum entanglement of particles. Quantum entanglement can be described as non-classical correlations of different parties. It is very different than the classical description and can be explained by using the following analogy. Imagine an author writes a book of one hundred pages which includes the most precious arts or explains very important facts about the universe depending on one’s point of view. To make it more interesting or more realistic, he distributes each page of the book to one of his servants and asks them to read and understand the rules written in the book. That is, each servant has access only to one page of the book. If we assume the information on the pages is classical, every servant has one hundredth of the total information written in the book and if we let them communicate with each other, they can in principle reconstruct the written information. However, the situation is very strange in the quantum world. If the information in the book is written using entanglement principle of the quantum world, then none of the servants has any definite idea about the partial information on his page. It is as if the pages are empty. All the information about the content of the book is written on correlations of the pages, not physically on each page. So, the servants can have no idea, if they only look at their pages.
Einstein vs. Bohr
To understand this strange feature of quantum entanglement we should review the historical development of the concept. One of the earliest objections came from Einstein, who was one of the developers of quantum theory. Although he explained the photoelectric effect by introducing the concept of quantization of light, he did not believe in some of its consequences. Mainly, he was not sure about the completeness of quantum theory because of its contradictions with common sense and the theory of relativity. The famous 1927 Solvey Con ference was a turning point for debates between Einstein and Niels Bohr, who was also one of the developers of quantum theory and the Copenhagen interpretations of the theory.
Einstein tried to show this incompleteness by proposing different Gedanken (thought) experiments. Each of these questions was answered rigorously by Bohr. However, Einstein was never convinced by Bohr about the completeness of the theory. The last one of these Gedanken experiments was one related to our concept, quantum entanglement. It is called the EPR paradox and takes its name from the authors of the famous paper “Can a quantum mechanical description of physical reality be considered complete?” by Einstein, Podolsky and Rosen in 1935.
Mainly, the paper was about faster-than-light communication between physically separated objects, two particles. If two particles are generated from a source affected by the existence of a conservation law, like the conservation of energy, or linear or angular momentum, the conserved property is carried by the particles independent of their separation. If the conserved quantity is observed by measuring one of the particles, the other particle arranges itself according to the result of this measurement independent of the distance between particles. According to Bohr, this arrangement happens instantaneously at the time of measurement, which conflicts with Einstein’s theory of special relativity that says nothing can travel faster than light. Apparently, the knowledge of the result of the first measurement is carried somehow to the second particle. Bohr’s reply is now called the Copenhagen interpretation of quantum mechanics. He takes this property as a postulate of quantum mechanics by saying that the state of the particles includes all information about them. After this explanation Einstein never replied again.
If we look more closely at the proposed experiment, we can deduce that in reality information is not transferred faster than light because although the measurement result of the second particle is decided by the first measurement, this information is hidden for the second particle. The result of the second measurement makes sense only if the result of the first measurement reaches the second one. Otherwise, the second measurement can be described as a random outcome of possible results. Now it makes sense if we return to the book description. Here our book has only two pages. Each page is given to one servant. If they only look at their pages there is no information, which means that measurement results are random.
However, if the two servants work together and share their measurement results, then the initial information can be reconstructed.
Einstein’s point of view can be described in the following example. Imagine we have two coins with the usual heads and tails on different sides. Let us assume that there is a conservation law deduced from everyday experiments stating that if we flip these two coins we always have two opposite results; that is, if we get tails from the one that we measured, the other one is heads for sure and vice versa. In the real world, these coins can be identified as electrons, photons or atoms. Heads/tails corresponds to the spin components for electrons, polarization directions for photons or ground/excited states for atoms. Now, imagine these two coins are separated by a large distance.
Einstein says that as soon as separation occurs the result of flipping is decided but this result is hidden from us. One can measure or learn it by performing a measurement or looking at each coin. Moreover, looking at only one coin is enough to determine the measurement result of the other coin, since the results are correlated. Conjecturing that the side of the coin is determined at the time of measurement is against the causality principle of the theory of relativity which says that cause and effect cannot be simultaneous. However, I am of the opinion that reality is closer to what Bohr described. That is, the result of the measurement is decided at the measurement time not at the separation time. Before the measurement, each coin shows both heads and tails at the same time. The information, deduced at the point of measurement when one of the coins is measured, is transferred faster than light, in other words, at infinite speed.
The nature of each coin is also very strange before the measurement because it includes both sides at the same time with equal probabilities, but a classical coin has only one side at one time, either heads or tails. Here the classical coin means the flipped or measured coin. This property of the quantum world is called parallelism. As in the famous case of SchrÃ¶dinger’s cat, sometimes two extreme situations can happen at the same time. SchrÃ¶dinger’s cat is a very special cat which is dead and alive at the same time, like a quantum coin. However, when one measures such a cat, that is, observes the cat, its nature collapses to one of the known situations, either a dead cat or a live cat. This measuring process happens systematically due to interactions with its surroundings and is called decoherence.
Although the quantum world is very strange and different than the classical world, it encapsulates more reality than we experience in our everyday life. In the near future, we can expect that ways of looking at the world will be different than the present mechanical, deterministic, and materialistic view because of the unexpected outcomes of the quantum world. If you know how to look, you can already feel this change.