About 100 years ago, scientists first thought about the nature of some unusual properties of light. For example, the light coming from gases when they are heated in a test tube. If you look at this light through a lens, you will notice something unusual. It’s not the spectrum in which colors smoothly transition into one another, reflected in a crystal glass, but the distinct lines, whose colors do not mix like in a rainbow. We are talking about vertical rays of light, similar to pencils – each of their colors. However, the scientists could not explain such a strange property of light. Search of answers unsuccessfully proceeded, while physicist Niels Bohr in the beginning of XX century has not put forward the most incredible and fantastic hypothesis. Bohr was convinced that the solution of clear lines lies in the heart of matter, the structure of the atom.

The fantastic hypothesis

According to the scientist, atoms resemble tiny models of the solar system, as electrons rotate around the nucleus, like planets. But the electrons, unlike the planets, move on one particular orbit and no other. Bohr argued that when an atom heats up, electrons come into motion and jump from one orbit to another. In this case, each jump is accompanied by the release of energy in the form of light with a certain wavelength. This is where those strange vertical lines and the concept of “quantum leap” come from.

In the National Geographic documentary on quantum theory, physicist Brian Green talks about the amazing properties of quantum leap, which is that the electron moves from one orbit to another at once, as if without crossing the space between them. It is as if the Earth changed its orbits with Mars or Jupiter in an instant. Bohr believed that because of the strange properties of electrons in the atom, they emit energy in certain, indivisible portions called quanta. That is why electrons can move strictly on certain orbits and can be either at one point or in another, but not in the middle. In everyday life we do not encounter anything like this.

If a baseball was in two places at the same time, we might believe that we are cheated by a wizard. But in quantum mechanics, having a particle in two places at the same time is exactly what makes us think the experiment is true.

However incredible Bohr’s assumption may seem, physicists quickly found a great deal of evidence in favor of his theory – electrons do behave according to completely different laws than planets of the solar system or ping-pong balls. The discovery of Bohr and his colleagues, however, contradicted the well-known laws of physics and soon led to collision with the ideas expressed by Albert Einstein.

Quantum entanglement

Einstein could not accept the uncertainty of the universe arising from quantum mechanics. The physicist believed that an object exists not only when it is observed (as Niels Bohr argued), but all the rest of the time. The scientist wrote: “I want to believe that the moon shines even when I am not looking at it. The very idea that the reality of the universe is determined when we open and close our eyes seemed unthinkable to him. In Einstein’s opinion, quantum theory lacked something that would describe all the properties of particles, including their location even when they are not observed. And in 1935 it seemed to Einstein that he found a weak point in quantum mechanics. It was an incredibly strange phenomenon, contrary to all logical concepts of the universe – quantum entanglement.

Quantum entanglement is a theoretical assumption derived from the equations of quantum mechanics, according to which two particles can be entangled if they are quite close to each other. Their properties become interconnected in doing so.

But even if these particles are separated and sent to different ends of the world, as quantum mechanics suggests, they can still remain confused and inextricably linked. Einstein thought such a bond between the particles was impossible, he called it that – “a supernatural bond at a distance”. The scientist assumed that tangled particles could exist, but believed that there was no “supernatural distance bond”. On the contrary, everything is predetermined long before measurement.

Niels Bohr, in turn, relied on equations that prove that particles behave like two wheels, which can instantly bind random results of their rotation, even being at a huge distance from each other. So who is right?

To determine whether there is indeed a “supernatural bond” between the tangled particles as between the rotating wheels, or whether there is no bond and the properties of the particles are predetermined in advance, as with a pair of gloves, the physicist John Bell succeeded. Using complex mathematical calculations, Bell showed that if there is no supernatural bond, then the quantum mechanics is wrong. However, the theoretical physicist also proved that the problem can be solved by building a machine that would create and compare many pairs of tangled particles.

Based on the instructions of Bell Physicist, Quantum Mechanics Specialist John Clauser assembled a machine capable of doing this work. Klauser’s machine could measure thousands of pairs of tangled particles and compare them on many parameters. The results made the scientist think he had made a mistake. Soon, the French physicist Alain Aspe got to the heart of Einstein’s and Bohr’s argument.

In Aspe’s experience, measuring one particle could only directly affect the other if the signal from the first to the second particle would pass at a speed exceeding the speed of light. Which, as we know, is impossible. Thus, there was only one explanation left: the supernatural connection. Moreover, our experiments proved that the mathematical basis of quantum mechanics is correct.

The entanglement of quantum states is a reality.

It turns out that quantum particles can be connected despite huge distances, and measurement of a single particle can really affect its distant pair, as if the space between them never existed. But no one can answer the question of how this connection works today.