The Observer's Riddle: 5 Famous Quantum Experiments

No one in the world understands quantum mechanics - this is the main thing you need to know about it. Yes, many physicists have learned to use its laws and even predict phenomena using quantum calculations. But it is still not clear why the presence of an observer determines the fate of the system and forces it to make a choice in favor of one state. The Theories and Practices selected examples of experiments, the outcome of which is inevitably influenced by the observer, and tried to figure out what quantum mechanics is going to do with such interference of consciousness in material reality.

Schrödinger's Cat

Today, there are many interpretations of quantum mechanics, the most popular of which remains Copenhagen. Its main provisions were formulated in the 1920s by Niels Bohr and Werner Heisenberg. And the central term of the Copenhagen interpretation has become the wave function - a mathematical function that contains information about all possible states of a quantum system in which it simultaneously resides.

According to the Copenhagen interpretation, the state of the system can be definitely determined, and only observation can distinguish it from the rest (the wave function only helps to mathematically calculate the probability of detecting a system in one state or another). We can say that after observation, the quantum system becomes classical: it instantly ceases to coexist in many states at once in favor of one of them.

This approach has always had opponents (remember at least "God does not play dice" by Albert Einstein), but the accuracy of calculations and predictions took its toll. However, in recent years, supporters of the Copenhagen interpretation have become less and less, and not the least reason for this is the same mysterious instantaneous collapse of the wave function during measurement. Erwin Schrödinger's famous thought experiment with a poor cat was just intended to show the absurdity of this phenomenon.

So, let's remind the content of the experiment. A live cat, an ampoule with poison and some mechanism that can trigger the poison into action at a random moment is placed in a black box. For example, one radioactive atom, the decay of which will break an ampoule. The exact decay time of the atom is unknown. Only the half-life is known: the time during which the decay will occur with a probability of 50%.

It turns out that for an external observer, the cat inside the box exists in two states at once: it is either alive if everything is going well, or dead if the decay has occurred and the ampoule has broken. Both of these states are described by the wave function of a cat, which changes over time: the further, the more likely it is that radioactive decay has already occurred. But as soon as the box opens, the wave function collapses, and we immediately see the outcome of the flaying experiment.

It turns out that until the observer opens the box, the cat will forever balance on the border between life and death, and only the observer's action will determine his fate. Here is the absurdity pointed out by Schrödinger.

Electron diffraction

According to a survey of leading physicists conducted by The New York Times, the experiment with electron diffraction has become one of the most beautiful in the history of science. What is its essence?

There is a source that emits a stream of electrons towards the screen-photographic plate. And there is an obstacle in the way of these electrons - a copper plate with two slits. What kind of picture on the screen can you expect if you think of electrons as just small charged balls? Two overexposed stripes opposite the slits.

In reality, a much more complex pattern of alternating black and white stripes appears on the screen. The fact is that when electrons pass through the slits, they begin to behave not like particles, but like waves (just like photons, particles of light, can simultaneously be waves). Then these waves interact in space, somewhere weakening, and somewhere reinforcing each other, and as a result, a complex picture of alternating light and dark stripes appears on the screen.

In this case, the result of the experiment does not change, and if electrons are sent through the slit not in a continuous flow, but one by one, even one particle can be a wave at the same time. Even one electron can simultaneously pass through two slits (and this is another of the important provisions of the Copenhagen interpretation of quantum mechanics - objects can simultaneously exhibit both their "usual" material properties and exotic wave properties).

But what does the observer have to do with it? Despite the fact that with him the already complicated story became even more complicated. When in such experiments physicists tried to fix with the help of devices through which slit the electron actually passes, the picture on the screen changed dramatically and became "classical": two illuminated areas opposite the slits and no alternating stripes.

It was as if the electrons did not want to show their wave nature under the watchful eye of an observer. We adjusted to his instinctive desire to see a simple and understandable picture. Mystic? There is also a much simpler explanation: no monitoring of the system can be carried out without physical impact on it. But we will return to this a little later.

Heated fullerene

Experiments on particle diffraction were performed not only on electrons, but also on much larger objects. For example, fullerenes - large, closed molecules composed of tens of carbon atoms (for example, a fullerene of sixty carbon atoms is very similar in shape to a soccer ball: a hollow sphere sewn from pentagons and hexagons).

Recently, a group from the University of Vienna, led by Professor Zeilinger, tried to introduce an element of observation into such experiments. To do this, they irradiated moving fullerene molecules with a laser beam. Then, heated by an external influence, the molecules began to glow and thus inevitably found their place in space for the observer.

Along with this innovation, the behavior of molecules has also changed. Before the start of total tracking, fullerenes quite successfully avoided obstacles (showed wave properties) like the electrons from the previous example passing through an opaque screen. But later, with the appearance of an observer, fullerenes calmed down and began to behave like completely law-abiding particles of matter.

Cooling measurement

One of the most famous laws of the quantum world is the Heisenberg uncertainty principle: it is impossible to simultaneously establish the position and speed of a quantum object. The more accurately we measure the momentum of a particle, the less accurately its position can be measured. But quantum laws operating at the level of tiny particles are usually invisible in our world of large macro objects.

Therefore, the more valuable are the recent experiments of the group of Professor Schwab from the USA, in which quantum effects were demonstrated not at the level of the same electrons or fullerene molecules (their characteristic diameter is about 1 nm), but on a slightly more tangible object - a tiny aluminum strip.

This strip was fixed on both sides so that its middle was suspended and could vibrate under external influence. In addition, next to the strip there was a device capable of registering its position with high accuracy.

As a result, the experimenters discovered two interesting effects. First, any measurement of the position of the object, observation of the strip did not pass without leaving a trace for it - after each measurement, the position of the strip changed. Roughly speaking, the experimenters determined the coordinates of the strip with great accuracy and thereby, according to the Heisenberg principle, changed its speed, and hence the subsequent position.

Secondly, which is quite unexpected, some measurements also led to the cooling of the strip. It turns out that the observer can change the physical characteristics of objects only by his presence. It sounds incredible, but to the credit of physicists, let's say that they were not taken aback - now Professor Schwab's group is thinking how to apply the discovered effect to cooling electronic microcircuits.

Fading Particles

As you know, unstable radioactive particles decay in the world not only for the sake of experiments on cats, but also quite by themselves. Moreover, each particle is characterized by an average lifetime, which, it turns out, can increase under the watchful eye of an observer.

This quantum effect was first predicted in the 1960s, and its brilliant experimental confirmation appeared in a paper published in 2006 by the group of Nobel laureate in physics Wolfgang Ketterle of the Massachusetts Institute of Technology.

In this work, the decay of unstable excited rubidium atoms (decay into rubidium atoms in the ground state and photons) was studied. Immediately after the preparation of the system, the excitation of the atoms began to be observed - to shine through them with a laser beam. In this case, the observation was carried out in two modes: continuous (small light pulses are constantly fed into the system) and pulsed (the system is irradiated from time to time with more powerful pulses).

The results obtained are in excellent agreement with theoretical predictions. External light influences really slow down the decay of particles, as if returning them to their original state, far from decay. In this case, the magnitude of the effect for the two investigated regimes also coincides with the predictions. And the maximum life of unstable excited rubidium atoms was extended 30 times.

Quantum mechanics and consciousness

Electrons and fullerenes cease to show their wave properties, aluminum plates are cooled, and unstable particles freeze in their decay: under the omnipotent gaze of an observer, the world is changing. What is not evidence of the involvement of our mind in the work of the world around? So maybe Karl Jung and Wolfgang Pauli (Austrian physicist, Nobel laureate, one of the pioneers of quantum mechanics) were right when they said that the laws of physics and consciousness should be considered as complementary?

But this is only one step left before the duty recognition: the whole world around is an illusory product of our mind. Creepy? ("Do you really think that the moon exists only when you look at it?" - Einstein commented on the principles of quantum mechanics). Then let's try again to turn to physicists. Moreover, in recent years they are less and less fond of the Copenhagen interpretation of quantum mechanics with its mysterious collapse of the wave of a function, which is being replaced by another, completely mundane and reliable term - decoherence.

The point is this: in all the experiments described with observation, the experimenters inevitably influenced the system. It was illuminated with a laser, measuring instruments were installed. And this is a general, very important principle: you cannot observe a system, measure its properties, without interacting with it. And where there is interaction, there is a change in properties. Especially when the colossus of quantum objects interact with a tiny quantum system. So the eternal, Buddhist neutrality of the observer is impossible.

This is exactly what the term “decoherence” explains - a thermodynamically irreversible process of violation of the quantum properties of a system when it interacts with another, large system. During such interaction, the quantum system loses its original features and becomes classical, "obeys" a large system. This explains the paradox with Schrödinger's cat: the cat is such a large system that it simply cannot be isolated from the world. The very statement of the thought experiment is not entirely correct.

In any case, in comparison with reality as an act of creation of consciousness, decoherence sounds much more relaxed. Even, maybe, too calm. Indeed, with this approach, the entire classical world becomes one big decoherence effect. And as the authors of one of the most serious books in this field claim, statements like “there are no particles in the world” or “there is no time at a fundamental level” also logically follow from such approaches.

Creative Observer or Omnipotent Decoherence? You have to choose between two evils. But remember - now scientists are more and more convinced that the very notorious quantum effects lie at the heart of our thought processes. So where observation ends and reality begins - each of us has to choose.