100 years of quantum physics: it works, but scientists still don't know what it is

Leading physicists of the world gathered on a German island Helgoland and disagreed on the definition of the term quantum physics. 

In honor of the centenary of quantum theory the journal Nature conducted the largest ever survey among quantum physicists. More than 15 thousand researchers were asked questions. More than 11 thousand responses were received. 

The results demonstrated, that scientists are confident in their knowledge, but disagree about the interpretation. Paradoxically, quantum theory works very well in practice and serves as the basis for such devices as MRI, lasers and computers. But even after 100 years, physicists still cannot come to a consensus on the definition of quantum physics. 

Another Nobel laureate disagrees Alain Aspect. It is somewhat unusual for people from the same field to disagree about the fundamental interpretation underlying their work.

Quantum mechanics, as a branch of physics, describes the behavior of atoms, electrons, photons, and other subatomic particles. It is a world governed by probabilities, uncertainty, and phenomena that defy conventional intuition. In 1900, the German physicist Max Planck proposed that energy comes in certain portions — quanta. In 1905, Albert Einstein developed this theory and suggested that light consists of photons. Over the next 20 years, experiments continued to develop this idea. By 1925, Werner Heisenberg had developed a version of quantum theory based on observables, and a year later Erwin Schrödinger proposed wave mechanics — a detailed mathematical description of the evolution of quantum systems in time.

The divergence of views among modern physicists on the definition of quantum theory is based on a fundamental question: what happens when you observe a quantum system? The classic example is usually cited as an invented Erwin Schrödinger’s cat of uncertainty. Inside a closed box a quantum event with a 50/50 probability determines whether the poison bubble will be released and the cat will die. You don’t know what happened and the cat is both alive and dead until someone opens the box and sees the result.

According to quantum mechanics, as long as the box is not opened and the system is not observed, the cat is in a superposition of life and death. Only upon close examination does one certain outcome become apparent.

But what actually happens when the box is opened and the results are examined? As many as five competing hypotheses have been put forward in this regard. According to the Copenhagen interpretation put forward by physicists Niels Bohr and Werner Heisenberg in 1927, opening a box creates reality. A particle has properties only when measured by an observer; they are not its intrinsic properties. This approach essentially states that quantum mechanics does not define a physical reality that exists independently of measurement. 

Instead, it describes what we can learn from measurement. Critics, however, argue that this definition sidesteps important questions, such as what is measurement and when is the outcome determined?

The multinational interpretation states, that all possible outcomes are indeed realized, but each in a separate branching universe. Thus, in Schrödinger’s imaginary experiment, the cat is both alive and dead, but in two different worlds that do not come into contact with each other.

The de Broglie-Bohm theory introduces the notion of «pilot wave», which makes all quantum phenomena deterministic rather than random. However, it faces the problem of instantaneous effects exceeding the speed of light, which is incompatible with Einstein’s theory of relativity.

The spontaneous collapse hypothesis assumes that quantum mechanics — is only an approximation to another theory. This hypothesis is well grounded mathematically, but has no experimental evidence.

Finally, epistemic interpretations assume that quantum states are only information available to an external agent. This also creates an unpleasant problem: reality can only exist subjectively, and there is no objective reality.

The most popular among the surveyed physicists was The Copenhagen interpretation is supported by 36% of quantum physicists surveyed. However, only a small proportion of them were confident that it was correct; the majority considered it merely adequate or useful. This was followed by epistemic approaches (17%) and multiworld theory (15%).

About 10% of respondents chose the option «other» and provided free-form answers, while a small number stated that none of the interpretations seemed adequate or that an interpretation was not necessary at all. Overall, only 24% of all respondents were confident that their interpretation was correct.

Source: ZME Science