Quantum theory of symmetrically in time

Scientists at the Université Libre de Bruxelles (Belgium) have developed a completely symmetrical formulation of quantum theory, which establishes a precise relation between asymmetry and the fact that we can remember the past but not the future.

The laws of classical mechanics are independent of the direction of time, but that the same is true in quantum mechanics has been a subject of debate. Although it has been agreed that the laws that govern the isolated quantum systems are symmetric in time, the measurement changes the State of a system in accordance with rules that only seem to work forward, and there are differences in opinion about the interpretation of this effect.

Now Belgium theoretical physicists have developed a fully symmetrical formulation in the time of the quantum theory, which sets an exact relationship between this asymmetry and the fact that we can remember the past but not the future - a phenomenon that the physicist Stephen Hawking He has named the "psychological" arrow of time. The study offers new insights into the concepts of free choice and causality, and suggests that causality does not have why to be considered as a fundamental principle of physics.

Also expands a fundamental theorem of quantum mechanics due to Eugene Paul Wignerthat points to new directions for the search for physics beyond the known models. The findings of Ognyan Oreshkov and Nicolas Cerf This week they have been published in the magazine Nature Physics.

The idea that our choices in the present can influence events in the future, but not in the past, is reflected in the rules of the quantum theory standard as a principle that quantum theorists call "causation", says the press release from the University, collected by AlphaGalileo.

To understand this principle, the authors of the new study analyzed what the concept of choice in the context of the quantum theory actually means. For example, we think that an experimenter can choose which measurement carried out in a given system, but not the result of the measurement. Accordingly, in accordance with the principle of causality, the choice the measurement can be correlated with results of measurements only in the future, while the result a measurement you can correlate with results of measurements both past and future.

The researchers argue that the property that determines that we interpret the variable that describes the extent as dependent the choice of the experimenter, while the result isn't, is that it can be known before that the measurement is made.

From this perspective, the principle of causality can be understood as a limitation to the information available on different variables at different times. This limitation is not symmetric in time as both the choice of measurement and the result of a measurement can be known ex post. This, according to the study, is the essence of the asymmetry in the standard formulation of the quantum theory.

"The quantum theory It has been formulated on the basis of asymmetric concepts that reflect the fact that we know the past and are interested in the prediction of the future. "But the concept of probability is independent of time, and from a perspective of Physics makes sense try to formulate the theory in terms essentially symmetrical", says Ognyan Oreshkov, the lead author of the study.

To do this, the authors propose to adopt a new notion of measure that is not only defined by the variables of the past, but that may depend on the variables of the future also. "The approach we propose, measurements are not interpreted as"free elections"from agents, but simply describe the information on possible events in different regions of space-time", says Nicolas Cerf, co-author of the study and director of the Center for quantum information and communication of the University.

In the symmetric formulation in the time of the quantum theory that follows from this approach, the principle of causality and the psychological arrow of time arise from what physicists call boundary conditions - parameters based on which the theory makes predictions, but whose values may be arbitrary in principle. Thus, for example, according to the new formulation, it is conceivable in some parts of the universe that causality can be raped.

Another consequence of the symmetric formulation in time is an extension of a fundamental theorem of Wigner, that characterizes the mathematical representation of physical Symmetries and is fundamental for the understanding of many phenomena, such as which elementary particles can exist.

The study shows that in the new formulation symmetries can be represented in ways not allowed by the standard formulation, which could have far-reaching physical consequences. One speculative possibility is that such symmetries can be relevant in a theory of quantum gravity, because they have the form of the transformations that will have guessed that they occur in the presence of black holes.

"Our work shows that if we believe that the symmetry of time must be a feature of the fundamental laws of physics, we must consider phenomena beyond the conceivable in" standard quantum theory. There are such phenomena and where we could find them is a big open question,"explains Oreshkov.