The theory “the arrow of time”


Coffee cools, buildings crumble, eggs break and stars burn out in the universe, which seems doomed to transition to gray monotony, known as thermal equilibrium. Astronomer and philosopher sir Arthur Eddington (Arthur Eddington) in 1927 stated that the gradual dissipation of energy is a proof of the irreversibility of the “arrow of time”.

But to the bewilderment of generations of physicists, the concept of the arrow of time is not consistent with the basic laws of physics, which in time act as in the forward direction and in the opposite direction. Under these laws, if someone knew the paths of all particles in the universe and drew them back, the energy would accumulate and not dissipate: cold coffee would start to heat up, building to rise from the ruins, and the sunlight would go back to the Sun.

“In classical physics we had difficulties,” says Professor Sandu Popescu (Popescu Sandu), who teaches physics at the British University of Bristol. — If I knew more maybe I could turn the events around and collect all the molecules of the broken eggs?”

Of course, he said, the arrow of time is not controlled by human ignorance. And yet, since the birth of thermodynamics in 1850-e years the only known method of calculating the distribution of energy was the formula of the unknown statistical distribution of the particle trajectories and demonstrate that over time the ignorance lubricates picture of things.

Now physicists reveal a more fundamental source for the arrow of time. The energy is dissipated, and objects come into balance, they say, because elementary particles in interaction go awry. This strange effect they called “quantum mixture” or confusion.

“Finally we can understand why a Cup of coffee in the room comes to equilibrium with it,” says quantum physicist from Bristol Tony Shorts (Tony Short). — Confusion between the state of the coffee Cup and the state of the room.”

Popescu, Short and their colleagues Noah Linden (Noah Linden and Andreas winter Andreas Winter) reported their discovery in the journal Physical Review E in 2009, arguing that objects come into balance, or into a state of uniform energy distribution, for indefinite time due to quantum-mechanical mixing with the environment. A similar discovery a few months earlier made Peter Reimann (Peter Reimann) from Bielefeld University in Germany, published their findings in Physical Review Letters. Short and colleagues supported their arguments in 2012, showing that the entanglement causes the equilibrium in finite time. But in a paper published in February on the website arXiv. org, two separate groups have taken the next step, calculating that most physical systems are balanced quickly for the time is directly proportional to their size. “To show that it is applicable to our real physical world, the processes must occur within a reasonable time frame,” says Short.

Trend coffee (and everything else) to come in equilibrium is “very intuitive,” said Nicolas Brunner (Brunner Nicolas), working quantum physicist at the University of Geneva. “But when you explain the reasons for that we first have a solid Foundation taking into account the microscopic theory”.

If a new direction of research is correct, then the story of time’s arrow begins with the quantum-mechanical idea that at its core nature is initially uncertain. Elementary particle devoid of specific physical properties, and it is determined only by the probabilities of being in certain States. For example, at some point, the particle may with a 50 percent chance of spinning clockwise and 50% counterclockwise. Experimentally proven theorem of Northern Irish physicist John bell (John Bell) States that there is no “true” States of the particles; the probability is the only thing that you can use to describe it.

Quantum uncertainty inevitably leads to confusion — the alleged source of the arrow of time.

When two particles interact, they cannot describe individual, independent probabilities of developing called a “pure state”. Instead, they become entangled components of more complex probability distributions, which describe the two particles together. They can, for example, indicate that the particles are rotated in opposite directions. The whole system is in a pure state, but the state of each particle is “mixed” with the state of the other particle. Both particles can move at a distance of several light years from each other, but the rotation of one particle will correlate with the other. Albert Einsten well described this as “spooky action at a distance”.

“Entanglement is in some sense the essence of quantum mechanics, or the laws governing interactions at the subatomic scale, Brunner said. This phenomenon is the basis of quantum computing, quantum cryptography and quantum teleportation.

The idea that mixing can explain the arrow of time, for the first time 30 years ago came up with Seth Lloyd (Seth Lloyd), when he was 23-year-old graduate of the faculty of philosophy of the University of Cambridge with a degree from Harvard in physics. Lloyd realized that quantum uncertainty and its propagation as particles become increasingly entangled, could replace the uncertainty (or ignorance) of a person in the old classical proofs and to become a true source of time’s arrow.

Using a little-known quantum-mechanical approach in which information units are the basic building blocks, Lloyd a few years studying the evolution of particles in terms of shuffling ones and zeros. He found that as more and more particles are mixed with each other, the information describing them (for example, 1 to rotate clockwise and 0 — counterclockwise), go to the description of the system of entangled particles as a whole. Particles as if gradually lost their autonomy and became pawns of the collective state. Over time, all the information goes into these collective aggregations and the individual particles it is not at all. At this point, as Lloyd discovered, particles pass into a state of equilibrium, and their States stop changing, like a Cup of coffee cools to room temperature.

“What’s really going on? Things become more vzaimosvyazannymi. The arrow of time is the arrow of growth correlations”.

This idea is contained in the doctoral thesis of Lloyd’s in 1988, was not heard. When a scientist sent an article about it in the journal, he said that “not physics”. The theory of quantum information “was deeply unpopular” at that time, says Lloyd, and questions about the arrow of time “there were a lot of loonies and tronuvshegosya mind the Nobel prize.”

“I was damn close to becoming a taxi driver” — he said.

Since then, advances in the field of quantum computing has turned the theory of quantum information one of the most active fields of physics. Currently, Lloyd works as a Professor at mit, he is recognized as one of the founders of this discipline, and his forgotten idea revived by the efforts of physicists from Bristol. New proofs are more common, scientists say, and is applicable to any quantum system.

“When Lloyd proposed the idea in his dissertation, the world was not ready for it,” says the head of the Institute for theoretical physics at ETH Zurich Renato Renner (Renato Renner). — No one understood him. Sometimes you need to the idea came at the right time”.

In 2009, the evidence of the Bristol team of physicists found a response among theorists of quantum information, which has opened new ways to use their methods. They showed that as objects interact with the environment — like particles in a Cup of coffee interacts with the air — information about their properties “leaks out and spreads on this environment,” explains Popescu. This local loss of information leads to the fact that the state of the coffee remains unchanged even if the pure state of the entire room continues to change. With the exception of a rare random fluctuation, he says, “its state stops changing in time”.

It turns out, the cold Cup of coffee can’t spontaneously heat up. In principle, as the evolution of a pure state of the room, coffee can suddenly stand out from the air and return to a clean state. But mixed States much more than clean, almost and coffee can never go back to a clean state. To see this, we have to live longer than the universe. This statistical unlikelihood does the arrow of time irreversible. “In fact, the mix opens up a huge space,” says Popescu. — Imagine you are in the Park, the gate in front of you. As soon as you enter them, you get out of balance, find yourself in a huge space and get lost in it. To the gate you will never return”.

In the new history of the arrow of time information is lost in the process of quantum entanglement, and not because of subjective lack of human knowledge about what leads to balance a Cup of coffee and the room. The room in the end is balanced with the external environment, and the environment is moving more slowly towards equilibrium with the rest of the universe. The giants of thermodynamics of the 19th century regarded this process as a gradual dissipation of energy, which increases the total entropy, or chaos of the universe. Today, Lloyd, Popescu and other professionals from this field use the arrow of time differently. In their view, information is becoming more diffuse, but never disappears completely. Although entropy increases locally, the overall entropy of the universe remains constant and is zero.

“The overall universe is in a pure state,” says Lloyd. But parts of it are intertwined with the rest of the universe, come in a mixed state”.

But one mystery of the arrow of time remains unsolved. “In these works there is nothing that explains why you start with gates, says Popescu, returning to the analogy with the Park. In other words, they do not explain why the initial state of the universe was far from equilibrium”. The scientist suggests that this question relates to the nature of the Big Bang.

Despite recent progress in calculations of the trim time, the new approach still can not become a tool for the calculation of thermodynamic properties of specific things like coffee, glass or unusual States of matter. (Some experts in traditional thermodynamics say that I know very little about the new approach). “The fact that you need to find the criteria for which things behave like window glass and which as a Cup of tea,” Renner says. — I think that will see new work in this direction, but there was still a lot.”

Some researchers expressed doubt that this abstract approach to thermodynamics will ever be able to explain how to behave in specific monitored objects. But the conceptual advances and a new set of mathematical formulas are already helping researchers to ask certain theoretical questions in the field of thermodynamics, for example, about the fundamental limitations of quantum computers, and even about the ultimate fate of the Universe.

“We are all thinking more and more about what you can do with quantum machines,” says Paul Skrzypczyk (Paul Skrzypczyk) from the Institute of photonic Sciences in Barcelona. — Suppose the system is in equilibrium, and we want to make it work. How much useful work we can extract? How can I intervene to do something interesting?”

The theoretical cosmology from the California Institute of technology Sean Carroll (Sean Carroll) applies the new formula in his latest work on the arrow of time in cosmology. I am interested in the long-term fate of cosmological space-time, ” says Carroll, who wrote the book From Eternity to Here: The Quest for the Ultimate Theory of Time (From infinity in here. The quest for the ultimate theory of time). — In this situation, we still don’t know all the right laws of physics, so it makes sense to turn to the abstract level, and here, I think, will help us this quantum mechanical approach”.

After twenty-six years after the failure of big ideas Lloyd about the arrow of time he enjoyed watching her rebirth and tries to apply the ideas of the last work by the paradox of the information that falls into a black hole. “I think right now still speak about that in this idea there is physics,” he says.

And philosophy even more.

According to scientists, our ability to remember the past but not the future that is confusing the manifestation of time’s arrow, can also be regarded as an increase in correlations between the interacting particles. When you read a note on a sheet of paper, the brain correlates with the information using photons that fall into your eyes. From this moment you can remember, what is written on paper. As Lloyd notes, “the present can be described as the process of establishing correlations with our environment”.

The backdrop for sustainable growth weaves in all the universe is, of course, time itself. Physicists stress that despite great advances in understanding how changes occur in time, they will not step closer to understanding the nature of time itself or why it differs from the other three dimensions of space (conceptually and in equations of quantum mechanics). Popescu calls this mystery “one of the greatest unknowns in physics.”

“We can discuss what hours ago our brain was in a state that was correlated with fewer things,” he says. — But our perception of time is passing — is quite another matter. Most likely, we need a new revolution in physics that will tell you about it.”

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