Coffee cools, buildings collapse, eggs shatter, and stars go out in a universe that is destined to shift to a drab monotony known as thermal equilibrium. Astronomer and philosopher Sir Arthur Eddington (Arthur Eddington) stated in 1927 that the gradual dissipation of energy is proof of the irreversibility of the "arrow of time."
But to the bewilderment of entire generations of physicists, the concept of the arrow of time does not correspond to the basic laws of physics, which in time act both in the forward direction and in the opposite direction. According to these laws, if someone knew the paths of all particles in the universe and reversed them, energy would accumulate rather than dissipate: cold coffee would begin to heat up, buildings would rise from the ruins, and sunlight would be directed back towards the Sun.
“We had difficulties in classical physics,” says Professor Sandu Popescu, who teaches physics at the University of Bristol in Britain. "If I knew more, could I turn the tide and put together all the molecules in the broken egg?"
Of course, he says, the arrow of time is not ruled by human ignorance. And yet, since the inception of thermodynamics in the 1850s, the only known way to calculate the propagation of energy has been a formula for the statistical distribution of unknown particle trajectories and the demonstration that ignorance blur the picture over time.
Physicists are now uncovering a more fundamental source of the arrow of time. Energy dissipates, and objects come to equilibrium, they say, because elementary particles get entangled during interaction. They called this strange effect "quantum mixing," or entanglement.
"We can finally understand why a cup of coffee in a room comes into balance with it," says Bristol-based quantum physicist Tony Short. "There is a confusion between the state of the coffee cup and the state of the room."
Popescu, Short and their colleagues Noah Linden and Andreas Winter reported their discovery in Physical Review E in 2009, stating that objects come to equilibrium, or evenly distributed energy, indefinitely long time due to quantum mechanical mixing with the environment. A similar discovery a few months earlier was made by Peter Reimann of Bielefeld University in Germany, having published his findings in Physical Review Letters. Short and colleagues backed up their case in 2012 by showing that entanglement induces equilibrium in finite time. And in a paper published in February on the arXiv. org, two separate teams have taken the next step, calculating that most physical systems quickly balance out in a time directly proportional to their size.“In order to show that this applies to our real physical world, the processes must take place in a reasonable time frame,” says Short.
The tendency of coffee (and everything else) to come to equilibrium is "very intuitive," says Nicolas Brunner, a quantum physicist at the University of Geneva. "But in explaining the reasons for this, we have for the first time solid foundations with regard to microscopic theory."
Promotional video:
If the new line of research is correct, then the history of the arrow of time begins with the quantum mechanical idea that nature is fundamentally indeterminate. An elementary particle is devoid of specific physical properties, and it is determined only by the probabilities of being in certain states. For example, at a certain moment, a particle can rotate clockwise with a 50% probability and counterclockwise with a 50% probability. The experimentally tested theorem of the Northern Irish physicist John Bell states that there is no "true" state of particles; probabilities are the only thing that can be used to describe it.
Quantum uncertainty inevitably leads to confusion - the alleged source of the arrow of time.
When two particles interact, they can no longer be described by separate, independently evolving probabilities called "pure states". Instead, they become entangled components of a more complex probability distribution that describe two particles together. They can, for example, indicate that particles are spinning in opposite directions. The system as a whole is in a pure state, but the state of each particle is "mixed" with the state of the other particle. Both particles can move several light years apart, but the rotation of one particle will correlate with the other. Albert Einstein described it well as "spooky action at a distance."
"Entanglement is, in a sense, the essence of quantum mechanics," or the laws governing interactions on a subatomic scale, says Brunner. This phenomenon is at the core of quantum computing, quantum cryptography, and quantum teleportation.
The idea that confusion can explain the arrow of time first came to Seth Lloyd's mind 30 years ago when he was a 23-year-old Philosophy graduate from Cambridge University with a Harvard degree in physics. Lloyd realized that quantum uncertainty and its propagation as particles become more entangled could replace human uncertainty (or ignorance) in old classical proofs and become the true source of the arrow of time.
Using a little-known quantum mechanical approach, in which units of information are the basic building blocks, Lloyd spent several years studying the evolution of particles in terms of shuffling ones and zeros. He found that as the particles became more and more mixed with each other, the information that described them (for example, 1 for clockwise rotation, and 0 for counterclockwise), will go to describe the system of entangled particles as a whole. The particles seemed to gradually lose their independence and became pawns of the collective state. Over time, all information is transferred to these collective clusters, while individual particles do not have it at all. At this point, as Lloyd discovered, the particles enter a state of equilibrium, and their states stop changing, like a cup of coffee cools down to room temperature.
“What's really going on? Things are becoming more interconnected. The arrow of time is the arrow of the growth of correlations."
This idea, spelled out in Lloyd's doctoral dissertation in 1988, went unheeded. When the scientist sent an article about this to the editorial office of the journal, he was told that "there is no physics in this work." Quantum information theory "was deeply unpopular" at the time, Lloyd says, and questions about the arrow of time "were the lot of psychos and mind-boggling Nobel laureates."
“I was pretty damn close to becoming a taxi driver,” he said.
Since then, advances in quantum computing have made quantum information theory one of the most active areas of physics. Lloyd is currently a professor at the Massachusetts Institute of Technology, he is recognized as one of the founders of this discipline, and his forgotten ideas are being revived by the efforts of physicists from Bristol. The new evidence is more general, scientists say, and applies to any quantum system.
“When Lloyd presented the idea in his dissertation, the world was not ready for it,” says Renato Renner, head of the Institute for Theoretical Physics at the Swiss Higher Technical School of Zurich. - Nobody understood him. Sometimes you need ideas to come at the right time."
In 2009, evidence from a team of Bristol physicists resonated with quantum information theorists, who discovered new ways to apply their methods. They showed that as objects interact with their environment - like particles in a cup of coffee interact with air - information about their properties "leaks and spreads through this environment," explains Popescu. This localized loss of information causes the state of the coffee to remain unchanged, even if the clean state of the entire room continues to change. With the exception of rare random fluctuations, the scientist says, "his state stops changing over time."
It turns out that a cold cup of coffee cannot spontaneously heat up. Basically, as the clean state of a room evolves, coffee can suddenly escape from the air in the room and return to a clean state. But there are many more mixed states than pure states, and practically coffee can never return to a pure state. To see this, we will have to live longer than the universe. This statistical low probability makes the arrow of time irreversible. “Basically, mixing opens up a huge space for us,” says Popescu. - Imagine that you are in a park with a gate in front of you. As soon as you enter them, you are out of balance, find yourself in a huge space and get lost in it. You will never return to the gate."
In the new history of the arrow of time, information is lost in a process of quantum entanglement, not because of a subjective lack of human knowledge about what brings a cup of coffee and a room into balance. The room eventually balances with the outside, and the environment moves even more slowly toward equilibrium with the rest of the universe. The thermodynamic giants of the 19th century viewed this process as a gradual dissipation of energy that increases the total entropy, or chaos, of the universe. Today, Lloyd, Popescu and others in the field view the arrow of time differently. In their opinion, information becomes more and more diffuse, but never completely disappears. Although the entropy grows locally, the total entropy of the universe remains constant and zero.
“Overall, the universe is in a pure state,” says Lloyd. "But its individual parts, intertwining with the rest of the universe, come into a mixed state."
But one mystery of the arrow of time remains unsolved. “There is nothing in these works that explains why you start at the gate,” says Popescu, returning to the park analogy. "In other words, they don't explain why the original state of the universe was far from equilibrium." The scientist hints that this question relates to the nature of the Big Bang.
Despite recent advances in equilibration time calculations, the new approach still cannot be a tool for calculating the thermodynamic properties of specific things like coffee, glass or unusual states of matter. (Some traditional thermodynamicists say they know very little about the new approach.) “The point is, you have to find criteria for which things behave like window glass and which things like a cup of tea,” Renner says. "I think I will see new work in this direction, but there is still a lot to be done."
Some researchers have questioned whether this abstract approach to thermodynamics will ever be able to accurately explain how specific observables behave. But conceptual advances and a new set of mathematical formulas are already helping researchers to ask theoretical questions from the field of thermodynamics, such as the fundamental limitations of quantum computers and even the ultimate fate of the universe.
“We're thinking more and more about what can be done with quantum machines,” says Paul Skrzypczyk of the Institute of Photonic Sciences in Barcelona. - Let's say the system is not yet in equilibrium, and we want to make it work. How much useful work can we extract? How can I intervene to do something interesting?"
Caltech cosmology theorist Sean Carroll applies the new formulas in his latest work on the time arrow in cosmology. "I am interested in the most long-term fate of cosmological space-time," says Carroll, who wrote From Eternity to Here: The Quest for the Ultimate Theory of Time (From infinity here. The search for a finite theory of time). "In this situation, we still do not know all the necessary laws of physics, so it makes sense to turn to the abstract level, and here, I think, this quantum-mechanical approach will help us."
Twenty-six years after the failure of Lloyd's grandiose idea of the arrow of time, he enjoys its renaissance and tries to apply the ideas of the last work to the paradox of information falling into a black hole. “I think now they will start talking about the fact that there is physics in this idea,” he says.
And philosophy even more so.
According to scientists, our ability to remember the past but not the future, which is a confusing manifestation of the arrow of time, can also be seen as an increase in correlations between interacting particles. When you read a note on a piece of paper, the brain correlates with the information through photons that enter your eyes. Only from this moment can you remember what is written on the paper. As Lloyd notes, "the present can be characterized as the process of establishing correlations with our environment."
The backdrop for the steady growth of weave throughout the universe is, of course, time itself. Physicists emphasize that despite great advances in understanding how changes occur over time, they have not come a step closer to understanding the nature of time itself or why it differs from the other three dimensions of space (conceptually and in the equations of quantum mechanics) … Popescu calls this riddle "one of the greatest unknowns in physics."
“We can discuss that an hour ago our brain was in a state that correlated with fewer things,” he says. “But our perception that time is passing is a completely different matter. Most likely, we will need a new revolution in physics that will tell about this."