Proof of time’s arrow at the atomic level

An article published in Physical Review Letters describes the results of a groundbreaking experiment performed by researchers in Brazil, Ireland and Germany on the arrow of time, a fundamental property of the universe

By Peter Moon  |  Agência FAPESP – The universe in which we live has two fundamental and immutable properties: the arrow of time and increasing entropy. These features are so inherent in our day-to-day lives that we almost never stop to think about them.

Time’s arrow refers to the irreversible flow of time from past to present and into the future. Entropy refers to growing disorder or chaos in the universe, which began as an infinitely small, hot, and dense point at the Big Bang and since then has expanded in an increasingly chaotic manner, forming gas clouds, galaxies, stars, planets, and eventually life, with no turning back.

Physicists have tested and confirmed the arrow of time and the increase in entropy in various environments and situations, but always macroscopically. Until recently, time’s arrow in connection with entropy had never been verified at the microscopic or quantum scale, i.e., at the level of atoms. In the microscopic world, the emergence of irreversibility intrigues physicists because the laws of quantum mechanics have no preferred direction in time: they do not distinguish between time’s arrow and its reverse, a return to the past.

This apparent incompatibility between time’s preferred direction and the microscopic laws of physics has fueled many a debate for decades. More controversy is likely as a result of a scientific article recently published in Physical Review Letters.

The paper details the results of a pioneering experiment performed by Brazilian, Irish and German physicists, proving for the first time that time’s arrow and its intrinsic relationship with entropy also occur in an isolated quantum system. The researchers did this by studying the nuclear spin of carbon-13 atoms in chloroform molecules. Spin in this context is a magnetic property analogous to that of a compass needle.

The experiment was performed in the laboratories of the Brazilian Center for Research in Physics (CBPF), in Rio de Janeiro, and the findings are associated with research conducted by the National Science & Technology Institute for Quantum Information (INCT-IQ), supported by FAPESP and the National Scientific & Technological Development Council (CNPq).

The researchers used nuclear magnetic resonance (NMR) to verify the emergence of time’s arrow in a microscopic environment, according to one of the authors of the article, physicist Roberto Menezes Serra, a professor at the Federal University of the ABC (UFABC) in Santo André, São Paulo State.

Because quantum systems have very low energy, the carbon-13 atoms were supercooled to 273.15°C, just above absolute zero, and submitted to a radiofrequency pulse. The intensity of this RF pulse was modulated in time at a frequency of 125 MHz, similar to that of FM radio waves. “The temperature of our system is known as spin temperature, and the spin lasted for fractions of a second during the experiment,” Serra said.

When the chilled nuclear spins interacted with radio waves at increasing intensities, they changed state and their internal energy rose. This rise occurred so fast that part of the energy absorbed by the spins became disorganized, like a wobble.

The process can be compared to the motion of a piston in an automotive engine as the gas released by fuel combustion expands in the cylinder.

When the RF pulse was switched off, part of the energy absorbed by the carbon atoms (the disorganized part) had to dissipate into the environment in the form of heat. The system then returned to its original state of thermal equilibrium.

To reveal the arrow of time, the strategy used in the experiment entailed switching the radio waves on and off very quickly at intervals of milliseconds. “We performed the procedure so fast that there was no time for the system to exchange energy with the environment in the form of heat,” Serra said. Thus, the researchers detected the production of entropy in a quantum system and observed how it increased at this microscopic scale.

Quantum fluctuations

The researchers then performed the same procedure but modulated the radio waves in the opposite direction, reducing the energy in the waves very quickly and hence decreasing the energy in the spin system. This can be compared to compression of the air-fuel mixture in the pistons of a combustion engine.

When they compared what happened to the carbon atoms’ nuclei while the energy of the radio waves rose and fell, they detected a subtle difference. If the laws that govern isolated quantum systems are symmetrical in time, this process should also have been symmetrical, but the experiment pointed to moderate asymmetry.

“The asymmetry is due to quantum fluctuations,” Serra said. Strange things happen in the microscopic world of atoms and particles. A vacuum is far from empty, for example. Subatomic particles can pop up in a vacuum out of nowhere. They appear and disappear without notice, as if by magic--hence Serra’s reference to quantum fluctuations.

In this experiment, what the researchers detected was a similar phenomenon, in which the fluctuations were associated with transitions between quantum states of nuclear spin. For the sake of illustration, imagine you take a few steps in any direction while holding a pendulum. It will continue to swing back and forth, but it will also wobble very slightly owing to imperceptible sideways movements as you walk.

The wobble is analogous to the quantum fluctuations in nuclear spin when the intensity of the radio waves was changed rapidly in the experiment.

The experiment proved that time’s arrow exists at the quantum level because of the asymmetry detected between what happened to nuclear spin when the energy in the radio waves increased and what happened when it decreased, i.e., when the process was reversed. The asymmetry originated in the transition between quantum states, showing entropy increasing in the system and demonstrating the arrow of time at the microscopic scale.

However, what is the point of all this? What practical applications could possibly flow from showing that time’s arrow exists in a quantum system? “In the groundbreaking experimental work we’ve done in quantum thermodynamics, our research group’s basic aim has been to understand thermodynamic phenomena at the quantum level. From the practical standpoint, we want to understand the limits of the new quantum technology at this microscopic scale,” Serra said.

This is one of the frontiers of science today. The expectation is that in the long run it will lead to the development of quantum computers, which will be many times more powerful than today’s computers. Another dividend will be quantum cryptography, with unbreakable codes and security assured by the laws of quantum mechanics. “Quantum computing is set to be one of the twenty-first century’s most important technologies,” Serra said.

The results of the experiment are described in the article “Irreversibility and the Arrow of Time in a Quenched Quantum System” (doi: http://dx.doi.org/10.1103/PhysRevLett.115.190601) by T. B. Batalhão, R. M. Serra et al., published by Physical Review Letters and available at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.190601.