Time’s Arrow Traced to Quantum Source

As a hot cup of coffee equilibrates with the surrounding air, coffee particles (white) and air particles (brown) interact and become entangled mixtures of brown and white states. After some time, most of the particles in the coffee are correlated with air particles; the coffee has reached thermal equilibrium.

As a hot cup of coffee equilibrates with the surrounding air, coffee particles (white) and air particles (brown) interact and become entangled mixtures of brown and white states. After some time, most of the particles in the coffee are correlated with air particles; the coffee has reached thermal equilibrium.

(…) The idea that entanglement might explain the arrow of time first occurred to Seth Lloyd about 30 years ago, when he was a 23-year-old philosophy graduate student at Cambridge University with a Harvard physics degree. Lloyd realized that quantum uncertainty, and the way it spreads as particles become increasingly entangled, could replace human uncertainty in the old classical proofs as the true source of the arrow of time.

Using an obscure approach to quantum mechanics that treated units of information as its basic building blocks, Lloyd spent several years studying the evolution of particles in terms of shuffling 1s and 0s. He found that as the particles became increasingly entangled with one another, the information that originally described them (a “1” for clockwise spin and a “0” for counterclockwise, for example) would shift to describe the system of entangled particles as a whole. It was as though the particles gradually lost their individual autonomy and became pawns of the collective state. Eventually, the correlations contained all the information, and the individual particles contained none. At that point, Lloyd discovered, particles arrived at a state of equilibrium, and their states stopped changing, like coffee that has cooled to room temperature.

“What’s really going on is things are becoming more correlated with each other,” Lloyd recalls realizing. “The arrow of time is an arrow of increasing correlations.”

The idea, presented in his 1988 doctoral thesis, fell on deaf ears. When he submitted it to a journal, he was told that there was “no physics in this paper.” Quantum information theory “was profoundly unpopular” at the time, Lloyd said, and questions about time’s arrow “were for crackpots and Nobel laureates who have gone soft in the head.” he remembers one physicist telling him.

“I was darn close to driving a taxicab,” Lloyd said.

Advances in quantum computing have since turned quantum information theory into one of the most active branches of physics. Lloyd is now a professor at the Massachusetts Institute of Technology, recognized as one of the founders of the discipline, and his overlooked idea has resurfaced in a stronger form in the hands of the Bristol physicists. The newer proofs are more general, researchers say, and hold for virtually any quantum system.

“When Lloyd proposed the idea in his thesis, the world was not ready,” said Renato Renner, head of the Institute for Theoretical Physics at ETH Zurich. “No one understood it. Sometimes you have to have the idea at the right time.”(…)

Read more at https://www.simonsfoundation.org/quanta/20140416-times-arrow-traced-to-quantum-source/

Neutrons Knock at the Cosmic Door

Figure 1 (Left) Neutron mirror apparatus. An ultracold neutron (UCN) enters a space between two mirrors that act as potential wells, giving rise to a discrete energy spectrum. A detector measures neutrons exiting the cavity formed by the mirrors. The bottom mirror sits upon a nanopositioning table that induces a vertical oscillation that produces dips in the neutron transmission at the resonances. (Right) Energy-level diagram for the neutrons in a gravitational field caught between the walls, which oscillate owing to the mirror motion (horizontal direction here is vertical in the apparatus). This, in turn, causes the neutrons to move up and down energy levels. A measurement of the energy-level spacing yields constraints on parameters of scenarios describing dark energy and dark matter, which would slightly shift the levels as indicated by the dashed lines.

Figure 1 (Left) Neutron mirror apparatus. An ultracold neutron (UCN) enters a space between two mirrors that act as potential wells, giving rise to a discrete energy spectrum. A detector measures neutrons exiting the cavity formed by the mirrors. The bottom mirror sits upon a nanopositioning table that induces a vertical oscillation that produces dips in the neutron transmission at the resonances. (Right) Energy-level diagram for the neutrons in a gravitational field caught between the walls, which oscillate owing to the mirror motion (horizontal direction here is vertical in the apparatus). This, in turn, causes the neutrons to move up and down energy levels. A measurement of the energy-level spacing yields constraints on parameters of scenarios describing dark energy and dark matter, which would slightly shift the levels as indicated by the dashed lines.

Wolfgang P. Schleich, Ernst Raselhttp://physics.aps.org/articles/v7/39

The quantum behavior of a neutron bouncing in the gravitational field of the Earth can improve what we know about dark energy and dark matter.

Spectroscopy has always set the pace of physics. Indeed, the observation of the Balmer series of the hydrogen atom led to the Bohr-Sommerfeld model about 100 years ago. A little later the discreteness of the spectrum moved Werner Heisenberg to develop matrix mechanics and Erwin Schrödinger to formulate wave mechanics. In 1947, the observation of a level shift in hydrogen by Willis E. Lamb ushered in quantum electrodynamics.

Now, a group led by Hartmut Abele of the Technical University of Vienna, Austria, reports, in Physical Review Letters [1] [http://arxiv-web3.library.cornell.edu/abs/1404.4099], experiments that once more take advantage of the unique features of spectroscopy to put constraints on dark energy and dark matter scenarios. However, this time it is not a “real atom” (consisting of an electron bound to a proton) that provides the insight. Instead, the research team observes an “artificial atom”—a neutron bouncing up and down in the attractive gravitational field of the Earth (Fig. 1). This motion is quantized, and the measurement of the separation of the corresponding energy levels allows these authors to make conclusions about Newton’s inverse square law of gravity at short distances.

Setup and results for the employed gravity resonance spectroscopy: Left: The lowest eigenstates and eigenenergies with conning mirrors at bottom and top separated by 30.1 µm. The observed transitions are marked by arrows. Center: The transmission curve determined from the neutron count rate behind the mirrors as a function of oscillation frequency shows dips corresponding to the transitions shown on the left. Right: Upon resonance at 280 Hz the transmission decreases with the oscillation amplitude in contrast to the detuned 160 Hz. Because of the damping no revival occurs. All plotted errors correspond to a standard deviation around the statistical mean. [http://arxiv-web3.library.cornell.edu/abs/1404.4099]

Setup and results for the employed gravity resonance spectroscopy: Left: The lowest eigenstates and eigenenergies with con ning mirrors at bottom and top separated by 30.1 µm. The observed transitions are marked by arrows. Center: The transmission curve determined from the neutron count rate behind the mirrors as a function of oscillation frequency shows dips corresponding to the transitions shown on the left. Right: Upon resonance at 280 Hz the transmission decreases with the oscillation amplitude in contrast to the detuned 160 Hz. Because of the damping no revival occurs. [arxiv]

The energy wave function of a quantum particle in a linear potential [2], corresponding, for example, to the gravitational field close to the surface of the Earth, has a continuous energy spectrum [3]. However, when a quantum particle such as a neutron is also restricted in its motion by two potential walls, the resulting spectrum is discrete.

Read also: “With neutrons, scientists can now look for dark energy in the lab

This elementary problem of nonrelativistic quantum mechanics is a slight generalization of the familiar “particle in a box” where the bottom of the box, which usually corresponds to a constant potential, is replaced by a linear one representing the gravitational field. Continue reading Neutrons Knock at the Cosmic Door