![]() ![]() When two particles interact, they can no longer even be described by their own, independently evolving probabilities, called “pure states.” Instead, they become entangled components of a more complicated probability distribution that describes both particles together. Quantum uncertainty then gives rise to entanglement, the putative source of the arrow of time. An experimentally tested theorem by the Northern Irish physicist John Bell says there is no “true” state of the particle the probabilities are the only reality that can be ascribed to it. For example, at a particular moment, a particle might have a 50 percent chance of spinning clockwise and a 50 percent chance of spinning counterclockwise. An elementary particle lacks definite physical properties and is defined only by probabilities of being in various states. If the new line of research is correct, then the story of time’s arrow begins with the quantum mechanical idea that, deep down, nature is inherently uncertain. “But when it comes to explaining why it happens, this is the first time it has been derived on firm grounds by considering a microscopic theory.” The tendency of coffee - and everything else - to reach equilibrium is “very intuitive,” said Nicolas Brunner, a quantum physicist at the University of Geneva. “To show that it’s relevant to our actual physical world, the processes have to be happening on reasonable time scales,” Short said. And, in work that was posted on the scientific preprint site in February, two separate groups have taken the next step, calculating that most physical systems equilibrate rapidly, on time scales proportional to their size. ![]() Short and a collaborator strengthened the argument in 2012 by showing that entanglement causes equilibration within a finite time. Similar results by Peter Reimann of the University of Bielefeld in Germany appeared several months earlier in Physical Review Letters. Popescu, Short and their colleagues Noah Linden and Andreas Winter reported the discovery in the journal Physical Review E in 2009, arguing that objects reach equilibrium, or a state of uniform energy distribution, within an infinite amount of time by becoming quantum mechanically entangled with their surroundings. “Entanglement builds up between the state of the coffee cup and the state of the room.” “Finally, we can understand why a cup of coffee equilibrates in a room,” said Tony Short, a quantum physicist at Bristol. Now, physicists are unmasking a more fundamental source for the arrow of time: Energy disperses and objects equilibrate, they say, because of the way elementary particles become intertwined when they interact - a strange effect called “quantum entanglement.” And yet, since the birth of thermodynamics in the 1850s, the only known approach for calculating the spread of energy was to formulate statistical distributions of the unknown trajectories of particles, and show that, over time, the ignorance smeared things out. Surely, he said, time’s arrow is not steered by human ignorance. ![]() “If I knew more, could I reverse the event, put together all the molecules of the egg that broke? Why am I relevant?” “In classical physics, we were struggling,” said Sandu Popescu, a professor of physics at the University of Bristol in the United Kingdom. By those laws, it seemed that if someone knew the paths of all the particles in the universe and flipped them around, energy would accumulate rather than disperse: Tepid coffee would spontaneously heat up, buildings would rise from their rubble and sunlight would slink back into the sun. The astronomer-philosopher Sir Arthur Eddington in 1927 cited the gradual dispersal of energy as evidence of an irreversible “arrow of time.”īut to the bafflement of generations of physicists, the arrow of time does not seem to follow from the underlying laws of physics, which work the same going forward in time as in reverse. Coffee cools, buildings crumble, eggs break and stars fizzle out in a universe that seems destined to degrade into a state of uniform drabness known as thermal equilibrium. ![]()
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