Scientific american a matter of time pdf

But if Wilczek's latest ideas on symmetry and the nature of time are correct, they would suggest the existence of a bona fide perpetual-motion machine— albeit one from which energy could never be extracted. He proposes that matter could form a "time crystal," whose structure would repeat periodically, as with an ordinary crystal, but in time rather than in space. Such a crystal would represent a previously unknown state of matter and might have arisen as the very early universe cooled, losing its primordial symmetries.

Wilczek describes his work in this article and in this one coauthored by Alfred Shapere of the University of Kentucky, that he posted on the physics preprint server, arXiv. Known for his pioneering work in developing quantum chromodynamics , the theory that explains how the particles inside atomic nuclei stick together, Wilczek, a professor at the Massachusetts Institute of Technology, says he got his latest idea two years ago while teaching a course on group theory. That branch of mathematics, which uses matrices to describe the symmetries inherent in families of elementary particles, also describes and classifies the structure of crystals.

Materials such as a liquid or a gas in equilibrium, made of uniformly distributed particles, exhibit perfect spatial symmetry—they look the same everywhere and in every direction. But at very low or minimum energies, most materials can't retain that symmetry, and they crystallize. The regular geometric pattern of a crystal lacks complete spatial symmetry; the structure does not look the same everyplace.

Because crystals have less symmetry than before, physicists say they exhibit spontaneous symmetry breaking. Equivalent processes occur in many domains of physics. A type of broken symmetry, which would be indicated by the presence of the Higgs boson now being hunted at the Large Hadron Collider, would explain why subatomic particles have mass.

Wilczek says he started wondering whether the concept of an ordinary three-dimensional crystal could be extended to four dimensions, with the extra dimension that of time. A time crystal would spontaneously break what Wilczek calls "the mother of all symmetries"—the symmetry of time translation, which holds physical laws remains the same regardless of what time it is. A time crystal would change with time but keep coming back to the same form it began with, like a clock whose moving hands periodically return to their original positions.

The difference from an ordinary clock or other periodic process is that a time crystal, as with a spatial crystal, would be a state of minimum possible energy. At first glance, that poses a contradiction. A time crystal by definition must change with time in order to break time translation symmetry. But a system with minimum energy ordinarily can't move. If it could, then additional energy could still be extracted, until the system achieved a true minimum energy, a motionless state.

He and Shapere showed that a material could have zero total energy yet still be in motion. They did so by mathematically reformulating the ordinary definition of kinetic energy one-half mass times velocity squared to a different but equally valid value that depends on a velocity in an alternative way for instance, adding an additional term such as velocity to the fourth power and changing the sign of the usual kinetic energy. Carroll agrees: "It's amusing to find a system that features motion in its ground state, but it certainly doesn't violate any truly cherished beliefs of physics.

I'm ready to believe that such a system could even be constructed in the real world. Once set in motion, a time crystal could remain in motion forever, with no outside force needed to keep it going.

This type of perpetual motion machine would not violate any known physical law because no energy could be extracted from the system without first adding energy. It seems like p. And of course, time is money. It is the partner of change, the antagonist of speed, the currency in which we pay attention. It is our most precious, irreplaceable commodity.

Yet still we say we don t know where it goes, and we sleep away a third of it, and none of us really can account for how much we have left. We can find ways to save time, but the amount remaining nonetheless diminishes steadily. It is already p. Time and memory shape our perceptions of our own identity. We may feel ourselves to be at history s mercy, but we also see ourselves as free-willed agents of the future. That conception is disturbingly at odds with the ideas of physicists and philosophers, however, because if time is a dimension like those of space, then yesterday, today and tomorrow are all equally concrete and determined.

The future exists as much as the past does; it is just in a place that we have not yet visited. Somewhere, it is p. Time is a river that carries me away, but I am the river; it is a tiger that destroys me, but I am the tiger; it is a fire that consumes me, but I am the fire.

This special issue of Scientific American summarizes what science has discovered about how time permeates and guides both our physical world and our inner selves. That knowledge should enrich the imagination and provide practical advantages to anyone hoping to beat the clock or at least to stay in step with it. It is now p. Synchronize your watches. Buy this issue. Follow Angela Cesaro on Twitter. Already a subscriber?

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