(Inside Science) -- One of the great mysteries of science is why time apparently only runs forward and not backward. Now scientists have found that using a "quantum time mirror," they could, in a certain sense, reverse time at the extremely tiny and sometimes counterintuitive quantum scale. The researchers said that such a phenomenon could be demonstrated in atomically thin sheets of carbon, and could one day help machines such as quantum computers examine themselves for errors.
"O, call back yesterday, bid time return," the Earl of Salisbury famously, and futilely, implored in Shakespeare's "Richard II." This entreaty is so poignant because the audience knows time inexorably marches from the past to the future but not the other way.
However, the fundamental laws of physics that currently best describe the universe, from quantum mechanics to general relativity, all for the most part work fine if the arrow of time points either forward or backward. This conundrum of why time acts like a one-way lane when it can theoretically behave as a two-way street was described by 19th-century Austrian scientist Josef Loschmidt, and is known as Loschmidt's paradox.
But milk does not stir itself out of coffee, omelettes do not turn back into eggs and vases do not automatically unsmash themselves. According to 19th-century Austrian physicist Ludwig Boltzmann's view of the second law of thermodynamics, there is a natural tendency for any isolated system to degenerate into a more disorderly state -- basically, things fall apart.
Still, Loschmidt argued that it should be possible to essentially reverse time, imagining a "daemon" that could, for instance, stop all particles of a gas and flip their velocities so they headed exactly back to where they came from. "Boltzmann responded to Loschmidt's argument saying that, if possible, Loschmidt should simply perform what he proposed," said study senior author Klaus Richter, a condensed matter physicist at the University of Regensburg in Germany.
As if seizing that challenge, scientists actually have created versions of Loschmidt's daemon over the years. For instance, in 1950, researchers used pulses of radio waves to successfully flip the ways in which ensembles of atomic nuclei spun, making them essentially devolve back to their original states. These "spin echoes" are now central to scanning techniques such as MRI.
Previously, scientists also created "time mirrors" for acoustic, electromagnetic and even water waves. These mirrors are generally arrays of antennas or microphones that record incoming waves and broadcast reversed versions of the original signals back at their source.
However, Richter said, it had seemed all but impossible to create time mirrors that could work in the bizarre realm of quantum physics, where matter and energy can behave in spooky ways -- for instance, particles can exist in two or more places at once, or spin in opposite directions simultaneously. Previous time mirrors worked by recording the signals they eventually reversed, but quantum phenomena are notoriously fragile, and the act of measuring them disrupts them.
Now Richter and his colleagues suggest they have designed a quantum time mirror that could get implemented using sheets of graphene, which each consist of a single layer of carbon atoms arranged in a honeycomb pattern. Graphene has attracted much industrial research in recent years due to its extraordinary qualities, such as how it is roughly 200 times stronger than steel by weight, as well as transparent and highly electrically conductive.
In quantum physics, particles can behave like waves. In the crystal lattice of graphene, electric charge can travel in waves, consisting of either negatively charged electrons or the positively charged absences of electrons known as "holes."
In graphene, electrons and holes move at equal, constant speeds, but in opposite directions. The reverse nature of their behavior made graphene potentially ideal for use in a time mirror, Richter said.
The researchers calculated that firing a short laser pulse at graphene could instantaneously trigger a "population reversal," switching electrons to holes and vice versa. This inversion of behavior is "effectively a propagation back in time," Richter said. Such a quantum time mirror could be made with current state-of-the-art technology, the researchers added.
Although the work is not intended to test the limits of time reversal, such fundamental restrictions may appear "once the experimental drawbacks are surmounted one by one," said quantum physicist Horacio Pastawski at the National University of Córdoba in Argentina, who did not participate in this study. He noted his own research explores how time reversal may become impossible once a system becomes complex and chaotic enough.
Richter stressed this quantum time mirror only works for relatively tiny systems protected from environmental disturbances. "It's completely impossible to reverse time this way with, say, a person," he said. "Still, an interesting future direction one may ask is how large a system could be made that can experience reversal with a quantum time mirror."
There are other materials that scientists could use to devise quantum time mirrors, such as so-called topological insulators, whose interiors are electrically insulating but whose surfaces are electrically conductive, Richter said. However, "graphene is well-studied, and scientists can now produce ultra-clean graphene, and time mirrors need to be quite clean," he said.
Richter suggested that quantum time mirrors could one day help probe activity within advanced devices such as quantum computers, which could in principle carry out more calculations in an instant than there are atoms in the universe.
Theoretical physicist Joshua Bodyfelt at Massey University in Auckland, New Zealand, who did not take part in this research, noted that another potential application was in encoding secret messages. "Perhaps you could encode data in pulses of data and perform time reversals to recapture the data," Bodyfelt said.