For the first time, physicists have shown that light travels forwards and backwards simultaneously in time. The new technique could help scientists improve quantum computing and understand quantum gravity.
By splitting a photon, or a packet of light, using a special optical crystal, two independent teams of physicists have achieved what they describe as a “quantum time reversal”, in which one photon exists at a time in forward and backward temporal states.
The effect results from the convergence of two strange principles of Quantum mechanics, the counter-intuitive rules that govern the behavior of toddlers. The first principle, quantum superposition, allows tiny particles to exist in many different states, or different versions of themselves, at once, until they are observed. The second – charge, parity, and time-reversal (CPT) symmetry – states that any system containing particles will obey the same physical laws even if the charges, spatial coordinates, and time-reversal motions of the particles are reversed as through a mirror.
By combining these two principles, the physicists produced a photon that appeared to simultaneously travel along and against the arrow of time. They published the results of their twin experiments October 31 and November 2 on the arXiv preprint server, which means the results have not yet been peer-reviewed.
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“The concept of the arrow of time gives a word to the apparent unidirectionality of time that we observe in the macroscopic world that we inhabit”, Teodor Stromberg, a University of Vienna physicist who was the first author of one of the papers, told Live Science. “This is actually in tension with many fundamental laws of physics, which are generally symmetric in time, and therefore have no preferred time direction.”
The second law of thermodynamics states that the entropy of a system, a rough analogue of its disorder, must increase. Known as the “arrow of time”, entropy is one of the few quantities in physics that determines time to go in a particular direction.
This trend of growing disorder in the universe explains why it is easier to mix ingredients than to separate them. It is also through this increasing disorder that entropy is so intimately tied to our sense of time. A famous scene from Kurt Vonnegut’s novel “Slaughterhouse-Five” shows how entropy makes a direction of time look differently by playing WWII upside down: bullets are sucked in by wounded men; fires are reduced, gathered into bombs, stacked in neat rows, and separated into composite minerals; and the inverted arrow of time undoes the disorder and devastation of war.
However, since entropy is primarily a statistical concept, it does not apply to single subatomic particles. In fact, in every particle interaction scientists have observed so far – including up to 1 billion interactions per second that occur inside the world’s largest atom breaker, the Large Hadron Collider – CPT symmetry is maintained. Thus, particles appearing to be advancing in time are indistinguishable from those of a mirror system of antiparticles receding in time. (Antimatter was created with matter during the Big Bang and does not go backwards in time; it simply behaves as if following an opposite arrow of time to normal matter.)
The other factor at play in new experiences is layering. The most famous demonstration of quantum superposition is Schrödinger’s cat, a thought experiment in which a cat is placed inside a box sealed with a vial of poison whose release is triggered by the radioactive decay of a alpha particle. Radioactive decay is a quantum mechanical process that occurs randomly, so it is initially impossible to know what happened to the cat, which is in a superposition of states, simultaneously dead and alive, until the box is opened and the result observed.
This layering of states allows a particle to exist in both forward and backward temporal states, but witnessing this feat experimentally is tricky. To achieve this, the two teams designed similar experiments to split a photon along a superposition of two distinct paths through a crystal. The superimposed photon traveled one path through the crystal normally, but another path was configured to change the polarization of the photon, or where it points in space, to move as if traveling back in time.
After recombining the overlapping photons by sending them through another crystal, the team measured the polarization of the photons through a number of repeated experiments. They found a quantum interference pattern, a pattern of light and dark stripes that could only exist if the photon had been split and moved back and forth in time.
“The layering of processes we’ve done is more like an object spinning clockwise and counterclockwise at the same time,” Strömberg said. The researchers created their time-reversed photon out of intellectual curiosity, but follow-up experiments showed that time flips. can be paired with reversible logic gates to allow simultaneous computation in both directions, paving the way for quantum processors with vastly improved processing power.
Theoretical possibilities also arise from the work. A future theory of quantum gravity, which unites general relativity and quantum mechanics, should include particles of mixed time orientations like the one in this experiment, and could allow researchers to peer into some of the most mysterious phenomena in the universe. .
“A nice way to put it is to say that our experiment is a simulation of exotic scenarios where a photon could move forwards and backwards in time,” said Giulio Chiribella, a physicist at the University of Oxford and lead author of the other article. , said Live Science. “What we’re doing is analogous to some experiments that simulate exotic physics, like black hole physics or time travel.”
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