In a world first, physicists move light back and forth in time simultaneously

The experiment could help to form a unified theory of quantum gravity.
Chris Young
3d rendering, abstract cosmic background
3d rendering, abstract cosmic background


Scientists have, for the first time ever, made light appear to move simultaneously forward and backward in time.

The new method, achieved by an international group of scientists, could help create novel quantum computing techniques and give scientists a better understanding of quantum gravity, a report from LiveScience reveals.

It was achieved thanks to a combination of two principles that form a part of the bizarre world of quantum mechanics.

What is a "quantum time flip"?

Two independent teams of scientists conducted their experiments side by side and achieved what they describe as a "quantum time flip", in which a photon exists both in forward and backward time states at the same time.

Essentially, this was achieved via the convergence of quantum superposition and charge, parity, and time-reversal (CPT) symmetry, both of which are principles of quantum mechanics — meaning they describe the physical properties of atoms and subatomic particles.

The former, quantum superposition, is a phenomenon that sees tiny particles exist in several different states until they are observed, while the latter, CPT, is a principle that states any system containing particles will obey the same physical laws, even if their charge, spatial coordinates, and movements are precisely reversed.

Superposition is famously described by the thought experiment Schrödinger's cat, in which a hypothetical cat is considered simultaneously alive and dead due to the fact that its life is in the hands of a random subatomic event that both takes place and doesn't take place until it's observed.

Each team published the results of their twin experiments in an Oct 31 paper and a Nov. 2 paper, both of which are yet to be peer-reviewed.

New experiments could help to develop a unified theory of quantum gravity

During their experiments, the scientists placed a particle of light, a photon, in superposition, to allow it to travel both forward and backward in time.

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To do so, both teams split a photon along a superposition of two separate paths through a crystal. While the superposed photon moved in a regular manner through the crystal, another path was also devised to change the photon's polarization — where it points in space — to make it move as though it were traveling backward in time.

The scientists then recombined the superposed photons by making them move through another crystal. They then measured the photon polarization and found a quantum interference pattern. This pattern, made up of light and dark stripes, would only be possible if the photon were moving in both directions.

Their findings could allow for more enhanced processing in quantum computing, as they showed that time flips could be linked to reversible logic gates to allow simultaneous computation in both directions. The work also adds to the scientific community's understanding of the world of quantum mechanics and could help in the quest for a unified theory of quantum gravity that marries the general theory of relativity with the principles of quantum mechanics.