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Scientists can now reverse time

The major question now is if these findings can be transferred to larger systems, says Fourie.

The major question now is if these findings can be transferred to larger systems, says Fourie.

Published Mar 27, 2023


Almost all people experienced a time in their lives when they wished they could turn back the clock. Sometimes it is to change certain poor decisions that have been made, fix a mistake in the past, to relive special moments of joy, or to properly say goodbye to a loved one.

Although the ability to turn back the clock has long been embraced by science fiction and debated by theoretical physicists, it is impossible in the classical physical world. We are all too familiar with the directionality of time and that all events follow a unidirectional path. Once an event has occurred, it cannot be reversed because we are not able to manipulate time as an entity.

However, an international team of scientists claim that they have succeeded in speeding up the clock, slowing it down, and even reversing the clock by taking advantage of the unusual properties of the quantum world.

Austrian and Spanish Scientists from the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences and the University of Vienna published a series of papers describing their ability to “accelerate, decelerate and reverse the flow of time within arbitrary, even uncontrolled quantum systems.”

In the quantum realm the well-known laws of physics and limitations that we have learned in school do not apply in the same manner as in the classical world. The invisible quantum universe is made up of subatomic particles called qubits that are the foundation for all reality and can exist in more than one state simultaneously. The unitarity of quantum mechanics entails that an inverse of a given time evolution always exist, even though it may be unknown. In quantum physics a particle can thus be in one state, in another or in both simultaneously (called superposition) and the action on one particle instantaneously affects the other, even if they are separated by vast distances (called entanglement).

The research team have now developed a method to rejuvenate these quantum particles or turn the clocks of these quantum particles backward. They developed a “rewind protocol” to revert any particle (electron, proton or muon) to a previous state, almost like the rewind button on the remote control when watching a movie at home.

The researchers were able to experimentally revert a quantum of light or photon to its original state before passing through a crystal. This was achieved through the use of an experimental device known as a “quantum switch.”

The “quantum switch” forces the target system to evolve in a superposition of its free evolution and perturbed evolution. This superposition of time evolutions are then used to “rewind” the system backwards in time without knowing its associated initial and final states. The light particle is thus returned to its state at the beginning of its journey.

What makes this breakthrough so remarkable is that the particles can return to a state that is unknown to the observer. It is thus possible to return a particle to a previous state without any knowledge about “the system, its internal dynamics, or even the details of the interaction between the system and the experimenter.”

The experiment by the researchers is based on the more than one hundred years-old special relativity theory of Einstein that real-life time travel entails time dilation. This approach reasons that: “A twin travels into space at high speed while his sibling remains on Earth. When the space traveller returns, he or she will have aged less than the one who stayed home. The traveller can claim that the trip into space took less time than the one measured on Earth.”

Einstein was the first scientist to realise that time is not a constant as many people of his time believed. Instead, time slows down the faster a person moves through space. Einstein, therefore, re-envisioned space itself and created the term “spacetime”, thus fusing the three dimension of space and the one dimension of time into one. Space is not flat and rigid holding all objects in the universe, but curved and malleable with gravitational dips around masses that pull objects in. The closer an object is to the gravitational centre, the faster the object moves through space and the slower time moves.

This effect can be observed in GPS satellites that orbit at 20 000 kilometres above the surface of the Earth. The highly accurate clocks of the satellites gain an average of 38 microseconds per day due to time dilation caused by the diminished gravitational force. This “error” is constantly corrected since 38 microseconds result in a positioning error on Earth of almost 10 kilometres.

Although observable, Einstein’s theory is difficult to extend because “enormous amounts of energy or proximity to a black hole are needed to observe significant effects. It is also limited in that it can slow down time, but not reverse or accelerate it.” The team of researchers wanted to demonstrate that “these limitations disappear when one leaves the realm of classical relativistic physics and enters the realm of non-relativistic quantum mechanics.”

This is exactly what they claim they have achieved by creating a mechanism to propagate any qubit to its state before the experiment began. Reversing the qubit states of a quantum processor could effectively allow researchers to undo errors during their development.

The research findings of the researchers are not a time machine. Time passes regardless. If you want to revert a particle to its state five minutes ago, it will take five minutes to complete the process. If you want a system to age 10 years, it will take 10 years. Time cannot be created out of nothing.

The researchers, however, were able to speed up time by transferring evolutionary time between identical physical systems. In an experiment with ten systems that lasted a year, they were able to take away one year from each of the first nine systems and give it to the tenth, allowing it to age 10 years in one year.

The major question now is if these findings can be transferred to larger systems? One of the encouraging aspects of the work of the researchers is that the researcher does not have to know the key information and constraints of the quantum system he or she is studying. This suggests that the protocol is not just restricted to photons. But, the experiment involved particles that can store only one bit of information. While it may be theoretically possible to replicate the experiment with a human being, it is not practical at all.

If a person could be put in sealed a box with absolutely no external influences, it would be theoretically possible to go back in time. However, the probability of success would be extremely low. What complicates it further is the time needed to complete the process, which depends on the amount of information the system is able to store. A human being is a complex physical system consisting of a massive amount of information. It would thus take millions of years to rejuvenate a person or go back in time for only one second. Furthermore, the system is currently only able to change the state of a particular particle.

Even if we succeed to use the intense gravitational acceleration caused by black holes or develop a propulsion system that can enable the traveller to travel close to the speed of light (about 1 billion km/h), scientists believe it is a one way voyage into the future and that time dilation will not allow people to travel back to the past. However, Einstein proposed that time travel into the past may be possible through a stable Einstein-Rosen bridge, a type of wormhole. Wormholes are theoretical areas of spacetime, warped to such an extent that they connect distant points in space.

Although an exciting project, scaling up the “rewind protocol” could prove extremely difficult, if possible at all. Stephen Hawking said in his book, “Black holes and Baby Universes”: “The best evidence we have that time travel [into the past] is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.” Or we are just not able to recognise them yet!

Professor Louis C H Fourie is an Extraordinary Professor in Information Systems University of the Western Cape.