Could the famous "grandfather paradox" described by Rene Barzhavel in 1943 become a reality? Theoretically, there are no direct prohibitions against traveling to the past. Einstein's theory of general relativity, which describes gravity as the curvature of space and time in energy and matter, allows such journeys.
However, they require an extremely strong gravitational field, such as generated by a rotating black hole. Only an object of such strength can deform matter in such a way that space curves to the other side. Stephen Hawking and many other physicists believed that a closed time-like curve was absurd because time-travel of any macroscopic object inevitably creates paradoxes that break causality.
A few years ago, University of Queensland physicist Tim Ralph and his PhD student Martin Ringbauer explored the "grandfather paradox" from the point of view of quantum mechanics. The essence of the paradox is to go back in time and kill your grandfather, thus preventing your own birth. According to the hypothesis, the grandfather must have survived the murder because otherwise, the time traveler creates an alternate line where he will never be born. Instead of a man walking through time to kill his ancestor, imagine an elementary particle traveling back in time to activate a switch on the particle generating machine that created it.
If a particle flips the switch, the machine will emit a particle that has started a time-travel loop. Thus, if the switch is not moved, the machine emits nothing. In this scenario, there is no deterministic certainty, only the probability distribution. If the particle were a person, it would have a 50% chance of killing its grandfather, which is good enough in probabilistic terms to close the causal loop and escape the paradox. As strange as it may be, this solution follows the known laws of quantum mechanics.
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