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Quantum Time Machine Solves Grandfather Paradox

A new kind of time travel based on quantum teleportation gets around the paradoxes that have plagued other time machines. Of all the weird consequences of quantum mechanics, one of the strangest is the notion of postselection: the ability to trigger a computation that automatically disregards certain results.

Here’s an example: suppose you have a long expression in which there are a frighteningly large number of variables. The question you want answering is which combination of variables makes the expression logically true. And the conventional way to solve it is by brute force: try every combination of variable until you find one that works.

Postselection makes the solution easy to find by simply allowing the variables to take any value at random and then postselect on the condition that the answer must be true. This is controversial because it leads to all kinds of fantastical predictions about the power of quantum computers. Nobody is quite sure if these kinds of computations are possible or how to achieve them but quantum mechanics seems to allow them.

MIT researchers say that if you build a time machine combinig postselection with another strange quantum behaviour such as teleportation, this uses the phenomenon of entanglement to reproduce in one point in space a quantum state that previously existed at another point in space.

The idea is to use postselection to make this process happen in reverse, ensuring that only a certain type of state can be teleported. This immediately places a limit on the state the original particle must have been in before it was teleported. In effect, the state of this particle has travelled back in time.

What’s amazing about this time machine is that it is not plagued by the usual paradoxes of time travel, such as the grandfather paradox, in which a particle travels back in time and some how prevents itself from existing in the first place. This time machine gets around this because of the probabilistic nature of quantum mechanics: anything that this time machine allows can also happen with finite probability anyway, thanks to these probabilistic laws.

Another interesting feature of this machine is that it does not require any of the distortions of spacetime that traditional time machines rely on. In these, the fabric of spacetime has to be ruthlessly twisted in a way that allows the time travel to occur. These conditions may exist in the universe’s extreme environments such as inside black holes but probably not anywhere else.

Postselection can only occur if quantum mechanics is nonlinear, something that seems possible in theory but has never been observed in practice. All the evidence so far is that quantum mechanics is linear. In fact some theorists propose that the seemingly impossible things that postselection allows is a kind of proof that quantum mechanics must be linear.

However, if nonlinear behaviour is allowed, time travel will be possible wherever it takes place. It is possible for particles (and, in principle, people) to tunnel from the future to the past.

Source: Technology Review | The paper of this research is available via arXiv.org