“Man! If only I could turn back time!” According to an article in the science magazine Scientific Reports, physicists have apparently succeeded in doing just that – at least in the quantum realm and with very small particles. However, it’s still impossible to manipulate the wheel of time, because the Second Law of Thermodynamics distinguishes between the past and the future.

Most other physical laws are reversible. But when the Second Law comes into play, nature behaves very stubbornly, and everything progresses in only one direction. The house of cards collapses, it doesn’t build itself. Without external influences, heat will flow from warm to cold bodies, but not in the other direction (this is also ultimately the reason why perpetual motion machines are impossible).

In the quantum realm, when objects are small enough and time intervals short enough, many of the old rules no longer apply. Here, for example, something can be created out of nothing, two bodies can occupy the same space at the same time, and two particles light-years apart from each other can change their state simultaneously, without any communication between the two. If that sounds weird to you, you’re in good company: Einstein could never get used to the ramifications of quantum physics his whole life.

But quantum physics has one important advocate: reality. Calculations produce results that agree with the real world, so they can’t be too far off the mark. Quite the opposite, really, we have to learn to accept that the world is rather strange at extremely small scales.

And that apparently applies to time too. In their work, the researchers considered a thought experiment in which they observe a single electron that could be located anywhere in interstellar space. Its state is described by the Schrödinger equation that, in principle, permits reversibility. However, the universe is constantly expanding and so is, of course, the space where the electron is located. Just after a fraction of a microsecond, the space where the electron exists has expanded irreversibly and its position has become “smeared.”

But there is a mathematical operation, a transformation, which can bring the electron back to its original state, that is, transport it into the past, at least if only a short amount of time has passed. Because of statistical fluctuation, this can happen for real in the cosmic background radiation. The physicists calculated that, if ten billion electrons were observed throughout the current life of the universe (13.7 billion years), the audacious jump into the past would happen only once over that entire period. The electron also would travel only one ten-billionth of a second into the past.

You see, the wheel of time almost always turns in the right direction.

But it is possible to transfer the operation to a quantum computer. In a system made from two qubits, the researchers were able to reverse time and restore the lost order with an 80 percent success rate; with three qubits, the success rate was 50 percent. The researchers put the fact that the success rate was not higher down to the technology of quantum computing still being in its infancy, so their results should improve.

What does this mean in practice? You won’t be able to travel back to the past, unless you’re a quantum computer. And if you are a quantum computer and you are reading this and understanding it, then please spare this humble author from any harm in your imminent takeover of the Earth. Thanks!