Physicists Calculate How Far Time Could Run Backward: One Femtosecond

A new mathematical study asks how far time could effectively be made to run backward in a quantum system, and the answer is almost nothing. According to ScienceAlert, physicists Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research found that in the simplest model they examined, the reversal could last only about a femtosecond, or one quadrillionth of a second.

The work is purely theoretical and appears in a preprint uploaded to arXiv. No experiment has made time run backward, and the researchers used mathematics because the idea has not been realized in a lab.

What exactly did the physicists study?

They revisited a theoretical device that Aharonov and colleagues proposed in 1990, called a quantum time-translation machine. ScienceAlert reported that it would use the strange rules of quantum mechanics to make a system behave as though time had sped up, stopped or run backward. Now, 36 years later, the pair set out to measure how far that negative time could go, working through four slightly different versions of the machine.

How could a clock appear to run backward?

The idea rests on superposition, in which a system exists in a combination of possible states at once. Picture a clock in two positions in a gravitational field at the same time, with gravity strong enough in one place to slow time compared with the other. When the two versions of the clock are brought back together, they interfere with each other.

By picking out one particular outcome of that interference, a step called post-selection, physicists can in theory make the clock act as if it had experienced a span of time outside the range of either path. That could be more time than either path saw, no time at all, or a negative amount, as if the clock had run in reverse.

How would anyone prove time had run backward?

The researchers chose atomic decay as the test. When an atom absorbs energy, an electron jumps to a higher level, and later it drops back, releasing a photon in a process called spontaneous emission. Under ordinary conditions this only goes one way.

ScienceAlert gave an example: start with 100 excited atoms and wait until only 80 remain excited. If the theoretical machine then leaves more than 80 excited, it has reversed part of the decay. To keep the emitted light from escaping, the researchers imagined sealing the atoms in a cell along with the light and walls that could absorb it.

Why is the limit so short?

Even in a sealed cell, the atoms interact with the surrounding electromagnetic field as they emit light, leaving traces of what happened. For the simplest version of the machine, reversal works only while the two possible histories stay quantum-coherent, which means they can still interfere. In the researchers' optical example, that window lasts around one femtosecond.

After that, the environment has retained too much information about the emission. ScienceAlert's summary is that the limit comes not from time itself but from the environment's memory of the light. More elaborate machines could in theory reverse longer intervals, but only by adding many more quantum branches, with increasingly improbable chances of success.

Why does it matter?

For ordinary readers, nothing changes: no device can turn back the clock, and the result does not suggest one is coming. The finding matters as a piece of basic physics. It suggests that if effective time reversal is possible at all in these models, the surrounding quantum environment sets a hard practical ceiling on it. Because the study is a theoretical preprint, its conclusions rest on the researchers' models and have not been tested in an experiment.

Around Prague, based on reporting by ScienceAlert

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