Physicists Find Mathematical Limit on Reversing Time
Two physicists calculate that quantum effects cap how far time can effectively run backward at about a femtosecond, one quadrillionth of a second, in the simplest model they studied. The work is a theoretical preprint on arXiv, not an experiment.
Physicists Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research report that time could effectively run backward for only about a femtosecond, or one quadrillionth of a second. Their calculation sets a mathematical limit, and it applies to the simplest model they explored. The work appears in a preprint on arXiv, so it has not yet been through formal peer review.
The result is theoretical. Nobody has built the device involved.
A machine that bends time
Aharonov first proposed the idea in 1990 with colleagues. They described a theoretical device called a quantum time-translation machine. It would use the odd properties of quantum mechanics to make a system behave as though time had sped up, stopped, or run backward.
The idea rests on superposition, where a system exists in a combination of different possible states at once. Picture a clock in superposition at two positions in a gravitational field, with enough difference in gravity to change how fast time passes. Deeper in the field, time runs slower. Farther out, it runs faster.
When the two branches of the clock are brought back together, they interfere with each other. Physicists can then pick one particular outcome of that interference, a step called post-selection. In theory, the clock then behaves as if it experienced more time than either branch, no time, or a negative amount of time.
How far back can it go?
Thirty-six years after the original proposal, Aharonov and Elitzur returned to ask how far that negative time could extend. Because no one has realized the machine in a lab, they used mathematics to study four slightly different configurations.
Their finding is that the quantum environment itself caps the effect. In the simplest model, the cap is about a femtosecond.
Proving time ran backward
The researchers first had to settle how anyone would show that time had run backward. They turned to atomic decay. When an atom absorbs energy, one of its electrons jumps to a higher energy level, leaving the atom in an excited state.
Atoms tend to settle back. The electron drops to a lower level and releases the extra energy as a photon, a process called spontaneous emission. Under ordinary circumstances it only goes one way.
So the pair proposed starting with atoms that had already begun to decay, then testing whether their theoretical machine could reverse the process. In their example, 100 excited atoms are left to emit until only 80 remain excited. Those atoms then pass through the machine.
