Our experience of time seems uniform and unidirectional, but relativity allows it to be a bit more loosey-goosey than you might expect.
For example, in a strong gravitational field, time runs more slowly compared to time farther away – as though gravity itself were dragging it out.
Likewise, if you travel fast enough, time will pass more slowly for you compared to someone who stays behind.
In these examples, time still only flows in one direction.
But looking beyond relativity, into the realm of quantum mechanics, another possibility emerges – one in which time might not merely slow down, but effectively run backward.
What happens when it does, though? A new mathematical exploration of this question has returned a surprising answer: The quantum environment itself imposes a limit on how far back time can effectively run.
And it's extremely short.
For the simplest model explored by physicists Yakir Aharonov of Tel Aviv University and Avshalom C. Elitzur of the Israel Institute for Advanced Research, time could only effectively run backward for about a femtosecond – or one quadrillionth of a second.
Their work is detailed in a preprint uploaded to arXiv.
To read more, click here.