The concept of negative time sounds implausible – or the basis for time travel. But while many scientists are skeptical of its existence, recent research has hinted to there being more to this wacky phenomenon than previously thought.
Negative time is a peculiar quirk of quantum mechanics, like the possibility of an object being in two places at one time (think: Schrodinger’s cat) or two particles existing in the same state when far apart (aka quantum entanglement). Quantum mechanics is the world of atoms, electrons, and photons and at times, can appear to be at odds with what we see in the world around us. As for negative time, it refers to a period of time that is less than zero.
The concept was explored earlier this year by scientists at the University of Toronto. As IFLScience reported at the time, researchers released a study on the preprint server arXiv (meaning it is yet to be peer-reviewed) that demonstrates how objects can emit light in so-called negative time. The piece of research involved looking at how long it takes a pulse of light to travel through a cloud of atoms.
As light passes through the cloud, the atoms temporarily absorb the photons, triggering an “excited” state before releasing the photons. The team measured the amount of time atoms remained in this excited state. Curiously, there were instances where the time was negative, i.e. less than zero.
“A negative time delay may seem paradoxical but what it means is that if you built a ‘quantum’ clock to measure how much time atoms are spending in the excited state, the clock hand would, under certain circumstances, move backward rather than forward,” co-author Josiah Sinclair told Spektrum, Scientific American reports.
It is worth stressing that this piece of research is still awaiting peer-review and it has been met with a lot of skepticism. Sabine Hossenfelder, a theoretical physicist based in Germany, told viewers in a YouTube video, “This negative time [in the experiment] has nothing to do with the passage of time – it’s just a way to speak about how a bunch of photons travel through a medium and how their phases shift.”
This comes down to another confusing feature of quantum mechanics – photons can behave both like a particle and as a wave, and follow probabilistic rules that mean they can appear in different states at once.
Ultimately, Einstein’s theory of special relativity stating nothing can travel faster than the speed of light remains unchallenged and as the researchers are keen to stress, this is not a basis for time travel. However, it does highlight the complex and paradoxical-seeming characteristics of quantum mechanics. As Hossenfelder goes on to say in her video, this negative time experiment is “very interesting work” and could have practical implications, for example, when it comes to optical fibers.
