Quantum mechanics: it's the realm of science where nothing is normal, and everything seems to undermine the fundaments of our common understanding of reality. Nonetheless, we simple humans tick on.
But quantum physicists, who pride themselves on staring into the abyss and gleaning its spooky secrets, have just discovered another baffling phenomenon to make your mind melt: "negative time."
As detailed in a yet-to-be-peer-reviewed study covered by Scientific American, a team of researchers say they've observed photons exhibiting this bizarre temporal behavior as the result of what's known as atomic excitation.
What essentially happened, as SciAm explains, is that when the photons were beamed into a cloud of atoms, they appeared to exit the medium before entering it. Trust us: we're just as confused as you are.
"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," Josiah Sinclair from the University of Toronto, whose early experiments formed the foundation of the study, but wasn't directly involved, told the magazine.
Photons — massless particles that form what we know as visible light — can get absorbed by the atoms they travel through. When this happens, the energy they carry causes the atoms' electrons to jump to a higher energy state. This is the atomic excitation we alluded to earlier.
But the atoms can also de-excite, returning to a ground state. One of the ways this happens is that the energy gets re-emitted as photons. To an observer, this looks like the light that traveled through the medium was delayed.
According to the researchers, they were stunned that there was no "expert consensus" on what actually happened to an individual photon during this delay.
"At the time, we weren't sure what the answer was, and we felt like such a basic question about something so fundamental should be easy to answer," Sinclair told SciAm.
So, like any good scientists, they conducted a series of experiments.
In them, photon pulses were shot through a cloud of atoms at near absolute zero temperatures. And here is where the weird stuff happened: in instances when the photons passed through without being absorbed, they found that the ultracold atoms were still excited for the exact amount of time as if they actually had absorbed them.
Conversely, in cases when the photons were absorbed, they would be re-emitted without delay, or before the ultracold atoms could de-excite.
No laws of physics are being broken here. What's really happening is that the photons are somehow traveling through the atom cloud faster when they excite the atoms — or when they should be absorbed by them, in other words — than when the atoms remain unaffected. Since the photons don't carry information, causality remains intact, SciAm notes.
But the inherent uncertainties involved at the quantum level have the effect of mystifying the whole process. Namely, the phenomenon of superposition, wherein quantum particles like photons can be in two different states at the same time. To a detector measuring when they enter and exit a medium, this means that the photons can produce a positive value as well as a negative one. And thus, negative time.
This doesn't change our understanding of time, the researchers say. On the other hand, it does suggest, at least where the field of optics is concerned, that negative time does have "more physical significance than has generally been appreciated" regarding the transmission of photons, they wrote in the study.
More on hard science: Something Wild Just Happened at the CERN Particle Accelerator
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