Negative Time: Another Curious Wrinkle in the Always Surprising Quantum Universe

 

By James Myers

There are many odd and counterintuitive properties of the quantum universe, and more questions continue to arise about the smallest-scale realm of atoms and the tiniest units of energy. Negative time can now be added to the list of quantum curiosities that scientists are puzzling over.

When a beam of light travels through any physical material, the light particles travel more slowly between their entry and exit points than they would otherwise. This time difference is called a “group delay,” because the light’s energy excites the material’s atoms that provide resistance and change the light’s frequency. But in a 2022 experiment, scientists at the University of Toronto shot a beam of photons through a cloud of ultracold rubidium atoms and observed the light travelling faster than expected, even while the atoms were still excited.

The effect was as if the light particles, which are called photons and have no mass or resistance of their own, had excited the rubidium atoms even before entering the cloud. It was like a time reversal or negative time, where the photons exited before entering and lost none of their energy to the rubidium atoms.

 

 

Published in the journal PRX Quantum, scientists who conducted the 2022 experiment wrote, “As unexpected as ‘excitation without loss’ might be, even less intuitive is the implication that lost photons are scattered long before the spontaneous lifetime of the atom has elapsed.” They concluded, “These results cry out for a fully quantized theoretical treatment as well as further experiments varying the optical depth, bandwidth, and pulse shape in order to further elucidate the strange history of transmitted photons.”

When our experience of time is that cause always comes before effect, why did the order of time seem to be reversed in the quantum experiment?

After developing the scientific apparatus to conduct further experiments, in September 2024 the scientists, together with a theoretical physicist from Griffith University, published a paper entitled “Experimental evidence that a photon can spend a negative amount of time in an atom cloud.” The results of the latest tests “suggest that negative values taken by times such as the group delay have more physical significance than has generally been appreciated.”

 

 

The recent experiment produced two surprising results when the photons passed through the cloud of rubidium atoms. One was that sometimes, but not always, when the atoms  were excited by the energy of the photons, the photons themselves were unaffected, as if none of their energy had been absorbed by the atoms. 

The other surprise was that when the photons did lose energy to the atoms, they exited the cloud faster than expected, even while the atoms remained excited.

The surprising results overturn long-held ideas of time sequences in physical actions. If the transfer of energy from light is the cause of the rubidium atoms’ excitement, why were the photons of light unaffected (in the present) by the energy transfer (that occurred the past)? The normal sequence of cause and effect also seems to have been overturned when the light’s energy left the cloud, but the rubidium atoms remained excited – an effect seemingly without a cause.

 

 

As one of the scientists, Josiah Sinclair, explained to Scientific American, “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.”

Normally, the group delay of photons and the material they’re passing through is positive, meaning that light slows down when it interacts with the atoms. However, the recent experiment demonstrated that when the light’s energy is tuned to frequencies close to those at which the atoms naturally resonate with one another, the group delay becomes negative and the correlation in the group delay is lost. 

The reasons for this result when light and atomic frequencies are harmonized is the subject of much scientific interest.

The scientists observed that the time spent by the photons as atomic excitations was directly related to the group delay. This suggests that even though photons have no mass or resistance of their own, the light’s action might be a consequence of the physical process as much as the physical process is the consequence of the energy of the light’s photons.  What comes first in this physical sequence: are the photons the cause or are the physical atoms the cause?

What are the potential consequences of the experimental observations?

The Quantum Insider reports that, “Although the study doesn’t directly address quantum computing, the findings could have implications for improving quantum memory and communication systems by enhancing the control of photon-atom interactions.”

Major advances are being made in the quest to develop a fully-functional quantum computer and practical applications for which the machine’s speed and accuracy will far surpass today’s most powerful computers. Difficulties persist, however, in controlling quantum circuits and maintaining their connections, due to interference caused by energy from the external environment and the interactions of atoms that carry the quantum signals.

Improved control of quantum signals and the photons of light in which they are stored suggests a potential to overcome the connection problems that plague quantum circuitry. However, the scientists are conducting further tests on negative time to eliminate other possible causes of the curious phenomenon and the results might contribute to an understanding of how negative time could be practically applied to quantum computing.

While the experiments might not upend our understanding of time as we experience it, they raise fundamental questions about differences in quantum time at the smallest scale of the universe. It’s important to note, however, that the photons used in the experiment did not carry information and therefore did not violate Einstein’s theory of general relativity which states that the speed of light is the maximum at which information can be transmitted. 

Regardless, could better knowledge of energy at its smallest scale lead to important discoveries at the largest scales? As time releases its secrets to science, perhaps only time will tell the answer to this question.