Time could be ‘negative’ as photons observed leaving a material before entering it

Time could be ‘negative’ as photons observed leaving a material before entering it

Time could be ‘negative’ as photons observed leaving a material before entering it

Can you exit a room before entering it? This may sound like a crazy idea but in “negative time” it could be possible, at least for photons. 



In their recently published study, a team of researchers from the University of Toronto claims that they observed photons leaving a material before they entered it. This is the first time scientists have come across evidence of negative time.

“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, a postdoc research fellow at MIT who previously worked with the study authors, told Scientific American.

Spotting negative time delay

For the last seven years, the study authors have been studying atomic excitation, a phenomenon that occurs as a result of interaction between light and matter.

During atomic excitation, electrons in an atom absorb energy from the light and jump to a higher energy level. However, this energy boost is not permanent and soon electrons return to their previous levels and release photons.

This process causes a delay, making light take longer to pass through the material compared to if it weren’t absorbed and reemitted by the atoms.

The study authors conducted an experiment to examine this time delay (a.k.a. group delay) in photons. They shot photons into extremely cold rubidium atoms and studied atomic excitation. This experiment led to a surprising observation.

The researchers noticed that some photons passed through the atoms faster than the atomic excitation process could complete. This resulted in a negative transit time, making it seem like the photons exited the material before they actually entered it. 

It sounds crazy, I know. It took a positive amount of time, but our experiment observing that photons can make atoms seem to spend a *negative* amount of time in the excited state is up!” Aephraim Steinberg, one of the study authors and a quantum physicist at the University of Toronto, posted on X

Negative time affects physics but not us

This experiment was quite special and tedious. For instance, the apparatus in which photons interacted with ultracold rubidium atoms, took three years to develop. 

The results of this experiment “suggest that negative values taken by times such as the group delay have more physical significance than has generally been appreciated,” according to the researchers.

An important point to note is that the photons in the experiment didn’t carry any information about the nature of time. So the results of this experiment don’t contradict our understanding of time and photons within the framework of the special theory of relativity.

It won’t affect our experience with time, and therefore, the concept of time will continue to remain the same for us. 

However, since negative time delay does exist in the case of photons, it is now likely to influence further research aimed at understanding the details of the quantum world. 

Source: Interesting Engineering

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Time could be ‘negative’ as photons observed leaving a material before entering it

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