Negative time exists. This evidence is the latest gift that quantum physics has given us

Quantum physics is strange. Contrary to our intuition. This is precisely where its difficulty lies. However, the phenomena it contains are extraordinarily fascinating. The quantum phenomenon we know as ‘wave-particle duality’ identifies the wave nature of lightand, at the same time, the double manifestation of photons and other subatomic particles such as waves and particles. This is undoubtedly one of the most exotic mechanisms in quantum mechanics, but entanglement is even more surprising.

This phenomenon has no equivalent in classical physics, and consists of the state of the quantum systems involved, which may be two or more, being the same. This means that these objects are actually part of the same system, even if they are physically separated. In fact, the distance does not matter. If two particles, objects or systems are entangled through this quantum phenomenon, when we measure the physical properties of one of them we will be instantly conditioning the physical properties of the other system with which it is entangled. Even if it is on the other side of the universe.

These two mechanisms are nothing more than an appetizer of the quantum phenomenon in which we are about to investigate. And it is even more disturbing than ‘wave-particle duality’ or quantum entanglement. Our starting point is a surprising experiment carried out by a group of researchers from the University of Toronto (Canada) led by physicist Daniela Angulo. What they have observed is that photons can spend a negative amount of time passing through a cloud of atoms at a very low temperature. This simply means that they appear to leave a material before having entered it.

The idea of ​​negative time is contrary to our intuition

The germ of this experiment emerged in 2017. Two experimental physicists from the University of Toronto, Aephraim Steinberg and Josiah Sinclair, were interested in studying the interaction that occurs between matter and light, and decided to approach their project by analyzing a phenomenon known as atomic excitement. It consists, broadly speaking, of the fact that when photons pass through a medium and are absorbed, the electrons that revolve around the atoms in that medium jump to higher energy levels.

The curious thing is that when the electrons that have been previously excited return to their original state, they release the energy they have absorbed in the form of photons. And, surprisingly, this mechanism introduces a delay in the time taken by light to traverse the medium. These physicists set out to measure with the greatest possible precision that time delay and find out if it depends on the fate of each particular photon. “At the time we weren’t sure what the answer was, but we thought that such a basic question about something so fundamental should be easy to answer,” confesses Josiah Sinclair.

After three years of planning, this team of researchers designed a device that should allow them to take the measurements they had in mind with great precision. Their plan was to shoot photons through a cloud of ultracold rubidium atoms and then measure their atomic excitation. The most shocking thing is that after carrying out their experiment they had two surprises. The first was that the photons sometimes passed through the cloud without problem, but the rubidium atoms became excited as if they had absorbed those photons.

But the strangest thing was that when the photons were absorbed they seemed to be re-emitted instantaneously and long before the rubidium atoms recovered their initial state. It seemed that photons on average were leaving atoms faster than expected. After consulting this result with Howard Wiseman, a theoretical and quantum physicist at Griffith University (Australia), these researchers concluded that the time that the transmitted photons spent in the atomic excited state coincided exactly with the delay acquired by the light. Curiously, this happened even if the photons were re-emitted before the atomic excitation had decreased.

“A negative time delay may seem paradoxical, but it means that if you build a quantum clock to measure how much time atoms spend in an excited state, the clock hand will, under certain circumstances, move backwards instead of forwards,” Sinclair explains.. This simply means that in this scenario the time in which the photons were absorbed by the atoms is negative. This phenomenon has no a priori impact on our perception of time, but it reminds us how strange quantum mechanisms are. The paper by these physicists has not yet been peer-reviewed, but is available in the open access repository arXiv. If you want to dig a little deeper into this experiment, I suggest you take a look at it. It is very worth it.

Image | Pixabay

rmation | arXiv

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