The second law of thermodynamics states that everything tends to move from order to disorder, a process known as entropy, which determines the arrow of time. A stronger time arrow means it will be more difficult for the system to return to a more ordered state.

Using previous work with salamanders, researchers at the City University of New York and Princeton examined how the arrow of time is represented in interactions between amphibian neurons. Their work will soon be published in the journal Physical Review Letters.

On the one hand, it is somewhat clear that the arrow of time must be determined biologically. “To be alive, practically, you need to have an arrow of time, because you evolve by constantly moving from an infant to an adult,” say the study’s authors. But everything is not so simple.

The scientists analyzed the results of a separate 2015 study in which salamanders watched two different movies. One of them showed a floating fish scene, similar to what salamanders might encounter in everyday life. As in the real world, the video had a clear arrow of time—meaning it would look different looking forward when looking backwards. Another video featured just a gray screen with a black horizontal bar in the middle of the screen that randomly, flickering rapidly moving up and down. There was no obvious time arrow in this video. The question for the researchers was whether they could detect signs of “local irreversibility” in interactions between small groups of neurons in the salamander retina in response to this stimulus.

The researchers found that whatever movie the salamanders were watching, interactions between pairs of simple neurons primarily determined the arrow of time. What’s more, the authors found a stronger time arrow for neurons when salamanders watched a video with a gray screen and black bars—in other words, a video without a time arrow evoked a greater time arrow for neurons.

“We naively believed that if the stimulus had a stronger time arrow, it would be displayed on the retina,” the authors write. “But it was the other way around. That’s why it surprised us.” Researchers cannot say exactly why this is so.

Source: Ferra

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