A new study looks at interactions between microscopic neurons in salamanders to figure out how the “arrow of time” is generated biologically. Time itself is a universal aspect of life. But what is time, and why do we perceive it to have direction, with a past and a future? Scientists have broken down this “arrow of time” to a microscopic physical level in a new study.
According to the second law of thermodynamics, everything tends to move from order to disorder, a process known as entropy that defines the time arrow. A stronger time arrow indicates that it will be more difficult for a system to return to a more ordered state.
“Everything we perceive as a difference between the past and the future stems fundamentally from that one principle about the universe,” said lead author Christopher Lynn. Lynn stated that he wanted to “understand how the arrows of time we see in life” fit into this larger concept of entropy on the scale of the entire universe.
Lynn and colleagues from the City University of New York Graduate Center and Princeton examined how the arrow of time is represented in interactions between amphibian neurons in response to watching a movie. Their findings will be published in the journal Physical Review Letters soon.
On the one hand, it seems natural that an arrow of time would be produced biologically. “You almost have to have an arrow of time to be alive because you develop from a baby to an adult, and you’re constantly moving and taking in stimuli,” Lynn explained. Indeed, entropy is irreversible here—there is no going back.
What the team found was anything but intuitive, however.
Lynn and colleagues examined a separate 2015 study in which researchers showed salamanders two different movies. One depicted a scene with fish swimming around, similar to what a salamander might encounter in the outdoors. The video, like in real life, had a clear arrow of time—that is, if you watched it backwards, it would look different than if you watched it forwards. The other video was only a grey screen with a black, horizontal bar in the middle of the screen that moved up and down randomly and jitterily. There was no obvious time arrow in this video.
Is the arrow of time produced more strongly by the more complicated interactions, or by the simpler dynamics?
The researchers wanted to know if they could detect “local irreversibility” in interactions between small groups of retinal neurons in response to this stimulus. Would irreversible interactions—those that would look different if played backward, having a “arrow of time”—appear in simpler or more complex interactions between neurons?
The researchers discovered that no matter which movie the salamanders watched, the interactions between simple pairs of neurons primarily determined the arrow of time. Indeed, the authors discovered a stronger arrow of time for the neurons when salamanders watched the video with the grey screen and black bar—that is, the video with no arrow of time in its content elicited a stronger arrow of time in the neurons.
“We naively assumed that if the stimulus has a stronger time arrow, it would show up on your retina,” Lynn explained. “But it was the inverse. That’s why it took us by surprise.”
While the researchers cannot say for certain why this is the case, Lynn speculated that it could be because salamanders are more accustomed to seeing things like the fish movie, and processing the more artificial movie required more energy. More energy is consumed in a more disordered system with a larger arrow of time. “Being alive will always define an arrow of time,” Lynn said, regardless of the stimulus.
According to Lynn, some research suggests that the perception of the arrow of time in the human brain is related to how hard people think. Lynn expressed his hope that future research will shed more light on this concept.
“If it’s not the arrow of time in the stimulus that’s driving the arrow of time in the retina, what is?” he wondered.
Finally, the experience of time passing in a physical body is more complex than it appears. Scientists can apply this approach of looking for local irreversibility to other contexts where more unexpected interactions with time may be discovered.
“It doesn’t just apply to neurons—you could apply it to flocks of birds or anything where multiple things interact, like bacterial populations,” Lynn explained.
“And the answers could be completely different; the arrow of time could come from completely different locations, from completely different types of interactions.”
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