Unravelling the mysteries of the “arrow of time” through a salamander’s eye view

The physics behind why time goes forward, the “arrow of time”, is largely a mystery. But new research coming out of the US City University of New York (CUNY) might have shed some light on time’s arrow.

The researchers set out to look at time’s arrow by analysing a salamander’s retina at a microscopic level in order to see if time’s movement – invisible to the naked eye – can be seen.

While some people might think of time as a human construct broken into everything from seconds to eons, time is the fourth dimension – a measurable quantity that goes in only one direction (as far as we know right now). But why?

Why is it clear to us what is in the past and what is in the future? Why does a video of a glass falling off a table seem normal, whereas the same video in reverse looks unnatural?

These questions all relate to the “arrow of time” – the name given to the physical flow of time.

Fundamentally, the arrow of time emerges from the second law of thermodynamics. This law is the principle that the microscopic arrangements of physical systems tend to increase in something called “entropy.”

Increasing “entropy” means that these systems tend to go from order to disorder (or increase in randomness).

By the same token, the more disordered a state becomes, the less likely it is to make its way back to an ordered state. This second law of thermodynamics is the reason why the universe has a flow of time in one direction.

Entropy and the Arrow of Time clip on YouTube produced by The Royal Institution (UK)

But the exact reasons for this phenomenon in the interactions of particles, atoms, molecules and cells is what the CUNY Graduate Centre Initiative for the Theoretical Sciences (ITS) aimed to uncover. The findings, published in the journal Physical Review Letters, could have important implications in many fields from physics to neuroscience and biology.

Christopher Lynn, a postdoctoral fellow in the ITS program and first author of the paper says the team had two basic questions.

“If we looked at a particular system, would we be able to quantify the strength of its arrow of time?

“And would we be able to sort out how it emerges from the micro scale, where cells and neurons interact, to the whole system?”

“Our findings provide the first step toward understanding how the arrow of time that we experience in daily life emerges from these more microscopic details.”

Read more: A primer to the theory of (almost) everything – PODCAST

The team sought to break down the arrow of time into components by observing specific parts of a system and its interactions.

For example, the team looked at the neurons that function within a retina. Taking a single moment in time, the physicists were able to determine four different parts of the flow of time in the retinal neuron activity.

Then they analysed existing experiments on the response of neurons in a salamander’s retina when shown different videos, to see if they could identify these four pieces in other moments.

One movie showed a single object moving randomly across the screen. Another showed a complex array of natural scenes.

In both cases, the arrow of time emerged out of simple interactions between pairs of neurons. The arrow of time was also more pronounced while the salamander was watching the random motion, rather than the natural scenes.

Lynn believes these findings could show how the arrow of time in our senses may become aligned with the external world.

“These results may be of particular interest to neuroscience researchers,” says Lynn. “They could, for example, lead to answers about whether the arrow of time functions differently in brains that are neuroatypical.”

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