More on the arrow of time


One of the fundamental questions about our Universe is the issue of why time only goes in one direction. Almost all of our physics works equally well with time going either forward or backward. Generally when theories show this sort “reflection symmetry” there is a physical consequence. Yet it’s not the case when we look at time and there’s no simple reason to explain why that is.

Many see the mono-direction of time (the arrow of time) as a direct consequence of the second law of thermodynamics (that says that entropy is maximized in thermodynamic interactions). The idea is that the fact that we can’t “un-stir a cup of coffee” causes the arrow of time directionality that we observe.

But there’s some new research that may shed some more light on the issue:

“In their study, Feng and Crooks have developed a method to accurately measure ‘time asymmetry’ (which refers to our intuitive concept of time, that the past differs from the future, in contrast with time symmetry, where there is no distinction between past and future). They began by investigating the increase in energy dissipation, or entropy, in various arrangements.

The scientists’ method of measuring time asymmetry is best explained in the context of an experiment. In the macroscopic world, where glasses of milk are spilled, time asymmetry is obvious. But on the microscopic scale, because the amount of energy involved is so small, it’s more difficult to tell that entropy is increasing, and that time is moving forward and not backward. In fact, during some intervals, entropy might actually decrease. So even though overall entropy is still increasing on average, in accordance with the second law, the direction of time is not obvious at every moment in the experiment. Further, the scientists show that even an average entropy increase does not necessarily ensure time asymmetry, but can arise in an arrangement that appears time-symmetric.

Feng and Crooks wanted their new measurement method to explain how time can move forward even at points when entropy is decreasing. To do this, they analyzed the folding and unfolding of a single RNA molecule attached to two tiny beads. By controlling the distance between one bead and an adjacent optical laser trap, the scientists could stretch and compress the RNA molecule. Initially, the RNA starts in thermal equilibrium, but, as it’s alternately stretched and compressed, the total entropy of the RNA and the surrounding bath increases on average. “

Read the full article here.

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