The times they are a changin,' and so is everything else.
That's the takeaway from a paper titled “Irreversibility and the Arrow of Time in a Quenched Quantum System,” recently published in Physical Review Letters.
The paper, headed by Tiago Batalhão of the Federal University of ABC, Brazil, says what seems like common knowledge — the arrow of time points in one direction, and once a system undergoes thermodynamic changes it can't reverse those changes, even in quantum physics.
We see this in our daily lives on the macro level. You can't un-break an egg (though, interestingly, you can un-boil it). But in quantum mechanics, many of the equations we use to describe the laws of physics are time symmetric. That means a quantum system in thermodynamic equilibrium should be able to change from one state into another and back again.
Everything should be reversible.
But that's not what scientists found when they put it to the test, according to the paper. Using liquid chloroform, static magnetic fields and a collection of complex measurements, they found that their quantum system's entropy increased.
Entropy is often referred to as the measure of disorder within a system. According to the second law of thermodynamics, an isolated system will never have its entropy decrease. In other words, you'll never have a system spontaneously become more orderly. However, entropy in a system can increase — things break down and become less orderly over time. This means that entropy creates a one-way street for a system — it can get more disorderly but never orderly (at least, not on its own).
This hadn't been observed at the quantum level before Batalhão's team conducted their experiments. And they aren't exactly sure at what point time became an asymmetric factor in the change of quantum states. They suggest that selecting an initial thermal equilibrium (a starting point of sorts) of a quantum state also selects a particular value of entropy and time-reversal invariance is broken.
If we could create a quantum state with true equilibrium for which entropy production isn't a factor throughout the process, it should then be time-symmetric. All changes in such a system would be reversible. But we're not capable of creating such a system now.
We still can't be certain at what point time kicks in and says “sorry, pal, but you can't go back the way you came in.” But irreversibility seems to be a fundamental trait in our universe. Just try not to think about that before you make your next decision. There's no going back now.