For the first time, an experiment has confirmed that the laws of thermodynamics hold true even at the quantum level – which means that even in the quantum world, you can’t unspill that glass of milk.

The reason time runs the way it does in our everyday lives is because of the second law of thermodynamics, which states that over time all systems become more disordered, or increase in entropy. And that process is irreversible, which is why time only moves forward. But theoretical physicists had predicted that on the quantum level, the process might go both ways.

That’s because when you start dealing with really, really small particles, the laws of physics – such as the Schrödinger equation – are 'time-symmetric' or reversible. "In theory, forward and backward microscopic processes are indistinguishable," writes Lisa Zyga for Phys.org.

Now physicists led by the Federal University of ABC in Brazil have performed an experiment that confirms that those theories don’t match up with the reality, with thermodynamic processes remaining irreversible even in quantum systems. But they still don’t understand why that’s the case.

"Our experiment shows the irreversible nature of quantum dynamics, but does not pinpoint, experimentally, what causes it at the microscopic level, what determines the onset of the arrow of time," one of the researchers, Mauro Paternostro from Queen's University in Ireland, told Phys.org. "Addressing it would clarify the ultimate reason for its emergence."

So how do you go about testing the laws of thermodynamics in a quantum system? Basically scientists need to be able to set up an isolated quantum system and observe the reversal of a natural process – which is tricker than it sounds.

For this experiment, the researchers used a bunch of carbon-13 atoms in liquid chloroform, and flipped their nuclear spins using an oscillating magnetic field. They then used another magnetic pulse to reverse the spins again.

"If the procedure were reversible, the spins would have returned to their starting points – but they didn’t," writes Zyga.

Instead what they saw was that the alternating magnetic pulses were applied so quickly that sometimes the atoms’ spin couldn’t keep up, which lead to the isolated system getting out of equilibrium.

The physicists confirmed that after the experiment the entropy was indeed increasing, which shows that the process of thermodynamics was irreversible, regardless of how small the particles involved were.

All of that basically means that the one-way arrow of time exists even for the tiniest particles in the Universe, defying the microscopic laws of physics. And it suggests that something else is getting involved to stop quantum systems from being reversible.

The physicists are now interested in figuring out what that is, and they believe the new insight into quantum systems could help advance the march towards quantum computers and other quantum devices.

"Any progress towards the management of finite-time thermodynamic processes at the quantum level is a step forward towards the realisation of a fully fledged thermo-machine that can exploit the laws of quantum mechanics to overcome the performance limitations of classical devices," said Paternostro.

For now though, we can take away from this research the knowledge that we can’t move backwards in time, as much as we might want to. The past really has passed… even on the atomic scale.

The research has been published in Physical Review Letters.