For the first time, an experiment has confirmed that the laws of thermodynamics apply even at the quantum level – which means that even in the quantum world, you can’t unspill that glass of milk.
The second law of thermodynamics states that all systems become more disordered, or increase in entropy, over time, which is why time runs the way it does in our daily lives. And because that process is irreversible, time only moves forward. However, theoretical physicists predicted that the process could go both ways at the quantum level.
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 from Brazil’s Federal University of ABC have conducted an experiment that confirms that those theories do not correspond to reality, with thermodynamic processes remaining irreversible even in quantum systems. But they still don’t understand why this is 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 put thermodynamic laws to the test in a quantum system? Basically, scientists must be able to isolate a quantum system and observe the reversal of a natural process – which is more difficult than it sounds.
The researchers used a bunch of carbon-13 atoms in liquid chloroform to flip their nuclear spins using an oscillating magnetic field for this experiment. They then used a second magnetic pulse to reverse the spins once more.
“If the procedure were reversible, the spins would have returned to their starting points – but they didn’t,” writes Zyga.
Instead, they observed that the alternating magnetic pulses were applied so quickly that the atoms’ spin couldn’t keep up, causing the isolated system to become out of equilibrium.
The physicists confirmed that the entropy increased after the experiment, demonstrating that the thermodynamic process was irreversible, regardless of how small the particles involved were.
All of this basically means that the one-way arrow of time exists even for the smallest particles in the Universe, defying the laws of physics at the microscopic level. And it suggests that something else is getting involved to stop quantum systems from being reversible.
The physicists are now trying to figure out what that is, and they believe that the new insight into quantum systems will help accelerate the march toward 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 realization 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.
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