Time Moves Forward Even In The Quantum World

Time flies like an arrow, fruit flies like a banana. dani3315/Shutterstock

Physicists have proven that the laws of thermodynamics work in the quantum world. This discovery has huge implications for technologies currently being developed, such as quantum computers. 

Researchers created an experiment in which they showed the irreversibility of a quantum mechanical process. They created an isolated quantum system and measured the change in entropy – defined as a gradual decline into disorder – when applying an oscillating magnetic field. If the process was reversible, the entropy wouldn’t increase and move towards disorder, but in reality it does. The team link this finding to the concept of the arrow of time.

We don’t know why time passes. We think that the arrow of time has a direction due to the second law of thermodynamics. The law states that the entropy of the universe always increases, with the entropy being the level of disorder of a system. Basically, the second law says that you can’t perfectly put back together a broken vase. If we see a broken vase, we know that it was broken in the past. 

Quantum mechanics has so far avoided being affected by thermodynamics. Most of the quantum laws are perfectly symmetric in time. “Quantum vases” break apart and jump back together and both situations are perfectly allowed. But this experiment showed that thermodynamics affects the quantum world as well and that the arrow of time arises naturally from the fundamental laws of the universe. 

In the experiment, the scientists measured the entropy of a sample of liquid chloroform. The substance is useful because the spin of the nucleus of the hydrogen atom couples with the spin of the nucleus of the carbon atom. A variable magnetic field was applied to the system, and every time the magnetic field would reverse, the spin would flip. 

The changes in the magnetic field were so rapid that the spins stopped keeping up with it and they ceased being in equilibrium, letting the entropy of the system increase. 

The researchers think that the lack of equilibrium arises directly from the initial condition of the system. The laws of quantum mechanics always start with systems in perfect equilibrium, but creating such a system in reality is very difficult and all the processes we have observed so far are not truly in equilibrium. "Full and perfect reversibility is an abstraction that might be approximately achieved in very controlled situations," Mauro Paternostro, co-author of the study, told IFLScience.

In the paper, published in Physical Review Letters, the scientists comment that they are still unsure where the arrow of time comes from. The laws of thermodynamics are the shackles of physics. We cannot get energy from nothing and we cannot use energy without waste. The laws imply everything we hate about the universe: things require effort, they break, and we age and die. 

"In a sense, the production of entropy is a detrimental byproduct that we aim at controlling and quenching," added Patermostro. "A quantum process that achieves a reduced amount of entropy production could lead to better manipulation of information (at the quantum limited level). We believe that the study of thermodynamics at the quantum-process level will enable the understanding of fundamental mechanisms for energy dissipation."

Understanding entropy and thermodynamics lets us improve our machinery and invest in better technology. Mother Nature might win in the end, but we’re going to give her a run for her money.

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