Charles Darwin would have been proud. His ideas on natural selection are being used to resolve one of the deepest questions in quantum mechanics: how does an objective reality emerge from the quantum world?
In quantum mechanics, particles exist in so-called superposition states, in which they have many mutually contradictory properties. It鈥檚 only when an observer measures the properties that the particle somehow settles into one of these multiple options. 鈥淨uantum mechanics is a beautiful theory, but it鈥檚 a pain in the neck when you鈥檙e trying to reconcile it with what you see around you,鈥� says Robin Blume-Kohout at the California Institute of Technology in Pasadena.
At a meeting in Copenhagen in 1927, two of the founders of quantum mechanics, Niels Bohr and Werner Heisenberg, suggested that until quantum particles are observed they exist as 鈥渨ave functions鈥� that can contain a superposition of many properties. But when an observer makes a measurement, the wave function collapses 鈥� yielding a particle that behaves classically.
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This view makes many uncomfortable, however, for it raises the question of whether our universe exists when nobody is looking at it (New 杏吧原创, 23 June, p 30). Blume-Kohout and his colleague Wojciech Zurek, at the Los Alamos National Laboratory in New Mexico, think that an objective universe does exist, as long as an environment is there to act as a witness.
They and their colleagues have formulated a mechanism which takes out the role of the observer. They dub this theory 鈥渜uantum Darwinism鈥� because the environment decides which quantum properties are the fittest and will ultimately survive to be viewed by people.
鈥淭he environment decides which quantum properties are the fittest and will ultimately survive to be viewed by people鈥�
We already know that the environment affects quantum particles 鈥� just ask any physicist trying to build a quantum computer, says Blume-Kohout. For instance, fragile quantum states are easily disrupted by heat from the surroundings.
鈥淚nstead of thinking of the environment as something negative that makes it hard for us to measure quantum information, we realised it is actually the thing that allows us to measure reality,鈥� says Blume-Kohout. 鈥淎fter all, if I want to measure the properties of an ion in the lab I don鈥檛 reach out and touch it 鈥� I interact with the electromagnetic field between us, and the environment carries information to me.鈥�
According to quantum Darwinism, a given environment will make some quantum properties more stable than others. As the quantum system interacts with the environment, many copies of that stable, 鈥渇itter鈥� state will be created throughout the environment. 鈥淵ou can think of the environment as an active witness: a reporter who doesn鈥檛 just passively observe, but chooses what information to report,鈥� says Blume-Kohout.
When humans make measurements, it鈥檚 most likely they will interact with one of these stable recorded copies, rather than directly with the actual quantum system. That explains why, when multiple observers make their measurements, they all see the same result. 鈥淭hat鈥檚 how objective reality emerges,鈥� says Blume-Kohout.
To test the idea, Blume-Kohout and Zurek built a computer model of an oscillating quantum object interacting with its environment. 鈥淵ou can think of our quantum oscillator as a bowling ball on a chain, swinging back and forth like a pendulum,鈥� says Blume-Kohout.
At the start of the experiment the bowling ball simultaneously exists in every possible location through which it can move. The environment is modelled by a thousand other oscillators, like ping-pong balls suspended by strings, each controlled to swing with different frequencies ().
Blume-Kohout and Zurek set the bowling ball swinging, as dictated by quantum mechanics. They then measured what 鈥� if any 鈥� quantum information about the bowling ball could be found by examining the ping-pong balls. What they found was that the bowling ball鈥檚 wave function appeared to have collapsed, and each ping-pong ball contained the same record of the bowling ball鈥檚 position. 鈥淚t made no difference if we looked at five ping-pong balls or 999 鈥� we still retrieved the same position information, suggesting that multiple redundant records had been made.鈥� The environment was indeed acting as the arbiter of reality.
Klaas Landsman, at Radboud University in Nijmegen, the Netherlands, likes the quantum Darwinism explanation. 鈥淢ost attempts at explaining wave-function collapse are philosophical, but this is a down-to-Earth answer based on the hard-boiled calculations and simulations,鈥� he says. 鈥淚 think it鈥檚 the most satisfying answer yet.鈥�
