We all have times we wish we could go back in time and make a different decision. Now that appears to be possible – for single atoms, at least. Physicists at the Australian National University have confirmed one of the most profound thought experiments of quantum physics. It appears to show that present actions can affect past events.
Andrew Truscott and his team showed that if you offer a speeding helium atom two possible paths, the route it takes appears to be retroactively determined by the act of measuring the atom at the end of its journey. The team reported the strange discovery in Nature Physics in May.
“It’s a fantastic experimental tour de force,” says Radu Ionicioiu, a physicist at Bucharest’s Horia Hulubei National Institute for Physics and Nuclear Engineering. “But how to interpret it? If you ask 10 people, you'll get 11 opinions.”
We have long known that quantum particles such as photons have a split personality. Sometimes a photon will behave like a particle, bouncing like a pebble off a wall. But photons can also behave like waves, rippling as they pass between narrow gaps. Weirdly, the observer appears to determine the characteristics the photon adopts. Test it for waviness and it will behave like a wave; look for particle-like behaviour, and that’s how it will manifest.
Ioniciou sums up the paradox: “The question is, how can the same thing behave both ways, depending on what sort of question you ask?”
In the late 1970s John Wheeler, one of the giants of 20th century physics, was pondering this when he realised quantum behaviour implied an even weirder possibility – that an observer’s influence on a photon’s behaviour could travel back in time.
“The experiment says that there are some things, such as the path followed by a particle, that you just cannot know"
In Wheeler’s “delayed-choice” thought experiment a photon is presented with a ‘choice’ — to act as a particle or as a wave. The photon is then measured a bit later to find out what choice was made. In 1978 Wheeler proposed sending a photon towards a crossroads (in this case a two-way mirror). Wheeler realised that if the experimenter recombined the two pathways (using a second mirror) at a point further along the track – that this should retroactively determine the path the photon takes at the crossroads.
He hypothesised that if the experimenter does NOT recombine the paths, then the photon should act like a solid particle, bouncing down one of the paths. But if the experimenter does recombine the paths, then the photon should act like a wave. When a wave of water strikes a forked channel it splits into two and ripples down both paths at once. The photon should do the same.
Performing Wheeler’s experiment seemed almost impossible at first, as it required rejigging the apparatus while the speeding photon was in mid-flight. A version of the experiment was finally achieved in 2007 when a French team built a 48-metre-long path for their light beam, which allowed just enough time for the equipment to be switched around after the photon passed the mirror crossroads, but before it reached the detectors.
Physicists have now taken the experiment to a different level. They have shown that Wheeler’s time experiment works not only with ephemeral photons of light, but also with matter in the form of single atoms. It has long been known that single atoms can also display wave-particle duality.
To carry out their experiment, Truscott and his team collected about a thousand helium atoms, cupping them with lasers, and cooled them to one billionth of a degree above absolute zero. The crowded atoms bumped and jostled, knocking each other out of the laser trap, until eventually a single atom remained.
The team then allowed the atom to fall towards crisscrossing laser beams. The lasers split the atom’s trajectory into two possible paths. After the atom passed the crossroads, the equipment randomly switched to a set-up that either recombined the two possible paths, or did not.
The atom behaved in the same way as the photon. If the paths were recombined this produced an interference pattern typical of a wave, showing the atom travelled down two paths at once. If the paths were not recombined, the atom banged into one of the detectors at the end of each track, in the same way a pebble would.
So, which path did the atom take? Or did it take both? The same experiment gives two contradictory results – as Wheeler predicted it would.
For Truscott, the big lesson from the experiment is that we can’t apply conventional intuition to quantum systems – they do not recognise human ideas such as “the past”. “The experiment says that there are some things, such as the path followed by a particle, that you just cannot know," he says. You can only talk about the probability that the particle is in a particular place at a particular time, he says.
Joan Vaccaro, a quantum physicist at Griffith University, says much of the confusion arises because people insist on describing quantum objects as either particles or waves. She suggests resurrecting British physicist Sir Arthur Eddington’s 90-year-old idea: “He coined this term ‘wavicle’ to give quantum systems a different name, instead of referring to them as either a particle or wave, when it shouldn't be either.”
For Ionicioiu, studying quantum mechanics is like entering another world, “one with different rules.” He quotes Richard Feynman, a PhD student of Wheeler’s, who said in his Lectures on Physics: "The ‘paradox’ is only a conflict between reality and your feeling of what reality ought to be."
Cathal O'Connell is a science writer based in Melbourne.
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