Back From the Future: Can Measurements Performed in the Future Influence the Present?


Zeeya Merali in Discover:

[Jeff] Tollaksen and [Yakir] Aharonov proposed analyzing changes in a quantum property called spin, roughly analogous to the spin of a ball but with some important differences. In the quantum world, a particle can spin only two ways, up or down, with each direction assigned a fixed value (for instance, 1 or –1). First the physicists would measure spin in a set of particles at 2 p.m. and again at 2:30 p.m. Then on another day they would repeat the two tests, but also measure a subset of the particles a third time, at 3 p.m. If the predictions of backward causality were correct, then for this last subset, the spin measurement conducted at 2:30 p.m. (the intermediate time) would be dramatically amplified. In other words, the spin measurements carried out at 2 p.m. and those carried out at 3 p.m. together would appear to cause an unexpected increase in the intensity of spins measured in between, at 2:30 p.m. The predictions seemed absurd, as ridiculous as claiming that you could measure the position of a dolphin off the Atlantic coast at 2 p.m. and again at 3 p.m., but that if you checked on its position at 2:30 p.m., you would find it in the middle of the Mediterranean.

And the amplification would not be restricted to spin; other quantum properties would be dramatically increased to bizarrely high levels too. The idea was that ripples of the measurements carried out in the future could beat back to the present and combine with effects from the past, like waves combining and peaking below a boat, setting it rocking on the rough sea. The smaller the subsample chosen for the last measurement, the more dramatic the effects at intermediate times should be, according to Aharonov’s math. It would be hard to account for such huge amplifications in conventional physics.

For years this prediction was more philosophical than physical because it did not seem possible to perform the suggested experiments. All the team’s proposed tests hinged on being able to make measurements of the quantum system at some intermediate time; but the physics books said that doing so would destroy the quantum properties of the system before the final, postselection step could be carried out. Any attempt to measure the system would collapse its delicate quantum state, just as chasing dolphins in a boat would affect their behavior. Use this kind of invasive, or strong, measurement to check on your system at an intermediate time, and you might as well take a hammer to your apparatus.

By the late 1980s, Aharonov had seen a way out: He could study the system using so-called weak measurements. (Weak measurements involve the same equipment and techniques as traditional ones, but the “knob” controlling the power of the observer’s apparatus is turned way down so as not to disturb the quantum properties in play.) In quantum physics, the weaker the measurement, the less precise it can be. Perform just one weak measurement on one particle and your results are next to useless. You may think that you have seen the required amplification, but you could just as easily dismiss it as noise or an error in your apparatus.

The way to get credible results, Tollaksen realized, was with persistence, not intensity. By 2002 physicists attuned to the potential of weak measurements were repeating their experiments thousands of times, hoping to build up a bank of data persuasively showing evidence of backward causality through the amplification effect.

Just last year, physicist John Howell and his team from the University of Rochester reported success.