A particle physicist wants to put more than his reputation in the firing line. Michael Brooks reports
Enlightenment in the barrel of a gun

Sit in front of a machine gun linked to a quantum particle and pull the trigger. This rather drastic experiment, called quantum suicide, could result in a pool of blood or you could survive having resolved one of the great debates of physics. The intelligent money — or at least that belonging to traditional particle scientists — is on the pool of blood, but the odds are shortening.

In the experiment, proposed last month by Princeton physicist Max Tegmark, pulling the trigger prompts a measurement of the particle’s spin; if the result is `clockwise' the gun fires a bullet, if it is `anticlockwise' there is a loud click. A quantum particle can actually spin both ways at once — such contradictory behaviour is known as `superposition', and is the natural state for quantum objects. But on measurement, according to established wisdom, the electron takes on one state or the other:repeatedly pulling the trigger causes a random pattern and after one or two measurements, the brave scientist is no more.

The orthodox view, the Copenhagen Interpretation, was put forward by the Danish physicist Neils Bohr in the 1920s. In the Copenhagen world, nothing exists until it is observed. This idea has a few shortcomings (it gives extraordinary powers to the observer for a start), but it has gone unchallenged as the `right' view for nearly 70 years.

Tegmark believes in the `Many Worlds' interpretation of quantum theory, first proposed in 1957 by a graduate student called Hugh Everett. Ridiculed for 40 years, Everett’s ideas are suddenly being taken seriously. Advances on the practical side of quantum mechanics have begun to pile up experimental evidence in their favour, and quantum suicide could be the final vindication of Everett’s work.

Everett proposed that the entire universe is one giant quantum mechanical system, and that there can be no definite outcome of a measurement within it. Although an individual who is part of the system cannot be aware of more than one result, every possible event can and does take place. Each conscious individual exists in their own world, with an individual perspective on a larger reality.

Quantum suicide, according to the Copenhagen Interpretation, will result in a pool of blood. That would also be how a distant observer might see things according to Many Worlds. But the scientist with his head at the end of the gun barrel will never die, Tegmark says. Every possible outcome happens, according to Many Worlds, but there’ll be no chance for the scientist to know about the ones where he gets killed. All he will ever be aware of is the `click' and after 10 or so clicks he will be convinced that a reality exists where he can survive this process forever.

Many Worlds calls for a radical shift in thinking, and Tegmark has noticed a generation gap between those who follow Copenhagen and those who believe in Many Worlds. `Young postdoctoral researchers are surprisingly relaxed about these matters. When I ask them if they think Many Worlds is correct, they tend to say `obviously’.'

John A Wheeler of the University of Texas, who counts Richard Feynman and Everett himself among his doctoral students, says he now believes in Many Worlds about `once every month'. Tegmark also cites the Cambridge University cosmologist Stephen Hawking among those who are `pretty sure' they agree.

The quantum suicide experiment may be the only way to be sure about Many Worlds, but although Tegmark says it is entirely feasible using currently available technology, so far, there have been no volunteers. `You’d be insane to try it,' he admits.

15 October 1997

This article was printed in The Guardian.