Miércoles 15 de Agosto de 2007, Ip nº 203

Free will - is our understanding wrong?
Por Zeeya Merali

REDEFINE the concept of free will? Only a Nobel laureate would have the nerve. Last year, the Dutch physicist Gerard 't Hooft announced that the weird effects that spring from quantum mechanics arise from a deeper deterministic reality based on classical physics. People objected that his theory appeared to rob us of free will, and now 't Hooft has responded by moving the goalposts. No, we don't have free will as it is commonly understood, he says - but that's because the way it is commonly understood is wrong.

't Hooft, of the University of Utrecht in the Netherlands, shared a Nobel prize in 1999 for laying the mathematical foundations for the standard model of particle physics. Like Einstein, he was troubled by the indeterminism at the heart of quantum mechanics, according to which particles do not have clearly defined properties before you measure them, and you can never predict with certainty what the outcome of your measurements will be. So 't Hooft constructed a deterministic alternative which showed that fundamental states which exist on the smallest scales do start out with clearly defined properties. Information about these states gets blurred over time, until we are no longer able to tell how they initially arose - leading to their apparently probabilistic quantum nature, he says.

However, mathematicians John Conway and Simon Kochen at Princeton University showed that if 't Hooft's theory is true, then people's ability to make instantaneous, unpredictable choices on a whim is similarly constrained - we don't have free will (New Scientist, 4 May 2006, p 8).

The revelation has been a stumbling block for his theory, 't Hooft admits. "It's not the mathematics that loses other physicists," he says. "It's this metaphysical worry about free will. Why worry at all about a notion so flimsy as 'free will' in a theory of physics?"

Imagine you are holding a cup of coffee. "I can't change my mind in an instant about whether to drink the coffee or hurl it across the room. My decision must have roots in brain processes that occurred in the past," he says. "What's important is that I have freedom to calculate what happens if I throw my coffee cup. Equally, I have the freedom to calculate the effects after I drink from my cup." What we lack is the freedom to instantaneously switch between which of these initial states we start from. 't Hooft calls his new formulation the "unconstrained initial conditions postulate".

Hans Halvorson, a philosopher of physics also at Princeton University, agrees that our ideas of free will need to be revised. "It's likely that our naive gut reaction about what free will is may need to be radically rewritten in just such a way, if we really want to consider what's happening at the deepest levels," he says.

Conway and Kochen say a deterministic theory denies us the freedom to choose what to measure about a particle's characteristics. The only way 't Hooft's theory matches experimental results, they say, is if nature is conspiring to prevent physicists measuring certain characteristics of a quantum particle by changing its properties at the same moment that they decide what to measure.

't Hooft sees nothing mysterious about this. Any decision about what to measure must have been influenced by environmental factors in your recent past, and it will take time to enact your choice as you modify your measuring apparatus. It's safe to assume that in this time, the particle you plan to measure will also be influenced by these environmental factors - a disruption that accounts for nature's ability to tweak what you are able to measure, he says.

However, Antoine Suarez, a physicist at the Center for Quantum Philosophy in Zurich, Switzerland, remains troubled. "If 't Hooft is really correct, then the work for which he is famed was not carried out as a result of his free will. Rather, he was destined to do it from the beginning of time," he says. "In that case, maybe his Nobel prize should rightfully have been presented to the big bang instead."

Suarez has performed an experiment that he claims proves 't Hooft wrong. 't Hooft's deterministic theory and his redefinition of free will rely on fundamental states obeying causal laws, so that a chain of events can be calculated precisely, given the starting conditions. By bringing the effects of special relativity into play in a standard entanglement experiment, Suarez and his colleagues were able to check how time flow interacts with the quantum world (see "Effects without causes"). "We tested the very concept of time," says Suarez.

The result was a resounding success for quantum mechanics, says Suarez. His team showed that the well-behaved time-ordering 't Hooft needs simply doesn't exist: there is no causality at a deep level. Suarez is submitting his paper to Foundations of Physics, a journal that is edited by 't Hooft. "I think it will spark an interesting debate," Suarez says.

't Hooft is ready to meet that challenge. Although his theory cannot yet explain the results, he is confident that it will eventually do so. "After all, we know that quantum mechanics produces eccentric results," he says. "That's exactly why I am looking for an alternative."

Effects without causes

To test 't Hooft's deterministic theory, Antoine Suarez at the Center for Quantum Philosophy in Zurich, Switzerland, and his colleagues performed an entanglement experiment with a relativistic twist.

Entangled particles are inextricably intertwined, so that making a measurement on one instantaneously affects its partner. In standard experiments, two entangled photons, A and B, follow different paths until they come to a beam splitter, which allows the photon to follow either a longer path or a shorter one to continue its journey (see Diagram). In every case, A and B make the same choice, proving they are entangled.

A deterministic theory can explain the result if A hitting the beam splitter somehow affects the environment of B, encouraging B to take the corresponding path - a straightforward causal link. To test this, the team exploited an effect of special relativity, which causes two events to appear to occur in a different order to different observers if those observers are moving relative to one another.

Suarez's set-up uses a pair of beam splitters that are moving apart. An observer sitting at the first beam splitter, BS1, would observe photon A hitting BS1 - and making its path choice - before photon B hit BS2. An observer sitting at BS2, would see the reverse - that photon B made its choice before A (www.arxiv.org/abs/0705.3974).

If a deterministic theory such as 't Hooft's is correct, any entanglement should disappear. This is because it is not possible for either photon to "tip off" its partner about its choice before its partner chooses its own path, since both photons are making their choices both before and after their partner, depending on which beam splitter you observe from. Yet the team continued to observe entanglement. "Quantum mechanics beat both time and 't Hooft," says Suarez.

  01/08/2007. New Scientist Magazine.