January 18, 2010


Sci-phi: Bernard d’Espagnat (Mathew Iredale, June 10, 2009, TPM)

“I worked with Bell when he discovered his inequalities. We were at CERN together. He worked on them in his spare time, they were not part of the work of CERN. At home, in his leisure time, he studied the big problems. In that respect we were quite similar. But we were different in that his intuition was on the realist side, on Einstein’s side. He really thought that he had found, with his inequalities, a test between realism and quantum mechanics and he really thought that the experimental answer would be that realism is right and quantum mechanics is wrong. And I thought the contrary. But these were just guesses.”

John Bell formulated his inequalities as a way of testing local realism. Local realism is the commonsense idea that results of measurements are predetermined by the properties that objects carry prior to and independent of observations (the reality part) and that these results are independent of any action (the locality part). Put simply, there exists an external reality independent of observation in which nothing travels faster than light. This was Einstein’s view.

In order to determine who was right, Bell and d’Espagnat realised that they had to test Bell’s inequalities experimentally. Either the experimental results would obey Bell’s inequalities, and thus exhibit a failure of quantum mechanics, or they would violate Bell’s inequalities, and force scientists to reject Einstein’s (and Bell’s) local realist view.

“I had the luck to discover in my university a young physicist, Alain Aspect, who was looking for a thesis subject and I suggested that testing the Bell inequalities might be a good idea. I also suggested that he go and talk to Bell, who convinced him it was a good idea and the outcome of this was that quantum mechanics won.”

This “win” for quantum mechanics has had far reaching consequences, leading to a clear confirmation of the phenomenon of “non-local entanglement”, which in turn was an important step in the later development of quantum information science, a flourishing contemporary domain of research combining physics, information science, and mathematics. This is something of which d’Espagnat is justly proud.

It also meant that physicists had to abandon, once and for all, the concept of local reality. And this raised once again the problem of interpretation: just what is quantum mechanics describing?

“I think that quantum physics is most easily interpreted precisely as a tool that enables us to describe human experience. That is, the questions I raised earlier about uncertainty relationships really arise because intuitively we believe in an ontological reality and we believe that we are able to describe it and that science can describe it. But quantum mechanics describes not what really exists but what we see or what we would see in such and such circumstances.”

In other words, the question “Is an electron a particle or a wave?” is the wrong question to ask as it presupposes ontological reality. Rather, in the light of quantum mechanics, one should say, under certain experimental conditions electrons exhibit wave-like behaviour, and under other experimental conditions, particle-like behaviour. Anything more is pure speculation. For d’Espagnat, quantum mechanics is a predictive theory rather than a descriptive theory.

“The actions of quantum mechanics are most easily stated as predicting what we shall see in certain circumstances and as Bohr said they are objective in the sense that what they predict is valid for you, for me, for everybody and at every place and every time. So they are scientifically quite objective but they are not ontologically objective.”

This difference in the concept of objectivity led d’Espagnat to distinguish between what he terms strong objectivity and weak objectivity.

“Strong objectivity is ontological objectivity; statements of classical physics could very well be interpreted in terms of strong objectivity. When you consider Newton’s inverse square law, for example, it describes the things themselves but it does not mention you, me or anybody at all. Weak objectivity refers to statements that are objective in the sense that they are valid for everybody, but they also fundamentally involve us. They are of the form, if you do such and such a thing you will observe such and such a thing. So, this is weak objectivity.”

This, for D’Espagnat, is the most important, distinctive feature of quantum mechanics: that it is weakly objective. Not that it involves indeterminism, although that is the feature of quantum mechanics that has attracted the most attention, especially from philosophers. But rather that human interaction is a fundamental part of quantum mechanics; our knowledge of reality fundamentally involves us.

“I think that our scientific knowledge finally bears, not on reality-in-itself – alias ‘the Real’, alias ‘the ground of everything’ – but just on empirical reality, that is, on the picture that, in virtue of its structure and finite intellectual capacities, the human mind is induced to form of reality-in-itself. I even claim that we must drop the view according to which objects, be they elementary or composite, exist by themselves and are at any time at some definite place in space.”

All roads lead to Hume.

Posted by Orrin Judd at January 18, 2010 7:51 PM
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