Most physicists tend to shy away from the big philosophical questions, but not Bernard d’Espagnat, Professor Emeritus of Theoretical Physics at the University of Paris-Sud. For the last forty years d’Espagnat has discussed the philosophical implications of his work in quantum physics, implications which have led him to state that we cannot know reality “as it really is”, but only what we can learn through experiments. For d’Espagnat there is forever a “veiled reality” that science cannot know but can only glimpse through experiments.
D’Espagnat’s work, a compelling mixture of adept philosophy and cutting edge science, brought him to the attention of The Templeton Foundation; earlier this year he was awarded the 2009 Templeton Prize, founded by the late Sir John Templeton in 1972 to honour those who have made an exceptional contribution to “affirming life’s spiritual dimension, whether through insight, discovery, or practical works.”
Since its establishment the Prize – currently worth £1million ($1.5million) – has been awarded to physicists, philosophers, theologians, philanthropists, and various others who have sought to answer one or more of what Templeton called “the Big Questions”. All have been seekers of wisdom, humbled by the complexity of the human condition but determined to chart a path forward with their ideas and deeds. D’Espagnat clearly fits into this category.
“Partly from my own intuition and partly from my physics I came to the view that there is something that is greater than us in every respect,” d’Espagnat told me when he came to London to collect his prize. “This of course fits with the idea of Sir John Templeton, the idea of humility. He said that God exists, which is another formulation of what I just said, that there is something greater than us. But he also said that we don’t really know what God is. I’m told that when representatives of diverse religions told him ‘well, God has this and this property’ he answered ‘well how do we know?’ This position of humility is also mine – in respect to theology of course, but I extend it to physics, with the notion of veiled reality.”
D’Espagnat’s interest in philosophy began early in life.
“My mother was religious, she was Catholic. But at the same time very much interested in philosophy. My father was a non-believer. I never heard them discuss the point with each other, not in my presence. Mother attended mass but Father did not. And so, unlike those who are brought up in totally religious households, this led to my realisation that there were deep problems. And this tended me towards an interest in philosophy.”
His interest in physics can also be traced to his childhood; he was just 17 when he read a book on quantum mechanics by Louis de Broglie “and found that it was very interesting, it raised questions.”
His interest in quantum mechanics having been awoken by de Broglie, it was only fitting that it was under de Broglie that he should study for his PhD. Thus began a career which saw him work with some of the greatest physicists of the last century, including Enrico Fermi at Chicago, Niels Bohr at Copenhagen and John Bell at CERN in Geneva.
Very early on he discovered that there were certain problems that were not taken to be the proper concern of science.
“I was really puzzled by the notion of uncertainty relations; was there really uncertainty or was there indeterminacy? Some people seemed to think that there was uncertainty in the sense that you could never measure at the same time the position and the motion of a particle. They thought that the particle always has a definite position and simultaneously a definite motion, but that you cannot measure them simultaneously because when you measure one you disturb the other one and vice versa. This was one possibility. But the other possibility was that the particle simply does not possess simultaneously these two features of position and motion. Nowhere in the books, in the teachings of the professors, was this thing clearly stated.
“I think that most physicists think ‘well this is a philosophical problem so we shouldn’t go into it. Best to keep away. Do serious physics and don’t worry about these things that are outside physics.’ I think that was the general consensus. So, I couldn’t find in any books definite answers to this question, and this excited me to think that the question was interesting and this encouraged me to study the foundations of quantum mechanics. Then in my leisure time as a physicist – because at CERN these were not the problems that were investigated – I pondered this and studied various problems in which this question appears. This lead me progressively to the idea that Bohr probably was right in that the real objective of physics is not to describe reality as it really is but to describe how we apprehend it. This then led me to part with Einstein who at the time was really an ontological realist.”
D’Espagnat’s thinking was very much influenced by his work concerning quantum mechanics, especially the work he carried out on a set of mathematic expressions known as Bell’s inequalities.
“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. To state that we see them so because the structure of our senses makes us perceive reality in this form seems to be nearer to the truth. Admittedly this conception of mine is not the one the bulk of the scientists’ community favours but it is quite far from just being my personal one.”
On Physics and Philosophy, Bernard d’Espagnat (Princeton University Press)
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