Meeting the universe half way

In her book Meeting the Universe Half Way the theoretical physicist Karen Barad (2007) draws on quantum theory and the philosophy of the Nobel prize-winning physicist Nils Bohr, to develop her thinking about the paradoxical relationship between the knower and the known and the sense we can make of the world through our engagement with it. She argues that Bohr’s philosophical reflections on his work in physics provides opportunities for linking the natural and social worlds in the sense that we are part of the natural world we seek to understand. She accepts that both Bohr’s views (he was regarded as too philosophical for a physicist!), and her own interpretation of them are contested, but I will explore them nonetheless because both perspectives are interesting and helpful in the context of the discussion on this blog about systematic ways of comprehending the social. Her ideas are interesting in terms of furthering the discussion about what it means to be scientific.

In order to explain Bohr’s philosophy she discusses and represents in great detail two now famous manifestations of quantum physics: the slit experiment conducted by a variety of scientists since the original claim by Thomas Young in 1806, that light demonstrates the properties of both waves and particles and the principle of uncertainty/complimentarity debated by Bohr and Werner Heisenberg.

Briefly to explain the slit experiment and what it reveals about the behaviour of matter: when particles of light are fired towards a light receptor through a barrier which has a single slit cut into it they appear to go in a straight line and strike the receptor directly in line with the firing mechanism. When a second slit is opened up in the barrier next to the first, then the particles appear to interfere with each other and demonstrate diffraction patterns of light and dark lines on the receptor much like the patterning produced by waves becoming superimposed over each other. From a classical perspective, particles are entities which occupy a given amount of space at a particular time. Waves, on the other hand, are disturbances which propagate in a medium such as water or air and can overlap at the same point in space. If this happens then their amplitude can be combined or they can cancel each other out. We may all have noticed the way that wave patterns interfere with each other when we throw two stones into a pond.

So if the experiment is conducted with one slit, then a physicist would conclude the light behaves like a continuous stream of sequential particles. When conducted with two slits, then light particles (or other experiments have demonstrated the same phenomenon given certain physical conditions with other types of matter, whether it electrons, neutrons or atoms) behave very differently. This wave/particle contradiction is important because:

This situation is paradoxical to the classical realist mind-set because the true ontological mature of light is in question: either light is a wave, or it is a particle; it cannot be both. (2007: 198)

Barad directly addresses the point usually made about the quantum paradox, that insights gleaned from microscopic entities have little relevance to considering Nature at the grander scale and disputes the interpretation that they have little relevance for the world we live in. She points out that when the wave nature of light or matter is insignificant i.e. when the wave length is small in relation to other important dimensions, then classical physics provides a useful shortcut for calculations. She regards classical formalisms as an often helpful approximation to the more elaborate calculations of quantum physics which operate at all scales and are a much more comprehensive account of what we observe in Nature. She concludes that: ‘As far as we know, the universe is not broken up into two separate domains (i.e. the microscopic and the macroscopic) identified with different length scales and different sets of physical laws for each.’ (2007: 85) Quantum effects are small if the mass of the object is large. However, Barad argues that quantum physics does not complement Newtonian physics, but supersedes it (Ibid: 110).

Additionally she treats Heisenberg’s uncertainty principle, an idea he explored in a famous paper he wrote in 1927 which involved a gedanken, or thought experiment. The paper considers the hypothetical detection of an electron by a photon using a gamma ray or high-energy spectrum microscope. Heisenberg reflects upon the way that the photon would disturb the electron in trying to measure it in a discontinuous way (owing to the Planck constant). Heisenberg concludes from this that there is an epistemic principle of quantum physics, that there is a limit to what we can know about the momentum or the position of an electron because of the incalculability of the technique of measurement.  Bohr was dissatisfied with this explanation and argued that it was not an epistemic difficulty, but an ontological one; that is to say, what we take reality to be. While Heisenberg argued that it was impossible for a physicist to know simultaneously the momentum and position of the same particle using quantum calculations, Bohr argued instead that particles do not have determinate values of position and momentum simultaneously. In other words, and according to Bohr, in order to measure a particle’s position, then one set of apparatus and measurements are required, and in order to calculate momentum, then an entirely different configuration of equipment and measurements are required. The two sets of apparatus are mutually exclusive: in being precise with one measurement the experimenter is electing to be imprecise about the other. In an addendum to his paper, Heisenberg later accepted this qualification of his work:

In this connection Bohr has brought to my attention that I have overlooked essential points in the several discussions in this paper. Above all, the uncertainty in our observation does not arise exclusively from the occurrence of discontinuities, but is tied directly to the demand that we ascribe equal validity to the quite different experiments which show up in the corpuscular theory on the one hand, and in the wave theory on the other (i.e. that we acknowledge complementarity, that is, the necessity of considering mutually exclusive experimental conditions). Quoted in Wheeler and Zurek (1983: 83)

This movement in Heisenberg’s position has been largely forgotten, even by some physicists, Barad insists, in favour of what is now popularly thought of as the uncertainty principle. Both Bohr and Heisenberg are contributors to what is broadly known as the Copenhagen interpretation quantum physics, although, as Barad points out, this comprises a variety of contributors and positions superimposed one on the other: there is no one comprehensive, coherent and determinate position on the variety of theories which make up quantum physics.

Barad goes on to draw some quite profound philosophical conclusions from this insight, drawing on Bohr’s work and his theories about the natural world. For Bohr quantum physics problematizes the strict determinism of Newtonian physics which states that if it were possible to measure the initial conditions of any particle, i.e. the position and momentum and the complete set of forces operating on it, then it’s entire trajectory, past and future, is determined. A Newtonian world view also assumes observer independence passively gazing on reality, the Cartesian separation of knower from the known. In contrast, she argues, Bohr called into question both of these assumptions:

that the world is composed of individual objects with individually determinate boundaries and properties whose well defined values can be represented by universal concepts that have determinate meanings independent of the experimental practice, and… that measurements…can be properly assigned to the premeasurement properties of objects as separate from the agencies of observation. (Ibid: 107)

For Bohr there was no separating Nature from the arrangements to measure Nature, which he expressed in his own words in his book Atomic Theory and the Quantum of Action (1934/2011) thus:

the very recognition of the indivisibility of physical processes, symbolized by the quantum of action, has justified the old doubt as to the range of our ordinary forms of perception when applied to atomic phenomena. Since, in the observation of these phenomena, we cannot neglect the interaction between the object and the instrument of observation, the question of the possibilities of observation again comes to the foreground. (Bohr, 1934/2011: 93).

Despite criticisms that Bohr is an ‘anti-realist’ in his philosophical stance, i.e. that he is somehow denying the materiality of Nature or objective standards in science, Barad claims that Bohr is claiming no such thing. To chose one configuration of apparatus to measure the momentum of a particle gives replicable results, and to chose a different apparatus to measure its position gives other replicable results leaving the other quantities indeterminate. These measurements are objective in the sense that they are calculable and reproducible by other scientists. Bohr’s point is that it is the specific nature of the material arrangements that are responsible for producing some values to the exclusion of others. These arrangements evolve through a history of what she terms socio-material practices, which I take to mean the development of methods involving objects and human reflection on objects, which determine the development of any social practice, including of course science. In keeping with the title of her book, Meeting the Universe Halfway, Barad is claiming, based on Bohr’s thinking, that it is human practices which make the world intelligible to us. We engage with the world with the view of producing accurate descriptions of it of which we are part (part of the world and part of the descriptions) – this is very different from the idea that there is some idealized, human-independent reality which we can accurately and flawlessly represent. We form, and are formed by the natural world we seek to discover. Our consciousness, our methods, the tools we bring to bear and the objects we attempt to study are all part of one mutually constitutive phenomenon which is non-separable and emerges in the intra-action. Barad borrows another concept from physics which I don’t have time to explore thoroughly here to describe this phenomenon, that of entanglement. We and the world we live in are entangled. This is what Barad calls her agential realist position: the world becomes real to us through our intra-action with it:

Our (intra)actions matter – each one reconfigures the world in its becoming – and yet they never leave us; they are sedimented in our becoming, they become us. And yet even in our becoming there is no ‘I’ separate from the intra-active becoming of the world. Causality is an entangled affair… (2007: 394).

Barad’s understanding of Bohrian physics, depending as it does on the importance of the inseparability of a generative paradox of self and others and self and the world, seems to me to be very close to the pragmatism of Mead and Dewey, and the process sociology of Elias, which we have explored elsewhere on this blog. At the heart of this paradox is, in Dewey’s terms (1925/1997), the object and the experience of the object, which for the pragmatist philosophers were inseparable.


Barad, K. (2007) Meeting the Universe Halfway: Qunatum Physics and the Entanglement of Matter and Meaning, Durham: Duke University Press.

Bohr, N. (1934/2011) Atomic Theory and the Description of Nature, Cambridge: Cambridge University Press.

Dewey, J. (1925/1997) Experience and Nature, New York: Open Court Publishing.

Wheeler, J.A. and Zurek, W. H. (1983) (eds) Quantum Theory and Measurement, Princeton NJ: Princeton University Press.


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