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the moving electron in space and time. Under his new criteria, to observe the path involves measuring the electron's position at each successive point. However, hitting the electron with a gamma ray photon in the act of measuring its position disturbs it, therefore its future trajectory cannot be predicted with certainty. In the case of an atomic electron 'orbiting' a nucleus, a gamma ray photon is energetic enough to knock it out of the atom, and only one point in its 'orbit' is measured and therefore known. Since the uncertainty principle forbids an exact measurement of both the position and velocity that define the path of an electron or its orbit in an atom, there simply is no path or orbit. The only thing that is known for certain, says Heisenberg, is one point along the path, and 'therefore here the word "path" has no definable meaning'.47 It is measurement that defines what is being measured.

There is no way of knowing, argued Heisenberg, what happens between two consecutive measurements: 'It is of course tempting to say that the electron must have been somewhere between the two observations and that therefore the electron must have described some kind of path or orbit even if it may be impossible to know which path.'48 Tempting or not, he maintained that the classical notion of an electron's trajectory being a continuous, unbroken path through space is unjustified. An electron track observed in a cloud chamber only 'looks' like a path, but is really nothing more than a series of water droplets left in its wake.

Heisenberg was desperately trying to understand the sort of questions that it was possible to answer experimentally after his discovery of the uncertainty principle. It was an unspoken basic tenet of classical physics that a moving object possessed both a precise location in space at a given time and a precise momentum, irrespective of whether it was measured or not. From the fact that the position and momentum of an electron cannot be measured with absolute accuracy at the same time, Heisenberg asserted that the electron does not possesses precise values of 'position' and 'momentum' simultaneously. To talk as if it did, or that it has a 'trajectory', is meaningless. To speculate about the nature of reality that lies beyond the realm of observation and measurement is pointless.

In later years, Heisenberg repeatedly chose to highlight the moment he remembered his talk with Einstein in Berlin as the crucial juncture on his journey to the uncertainty principle. Yet as he travelled the road to discovery that ended in the depths of a winter's night in Copenhagen, others had walked parts of the route with him. His most influential and valued companion was not Bohr, but Wolfgang Pauli.

As Schrödinger, Bohr and Heisenberg were locked in debate in Copenhagen in October 1926, Pauli was in Hamburg quietly analysing the collision of two electrons. He discovered, aided by Born's probabilistic interpretation, what he described in a letter to Heisenberg as a 'dark point'. Pauli had found that when electrons collide their respective momenta 'must be taken as controlled' and their positions 'uncontrolled'.49 A probable change in momentum was accompanied by a simultaneous but indeterminable change in position. He had found that one could not 'ask simultaneously' about momentum (q) and position (p).50 'One can see the world with the p-eye and one can view it with the q-eye,' Pauli stressed, 'but if one opens both eyes together, then one goes astray.'51 Pauli took it no further, but his 'dark point' lurked in the back of Heisenberg's mind as he and Bohr grappled with the problem of interpretation and wave-particle duality in the months before the discovery of the uncertainty principle.

On 23 February 1927, Heisenberg wrote a fourteen-page letter to Pauli summarising his work on the uncertainty principle. He relied on the critical judgement of the Viennese 'Wrath of God' more than most. 'Day is dawning in quantum theory', replied Pauli.52 Any lingering doubts vanished and, by 9 March, Heisenberg had turned the contents