Conceptions of Reality — Einstein and Bohr (3)

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Continued from here.

Einstein had proposed a contrivance by which to measure the speed of an atomic particle. Bohr riposted that clock are affected by gravity. hence, one could not hope to measure this property of an atomic particle without changing it in the very act of ascertainment.

Einstein was appalled by the idea that a complete picture of nature — a synopsis of all features of the phenomena of the universe — should not be within our reach.

It is actually quite astounding that Einstein could not accept an idea as simple and plausible as the inevitable selectivity of our knowledge of the universe. 

Even over great distances, atomic particles remain connected to one another — this is a core insight of Bohr's physics. Concepts, he thought, like space and time would not work on the atomic scale. The difficulty was, of course, that Bohr could not follow up this "destructive" insight with something workable to replace these categories. At the same time, Einstein was very attached to space-time and unwilling to give up these concepts easily.

In 1905, Einstein had shown that light could be conceived in terms of both waves and particles (quanta or photons). And this duplex nature holds true of atoms as well.  So, to clarify the situation, it was desirable to account for these phenomena more distinctly. Efforts were made to measure the properties of the atom with greater reliability. But it turned out: observation will inevitably disturb the atomic particle.

Bohr discovered complementarity, i. e. the fact that the form in which atomic particles appear (waves or particles) is affected by interaction with different kinds of measuring apparatuses.

Later crucial experiments, notably by Alain Aspect, provided empirical support for Bohr's hypothesis that the act of observation affects the observed atomic particles.

Einstein hoped that "a mouse could not affect the universe simply by looking at it" and that a crucial experiment could show that local causality determined observational outcomes.  But he was proven wrong. As John Wheeler explains a Bohrian phenomenon becomes a Bohrian phenomenon "when the dragon bites", not before it. The dragon being a nondescript, premature, undeveloped set of conditions awaiting closure. To the extent that it is accessible to us, reality is dependent on observation; to us, it is constituted by it.

The road to the thing in itself, to the Kantian Ding an sich, is barred to the human mind.