Things We May Never Know


The quest to understand the meaning of quantum mechanics began in earnest in 1927. Physicists fell into two camps. Heisenberg and his colleagues believed that the particle nature of electromagnetic waves and matter described in his matrix representation was paramount. Schrodinger’s followers argued that the physics of waves underlay quantum behaviour. Heisenberg had also shown that our understanding was fundamentally limited by his uncertainty principle. He believed that both the past and future were unknowable because of the intrinsic uncertainty of all the parameters describing a subatomic particle’s movement.

Another man tried to pull everything together. Bohr was the scientistwho had a decade earlier explained the quantum energy states of electrons in the hydrogen atom. When Heisenberg came up with his uncertainty principle in 1927 he was working in Copenhagen at Bohr’s institute. Bohr apparently returned from a skiing trip to find Heisenberg’s draft paper on his desk and a request to forward it to Albert Einstein. Bohr was intrigued by the idea, but complained to Einstein that Heisenberg’s imagined test was flawed as it didn’t consider the wave properties of matter. Heisenberg added a correction that included the scattering of light waves, but his conclusion still held firm. Uncertainties were inherent in quantum mechanics.

In Bohr’s view, wave and particle aspects of a real entity were complementary characteristics. They are two sides of the same coin, In the same way that some illusions trick our eyes into seeing two different in a black and white pattern. The real electron, proton or neutron is neither one or not the other, but the composite of both. A given trait only appears when an experimenter intervenes and selects which aspect to measure. Light appears to behave like a photon or electromagnetic wave because that is the sign we are looking for. Because the experimenter disturbs the pristine system, Bohr argued, there are limits to what we know about nature. The act of observation generates the uncertainties that Heisenberg spotted. This line of reasoning became known as the ‘Copenhagen interpretation’ of quantum mechanics.

The uncertainty principle which states that one cannot measure both the position and momentum of any subatomic particle at the same time, is central, Bohr realized. Once one characteristic is measured precisely the other becomes less well known. Heisenberg believed that the uncertainty arose due to the mechanics of the measurement processof itself. To measure a quantity we must interact with it, such as by bouncing photons off a particle to detect its movement. That interaction changes the system, Heisenberg realized, making its subsequent state uncertain.

Bohr’s understanding was quite different. The observer is part of the system being measured, he argued. It doesn’t make sense to describe the subject without including the measuring device. How can we describe a particle’s motion by considering it alone if it is being bombarded with photons in order to track it? Even the word ‘observer’ is wrong, said Bohr, because it suggests an external entity. The act of observation is like a switch, which determines the system’s final state. Before that point we can only say that the system has some chance of being in some possible state. What happens when we make a measurement? Why does light passing through two slits interfere like waves one day, but switch to particle like behaviour the next if we try to catch the photon as it passes through one slit? According to Bohr, we choose in advance how it turns by deciding bow we would like to measure it.

Here Bohr turned to Schrodinger’s equation and his concept of the wave function, containing everything we can know about a particle. When an object’s character is fixed, say as a particle or a wave, by an act of observation we say that the wavefunction has ‘collapsed’. All the probabilities, bar one, vanish. Only the outcome lingers. So a beam of light’s wavefunction is a blend of two probabilities, whether it behaves as a save or a particle. When we detect the light, the wavefunction collapses to leave one form, not because it switches its behaviour but because light truly is both.

Heisenberg initially rejected Bohr’s picture. He clung to his original imagery of particles and energy jumps. The two fell out. Heisenberg apparently burst into tears at one point during an argument with Bohr. The stakes of the young man’s career were high. Matters improved later in 1927 when Heisenberg secured a job at the University of Leipzig. Bohr presented his complementarity idea to great acclaim at a conference in Italy and many physicists took it up. By October, Heisenberg and Max Born were talking of quantum mechanics as having been fully solved.

Not everyone agreed, notably Einstein and Schrodinger, who remained unconvinced by Bohr’s doctrine for the rest of their days. Einstein believed that particles could be measured precisely. The idea that real particles were governed by probabilities unsettled him. These would not be needed in a better theory, he argued. Quantum mechanics must be incomplete. Even today physicists struggle to comprehend the deep meaning of quantum mechanics. Some have tried to offer new explanations, although one has overturned Bohr’s. The Copenhagen view has stood the test of time because of its explanatory power.


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