Our language is inadequate to describe quantum reality
This is the fifth in a series of articles exploring the birth of quantum physics.
“Heaven knows what seeming nonsense the truth may not reveal tomorrow.”
This is how the great mathematician and philosopher Alfred North Whitehead expressed his frustration with the onslaught of weirdness coming from emerging quantum physics. He wrote this in 1925, just as things were getting really weird. By then, light had been shown to be both a particle and a wave, and Niels Bohr had introduced a strange model of the atom that showed how electrons were stuck in their orbits. They could only jump from one orbit to another either by emitting photons to go to a lower orbit or absorbing them to go to a higher orbit. Photons, in turn, were particles of light that Einstein assumed existed in 1905. Electrons and light danced to a very unique tune.
When Whitehead spoke, the wave-particle duality of light had just been extended to matter. In trying to understand Bohr’s atom, Louis De Broglie proposed in 1924 that electrons were also waves and particles, and that they fit into their atomic orbits like standing waves—the kind you get by vibrating a single-ended string addicted. Everything ripples, then, although the ripple of objects quickly becomes less noticeable as size increases. For electrons this ripple is crucial. It’s much less important than, say, a baseball.
Two fundamental aspects of quantum theory emerge from this discussion, and they are fundamentally different from traditional classical reasoning.
First, the images we construct in our minds when we try to visualize light or particles of matter are not adequate. Language itself struggles to handle quantum reality, as it is limited to verbalizations of those mental images. As the great German physicist Werner Heisenberg wrote, “We wish to speak somehow of the structure of atoms and not just of ‘facts’… But we cannot speak of atoms in ordinary language.”
Second, the observer is no longer a passive player in describing natural phenomena. If light and matter behave as particles or waves depending on how we set up the experiment, then we cannot separate the observer from the observed.
In the quantum world, the observer plays a crucial role in determining the physical nature of what is being observed. The notion of an objective reality, existing independently of an observer – given in classical physics and even in the theory of relativity – is lost. To a certain extent that is debatable; the world out there, at least within the realm of the very small, is what we choose it to be. Richard Feynman said it best:
“Things on a very small scale behave like nothing of which you have any direct experience. They don’t behave like waves, they don’t behave like particles, they don’t behave like clouds, or billiard balls, or weights on springs, or anything you’ve ever seen.”
Given the strange nature of the quantum world, progress could only be made through radically new approaches. In the space of two years in the 1920s, a whole new quantum theory was invented. This was quantum mechanics, which could describe the behavior of atoms and their transitions without invoking classical pictures like billiard balls and miniature solar systems. In 1925, Heisenberg produced “matrix mechanics,” a completely new way of describing physical phenomena.
Heisenberg’s construct was a brilliant liberation from the constraints imposed by classical inspired imagery. It did not include particles or orbits, only numbers describing electron transitions in atoms. Unfortunately, it was also very difficult to calculate with – even for the simplest atom, hydrogen. Enter another brilliant young physicist. (There were many of them in those days, all in their 20s and under Bohr’s tutelage.) The Austrian Wolfgang Pauli showed how matrix mechanics could be used to obtain the same results as Bohr’s model— it for the hydrogen atom. In other words, the quantum world required a way of description completely foreign to our everyday intuition.
The only certainty is uncertainty
In 1927, Heisenberg followed his new mechanics with a breakthrough in the nature of quantum physics, further distancing it from classical physics. This is the famous Uncertainty Principle. It asserts that we cannot know the values of some pairs of physical variables (such as position and velocity, or rather, momentum) with arbitrary precision. If we try to improve our measure of either, the other becomes more imprecise. Note that this limitation is not due to the act of observation, as is sometimes claimed. Heisenberg, trying to create an image to explain the mathematics of the Uncertainty Principle, claimed that if we, say, shine light on an object to see where it is, the light itself will push it away and its position will inaccurate life. That is, the act of observation interferes with what is observed.
Although this is true, it is not the origin of quantum uncertainty. Uncertainty is built into the nature of quantum systems, an expression of the elusive wave-particle duality. The smaller the object – that is, the more localized it is in space – the greater the uncertainty in its moment.
Again, the point here is to explain in words a behavior about which we have no intuition. However, the math is very clear and effective. In the world of many little ones, everything is unclear. We cannot ascribe forms to objects in that world, as we are accustomed to do to the world around us. The values of the physical quantities of these objects—values such as position, momentum, or energy—are not known beyond a level dictated by the Heisenberg relation.
Knowability, understood here as the possibility of having absolute knowledge of something, becomes more difficult than abstraction in the quantum world. It becomes impossible. For those interested, Heisenberg’s expression for the position and momentum of an object is ∆x ∆p ≥ h/4π, where ∆x and ∆p are the standard deviations of position x and momentum p, and h is Planck’s constant. If you try to decrease ∆x, that is, increase your knowledge of where the object is in space, you decrease your knowledge of its momentum. (In objects moving slowly relative to light, momentum is just mv, mass times velocity.)
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Quantum uncertainty was a devastating blow to those who believed that science could provide a deterministic description of the world: that action A causes reaction B. Planck, Einstein and de Broglie were incredulous. So was Schrödinger, the hero of the wave description of quantum physics, which we will cover next week. Can nature be so absurd? Ultimately, Heisenberg’s relation was telling the world that even if you knew the initial position and momentum of an object with infinite precision, you would not be able to predict its future behavior. Determinism, the cornerstone of the classical worldview of mechanics, of planets revolving around stars, of objects falling predictably to earth, of light waves propagating through space and reflecting off surfaces, had to be abandoned in favor of a description probabilistic reality.
This is where the real fun begins. It’s when the worldviews of giants like Einstein and Bohr clash between new holdings of uncertainty about the nature of reality. About a century ago, the world, or at least our understanding of it, became something else entirely. And the quantum revolution had just begun.