These are the eight articles in a series of articles that explore the birth of quantum physics.
Scientists have worldviews. This is not too surprising, given that they are people, and humans have worldviews. You have a way of thinking about politics, religion, science, and the future, and that way of thinking informs how you move in the world and the choices you make.
It is often said that you know a person’s true colors by seeing how they respond to a threat. This threat can be of various types, from an intrusion into your home, to an intellectual threat against your belief system. In the past weeks, we’ve been discovering how quantum physics changed the world, looking back at its early history and the strange new world of unexpected laws and rules that dictate what happens at the level of particles and smaller physical components. Today, we look at how this new science affected the worldview of some of its makers, particularly Albert Einstein and Erwin Schrödinger. At stake were these physicists nothing less than the true nature of reality.
loss of meaning
In a letter to Schrödinger from December 1950, Einstein wrote:
“If one wants to consider quantum theory definitive (in principle), then one must think that a more complete description would be useless because there would be no laws to it. If that were the case, then physics could only claim the interest of shopkeepers and engineers. Everything will be a miserable mess.”
Until the end of his life, Einstein could not resign himself to the new vision of the world coming from quantum physics—that set of beliefs that told us, in essence, that reality was only partially known to us humans, and that the essence of nature is hidden from our power of reasoning. Werner Heisenberg uncertainty principle Seal the fate of deterministic physics. Unlike a falling stone, or a planet orbiting a star, in the quantum world we can only know the beginning and the end of a story. Everything in between is unknown.
Physicist Richard Feynman devised a beautiful way of expressing this strange fact through his theory A pathway-integrated approach to quantum physics. In Feynman’s formulation, to calculate the probability of a particle starting here and ending there, you must sum all the available paths it can take to that end. Every path is possible, and each has the probability of being the first. But unlike a falling rock or a planet orbiting a star, we can’t know which path a particle takes. The idea of a path between two points loses its meaning.
Einstein had none of that. For him, nature must be rational, in the sense that it must be subject to a logical description. By logic, he meant that an organism follows a simple causal behavior dictated by deterministic evolution. He thought quantum physics was missing something fundamental, and he figured something would bring reason back into physics.
So, in 1935, Einstein, along with colleagues Boris Podolsky and Nathan Rosen—together they became known as EPR—published a paper attempting to expose the absurdities of quantum mechanics. The title says it all: “Can the quantum mechanical description of physical reality be considered complete?”
EPR acknowledged that quantum physics worked, because it could explain the results of experiments with great precision. Their problem was with completion For the quantitative description of the world.
They proposed an operational criterion for defining the elements of our perceived physical reality: it can only be described by those physical quantities that can be predicted with certainty (one probability), and without disturbing the system. In the sense that there must be a physical reality that is completely independent of how it is investigated. For example, your height and weight are components of physical reality. It can be measured with certainty, at least within the accuracy of the measuring device. They can also be measured simultaneously, at least in principle, without any mutual overlap. You do not gain or lose weight when measuring your height.
When quantum effects dominate, such clean independence is not possible for some very important pairs of quantities, as shown by Heisenberg’s uncertainty principle. EPR rejected this. They could not accept that the act of measurement harms the concept of a reality independent of the observer. The act of measurement creates the fact that a particle is located at a specific location in space, according to quantum mechanics, but EPR found this idea absurd. What is real, they insisted, must not depend on who or what is searching.
To make their point, the EPR considered a pair of identical particles, such as A and B, moving at the same speed but in opposite directions. The physical properties of particles are fixed when they interact for a certain time before they fly away from each other. Suppose the detector measures the position of particle A. Since the particles have the same speeds, we also know where particle b is. If the detector is now measuring the velocity of particle B at that spot, we know its location and velocity. This seems to contradict Heisenberg’s uncertainty principle, since information about a particle’s position and velocity is apparently obtained simultaneously. Moreover, we know a particle’s property (B’s position) without noticing it. According to the EPR definition, this property is part of physical reality even if quantum physics insists we cannot know it before we measure it. Obviously, as the EPR argued, quantum mechanics must be an incomplete theory of physical reality. EPR closed their article in hopes that a better (more complete) theory would restore realism to physics.
Niels Bohr, champion of the worldview that quantum physics is weird and that’s okay, responded within six weeks. Bohr invoked his concept of integration, confirming that in the quantum realm we cannot separate what is detected from the detector. The interaction of the particle with the detector introduces uncertainty in the particle but also in the detector, since the two are correlated. The measurement process, then, determines the measured property of a particle in unpredictable ways. Before measuring, we cannot say that a particle has any property at all. In this case, we also cannot attribute physical reality to this property in the sense defined by the EPR.
As Bohr writes,
The limited interaction between the subject and the measuring agencies necessitates a final abandonment of the classical idealism of causality and a radical revision of our attitude to the problem of physical reality. In essence, a particle only acquires a specific property such as position or momentum due to its interaction with a measuring device. Before measuring, we cannot say anything about this particle. Therefore, we cannot say anything about the physical reality of a particle before it interacts with something.”
Quantum Einstein’s Ghost
Einstein wanted a truth that could be known all the way down to the quantum level. There was no reason to expect this, Bohr insisted. Why should the world of the very young obey similar principles to the world we are used to? Schrödinger was also upset. In response to Bohr’s paper, he wrote his own letter where he introduced his famous cat, whom we will meet soon.
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The missing piece connecting the dots here is a clue tangleIt is a key concept in quantum physics. Very hard to swallow, it states that two or more objects can be connected, or entangled, in ways that defy space and time. In this case, knowing something about one element of the pair will tell us something about the other, even before anyone measures it. And this happens instantly, or at least faster than light travels between the two. This was what Einstein called “frightening action at a distance.” We can see where it came from. He performed a spectacular exorcism at a distance of Newtonian gravity, showing that gravitational pull can be explained as a result of the curved geometry of space-time around a massive object. Einstein wanted to do the same for quantum physics. But the quantum specter, as we now know it, is here to stay. We’ll see why next time.