Einstein's quantum ghost is here to stay

Einstein’s quantum ghost is here to stay

This is the eighth in a series of articles exploring the birth of quantum physics.

Scientists have worldviews. Not too surprising, given that they are human 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 someone’s true colors by seeing how they react to a threat. This threat can be of many different types, from breaking into your house to an intellectual threat against your belief system. In recent weeks, we’ve explored how quantum physics has changed the world, looking at its early history and the strange new world of unexpected laws and rules that dictate what happens at the level of molecules and smaller material components. Today, we look at how this new science affected the worldview of some of its own creators, notably Albert Einstein and Erwin Schrödinger. At stake for these physicists was nothing less than the true nature of reality.

Loss of meaning

In a December 1950 letter to Schrödinger, Einstein wrote:

“If one wants to regard quantum theory as final (in principle), then one must believe that a more complete description would be useless, since there would be no laws for it. If it were so, then physics could only claim the interest of businessmen and engineers; the whole thing would be a disaster.

Until the end of his life, Einstein could not resign himself to the new world view that came from quantum physics – that set of beliefs that basically told us that reality was only partially known to us humans, and that the very core of nature was hidden from our reasoning powers. of Werner Heisenberg The principle of uncertainty sealed 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. All of them are unknown.

Physicist Richard Feynman created a beautiful way to express this bizarre fact with his path integral approach to quantum physics. In Feynman’s formulation, to calculate the probability of a particle starting here and ending up there, you must sum up all the available paths it can take to that end. Each path is possible and each has a probability of being the one. But unlike a falling rock or a planet orbiting a star, we cannot know which path the particle is taking. The very notion of a path between two points loses its meaning.

Einstein would have none of it. For him, nature had to be rational, that is, it had to be subject to a description that made sense. By making sense, he meant that an object follows a simple causal behavior dictated by a deterministic evolution. He believed that quantum physics was missing something essential and that he had found that something would restore sanity to physics.

So in 1935, together with colleagues Boris Podolsky and Nathan Rosen – together they became known as EPR – Einstein 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 recognized that quantum physics worked because it could explain the results of experiments with great precision. Their problem was with completeness of the quantum description of the world.

They proposed an operational criterion to determine the elements of our perceived physical reality: it could be described only by those physical quantities that could be predicted with certainty (a probability of one) and without disturbing the system. That is, there should be a physical reality that is completely independent of how we verify it. For example, your height and weight are elements of physical reality. They 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 interference. You don’t gain or lose weight when your height is measured.

When quantum effects dominate, this clean independence is not possible for certain very important pairs of quantities, as expressed in Heisenberg’s uncertainty principle. The EPR rejected this. They could not accept that the act of measurement compromises the notion of observer-independent reality. The act of measuring creates the reality of a particle being at a particular location in space, according to quantum mechanics, but EPR found this idea absurd. What is real should not depend on who or what they are looking for, they insisted.

To illustrate their point, EPR considered a pair of identical particles, say A and B, moving at the same speed but in opposite directions. The physical properties of the particles were fixed when they interacted for a certain time before flying away from each other. Suppose a detector measures the position of particle A. Since the particles have the same velocities, we also know where particle B is. If a detector now measures the velocity of particle B at that point, we know both the position and the velocity. This seemed to conflict with Heisenberg’s Uncertainty Principle, since information was apparently obtained about a particle’s position and velocity simultaneously. Moreover, we know the property of a particle (the position of B) without observing it. According to the EPR definition, this property is part of physical reality, even though quantum physics insists that we could not know it before measuring it. Clearly, EPR argued, quantum mechanics must be an incomplete theory of physical reality. EPR concluded his article in the hope that a better (more complete) theory would restore the realism of physics.

Niels Bohr, the champion of the quantum-physics-is-weird-and-okay worldview, responded within six weeks. Bohr invoked his notion of complementarity, which states that in the quantum world we cannot separate what is detected by the detector. The interaction of the particle with the detector induces an uncertainty in the particle but also in the detector, since the two are correlated. Therefore, the act of measurement establishes the measured property of the particle in unpredictable ways. Before the measurement, we cannot say that the particle had any property. That being the case, we cannot even attribute physical reality to this property in the sense defined by EPR.

As Bohr writes,

“The finite interaction between the object and the measuring agents implies the need for the definitive abandonment of the classical ideal of causality and a radical revision of our attitude towards the problem of physical reality. Essentially, a particle only acquires a concrete property, such as position or momentum, due to its interaction with a measuring device. Before the measurement, we cannot say anything about that particle. So we can’t say anything about the physical reality of the particle before it interacts with something.”

Einstein’s quantum ghost

Einstein wanted a reality that could be known down to the quantum level. Bohr insisted that there was no reason to expect this. Why should the world of the very small obey similar principles to the world we are used to? Schrödinger was upset too, though. In response to Bohr’s paper, he wrote his own in which he introduced his famous cat, which we will meet shortly.

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The missing piece that connects the dots here is the notion of tangle, a key concept in quantum physics. It’s a pretty hard idea to swallow, stating that two or more objects can be connected, or entangled, in ways that defy space and time. In this case, knowing something about one item in a pair will tell us something about the other, even before anyone measures it. And this happens instantaneously, or at least faster than light could travel between the two. This was what Einstein called “spooky action at a distance.” We can see where he was coming from. He spectacularly exorcised action at a distance from Newtonian gravity, showing that the pull of gravity could be explained as the result of a curved space-time geometry around a massive object. Einstein wanted to do the same for quantum physics. But the quantum ghost, we now know, is here to stay. We’ll see why next time.

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