How philosophy turned into physics and reality turned into information
The Nobel Prize in physics this year has been awarded "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science."
To understand what this means, and why this work is important, we need to understand how these experiments settled a long-running debate among physicists. And a key player in that debate was an Irish physicist named John Bell.
In the 1960s, Bell figured out how to translate a philosophical question about the nature of reality into a physical question that could be answered by science—and along the way broke down the distinction between what we know about the world and how the world really is.
Quantum entanglement
We know that quantum objects have properties we don't usually ascribe to the objects of our ordinary lives. Sometimes light is a wave, sometimes it's a particle. Our fridge never does this.
When attempting to explain this sort of unusual behavior, there are two broad types of explanation we can imagine. One possibility is that we perceive the quantum world clearly, just as it is, and it just so happens to be unusual. Another possibility is that the quantum world is just like the ordinary world we know and love, but our view of it is distorted, so we can't see quantum reality clearly, as it is.
In the early decades of the 20th century, physicists were divided about which explanation was right. Among those who thought the quantum world just is unusual were figures such as Werner Heisenberg and Niels Bohr. Among those who thought the quantum world must be just like the ordinary world, and our view of it is simply foggy, were Albert Einstein and Erwin Schrödinger.
At the heart of this division is an unusual prediction of quantum theory. According to the theory, the properties of certain quantum systems that interact remain dependent on each other—even when the systems have been moved a great distance apart.
In 1935, the same year he devised his famous thought experiment involving a cat trapped in a box, Schrödinger coined the term "entanglement" for this phenomenon. He argued it is absurd to believe the world works this way.
The problem with entanglement
If entangled quantum systems really remain connected even when they are separated by large distances, it would seem they are somehow communicating with each other instantaneously. But this sort of connection is not allowed, according to Einstein's theory of relativity. Einstein called this idea "spooky action at a distance."
Again in 1935, Einstein, along with two colleagues, devised a thought experiment that showed quantum mechanics can't be giving us the whole story on entanglement. They thought there must be something more to the world that we can't yet see.
But as time passed, the question of how to interpret quantum theory became an academic footnote. The question seemed too philosophical, and in the 1940s many of the brightest minds in quantum physics were busy using the theory for a very practical project: building the atomic bomb.
It wasn't until the 1960s, when Irish physicist John Bell turned his mind to the problem of entanglement, that the scientific community realized this seemingly philosophical question could have a tangible answer.
Bell's theorem
Using a simple entangled system, Bell extended Einstein's 1935 thought experiment. He showed there was no way the quantum description could be incomplete while prohibiting "spooky action at a distance" and still matching the predictions of quantum theory.
Not great news for Einstein, it seems. But this was not an instant win for his opponents.
This is because it was not evident in the 1960s whether the predictions of quantum theory were indeed correct. To really prove Bell's point, someone had to put this philosophical argument about reality, transformed into a real physical system, to an experimental test.
And this, of course, is where two of this year's Nobel laureates enter the story. First John Clauser, and then Alain Aspect, performed the experiments on Bell's proposed system that ultimately showed the predictions of quantum mechanics to be accurate. As a result, unless we accept "spooky action at a distance," there is no further account of entangled quantum systems that can describe the observed quantum world.
So, Einstein was wrong?
It is perhaps a surprise, but these advances in quantum theory appear to have shown Einstein to be wrong on this point. That is, it seems we do not have a foggy view of a quantum world that is just like our ordinary world.
But the idea that we perceive clearly an inherently unusual quantum world is likewise too simplistic. And this provides one of the key philosophical lessons of this episode in quantum physics.
It is no longer clear we can reasonably talk about the quantum world beyond our scientific description of it—that is, beyond the information we have about it.
As this year's third Nobel laureate, Anton Zeilinger, put it: "The distinction between reality and our knowledge of reality, between reality and information, cannot be made. There is no way to refer to reality without using the information we have about it."
This distinction, which we commonly assume to underpin our ordinary picture of the world, is now irretrievably blurry. And we have John Bell to thank.
What is quantum entanglement? A physicist explains the science of Einstein's 'spooky action at a distance' Provided by The Conversation
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What is quantum entanglement? A physicist explains the science of Einstein's 'spooky action at a distance'
The 2022 Nobel Prize in physics recognized three scientists who made groundbreaking contributions in understanding one of the most mysterious of all natural phenomena: quantum entanglement.
In the simplest terms, quantum entanglement means that aspects of one particle of an entangled pair depend on aspects of the other particle, no matter how far apart they are or what lies between them. These particles could be, for example, electrons or photons, and an aspect could be the state it is in, such as whether it is "spinning" in one direction or another.
The strange part of quantum entanglement is that when you measure something about one particle in an entangled pair, you immediately know something about the other particle, even if they are millions of light years apart. This odd connection between the two particles is instantaneous, seemingly breaking a fundamental law of the universe. Albert Einstein famously called the phenomenon "spooky action at a distance."
Having spent the better part of two decades conducting experiments rooted in quantum mechanics, I have come to accept its strangeness. Thanks to ever more precise and reliable instruments and the work of this year's Nobel winners, Alain Aspect, John Clauser and Anton Zeilinger, physicists now integrate quantum phenomena into their knowledge of the world with an exceptional degree of certainty.
However, even until the 1970s, researchers were still divided over whether quantum entanglement was a real phenomenon. And for good reasons—who would dare contradict the great Einstein, who himself doubted it? It took the development of new experimental technology and bold researchers to finally put this mystery to rest.
Existing in multiple states at once
To truly understand the spookiness of quantum entanglement, it is important to first understand quantum superposition. Quantum superposition is the idea that particles exist in multiple states at once. When a measurement is performed, it is as if the particle selects one of the states in the superposition.
For example, many particles have an attribute called spin that is measured either as "up" or "down" for a given orientation of the analyzer. But until you measure the spin of a particle, it simultaneously exists in a superposition of spin up and spin down.
There is a probability attached to each state, and it is possible to predict the average outcome from many measurements. The likelihood of a single measurement being up or down depends on these probabilities, but is itself unpredictable.
Though very weird, the mathematics and a vast number of experiments have shown that quantum mechanics correctly describes physical reality.
Two entangled particles
The spookiness of quantum entanglement emerges from the reality of quantum superposition, and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.
To create entangled particles you essentially break a system into two, where the sum of the parts is known. For example, you can split a particle with spin of zero into two particles that necessarily will have opposite spins so that their sum is zero.
In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published a paper that describes a thought experiment designed to illustrate a seeming absurdity of quantum entanglement that challenged a foundational law of the universe.
A simplified version of this thought experiment, attributed to David Bohm, considers the decay of a particle called the pi meson. When this particle decays, it produces an electron and a positron that have opposite spin and are moving away from each other. Therefore, if the electron spin is measured to be up, then the measured spin of the positron could only be down, and vice versa. This is true even if the particles are billions of miles apart.
This would be fine if the measurement of the electron spin were always up and the measured spin of the positron were always down. But because of quantum mechanics, the spin of each particle is both part up and part down until it is measured. Only when the measurement occurs does the quantum state of the spin "collapse" into either up or down—instantaneously collapsing the other particle into the opposite spin. This seems to suggest that the particles communicate with each other through some means that moves faster than the speed of light. But according to the laws of physics, nothing can travel faster than the speed of light. Surely the measured state of one particle cannot instantaneously determine the state of another particle at the far end of the universe?
Physicists, including Einstein, proposed a number of alternative interpretations of quantum entanglement in the 1930s. They theorized there was some unknown property—dubbed hidden variables—that determined the state of a particle before measurement. But at the time, physicists did not have the technology nor a definition of a clear measurement that could test whether quantum theory needed to be modified to include hidden variables.
Disproving a theory
It took until the 1960s before there were any clues to an answer. John Bell, a brilliant Irish physicist who did not live to receive the Nobel Prize, devised a scheme to test whether the notion of hidden variables made sense.
Bell produced an equation now known as Bell's inequality that is always correct—and only correct—for hidden variable theories, and not always for quantum mechanics. Thus, if Bell's equation was found not to be satisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement.
The experiments of the 2022 Nobel laureates, particularly those of Alain Aspect, were the first tests of the Bell inequality. The experiments used entangled photons, rather than pairs of an electron and a positron, as in many thought experiments. The results conclusively ruled out the existence of hidden variables, a mysterious attribute that would predetermine the states of entangled particles. Collectively, these and many follow-up experiments have vindicated quantum mechanics. Objects can be correlated over large distances in ways that physics before quantum mechanics can not explain.
Importantly, there is also no conflict with special relativity, which forbids faster-than-light communication. The fact that measurements over vast distances are correlated does not imply that information is transmitted between the particles. Two parties far apart performing measurements on entangled particles cannot use the phenomenon to pass along information faster than the speed of light.
Today, physicists continue to research quantum entanglement and investigate potential practical applications. Although quantum mechanics can predict the probability of a measurement with incredible accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.Quantum entanglement: the 'spooky' science behind physics Nobel
Provided by The Conversation
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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