This is the fourth article in a four-part series on quantum entanglement. In the first, we discussed the basics of quantum entanglement. We then discussed how quantum entanglement can be used practically in communications and sensing. In this article, we take a look at the limits of quantum entanglement, and how entanglement on the large scale might even challenge our very basis of reality.
We can all agree that quantum entanglement is weird. We don’t worry too much about it, though, beyond some of its more practical applications. After all, the phenomenon plays out on scales that are vastly smaller than our everyday experiences. But perhaps quantum mechanics and entanglement are not limited to the ultra-small. Scientists have shown that macroscopic (albeit small) objects can be placed in entanglement. It begs the question: Is there a size limit for quantum entanglement? Carrying the idea further, could a person become entangled, along with their consciousness?
Asking these questions not only lets us probe the limits of quantum mechanics, but it could also lead us to a unified theory of physics — one that works equally well for anything from electrons to planets.
For the past five years or so, physicists have been trying to put larger objects into entangled states. These are not just single particles; they are, rather, collections of thousands or even billions of atoms.
In 2021, two independent groups of physicists, one at the Aalto University in Finland and another at the University of New South Wales in Australia, were able to put two small “drums” into entanglement. These drums were only 10 microns across — small, but nevertheless macroscopic. For their efforts, the teams won the Physics World Breakthrough of the Year.
Scientists at the National Institute of Standards and Technology were able to directly observe entanglement between macroscopic drum systems. And a group from the Niels Bohr Institute at the University of Copenhagen put two different macroscopic objects in quantum entanglement with one another: A drum a few millimeters long was entangled with a cloud containing a billion cesium atoms.
Although these objects are still very small, they contain large collections of atoms. Systems with a large number of particles lead to a more complicated entanglement. They also illustrate how entanglement can move to the macroscopic world, and in doing so, they push us to ask: Is there a limit to how large an object can be that is placed in entanglement?
There may not be a theoretical limit, although as the objects get larger, the role of gravity, which affects their wave function, grows. It is in any event an interesting question, one that leads us into the realm of the metaphysical. For example, can people — consciousness and all — become entangled?
Nobel Prize-winning physicist Eugene Wigner pondered the role of consciousness in quantum physics in the early 1960s. At the time, many physicists did not think there was anything special about consciousness or the human mind. But Wigner disagreed. He looked at quantum mechanics and argued that consciousness was required in order for a wave function to collapse — that is, for anything to be in a specific state.
To illustrate this, he came up with the following thought experiment, often referred to as Wigner’s Friend.
Let’s say we have a scientist, named Debbie, in an isolated lab. Debbie measures a system in which, say, the spin of an electron either can be up or down.
Outside of her sealed-off lab, another scientist, Bob, does not know the measurement Debbie has made. From his perspective, the electron’s wave function has not collapsed — it is still in a superposition of up and down. Similar to Schrödinger’s Cat, from Bob’s perspective, Debbie has both made an observation of spin up and spin down. Only when he opens up the laboratory door, and Debbie tells him the measurement she made, does he see the wave function collapse.
So when does the wave function collapse: when Debbie makes her observation, or when Bob does? Is there one objective truth in science? If so, the observations that Debbie and Bob make should agree. But if two observers see different things, the foundations of our science are called into question.
If this all seems ridiculous, that was precisely Wigner’s point. Consciousness changes things, he argued. It is special. Some people argue that resolving Wigner’s Paradox is essential for a complete understanding of quantum mechanics, including if it can be reconciled with the macroscopic world.
Quantum mechanics and the nature of reality
In 2020, scientists at the University of Brisbane in Australia expanded upon Wigner’s Paradox to include quantum entanglement. Not only that, they actually put it to the test. Their experiment asked the question: Can observers agree upon one “truth”?
Their experiment goes something like this: Two scientists in two enclosed labs — let’s call them Charlie and Debbie — measure a pair of entangled photons. No one but Charlie and Debbie now knows the result of this experiment. Outside of the lab, there is another pair of “super-observers,” Alice and Bob. From their perspective, the photons are still in a superposition of states. More than that, Charlie and Debbie themselves become entangled. This means, in essence, that Charlie and Debbie are entangled with their particles, and thus entangled with each other. Therefore, whenever Charlie makes an observation, Debbie will make the same observation, and vice versa.
Now Alice and Bob randomly choose to either open the door to their friends’ labs and ask them what they saw, or perform some other experiment.
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Let’s pause and think about what we know, or at least what we think we know, about the real world. First, if Charlie and Debbie make an observation, we assume that it points to one truth. In other words, what they saw really happened. Second, Alice and Bob have the freedom of choice to open the door and ask Charlie and Debbie what they saw or perform another experiment. And finally, the choice that they make shouldn’t affect the results that Charlie and Debbie already saw. In the macroscopic world, all of these statements seem like they should be true.
When they actually performed this experiment, the researchers didn’t use people but “simple observers” — entangled photons that have both an up and down polarization until they are observed. The observation in this experiment happens when the photon chooses one of two paths, depending on its polarization. When that choice is made the photon is, in essence, observed. This choice of path plays the role of Charlie and Debbie’s observation in the experiment. Photon detector measurements play the role of the “super-observers,” Alice and Bob. They choose to either detect the photon (the equivalent of asking Charlie and Debbie what they saw) or not. In this way, the experiment makes its own measurements.
If what we believe to be right (based on our experiences in the macroscopic world) is indeed true, the experiment should show a certain amount of correlations between the paths. If quantum mechanics is correct, we would actually see more correlations between the results. In other words, our idea of reality — that there is a universal truth in observations, that we have freedom of choice, and that this choice cannot affect what happens in the past or at a distance — is not consistent with quantum mechanics.
So what did their experiment show? The number of correlations they saw was consistent with what quantum mechanics would predict.
Now, one might argue that this experiment only used “simple observers.” Things might change if we could perform an experiment where the observers were actual, conscious people. But then ask yourself: Why? Why would consciousness change the results of the experiment? What is so special about consciousness?
Is your mind blown yet? It should be.
We might never reach the stage where we could perform such an experiment, but thinking about it raises several interesting questions. Why is what we believe about how the world works inconsistent with quantum mechanics? Is there an objective reality, even on the macroscopic scale? Or is what you see different than what I see? Do we have a choice in what we do?
At least one thing is for sure: We are not seeing the whole picture. Maybe our understanding of quantum mechanics is incomplete, or maybe something changes when we scale it to the macroscopic world. But perhaps our role as conscious observers of the world around us is, indeed, unique.