If you’ve seen the recent trailer for the forthcoming installment of the Marvel Cinematic Universe titled *The Marvels*—which brings together three of Marvel’s most prominent heroes, Captain Marvel, Ms. Marvel, and Monica Rambeau—you might have heard a brief mention of “entanglement.”

Describing the initial predicament of the film, Rambeau, played by Teyonah Parris, describes the three heroes’ powers as being “entangled,” with them exchanging places seemingly when they attempt to use them. This could be a throwaway line if it weren’t for the fact that Rambeau’s and Ms. Marvel’s abilities are light-based, and the quantum phenomenon of entanglement is perhaps best demonstrated with photons, or particles of light.

As such, the mention of entanglement links the plot of the movie to one of the most counterintuitive and seemingly mysterious aspects of quantum physics.

Upon its inception, entanglement was so troubling to physicists that Albert Einstein once described it to fellow physicist Max Born as “spukhafte Fernwirkungen” or, in English, “spooky actions at a distance.”

With no superheroes to seek answers from, *Popular Mechanics *turned to the next best thing: Chad Orzel, an associate professor in the Department of Physics and Astronomy at Union College in Schenectady, New York, and author of the book *How to Teach Quantum Physics to Your Dog**, *to explain what entanglement is and why it has been so troubling.

## What is quantum entanglement, and why is it so “spooky?”

To discover why entanglement was concerning to physicists, it is necessary to first establish what it is.

“The idea of entanglement is that you can have these quantum systems that can be separated into two places, say two particles, and they have some joint property about them,” Orzel said. “There’s also this thing in quantum mechanics that says those states should be indeterminate until you actually measure them.”

To consider this, think of a quantum property called “spin,” which you can consider as a magnetic version of angular momentum. If one particle in an entangled pairing spins “up,” then the other must spin “down.” But, here is the catch: When you send these particles out from your entangling experiment, neither is spun up or down—their spin isn’t yet determined.

It takes a measurement or an interaction with, say, another physical system like a magnetic field to make one particle assume a spin value. And when that happens, the partner particle *instantly *assumes a corresponding value. That’s not so scary, right? Wrong. It is very scary indeed, or at least it was around 100 years ago.

You see, this instant action happens at a distance—*any* distance. It happens instantaneously even if the particles are located at opposite ends of the universe, separated by a gulf of around 93 billion light years.

“This correlation between states of a system doesn’t seem to respect the distance between particles. No matter how far apart the particles are, doing something to determine the state of one also determines the state of the other,” Orzel explains. “And that’s an idea that really confounds our classical picture of how the world ought to work.”

Einstein in particular was concerned because a theory he posited in 1905 called “special relativity” explicitly hinges on the fact that the universe has a speed limit for particles with mass, which limits how fast information can be exchanged. This limit is the speed of light in a vacuum, around 300 million meters per second (3 x 10⁸ m/s or 3 followed by 8 zeroes m/s).

So by this rule, if the first particle assumes a spin value of “up,” the second particle on the opposite side of the universe shouldn’t know to make itself “down” until 93 billion years later. And that isn’t even taking into account the fact that the universe is rapidly expanding.

## Local realism goes out the window

To Einstein, the message that entanglement conveyed was clear: he believed it demonstrated that quantum physics was an incomplete theory. The great physicist put forward the idea that there must be so-called “hidden variables” within quantum systems. These hidden variables would be present, Einstein and other physicists thought, when the entangled system was created and would determine what state the system would adopt, thus removing quantum indeterminacy and the troubling randomness of quantum systems.

This preserved the idea of “local realism,” which had been at the heart of classical or “deterministic” physics since its inception. Very simply, local realism is made of two principles: “real” means objects have definite properties that are independent of measurement, and “local” means only the immediate surroundings of an object can influence it — mixing in special relativity means no influence proceeds faster than light.

The back and forth on this issue proceeded for around 30 years, with many quantum physicists adopting a “shut up and calculate” approach to quantum phenomena that, for the most part, put to one side the more philosophical implications of the theory. This was until the mid-1960s when CERN physicist John Bell started to think deeply about what it means to take measurements of quantum systems and to find them correlated in certain ways.

“Bell worked out a situation in which you could test the kind of deterministic theory that Einstein would have liked to have with definite states,” Orzel said. “Bell proved this theorem, now called ‘Bell’s theorem,’ suggesting there’s a limit on what results you can possibly get with states that have definite variables.”

Over the next three decades after this, physicists would set about trying to confirm or disprove Bell’s theorem experimentally by violating so-called Bell inequalities, which wouldn’t be possible if hidden variables lurked within a system. Foremost among these physicists were John Clauser, Alain Aspect, and Anton Zeilinger. In 2022, the trio won the Nobel Prize in Physics for establishing the violation of Bell’s inequalities, showing entanglement does indeed defy local realism.

“That convinced everybody that this weird quantum element where measurements ‘here’ determine the outcomes of measurements ‘there’ seems to be absolutely true,” Orzel said. “That was both weird and interesting, and it set physicists to really figure out what the consequences of that are. This becomes the foundation of the quantum information field, which really takes off in an experimental sense, starting in the 80s.”

It took from 1935 to the 1990s for entanglement to “go mainstream,” but after this, potential practical applications of this once “spooky” aspect of nature began to present themselves.

## How useful is quantum entanglement?

When *The Marvels* releases in November 2023, its titular heroes will likely use the phenomenon of entanglement to counter a cosmic threat, but here on Earth, in the real world, entanglement can be used to address more mundane but important issues like privacy and confidentiality. In fact, entanglement could be just as useful to super-spy Nicholas J. Fury, played by Samuel L. Jackson in the Marvel cinematic universe, as it is to a cosmic hero like Captain Marvel.

Orzel describes how entanglement can be used in quantum cryptography to scramble and decode messages between a widely separated sender and receiver, canonically known as “Alice” and “Bob,” respectively.

“Alice and Bob have a set of entangled particles, and they can compare them by doing the measurements that John Bell envisioned,” Orzel said. “Alice gets a random string of ones and zeros [if, for example, an “up” spin value corresponds to a one and “down” corresponds to a zero], and she can confirm that Bob has a corresponding string of ones and zeros.”

With the application of some mathematical operations, this random string of numbers could be converted into a key unique to Alice and Bob. To see how this is effective, we can introduce what every good spy-thriller or Marvel flick needs: an antagonist. In this case, it isn’t Thanos, Kang, or Loki who is the threat but an interloper called “Eve,” for “eavesdropper.”

Eve attempts to intercept the passage of the quantum key between Alice and Bob so she can then also interpret the messages that pass between them. Eve is foiled, however, by the fact that measurement resolves the quantum state of the particles that Bob and Alice use. This means that when Alice and Bob check the random set of ones and zeros with each other, they see that they are no longer correlated. Therefore, they know their key has been intercepted, and they can discard it.

## The limits of quantum entanglement

Orzel pointed out, though, there is a significant limit to what can be transmitted this way. He says that because the state of quantum systems is completely random and indeterminate, Alice can’t choose in advance what “message” she will send to Bob.

Remember how Einstein suspected quantum mechanics was incomplete because information shouldn’t be able to transmit instantaneously? Well, the random nature of quantum mechanics means that the instantaneous transmission between particles when one is measured can’t transmit information in a usable way.

“Alice can do measurements, but she has no power to influence the outcome of the measurements,” Orzel added. “So she can measure this particle, and it will be either a zero or a one. And then Bob will correspondingly have either a one or a zero. But she can’t pick which number she wants it to be. There’s no way to use that to send a message.”

Entanglement is also challenging to use because it is so easily disturbed. Orzel gives the example of measuring entanglement in terms of photons of light that can be polarized in certain directions, with horizontal polarization being one and vertical polarization being zero.

“Entanglement is easily destroyed in the sense that if I’m sending a photon from one place to another, and it’s sent out with the presumption that you’re going to do vertical versus horizontal measurements, and something along the way rotates the polarization of that photon, which is not a hard thing do,” he says. “When you do the measurement at the other end, it will look like there’s no correlation. But it looks like there’s no correlation because you’re not doing the right measurements anymore.

“It’s a subtle thing that it’s not broken; the photons are not disentangled. It’s just they’re not entangled with the same set of measurements that you had at the beginning.”

Entanglement may not be as mysterious as it was in 1935, but it is certainly still counterintuitive compared to our everyday picture of the universe. With its active disregard for distance and its ability to kick-start instantaneous action, it’s little wonder it has been chosen as a cosmic MacGuffin to bring together superheroes from opposite ends of the Marvel Universe.

Robert Lea is a freelance science journalist focusing on space, astronomy, and physics. Rob’s articles have been published in *Newsweek*, *Space*, *Live Science*, *Astronomy* magazine and *New Scientist*. He lives in the North West of England with too many cats and comic books.