"Do you guys just put the word 'quantum' in front of everything?"
– Scott Lang, Ant-Man and the Wasp
My first exposure to the word "quantum," like many people who were alive for the entirety of the 1990s, was from the title of the show Quantum Leap. That phrase is often used as a synonym for a "huge advancement" of some sort, despite quantum actually referring to the tiniest possible things in the known universe.
Luckily for science nerds such as myself, this mistake was not made in the MCU's 20th entry. Instead, we get a bunch of other quantums: Hank Pym (Michael Douglas) and Hope van Dyne (Evangeline Lilly) construct a "Quantum Tunnel," Scott Lang (Paul Rudd) and Janet van Dyne (Michelle Pfeiffer) become "quantum entangled," and Ava Starr, aka Ghost (Hannah John-Kamen) "quantum phases" through solid matter.
Quantum Mechanics — the physics that happens on subatomic scales — is wildly unlike our idea of reality. Its rules might seem like they're straight out of a comic book, but no. The universe does actually work like that.
At least, it works like that for the teeny tiny.
While the Quantum Tunnel is a piece of fiction, quantum tunneling is very much not. It's totally possible for a subatomic particle to use the quirks of QM to "tunnel" through solid matter.
It all comes down to the fact that subatomic particles, like electrons, are actually waves, too. Light, you might recall, is also both a particle and a wave, but matter-waves are a little (or a lot) different. The important thing to take away is that matter has a wave component to its existence.
It's these matter-waves that restrict where electrons are allowed to hang out while orbiting an atom's nucleus. It's basically the most stringent restraining order in the entire universe; like, a stalker can't just not get within 500 yards of you, they can only stay exactly 500, or 900, or 1,002 (etc.) yards away.
This is why atoms are almost entirely empty space. But you might notice that, after running head-first into the nearest wall, it very much does not feel like it.
Electrons are also governed by a rule (the "Pauli Exclusion Principle") which states they aren't allowed to exist in the exact same place at the same time. Electrons that get too close together exert a lot of pressure keeping each other at a safe distance.
While there is also a little bit of electrostatic repulsion caused by electrons all being the same charge, the P.E.P. is why you cannot throw a sheet over your head and do some proper house haunting.
But here's where things get weird... er. On a subatomic scale, you can never know both where a particle is and where it's going; the more accurately you measure its position, the less accurately you know its trajectory (and vice versa). So we don't just have a matter-wave on our hands, we have a "probability density wave".
In other words, the best we can do is describe where a particle probably is at any given moment. Hypothetical-cat murderer Erwin Schrӧdinger provided us the math for that. So, a particle always has a non-zero — though super duper tiny — chance of revealing itself where it 'shouldn't' be able to get to.
Like on the other side of a solid wall.
This is quantum tunneling. It has nothing to do with actual tunneling. Sometimes scientists are lousy at naming things.
The faster a particle moves (i.e. the more energy it has), the greater the probability it could tunnel through some 'impassable' barrier. Which means your best chance of getting through that solid wall requires you run as fast as you can. You'll have to do it over and over again for longer than the current age of the universe to have even a small chance of success, but it could happen.
Side note: Real-world applications of quantum tunneling include both the natural — e.g. stellar nuclear fusion (of which I wrote about previously, here) — and the man-made, including solid state devices like transistors. Yes, dear reader. The science upon which all of Silver-Age Iron Man's tech is derived? It's quantum.
Ava's "quantum phasing" is actually in line with a macroscopic version of quantum tunneling. She's (sometimes inadvertently) altering her personal wave so that the probability of "tunneling" is near 100 percent, rather than 0.a-very-long-string-of-zeroes-1 percent.
This actually, in my mind, best explains the cool VFX of Ghost's multiple images — if she doesn't have complete control of her wave function, then it's totally reasonable that she'd appear to show up in multiple places before settling on one.
It's better than that completely made-up "molecular disequilibrium", anyway. But what do I know? I wasn't the science consultant on the film.
There's another weird way probability pops up in QM, and that is Quantum Entanglement. This has nothing to do with being able to take over another person's body for plot purposes (and comedic effect). Instead, it's a way for one particle to affect another at seemingly instantaneous speeds, no matter how far apart they are.
When physicists say two "particles" (as much as you can be a particle on a quantum scale) are entangled, what they mean is their states of being depends on one another.
Let's imagine — for the sake of simplicity — we have two quANTum particles (I hear your "ughs" and feast upon them): ANT 1 and ANT 2. The entangled property will be their "color," where if one is red, the other must be blue, and vice versa. And those are the only two possible colors…
The process real-world particles go through to become entangled varies, but scientists can do it in a lab. We'll pretend we have scientists that can do that for us. After entanglement, and without anyone actually checking either ANT's color, each is put in a separate, opaque box.
Now, you take the box with ANT 2 and hitch a ride elsewhere — say, the other side of the Milky Way. I can know with 100 percent certainty what color ANT 2 is just by opening ANT 1's box.
Before I open my box, ANT 1 is neither red nor blue; it merely has the possibility to become either after observation. Its state of being, described mathematically by one of those quantum wave functions, is currently a weird combination (in jargon-speak, a "superposition") of both red and blue.
It's my looking at ANT 1 that collapses its wave function into a single state, making it choose red or blue. This immediately tells ANT 2 - all the way on the other side of the galaxy - which color to become.
If this makes your brain hurt, I'm sorry. Quantum mechanics tend to do that.
At this point, if you open your box to check ANT 2's color, you break the entanglement. So you'll have to wait thousands upon thousands of years for my radio signal to traverse the Milky Way in order to actually the get the information I already know.
So quantum entanglement is no solution for FTL communication, but it has applications in cryptography (keeping data secure) and the next stage in supercomputing, both of which Marvel's heroes could probably use when they're not solving the world's problems just by punching things.
They could also certainly use this magic quantum healing energy, which apparently does not vaccinate against turning into ash because a giant purple man can snap his fingers through a giant metal glove.