Why don’t molecules ever stop moving?

Our bodies are always moving, even when we don’t realize it. We churn through a solar system on a wobbly planet, stand upon shifting tectonic plates, and rise and fall and toss and turn all day and night in a symphony of autonomic and somatic functions. Dizzying though this may seem, life is always in motion, right down to its fundamental building blocks — molecules.

Molecules can be thought of as atoms that share electrons. They form electronic bonds that can be visualized like springs, and they jiggle around at a rate that corresponds with the heat energy to which they’re exposed. Although molecules are neither alive nor dead, they never cease moving, even when scientists try their damndest to sap them of energy. To unpack why molecules never stop moving, and before we get lost in the fundamental weirdness of quantum mechanics, we must first clarify something a bit more straightforward: What is temperature?

In daily life, blustery weather might chill you to the bone, and a cup of tea could warm your lips. When we talk about temperature, we often do so in reference to our own comfort level, body thermoregulation, or climate change. But on a submicroscopic level, temperature measures the average kinetic energy of the molecules or particles in an object or substance. The hotter they get, the more they move; therefore, to “stop” them, we’d just need to take away all their heat energy to achieve absolute zero, or 0 Kelvin — right?

Only, “you can never really completely isolate a molecule from its environment,” said Justin Caram, an associate professor of chemistry at the University of California, Los Angeles, said in a call with Popular Science. “Whether it’s knocking into other molecules in the air — or atoms or whatever — or it’s absorbing light and reemitting light, it’s always interacting with its environment.” Caram added, “You can temporarily cool things down so that they move very, very, very little — and that’s how we define a very low temperature, right? But by the principles of quantum mechanics, you can never completely eliminate all of the motion in the system.”

But… why?

Let’s start with a classically familiar factor in play here: the observer effect. In this case, simply attempting to measure the temperature of something can affect its temperature, because “the molecule can interact with other things, including the measurement apparatus,” explained A. F. J. Levi, an engineering, physics, and astronomy professor at the University of Southern California, in an email to Popular Science. But things get a lot weirder in quantum mechanics, where the uncertainty principle enters the picture, per Levi — who, by the way, called our broader question about molecular movement “deceptively simple.” 

When we talk about how molecules move, this motion “can be separated into the center of mass and relative motion between atoms,” Levi explained. “Because the molecule consists of atoms bound together, at least one lowest-energy bound state is assumed to exist (the idea that a lowest energy ‘ground state’ exists is a very important assumption justified by experiment).” 

Levi went on: “The mathematics of quantum mechanics can be thought of as the linear algebra of non-commuting operators, and this leads directly to the uncertainty principle, which does not allow the lowest-energy bound state of a molecule to simultaneously have zero momentum and a definite, precisely measured position. Physically, it is impossible to precisely measure a molecule’s position without giving it momentum.” 

Levi’s response initially made my brain fuzzy, so I got Caram on the phone a second time to discuss the matter some more. That’s when he offered a Chemistry 101-level explanation of Heisenberg’s uncertainty principle.

“The uncertainty principle just says that position and momentum don’t — okay, there’s a mathematical term. They say they don’t commute. But what that really means is that you can’t measure them simultaneously.” A more accurate way to put that, per Caram, has less to do with measurement. Fundamentally, he explained, “an object cannot possess both properties at the same time.”

This is because quantum theory tells us everything is a wave, including particles, when you look closely enough.

As I start to go existential about wave–particle duality on the phone, Caram offers some words of comfort: “This is just one of those fundamental, really upsetting weirdnesses about quantum mechanics,” Caram said. “[We] have to describe matter as waves. Like, that’s a weird thing; it doesn’t have much meaning to you or me just thinking about it, but the mathematics and the observations work out.”

I ask Caram if he ever thinks about the constant motion going on within our bodies, at molecular and macro levels, and he tells me he tries not to think about it. “It’s a little unsettling to think of all the things that our bodies are doing. Yeah, I don’t know. I don’t know how to answer that, other than to say: I like to not think about it that much, because sometimes I do worry, ‘What if it’ll just stop?’”

Okay! Enough! Beyond the philosophical, molecular motion also connects to the sometimes-buzzy realm of quantum computing.

At UCLA, Caram said that “one of the research areas we work in is related to developing molecules that move as little as possible.” He went on: “Obviously, like I said, there are limits to that — fundamental limits — but the more you’ve slowed down those molecules and controlled their states, the more you can sort of do quantum algorithms and make the quantum computers work better.” 

While humans can’t stop molecules from moving altogether, we’re getting good at slowing them down a whole lot. In fact, “we have achieved colder places on Earth than we have than anywhere in deep space,” according to Caram, “because deep space is always permeated with something called microwave background radiation, and so it has a temperature.” (That temperature is about 2.7 Kelvin.) 

Compare that to the coldest temperature recorded in a lab to date, which is 38 trillionths of a Kelvin — or around -273.15 degrees Celsius. German scientists achieved this feat in 2021 for only a couple of seconds, when they trapped rubidium atoms in a vacuum and mimicked zero gravity at a drop tower at the University of Bremen.

As humans go to great lengths to push the limits of cold, the universe itself is cooling. It was “born with a given amount of energy” in all sorts of forms, “kinetic, chemical, nuclear, whatever,” said Caram, citing the Big Bang theory and the process by which things exchange heat with their environment, known as thermalization. Indulging me, Caram said his “very philosophical” view is that “in the endless, infinite timescale, you know, as the universe expands — we’ll just all sort of slowly move towards some time where things move less and less. That’s the heat death of the universe,” explained Caram, who cautioned me not to worry about it because there really are more pressing things to ponder today. Still, the concept that everything has been spreading out since the “initial kick” of the Big Bang suggests that our lives on Earth are profoundly and uniquely active. (Unless the universe, uh… starts contracting somehow.)

“What we’re really doing is a sort of an extreme fluctuation, where things happen to be moving a little bit more to give rise to complexity” before everything just falls apart, said Caram. “But it will never stop moving entirely, because, like I said, you can never really stop moving.”

This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.

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