CUriosity: What is the smallest thing in the universe?

In CUriosity, experts across the 51�Թ�� campus answer pressing questions about humans, our planet and the universe beyond.

Previously, astrophysicist Jeremy Darling tackled: “What is the biggest thing in the universe?” This week, Ethan Neil, associate professor in the Department of Physics, answers: “What is the smallest thing in the universe?”

Man wearing hard hat works in a long tunnel with machinery running lengthwise

Part of the tunnel that makes up the Large Hadron Collider in Switzerland. Scientists use colliders like this one to smash together particles at incredible speeds, becoming what physicist Ethan Neil calls "the world's best microscopes." (Credit: CERN)

As with everything in physics, the answer may melt your brain—just a little. It also hinges on how you define “small,” said Ethan Neil, a theoretical physicist who studies incredibly small things.

Does smallest mean, for example, the object with the least mass? Or is it more about size, how much space an object takes up?

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��Previously in CUriosity

Image of purple strands forming a web-like pattern around dark voids

What is the biggest thing in the universe?

Or read more CUriosity stories here

As Neil put it: “The question is more complicated than it seems on the surface, partly just due to the weirdness of quantum physics. The world is unintuitive when we get to very short distance scales.”

Let’s start with mass. Neil explained that the universe, at least as we know it, is made up of elementary particles like electrons and quarks, small things that can’t be broken down into even smaller stuff. Think of them as the basic ingredients for making everything in the cosmos.

Physicists capture the family tree of these particles in a theory that dates back to the 1960s known as the Standard Model. Within that tree, the electron is superbly petite. Writing out its mass in kilograms, you’d get 0.000000000000000000000000000000911 (that’s 30 zeros). Another elementary particle, the electron neutrino, has an even smaller mass—although no one knows exactly how small. The sun ejects neutrinos constantly and, at this moment, trillions are moving through your body.

The question of size, however, is where things really get weird.

“In the Standard Model, things like the electron don’t have any size,” Neil said.

In other words, you could zoom in and in on them and never see anything. But how sure are scientists that electrons are truly infinitely small?

Using facilities like the at CERN in Switzerland, scientists have probed the universe down to really small scales. So far, they’ve been able to observe the universe down to about 20 zeptometers.

Or, as Neil put it: “If a single atom was the size of a human being, 20 zeptometers would be the size of an atom.”

If an electron has size, it has to be smaller than that. But theoretical physicists like Neil have also thought about what could exist at even smaller scales. That includes at the Planck length, a distance that, in meters, would take a decimal point followed by 34 zeros to write out.

At that scale, Neil explained, the inherent randomness and uncertainty of the universe dominates so much that concepts like size and distance become more or less meaningless. In fact, physicist John Baez predicted that if you tried to measure something that small, you’d concentrate enough energy to form a black hole.

That doesn’t, however, mean that there’s nothing there. One popular theory suggests that the elementary particles themselves are made up of vibrating strings that are about the size of the Planck length—meaning that everything you know could be the product of a concerto played by an orchestra of impossibly tiny violins.��

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