What Is the Smallest Small?

Looking for limits is often instructive. We’ve made remarkable progress, as a species, by answering questions like, “What’s beyond the horizon?” “How old is the universe?” And, of course, “How spicy is too spicy?”

When the limits get small, the stakes get high. “What is the smallest thing?” weighs in at a svelte five words, but seeking the answer has had huge implications. The quest has improved our understanding of the fundamental nature of reality, no less — and much else besides.

The smallest thing we know has become a lot smaller in the past couple of decades, thanks in large part to particle accelerators, says physicist Roger Moore (pictured). The biggest and most powerful accelerator is the Large Hadron Collider at CERN, the European Organization for Nuclear Research, where the fabled Higgs boson particle was discovered in 2012.

Moore contributed to that discovery (as did U of A physicists Doug Gingrich and James Pinfold). Moore is now a member of the IceCube experiment at the South Pole, searching for neutrinos that can achieve energies similar to and beyond those observed at CERN. 

So who better to ask about the smallest small?

The Unbearable Weirdness of Quantum 

There’s one thing you need to know about the bizarre and baffling world of quantum physics. Or is it two things? In quantum physics, the smallest thing is also the heaviest thing. And a given thing will be smaller the faster it is moving.

So what, then, is the smallest-heaviest-fastest thing we’ve found? There are two contenders, but choosing between them isn’t as easy as it might seem, says Moore.

“Determining the smallest thing depends on what you mean by ‘smallest thing.’ ”

Contender #1: The Top Quark

In one corner of the tiniest boxing ring you’ve never seen is the top quark, sometimes called the truth quark, which was discovered in 1995. 

“It’s the fundamental particle with the highest mass,” says Moore. “It has a mass of 175 gigaelectron volts, whereas the Higgs boson has a mass of 125 gigaelectron volts. So you could argue the top quark is the smallest particle.” (Put another way, a top quark is about 182 times heavier than a proton, while a Higgs boson is about 133 times heavier.)

With a name like “top quark,” you’d think it’s a clear winner as the smallest small. But the top quark and the Higgs boson have been observed only in particle accelerators built specifically to see them. In the wild, physicists have been able to catch fleeting glimpses of particles with even higher energies — and therefore higher masses, shorter wavelengths and smaller sizes.

Contender #2: Neutrinos and Cosmic Rays

These even smaller particles include high-energy neutrinos — which Moore began working on after the discovery of the Higgs boson — and cosmic rays, which are primarily protons travelling at nearly the speed of light.

“We have seen high-energy neutrinos with energies up to 1,000 times higher than anything we can achieve at the Large Hadron Collider, and cosmic rays have even higher energy than that.” 

There is, of course, a catch: both particles are extremely difficult to detect and measure. They exist on Earth only for a fraction of a scintilla of a microcosm of a second and, until just months ago (see “At the Frontiers of Physics” below), their origins were still mostly mysterious to physicists.

Which leaves us with three questions, all with mercifully straightforward answers.

Question 1: What does Moore see as the smallest thing we’ve ever found?

“You could say the best definition of ‘small’ is for something that’s stationary,” he says, “in which case it’s the top quark. If you could make anything move fast enough, you could make it small.”

Question 2: Is this the final frontier in small, or might we find something tinier still?

“There is definitely a chance that there are weirder and smaller things out there,” Moore says.

“Gravity, dark matter and the ‘lightness’ of the Higgs boson itself — ironically, despite it being one of the most massive particles we have found! — are all problems with our current understanding of physics that need to be solved. And some or all are likely to involve higher mass and hence, arguably, smaller particles.”

Question 3: Why is the world of atomic and subatomic particles so strange and baffling?

“Why,” he says, “is a hard question to answer in science.”