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What Is the Smallest Small?

Enter the weird and wonderful world of quantum physics

By Lewis Kelly

April 19, 2023 •

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.”

At the Frontiers of Physics

Scientists made a big leap forward recently in locating the source of some of the smallest building blocks of the universe. 

“This is the cutting edge of science,” says U of A physicist Roger Moore, part of an international team working on the IceCube experiment in Antarctica. “We now have the first real evidence that this is where at least some of the extremely high-energy cosmic rays and neutrinos come from.”

The team announced in Science last November that neutrinos had been detected coming from a supermassive black hole that powers the nearby active galaxy, NGC 1068, also known as Messier 77.

While research like this might seem obscure, similar basic research underpins a lot that we take for granted today.

“Our modern world is built on what we learned from equally esoteric experiments,” Moore says. He points to Ernest Rutherford’s famous 1911 experiment bouncing radioactive particles off gold foil, which revealed the nuclear structure of the atom. “The knowledge of atoms and quantum mechanics we learned from those experiments led to semiconductors, MRI scanners, computers and many other transformative inventions,” he adds. 

“Today’s physics research may seem removed from everyday life, but it is laying the foundations of the world of our grandchildren, just as Rutherford’s scattering experiment did for ours.”

Go Deeper

Ant Man (Album/Alamy Stock)
Ant Man (Album/Alamy Stock)
Could Ant Man ever be real?

Ok, we’ve covered the serious stuff. 

Now let’s get to the big question. 

Will humanity’s quest for ever smaller objects eventually let us shrink things to the “quantum realm,” à la Marvel superhero Ant Man? (If you’re a fan, you might want to stop reading here.)

We asked U of A physics expert Roger Moore who, despite his expertise and international reputation, is not above answering a silly question. 

“No, unfortunately physics does not work like that!” 

He can explain why in immense — or is that minute? — detail. Essentially it comes down to the fundamental laws of physics at the human scale, which are completely different from those at the quantum scale. Shrinking a human body, Moore says, would require finding a way to break these essentially unbreakable rules of physics, including the consistent speed of light. 

“These are fundamental constants and, as far as we know, cannot be altered.”

Even the structure of space and time could be different at the quantum scale, he says. “If you reach this point then all bets are off, since we have literally no idea what, if any, that structure is.”

“This is one of the issues with films like Ant Man or the earlier Fantastic Voyage: you cannot shrink atoms, since their size is governed by fundamental physics. So, while it may make for a fun science fiction story, it’s almost entirely fiction, with little to no science.”

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