Name: How Big Is a Proton Anyway? This Decade-Long Debate May Have Just Been Solved
Uploaded: 2020-03-30T02:41:57.000Z
Duration: 4 min 40 s
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The subatomic realm can be a confusing place, but you would think that we've studied atoms

long enough to at least understand their most basic properties. Things like how big a proton

is. And based on several experiments, scientists had thought they had a pretty good handle

on it too. Until an ingenious 2010 experiment came back with a very different number for

the size of a proton, calling what we thought we knew into question. Now, after almost a

decade of reexamination, scientists think they've solved what's known as the proton-radius

puzzle. Before we really dive into the details of

the latest findings, I have to set the record straight on atoms. A lot of the things you

were taught about them in grade school are oversimplified. The proton itself is not a

smooth billiard-ball, but more like a cloud of quarks held together by gluons. The quarks

a proton is made up of give it its positive charge, and the threshold of that positive

charge can be thought of as the proton's size.

For decades, scientists have used two approaches to find the radius of a proton's boundary.

One method involves firing electrons at atoms, often hydrogen, which in its simplest form

is a single proton in the nucleus with one orbiting electron. Based on how the electrons bounce

off the nucleus, scientists can determine where the proton's positive charge starts

to fade. The other method measures how much energy it takes to excite an atom's electron

from one state to the next, and again, hydrogen is often the atom of choice. In its lower

energy state, hydrogen's electron doesn't just orbit around the proton, but actually

spends some time inside the proton. I told you your grade school ideas about atoms are

all wrong. Anyway, because electrons have a negative

charge, when one is inside the proton, the proton's positive charge pulls it in opposite

directions, reducing the electrical attraction between them. This lowers the energy needed

to excite the electron to its next energy level. So the thinking goes that the bigger

the proton, the more time an electron will spend inside it, and the weaker the atom will

be bound together. By measuring just how much energy it takes for an electron inside a proton

to hop to the next energy state, scientists can deduce the size of the proton.

Over the years, these two methods came more or less to the agreement that the proton's

radius was about 0.8768 femtometers, and all was well until about a decade ago when someone

had the bright idea of artificially swapping out hydrogen's electron with a muon. Muons

are like electrons in every way, except they're 207 times more massive. That added weight

means that the muon spends more time inside the proton, making its switch to a higher

energy state millions of times more sensitive to the proton's size than the electron is in

regular hydrogen. By measuring the proton using muonic hydrogen, they came back with

a result 4% smaller than the previously accepted size, a difference that's not insignificant.

But the sensitivity of the method was too much to ignore. So scientists had a problem.

Were their previous measurements off? Or was this a hint at something more tantalizing?

Maybe muons and protons interacted in  ways that made the protons shrink, or muons somehow

behaved differently than electrons. Maybe the discrepancy would reveal some heretofore

unknown physics, or even new elementary particles. Which brings us to September of 2019, when

scientists at York University in Toronto announced the results of an experiment that used regular,

electronic hydrogen like most experiments before. Like those past experiments, they

excited the electron into a higher energy state. Only this time, the scientists used

a high-precision measuring technique called frequency-offset separated oscillatory fields,

which they modified for this experiment. Their results showed the proton's radius

was right around 0.833 femtometers, consistent with the muonic hydrogen experiment from 2010.

So the results are bittersweet. It looks like the discrepancy in the proton's size was

just down to measurement error, and not a hint of some undiscovered realm of particle

physics. But at least the proton's size is finally settled. Then again, we've said

that before, haven't we? Even I oversimplified protons for the sake

of this video. They're not made of three quarks that are stable all the time, but tons

of quarks and antiquarks that are forming and annihilating each other constantly. The

net difference though is three more quarks than antiquarks. So this experiment doesn't

reveal any new particles that we'll be adding to the standard model, but if you'd like

to brush up on what particles already make it up, check out Maren's video here. Make

sure you subscribe to Seeker for your source of positive news, like protons, and as always,