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,
thanks for watching.