Scientific American - USA (2022-02)

6 Scientific American, February 2022

MUON MATTERS

In “The Unseen Universe,” Marcela Care-

na describes apparent discrepancies be-

tween the observed behavior of muons—

one of the three types of charged leptons—

and calculations based on the Standard

Model of particle physics. If the discrepan-

cies for muons are real, I would also expect

discrepancies for tau leptons.

Van Snyder La Crescenta, Calif.

As a shade-tree quantum mechanic, I won-

der if the vibrations of muons are the ef-

fect of waves of gravity—the crests and

troughs of the waves.

Jim Jakubcin via e-mail

In her article, Carena states, “Starlight is
the result of the electromagnetic force act-
ing between the charged protons and elec-
trons, liberating light energy at the hot
surface of the star. The heat source of these
stars, including our sun, is the strong
force, which acts on the protons and neu-
trons to produce nuclear fusion.”
I wonder what heat is. Are heat and
heat energy the same? Is the gluon the
source of the heat of nuclear fusion? I also
wonder what light is. Are light and light
energy the same? Is light a photon travel-
ing through space?
Hiroyuki Uchida Tokyo

CARENA REPLIES: Snyder brings up a

very good question. Indeed, the simplest

explanation for the results of the Muon g-

experiment introduces a new force that af-

fects both muons and tau leptons but not

electrons (the third type of charged lepton).

This is because the rules of quantum me-

chanics are not consistent with the sim-

plest kind of new force unless that force af-

fects at least one other kind of particle be-

sides the muon. Several other explanations

for the Muon g-2 results also involve tau

leptons, so it is important to think about

how we might see something in an experi-

ment with these particles. Tau leptons are

much more unstable than muons and

harder to produce in the first place, but

one could imagine building a particle ac-

celerator optimized for producing them—

a “tau factory”—in the future.

A shade-tree quantum mechanic sounds

like a very nice occupation! We think the

effects of gravitational waves are too weak

to be relevant to the Muon g-2 experiment,

but what Jakubcin suggests is not far

away from what string theorists are trying

to do. They claim that everything we see is

the result of vibrating strings, such that

gravity is one kind of vibration, whereas

the effects we see with muons are other

kinds. If such a unified picture turns out to

be correct, it might ultimately be the expla-

nation for why there should be new exotic

particles or forces affecting Muon g-2.

Uchida asks questions that deserve

long answers, but here is a short summa-

ry: When we say “heat,” we almost always

mean the kinetic energy of particles, which

could be atoms, molecules or a plasma of

charged elementary particles. Kinetic ener-

gy just means that the particles are rapid-

ly moving. Charged particles (such as elec-

trons) can emit photons, which are parti-

cles of light. This is a way to convert energy

in the form of heat into energy in the form

of light, as occurs in an old-fashioned

lightbulb or at the surface of the sun.

Heat in our sun originally arose from

gravity, which, billions of years ago, caused

the gas that the sun is made of to collapse

into a giant hot ball. Once this hot ball

formed, however, the sun began to produce

heat from a process called nuclear fusion.

In the first step of this fusion process,

two protons inside the sun fuse into a deu-

teron while giving off a neutrino and a pos-

itron—the antiparticle of the electron. Then

the deuteron captures another proton to

form a helium isotope and emits a gamma-

ray photon. The deuteron is a bound state

of a proton and a neutron, and the binding

comes from the strong force, which is car-

ried by gluons. So indeed, fusion energy

production in the sun requires gluons.

In addition, because one of the two

protons must be converted to a neutron,

the process requires a W boson, the carri-

er of what we call the weak force. It is ac-

tually a good thing for us that the weak

force is weak because this is why the first

step of fusion in the sun proceeds very

slowly—meaning our home star will

shine steadily for billions of years in-

stead of exploding in a much shorter time.

AGE AND FALLING RISKS

“When Health Takes a Tumble,” by Clau-

dia Wallis [The Science of Health], dis-

cusses a recent increase in dangerous falls

among the elderly. As a 78-year-old retired

physician, I think that perhaps one of the

causes of these falls is the gradual dimi-

nution of proprioception—at least it is in

my case. My balance is good (I walk on un-

even, rocky ground every day), and my

strength, though certainly decreased, is

still adequate. But I have noticed my pro-

prioception is off by just a bit, which is

enough to cause problems. I drop items

because my grip is not quite tight enough

and stub my toe on thresholds and throw

rugs. I also tend to bang plates and cups

when placing items on the counter.

Patrick Laughlin via e-mail

WALLIS REPLIES: Research confirms that

proprioception—the sense of where one’s

body is in space—does decline with age, as

October 2021

LETTERS

[email protected]

“Perhaps one of the causes of falls

among the elderly is the gradual

diminution of proprioception.”

patrick laughlin via e-mail