Scientific American 201905

8 Scientific American, May 2019

LETTERS

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by peripheral vascular dysfunction, as the
article describes, can’t that also happen in
people in whom testing shows them to
have some coronary artery stenosis (block ­
age or narrowing of the arte ries)? In those
cases, perhaps the stenosis is a red herring,
and coronary artery bypass grafting is the
wrong treatment. That could explain some
of the patients who don’t fare well after
bypass surgery.
Steve Wise
Charlotte, N.C.

BAIREY MERZ REPLIES: Wise makes a

good point. Recent evidence from the

ORBITA trial demonstrates that angina

is not improved by stenting, suggesting

that ischemia symptoms may, in fact,

more often be caused by coronary micro­

vascular dysfunction.

TRICK OR FEAT

In “The Particle Code,” Matthew von Hip­

pel consistently refers to mathe ma tician

Alexander  B. Goncha rov’s obser vation

that mathematical pe ri od theory can be

used to simplify loop compu tations in

particle physics as a “trick.” But that ob­

servation shows such compu tations can

be decomposed into an al pha bet of sim­

pler functions, which can be assembled

only according to a simple grammar.

That seems quite pro found to me. Could

the alphabet and the grammar corre­

spond to some un derlying structure in

particle theory?

Doug Hoover

Sunnyvale, Calif.

VON HIPPEL REPLIES: There may well be
a deeper meaning to the kinds of “al pha ­
bets” that appear in particle physics cal­
culations. Physicists suspect they are re­
lated to a result of Nobel laureate Lev Lan­
dau, called the Landau equations. But this
is still not fully understood.

ERRATUM

In “The Particle Code,” by Matthew von

Hippel, the lower graph in the box show­

ing “Euler’s formula visua lized as a circle,

then projected through time” inaccurate­

ly represented the curve for cosine (x).

The corrected illustration can be seen at

http://www.scientificamerican.com/eu lers­

formula­projected

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