Catching the Waves
Thank you for Robert Naeye’s excellent
article “Pulsar Timing Arrays” (S&T:
Jan. 2019, p. 22). The illustration on
page 24 shows a diagram of gravita-
tional waves squeezing space in one
direction and stretching in the perpen-
dicular direction, then vice versa in the
next half-cycle. But one question comes
to mind: Does that mean that gravita-
tional waves are polarized?
Vad Falcone
Leeds, United KingdomCamille Carlisle replies: You have
an astute eye! I ran your question
by LIGO’s spokesperson, and he said that
yes, all gravitational waves are in some
polarization or another. The general theory
of relativity permits two polarization states,
called plus (+) and cross (x) states. The two
polarizations are 45 degrees apart, rather
than the 90 degrees for polarized light. In
general, waves will be a superposition of NEW^
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Expectation Bias
I enjoyed Tom Dobbins’s discussion of
expectation bias (S&T: Mar. 2019,
p. 52). His mention of the Heinkel
He-113 reminded me of Johann von
Goethe’s famous quote, which sums
things up: “Man sieht nur, was man
weiß” or “You only see what you know.”
Mark L. Mitchell
Hockessin, DelawareAccording to Scale
Dave Nakamoto’s letter (S&T: Mar.
2019, p. 7) gave a nice analog for com-
prehending the vast emptiness around
and within the Local Group. Even more
locally, if the Sun were the size of a
poppy seed, the Alpha Centauri system
(our closest stellar neighbors) would be
about 9 miles away. Except for our star’s
own planetary, Kuiper, and Oort debris,
there’s a lot of space out there.
Mike Douglass
Poolesville, MarylandJohnny Horne, in his review of Daystar’s Solar Scout (S&T: Mar. 2019, p. 58), men-
tioned that “I experienced slightly better seeing when I used the SS60C outside on
my lawn,” and in the early morning hours. Long ago, I had to do my PhD observa-
tions of the solar chromosphere at the Sacramento Peak Observatory in the hour or
so after sunrise, before the atmosphere heated up. And lore has it that seeing was
better longer there when the lawn was being watered. That led Hal Zirin at Caltech
to build the Big Bear Solar Observatory in a lake on an artifi cial island, which led
to much longer periods of good seeing during the day. Great solar movies resulted.
I’m looking forward to using Big Bear’s 1.6-meter Goode Solar Telescope again
this November 11 for the transit of Mercury.
Jay M. Pasachoff • Pasadena, Californiaplus and cross states. Alternative gravity
theories predict additional polarization
patterns to the two that GR permits.
To detect the polarization of gravitational
waves, scientists need detectors at
different angles — but the two LIGO sites
are essentially aligned. It’s only now with
the addition of Europe’s Virgo detector
that scientists can start to look at the
polarization of gravitational-wave signals,
but they will need at least four sites to
analyze the polarization comprehensively.
Fortunately, the Japanese are building the
KAGRA detector, which is scheduled to
begin observing in late 2019. Check out
https://is.gd/WZRapGand https://is.gd/
Bov6fz for more info.More Bang
I enjoyed reading Faye Flam’s “What
Came Before the Big Bang?” (S&T: Feb.
2019, p. 16) and found the various
speculations relating to the Big Bang
and the multiverse theory very interest-
ing. However, after fi nishing the article
I wondered whether the physicists
whose views were presented had perhaps
stepped outside the bounds of physics
and into the realm of philosophy.
One problem with the multiverse
theory is that it implies we can stop
using science to understand why our
particular universe exists, because our
universe had to come about in an infi -
nite number of universes. It also implies
that there must be an infi nite number
of universes exactly like ours. To me,
that is no more satisfying than justify-
ing our universe by simply saying that
God made it this way. Both scenarios
are equally not falsifi able and therefore
not science by defi nition.
Angelo DiDonato
Macomb, MichiganDated Material
In her feature “Secrets of Polaris” (S&T:
Mar. 2019, p. 14), Camille Carlisle
notes, “In 1912 Henrietta Swan Leavitt
discovered that the brighter a Cepheid
is intrinsically, the longer it takes to
cycle through a pulsation period.”
Actually, Leavitt discovered the Cepheid
period-luminosity relation in 1907, not
the frequently cited 1908 or 1912. In ap Located in Big Bear Lake in Southern California, Big Bear Solar Observatory is operated by the
New Jersey Institute of Technology.8 JUNE 2019 • SKY & TELESCOPE
Solar Seeing