Scientific American Space & Physics 2020 02-03.pdf

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FEBRUARY/MARCH 2020 | SCIENTIFICAMERICAN.COM
Plus:
Space
&
Physics
PROBING
BLACK HOLES
AT THE
HEART OF
COLOSSAL
GALAXIES
SHOULD WE
BELIEVE IN THE
MULTIVERSE?
QUANTUM
HEAT TRANSFER
IN A VACUUM
Interstellar
Interlopers
OBJECTS ENTERING OUR
SOLAR SYSTEM ARE UPENDING
SOME LONG-HELD
ASTRONOMICAL ASSUMPTIONS
WITH COVERAGE FROM
FROM
THE
EDITOR
&PHYSICS
Your Opinion
Matters!
Help shape the future
of this digital magazine.
Let us know what you
think of the stories within
these pages by emailing us:
editors@sciam.com.
SPACE
The Majesty of Cosmic Chaos
I must have been in about eighth grade when I first learned about one of the most recognizable cosmic formations
that humans have ever observed. The Horsehead Nebula, a sculpted pillar of dust and gas that forms the far edge of
the Orion B molecular cloud, is part of a massive stellar nursery where gravity, magnetic forces and radiation winds
force matter together to birth new stars. That such a volatile, powerful place in the universe could also resemble the
majestic beauty of a horse’s head had me hooked. Now some astronomers hypothesize that colossal black holes,
too, may arise from so-called nurseries. As Charlie Wood details in this issue, a handful of candidates for this type
of black hole formation have been discovered using LIGO observations, but much more data is needed (see “Black
Hole Factories May Hide at Cores of Giant Galaxies”).
Elsewhere in these pages, Alexandra Witze details the ways that the appearance of two recent alien objects in our
solar system are overturning long-held astronomical assumptions (see “Two Interstellar Intruders Are Upending
Astronomy”). And XiaoZhi Lim reports on new “supercool materials” that can absorb heat and reradiate it directly
through Earth’s atmosphere and into space (see “The Supercool Materials That Send Heat to Space”). As always,
enjoy this issue!
Andrea Gawrylewski
Senior Editor, Collections
editors@sciam.com
LIZ TORMES
Objects entering our
solar system are
upending some long-held
astronomical assumptions
ESO/ M. KORNMESSER
On the Cover
2
WHAT’S
INSIDE
4.
Space Heater:
Scientists Find
New Way to
Transfer Energy
through a Vacuum
Nanoscale experiments
reveal that quantum
effects can transmit
heat between objects
separated by
empty space
6.
Swirling Magnetic
Fields Hint at
Origins of Spiral
Galaxy Shapes
The formation of spiral
galaxies remains
an open question in
astronomy, but a new
study offers a fresh
look into how these
structures emerge
8.
Japan Will Build
the World’s Largest
Neutrino Detector
Cabinet greenlights
$600-million Hyper-
Kamiokande experiment,
which scientists
hope will bring
revolutionary discoveries
NASA AND SOFIA; NASA, JPL-CALTECH AND ROMA TRE UNIVERSITY
February-March 2020
Volume 2
•
No. 12
NEWS
OPINION
26.
Long Live
the Multiverse!
The idea that our universe is
just part of a much vaster
cosmos has a long history—
and it’s still very much with us
28.
Multiverse Theories Are
Bad for Science
New books by a physicist and
science journalist mount
aggressive but ultimately
unpersuasive defenses
of multiverses
31.
The First Alien
When did we start talking
about life from elsewhere?
33.
The Simple Truth
about Physics
Theoretical models
can be complex—but
the most successful ones
are usually not
35.
Celestial Movement
The sky is always changing.
To appreciate this ever
changing view, grab
these sky maps,
go outside at night and
look up!
Sky maps: February, p. 38;
March, p. 39.
3
ESO/ M. KORNMESSER
FEATURES
14.
Two Interstellar Intruders
Are Upending Astronomy
Researchers grapple with the meaning of
the first objects entering our solar system
from beyond
17.
The Supercool Materials That Send
Heat to Space
Paints, plastics and even wood can be
engineered to stay cool in direct sunlight—but
their roles in displacing power-hungry air
conditioners remain unclear
23.
Black Hole Factories May Hide
at Cores of Giant Galaxies
Gravitational-wave astronomers are probing the
origins of abnormally massive black holes—and
with them, the inner workings of their colossal
galactic homes
GETTY IMGAES
10.
Newfound “Ablating”
Exoplanets Could
Reveal Alien Geology
By probing close-in
worlds, the discovery will
help astronomers better
understand how planets
form and evolve
12.
Evidence of New X17
Particle Reported,
but Scientists
Are Wary
Could the mysterious
particle be our window
into studying
dark matter?
NEWS
Space Heater:
Scientists Find
New Way to Transfer
Energy through
a Vacuum
Nanoscale experiments reveal
that quantum effects can transmit
heat between objects separated
by empty space
Early in life, most children learn that
touching a hot stove or even being
near a roaring fire can burn them.
Whether conveyed via direct contact
or rays of light zipping through space,
the often painful lessons of heat
transfer are as intuitive as they are
unforgettable. Now, however, scien-
tists have revealed a strange new
way that warmth can wend its way
from point A to B. Through the
bizarre quantum-mechanical proper-
ties of empty space, heat can travel
from one place to another without
the aid of any light at all.
Generally speaking, heat is the
energy that arises from the motions
of particles—the faster they move,
the hotter they are. On cosmic scales,
most heat transfer occurs through a
vacuum via photons, or particles of
light, emitted by stars—this is how
the sun warms our planet despite
being some 150 million kilometers
away. Here on Earth, heat flow is
often more intimate, taking place via
direct contact between materials
and helped along by the wavelike
collective vibrations of atoms known
as phonons.
Phonons, it was long thought, could
not transfer heat energy through
empty space; they require two
objects to touch or, at least, to be
in mutual contact with a suitable
medium such as air. This principle is
how thermoses keep their content
hot or cold: they use a wall enclosing
a vacuum to insulate an inner
An artist’s impression of a virtual particle
popping in and out of existence in a vacuum.
Such quantum fluctuations are at the heart of a
newfound way for heat to move between objects.
chamber. Yet scientists have specu-
lated for years about the possibility
that phonons might impart heat
across a vacuum, enamored by the
mind-boggling fact that quantum
mechanics dictates space can never
be truly empty.
Quantum mechanics suggests the
universe is inherently fuzzy—for
ANDREW AGS
GETTY IMAGES
4
NEWS
example, try as one might, one can
never pin down a subatomic particle’s
momentum and position at the same
time. A consequence of this uncer-
tainty is that a vacuum is never
completely empty but instead buzzes
with quantum fluctuations—so-called
virtual particles that constantly pop
in and out of existence. “Vacuum is
never totally vacuum,” says Xiang
Zhang, a physicist at the University
of California, Berkeley, and senior
author of the new study on phonon
heat transfer, which appeared in
Nature
on December 11, 2019.
Decades ago scientists found that
virtual particles were not just theoreti-
cal possibilities but could generate
detectable forces. For instance, the
Casimir effect is an attractive force
seen between certain objects in
proximity, such as two mirrors placed
close together in a vacuum. These
reflective surfaces move because of
the force generated by virtual photons
blinking in and out of existence.
If these ephemeral quantum
fluctuations could give rise to real
forces, theorists mused, perhaps they
could also do other things—such as
transfer heat sans thermal radiation.
To envision how phonon heating via
quantum fluctuations might work,
picture two objects with different
temperatures separated from one
another by a vacuum. The phonons
in the warmer object could impart
thermal energy onto virtual photons
in the vacuum, which could then go
on to transfer such energy to the
cooler object. If both objects are
essentially collections of jiggling
atoms, the virtual particles could act
like springs to help carry vibrations
from one to the other.
The question of whether quantum
fluctuations could really help pho-
nons transfer heat across a vacuum
“had been argued about by theorists
for a decade or so, sometimes with
wildly different estimates for the
strength of the effect—the calcula-
tions are quite tricky,” says physicist
John Pendry of Imperial College
London, who did not participate in
this study. In general, this prior work
predicted that researchers could
only see the effect between objects
separated by a few nanometers
(billionths of a meter) or less, he
explains. At such tiny distances,
electrical interactions or other
nanoscale phenomena between
objects might easily obscure this
phonon effect, Pendry says—making
it very hard to test.
To meet that challenge, Zhang and
his colleagues spent four years of
painstaking trial and error in crafting
and perfecting experiments to see
if they could achieve phonon heat
transfer across greater distances in
a vacuum, on the scale of hundreds
of nanometers. For example, the
experiments involved two silicon
nitride membranes, each roughly 100
nanometers thick. The extraordinarily
thin and light nature of these sheets
makes it easier to see when energy
from one has an effect on the
motions of the other. Vibrating atoms
in the sheets make each membrane
flex back and forth at frequencies
that depend on their temperature.
If the sheets were both the same
size but disparate temperatures,
Zhang’s team realized, they would
quiver at different frequencies. With
all these details in mind, the scien-
tists tailored the sizes of the mem-
branes so that even though they
started at different temperatures
(13.85 and 39.35 degrees Celsius,
respectively), they both vibrated
about 191,600 times per second.
Two objects resonating at the same
frequency tend to exchange energy
efficiently—one well-known example
of resonance can be seen when an
opera singer hits the right note to
cause a champagne glass to reso-
nate and shatter.
In addition, the researchers made
sure the membranes were within
about a few nanometers of being
perfectly parallel to each other, all
to help precisely measure the forces
one might exert on the other. They
also took care that the membranes
were extremely smooth, with surface
variations no greater than 1.5 nano-
meters in size. Clamped to a surface
in a vacuum chamber, one mem-
brane would be mated to a heater,
whereas the other would be con-
nected to a cooler. Both would be
coated with a gossamer-thin layer
of gold for reflectivity and bathed in
faint laser beams to detect their
oscillations—and thus their tempera-
ture. In trial after trial, the scientists
checked to ensure the membranes
did not exchange heat through the
surface they were clamped onto or
through any emission of visible light
or other electromagnetic radiation
across the vacuum.
“This experiment required very
sensitive control of temperature,
distance and alignment,” Zhang says.
“We once had trouble running the
experiment in the summertime
5

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