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OCTOBER/NOVEMBER 2020 | SCIENTIFICAMERICAN.COM
Plus:
Space
&
Physics
HOW MANY
ALIENS ARE
REALLY OUT
THERE?
MISSION
TO MARS:
PERSEVERANCE
TIME’S ARROW
MAKES MUSIC
New
Quantum
Restraints
on Reality
AN EXPERIMENTAL
TWIST ON THE
SCHRÖDINGER’S
CAT PARADOX
COULD OVERTURN
CHERISHED
ASSUMPTIONS
IN METAPHYSICS
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 Most Confused of the Scientific Branches
Quantum researchers seem to have more theories than they know what to do with. Of the handful of options, take,
for example, the many-worlds view, which posits that when a quantum observation is made, reality splits into parallel
universes, each representing all potential outcomes. Or there is the relatively new QBism camp, members of which
argue that quantum mechanics is subjective to the individuals making predictions about how they will measure an
experiment. On top of these conflicting theories, any new experimental data invariably support one possible explana-
tion and contradict another. What to make of this confounding research situation? Where some see an impasse,
others see opportunity. At the very end of this issue’s cover story, Michele Reilly, co-founder of a quantum computing
company based in New York City, tells our reporter that such confusion opens the door for novel experiments, of both
the theoretical variety and the physical (see “This Twist on Schrödinger’s Cat Paradox Has Major Implications for
Quantum Theory”). If that’s not a pure emblem of the scientific method, then I don’t know what is.
Elsewhere in this issue, Anil Ananthaswamy examines the latest estimates of alien life in the universe—estimates
that ride on 18th-century statistics (see “How Many Aliens Are in the Milky Way? Astronomers Turn to Statistics for
Answers”). And Alexandra Witze gives a dazzling overview of NASA’s latest rover project: Perseverance (see “NASA
Has Launched the Most Ambitious Mars Rover Ever Built: Here’s What Happens Next”). If any field needed the
gumption to keep going for the long haul, it’s space and physics. Enjoy!
Andrea Gawrylewski
Senior Editor, Collections
editors@sciam.com
LIZ TORMES
On the Cover
An experimental twist on the
Schrödinger’s cat paradox
could overturn cherished
assumptions in metaphysics
ANDRZEJ WOJCICKI
GETTY IMAGES
2
WHAT’S
INSIDE
October–November 2020
Volume 3
•
No. 5
OPINION
28.
Unidentified Aerial
Phenomena, Better
Known as UFOs,
Deserve Scientific
Investigation
UAP are a scientifically
interesting problem.
Interdisciplinary teams
of scientists should
study them
31.
Could We Force
the Universe to Crash?
If we’re all living in a
simulation, as some have
suggested, it would be
a good, albeit risky, way
to find out for sure
33.
A Movie of the
Evolving Universe
Is Potentially Scary
The Vera C. Rubin
Observatory will reveal
all sorts of short-term
changes in the cosmos—
and some could have
dire consequences
for humanity
YUICHIRO CHINO
GETTY IMAGES
NEWS
8.
Time’s Arrow Flies
4.
Quantum Tunneling through 500 Years
Is Not Instantaneous, of Classical Music,
Physicists Say
Physicists Show
A statistical study
A new experiment
of more than 8,000
tracks the transit time
compositions shows
of particles burrowing
how the flow of time
through barriers,
distinguishes music
revealing previously
from noise
unknown details of
a deeply counterintuitive
11.
Higgs Boson Gives
Next-Generation
phenomenon
Particle Its Heft
6.
Scientists Unveil
Experiments at the
First Ever Pictures
Large Hadron Collider
of Multiple Planets
suggest that muons and
around a Sunlike Star
other “second-generation
The two giant worlds,
particles” obtain their
each much larger than
mass from interacting
Jupiter, constitute only
with the Higgs, further
the third multiplanet
strengthening the
system ever imaged
Standard Model
13.
Mystery over
Universe’s Expansion
Deepens with
Fresh Data
A long-awaited map
of the big bang’s
afterglow fails to
settle a debate over
how fast the universe
is expanding
14.
This Photo
of the Sun
Is the Closest
Ever Taken
Close-up reveals
a surface dancing
with “campfires”
ZIHAO CHEN
GETTY IMAGES
FEATURES
16.
This Twist on Schrödinger’s Cat Paradox
Has Major Implications for Quantum Theory
A laboratory demonstration of the classic “Wigner’s
friend” thought experiment could overturn cherished
assumptions about reality
21.
How Many Aliens Are in the Milky Way?
Astronomers Turn to Statistics for Answers
The tenets of Thomas Bayes, an 18th-century
statistician and minister, underpin the latest
estimates of the prevalence of extraterrestrial life
25.
NASA
Has Launched the Most
Ambitious Mars Rover Ever Built:
Here’s What Happens Next
Perseverance will stow away rocks for eventual
delivery to Earth and will listen for Martian sounds
for the first time
3
NEWS
A new experiment tracks
the transit time of particles
burrowing through barriers,
revealing previously unknown
details of a deeply counter-
intuitive phenomenon
YUICHIRO CHINO
GETTY IMAGES
Quantum Tunneling
Is Not Instantaneous,
Physicists Show
Although it would not get you past a
brick wall and onto Platform 9 ¾ to
catch the Hogwarts Express, quan-
tum tunneling—in which a particle
“tunnels” through a seemingly insur-
mountable barrier—remains a con-
founding, intuition-defying pheno-
menon. Now Toronto-based ex-
perimental physicists using rubidium
atoms to study this effect have mea-
sured, for the first time, just how
long these atoms spend in transit
through a barrier. Their findings
appeared in
Nature
on July 23.
The researchers have showed
that quantum tunneling is not instan-
taneous—at least, in one way of
thinking about the phenomenon—
despite recent headlines that have
suggested otherwise. “This is a
beautiful experiment,” says Igor Litvi-
nyuk of Griffith University in Austra-
lia, who works on quantum tunneling
but was not part of this demonstra-
tion. “Just to do it is a heroic effort.”
To appreciate just how bizarre
quantum tunneling is, consider a ball
rolling on flat ground that encoun-
ters a small, rounded hillock. What
happens next depends on the speed
4
NEWS
of the ball. Either it will reach the top
and roll down the other side, or it will
climb partway uphill and slide back
down because it does not have
enough energy to get over the top.
This situation, however, does not
hold for particles in the quantum
world. Even when a particle does not
possess enough energy to go over
the top of the hillock, sometimes it
will still get to the opposite end. “It’s
as though the particle dug a tunnel
under the hill and appeared on the
other side,” says study co-author
Aephraim Steinberg of the Univer-
sity of Toronto.
Such weirdness is best under-
stood by thinking of the particle in
terms of its wave function, a mathe-
matical representation of its quan-
tum state. The wave function evolves
and spreads. And its amplitude at
any point in time and space lets you
calculate the probability of finding
the particle then and there—should
you make a measurement. By defini-
tion, this probability can be nonzero
in many places at once.
If the particle confronts an energy
barrier, this encounter modifies the
spread of the wave function, which
starts to exponentially decay inside
the barrier. Even so, some of it leaks
through, and its amplitude does not
go to zero on the barrier’s far side.
Thus, there remains a finite probabil-
ity, however small, of detecting the
particle beyond the barrier.
Physicists have known about
quantum tunneling since the late
1920s. Today the phenomenon is at
the heart of devices such as tunnel-
ing diodes, scanning tunneling
microscopes and superconducting
qubits for quantum computing.
Ever since its discovery, experi-
mentalists have strived for a clearer
understanding of exactly what hap-
pens during tunneling. In 1993, for
example, Steinberg, Paul Kwiat and
Raymond Chiao, all then at the
University of California, Berkeley,
detected photons tunneling through
an optical barrier (a special piece of
glass that reflected 99 percent of
the incident photons; 1 percent of
them tunneled through). The tunnel-
ing photons arrived earlier, on aver-
age, than photons that traveled the
exact same distance but were unim-
peded by a barrier. The tunneling
photons seemed to be traveling
faster than the speed of light.
Careful analysis revealed that it
was, mathematically speaking, the
peak of the tunneling photons’ wave
“This is a beautiful experiment.
Just to do it is a heroic effort.”
—Igor
Litvinyuk
functions (the most likely place to
find the particles) that was traveling
at superluminal speed. The leading
edges of the wave functions of both
the unimpeded photon and the tun-
neling photon reach their detectors at
the same time, however—so there is
no violation of Einstein’s theories of
relativity. “The peak of the wave func-
tion is allowed to be faster than light
without information or energy travel-
ing faster than light,” Steinberg says.
Last year Litvinyuk and his col-
leagues published results showing
that when electrons in hydrogen
atoms are confined by an external
electric field that acts like a barrier,
they occasionally tunnel through it.
As the external field oscillates in
intensity, so does the number of tun-
neling electrons, as predicted by
theory. The team established that
the time delay between when the
barrier reaches its minimum and
when the maximum number of elec-
trons tunnel through was, at most,
1.8 attoseconds (1.8 × 10
–18
sec-
ond). Even light, which travels at
about 300,000 kilometers per sec-
ond, can only travel over three
ten-billionths of a meter, or about
the size of a single atom, in one atto-
second. “[The time delay] could be
zero, or it would be some zeptosec-
onds [10
–21
second],” Litvinyuk says.
Some media reports controver-
sially claimed that the Griffith Uni-
versity experiment had shown tun-
neling to be instantaneous. The
confusion has a lot to do with theo-
retical definitions of tunneling time.
The type of delay the team mea-
sured was certainly almost zero, but
that result was not the same as say-
ing the electron spends no time in
the barrier. Litvinyuk and his col-
leagues had not examined that
aspect of quantum tunneling.
Steinberg’s new experiment claims
to do just that. His team has mea-
sured how long, on average, rubidium
atoms spend inside a barrier before
they tunnel through it. The time
is on the order of a millisecond—
5
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