Even if you could measure three black
holes’ locations as precisely as physically possible, you still might not know
where the black holes would go. Such a trio’s complex dance can be so chaotic
that the motions are fundamentally unpredictable, new computer simulations
show.

The paths of three black holes orbiting each other can be calculated based on their positions and velocities at one point in time. But in some cases, the orbits depend so sensitively on the black holes’ exact positions that the uncertainty of quantum physics comes into play. Tiny quantum uncertainties in specifying the locations of objects can explode as the black holes’ gyrations continue over tens of millions of years, astrophysicist Tjarda Boekholt and colleagues report in the April Monthly Notices of the Royal Astronomical Society. So the distant future of the black holes’ orbits is impossible to foresee.

Such extreme sensitivity to initial conditions is known as chaos. The new study suggests, in the case of three black holes, “quantum mechanics imprints into the universe chaos at a fundamental level,” says astrophysicist Nathan Leigh of Universidad de Concepción in Chile, who was not involved with the research.

In chaotic systems, tiny changes can generate
wildly different outcomes. The classic example is a butterfly flapping its wings, thereby altering weather patterns, possibly
producing a distant tornado that otherwise wouldn’t have formed (SN: 9/16/13). This chaos also shows up
in the orbits of three black holes and other collections of three or more objects,
making such orbits difficult to calculate, a conundrum known as the three-body
problem.

To test whether the black holes’ motions
were predictable, Boekholt, of the University of Coimbra in Portugal, and
colleagues checked if they could run computer simulations of the orbits both
forward and backward and achieve the same result. Starting with a given set of
locations for three initially stationary black holes, the researchers evolved
those orbits forward in time to an end point tens of millions of years in the
future. Then, they rewound the simulation, reversing the motions to see if the
black holes ended up where they started from.

Computer simulations have a limited level
of accuracy. In this case, for example, the locations of black holes were known
only to a certain number of decimal places. That tiny imprecision can balloon
over millions of years of the simulation.

According to quantum mechanics, it is
impossible to determine the position of any object better than an utterly tiny
distance called the Planck length, about 1.6
times 10^-35 meters, or 16 billionths of a trillionth of a trillionth
of a millimeter (SN: 4/8/11). Yet even
with accuracy the size of the Planck length, the researchers found that about 5
percent of the time the three black holes wouldn’t return to the same spots
when the simulation was reversed. That means, even if you measured where the black
holes were to the quantum mechanical limit, you couldn’t rewind to find out
where they had come from.

“These systems are fundamentally
irreversible,” says Boekholt. “You can’t go forwards and backwards for these 5 percent
of systems in nature. And that was quite a surprising result.”

The result is theoretical and can’t be applied to real black holes, says astrophysicist Nicholas Stone of the Hebrew University of Jerusalem. For example, measurement errors would swamp the importance of quantum physics. But that doesn’t detract from the study’s importance, he says: “It is still quite interesting from a conceptual perspective.”

A microscopic speck of green algae can
trot like a horse. Or gallop. Biophysicist Kirsty Wan  compares the gaits of creatures large and
small.  

Moving diagonally opposite limbs, or flagella
in this case, in unison —
that’s a trot, Wan says. Her lab, at the University of Exeter in England, is
working on the conundrum of how single-celled creatures, with no nervous system
or brain, coordinate “limbs” to create various gaits. Some of those movements
get far trickier than trots and gallops.

Her work echoes that of 19th century
photographer Eadweard Muybridge, who used a then-novel imaging technique to
reveal hoof positions obscured in the blur of a horse galloping. Wan now creates
Muybridge moments for microalgae.
Using a range of microscopy analytics on what she calls “my private collection
of weird algae,” Wan and colleagues have documented microalgae that coordinate from
four to 16 flagella.

In some four-limbed cells, flagella can move
in neighborly pairs, pulling back in a sort of double-vision breaststroke. To
these microscopic critters, water feels thicker than the splashy stuff that giant
humans easily swish aside. So the algal breaststroke has little glide. It’s more
like a slog through molasses.

Wan looked hard for microalgae with
eight flagella and found three species. One, Pyramimonus octopus, has a gait unlike any Muybridge ever saw. Wan
calls it rotary breaststroke. Flagella across from each other in the array of eight
will curl in for the stroke as their neighbors are uncurling a few beats behind.

P.
octopus is a twitchy microbe that
goes through “shocks,” Wan says. An alga swims along, then “like a knee-jerk
reaction,” it changes direction, though she can’t see what spooks it. In
comparison, when she watches a two-flagella Chlamydomonas
species, “sometimes it twirls; sometimes it spins,” but there’s nothing so dramatic
as the abrupt pullback.

The trickiest example she’s seen may have been lost to science. Wan once grew the Arctic’s P. cyrtoptera, the only microalgal species she knows of with an array of 16 flagella. Sometimes opposite pairs of flagella stroke in unison as the motion ripples around and around the array in a gait she calls a “wave.” Her colony died, however, and so did her supplier’s. “I hope it still exists somewhere in the world,” she says. “Otherwise, I might have … taken the last footage.”

Saturn’s auroras may heat
its atmosphere like an electric toaster.

Measurements from NASA’s Cassini
spacecraft’s final orbits show that Saturn’s upper atmosphere is hottest where its auroras shine, a finding that could help solve a long-standing
mystery about the outer planets.

Saturn’s upper atmosphere is
much hotter than scientists first expected based on the planet’s distance from
the sun. In fact, all the gas giant planets — Saturn, Jupiter, Uranus and
Neptune — were thought to have chilly upper atmospheres of around 150 kelvins (–123° Celsius). But data from the Voyager spacecraft, which flew past the outer planets in the 1970s and 1980s (SN:
8/7/17), showed surprisingly toasty upper atmospheres of 400 to 600 kelvins
(125° to 325° C).

Planetary scientists dub this
mismatch an “energy crisis.” Something injects extra energy into the gas
giants’ atmospheres, but no one knew what. “Trying to explain why these
temperatures are so high has long been a goal in planetary atmospheric
physics,” says planetary scientist Ron Vervack of Johns Hopkins University’s
Applied Physics Laboratory.

Data from the Cassini
spacecraft’s waning weeks might point to an answer, planetary scientist Zarah
Brown of the University of Arizona in Tucson and colleagues report April 6 in
Nature Astronomy.

After orbiting Saturn for 13
years, Cassini finished its mission with a daredevil series of dips between the
planet and its rings before plunging into Saturn’s atmosphere in September 2017 (SN: 9/15/17). During those
final orbits, the spacecraft probed the planet’s upper atmosphere by watching stars
in the background. Measuring the amount of starlight that the atmosphere blocks
told Brown and colleagues how dense the atmosphere is at different points, a
clue to its temperatures.

Using 30 of these stellar
measurements, 22 of which came from the last six weeks of Cassini’s mission,
the team mapped Saturn’s atmospheric temperatures across the whole planet and
at different depths. “For the outer planets, this is an unprecedented data
set,” says planetary scientist Tommi Koskinen, also of the University of
Arizona.

The atmosphere was hottest around
60° N and 60° S latitudes — roughly
where Saturn’s glowing auroras show up (SN: 2/16/05). Auroras are brilliant lights
that sparkle when charged particles from the sun interact with a planet’s
magnetosphere, the region defined by a planet’s magnetic field. Unlike Earth’s
visible auroras, Saturn’s auroras glow mainly in ultraviolet light.

The auroras’ light doesn’t
emit much heat on its own, but is accompanied by electric currents that can generate
heat like the wires in a toaster. This process, called Joule heating, also
happens in Earth’s atmosphere.

If Jupiter, Uranus and
Neptune’s auroras also coincide with extra heat, then auroras may solve the
mystery of hot atmospheres across the solar system. The same process could even
take place on exoplanets, Brown says.

Vervack, who has worked with
the Voyager dataset but was not involved in the new work, thinks this study
marks “a big step in our understanding” of these hot upper atmospheres.

“The real test of whether
they’re right will be when you go out to Uranus or Neptune,” whose magnetospheres
are more complicated than Saturn’s, he says. “Being able to see how our
understanding from Saturn holds up when we get to these more complicated systems
is going to be really key to knowing if we’ve licked this problem or not.”