34 March/April 2022
Physics
7Absolute Zero Is Certain Death
for Our UniverseThe Big Freeze hypothesis states that
the heat death of the universe will
result when all of space approaches
absolute zero, or a state of no
thermodynamic free energy. A 2020
study published in the journal Monthly
Notices of the Royal Astronomical
Society posits that heat death will
arise in the distant future, when
low-mass stars like the sun exhaust
their nuclear fuel and become white
dwarf stars—extremely dense
remnants of the stars’ cores. After
trillions of years, the white dwarf
stars will become black dwarf stars,
stellar remnants that have cooled
down so much that they are no longer
emitting heat or light.
Silicon nuclei in the black dwarf
stars’ cores will fuse to produce nickel
nuclei which will emit positrons (theantiparticle of an electron, meaning
it has the same mass as an electron,
but a positive charge), and in turn will
decay into iron nuclei. In the most
massive black dwarf stars, those
positrons will eventually “annihilate”
enough electrons at the core, causing
a gravitational collapse that will fuel
a colossal explosion. The creation of
each black dwarf star—along with
black hole evaporation and darkened
galaxies—will leave behind iron
chunks of former planets, comets and
stars, and trace subatomic particles
and energy. As space expands, that
energy will cool to absolute zero,
bringing on the heat death of our
universe. But don’t worry too much:
Experts estimate that this final
hurrah won’t occur for another 10^1100
years.—Courtney Lindertemperature we could ever theoretically reach,
according to the laws of thermodynamics. Some
researchers seek absolute zero for use in preci-
sion instruments that can test the fundamental
laws of physics, while others do so to model some-
thing called the Cold Big Bang, when all matter
exploded into being and the universe began oper-
ating under observable laws of matter and energy.
In this latter sense, looking at a system at abso-
lute zero—one almost completely without kinetic
energy—would be close to observing the very
beginning of physics.
When conditions move closer to absolute
zero, particles begin to behave in abnormal,
unpredictable ways, affecting the properties of
elements and compounds. Nitrogen freezes into
an unstable solid at 63.15 Kelvin, or –210 degrees
Celsius, and liquid helium becomes a frictionless
“superf luid” at around 2 Kelvin. At cool enough
temperatures, some particles even take on
special wavelike characteristics, forming a state
of matter called a “Bose-Einstein condensate”
in which a mass of individual particles enter the
same quantum state to become a single f luidlike
cloud of atoms.
In the distant future, the universe will
approach absolute zero at the end of everything
(see sidebar). However, it’s impossible for sci-
entists to create absolute zero conditions in a
lab, because removing all heat from an object
would require an enormous amount of energy.
To simulate an extremely cold temperature, the
researchers in Germany applied a magnetic field
to an atomic cloud to slow the atoms within and
lower the system’s temperature. The magnetic
field was generated by a current running through
a chip that was used to trap and cool the atoms
beforehand.
The field acted as a lens affecting all the
atoms collectively. The researchers tuned their
lens toward a distant focal point to slow down
the atomic cloud’s expansion. With the cor-
rect magnetic field magnitude and timing, the
atomic cloud’s expansion came close to a stand-
still, reducing the effective temperature to an
astonishing 38 picoKelvins (that’s 38x10-12 Kel-
vins). Because no thermometer can detect such
a minute amount of energy, the researchers cal-
culated the figure based on the particles’ lack of
kinetic movement.
The magnetic field was tuned using careful
calibration, as if it were a specialized pair of pre-
scription glasses. Just as eyeglass lenses can focus
near or far, so can the scientists tune a magnetic
field. In this experiment, the scientists tuned
their magnetic field to an infinite distance away,
collimating the cloud’s expansion in all three
dimensions. With the expansion of the atomic
cloud reduced to .00005 meters per second, record-
ing the kinetic temperature required two seconds
of observation, which was only possible by doing it
in ZARM’s drop tower in weightless free fall.
The group of researchers, from the Univer-
sity of Bremen, Leibniz University Hannover, the
Humboldt University of Berlin, and the Johannes
Gutenberg University Mainz, say they hope to
eventually achieve a top potential “weightless-
ness” period of up to 17 seconds.
Although brief, this stretch of time could
allow them to test the fundamentals of phys-
ics in the purest possible conditions and
also help other researchers examine the
Cold Big Bang and other universal mysteries
in a context that explores the state of the world
just as it began.