The
Coolest Stuff in the Universe – Approaching Absolute Zero
We
went to a lecture a while
back (March
15, 2016),
at the University of Alberta, by Nobel laureate Dr. William Phillips,
of NINST (National Institute of Technology), in the
U.S..
He is also a professor at the University of Maryland. The
professor's Nobel was for work in “laser cooling”, a technique
used to bring the temperature of certain gases down to very close to
absolute zero.
Time and Motion
One
of the key reasons for wanting to cool matter down to very low
temperatures, is to make more accurate clocks. We measure time via
motion – that can be anything from:
the
motion of the Earth around the sun (year),
to
the motion of the moon around the Earth (month),
to
the swing of a pendulum (seconds),
to
the oscillations of a spring (less
than a second),
to
the oscillations of atoms (millionths
of a second).
The
random thermal motion that we call heat makes it difficult to measure
time with great precision, so by eliminating as much of this heat as
possible, we can measure time more accurately.
Another
way that motion affects time, is via Einstein's equations of special
relativity. As the saying goes, “moving clocks run slow”. At
the highest levels of accuracy, we use devices such as atomic clocks,
so we are interested in the realm of the very small, when we talk
about cooling matter to these very low temperatures, millions of a
degree above absolute zero. When
using atoms as clocks, it is helpful to keep the state of motion of
the atoms as low as possible, and thus it is necessary to keep the
“clock” as cool as possible.
One
might wonder if these esoteric concerns of physicists have any
practical effect on everyday society. In fact they do, in an
application that is vital to the entire world - the geographical
positioning system or GPS. Positions on Earth are determined via the
timing of signals from multiple satellites, so extreme precision in
timing is needed to produce extreme precision in geographical
positioning. Super accurate clocks are also needed to efficiently
coordinate computer networks, as well as for various types of
advanced scientific research (particle physics, astrophysics).
Temperatures
and Cooling
Getting
back to temperatures and cooling, the professor (who was quite a
showman), demonstrated how the temperature regime at which we humans
live (about 290 Kelvin) is quite special. Via the use of a fair bit
of liquid nitrogen (77 Kelvin), he showed how matter's
characteristics change, when cooled down far below our special
temperature range. Here are a few examples:
Inflated
balloons collapsed to pancake thinness after immersion in liquid
nitrogen, then re-inflated after being re-exposed to room
temperature. In fact, several popped, perhaps because of the strain
on the rubber, caused by the temperature changes.
Rubber
balls go from bouncy before immersion, to smashy afterwards.
After
pouring liquid nitrogen into a plastic bottle, then sealing it
tightly, the bottle was placed under an overturned wastepaper
basket. After some minutes, the pressure of the warming nitrogen
burst the bottle, which was safely under the wastepaper basket, and
the gas then blew the overturned basket to the ceiling. Quite a
show, that was (no picture, though)
And,
of course, just the act of spilling liquid nitrogen on the floor was
“pretty cool”, as it quickly boils off in a dramatic display of
vapour.
But
liquid nitrogen only takes you only part of the way to absolute zero
, which is the theoretically coldest temperature that can be
achieved. All motion stops at that point (strictly not true, due to
quantum mechanical effects). Liquid nitrogen is 77, on the Kelvin
scale. A number of other atoms can be used, whose liquid or solid
states are at much lower temperatures, helium for example, which
gets to a few degrees above absolute zero.
Most
of this initial cooling is via evaporative cooling.
But
how do you cool matter to temperatures barely above absolute zero?
One way is through the technique of laser cooling. In fact,
Professor Philloips won the Nobel for his work on laser cooling.
The
principle behind this, is to use radiation pressure to slow down
atoms of a gas (usually
cesium).
This is the same effect as that produced by the sun's
light
on a comet's tail, causing the gas molecules to stream away from the
sun, as they are boiled off, while in the inner solar system.
To
cool gas (slow down the atoms), laser light is pumped into the gas,
so as to resonate at the natural vibration frequency of the atoms.
Doppler
shifted atoms can be selected for cooling, with different laser
frequencies. When the atoms absorb the appropriate frequency of
radiation, they are slowed down, somewhat like marbles in a viscous
fluid.
This
has been given the nickname “optical molasses”.
Of
course, when the radiation is re-emitted, the atoms speed up (gain
heat). But this emmission process is in random directions, while the
absorbtion that slows the atoms can be tuned in one direction. The
cumulative effect allows the process to lead to very low
temperatures.
For
example, sodium atoms can be cooled to 240 micro-Kelvins via this
method. Even lower temperatures have been acheived, such as 700
nano-degrees (billionths of a Kelvin). At the lowest temperatured,
the atoms might only be moving at speeds on the order of a centimeter
per second, rather than the hundreds
of meters per second typical of gases at “normal” temperatures.
But
that's not the end – Bose-Einstein condensates have been used to
get to even lower temperatures, less than one nano-degree above
absolute zero.
It
is worth noting that the coldest “natural temperature” is about 3
degrees Kelvin, the residual background heat from the big bang. So,
short of what aliens might be doing, this really is the coolest stuff
in the universe.
Atoms
can both be cooled and trapped this way. Magnetic containment is
also used, to keep the atoms from warming as they would
interact
with normal conainers. The professor showd the audience an example
of a toy that uses magnetic levitation, to keep a gyroscope suspended
in mid-air, to give a concrete demonstration of this phenomenon.
Between
laser cooling and magnetic containment, atomic
clocks have been made that are accurate to one part in 10 to the -18.
That is equivalent to one second every 10 billion years, which is
about the estimated current age of the observable universe.
–------------------------------------------------------------------
Science is cool (especially the science of super-cooling matter),
but so is Science Fiction. So, you should consider reading the books
described below. :)
The Witches’ Stones Series
The Witches' Stones, Book 1 -
Rescue from the Planet of the Amartos
Young
Earth woman and spaceship mechanic, Sarah Mackenzie, has unwittingly
triggered a vast source of energy, the Witches' Stones, via her
psychic abilities, of which she was unaware. She becomes the
focal point of a desperate contest between the authoritarian galactic
power, known as The Organization, and the democratic Earth-based
galactic power, known as The Terran Confederation. The
Organization wants to capture her, and utilize her powers to create a
super-weapon; the Terra Confederation wants to prevent that at all
costs. The mysterious psychic aliens, the Witches of Kordea
also become involved, as they see her as a possible threat, or a
possible ally, for the safety of their own world.
A small but
fast scout-ship, with its pilot and an agent of the Terra
Confederation, Coryn Leigh, are sent to rescue her from a distant
planet at the very edge of the galaxy, near space claimed by The
Organization. Battles, physical and mental, whirl around the
young woman, as the agent and pilot strive at all costs to keep her
from the clutches of the
Organization.https://www.amazon.com/dp/B008PNIRP4
https://www.amazon.co.uk/dp/B008PNIRP4
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https://www.amazon.de/dp/B008PNIRP4
The Witches' Stones, Book 2 -
Love and Intrigue, Under the Seven Moons of Kordea
Sarah has
taken refuge on the planet of Kordea, where she is also learning how
to control her psychic abilities, through the tutelage of the Witches
of Kordea. Coryn Leigh has now taken up the position of
Confederation diplomat to the Kordeans, but he is also charged with
keeping the Mackenzie girl safe at all costs. During their time
on the planet, an attraction between them grows, though they try to
deny it, to themselves and each other.
But The
Organization has plans of its own, including threatening the
destruction of the planet Kordea, via destabalizing the orbit of
Lina, one of its many moons. The Organization proves that its
threats are in deadly earnest, so, ultimately Sarah, Coryn and the
Witches of Kordea must take the fight to the enemy. Thus is
borne a dangerous mission, to a planet where their foe has
based the weapon that threatens Kordea, and ultimately, the balance
of power throughout the galaxy. Sarah and Coryn agree that the
machine must be destroyed, even at the possible cost of their own
lives and growing love.
The Witches' Stones, Book 3 -
Revenge of the Catspaw
Sarah
and Coryn have become married, under the traditions of the Witches of
Kordea. But the marriage is performed by the Eldest of the most
important coven, a rare honour, that comes with a blessing and a
curse. The slow working out of this blessing and curse forms
the backdrop to the story.
Having come so close to their goal
of enhancing their weaponry via Witches' Stone power, The
Organization will not give up. In order to lure Sarah into
their trap, and thus have her become their Catspaw (someone who is
forced into helping another, against their will) they need bait, and
Coryn becomes the bait. He also comes under the domination of a
particularly nasty Elite of The Organization, one "Evil Evilla"
Copoz.
Sarah, and a picked group of companions, must re-enter
The Organization space, this time to the very heart of the empire, to
rescue her husband, as he has done for her in the past. They do
so at great peril, but nothing can stop the terrible Revenge of the
Catspaw.https://www.amazon.com/dp/B01G2I37G2
https://www.amazon.co.uk/dp/B01G2I37G2
https://www.amazon.ca/dp/B01G2I37G2
https://www.amazon.de/dp/B01G2I37G2
