Absolute zero. - 273,15 degrees
Celsius, - 459,67 degrees Fahrenheit, and zero point on the Kelvin scale. The point
where there is no more energy in a system, the absolute coldest anything can be. The point
where everything, down to the tiniest particles stops moving... Well, that's how it would be in classical
physics but it's actually not possible to ever reach absolute zero. So, why is that?
Temperature is our way of measuring the kinetic energy of particles in matter. The faster
atoms vibrate, the higher the temperature is, and temperature drops when atoms slow down. Logically,
once atoms stop vibrating entirely that's the coldest matter can be, so that is absolute zero.
This works within the framework of classical mechanics, the collection of mathematical
theories that began with Isaac Newton, who first formulated the laws of motion that
describe how things behave on the macroscopic scale, from planets to stars to humans and our
machines. In the beginning of the 20th century, physicists discovered that these theories were
no longer sufficient to describe and predict what happens on the very smallest scales, on the
atomic scale and beyond, down into the world of elementary particles and quantum mechanics,
the theory in physics that aims to describe how matter and energy behave on the most fundamental
level. Quantum physics tells us that even if the lowest temperature possible could be reached,
particles will always still have some kinetic energy that cannot be removed and this is called
zero point energy. This arises from the Heisenberg uncertainty principle, which states that you can
never be entirely sure of all properties of any given particle. For example, if you try to measure
the speed or rather the energy of a particle, you can never be entirely sure of its position. If
you measure its position you can never be entirely sure of its energy. And so particles always
sort of vibrate a tiny little bit even in their lowest energy state. I know that seems counter
intuitive but that's just quantum physics for ya. The temperature of space itself is about 2.7
Kelvin, and this is because the space between stars and planets and clouds of Interstellar dust
and gas does not hold a lot of particles that can interact with each other and heat things up.
The average density of the Universe is about 6 protons per cubic meter, and since that's an
average, interstellar space between stars and cosmic clouds is even emptier and colder. The
reason space itself still has a temperature a few degrees above 0 Kelvin is because of
the cosmic microwave background radiation, the leftover glow of an event called the Last
Scattering, that occurred about 400,000 years after the Big Bang itself. At this point,
the Universe stopped being an opaque soup of of particles bouncing around, now that its
temperature had dropped to about 3,000 Kelvin, and so it was cold enough that electrons could
bond to protons and form hydrogen atoms. This stopped electrons from endlessly scattering
light and so photons could travel freely, and this radiation is still detectable
as the cosmic microwave background. The coldest place in our Solar System is,
surprisingly, on the Moon. There are craters at the Moon's poles where sunlight never reaches
the bottom, and so the photons coming from the Sun can't interact with the surface and can't
heat it up even a little bit, and this has been this way for billions of years. It's estimated
that these dark places are only about 25 Kelvin. Even the average surface temperature of dwarf
planet Pluto is estimated at about 40 Kelvin, which is quite a bit warmer. If you want to
consider that warm. For your reference, the lowest temperature ever measured on Earth was 184
Kelvin. That's - 89.2 Celsius 128. 6° Fahrenheit. That's almost cozy compared to 25 Kelvin. If we
travel far beyond the orbit of Pluto we may find an even colder place in the Oort Cloud, which
is supposed to be a giant spherical shell of icy pieces of space debris around the Sun. It's
believed most long period comets originate here, with their orbits of 200 years or more. Now
because the or cloud is supposedly made up of trillions of small, cold and dark objects, it's
currently pretty much impossible to determine its size and overall shape, and it seems only
loosely bound to the Sun. Other stars and the gravity of the Milky Way itself probably disturb
the cloud and pull objects out of it all the time, especially from the outer regions. The cloud may
extend as far as halfway to the nearest star to the Solar System, Proxima Centauri, which is about
four light years away. Temperatures in the Oort Cloud could be as cold as 5 Kelvin. The coldest
place that we we know of in the Universe is the Boomerang Nebula, at about 5,000 light years away
from Earth. It's formed from the gas ejected by a dying sunlike star at its heart. Now this star is
expelling gas so fast it expands and loses heat and cools down so quickly its temperature has
dropped to only 1 Kelvin. This is colder than the surrounding cosmic microwave background. The
only places in the Universe we know of are even colder are laboratories here on Earth, where
scientists have managed to cool matter down to only 38 picoKelvin. And yes that's a zero with 12
digits after it. So why are scientists doing this? The idea of an absolute minimal temperature goes
way back to the 1600's, and since then scientists have fine tuned ways to calculate absolute zero,
but they've also been trying to cool materials down to as close to this point as they can.
By 1845, Michael Faraday had managed to cool and find the temperatures at which most known
gasses at that time would liquefy. Now Faraday is mostly known for his work with electricity but
he was an accomplished chemist as well. Ammonia for instance becomes liquid below - 33° C and it
freezes at - 77° C. Chlorine liquefies at - 34° C and freezes below - 101° C. With the technology
available at the time Faraday managed to reach a temperature of- 130° C, but this was not cold
enough to liquefy gases like oxygen, nitrogen, and hydrogen. These gases would only liquefy at
high pressures and very low temperatures. Air, which mostly consists of nitrogen and oxygen, was
first liquefied in 1877, needing a temperature of -195° C. Oxygen was liquefied separately in 1883,
at a temperature of -218° C. Hydrogen was first liquefied in 1898 at - 249° C. Dutch physicist
Heike Kamerlingh Onnes was the first to succeed in liquefying helium in 1908 at- 269° C, or only 4.15
Kelvin. By reducing the pressure of the liquid helium he cooled it even further down to only
1.5 Kelvin. These were the coldest temperatures achieved on Earth at the time and Kamerlingh
Onnes received a Nobel Prize for his work in 1913. This was an important scientific milestone,
and Kamerlingh Onnes experimented with certain materials at these extremely low temperatures
and in doing so he discovered superconductivity. At they low enough of a temperature, certain
materials resistance to electrical current drops to zero. The neat thing with super conductivity
is that it's a quantum effect that becomes apparent only at extremely low temperatures.
In a normal good conductor that doesn't have a lot of resistance, like gold for instance, the
electrons that carry electric energy flow really well but they still get scattered around a little
bit by impurities and defects in the material, and the vibrations of the particles in the atoms
the material is made of. The energy that gets lost is why your electrical devices heat up when you
use them. Now if you make certain materials cold enough, something peculiar starts to happen with
the electrons. They form pairs, like they don't literally stick together but they become coupled
in some kind of way that makes that they can move through the material freely, and so they don't
bump into each other and they don't get scattered and so your material becomes superconducting.
This is only possible when the particles in the material don't vibrate too much and that is why it
has to be so cold. An electrical current through a loop of superconducting wire can essentially
flow forever without any power source because it never loses any energy. Now because a closed
electrical circuit forms a magnetic field, this field also becomes permanent as long as
you can keep it cold enough. Superconducting magnets have many applications but probably
the best known are MRI machines in hospitals, which use liquid helium to cool superconducting
coils to create a strong magnetic field in order to make high contrast images of the human
body. The Large Hadron Collider at the CERN in Switzerland also uses superconducting magnets
to accelerate particles in the machine. The big downside is that these magnets need to be kept
exceptionally cold and so it's only really possible with liquid helium and that requires very
big machines and is very expensive. Research into superconductivity is still ongoing and it turns
out that certain ceramic materials can also become superconducting at slightly higher temperatures.
Having to cool a material down to only 90 Kelvin is quite a bit more practical and a lot cheaper
to do with liquid nitrogen, but on the other hand ceramic materials are harder to make into
wires and coils. Scientists are still looking for materials that can become superconducting
at more reasonable temperatures. The Holy Grail of this research is superconductors at room
temperature, which would make them a lot more energy efficient than anything we have right now.
Another property of a superconductor is that it pushes away any outside magnetic field and so you
can let a foreign magnetic field like this small magnet float above the superconducting
material and this is interesting in the construction of super highspeed trains.
There is a related quantum effect that also happens at near absolute zero and that is
superfluidity. A superfluid really does seem to break the laws of physics as we know them.
If you would leave it in an open container it would slowly creep over the edges and leak out.
It would also seep through microscopically small cracks in its container, and if you would stir
it it would basically keep spinning forever, or at least as long as you can keep the material
cold enough. This is because a super fluid has zero viscosity. It has no friction with any other
material no matter what what you keep it in, and it has no friction with itself. A regular
liquid, like water, does tend to creep up the walls of the class or cup it's in but friction
with its container and itself makes that it stays nicely put if you leave it alone. The particles
in both the water and the container have too much energy and so they constantly bump into each
other which causes the friction and so the quantum effect that causes superfluidity is overcome. Now
superfluidity is a very specific phenomenon that only happens with liquid helium. Helium is quite
special because it basically never freezes. At normal atmospheric pressure you need to cool
it down to only 4 Kelvin in order to liquefy it in the first place, and it remains a liquid
pretty much all the way down to absolute zero, or at least as close as you can possibly get
to that point. Although you can technically solidify helium by applying a high amount
of pressure to it. The most common form of helium in nature is helium 4, with its core
made up of two protons and two neutrons, and with two electrons circling the core. Because
of helium 4's specific nature, if you cool it all the way down to 4 Kelvin, all the helium 4 atoms
fall into the same low energy state, and they all start to behave in the same way. They no longer
bump into each other and all move in unison, and so it flows out of its container and will
spin forever if you stir it. This really is completely amazing because it's a quantum effect
that we can see happening with just our eyes, even if it needs very specific circumstances
so you'll probably never find it in nature. One other quantum effect I want to talk about
that is also closely related to superfluidity and superconductivity and so also needs extremely
low temperatures, is the Bose-Einstein condensate. But first there's a couple more things
you need to know about quantum physics. In classical mechanics we like to imagine
particles as tiny little balls that interact with each other, but this is not what quantum physics
tells us. Because of the uncertainty principle it's not possible to know all properties of a
particle so it's more realistic to think of it as more of a vague blob or tiny cloud, or a little
wave packet. Because in quantum physics particles are waves and this wave function is where we
more or less expect the particle to be. Its true position can never be known with 100% certainty,
so the best approach is to imagine it as a cloud or a wave of probability with its own energy
identity. If you find this impossible to wrap your head around, that's just quantum physics for ya.
So, if we cool the atoms in a cloud of gas way down, into the nanoKelvins, their energies start
to approach their lowest point and these little wave packets grow bigger and their wavelengths
become longer. Now at about 50 nanoKelvin, these waves start to overlap and blend into
each other, and they start to behave as if they were one big atom. They don't become solid
to be clear, it's just that all these atoms lose their individual energy identity and you can't
tell one from the other and so they condense into this single collective quantum wave.
A Bose-Einstein condensate is effectively a different state of matter from solid, liquid,
gas, and plasma, but it's probably only possible in laboratories because we don't know of any
natural circumstances that could cause it. As such it could only be made in very small amounts
and so it's very fragile. Any interaction with anything outside of the extremely cold
environment would break the condensate and turn it back into a normal gas. Just so you
know, superconducting materials and superfluids are also different states of matter outside of
the four most common ones. The Bose-Einstein condensate was predicted in the 1920's by Albert
Einstein based on the work of Indian physicist Satyendra Nath Bose. It wouldn't be until 1995
that this exotic state of matter was created by a group of scientists under the leadership of
physicists Eric Cornell and Carl Wieman. They received a Nobel Prize for the research in 2001,
along with German physicist Wolfgang Kettelre, who also made a Bose-Einstein condensate just a
few months after the first one was made. Research into this is important to help us understand
quantum physics better, it may be helpful in developing nano technology and quantum technology,
and in studying the nature and behavior of black holes and neutron stars. It may even be useful
for research into String Theory which hopes to unify the theory of gravity and quantum
mechanics into one big Theory of Everything. One of the coolest things that a Bose-Einstein
condensate has been used for is slowing down light. The speed of light that you often see
and hear about is the speed it has in a vacuum, but many substances like regular old water
for instance, slow light down a little bit, at least from our point of view. From the
point of view of the photons themselves, they always keep going at the same speed. Weird,
yes, but that's just special relativity for ya. So, in simple terms, a Bose-Einstein condensate
can be made more opaque or more transparent by changing its frequency by pointing a laser at it.
If you then change the condensate's frequency at the same time you send a light pulse through it,
you can slow that light down and you can even stop it completely. This is what Danish physicist Lene
Hau did at Harvard University in the year 2000, and the year after she and her team effectively
stopped a light pulse inside a Bose-Einstein condensate. The important part here is that
this experiment shows that it's possible to store information in this brand new way
and that makes it extremely interesting for the making of quantum computers. As it is,
quantum computing is technology that's still in its infancy but new discoveries are being made
all the time. So who knows what the future will bring. Thank you for watching, I hope you learned
something new today, and I will see you soon :)
