Kind: captions
Language: en
this is a video about one of the most
important yet least understood Concepts
in all of physics it governs everything
from molecular collisions to humongous
storms from the beginning of the
universe through its entire Evolution to
its inevitable end it may in fact
determine the direction of time and even
be the reason that life exists to see
the confusion around this topic you need
ask only one simple
question what does the Earth get from
the Sun what does the Earth get from
Sun um well it's light rays what do we
get from the Sun warmth warmth light
vitamin D we get vitamin D from we do
get vitamin D from the ultraviolet rays
well a lot of energy what does the Earth
get from this energy yeah it's energy
energy nailed it every day the Earth
gets a certain amount of energy from the
Sun
and then how much energy does the Earth
radiate back into space relative to that
amount that it gets from the Sun
probably not as much I you know I don't
believe that it's it's just radiating
right back I would say less less I say
less I'd guess about 70% I think it's a
fraction I'd say 20% because because we
use some of it we use some of the energy
we consume a lot right but the thing
about energy is it never really goes
away you can't really use it up it would
have to break even wouldn't it same
amount yeah you know cause and effect it
be equal in some ways right for most of
the Earth's history it should be exactly
the same amount of energy in from the
Sun as Earth radiates into space wow
because if we didn't do that then the
Earth would get a lot hotter that'd be a
problem that'd be a big problem so if
that is the case y then what are we
really getting from the
Sun that's a good question h it gives us
a nice 10 it gives us a nice 10 I love
it we're getting something special from
the Sun I don't know what do we get
without the energy but nobody talks
about it to answer that we have to go
back to a discovery made two centuries
ago in the winter of 1813 France was
being invaded by the armies of Austria
Prussia and Russia the son of one of
Napoleon's generals was Sadi Carno a
17-year-old student on December 29th he
writes a letter to napole to request to
join in the fight Napoleon preoccupied
in battle never
replies but Carno gets his wish a few
months later when Paris is attacked the
students defend a chateau just east of
the city but they're no match for the
advancing armies and Paris Falls after
only a day of fighting forced to retreat
Carno is
devastated 7 years later he goes to
visit his father who's fled to Prussia
after Napoleon's downfall his father was
not only a general but also a physicist
he wrote an essay on how energy is most
efficiently transferred in mechanical
systems when his son comes to visit they
talk at length about the big
breakthrough of the time steam engines
steam engines were already being used to
power ships mine ore and excavate ports
and it was clear that the future
industrial and Military might of Nations
depended on having the best steam
engines but French designs were falling
behind those of other countries like
Britain so sad Carno took it upon
himself to figure out
why at the time even the best steam
engines only converted around 3% of
thermal energy into useful mechanical
work if he could improve on that he
could give France a huge advantage and
restore its place in the world so he
spends the next 3 years studying heat
engines and one of his key insights
involves how an ideal heat engine would
work one with no friction and no losses
to the environment it looks something
like
this take two really big metal bars one
hot and one cold the engine consists of
a chamber filled with air where heat can
only flow in or out through the bottom
inside the chamber is a piston which is
connected to a flywheel the air starts
at a temperature just below that of the
hot bar so first the hot bar is brought
into contact with the chamber the air
inside expands with heat flowing into it
to maintain its temperature this pushes
the Piston up turning the flywheel next
the hot bar is removed but the air in
the chamber continues to expand except
now without heat entering the
temperature decreases in the ideal case
until it is the temperature of the cold
bar the cold bar is brought into contact
with the chamber and the flywheel pushes
the Piston down and as the air is
compressed heat is transferred into the
cold bar the cold bar is removed the
flywheel compresses the gas further
increasing its temperature until it is
just below that of the hot bar then the
hot bar is connected again and the cycle
repeats through this process heat from
the hot bar is converted into the energy
of the flywheel
and what's interesting to note about
carno's ideal engine is that it is
completely reversible if you ran the
engine in Reverse first the air expands
lowering the temperature then the
chamber is brought into contact with the
cold bar the air expands more drawing in
heat from the cold bar next the air is
compressed increasing its temperature
the chamber is placed on top of the hot
bar and the energy of the flywheel is
used to return the heat back into the
hot bar are however many cycles were run
in the forward Direction you could run
the same number in reverse and at the
end everything would return to its
original state with no additional input
of energy required so by running an
ideal engine nothing really changes you
can always undo what you
did so what is the efficiency of this
engine since it's fully reversible you
might expect the efficiency to be 100%
but that is not the case each cycle the
energy of the flywheel increases by the
amount of heat flowing into the Chamber
from the hot bar minus the heat flowing
out of the chamber at the cold bar so to
calculate the efficiency we divide this
energy by the heat input from the hot
bar now the heat in on the hot side is
equal to the work done by the gas on the
piston and this will always be greater
than the work done by the piston on the
gas on the cold side which equals the
heat out and this is because on the hot
side the hot gas exerts a greater
pressure on the Piston than that same
gas when cold to increase the efficiency
of the engine you could increase the
temperature of the hot side or decrease
the temperature of the cold side or
both Lord Kelvin learns of carno's Ideal
heat engine and realizes it could form
the basis for an absolute temperature
scale
imagine that the gas is allowed to
expand an extreme amount so much that it
cools to the point where all the gas
particles effectively stop moving then
they would exert no pressure on the
piston and it would take no work to
compress it on the cold side so no heat
would be lost this is the idea of
absolute zero and it would make for a
100% efficient engine using this
absolute temperature scale the Kelvin
scale
we can replace the amount of heat in and
out with the temperature of the hot and
cold side respectively because they are
directly proportional so we can express
efficiency like this which we can
rewrite like
this what we have learned is that the
efficiency of an ideal heat engine
doesn't depend on the materials or the
design of the engine but fundamentally
on the temperatures of the hot and cold
sides to reach 100% efficiency you'd
need infinite temperature on the hot
side or absolute zero on the cold side
both of which are impossible in practice
so even with no friction or losses to
the environment it's impossible to make
a heat engine 100% efficient and that's
because to return the Piston to its
original position you need to dump heat
into the cold bar so not all the energy
stays in the
flywheel now in carno's time high
pressure steam engines could only reach
temperatures up to 160° C so their
theoretical maximum efficiency was
32% but their real efficiency was more
like 3% that's because real engines
experience friction dissipate heat to
the environment and they don't transfer
heat at constant temperatures so for
just as much heat going in less energy
ends up in the flywheel the rest is
spread out over the walls of the
cylinder the axle of the flywheel and is
radiated out into the
environment when energy spreads out like
this it is impossible to get it back so
this process is irreversible the total
amount of energy didn't change but it
became less usable energy is most usable
when it is concentrated and less usable
when it's spread
out decades later German physicist
Rudolph Claus studies carno's engine and
he comes up with a way to measure how
spread out the energy is he calls this
quantity
entropy when all the energy is
concentrated in the hot bar that is low
entropy but as the energy spreads to the
surroundings the walls of the chamber
and the axle will entropy increases this
means the same amount of energy is
present but in this more dispersed form
it is less available to do
work in 1865 colaus summarizes the first
two laws of thermodynamics like this
first the energy of the universe is
constant and second the entropy of the
universe tends to a maximum in other
words energy spreads out over
time the second law is core to so many
phenomena in the world it's why hot
things cool down and cool things heat up
why gas expands to fill a container why
you can't have a perpetual motion
machine because the amount of usable
energy in a closed system is always
decreasing the most common way to
describe entropy is as disorder which
makes sense because it is associated
with things becoming more mixed random
and less ordered but I think the best
way to think about entropy is as the
tendency of energy to spread out so why
does energy spread out over time I mean
most of the laws of physics work exactly
the same way forwards or backwards in
time so how does this clear time
dependence
arise well let's consider two small
metal bars one hot and one cold for this
simple model we'll consider only eight
atoms per bar each atom vibrates
according to the number of energy
packets it has the more packets the more
it vibrates so let's start with seven
packets of energy in the left bar and
three in the right the number of energy
packets in each bar is what we'll call a
state first let's consider just the left
bar it has seven energy packets which
are free to move around the lattice this
happens nonstop the energy packets hop
randomly from atom to atom giving
different configurations of energy but
the total energy stays the same the
whole time now let's bring the cold bar
back in with only three packets and
touch them together the energy packets
can now hop around between both bars
creating different
configurations each unique configuration
is equally likely
so what happens if we take a snapshot at
one instant in time and see where all
the energy packets are so stop look at
this now there are nine energy packets
in the left bar and only one in the
right bar so heat has flowed from cold
to hot shouldn't that be impossible
because it decreases
entropy well this is where ludvig
boltzman made an important Insight heat
flowing from cold to hot is not
impossible it's just
improbable there are
91,5 120 configurations with nine energy
packets in the left bar but 627,000
with more energy packets than it
started so why don't we observe this
happening around us well watch what
happens as we increase the number of
atoms to 80 per bar and the energy
packets to 100 with 70 in the left bar
and 30 in the right there is now only a
05% chance that the left solid ends up
hotter than it started and this trend
continues as we keep scaling up the
system in everyday solids there are
around 100 trillion trillion atoms and
even more energy packets so heat flowing
from cold to hot is just so unlikely
that it never
happens think of it like this Rubik's
cube right now it is completely solved
but I'm going to close my eyes and make
some turns at random if I keep doing
this it will get further and further
from being solved but how can I be
confident that I'm really messing this
Cube up well because there's only one
way for it to be solved a few ways for
it to be almost solved and quenti of
ways for it to be almost entirely random
without thought and effort every turn
moves the Rubik's Cube from a highly
unlikely state that of it being solved
to a more likely State a total
mess so if the natural tendency of
energy is to spread out and for things
to get Messier then how is it possible
to have something like air conditioning
where the cold interior of a house gets
cooler and the hot exterior gets
hotter energy is going from cold to hot
decreasing the entropy of the
house well this decrease in entropy is
only possible by increasing the entropy
a greater amount somewhere else in this
case at a power plant the concentrated
chemical energy and coal is being
released heating up the power plant and
its environment spreading to the turbine
the electric generators heating the
wires all the way to the house and
producing waste heat in the fans and
compressor whatever decrease in entropy
is achieved at the house is more than
paid for by an increase in entropy
required to make that
happen but if total entropy is
constantly increasing and anything we do
only accelerates that increase then how
is there any structure left on Earth how
are there hot Parts separate from cold
Parts how does life
exist well if the Earth were a closed
system the energy would spread out
completely meaning all life would cease
everything would Decay and mix and
eventually reach the same temperature
but luckily Earth is not a closed system
because we have the
[Music]
Sun what the Sun really gives us is a
steady stream of low entropy that is
concentrated bundled up energy the
energy that we get from the Sun is more
useful than the energy we give back it's
more compact it's more clumped together
plants capture this energy and use it to
grow and create sugars then animals eat
plants and use that energy to maintain
their bodies and move around bigger
animals get their energy by eating
smaller animals and so on and each step
of the way the energy becomes more
spread out okay interesting yeah oh wow
I did not know that there you go
ultimately all the energy that reaches
Earth from the Sun is converted into
thermal energy and then it's radiated
back into space but in fact it's the
same amount I know this is you you do
know this is I'm I'm a PhD physicist
okay anyway I trust
you the increase in can be seen in the
relative number of photons arriving at
and leaving the Earth for each Photon
received from the Sun 20 photons are
emitted and everything that happens on
Earth plants growing trees falling herds
stampeding hurricanes and tornadoes
people eating sleeping and breathing all
of it happens in the process of
converting fewer higher energy photons
into 20 times as many lower energy
photons without a source of concentrated
energy and a way to discard the spread
out energy life on Earth would not be
possible it has even been suggested that
life itself may be a consequence of the
second law of Thermodynamics if the
universe tends toward maximum entropy
then life offers a way to accelerate
that natural tendency because life is
spectacularly good at converting low
entropy into high entropy for example
the surface layer of seawater produces
between 30 to
680% more entropy when cyanobacteria and
other organic matter is present than
when it's not Jeremy England takes this
one step further he's proposed that if
there is a constant stream of clumped up
energy this could favor structures that
dissipate that energy and over time this
results in better and better energy
dissipators eventually resulting in Life
or in his own words you start with a
random clump of atoms and if you shine
light on it for long enough it should
not be so surprising that you get a
plant so life on Earth survives on the
low entropy from the Sun but then where
did the Sun get its low
entropy the answer is the universe if we
know that the total entropy of the
universe is increasing with time then it
was lower entropy yester yesterday and
even lower entropy the day before that
and so on all the way back to the Big
Bang so right after the big bang that is
when the entropy was lowest this is
known as the past hypothesis it doesn't
explain why the entropy was low just
that it must have been that way for the
universe to unfold as it has but the
early Universe was hot dense and almost
completely uniform I mean everything was
mixed and the temperature was basically
the same everywhere varying by at most
0.001% so how is this low entropy well
the thing we've left out is gravity
gravity tends to Clump matter together
so taking gravity into account having
matter all spread out like this would be
an extremely unlikely State and that is
why it's low
entropy over time as the universe
expanded and cooled matter started to
Clump together in more dense regions and
in doing so enormous amounts of
potential energy were turned into
kinetic energy and this energy could
also be used like how water flowing
downhill can power a
turbine but as bits of matter started
hitting each other some of their kinetic
energy was converted into heat so the
amount of useful energy decreased
thereby increasing entropy over time the
useful energy was used in doing so stars
planets galaxies and life were formed
increasing entropy all along the
universe started with around 10 to the
88 boltzman constants worth of entropy
nowadays all the stars in the observable
universe have about 9.5 * 10 80 the
interstellar and Intergalactic medium
combined have almost 10 times more but
still only a fraction of the early
universe
a lot more is contained in neutrinos and
in photons of the cosmic microwave
[Music]
background in 1972 Jacob beckenstein
proposed another source of entropy black
holes he suggested that the entropy of a
black hole should be proportional to its
surface area so as a black hole grows
its entropy
increases famous physicists thought the
idea was nonsense and for good reason
according to classical thermodynamics if
black holes have entropy then they
should also have a temperature but if
they have temperatures they should emit
radiation and not be black after
all the person who set out to prove
beckenstein wrong was Steven Hawking but
to his surprise his results showed that
black holes do emit radiation now known
as Hawking radiation and they do have a
temperature the black hole at the center
of the Milky Way has a temperature of
about 100 trillionth of a Kelvin
emitting radiation that is far too weak
to detect so still pretty black but
Hawking confirmed that black holes have
entropy and beckenstein was right
Hawking was able to refine beckenstein's
proposal and determine just how much
entropy they have the super massive
black hole at the center of the Milky
Way has about 10 to the 91 boltzman
constants of entropy that is a thousand
times as much as the early observable
universe and 10 times more than all the
other particles combined and that is
just one black hole all black holes
together account for 3 * 10 104 boltzman
constants worth of
entropy so almost all the entropy of the
universe is tied up in black holes that
means the early Universe only had about
3% of the the entropy it has
now so the entropy was low and
everything that happens in the universe
like planetary systems forming galaxies
merging asteroids crashing Stars dying
to life itself flourishing all of that
can happen because the entropy of the
universe was low and it has been
increasing and it all happens only in
One Direction we never see an asteroid
UNC crash or a planetary system unmix
into the cloud of dust and gas that made
it up there is a clear difference
between going to the past and the future
and that difference comes from entropy
the fact that we are going from unlikely
to more likely States is why there is an
arrow of
time this is expected to continue until
eventually the energy gets spread out so
completely that nothing interesting will
ever happen again this is the heat death
of the
Universe in the distant future more than
10 to the 100 years from now after the
last black hole has evaporated the
universe will be in its most probable
State now even on large scales you would
not be able to tell the difference
between time moving forwards or
backwards and the arrow of time itself
would
disappear so it sounds like entropy is
this awful thing that leads us
inevitably towards the dullest outcome
imaginable but just because maximum
entropy has low complexity does not mean
that low entropy has maximum complexity
it's actually more like this tea and
milk I mean holding it like this is not
very interesting but as I pour the milk
in the two start to mix and these
beautiful patterns emerge they arise in
an instant and before you know it
they're gone back to being featureless
both low and high entropy are low in
complexity it's in the Middle where
complex structures appear and Thrive and
since that's where we find ourselves
let's make use of the low entropy we've
got while we
can with the right tools we can
understand just about anything from a
cup of tea cooling down to the evolution
of the entire universe and if you're
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