[Music]
the mysterious
zero point energy the quantum vacuum
has been a misrepresented subject of
science fiction
and pseudoscience baloney for too long
let's talk about what vacuum energy
really can and really can't do
[Music]
it seems pretty crazy that space itself
might contain a higher density of energy
than the nucleus of the atom this is the
prediction of quantum field theory
that there exists an energy of the
vacuum resulting from the non-zero
zero point energies of the quantum
fields that fill our universe
for the electromagnetic field alone this
energy density has been estimated to be
up to a crazily high
10 to the power of 112 eggs per
centimeter cubed
on the other hand our observations of
the accelerating expansion of the
universe
suggests a vacuum energy density of only
10 to the power of
minus 8 eggs per centimeter cubed as we
discussed in our episode on the vacuum
catastrophe
this mismatch between the measured and
theoretical values of vacuum energy
is one of the greatest unsolved problems
in physics
despite this minor glitch quantum field
theory is arguably the most successful
theory in all of physics
in terms of sheer predictive power this
means we should take the idea of
zero point energy seriously and are
justifiably perplexed
at the mismatch between theory and
measurements but the scientific validity
of
zero point energy has also encouraged
some pretty terrible
pseudoscience and outright quackery on
the subject
from accessing this energy as a free
power source to manipulating it to
generate warp fields
or pushing against the vacuum energy in
proportionalist spaceship engines
today i want to debunk some of the
nonsense surrounding the quantum vacuum
let's start with the physics if the
vacuum has an energy density of
10 to the power of ridiculous ergs per
centimeter cubed
where is it why can't we pull infinite
free energy
out of nothing well the answer lies in
entropy
and the second law of thermodynamics
entropy can be thought of as a measure
of the
specialness of the arrangement of a
system of particles
higher entropy means a more disordered
arrangement
the universe tends towards disorder and
so a highly unusual arrangement will
decay
over time the entropy of a closed system
always increases and
that's the second law of thermodynamics
right there we harness that decay of
order
whenever we draw energy from a system
for example
the piston in a car engine only rises
when the interior
chamber becomes much hotter than the
exterior the combined volume of the
inside and outside of the piston chamber
is in a low entropy configuration it's a
special
unusual configuration of particles in
its return to
high entropy equilibrium energy is
extracted and your car accelerates
but try to drive your car in a dense
1500 kelvin atmosphere
and that piston has no reason to rise
it's already in equilibrium your car
doesn't go
anywhere and then it explodes so even if
the system contains a lot of energy
that energy may be inaccessible and this
would be the case
even with an extremely energetic quantum
vacuum
on its own the vacuum is the same
everywhere
it's already in perfect equilibrium
energy is extracted in the movement
towards equilibrium
in the increase of entropy no such
movement is possible for the vacuum
and so the vacuum contains no useful
energy
this is why so-called zero-point energy
machines
are baloney they claim to access an
inexhaustible source of energy
but no such source exists in order to
access vacuum energy
we need to introduce a disequilibrium
in fact we need to reduce the vacuum
energy in one region of space
the universe would then try to fill that
energy whole and we could
harness that to extract energy in fact
there's one way to do this with the
casimir effect
if you bring a pair of conducting plates
very close together
a proportion of the virtual particles
will be excluded
from between them by cutting off certain
frequency modes between the plates you
lower the vacuum energy
in that region the result is a pressure
differential
that produces a measurable force pulling
the plates together
now we talk about that in detail here
so what about building a casimir engine
unfortunately no while that initial pull
between the plates may seem like a free
lunch
to extract continuous energy you need to
pull the plates apart again
this takes as much energy as you
originally gained
another popular notion is that the
reduction of energy between casmir
plates
can be considered negative energy and so
it could be useful for all of those
wonderful things that negative energy
can do like
opening wormholes or creating an
alcubiary warp field
well if we define the average vacuum
energy as
zero then the cashmere effect produces
negative energy unfortunately the vacuum
energy between casimir plates is very
much
positive in an absolute sense the
gravitational effect of energy depends
on its absolute value
that means the vacuum energy between
casimir plates still produces
positive spatial curvature not the
negative curvature required for warp
drives
another popular use for the quantum
vacuum is as a
medium to push against for
proportionless engine systems
like the rf resonant cavity thruster aka
the em drive this notion is particularly
silly
the fact is any acceleration of a real
particle involves a transfer of momentum
between real particles
via virtual particles virtual particles
and hence the quantum vacuum
mediate all forces however it's not
possible to transfer momentum from a
real particle to the vacuum
without getting another real particle
out the other end
that momentum must be given up by the
vacuum to produce real particles again
those particles would become the
propellant carrying momentum away
in the case of the em drive the proposal
is that microwaves within the drive's
resonant cavity
push against the quantum vacuum if
particles are somehow
extracting momentum from the resonant
cavity then they're giving it up again
to real particles probably photons
on the outside this would certainly
produce thrust
but no more than the rather anemic
photonic thruster
so don't believe the hype vacuum energy
is real
and it's part of the fundamental
clockwork of the universe
but is probably very weak weighing in at
10 to the power of -8 eggs per
centimeter cubed
and regardless of its strength it's not
accessible to us
as an energy source or as a miracle
resource for fast space travel
sorry internet all that said the quantum
vacuum
does have its uses just ask the gecko
geckos are able to cling to almost any
surface by making use of
van der waals forces which are
essentially the same thing as the
casimir force
the pads of gecko feet are covered with
microscopic hairs
called setae these hairs split into
millions of
shaped ends they're around 0.2
micrometers in diameter
when a gecko presses its cute little
feet onto any surface
a fraction of these hairs are close
enough to the surface so that casimir
forces come into effect
geckos literally manipulate quantum
vacuum energy
to climb walls you know what this
suggests a good challenge question
let's say that adult geckos are able to
efficiently apply
two hundred thousand setae ends at any
one time to a surface
and each setter produces sufficient van
der waals force
to withstand 200 micronewtons of sheer
force
how many geckos do you need to catch and
put on a leash
in order to drag you up any wall using
only the power of the quantum vacuum
and i guess we should have an extra
credit question
so as we saw last week the theoretical
density of the vacuum energy
due to the electromagnetic field is
estimated by integrating the energy
in all possible frequency modes up to
some cutoff frequency
you get that the vacuum energy density
is proportional to the fourth power of
that cutoff frequency
based on the vacuum energy densities
we've already talked about
answer me this what cutoff frequency
is needed for this theoretical estimate
to give the energy density implied
by dark energy and
is this a possible maximum virtual
photon frequency
given the results of casimir experiments
why or why not email your answer to one
of these questions
to pbsspacetime gmail.com within two
weeks of the release of this video
use the subject line zero point
challenge because we filter by subject
line
make sure you show all of your work
we'll choose six correct answers
to win one of our t-shirts so you can
show off
your mastery of the mysteries of
space-time
you