Kind: captions
Language: en
[Applause]
how much energy is there in empty space
we believe there is 10 Theus e URS in
every cubic cimeter of empty space how
much is an urg it's a very small amount
you know if a if a mosquito flaps its
wings or a fly I guess flaps its wings
that's about one urg's worth of energy
so an urg is not very much uh we're
talking about a 100 millionth of an urg
in every cubic centimeter of space now
it might seem strange that completely
empty space contains energy but there
are two good reasons why scientists
think this is the case one is because
the expansion of the universe is
speeding up and that's thought to be
caused by dark energy which fills all
space and makes gravity repulsive we
always think the gravity always pulls
but according to Einstein the way
gravity works really depends on on the
material itself and energy tied to space
gravity pushes it doesn't pull if that
number is 10 Theus 8 URS per cubic cm
then you fit the data then that's the
dark energy it makes the universe
accelerate it's constant does everything
you want the second reason why empty
space contains energy is because it
isn't really empty there is the idea
that thanks to Quantum Mechanics we do
expect particles to come in and out of
existence and so space is full of energy
yeah there's there was nothing there
when we first looked nothing there when
we later looked could we say for sure if
we had looked in the middle that there
weren't particles there no there might
have been evidence for these so-called
virtual particles came from studying the
light emitted by energized hydrogen
atoms Neil's bore first used the
particular colors of light to devise his
Quantum model of the atom where the
electron orbits the nucleus in certain
allowed orbits each with its own
specific energy when the electron jumps
from a higher energy orbit to a lower
one it gives off light and the energy
and hence color of that light is
determined by the energy difference
between the two levels as quantum theory
developed B's orbits were superseded by
probabilistic electron clouds but the
energy levels remained mostly unchanged
then in 1947 Willis lamb and his grad
student Robert retherford attempted to
test the best version of quantum theory
at the time which was the dra equation
and they did this by measuring the
energy levels in hydrogen more precisely
than ever before now what they found did
not match up with the theory one of the
energy levels was actually not one but
rather two very closely spaced energy
levels and this is known as the lamb
shift they used micro waves to excite
the electron between these two levels so
why were there two energy levels where
there were supposed to be just one well
the answer is virtual particles for
example around the proton virtual
electron positron pairs pop in and out
of existence this is permitted by the
laws of quantum mechanics so long as
they don't stick around for too long
during their brief lives the negative
electron is attracted to the positive
proton while the positron is repelled
away from it this means the charge of
the proton is shielded by the virtual
electron surrounding it this affects the
energy of the 2s orbital in which the
electron is closer to the nucleus
differently from the 2p orbital and
that's what causes the energy levels to
split and this is just one contribution
to the energy difference the full
calculation involves drawing a series of
findan diagrams that show all the
possible interactions between real and
virtual particles and adding up the
contribution from each one and there's
an infinite chain of these different
diagrams so you might worry that you
predict nothing at all because you need
to add up an infinite number of numbers
to get the right answer but the point is
that every time you have these
two vertices these two places where the
photon hits the electron that's
associated with a number Alpha defined
structure constant which is 1 over 137
which is a small number so this leading
term this classical contribution is
already small and these more complicated
ones you notice there are more vertices
appearing and as the diagrams get more
and more complicated there are more and
more of these terms so you get alpha
squar alpha cubed Alpha to the 4th and
the numbers become smaller and smaller
and smaller so the more complicated
diagrams the more intricate effects of
the virtual particles become less and
less important using the dur equation
but adding in the effects of virtual
electrons positrons and photons we can
actually calculate the values of atomic
parameters with better accuracy than one
part in a billion that is is incredible
in fact physicist Lawrence Krauss calls
it the best most accurate prediction in
all of science so with the spectacular
usefulness of virtual particles You'
think it would be pretty hard for anyone
to doubt their existence but then again
they have never been directly observed
in fact theoretically that's impossible
explaining virtual particles is is
always tricky
because it it's very hard to think of
them in any sort of real way they can go
in the opposite direction of their
momenta and and do these things which
are not consistent with with a real
particle so virtual particles have a
strange sort of existence where they're
indispensable for calculations obviously
but you can never directly observe them
but maybe this is not actually a real
problem it's just the result of thinking
about reality in the wrong way maybe as
the fundamental building blocks of our
universe we shouldn't be talking about
particles at all but rather Fields the
truth is that Quantum field theory is
according to our best current modern
understanding the right way to talk
about nature at a fundamental level uh
when we talk about particle physics when
we talk about electrons and gluons and
quarks to the modern physicist these are
all vibrations in fields in exactly the
same way that the photon is a vibration
in the electromagnetic field just like
there's an electromagnetic field there's
also an electron field there's a
neutrino field field there's an up Quark
field and a top Quark field Etc and
these fields fill space and usually
they're just sitting there fairly
quietly but when they vibrate and you
look at it you see that in terms of
individual particles in this picture
virtual particles are quantum
fluctuations little disturbances in the
field they play a role in interactions
but they aren't measurable as real
particles themselves and since they
exist everywhere they should imbue the
vacuum with energy if you were to try to
estimate the amount of energy due to the
virtual particles in empty space what
kind of number do you get for the energy
of empty space yeah this is a huge
problem because the back of the envelope
estimate says there should be about 10
to the 112 herbs per every cubic cm
compared to the 10us 8th that we
actually get in other words you predict
10 to the 120 times the energy density
of empty space that you actually observe
so obviously your back of the envelope
calculation is wildly wildly Incorrect
and no one is really quite sure why the
way we see the universe responding the
cosmological constant the way we expect
the vacuum to behave are the most
discrepant things that we have in
physics if they are the same thing how
is it possible that both the best and
the worst prediction in science are both
based on the same underlying physics of
virtual particles I mean one experiment
seems to show that the theory is
perfectly correct while another shows
that we could not be more wrong you know
there are a lot of areas of human
endeavor where we fear being so wild ly
incorrect but in science this
discrepancy is actually really exciting
because it shows us a piece of the
puzzle that we haven't figured out just
yet that we have to solve this problem
if we are to fully understand the
universe and in that way this is the
best clue we have at the work that still
needs to be done so yeah we don't know
but that's always good it's always good
to say you don't know right I think so
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