[Music]
there is
nothing more important in physics than
the speed of light its abbreviation
c literally means constant like the
constant
the one that matters but here's a secret
the speed of light changes all the time
it's about 25 slower in water 35
in glass and more than twice as slow
when traveling through a diamond
sea itself measures the speed of light
in a vacuum about 300 million meters per
second
and it's literally the fastest anything
in the universe can go and you know what
in nature light is always fast because
25
slower than the fastest thing in the
universe is still stupid fast but in the
lab
scientists have done something
extraordinary created beams of light
that are slower than a car and then with
the literal flick of a switch
they can freeze that beam in place if
you're thinking that sounds tricky
you're right but if scientists can
master it this could be the key to our
computing future the key to catching a
beam of light is called
electromagnetically induced transparency
and it was first observed in 1991 it's a
way of taking something opaque
and making it perfectly transparent at a
given wavelength
using nothing but a couple beams of
light which weirdly enough is possible
to do it you need an opaque cloud of gas
say sodium gas that's super cold so
atoms aren't moving very much that cloud
also needs to be in a vacuum so there's
no outside interference
and the atoms in the cloud need to have
electrons with three available energy
levels initially the electrons in each
atom are in the lowest energy level then
you shine a pair of lasers into the gas
the first is called the probe and it
emits light with just the right amount
of energy to take electrons from the
first energy level
to the third if you were to shine it
into the cloud by itself it would all
get instantly absorbed by the electrons
jumping up to that third energy level
so the cloud would stay opaque because
being opaque just means you don't let
light through now there's another laser
called the coupling beam
and it has the right amount of energy to
take electrons from the second energy
level to the third except
if all the electrons are in the lowest
energy level there aren't any to move
from level two to three so the beam just
passes through untouched so if you shine
the light separately
only the coupling beam goes through the
cloud but when you shine the two beams
together
things get weird if you turn the
coupling beam on first
and then the probe beam the probe can
suddenly shine straight through
what happens is the coupling beam is
trying to shift the electrons from level
two to three
but at the same time the probe beam is
trying to shift them from one to three
faced with these two tugs the electron
basically gets trapped between them
effectively ending up in level one and
level two at the same time and when that
happens the electron doesn't absorb
photons from either
beam of light physicists call this
situation a dark state which
sounds ominous but it just means that
the electron ends up in the dark about
the presence of either laser now that
the light's not getting absorbed the
cloud becomes transparent to the laser
and the probe beam can travel straight
through it's a neat trick but it doesn't
come for free the probe can
pass through but it does so super slowly
see the probe beam is made up of a bunch
of light waves at frequencies that are
really similar but not quite the same as
they enter the cloud
those waves slow down at different rates
and start to interfere with each other
and that causes the entire probe beam to
slow down
like instead of traveling at a billion
kilometers an hour it's suddenly going
more like
25 slow enough that you could keep up
with this beam of light
on a bike that sudden slow down also
bunches up the light beam into a tiny
sluggish package but physicists weren't
done once they'd slowed light to a crawl
in some cases they went on to stop light
altogether they accomplished that by
turning
off the coupling beam while that
scrunched up package of light was making
its way through the gas
at that point the package completely
vanished once again the energy in that
light got absorbed into the sodium atoms
but what's incredible
is that that light wasn't just gone when
they turned the coupling beam back on
a package of light emerged with the same
exact phase and energy
as it had before somehow the sodium
atoms were
storing information about that beam of
light so far
scientists have been able to freeze
light for up to 1.5 seconds but it's not
just for kicks this might also have
applications in fields like
quantum computing and cryptography for
instance the last time you bought a
computer
one of the specs you probably had to
pick was the amount of memory or ram
that you wanted the ability to store
something for later is kind of a
fundamental part of what makes computers
useful but the quantum computers built
so far don't really have memory since
quantum information is more complex than
ones and zeros
these machines can't store information
like a regular computer does
electromagnetically induced transparency
seems to offer one way to solve that
problem after all stopping light is all
about getting atoms to hold on to
information in the configuration of
their electrons and that's exactly the
stuff quantum computers are working with
now having a lab's worth of equipment
keeping a bunch of sodium atoms near
absolute zero doesn't sound scalable
but there are other ways to induce
transparency some researchers have done
it using materials much closer to room
temperature
and in 2005 ibm developed a computer
chip
that could slow light by a factor of
300. that's a far cry from the 30
million time slowdown done using
ultracold atoms
and it's not the same as stopping light
but it's a step
and even if it's all a long way from
being useful there's definitely still
something incredible
about taking one of physics's most
important rules
and bending it to our will thanks for
watching this episode of scishow and a
special thanks to our patrons on patreon
who make it possible for us to make
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[Music]
scishow