Kind: captions
Language: en
1.3 billion years ago in a galaxy far
far away two black holes merged as they
violently spiraled into each other they
created traveling distortions in the
fabric of SpaceTime gravitational waves
in the last 10th of a second the energy
released in these waves was 50 times
greater than the energy being released
by everything else in the observable
universe combined it's like a a
inspiring kind of energy after spreading
out through the universe at the speed of
for over a billion years the waves
reached Earth where they stretched and
squeezed space such that two light beams
traveling in perpendicular pipes were
put slightly out of Step allowing humans
to detect the existence of gravitational
waves for the first
time that's a simple enough story to
tell but what I found out when I went to
visit Professor r adari at Caltech is
that it hides the absurdity of just what
was required to make that detection
there's a lot of things about
gravitational waves which are
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absurd is that is that's it yeah the
main problem with detecting
gravitational waves is that they're tiny
they stretch and squeeze Space by just
one part in 10 to the 21 that's the
equivalent of measuring the distance
between here and Alpha centuri and then
trying to measure variations in that
distance that are the width of a human
hair to detect such tiny Wiggles you
have to measure over as large a distance
as possible which is why the arms of the
interferometers are 4 km and even with
arms this long gravitational waves vary
the length of the Arms by at most 10- 18
M so the detector has to be able to
reliably measure distances just 110,000
the width of a proton it's the tiniest
measurement ever made so how is it
possible to measure that considering all
the other sources of vibrations and
noise in the environment like
earthquakes traffic and electrical
storms well for one thing the mirrors
are the smoothest ever created they
weigh 40 kg or 90 lb and are suspended
by silica threads just twice the
thickness of a hair to isolate them from
their environment and even then the only
way to be certain not to be tricked by
environmental noise was to build two
detectors far apart from each other in
reasonably quiet locations that allows
to distinguish between local noise which
would appear at only one site and
gravitational waves which would pass
through both sights almost
simultaneously I in a building that
contains a 1 to 100 scale of ligo the
gravitational wave detector the next
challenge is the laser um who who it's a
lot of stuff it is you need a laser that
can provide one and exactly one
wavelength you can imagine if your laser
wavelength is changing and you're trying
to use interference of light waves to
make this measurement it's never going
to work because it's something like
trying to measure this distance but your
ruler stick is constantly changing back
and forth you can't tell how many inches
this is all this equipment um at least
3/4 of it all we're trying to do is make
the laser more stable and by the end of
the day what we've achieved is something
which has a stability of one part in 10
the 20 what does that mean that's 100
billionth of a trillion that's kind of
what we end up with the best lasers for
this purpose have a wavelength of 1,64
NM that's infrared light but this
presents a problem how can you measure
10- 18 with 10- 6 length of light yes I
wish more people would ask this question
it's great for this animation to show
such a large shift in the wavelength but
the reality is it's only one trillionth
of a wavelength that the arms are
shifting in length it seems obvious that
you can measure half a wavelength
because that'll cause the light to
interfere with itself yeah but that's
fully that'll go from completely dark to
completely bright so are you looking at
like slightly darker and slightly
brighter yeah yeah and the limit here to
how good we can measure this difference
between dark and bright has to do with
the the fact that the light is discrete
it comes in discrete chunks which are
called photons the variation in the
number of photons hitting the mirrors at
any instant due to this Quantum
uncertainty is proportional to the
square root of the total number of
photons
what this means is the more photons you
use the smaller the uncertainty gets as
a fraction of the total this is why the
laser power in the arms is 1
megawatt that is enough energy to power
a th000 homes in a light beam and a
megawatt you know boom it won't even rip
your head off you just vaporized it be
just a smoking
stump even with a perfect laser and 1
megawatt of power anything the light
hits would interfere with it even the
air so all the air in the arms of the
detectors had to be eliminated and it
took 40 days to pump down to just a
trillionth of atmospheric pressure and
the tubes were heated up to the
temperature of an oven to expel any
residual gases they pumped out enough
air to fill up 2 and 1/2 million
footballs making it the second largest
vacuum in the world after the Large
Hadron Collider now here's something
most people don't think about which is
that gravitational waves stretch
SpaceTime so light traveling through
that face should be stretched as well if
everything is stretching how do you know
anything is stretching how do you know
anything is stretching that's the
conundrum it doesn't make any sense it
do this thing this whole thing is bogus
shut it
down I would send the laser beam down
this tube and then wait for it to come
back and then I would say well nothing
happened because uh the space got
stretched and the laser wavelength got
stretched it's it looks the same if it
got stretched or not stretched doesn't
make any sense well it's it's sort of a
matter of timing is how it works so the
amount of time it takes for light to go
down this tube and comes back uh is very
short however the wave the gravitational
wave when it comes through it's doing
this slow thing like this noise I made
um which is low it's a it's a slow
stretching it's only 100 times per
second and it's true when the wave comes
through um the light which is in there
it actually does get
stretched and and then that part doesn't
doesn't do the measurement for us but um
now that the space has stretched that
laser light is like come and gone it's
out of the picture we're constantly
shooting the laser back into the system
so the new Fresh Light now goes through
there and has to travel a bigger
distance than the light before and so by
looking at how this interference changes
with time and keeping the laser
wavelength from the laser itself fixed
were able to do the measurement so what
was needed to detect gravitational waves
well a megawatt of laser power to
minimize shot noise of exactly one
wavelength because we're trying to
measure just a trillionth of that
wavelength continually inserted to
replace older light that's been
stretched and squished in the world's
second largest vacuum chamber at just a
trillionth of atmospheric pressure
hitting the smoothest mirrors ever
created suspended by silica threads at
two distant sights to eliminate noise
with 4 km long arms to increase the
magnitude of gravitational waves to just
a thousandth of the width of a proton
you know what we already do daily in
here is what I would have said is
impossible if you'd asked me about it 20
years ago one of the things that was
most interesting for me to learn was
what is limiting the sensitivity of the
detectors today and it turns out it is
quantum mechanics and essentially you
can think of it like a Heisenberg
uncertainty principle you've got uh two
things and together their uncertainty
has to be bigger than a certain value
luckily for us we're only trying to
measure one thing here we're not trying
to measure two things at the same time
all we want to know is how much more did
this arm get stretched from that arm and
that's that's the key point which people
did not understand until recently the
way to build these systems is such that
they're extremely good in measuring one
thing and that all of the uncertainty
which comes from quantum mechanics uh is
completely crammed into this other thing
that we don't care about I feel like
we're getting down to these levels of
nature where it seems like nature
doesn't want you to go any further but
through our Ingenuity we're figuring out
ways to engineer Quantum noise I think
that's such a remarkable concept and I
look forward to the results that it's
going to bring I think the next logical
step is to go from two signals to
detecting all the black holes in the
universe all of the time it's not like a
alien civilization level of Technology
it's just we have to do a lot better
than what we're doing now but it's I see
it sort of
within within our grasp
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