how do you measure something that's really really small
well first you might try a measuring tape then calipers and if you've got a
big budget you might treat yourself to a scanning electron microscope
but there is one very clear problem with
all of those solutions a they're too small and b
they don't use lasers at lago they had the right idea so by
shooting a laser four kilometers down this tube they're able to get an
accuracy of one ten thousandth the charge diameter of proton
that's like measuring from here to the closest star with the accuracy of a hair
which i don't know i still don't have context for that but it got them a noble
prize for physics and they were the first ones to get gravitational waves
detected so awesome
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gravitational waves happen all the time every time a mass accelerates waves
ripple through space time so like i'm actually waving space time right now
but it only really becomes noticeable when crazy massive things start
accelerating like black holes colliding when einstein first predicted
gravitational waves in 1916 he thought they were a problem with his algebra and
then later when he believed that they were real he figured it would still be
impossible to detect them but of course that hasn't stopped people from trying
in the 50s joseph weber first tried to detect gravitational waves using the 6
000 pound chunk of aluminum the idea is that it would resonate
when gravitational waves went through it and that would be picked up by these piezoelectric crystals on the top
unfortunately it just wasn't possible to get the precision needed
in 1972 though ray vice wrote a paper
detailing just how an interferometer could be used to detect gravitational
waves basically he wrote a laser beam could go
through a beam splitter and then down two four kilometer long tubes at the end
of each of these tubes would be a mirror that sends the beam back where it would
be recombined by the beam splitter and then measured
normally when recombined in this manner the two beams would destructively
interfere but if a gravitational wave were to pass
through the detector it should physically distort space and time
causing the length of the two arms to very slightly change compared to each
other this would change the interference of the two beams ultimately changing the
signal on the photo detector at the end
in 2002 then caltech and mit joined
forces to create iligo a proof that this
could work and after extensive research and an upgrade to advanced ligo we were
able to successfully detect gravitational waves in 2015.
and by we of course i mean humans as a as a species not me i i
wasn't involved so behind us is the beam
splitter so it comes from the laser room back there which is why we have to have
these laser glasses on it gets split right around here and then gets sent off
the arms that way and that way it then travels back in and is detected
over there somewhere unsurprisingly taking measurements that precisely in
the real world is a lot easier said than done any particles in the air will cause
the laser to scatter and although they're only sending in 20 watts of
laser power the way it works is to have the beam trapped as a standing wave
inside so after 300 bounces or so that
amplifies it to about a hundred kilowatts with power like that even a
tiny speck of dust on the mirror could absorb enough heat to permanently damage
it meaning that the four kilometer tubes or arms have to be under an ultra high
vacuum so this is the concrete that goes around the ARM it doesn't actually help
with the laser tube but it's just to protect it from like cars animals stray
bullets or whatever
the real magic though happens in this three millimeter thick stainless steel
tube which is hold to one trillionth of an atm
even the vacuum outside the international space station can't hold a
candle to what they've got going on in here so how did they build it
well first they cooked or more accurately
baked now if regular stainless steel was used for this hydrogen and other
particles on the metal could contaminate the vacuum so to combat this every
inside surface needs to be heated to at least 170 degrees celsius and held there for
an extended amount of time now for the nuts and bolts that's pretty simple i
mean i could do that with my toaster oven at home but for something this
large what they had to do was pump a
quadrant of it full of electricity effectively using the resistance of the
metal to turn it into a heater and hold it at 170 for a month with the vacuum
pumps running to remove all the contaminants i guess that's probably why
they wouldn't let us inside we are the contaminants
so that's cool now the laser can make its way from one end to the other but
what about the mirrors on the end even something as small as a truck on
the highway or an earthquake in taiwan would create too much vibration for
gravitational wave detection which i guess is probably why they put
their air conditioner way off away from the building rather than up on top of it
like normal people this is one of the mirrors from iligo to
reduce the high frequency vibrations the mirror is hung like a pendulum by this
steel wire and the lower frequency vibrations are decreased by the springs
on the lower base but they obviously weren't done there in the
newer version the mirror is hung from four pendulums by glass fibers
and that is far from the end on the bottom of the table there's actually a
seismometer that measures any movement in the ground and then uses that
information to manipulate voice coils and static electricity to actively
cancel out the vibrations coming from the ground
at this point then the mirrors are basically perfectly still
but they still aren't done yet
for some frequencies their level of accuracy is being determined by quantum
mechanics and the heisenberg uncertainty principle but even this can be reduced
using quantum squeezing on a very basic level they can only know
so much about the amplitude or phase of the laser light but for this application
the amplitude of the light being detected matters a lot less
so they're able to perform a quantum squeeze and get better accuracy out of
the phase detection so they're using a ktp crystal that is a
potassium titanal phosphate crystal and this is able to transform one green
photon into two infrared photons or vice
versa energy is conserved here because the infrared light has less energy than the
green light so what they do here is pump a crystal
full of infrared light this is just to get a lot of green light and high
powered infrared lasers are easier to get and then that green light is sent
into another ktp crystal but when those photons are emitted they
are quantumly entangled now in a vacuum there naturally exists a
lot of noise particularly when there is very little light like around the dark
end of the detector so by injecting these entangled photons they're able to
remove the completely random vacuum noise and replace it with the entangled
photons that produce noise that they like
now you're probably thinking i am too holy sweet crap
how could they even verify that these are actually gravitational waves they're
detecting and the answer is by building another
detector on the other side of the country so that if a gravitational wave
is detected they can confirm that it isn't just a localized movement oh and
also by using two observatories to try and find where the astronomical event is
happening so with the accuracy of the
interferometer mostly figured out how the heck do they acquire all the data
and process it ah yes my friends we are finally getting to the computery part of
the video in here they collect data from 250
000 channels simultaneously and then to isolate the signals being created by the
detector from the computers they actually put the noisy bits of the
computer cpus power etc in another room
150 feet down the hall and they connect them using fiber optic pci express
extenders the most important thing with the computing here though is timing so every
processor's clock in this server room is
synced using a custom built system since the processor's timing has to be
precisely known in order to calculate the time of the events in the
interferometer but it's not just synced in this room there are also computers at
the very ends of the laser arms four kilometers away with their clocks
perfectly synchronized too and the 22 microsecond transmission time
delay taken into account oh and that second gravitational wave
observatory in louisiana you guessed right the processor clocks are perfectly
synchronized there too once the data is collected it gets sent
over to their server warehouse for analysis and very quick analysis is
important since if there's a large astronomical event taking place they'll
detect it here first and then they need to be able to tell their astronomers
where to point the telescopes in a timely manner
so all that processing is handled in here where they've got 6
000 processing cores 64 gpus almost 4
terabytes of RAM and close to 5 and a half petabytes of data stored on SSD
spinning and tape storage the data is then copied from here to
caltech servers where it is further analyzed and made available to more
scientists and then eventually the armchair physicists at home
and they're still not done future plans to improve their accuracy involve using
cryogenics to reduce the movement of the molecules on the mirrors detecting
changes in the earth's gravity to remove noise from movement below the earth's
crust implementing quantum squeeze that can be changed throughout the frequency
band and even building a space-based
gravitational wave observatory the goal is that with every little
adjustment they should be able to see even further into space collecting a bit
more data about how our universe works
so a huge thanks to ligo and particularly amber you rock for letting
us come and hang out in the observatory and a huge thanks to you guys for
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