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 space-time
gravitational waves
in the last tenth of a second
the energy released in these waves
was 50 times greater
then the energy being released by everything else
in the observable universe combined
It's like an awe-inspiring kind of energy
after spreading out through the universe at the speed of light 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 Rana Adhikari 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 absurd
*Humming simulation of gravitational waves
Is that.... is that it? [RA]: That's it, yeah.
The main problem with detecting
gravitational waves is that they're tiny
they stretched and squeezed space by
just one part in 10 to the 21.
That's the equivalent of measuring the
distance between here and Alpha Centauri
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
distance as possible, which is why the
arms of the interferometers are four
kilometers. And even with arms this long
gravitational waves vary the length of
the arms by at most 10 to the minus 18 meters
so the detector has to be able to
reliably measure distances just 1/10000
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 kilograms or 90 pounds 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 you to distinguish between local
noise which would appear only one side
and gravitational waves which would pass
through both sides
almost simultaneously
I'm in a building that contains a 1 to 100 scale
of LIGO, the gravitational wave detector.
The next challenge is the laser.
Whoa, whoa.
That's a lot of stuff.
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, at least
three-quarters 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 to the 20. What does that mean...
That's a hundred billionth of a trillion.
That's kind of what we end up with.
The best lasers for this purpose have a
wavelength of 1064 nanometers. That's
infrared light. But this presents a problem.
How can you measure 10 to the minus 18 with 10 to the minus 6
wavelength 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 will cause the
light to interfere with itself.
Yeah, but that's fully. That will go from
completely dark to completely bright
So are you looking at, like, slightly darker
and slightly brighter?
Yeah and the limit here at 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
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, that's a fraction of the total.
This is why the laser power in the arms is one megawatt.
That is enough energy to power a thousand homes, in a light beam.
And a megawatt, you know
*snap* boom, they won't even rip your head off.
Just, vaporized be just a smoking stump.
Even with a perfect laser and one
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 the oven to expel any residual gases.
They pumped out enough
air to fill up two and a half 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
space-time so light traveling through
that space 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!
This whole thing is bogus shut it down!
I would send a laser beam down this tube
and then wait for it to come back and
then i would say "well nothing happened"
because the space got stretched and the
laser wavelength got stretched
Its...It looks the same if you got it stretched or not stretched.
It doesn't make any sense
well 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 come back is
very short.
However the wave... the gravitational wave
when it comes through its doing the slow thing like
*low humming*
this noise I made, which is low. Its this
it's this slow stretching, its only a
hundred times per second. And it's true
when the wave comes through 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 is 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, we're 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 sites to eliminate noise,
with four kilometer long arms to
increase the magnitude of gravitational
waves to just a thousand 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 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, we've got two
things and together their uncertainty
has to be bigger than a certain value.
Luckily for us we are 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
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 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 the time.
It's not like an 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