On September 14, 2015, scientists at the Laser
Interferometer Gravitational-Wave Observatory detected gravitational waves directly for the
first time – a stunning achievement that led to the 2017 Nobel Prize in Physics. Why was
this significant? Well, here’s an analogy. Let’s imagine that human beings evolved without
the ability to see light. For thousands of years, we’d fumble in the dark, relying on our other
senses, until, one day, someone invented a machine that could perceive light for us. In
time, we’d see everything from the tips of our noses to the farthest-flung galaxies. This
analogy captures the magnificence of LIGO. It’s about much more than proving a
scientific prediction. LIGO enables us to perceive the physical universe
and understand reality on a new level. Like photons, gravitational waves travel at the
speed of light as they ripple across spacetime. Their signals are all around us. By listening
for gravitational waves with some of the most sensitive instruments ever built, scientists are
recording tremors of distant violent events – the formation of black holes, supernovae explosions
and, potentially, exotic phenomena we haven’t discovered yet. So, what are gravitational waves?
What causes them? And why is LIGO’s ability to detect them already transforming our understanding
of the universe? I’m Alex McColgan, and you are watching Astrum. Join me today as we learn about
gravitational waves, unpack the groundbreaking technology behind LIGO and anticipate some of the
stunning developments that lie around the corner. Gravitational waves are one of the stranger
implications of Albert Einstein’s General Theory of Relativity. As we’ve covered
previously, spacetime is a model that combines the three dimensions of space and the
fourth dimension of time into a single manifold. All objects with mass create curvature in
spacetime, and objects with a lot of mass create a lot of curvature, which we experience
as gravity. A simple way to visualize this is to think of a pool ball resting on an elastic surface
and a bowling ball resting on that same surface. The more massive bowling ball will create more
curvature. As objects move across spacetime, that curvature changes position with them. One
of the amazing consequences is that when objects of a certain mass accelerate, they can send
ripples across spacetime as gravitational energy. While this requires a special set of conditions,
namely, a very massive object undergoing acceleration, such a cataclysmic event would send
ripples, or gravitational waves, outward at the speed of light. Think of them like ripples on a
pond, but instead of water, they travel through the fabric of spacetime in all directions. And
as in the pond analogy, these disturbances become weaker as they radiate outward. To an observer,
the distance between objects would appear to expand and shrink as the gravitational
wave passes – mind-boggling to imagine. Yet although Einstein predicted the existence of
gravitational waves, he was pessimistic about our chances of ever detecting them. He thought that
these disturbances would be so small as to escape our ability to measure them. And who could blame
him? Many of the changes in distance that LIGO seeks to measure are one 10,000th the length
of a proton. Yes, you heard that correctly, 10,000 times smaller than a single proton. And
yet, these signals would come encoded with all kinds of information about their origins – when
they originated, how far they travelled and what kind of event produced them. This is where
LIGO comes in. It consists of two observatories funded by the United States National Science
Foundation and operated by MIT and CalTech. Among its driving forces are renowned physicists
Kip Thorne, Rayner Weiss and Barry Barish, all of whom shared the 2017 Nobel Prize for
their “decisive contributions” to the detection of gravitational waves. LIGO is essentially a
large-scale and very sensitive interferometer, an invention that’s been around since the 1880s.
An interferometer essentially measures what happens when light waves are combined from two
or more sources. For example, you could use an interferometer to test whether light travels at
different speeds through different substances, such as through air or water. Even a subtle
difference in speed will produce an interference pattern when the light waves combine – much like
what happens when two ripples on a pond intersect. If the peak of one ripple hits the valley of a
second ripple, they will subtract each other, producing a flat surface. However, if the peaks
line up exactly, it means the waves are in phase and add to each other. This is essentially
what the interferometer measures with light. By seeing how in or out of phase two light waves
are, an observer can infer the relative speed of the waves. And the larger and more powerful the
interferometer, the more sensitive it is. Here’s how it works: LIGO has two observatories located
in Hanford, Washington and Livingston, Louisiana. Why two? Well, you need at least two detection
sites to triangulate where the signals are coming from. Each observatory continuously fires a
powerful laser at a beam splitter positioned at a 45-degree angle. The laser beam has to
operate at around 750 kilowatts – powerful enough to vaporize you completely if you got in
its path. The splitter then splits the laser beam perpendicularly. The light in each arm travels
down a 4-kilometre vacuum cavity with a mirror at the end of it. The beams then bounce between
this mirror and a recycling mirror at the other end nearly 300 hundred times, increasing
the distance from 4 to 1,200 kilometres. Remember what we said, with interferometers,
bigger is better! After completing nearly 300 trips, the laser beams recombine at the beam
splitter and head to a photodiode, which is a light-sensitive semiconductor. If undisturbed, the
beams will be in phase, meaning their frequencies will subtract each other, and no light will arrive
at the photodiode. But if there’s a gravitational wave, the distance each beam travels will be
slightly different, and they’ll be out of phase. The photodiode will pick up a signal,
indicating the presence of a gravitational wave. Now, this is how it works in a perfect
world. But in reality, the interferometer is constantly picking up noise. To minimize
this, LIGO uses incredibly smooth 40 kg mirrors suspended by silica threads. Any particles in the
interferometer’s arms are also a problem, which is why LIGO pumps the air from its vacuum chambers
to one trillionth of atmospheric pressure. But there’s another problem: at these minuscule
levels, even quantum mechanics are a nuisance because they introduce randomness into photon
behaviour. LIGO mitigates this with an optical cavity which squeezes the light. This squeezing
minimizes the light’s phase noise and squeezes it into amplitude noise, which the interferometer
doesn’t measure. In other words, the quantum randomness will show up more in the height of
the waves. Quantum randomness is a fact of life: it can’t be eliminated, but it can be shifted,
much as you might move clutter from your bedroom floor to your closet. The chaos isn’t gone
– just out of sight for the moment. Plus, the goal isn’t to eliminate noise completely but
to get the best signal-to-noise ratio possible. That’s a pretty good overview of how LIGO works.
So, what has it discovered? As I mentioned earlier, LIGO detected its first signal in 2015.
Named GW150914, scientists studied the data and learned that it was caused by the merger of two
black holes about 1.6 billion light years away. These black holes, which were 29 and 36 solar
masses, became a binary and spiralled around each other until they merged and released a
blast in the final 20 milliseconds that was so powerful – now, get ready for this number
because this is what the scientists actually think - it contained 50 times the combined light
power of every star in the observable universe. At the risk of sounding crude, that is nuts. I’ve
read this fact many times over, and I still cannot comprehend what it means. Yet after travelling
for 1.6 billion years and finally reaching LIGO, the disturbance was so faint, it moved LIGO’s
4-kilometre arm one-thousandth the width of a proton. To visualize this, imagine the distance
between us and Proxima Centauri and changing it the width of a human hair. That is the
level of precision LIGO was able to detect. If that’s not one of the most astonishing
feats in human history, I don’t know what is. And this was just the first gravitational
wave LIGO detected. The second detection occurred 3 months later in December 2015. That
signal also came from a black hole merger, which took place 1.4 billion light years away.
Over its initial three runs, LIGO recorded more than 80 black hole mergers and, in August 2017,
it detected the merger of two neutron stars. Named GW170817, this signal was notable for
being the first gravitational wave to be corroborated by electromagnetic observations
from 70 observatories across the planet. This was a breakthrough not only in gravitational
wave detection but in multi-messenger astronomy. It turns out, LIGO was just warming up during
these three runs. As of May 2023, LIGO has begun its fourth run with better sensitivity
than ever. After its latest round of upgrades, which kept LIGO offline for 3 years, the
observatories now have more reflective mirrors, better mirror suspension and improved
light-squeezing with lower quantum uncertainty. And this time, LIGO also has the support of KAGRA,
a new interferometry observatory in Hida, Japan. KAGRA is located underground, making
it the world’s first subterranean gravitational wave observatory and also
the first to use cryogenic mirrors. During an engineering test run on May
18, LIGO’s scientists say they already received a signal that was possibly caused by
a neutron star being swallowed by a black hole. We’ll have to wait a while for confirmation, but
if these early results are any indication, LIGO is about to blow the doors off our understanding
of gravitational wave-generating phenomena. So, what other developments lie ahead? India
is preparing a collaborative project called LIGO-India, or INDIGO, which will help
LIGO triangulate better location data. In 2027 to 2028, LIGO will implement its LIGO
Voyager upgrade, which will achieve higher sensitivity with four times-heavier
mirrors and higher frequency lasers. And in the more distant future, a third-generation
facility has been proposed called Cosmic Explorer. This facility would feature two new
observatories with arms spanning 40 kilometres and 20 kilometres respectively. (Remember,
with interferometers, bigger is better!) But the proposal that really excites me is the
Laser Interferometer Space Antenna, or LISA. This would be the first space-based gravitational
wave observatory, which would utilize three spacecraft in a 2.5-million-kilometre-long
configuration. This interferometer would be so big and so precise, scientists hope
it would be adept at uncovering exotic and theoretical sources of gravitational waves, such
as cosmic strings and other speculative phenomena. In theory, it could help us stare directly into
the fabric of reality. With a planned launch date of 2037, we’re still over a decade away, but
it’s never too early to start counting the years. So, there you have it: an overview of LIGO and
how scientists are using gravitational waves to better understand the universe. They
give us evidence of extremely remote and ancient phenomena that cannot be measured by
any other means, and they can be a secondary way to measure observations made by other
instruments, like the Webb Telescope or Hubble. In time, this revolutionary field should allow
us to understand the nature of our universe, its history and even its future. I hope you’ve
found this episode as fascinating as I have. As I’ve watched my child grow, it’s been wonderful
seeing them explore the world around them through interacting with it. I’m sure this fact will
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about some other really unique telescopes, check out this playlist here. A big thanks to
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All the best, and see you next time.
