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5 Reasons You Should Care about the Discovery of Gravitational Waves

The world of physics was abuzz last week with the historic
announcement of the first-ever detection of gravitational waves. But
why is it such a big deal?
By Sabrina Stierwalt, PhD. on February 24, 2016

Last week marked the historic announcement of the first detection of
gravitational waves. A big press conference was held, and physicists
around the world celebrated. The discovery was even compared to
Galileo looking through a telescope for the first time. So why all the
fanfare? Why are gravitational waves such a huge deal?

Last week marked the historic announcement of the first detection of
gravitational waves. A big press conference was held, and physicists
around the world celebrated. The discovery was even compared to
Galileo looking through a telescope for the first time. So why all the
fanfare? Why are gravitational waves such a huge deal?

1. GRAVITATIONAL WAVES ARE AN ENTIRELY NEW WAY
OF OBSERVING THE UNIVERSE

Astronomers observe the universe across the electromagnetic
spectrum, from X-ray and ultraviolet through optical and down to
radio frequencies. Emission in each of these frequency ranges provides
different information and thus a different perspective on our
astronomical points of interest.
For example, we know that there are millions of stars clustered toward
the center of our Galaxy which emit mostly at optical wavelengths, but
there is also a lot of dust near the Galactic center as well. So to study
those dust-enshrouded stars, astronomers must observe them at either
infrared wavelengths (where the dust emits) or radio wavelengths
(which can penetrate through the dust more effectively than shorter
optical wavelengths). All of these wavelengths offer a unique
perspective on the universe, but they are all the same kind of light,
electromagnetic radiation, and so behave in similar, understood ways.
Gravitational waves are an entirely new phenomenon different from
anything on the electromagnetic spectrum. In 1915, Albert Einstein
proposed a radically different way of looking at gravity with his theory
of general relativity. Rather than thinking of gravity as a force pushing
and pulling massive objects in different directions, he described
gravity as being manifested in a curvature of spacetime. In other
words, the space (and time) around a massive object is curved, which
then dictates how passing objects can move through that space.
This may sound crazy, but we can actually observe many of the effects
predicted by Einstein’s theory. For example, general relativity informs
us that time passes more slowly by an ever so small margin down here
on Earth than it does for GPS satellites in orbit, an effect known as
time dilation, a result of the curvature of spacetime. Without adjusting
for this small time difference in our satellite communications, we
would never get to where we are trying to go.
A consequence of the general relativity framework is that when
objects accelerate through this warping of spacetime, they produce
ripples known as gravitational waves. These waves propagate through
space, compressing it in one direction and stretching it in another.
The frequencies predicted for these fluctuations are within the human
hearing range. We can hear gravitational waves and already scientists
and artists have teamed up to explore other artistic interpretations of
their sound.
So why did it take 100 years to detect them? These ripples are tiny, on
order of a thousandth of the size of a proton nucleus, so we need a
pretty violent event to occur to produce enough of them for us to
detect. We also need, of course, a very sensitive detector.

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 2. The Instrument That Made the Gravitational Wave Detection Is
the Most Precise Measuring System Ever Built

To detect such tiny distortions in spacetime, physicists use a technique
called laser interferometry. A focused beam of light is sent in different
directions to bounce back and forth between two sets of mirrors before
being sent to a detector. If a gravitational wave passes by the
interferometer during all of this bouncing, the distance between the
mirrors will change ever so slightly and this change will translate to a
difference in the two signals as measured by the detector.
Not only is the signal from gravitational waves incredibly weak, there is
also a significant amount of competing noise attempting to drown it out.
To increase the detectability of such a signal against the background
noise, the path the laser travels must be a long one. The Laser
Interferometer Gravitational-Wave Observatory (LIGO), the instrument
that made the historic detection, is four kilometers long on each side.
The detectors are further suspended in the air in hopes of isolating the
slightly faster wiggles due to gravitational waves from terrestrial
interference.
To further fight back against false detections, LIGO has not one but two
detectors: one in Hanford, Washington and the other in Livingston,
Louisiana. Detecting the same signal at both, widely separated locations
would mean that signal was likely not a local one. And that is exactly
what happened on September 14, 2015—a signal with the precise
characteristics predicted for gravitational waves was observed at both
detectors only milliseconds apart.

3. We Now Know That Massive Black Holes Can Merge to Create
Even Bigger Black Holes

Famous theoretical physicist and author Kip Thorne described the event
that produced the detected gravitational waves as a “violent storm in the
fabric of space and time”. Around 1.3 billion years ago, when
multicellular life was just beginning here on Earth, two black holes
orbiting each other began to close in on one another. As these dense
objects got closer, they began to accelerate to nearly half the speed of
light in the presence of their shared strong gravitational field – the
perfect mix for producing gravitational waves.
From fitting the waveform of the gravitational wave detection and
comparing it to simulations done with a supercomputer, astronomers
can tell that the two black holes were originally 29 and 36 times the
mass of our Sun. They merged to form a 62 solar mass black hole which
means that an amount three times the mass of our Sun was emitted away
as energy in the form of gravitational waves, all in the 20 milliseconds
it took for the collision to happen! That’s a power output of 50 times
greater than all of the power put out by all of the stars in the universe
put together.
Before this first detection, astronomers were not even sure that mergers
between black holes existed, and now the details of this particular event
are known to a high degree of certainty.

4. Hundreds of People Had to Take a Huge Risk to Make This
Detection Possible

Einstein predicted the existence of gravitational waves 100 years ago
but at the time, there was significant doubt that such weak signals could
ever be detected. In 1992, LIGO became the largest investment the
National Science Foundation had ever made. The investment was a risky
one–the existence of gravitational waves was only theoretical, and their
signal, even if they were real, would be impossible to detect without the
construction of an instrument larger than any other measurement device
previously built. In fact, the initial operations of LIGO between 2002
and 2010 came up empty-handed.
However, when the new and improved advanced LIGO came online with
its increased sensitivity, it detected a signal that was so clear it could be
seen by eye almost immediately.

5. With Our New Ears on the Universe, We Will Likely Make
Discoveries We Haven’t Even Thought of Yet

Arguably the most exciting part of this new discovery is that it’s only
the beginning! The ease with which LIGO detected the signal suggests
that there will be many more to come and that we won’t have to wait
long for them either. The LIGO detectors are not even at their full
planned sensitivity yet and so detections will get easier.
The success of LIGO will also likely help other, similar projects get
funded, and in fact it likely already has with the prime minister of India
voicing support for LIGO India. Other efforts include KAGRA in Japan,
VIRGO in Italy, and the already operational GEO600 in Germany.
Using multiple detectors in concert will further help pinpoint the origin
of the gravitational wave signals.
The LISA project (i.e., the Laser Interferometer Space Antenna), which
has experienced funding woes since NASA bowed out in 2011 and the
project was picked up entirely by European agencies, would also send
gravitational wave detectors into space. This would open up the
possibility of detecting  signals with much longer periods (on the order
of minutes or hours rather than milliseconds) not currently possible
with LIGO.
Gravitational waves are predicted to arise from co-orbiting black holes
like the system detected in September 2015, but also from binary
systems with other compact objects like neutron stars. But gravitational
waves are also fundamentally different from the electromagnetic
radiation that we know so well. So really, who knows what we will find?

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