An Optics student's charm about the latest discovery !
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| Arms of LIGO |
The world of Science is
celebrating the detection of a certain kind of ripple- Gravitational
waves ! The waves were predicted by Albert Einstein in 1916 while he
was working on one of the most beautiful theories in Physics called
the General Theory of Relativity. Almost 100 years later, the waves were
recently detected thus opening a new window to the universe. While
the whole prediction and discovery is amazing, what enchants me the
most is the core idea of the experimental setup.
The whole experimental setup
at LIGO (Laser Interferometer Gravitational wave Observatory), U.S.A.
equipped with the best in technology spans over a radius of 4 km. The
setup is a grand and a giant version of the humble Michelson
interferometer. Any student of Physics should be familiar with the
term and anyone with a sense of how light reflects and combines
should understand the idea pretty well.
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| Fringes from M.I |
I came across the concept of the Michelson interferometer during an Optics demonstration at College
and it has impressed me at different levels ever since. Till then,
nothing had been more beautiful than white light splitting into seven
colours, but that holy experiment changed everything. The setup is
simple yet elegant- A beam of light is split into two, allowed to
travel and bounce from two mirrors at right angles to each other and
recombined. A detector is then used to see how the recombined light
looks like. As the mirrors get adjusted/disturbed, the pattern keeps
evolving. For the first time, I saw streaks of bright and dark
fringes that seemed to magically emerge out of a couple of mirrors
illuminated by a laser beam. They looked just spectacular!
A good long time ago, when I was
still in high school, my mother bought me a laser pointer and I spent
an entire summer playing with it. I observed two things- the light
reached farther and the beam spot was small compared to the light
from a torch. Ten years later, as a masters student, I had to send a
slightly high powered laser beam into an M.I and understand the
reason behind those observations. It took a couple of more years for
me to appreciate the technical beauty of Holograms which remained a
fantasy for long. We did an M.I experiment at the dead of night in a
pitch dark room on a vibration free surface to produce a hologram. A
number of trials failed miserably but the end result was aces. We
widened our tired eyes to see if the hologram was actually there and
it was. Another sight worth witnessing! Later, during one of the
classes on Special Relativity, M.I again came into picture and this
time, it had incarnated itself as a tool to measure the speed of
light-A humongous number and a prodigious upper limit in space-time.
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| Setup at LIGO |
Fast forward another couple
of years, 2015 arrived with royalty. The year was special for two
reasons. It was declared the International Year of Light by UNESCO
and it also marked the 100th anniversary of the General
Theory of Relativity. On one side, Science folks were celebrating the
importance of light and light based technologies and on the other
side, a remarkable discovery was being anticipated that would make
Einstein's speculation into reality. On September 14, 2015
at 5:51 a.m. Eastern Daylight Time (09:51 UTC)
, the
signal was finally detected at LIGO observatories in Louisiana and
Washington, U.S.A.
The signal, which is
essentially a disturbance in the space time fabric came from the
collision of two black holes that took place 1.3 billion years ago.
According to General Relativity, two black holes lose energy as they
spin around each other and eventually merge together to form a single
massive black hole. The process which happens at half the speed of
light is highlighted by the conversion of a portion of the combined
mass into energy. This energy travels through the cosmic fabric as
gravitational waves.
The LIGO interferometer at the core is a Michelson interferometer that has arms 4 km in length and suspension mirrors that weigh 40 kg each. More mirrors are placed along the arms
to efficiently recycle power in the cavity to enhance the
sensitivity. Light beam emerging from a 200 W near infrared laser travels a
whopping distance of 1600 km (due to the cavity effects) before
reaching the detector.This setup is about 144,000 times bigger than
the M.I that was used to measure the speed of light in 1887. Just like the
original M.I, the interference pattern would change if the mirror
moves because of any disturbance.
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| Colliding Black holes |
The radiation from the
collided black holes hit the mirror and moved it by a distance as
small as the width of an atomic nucleus. The resulting interference
is at excellent agreement with the simulations and calculations of
the General Theory of Relativity thereby confirming the genius'
prediction. Though it was Einstein's contemplation, a whole bunch of
scientists and engineers had been instrumental in the development of
the idea over the past few decades. The discovery is certainly a big
leap pushing science to a new era.
There it goes, a favourite
experimental setup, BIGGER and BETTER !
