How can scientists measure or observe things that are an immense distance away? And how can scientists view objects in space that do not even produce light? These questions, and more, were answered in an announcement of the observation of gravitational waves that were previously only understood in theory.


Vintage postcard depicting the shape and locations of LIGO interferometers.

Photo by: Tobin, Flikr

Gravitational Waves — What in the world?

1.3 billion light years away — about 13,000 times the diameter of the Milky Way — two black holes collided. The effect of such a collision was felt on September 14, when planet Earth experienced the gravitational effects of the colliding black holes. Gravitational waves are distortions in spacetime, a combination of space and time that makes up the universe. The odd thing about these waves is that they have been known to us for quite a while, but we have never been able to observe them until now.

These waves were originally predicted by Albert Einstein’s theory of general relativity under the premise that mass curves spacetime. When such mass moves, spacetime is compressed and stretched in the form of ripples aptly named gravitational waves. Using Einstein’s equation, scientists can predict the pattern of the waves from certain events. The pattern calculated from the waves experienced on September 14 matched the merging of two black holes. The signal produced by such a monumental event so far away in the universe was difficult to detect because the effects are very tiny. Luckily, scientists have extremely precise equipment and procedures in place to detect these signals.

Experimental Procedure

The experiment that detected the minute effects of the colliding black holes used mirrors that moved by a few thousandths of the diameter of a proton. The Laser Interferometer Gravitational-Wave Observatory (LIGO) had two detectors, in Washington state and Louisiana. The detectors function based on whether or not two beams of lights are in sync  after traversing the detectors’ long arms, which look much akin to a giant letter “L”. When there is no detection of the gravitational waves, the two beams of light which pass through the detector will come out of the detector in sync with one another. However, if the two beams of light come out of sync, then there must have been a disturbance that moved the arms. When gravitational waves influence the detectors, one of the two beams of light stretches as a result of shifting arm positions.

The findings of the experiment definitely seemed to be gravitational waves, but at first there still existed a lot of uncertainty. This is because any kind of disturbance could potentially trigger the detectors to output out-of-sync light beams. In fact, to safeguard against fake signals, the research team regularly produces fake simulations of gravitational waves to see if the scientists can differentiate between analyzing data from gravitational waves, or merely just data from regular disturbances. In the case of the measurements made on September 14, there were no earthquakes or other type of disturbances, except for a lightning strike in Africa that was ultimately deemed too weak to register on the detectors. The researchers are so confident that they believe there is a less than 1 in 3.5 million chance that these results don’t represent gravitational waves. Hopefully the first of many such modern experimental physics results, gravitational waves will indubitably have a huge impact on how scientists study the cosmos.


An external view of the tunnel in which interferometer beams are housed.

Photo by: Tobin, Flikr

What about the future?

The overall meaning of the observation of gravitational waves is that much more is in store for the future of space observation. Currently, we observe the universe via electromagnetic radiation. Astronomers use visible light that we use to see things with our own eyes, as well as  lower energy radio waves and high energy x-rays to view things in space. However, there exists many bodies in space that cannot be seen using just electromagnetic radiation, such as black holes that do not emit electromagnetic radiation, but do give off gravitational waves. The scientists are hopeful to eventually detect previously invisible celestial bodies such as black holes and neutron stars.

Looking towards the future, researches are looking to make even more precise and accurate detectors. One example is the Evolved Laser Interferometer Space Antenna (eLISA). This detector would function based upon three orbiting satellites around the sun which would look for gravitational waves in space with functional arms that would be a million kilometers long, which is roughly a bit more than twice the distance from Earth to the Moon. Another project much more humble in nature is the Einstein Telescope, which would build three ten-kilometer-long arms arranged in a triangle. It is expected to be complete by the late 2020’s.

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