Gravitational waves have been theorized about since Albert Einstein came up with his Theory of Relativity in 1916. What are gravitational waves, and why are we suddenly finding them now, after 100 years of searching?
What are Gravitational Waves?
What happens when you throw a rock in a puddle? The impact of the rock creates ripples in the water that travel to the edge of the puddle and bounce back again and again until the energy used to create them is exhausted. The same thing happens in space — when a powerful catastrophic event occurs, such as the collision of black holes or neutron stars, it creates gravitational ripples that course through the fabric of space-time across the universe.
While Einstein may have theorized these waves back in 1916, we didn’t have proof of the existence of these gravitational ripples until 1974. Astronomers at the enormous Arecibo Radio Telescope in Puerto Rico found evidence of a binary pulsar star system — two massively dense stars in close orbit around one another. This was the kind of astronomical occurrence that Einstein had theorized would generate gravitational waves, so the astronomers began studying the movement of those two binary stars.
After 8 years, it was determined that the stars were moving closer together — just as Einstein’s theory of general relativity had predicted.
Now, that star system has been studied, measured, and monitored for more than 40 years and it’s become apparent that Einstein was correct, and occurrences like these are indeed generating gravitational waves.
We didn’t actually spot the waves themselves until 2016 because by the time the waves reach our little corner of the Milky Way Galaxy they are so weak that they’ve nearly completely dissipated.
The Death of Neutron Stars
For the first time since Einstein first theorized about gravitational waves, we’ve been able to observe their creation Researchers were able to watch the death spiral of a pair of neutron stars that were in orbit around each other similar to the pulsars that were originally observed in 1974. The collision of those stars created the first observable generation of gravitational waves in history.
This collision has also been hailed as the first known instance of one astronomical occurrence emitting two different types of waves. In this case, the collision of the neutron stars is emitting both gravitational and electromagnetic waves.
While we’ve been observing this kind of phenomena for decades, this is the first time that the astronomical community has gotten the chance to see the generation of these gravitational waves first hand.
LIGO, which is short for Laser Interferometer Gravitational-Wave Observatory, relies on a pair of detectors placed in two different locations in the country. One detector is in Hanford, Washington while the other calls Livingston, Louisiana home. As gravitational waves pass through Earth, they cause the legs of the detector to expand and contract. This chance is minuscule — a fraction of the diameter of a proton — but it is enough of a change that it can be monitored.
The detectors are placed nearly 2,000 miles apart, but a gravitational wave can cross that distance in approximately 10 milliseconds. The time difference between the two stations can help astronomers determine which direction, astronomically speaking, that the waves came from.
LIGO and other similar detectors are the main reason that we’re finally seeing so many gravitational waves. The effect that these waves have on the plant is almost imperceptible, and until the establishment of LIGO in the late 1990s, we simply didn’t have any equipment sensitive enough to measure the minute changes that the gravitational waves caused as they moved through our planet.
LIGO, as its name implies, relies on an interferometer to measure changes caused by gravitational waves. An interferometer relies on targeted light or radio waves to measure incredibly small things. In the case of a device that uses light, the laser or other light source is split into two even halves by a beam splitter. One half of the beam is projected onto a screen, and the other is projected at a mirror and then reflected back as the screen. This puts the second beam slightly out of phase from the first one.
Once the two beams meet, they overlap and interfere with each other. The pattern of the interference will depend on the distance between the screen and the mirror. By monitoring the interference pattern, LIGO can monitor gravitational waves as they pass through the planet because it causes the interference pattern to shift.
This is easily one of the most exciting astronomy discoveries in the past few decades. It provides a few more puzzle pieces that help us understand the universe around us a little bit better and may even enable us to study the expansion of the universe back to the Big Bang. While it may take researchers a while to make sense of these gravitational waves, the death of those two neutron stars has helped to pave the way toward a better understanding of our universe. This will become vital if we’re to make our way out into space and become the interstellar race that we’ve been trying to become since we first walked on the moon.