What do gravitational waves travel through?

Gravitational waves found - "ripples" of space-time

After decades of planning and efforts to gain political support, the LIGO detectors have been listening for gravitational waves since 2002. After eight years of silence, the detectors were switched off in 2010 and isolated even better against interfering signals.

When the measurements with the Advanced LIGO officially began on September 18, the scientists were hopeful that they would finally find meaningful signals.

As if by an irony of fate, by this point in time they had already stored a signal. The detectors had been in operation before the official measurements began and had already picked up an extremely promising signal. It first appeared at the detector in Louisiana and seven milliseconds later in Washington.

“When this signal was reported, we were pretty sure it was a relevant signal. Did we think this was too good to be true? Definitely! I was absolutely amazed. I couldn't believe it, ”says Reitze.


Using several Einstein equations, the scientists tried to draw conclusions from the observed waves as to which astrophysical event had triggered them. In the present case, the equations showed that two colliding black holes must have triggered the waves. After merging, they formed a new black hole that is just over 60 times as heavy as the sun.

Black holes, which are formed when massive stars collapse, are among the most bizarre objects in the known universe - if the term "objects" is even justified. To simplify matters, black holes can be described as lumps of matter that are so dense and compact that not even light can escape their gravitational field. In fact, black holes are not so much “things” or “objects”, but areas of space with strongly curved, bottomless spacetime. The fact that two such black holes merge can therefore be regarded as a very extraordinary event.

“It's a kind of restless vortex of curved space that changes at breakneck speed,” says Weinstein.

Before the collision recorded by LIGO, the two black holes had slowly orbited each other for millions or billions of years. As they slowly got closer and closer, their orbital speeds increased until they finally rotated around each other at about half the speed of light and emitted gigantic amounts of energy in the form of space-curving gravitational waves.

Then the black holes merged. In the last second before the merger, the swirling black holes emitted more energy than the entire universe emits in the sum of all types of radiation. After the merger, the newly formed black hole oscillated briefly and then calmed down. As this faded away, a signal called a “ringdown” was sent out, a “last breath” before the silence.

It was a huge event that could later be measured using infinitesimally small changes in the distance between two mirrors on earth.

"The data is amazingly good," said astronomer Scott Ransom of the National Radio Astronomy Observatory, who reviewed the team's manuscript, which was published in the Physical Review Letters. "Hardly anyone expected that the waves would be visible in the raw data of the detector, ie without statistical processing."

The scientists of the LIGO team are very sure that the signal is real. According to their calculations, it can only be expected once in 200,000 years that such convincing results will turn out to be false alarms. That cannot be said for all of the potential gravitational wave signals her team has collected so far. LIGO found at least one other candidate - on October 12th - another signal that could have been generated by merging black holes. But in this case, the scientists cannot safely rule out that it is a false alarm.


The discovery is the first direct observation of gravitational waves, but it is not the first evidence of their existence. In 1974 Joe Taylor and Russell Hulse found what was then a new and exotic object: a double pulsar, i.e. two neutron stars swirling around each other. The team observed the pulsars' orbits tightening and found that the only possible explanation for this behavior was the radiation of energy from the system in the form of gravitational waves.

For this discovery, with which the existence of gravitational waves was proven beyond doubt, Taylor and Hulse received the 1993 Nobel Prize in Physics.