Seven Billion-Year-Old Signal from Merging Black Holes Reaches Earth

gravitational waves confirm prehistoric collision of black holes

By Jonny Lupsha, Wondrium Staff Writer

Black holes that merged seven billion years ago produced a shockwave and its signal just reached Earth, BBC News reported. The energy released during their collision was picked up by laser detectors in the United States and Italy in May 2019. Gravitational waves are detected with laser technology.

Northern leg of LIGO interferometer on Hanford Reservation
Interferometers in the United States and Italy detected the signal from the shockwave produced by the merging of two black holes, which traveled seven billion years before reaching Earth. Photo by Umptanum / Wikimedia Commons / CC By SA 3.0

According to BBC News, scientists have just detected an energy signal that reached Earth from an incredible distance. “Imagine the energy of eight Suns released in an instant,” the article said. “This is the gravitational ‘shockwave’ that spread out from the biggest merger yet observed between two black holes. Researchers say the colliding black holes produced a single entity with a mass 142 times that of our Sun.”

The article also said that the signal was detected by an international collaboration of scientists that operate “three super-sensitive gravitational wave-detection systems in America and Europe.”

Science still has much to learn about the collision of black holes and the gravitational waves they cause, though what they’ve discovered so far is due to experimenting with reflecting wavelengths of light.

Oscillating Tidal Forces

One way to think about how gravitational waves affect the space around them is to imagine yourself in a swimming pool.

“If someone does a cannonball dive on the other side of the pool, it takes a while for the waves to reach you,” said Dr. Joshua N. Winn, Professor of Astrophysical Sciences at Princeton University. “Likewise, when black holes crash into each other, there’s a delay before the resulting distortions in space reach us. But, while a water wave might travel a few meters per second, gravitational waves travel at 300 million meters per second—the same speed as light.”

Additionally, Dr. Winn said, when a wave in a swimming pool reaches you, you might bob up and down. If you’re in space and a gravitational wave reaches you, your body stretches. First it stretches vertically, then horizontally, then the process repeats. These are “oscillating tidal forces.”

“One way to visualize the effect is to imagine a series of circular rings, arranged to make a cylinder,” he said. “Send in a gravitational wave along the axis of the cylinder [and] each ring gets alternately stretched one way, then the other. The result is a traveling pattern of distortions.”

However, these distortions are so incredibly small they’re barely even detectable, even with special equipment.


One member of the international collaboration of scientists that detected the gravitational wave is the Laser Interferometric Gravitational-Wave Observatory (LIGO). Dr. Winn said that interferometry refers to using the wave nature of photons to sense these incredibly small distortions.

“They use electromagnetic waves, from a laser, to detect gravitational waves,” he said.

Dr. Winn said that LIGO built tunnels in two perpendicular directions. Hypothetically, he imagined one direction was north and the other was west. Gravitational waves would change the relative lengths of the tunnels—like the stretching body in space, one tunnel would stretch and the other would constrict, then vice versa. LIGO monitors the relative lengths of each tunnel by running a laser through a beam splitter that would reflect half the light west and the other north.

“At the end of each tunnel is a highly reflective mirror, which bounces the light back to the beam splitter, and from there the light either goes back toward the laser or bounces to a photo detector,” he said. “If the two laser beams arrive at the detector in phase—that is, if each crest arriving from the west is met by a crest arriving from the north—then the beams interfere constructively.

“If, on the other hand, the two optical paths differ by half a wavelength, then the waves interfere destructively.”

Dr. Winn said the experimenters arrange for destructive interference; then if a gravitational wave reaches the equipment, it disturbs the interference and the light will flicker on and off. In simpler terms, with two perpendicular tunnels reflecting light split by a beam splitter, disturbances in the reflections can indicate a gravitational wave.

Edited by Angela Shoemaker, Wondrium Daily

Dr. Joshua N. Winn contributed to this article. Dr. Winn is the Professor of Astrophysical Sciences at Princeton University. After earning his PhD in Physics from MIT, he held fellowships from the National Science Foundation and NASA at the Harvard-Smithsonian Center for Astrophysics.