Einstein thought we'd never detect ripples in space-time. But 100 years later, a remarkable experiment is proving him wrong over and over.

Morgan McFall-JohnsenDec 6, 2020, 20:28 IST

A worker inspects quartz fibers that suspend a mirror inside the Virgo gravitational-wave observatory.EGO/Virgo Collaboration/Perciballi

Albert Einstein predicted that extremely massive objects, like black holes and neutron stars, send out ripples in space-time when they collide.
Einstein didn't think humans would ever detect these ripples, called gravitational waves.
But the Laser Interferometer Gravitational-Wave Observatory (LIGO) proved him wrong 100 years later.
Now a global network of observatories has detected 50 probable gravitational waves from violent space collisions.
The observatories are still getting stronger and more precise.

When incredibly massive objects collide violently in space, they send out ripples in space-time that reverberate through the cosmos for billions of years.

Long after the collisions happen, these gravitational waves - first theorized by Albert Einstein - pass through Earth. Over the last five years, a set of three miles-long devices in Washington, Louisiana, and Italy have been listening for these waves. The two US detectors make up the Laser Interferometer Gravitational-Wave Observatory (LIGO), and their partner in Italy is called Virgo.

Einstein predicted that noise and vibrations on Earth would prevent us from ever being able to detect gravitational waves. But these observatories proved him wrong. Scientists have detected cataclysmic collisions between black holes and neutron stars. They've found black holes that shouldn't exist. And they've identified the origins of nearly all the universe's gold, platinum, and silver.

Together, these experiments detected 39 new gravitational-wave events during just six months of observations last year, LIGO and Virgo researchers announced in October. All in all, scientists have now identified likely gravitational waves 50 times.

Here's how astrophysicists proved Einstein right about gravitational waves and wrong about our ability to detect them.

In 1916, Einstein predicted that collisions of massive objects, like black holes and neutron stars, would produce gravitational waves.

Albert Einstein at home in Princeton, New Jersey, 1944. Popperfoto/Getty Images

When they crash together, such objects should instantly convert several suns' worth of mass into pure gravitational-wave energy.

The waves could eventually pass through Earth, warping our space and time, but Einstein thought we would never detect them.

An animation exaggerates the space-time-bending effect of gravitational waves passing through Earth. NSF

In the late 1990s, researchers built two huge experiments in an attempt to pick up gravitational waves.

This L-shaped LIGO observatory in Hanford, Washington, is one of the world's first two gravitational-wave detectors. LIGO Laboratory/NSF

For the first 13 years, there was silence. But then the LIGO detectors sensed their first ripples in space-time, from the merger of two black holes 1.3 billion light-years away.

Technicians inspect the coating of a mirror inside the LIGO experiment. Caltech/MIT/LIGO Lab

For the first 13 years, there was silence. But then the LIGO detectors sensed their first ripples in space-time, from the merger of two black holes 1.3 billion light-years away.

LIGO scientists describe these signals as "chirps" because of the sound they make in the data.

Discoveries cascaded from there. In 2017, LIGO and its Italian companion, Virgo, sensed ripples from a merger of two neutron stars.

An illustration of two neutron stars colliding. NASA

Last year, the observatories detected waves from what appeared to be a black hole swallowing a neutron star nearly 1 billion years ago.

Artist’s depiction of a black hole about to swallow a neutron star. Carl Knox, OzGrav ARC Centre of Excellence

LIGO and Virgo use a clever method to detect these waves. First, each detector shoots out a laser beam and splits it in two.

The light waves from the laser beams return at equal lengths and line up in such a way that they cancel each other out.

But when a gravitational wave comes through, it warps spacetime - making one tube slightly longer and the other shorter.

A physicist measuring those changes in brightness is thus measuring and observing gravitational waves.

A technician inspects one of LIGO's mirrors by illuminating its surface with light at a glancing angle. LIGO

Scientists can learn about the event that made those waves based on the different arrival times of the two laser beams.

LIGO's sensitivity leads to a lot of false signals because of passing trucks or gusts of wind. Even movements of atoms in the detector's mirrors can mimic the signal of a gravitational wave.

A truck drives in front of a turbine in Wancourt, France, April 3, 2019. Pascal Rossignol/Reuters

LIGO's sensitivity leads to a lot of false signals because of passing trucks or gusts of wind. Even movements of atoms in the detector's mirrors can mimic the signal of a gravitational wave.

LIGO gets regular upgrades to make it more sensitive and powerful. The latest added a new instrument that squeezes light to reduce false alarms.

Researchers install a new quantum squeezing device into one of LIGO’s gravitational wave detectors. Lisa Barsotti

That photon squeezer helped the LIGO-Virgo collaboration detect 39 likely gravitational-wave events in just six months last year.

A supercomputer simulation depicts a pair of neutron stars colliding, merging, and forming a black hole. NASA Goddard

One of the events it detected revealed a type of black hole that physicists thought couldn't exist.

An artist's concept of a supermassive black hole and its surrounding disk of gas, where two smaller, embedded black holes orbit one another. Caltech/R. Hurt (IPAC)

Researchers are still sifting through data from the final five months of their 2019-2020 observations.

Engineers begin hardware upgrades inside the vacuum system of the detector at LIGO in preparation for 2019-2020 observations. LIGO/Caltech/MIT/Jeff Kissel

Japan joined the global gravitational-wave network this year with its own observatory: the Kamioka Gravitational-wave Detector (KAGRA).

The KAGRA system is housed in a giant L-shaped tunnel located 200 meters underground, November 6, 2015 in Hida, Gifu, Japan. The Asahi Shimbun via Getty Images

Its location in underground tunnels should insulate KAGRA from the background noise of wind and passing vehicles.

An illustration of the underground KAGRA gravitational-wave detector in Japan. ICRR, Univ. of Tokyo/LIGO Lab/Caltech/MIT/Virgo Collaboration

KAGRA is also the first detector to cryogenically chill its mirrors, reducing false signals from the moving molecules.

A 50-pound sapphire mirror for KAGRA. Photo courtesy: KAGRA Observatory, ICRR, The University of Tokyo

Overall, adding new observatories can help researchers detect more gravitational waves with more accuracy.

An aerial view of Virgo in the Italian countryside. The Virgo Collaboration/CCO 1.0

The new global network could ultimately detect 100 collisions per year, according to LIGO astrophysicist Vicky Kalogera.

The entrance of the KAGRA tunnel, August 2018. Photo courtesy: KAGRA Observatory, ICRR, The University of Tokyo

Starting in 2022, LIGO, Virgo, and KAGRA are set to spend a year listening for gravitational waves together.

Technicians install Advanced LIGO upgrades in Hanford, Washington. Cheryl Vorvick/LIGO Hanford Observatory

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Einstein thought we'd never detect ripples in space-time. But 100 years later, a remarkable experiment is proving him wrong over and over.

In 1916, Einstein predicted that collisions of massive objects, like black holes and neutron stars, would produce gravitational waves.

When they crash together, such objects should instantly convert several suns' worth of mass into pure gravitational-wave energy.

The waves could eventually pass through Earth, warping our space and time, but Einstein thought we would never detect them.

In the late 1990s, researchers built two huge experiments in an attempt to pick up gravitational waves.

For the first 13 years, there was silence. But then the LIGO detectors sensed their first ripples in space-time, from the merger of two black holes 1.3 billion light-years away.

LIGO scientists describe these signals as "chirps" because of the sound they make in the data.

Discoveries cascaded from there. In 2017, LIGO and its Italian companion, Virgo, sensed ripples from a merger of two neutron stars.

Last year, the observatories detected waves from what appeared to be a black hole swallowing a neutron star nearly 1 billion years ago.

LIGO and Virgo use a clever method to detect these waves. First, each detector shoots out a laser beam and splits it in two.

The light waves from the laser beams return at equal lengths and line up in such a way that they cancel each other out.

But when a gravitational wave comes through, it warps spacetime - making one tube slightly longer and the other shorter.

A physicist measuring those changes in brightness is thus measuring and observing gravitational waves.

Scientists can learn about the event that made those waves based on the different arrival times of the two laser beams.

LIGO's sensitivity leads to a lot of false signals because of passing trucks or gusts of wind. Even movements of atoms in the detector's mirrors can mimic the signal of a gravitational wave.

LIGO gets regular upgrades to make it more sensitive and powerful. The latest added a new instrument that squeezes light to reduce false alarms.

That photon squeezer helped the LIGO-Virgo collaboration detect 39 likely gravitational-wave events in just six months last year.

One of the events it detected revealed a type of black hole that physicists thought couldn't exist.

Researchers are still sifting through data from the final five months of their 2019-2020 observations.

Japan joined the global gravitational-wave network this year with its own observatory: the Kamioka Gravitational-wave Detector (KAGRA).

Its location in underground tunnels should insulate KAGRA from the background noise of wind and passing vehicles.

KAGRA is also the first detector to cryogenically chill its mirrors, reducing false signals from the moving molecules.

Overall, adding new observatories can help researchers detect more gravitational waves with more accuracy.

The new global network could ultimately detect 100 collisions per year, according to LIGO astrophysicist Vicky Kalogera.

Starting in 2022, LIGO, Virgo, and KAGRA are set to spend a year listening for gravitational waves together.