Scientists say they have proven the existence of gravitational waves — the ripples in space-time that stem from objects moving throughout the Universe. If true, it’s the first time these waves have been measured directly and marks one of the biggest scientific discoveries in decades. But there have been a few false alarms before.
The discovery was made by the large group of researchers who make up the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration. The LIGO observatories picked up the wave signals on September 14th, 2015, coming from two distant black holes merging 1.3 billion years ago. The black holes spun around each other several times per second before merging in a massive explosion — a process that generates huge gravitational waves. These waves peter out on the way to Earth, requiring delicate instrumentation that must adjust for any interference on our planet. Though LIGO’s observations are ground-breaking, other scientists will need to pore over the data to make sure it’s correct. The study was published today in Physical Review Letters. There will also be more studies in the coming weeks and months.
Gravitational waves were predicted by Albert Einstein in 1918, as part of his theory of general relativity. Finding them would confirm a big part of that theory — and would also be the first step toward a new way of observing the cosmos. Black holes and neutron stars are too dim to be seen by modern telescopes, but researchers might be able to measure the gravitational waves they produce. That would tell scientists more information about how these objects manipulate gravity, and may even lead to the discovery of new objects and concepts we’ve never even thought about. “It’s not just that we’re checking on a theory,” says Ira Thorpe, an astrophysicist at NASA, who was not involved with LIGO. “The real important part of this is it gives us a completely new tool for understanding our Universe.”
That is, assuming today’s finding is right. Scientists have been wrong about gravitational waves a few times before. In the 1960s, American physicist Joseph Weber claimed he had detected gravitational waves, but his findings were discredited when no one could reproduce them. And in 2014, a team using the BICEP2 telescope near the South Pole said they had found evidence of the waves by studying the early Universe; their results, however, turned out to be from cosmic dust. The tools that LIGO uses are very susceptible to outside interference, which means that today’s results could be influenced by interference generated here on Earth.
But for now, today is a day of celebration for scientists. “Ladies and gentlemen, we have detected gravitational waves. We did it,” said David Reitze, the LIGO executive director, at the press conference announcing the discovery. His declaration was met by cheers and applause by the scientists and press in attendance. “The things we’ve surmised and speculated about will become the subjects of detailed study,” Marc Kamionkowski, a theoretical physicist at Johns Hopkins University, told The Verge before the announcement. “It’s just amazing that they can detect this.”
What are gravitational waves?
An artist rendering of gravitational waves from two black holes. (NASA)
Einstein’s theory of general relativity completely altered how we perceive the Universe. Originally, scientists thought that space and time were two fixed and independent concepts. But Einstein’s theory combined space and time together into one four-dimensional model called space-time. And space-time isn’t fixed at all.
The theory also altered our understanding of gravity. According to Newtonian physics, all objects in the Universe are innately attracted to one another. Einstein instead proposed that objects actually warp the space-time around them, creating gravitational “pull.” Imagine a bowling ball on top of a stretched blanket. The ball bends the blanket, creating an impression in the fabric. Planets and stars are the bowling ball; space-time is the blanket. Now, imagine rolling a marble across that blanket with the bowling ball on it. The marble is going to follow the curve of the blanket down toward the ball. Smaller objects like asteroids act like the marble when near a bigger mass: they following the curvature of space-time the larger object creates.
When objects move through the Universe, they also warp space-time as they go. This causes ripples: gravitational waves. The movement of every object — from a person to a supermassive black hole — produces gravitational waves.
Most everyone in the scientific community believe gravitational waves exist, but no one has ever proved it. That’s because the signals from gravitational waves are usually incredibly weak. “When we move, the gravitational wave is so weak it is effectively zero,” says Kamionkowski. “The Earth going around the Sun produces a gravitational wave signal, but it is still very, very weak.”
But the more mass an object has, the bigger the wave it produces. Super-dense objects like black holes or neutron stars moving at rapid speeds can produce big enough gravitational waves that can be measured here on Earth. So that’s what LIGO’s scientists targeted.
The LIGO collaboration has two observatories in Louisiana and Washington State, both funded by the National Science Foundation. Each facility is shaped like a giant “L;” the “arms” of the L are two vacuum-sealed tubes stretching 2.5 miles long, with mirrors at each end. The mirrors are used to measure how gravitational waves warp space-time. When a gravitational wave passes, one mirror gets closer while the other retreats; scientists measure this phenomenon by bouncing lasers off the mirrors. Changes in the amount of time it takes a laser to bounce off a mirror indicate a gravitational wave.
The gravitational wave measurements from the black holes were also converted into audible form, what LIGO calls a “chirp.” Right as the black holes merge, the frequency of the resulting gravitational waves increases up until the moment of collision. As a sound, that movement becomes a high-pitched note that raises octaves really quickly. The chirp is perceptible in the recording below. Scientists shifted its frequency so it’s easier to hear.
The gravitational waves only change LIGO’s instruments by about one ten-thousandth the size of a proton. This means Earth isn’t the ideal place to look for waves, since movements from people or traffic can potentially cause interference. For instance, LIGO kept getting “readings” that were actually the result of cars rolling over a nearby bump in the road.
One of LIGO’s test mass mirrors. (LIGO)
But LIGO has tried to adjust for possible interferences. The collaboration even has three members dedicated to messing with the data. These scientists purposefully create fake signals to make sure the collaboration can discern what real measurements are supposed to look like. At one point, LIGO was ready to announce they had detected gravitational waves based on one of these fake readings. The team had to start all over again when they found out the data had been manipulated.
An artist rendering of what the LISA gravitational wave detector in space could look like. (NASA)
If LIGO’s measurements hold up, the collaboration could start its own ripple effect — one within the scientific community. Gravitational waves offer ways to study objects that don’t give off light — which could mean more research institutions might invest in ways to detect them. “When radio telescopes were first discovered, we found all these powerful radio sources we’d never known before,” says Kamionkowski. “We’re getting new eyes to look at the sky.”
Likely, gravitational wave detection will move beyond Earth into space. Thorpe is currently working on the LISA Pathfinder mission, a partnership between NASA and the European Space Agency. The project launched a spacecraft in December to test the technologies needed for future space-based gravitational wave detectors. Thorpe says that by working in space, the disturbances associated with Earth aren’t a problem. Also, there’s a lot more room to work with: a space detector could use laser beams millions of miles long to pick up gravitational wave movements. “By going to space, we can really take advantage of the fact that we can make our detector bigger,” he says.
Realistically, though, such technologies are decades away. But Thorpe says today’s discovery will help spur efforts like the LISA Pathfinder mission. “The success of a project like LIGO will inspire us to keep going and trying.”
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