Gravitation waves around black hole in space 3D illustration

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Gravitation waves around black hole in space 3D illustration

Photo by: andreusK

andreusK

How Gravity Can Make Waves – And How You’re Feeling Them Right Now

Einstein was the first to explain the force of gravity as warps and dents in the fabric of spacetime. He was also the first to realize that those warps and dents can make waves – literal waves of gravity. But he didn’t think we would ever get to measure them, because they would be so tiny.

April 07, 2022

Fast forward a hundred years, and we’re detecting gravitational waves all the time. Yay science!

The key insight that makes Einstein’s theory of gravity – known as general relativity – work is that spacetime is flexible. The distance between any two points in space or time can change depending on the situation. In particular, massive objects (like stars and planets) distort spacetime around them so much that distances get skewed. It’s this changed perception of distance that we experience as the force of gravity.

JERUSALEM, ISRAEL - FEBRUARY 11:  The original 100 year old documents of Albert Einstein's prediction of the existence of gravitational waves are put on display at the Albert Einstein Archives of the Hebrew University on February 11, 2016 in Jerusalem, Israel. This year marks the 100th anniversary of the first publication of Einstein's prediction of the existence of gravitational waves. A group of scientists has recently announced that they have finally discovered evidence of the existence of the waves.  (Photo by Lior Mizrahi/Getty Images)

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The original 100-year-old documents of Albert Einstein's prediction of the existence of gravitational waves.

Photo by: Lior Mizrahi

Lior Mizrahi

The original 100-year-old documents of Albert Einstein's prediction of the existence of gravitational waves.

If you put something heavy on a trampoline you can see this play out. The heavy object distorts the fabric of the trampoline around it, causing it to curve. If you try to walk in a straight line across the trampoline, you’re forced to negotiate the curvature, which alters your path.

Now imagine taking that heavy object and making it spin. Or vibrate. Or do pretty much anything. The rest of the trampoline won’t just sit there – it too will vibrate and jiggle in response to whatever the heavy object is doing.

Even if you’re sitting far away from the heavy object, on a relatively flat patch of the trampoline, you won’t be able to escape feeling its influence.

These are the gravitational waves.

Press conference on Scientists to Provide Update on the Search for Gravitational Waves 100 Years After Einstein's at Chinese University in Shatin. SCMP/Felix Wong (Photo by Felix Wong/South China Morning Post via Getty Images)

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Press conference on Scientists to Provide Update on the Search for Gravitational Waves 100 Years After Einstein.

Photo by: South China Morning Post

South China Morning Post

Press conference on Scientists to Provide Update on the Search for Gravitational Waves 100 Years After Einstein.

Since all forms of mass and energy warp spacetime – because they all deform spacetime – just about anything will cause gravitational waves. Wave your arm up and down. Congratulations, you just made a gravitational wave.

But gravity is by far the weakest of the four known forces of nature. Even if it were a billion billion times stronger, it would still be the weakest force. And gravitational waves are exceedingly tiny ripples on top of that already-feeble force. So the gravitational waves sloshing around the universe that are caused by your arm flapping are almost entirely nonexistent.

Instead, you need really big and heavy stuff to make a decent wave. Things like merging black holes or exploding stars will do the trick.

In 1887, American astronomer Lewis Swift discovered a glowing cloud, or nebula, that turned out to be a small galaxy about 2.2 million light years from Earth. Today, it is known as the “starburst” galaxy IC 10, referring to the intense star formation activity occurring there.  

  More than a hundred years after Swift’s discovery, astronomers are studying IC 10 with the most powerful telescopes of the 21st century. New observations with NASA’s Chandra X-ray Observatory reveal many pairs of stars that may one day become sources of perhaps the most exciting cosmic phenomenon observed in recent years: gravitational waves.

  By analyzing Chandra observations of IC 10 spanning a decade, astronomers found over a dozen black holes and neutron stars feeding off gas from young, massive stellar companions. Such double star systems are known as “X-ray binaries” because they emit large amounts of X-ray light. As a massive star orbits around its compact companion, either a black hole or neutron star, material can be pulled away from the giant star to form a disk of material around the compact object. Frictional forces heat the infalling material to millions of degrees, producing a bright X-ray source. 

  When the massive companion star runs out fuel, it will undergo a catastrophic collapse that will produce a supernova explosion, and leave behind a black hole or neutron star. The end result is two compact objects: either a pair of black holes, a pair of neutron stars, or a black hole and neutron star. If the separation between the compact objects becomes small enough as time passes, they will produce gravitational waves. Over time, the size of their orbit will shrink until they merge. LIGO has found three examples of black hole pairs merging in this way in the past two years.

  Starburst galaxies like IC 10 are excellent places to search for X-ray binaries because they are churning out stars rapidly. Many of these newly born stars will be pairs of young and massive stars. The most massive of the pair will evolve more quickly and leave behind a black hole or a neutron star partnered with the remaining massive star. If the separation of the stars is small enough, an X-ray binary system will be produced. 

  This new composite image of IC 10 combines X-ray data from Chandra (blue) with an optical image (red, green, blue) taken by amateur astronomer Bill Snyder from the Heavens Mirror Observatory in Sierra Nevada, California. The X-ray sources detected by Chandra appear as a darker blue than the stars detected in optical light.  

The young stars in IC 10 appear to be just the right age to give a maximum amount of interaction between the massive stars and their compact companions, producing the most X-ray sources. If the systems were younger, then the massive stars would not have had time to go supernova and produce a neutron star or black hole, or the orbit of the massive star and the compact object would not have had time to shrink enough for mass transfer to begin. If the star system were much older, then both compact objects would probably have already formed. In this case transfer of matter between the compact objects is unlikely, preventing the formation of an X-ray emitting disk.

  Chandra detected 110 X-ray sources in IC 10. Of these, over forty are also seen in optical light and 16 of these contain “blue supergiants”, which are the type of young, massive, hot stars described earlier. Most of the other sources are X-ray binaries containing less massive stars. Several of the objects show strong variability in their X-ray output, indicative of violent interactions between the compact stars and their companions.   A pair of papers describing these results were published in the February 10th, 2017 issue of The Astrophysical Journal and is available online here and here. The authors of the study are Silas Laycock from the UMass Lowell’s Center for Space Science and Technology (UML); Rigel Capallo, a graduate student at UML; Dimitris Christodoulou from UML; Benjamin Williams from the University of Washington in Seattle; Breanna Bin

When this massive companion star runs out fuel, it will undergo a catastrophic collapse that will produce a supernova explosion, and leave behind a black hole or neutron star. The end result is two compact objects: either a pair of black holes, a pair of neutron stars, or a black hole and neutron star. If the separation between the compact objects becomes small enough as time passes, they will produce gravitational waves. Over time, the size of their orbit will shrink until they merge. LIGO has found three examples of black hole pairs merging in this way in the past two years.



Photo by: X-ray: NASA/CXC/UMass Lowell/S. Laycock et al.; Optical: Bill Snyder Astrophotography

X-ray: NASA/CXC/UMass Lowell/S. Laycock et al.; Optical: Bill Snyder Astrophotography

When this massive companion star runs out fuel, it will undergo a catastrophic collapse that will produce a supernova explosion, and leave behind a black hole or neutron star. The end result is two compact objects: either a pair of black holes, a pair of neutron stars, or a black hole and neutron star. If the separation between the compact objects becomes small enough as time passes, they will produce gravitational waves. Over time, the size of their orbit will shrink until they merge. LIGO has found three examples of black hole pairs merging in this way in the past two years.



Right now, you are awash in gravitational waves generated by these titanic events. When a gravitational wave passes through you, it literally stretches and squeezes you. It is, after all, a wave of gravitational force.

But you’re unlikely to feel them. Even the strongest gravitational waves passing through the Earth right now can’t even nudge something more than the width of an atomic nucleus. That’s why it took hyper-precise experiments like LIGO (the Laser Interferometer Gravitational Wave Observatory) almost a quarter-century to fine-tune their machine before they got their first results in 2015.

Since then, LIGO and other machines around the world have recorded dozens of gravitational wave events from merging black holes and neutron stars, and plans are afoot to loft an observatory into space to find supermassive black holes, supernovae, and maybe even gravitational echoes leftover from the big bang itself.

Yay, science!

Dive Deeper into the Universe

Journey Through the Cosmos in an All-New Season of How the Universe Works

The new season premieres on Science Channel and streams on discovery+.

Paul M. Sutter

Paul M. Sutter is an astrophysicist at Stony Brook University and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space.

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