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The Universe Remembers Gravitational Waves — And We Can Find Them

gravitational waves from two merging black holes.
An artist's illustration of two black holes merging and creating ripples in spacetime known as gravitational waves.
(Image: © Shutterstock)

Gravitational waves slosh throughout the universe as ripples in space-time produced by some of the most cataclysmic events possible. 

With facilities like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo, we can now detect the strongest of those ripples as they wash over the Earth. But gravitational waves leave behind a memory — a permanent bend in space-time — as they pass through, and we are now on the verge of being able to detect that too, allowing us to push our understanding of gravity to the limits.

Related: Hunting Gravitational Waves: The LIGO Laser Interferometer Project in Photos

Waves of gravity

Despite the fact that it's over a century old, Einstein's theory of general relativity is our current understanding of how gravity operates. In this view, space and time are merged together into a unified framework known as (no surprises here) space-time. This space-time isn't just a fixed stage but bends and flexes in response to the presence of matter and energy. 

That bending, warping and flexing of space-time then goes on to tell matter how to move. In general relativity, everything from bits of light to speeding bullets to blasting spaceships want to travel in straight lines. But the space-time around them is warped, forcing them all to follow curved trajectories — like trying to cross a mountain pass in a straight line, but following the peaks and valleys of the topography.

What we call "gravity" is then the result of all that warping of space-time, and the fact that moving objects have no choice but to follow the curves and undulations of space-time around it.

And like any other flexible surface, space-time doesn't just bend and flex; it also vibrates.

If you stand on a trampoline, you'll bend the trampoline down. If anyone tries to walk on the trampoline near you, they will feel your "gravity" and be forced to follow a curving path. But far enough away from you, they won't even notice your gravitational influence.

But if you start jumping up and down on the trampoline, you'll send waves and tremors through the whole thing, and they can't help but be influenced by your motion.

Remembering the past

Gravitational waves act in the same way, transmitting energy through ripples in the fabric of space-time itself. These ripples originate from just about every kind of motion possible, but since gravity is so weak (it is the weakest force of nature billions of times over), and gravitational waves are weaker still, only the most energetic movements are capable of creating ripples able to be detected with instruments here on Earth.

So far, our gravitational-wave observatories LIGO and Virgo have spotted dozens of cataclysmic events, involving mergers of massive black holes and neutron stars. The gravitational waves from these events ripple throughout the universe, washing over the Earth. When they do, they ever-so-slightly (as in, less than the width of an atom) move things around.

Even you. Right now, you are being gently squeezed and stretched by gravitational waves from violent events billions of light-years away.

You might think that the event is over once the wave passes, like a breaker crashing onto you at the beach and washing onto the shore. But gravity is a tricky thing, and gravitational waves are trickier still.

Almost any kind of movement triggers the generation of a gravitational wave, from black holes smashing into each other to you waving your hand around. And even gravitational waves themselves. 

As gravitational waves ripple through space-time, they become a source of new gravitational waves, which become a source of new gravitational waves, which become a source of new gravitational waves, and so on. Each new generation of waves is weaker than the last, but the effect builds up into what scientists call a space-time "memory" — a permanent distortion of space-time left in the wake of a passing gravitational wave.

In other words, when gravitational waves wash over you, you don't just stretch and squeeze temporarily. When all is said and done, you are left permanently stretched.

Related: Images: Black Holes of the Universe

Looking to the future

Since the gravitational waves generated by gravitational waves are so weak, we haven't found any evidence for this space-time "memory" yet, but it should be there, lurking in the data taken by LIGO and Virgo. What we ought to see is a lasting shift in the position of the detectors, well after the passage of the confirmed gravitational-wave event.  

Recently, a team of astronomers examined what it would take to finally see a gravitational wave memory. Since each individual detection leaves behind only an incredibly weak memory, we won't be able to see such phenomena one by one. Instead, we have to add together multiple events to build up the evidence needed to signify a detection. 

And how many events will we need? The researchers predict that we will need to record around 2,000 individual black hole mergers before we'll be able to spot the permanent memory left behind. This number of detections won't happen anytime soon, but the next generation of gravitational-wave observatories, which will hopefully collect around 10 events per day, could find this memory within a year of observations.

This permanent space-time memory ought to be there — if our predictions from general relativity are correct. And if we don't find anything after a few years of searching, we'll have to re-examine our understanding of gravity and see if we forgot something.

Read more: "Thanks for the memory: measuring gravitational-wave memory in the first LIGO/Virgo gravitational-wave transient catalog"

Paul M. Sutter is an astrophysicist at The Ohio State University, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights

You can listen to the Ask A Spaceman podcast on iTunes, and on the Web at http://www.askaspaceman.com. Ask your own question on Twitter using #AskASpaceman, or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter. Follow us on Twitter @Spacedotcom or Facebook

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  • Mario Sanchez
    About gravitational waves -
    A contributor to our Group who sent this article reminded me:
    A single mechanical experiment, bringing boundless energy from the gravitational field to the quantum field, performed here on Earth, is worth more than a hundred trillion measurements of supposed accuracy at so many light years apart in the changing and illusory field of empty quantum holes.
    He concludes - Paul Sutter did not read your book on Energy extracted from the G-force, nor does he know that this experiment, which brings about boundless energy as Einstein predicted, must first define this Nature of continuous space time. And that this is still ... continuous! Waves are discontinuous impulses. Thus there is a confusion of incompatible terms.
    Reply
  • Aozota3232
    Mario Sanchez said:
    About gravitational waves -
    A contributor to our Group who sent this article reminded me:
    A single mechanical experiment, bringing boundless energy from the gravitational field to the quantum field, performed here on Earth, is worth more than a hundred trillion measurements of supposed accuracy at so many light years apart in the changing and illusory field of empty quantum holes.
    He concludes - Paul Sutter did not read your book on Energy extracted from the G-force, nor does he know that this experiment, which brings about boundless energy as Einstein predicted, must first define this Nature of continuous space time. And that this is still ... continuous! Waves are discontinuous impulses. Thus there is a confusion of incompatible terms.
    What is the book's title sir?
    Reply
  • Mario Sanchez
    Aozota3232 said:
    What is the book's title sir?
    "
    Aozota3232 said:
    What is the book's title sir?
    "Einstein, the Unified field, and the Free Energy". You might read excerpts about it, in my answer about Einstein included in this forum
    Reply
  • THarm23
    Waves aren't a motion which must be discontinuous. You're attempting to define a wave as an entity, but in this case the importance of the wave is it's motion. All this article is referring to is the prediction of a "gravitational memory" by reverberations of gravitational waves against one another. No one here is arguing about utilizing gravitational waves as energy. That's not possible given how little we know about gravity at the moment. These invaluable measurements serve to aid our understanding. Experimentation on light was a bit difficult when we didn't know how light behaved. However, if someone would like to present a hypothesis for this experiment and a sound method for it, please do. Harnessing energy from the weakest natural force would be a breakthrough, especially on a livescience forum.
    Reply
  • Mario Sanchez
    THarm23 said:
    Waves aren't a motion which must be discontinuous. You're attempting to define a wave as an entity, but in this case the importance of the wave is it's motion. All this article is referring to is the prediction of a "gravitational memory" by reverberations of gravitational waves against one another. No one here is arguing about utilizing gravitational waves as energy. That's not possible given how little we know about gravity at the moment. These invaluable measurements serve to aid our understanding. Experimentation on light was a bit difficult when we didn't know how light behaved. However, if someone would like to present a hypothesis for this experiment and a sound method for it, please do. Harnessing energy from the weakest natural force would be a breakthrough, especially on a livescience forum.
    dear Tharm23,
    It seems to me that in a discussion forum the respondent needs to read what was said, then study what was said. I did not propose a debate about hypotheses or proposals for experiments. I am making an experiment conclusion communication and offering the power companies the machines that Einstein predicted. Therefore, the debate is already at the level of finding out who puts the invention to use, because I can't find them. I am asking for advice on how to act to be received by those interested in clean, cheap, limitless energy to end the cries of climate danger and power outages.
    Reply
  • THarm23
    You're making an "experiment conclusion communication" with no experiment to conclude. What experiment was performed? None. A hypothesis for an "experiment conclusion communication" without an experiment. If you're Keen on a "gravitational energy- harnessing machine", make it. Or, study energy yourself without riding on the back of a book proposing conspiracy theories. You've put forth a claim with no evidence but a book you've read. You've cited your evidence by the same book. Let me tell you; no one knows for sure how gravity is produced. There's theories all the way down to a partical for it's interactions (graviton). Pick up a text book, and any of them will cite this same conclusion. If you wish to metaphorically build a theoretical car without knowing the mechanics of the motor, then I wish you luck. If you insist this is possible, do it. Stop wasting your time explaining something, put your Harvard degree into action. Until then, I'll continue my studies of the practical findings of physics, not wishing to build a house before I know how to pour concrete. Continue to preach free energy machines if you like, but it's my hope that everyone here is sensible enough to know that a generation struggling immensely with the creation of fusion energy isn't bothered with creating a generator that runs on unsubstantiated books, poor grammar, and leaps of theory.
    Reply
  • Mario Sanchez
    THarm23 said:
    You're making an "experiment conclusion communication" with no experiment to conclude. What experiment was performed? None. A hypothesis for an "experiment conclusion communication" without an experiment. If you're Keen on a "gravitational energy- harnessing machine", make it. Or, study energy yourself without riding on the back of a book proposing conspiracy theories. You've put forth a claim with no evidence but a book you've read. You've cited your evidence by the same book. Let me tell you; no one knows for sure how gravity is produced. There's theories all the way down to a partical for it's interactions (graviton). Pick up a text book, and any of them will cite this same conclusion. If you wish to metaphorically build a theoretical car without knowing the mechanics of the motor, then I wish you luck. If you insist this is possible, do it. Stop wasting your time explaining something, put your Harvard degree into action. Until then, I'll continue my studies of the practical findings of physics, not wishing to build a house before I know how to pour concrete. Continue to preach free energy machines if you like, but it's my hope that everyone here is sensible enough to know that a generation struggling immensely with the creation of fusion energy isn't bothered with creating a generator that runs on unsubstantiated books, poor grammar, and leaps of theory.
    Sorry for the lack of grammar that did not allow me to communicate.

    Maybe I'm the ignorant who doesn't know how ignorant I am. So I am asking the wise (you) to tell me how I can find the companies that can receive my invention, because after 16 iron models that I put into operation, at least I can go to a workshop and train the mechanical engineers of the companies to do it. The same as I already did.

    As for Harvard, I agree to listen to their wisdom. On the condition that I can show in practice what I have done, with the explanations I will have to give.



    Finally, I request the intervention of the forum administrator to allow me to find the companies to whom I will give the invention, and free me from miscommunication like this, where I say I already did the mechanics and the commentator is deaf to the information, and he says it is by lack Grammar. I repeat that I already made the machine and just need to find out who will profit from it.
    Reply
  • Mario Sanchez
    I return to explain:

    What does the intelligent American citizen think of the G-Force prophesied by Einstein to provide energy without consumption and without limit? This force will be discovered; and it can happen in another country; a patriotic citizen wants the inventor to bring this discovery to America; and will help to prevent it from being used by the enemies of the American dream. But the ignorant and arrogant citizen, ignoring how ignorant he is, will be a tremendous danger to America at this time. With his opposition, doubts and discussions of rights, about knowledge he has not attained and interests he supposes, he will be opening the door for knowledge thieves to destroy the greatest invention in history that would happen anyway. And you understand that?
    Reply