What are Gravitational Waves?

Article written: 8 Jun , 2015
Updated: 27 Feb , 2017
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When massive objects crash into each other, there should be a release of gravitational waves. So what are these things and how can we detect them?

Who wants to bet against Einstein? You? You? What about you?

Sure, there were a few bumps, but the guy’s track record on relativity is spotless. He explained the strange way that Mercury orbits the Sun. He guessed astronomers would see stars deflected by the Sun’s gravity during a solar eclipse. He predicted that gravity would redshift light, and it took physicists 50 years to finally come up with an experiment to verify it.

Based on his predictions, scientists confirmed galaxies warp light with their gravity, photons get time dilated when they pass near the Sun, and clocks that travel at high speeds experience less time than clocks on Earth.

They’ve even tested gravitational redshift, frame-dragging and the equivalence principle. Which is a word salad we’ll cover in the future, or for those of you who can’t wait, google.

Every time Bertie made a prediction about Relativity, physicists have been able to verify via experimentation. And so, according to this fuzzy man with the giant brain, when massive objects crash into each other, or when black holes form, there should be a release of gravitational waves.

So what are these things and how can we detect them?

First, a quick review. Mass causes a warp in space and time. The Sun’s “gravity” isn’t a pulling force, it’s really an indentation that the Sun causes in the space around itself.

Planets think they’re moving in a straight line, but they’re actually pulled into a circle while traveling through this warped spacetime. Go home planets, you’re drunk.

The idea is when mass moves or changes, Einstein said that there should be gravitational ripples produced in spacetime.

Our problem is that the size and effect of gravitational waves is incredibly small. We need to find the most catastrophic events in the Universe if we hope even detect them.

A supernova detonating asymmetrically, or two supermassive black holes orbiting each other, or a Galactus family reunion; are the magnitude of events we’re looking for.

The most serious attempt to detect gravitational waves is the Laser Interferometer Gravitational-Wave Observatory, or LIGO detector, in the United States. It has two facilities separated by 3000 km. Each detector carefully watches for any gravitational waves passing through by the length of time it takes for laser pulses to bounce within a 4km long sealed vacuum.

Laser Interferometer Gravitational-Wave Observatory  Hanford installation - each arm extends for four kilometres. Credit: Caltech.

Laser Interferometer Gravitational-Wave Observatory Hanford installation – each arm extends for four kilometres. Credit: Caltech.

If a gravitational wave is detected, the two observatories use triangulation to determine its magnitude and direction. At least, that was the plan from 2002 to 2010. The problem was, it didn’t detect any gravitational waves for its entire run.

But hey, this is a job for science. Unbowed, the steely-eyed researchers rebuilt the equipment, improving its sensitivity by a factor of 10. This next round starts in 2015.

Scientists have proposed space-based instruments that could provide more sensitivity and increase the chances of detecting a gravitational wave.

Physicists assume this is a question of “when”, not “if” that gravitational waves will be detected, as only a fool bets against Einstein. Well, that and gravitational waves have already been detected… indirectly.

By watching the extremely regular energy blasts coming from pulsars, astronomers track exactly how quickly they’re radiating their energy away due to gravitational waves. So far, all the observations perfectly match the predictions of relativity. We just haven’t detected those gravitational waves directly… yet.

So, good news! Assuming the physicists and Einstein are right, we should see the detection of a gravitational wave in the next few decades, wrapping up a series of predictions about how insanely strange our Universe behaves.

Should we dig deeper into relativity, Einstein and his predictions? Tell us in the comments below.

Fraser Cain is the publisher of Universe Today. He’s also the co-host of Astronomy Cast with Dr. Pamela Gay.


5 Responses

  1. MarcusAurelius says

    The lack of initial results makes me think that being inside a gravity well may dampen the gravity waves – much like how an oscillating string could have it’s amplitude reduced by tightening the string itself. Hopefully the upgrade finds some results.

    More Albie, please!

  2. Richard Kirk says

    The actual gravitational strains are pretty tiny. You are looking for a length change of the order of 1 part in 10^20, so it is not surprising we have not seen anything. We also want to detect this over the sort of timescale we expect from sudden astronomical events, which is about 0.001 Hz rather than the advanced-LIGO sensitivity peak of 100 Hz. You might catch the last moments of some compact mass object such as a neutron star spiralling into a black hole, but you would have to be pretty lucky to catch a local one in the act. There may be something else unexpected in this bit of the spectrum, but we would rather be looking around 0.001 Hz. It would be great if we could detect gravity waves; but, sadly, I suspect even with a-LIGO, we may not pick up anything significant. But it will be great if and when we finally do.

    Why do we believe gravity waves exist when we cannot detect them? The slowing down of rotational pulsars is pretty good indirect evidence, However, we mainly expect to detect gravity waves someday because physics would be a lot odder if they did not exist. If information cannot spread faster than light, it would be very strange if a gravitational field could go faster, even if we have no practical method of detecting it; and we would have a second absolute speed to the universe if it went slower, which would upset general relativity.

    At the moment, the non-existence of gravity waves would be an extraordinary result, which would need extraordinary data to back it up. I reckon it is going to take some while before gravitational detectors get that good. But, who knows?

  3. BlackWolfStanding says

    Just a random thought. Could Dark Energy be obscuring our observations of gravity waves? Should we not be looking for the effects and not the actual wave?

  4. TinkerLoco says

    Just replace the explanation “Time Dilation” with the effects of “Gravitational Waves” and it might be we have already found them, and even measured them.

    One of my favorite lines can be found in Stephen Hawking’s book, The Universe in a Nutshell, where it reads, “but others (including myself) felt that the beginning of the universe should be governed by the same laws that held at other times.” I would argue that throwing the explanation of “Time Dilation” on the table just because the theory is out there is NOT following the laws that held at other times. It’s more like hopeful thinking. I would also argue that if you look at any image taken of any one galaxy, it shows the relationship between Gravitational Waves and anything that has formed within that one galaxy, and there are a whole bunch of them out there. Enough to say there’s solid evidence of the relationship between Gravitational Waves, our planets, solar systems, and everything else. Hint: There just out there SPINNING, WEEEEE.

    • TinkerLoco says

      Like a unbalanced wheel on a car, you can’t tell it’s unbalanced until you reach higher speeds.

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