# About those gravitational waves they just discovered

First, what is a gravitational wave?

I find it interesting that some people are expressing difficulty in understanding what a gravitational wave is, as though everybody (who is not a physicist) has a perfectly good understanding of what any kind of wave is. We don’t need to go too deeply beneath the surface, as it were, to understand this well enough to be amazed at the discovery, but not well enough to get a job being a Gravitational Waveologist.

Imagine a perfectly flat pond. Imagine throwing a stone out into the middle of the pond. Now imagine ripples, tiny waves, spreading out from the point where the rock hit the pond.

Now imagine the smallest possible stone that could produce a visible wave on the water. It would be pretty small. The ripples would be pretty small.

Gravitational waves are just like that, but even smaller, so you can’t see them. Not that you would “see” them with your eyes, but rather, you can’t detect them, in any normal way.

They won’t make the moon wobble, or a bird fall out of the sky. You know about super sensitive satellites that orbit the earth detecting variations in gravity such as the decrease in the pull of a glacier on the nearby ocean when the glacier melts by way less than one percent, and that sort of thing. Gravitational waves are presumably passing through that satellite all the time, but it can’t detect them.

Since they had not been detected for a long time, it had not been 100% certain that they exist. However, evidence for their existence has been mounting, and they have, in a sense, been indirectly observed.

Gravitational waves were first postulated by Einstein as early as 100 years ago as part of his application of his General Theory of Relativity, though he later temporarily retracted the idea (but then put it back).

Russian scientists advanced ways of detecting them in the 1960s, and one scientist thought he had found them a bit later in time. Further methods were developed to try to see them. In 1974 one group detected the slowing down of a pulsar orbiting a neutron star in such a way that implicated “gravitational radiation,” and this work stood (and earned a Nobel Prize). That was probably the first actual “detection” of the waves, in a sense.

In the late 1970s, the LIGO project was started to build a gravitational wave detecting antenna. In 1996, the VIRGO project was started in Italy. The Germans had just built GTO600, and partnered with LIGO soon after.

Over the last five years, the LIGO wave detector was upgraded, and after a period of time being turned off, was started up again in September. And bingo, there was a gravitational wave. Detected. Probably.

From Science Magazine:

Long ago, deep in space, two massive black holes—the ultrastrong gravitational fields left behind by gigantic stars that collapsed to infinitesimal points—slowly drew together. The stellar ghosts spiraled ever closer, until, about 1.3 billion years ago, they whirled about each other at half the speed of light and finally merged. The collision sent a shudder through the universe: ripples in the fabric of space and time called gravitational waves. Five months ago, they washed past Earth. And, for the first time, physicists detected the waves, fulfilling a 4-decade quest and opening new eyes on the heavens.

The discovery marks a triumph for the 1000 physicists with the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of gigantic instruments in Hanford, Washington, and Livingston, Louisiana. Rumors of the detection had circulated for months. Today, at a press conference in Washington, D.C., the LIGO team made it official. “We did it!” says David Reitze, a physicist and LIGO executive director at the California Institute of Technology (Caltech) in Pasadena. “All the rumors swirling around out there got most of it right.”

The way you detect a gravitational wave is to place two objects some distance from a laser source. You hit both objects with the laser and measure the return time, which should be exactlyvthe same for each object. If either object moves, that might have been because of a gravitational wave.

Obviously objects are going to move. Temperature changes, vibrations in the earth, all those things, can cause the objects to vibrate or move in some way. So you figure all that out and make all those things impossible. Also, the objects have to be really far out in order to get a signal. In the case of LIGO, the objects are four kilometers out from the laser source.

Yes, there is a reason that LITO was first funded in 1979 and only now has a result!

Now, here’s the part I don’t like. I don’t like the fact that the device was turned off and upgraded, then, very soon after being turned on, found a gravitational wave, and apparently hasn’t found another since then. I’m worried that this is similar to when CERN detected faster than light neutrinos. Everybody knew there were no faster than light neutrinos, but the instruments detected them anyway. Eventually, it was discovered that something was going on with the way the instruments were wired up that made the detector wrong.

I asked Jeffrey Bennett, author of What Is Relativity?: An Intuitive Introduction to Einstein’s Ideas, and Why They Matter, how convincing the results are. He told me,

“I think it’s very convincing. The reason they’ve waited months from the actual detection to report the event was so that they could check every possible source of artifact that might be something other than a real signal. So at this point, it’s well over 99% likely that the detection is real. Remember that the detection was made by both LIGO sites — Louisiana and Washington — and the delay between the two signals agrees with the light travel time. As to the “coincidence” — it depends on how rare or common these events are. Currently, no one has a good prediction for how often 2 black holes should collide and merge somewhere in the universe. If such events are very rare — e.g., one every 10 years or one every century — then it would indeed be a surprising coincidence. But if these events happen, say, once a year or more, then we’d expect to get a signal within a few months of starting a machine like this. The real test, then, will be whether other similar signals are detected over the next few years. I suspect they will be, and so it will turn out that these events are relatively common…”

Is there something wrong with the LIGO detector?

See the link to Science Magazine above for discussion of how hard the team worked to eliminate alternative explanations. Quite a bit, and quite convincing. But the gravity of this situation, this new discover, is so great that it may be good to retain a small amount of healthy skepticism.

By the way, don’t confuse “gravitational waves” with “gravity waves.” Going back to the pond, the ripples or waves on the water. Those are technically called gravity waves, as are some of the patterns we see in clouds. This is simply wave energy within a gravitational field. Gravitational waves are a totally different thing.

ADDED: Neil dGrasse Tyson on this discovery:

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## 28 thoughts on “About those gravitational waves they just discovered”

1. bigcitylib says:

Well, I’ve heard some speculation that they’ve probably picked up several waves but were only 100% sure with this one, so went with it.

In a way, that might make me more skeptical.

2. Eric Lund says:

Experimental physicists have a much higher standard than most other scientists for declaring something significant. They require 5σ significance; by contrast, your typical p<0.05 standard is about 2σ. Remember that under the usual assumption of normally distributed errors, the number of standard deviations is related to the probability by squaring that number and putting it in an exponent, so your chances of getting a 5σ result by chance are negligible. I don’t doubt that there would be more candidate events at a lower significance level, but physicists have been burned by 3σ events that didn’t hold up under scrutiny, so they are only reporting the one case. Or perhaps there are candidate events which meet the 5σ standard but for which they are still working on ruling out the alternatives, a process which takes a few months (which is why they are only reporting now an observation made in September). And of course there are detailed theoretical predictions for what a real event should look like, so when they observed exactly what the theorists say a real event should look like, that gives them some confidence.

You are somewhat justified in your skepticism, however. People occasionally look for magnetic monopoles (e.g., a south pole without a north). Among other things, the existence of even one magnetic monopole would explain why the electron charge has the value it does. According to physics lore, one event consistent with a magnetic monopole was observed sometime in the early 1980s (it’s sometimes called the Valentines Day event because it was observed on 14 February of that year). To this day it remains the only such observation.

People more knowledgeable on the subject than I should be able to estimate how frequently LIGO can expect to observe events like this. If it’s only a few times a year or less, then it’s not surprising that they have only one to report so far, and perhaps they did get lucky in timing their observation. If it’s daily or more often, then the lack of events is a bit harder to explain, although it would then make sense to have seen an event right away.

3. Kevin O'Neill says:

Hulse and Taylor received the 1993 Nobel Prize in physics in large part for their indirect detection of gravity waves. The original 1993 press release includes this: “The good agreement between the observed value and the theoretically calculated value of the orbital path can be seen as an indirect proof of the existence of gravitational waves. We will probably have to wait until next century for a direct demonstration of their existence. Many long-term projects have been started for making direct observations of gravitational waves impinging upon the earth. The radiation emitted by the binary pulsar is too weak to be observed on the earth with existing techniques. However, perhaps the violent perturbations of matter that take place when the two astronomical bodies in a binary star (or a binary pulsar) approach each other so closely that they fall into each other may give rise to gravitational waves that could be observed here. It is also hoped to be able to observe many other violent events in the universe. Gravitational wave astronomy is the latest, as yet unproven, branch of observational astronomy, where neutrino astronomy is the most direct predecessor. Gravitational wave astronomy would then be the first observational technique for which the basic principle was first tested in an astrophysical context. All earlier observational techniques in astronomy have been based on physical phenomena which first became known in a terrestrial connection.”

As always, when there’s breaking news in physics, the place for a great first take is Matt Strassler’s “Of Particular Significance.” Today Matt has posted Advanced Thoughts on LIGO.

4. John says:

There are 2 detectors thousands of miles apart who saw the same thing at the same time. If I wanted to be skeptical it wouldn’t be about the hardware, but about whether the explanation is the right one.

5. Donal says:

I am trying to reconcile these two statements:

“… imagine the smallest possible stone that could produce a visible wave on the water. It would be pretty small. The ripples would be pretty small. … Gravitational waves are just like that, but even smaller,”

“Going back to the pond, the ripples or waves on the water. Those are technically called gravity waves, … Gravitational waves are a totally different thing.”

So, the visible waves are gravity waves? The barely detectable waves are gravitational waves?

Well, not really a proper comparison. But yes, the waves on the pond are gravity waves. I don’t know if a tiny pebble falling on a pond create gravitational waves.

7. Obstreperous Applesauce says:

So are these waves (from a black hole collision) in the fabric of space-time (whatever that is) or more like EM wave/particle emission thingamies? And is there even a difference?

8. Christopher Winter says:

@Kevin O’Neill: Thanks for posting the link. It is an excellent article — on a blog I was unaware of to date.

9. Brainstorms says:

So are these waves (from a black hole collision) in the fabric of space-time (whatever that is) or more like EM wave/particle emission thingamies? And is there even a difference?

Waves in the fabric of space-time. There is a difference; Mass produces the effect of gravity by curving space-time (in a dimension you can’t perceive, so don’t feel bad about not having a good grasp or mental model of it).

When two very massive bodies such as black holes coalesce, the associated disruption in the space-time curvature can produce wave-like “ripples” that propagate in space-time away from the source, similar to EM wave propagation from a radiating source. (That was Einstein’s great insight.) But it’s fundamentally different from EM waves.

10. Obstreperous Applesauce says:

Thanks!

“Mass produces the effect of gravity by curving space-time”

Somehow an image of that got buried in the scrap pile of my memory, so I didn’t put it together with space-time rippling. So mass doesn’t have gravity which in turn causes curvature…

Still a little unclear on Donal and Greg’s point on the distinction between gravity waves v. gravitational waves, however. Just as a mental exercise, if you scaled them up to a macroscopic scale, how would you see/experience their effects differently?

And also, I suppose I could just look it up, but since there is a knowledgeable crew here… I assume there is a particular (no pun intended) kind of movement of gravity ‘particles’ at the quantum level as the rippling propagates?

11. Donal says:

What confused me is that gravitational waves cannot be both “just like” gravity waves “but smaller” and “totally different.”

12. Brainstorms says:

So mass doesn’t have gravity which in turn causes curvature…

“Newtonian” physics (AKA “classical physics”) looks at it that way (sort of, and Newton is the one credited with the great insight that gravity is tied to the mass of objects), but it was Einstein who turned the classical model on its head with his “genius” view that mass actually curves space-time, and that this curvature is what we perceive as “gravity” — the effect of the curvature.

His 1915 General Relativity theory gave the succinct & elegant equation that marries the geometric curvature of space-time to the mass-energy that causes the curvature. “Mass tell space how to curve, and the curvature of space tells mass how to move”, as Neil explained.

So mass doesn’t “have gravity”; it’s actually energy (E=mc^2) that has the effect of curving space, the curvature of which causes masses to attract, a phenomenon we call gravity. More mass = more curvature = more “attraction”, which to us is just “heavier”.

If you scale it up to the size of stars, and imagined a star pulsating in mass (not something it could actually do), you would feel gravity near it also pulsating — its pull on you would vary over time, periodically. A gravitational wave would propagate, as space-time near the star would be changing its deformation periodically as a consequence, and that would ripple outwards from the star.

As that wave passes by you, you would not only feel the effect as the gravitational pull on you changing (which would make you speed up or slow down if you were orbiting such a pulsing star), you would experience another strange effect: Time, for you, would speed up & slow down as well. You would also shrink & stretch slightly. But those things would be so small that you wouldn’t perceive them.

Stars don’t do that, of course, but two black holes coalescing would give you one good, strong whack in space-time that would start a strong wave propagating outwards. It just wouldn’t be periodic (because the black holes won’t separate and do that repeatedly). But the wave would “ring” for a while as the local curvature around the black holes reacted to the disturbance and it “settled down” to a more homeostatic balance between the new mass distribution and the local curvature. That propagates, too.

The “movement” is the compression & stretching of space (and time). Two separated masses, while not moving relative to their local environment (much), would “move” in relation to each other as space between them “got shorter” and then “got longer”. Ticking (atomic) clocks at each point would also change their ticking rates as the wave passes through.

13. Brainstorms says:

What confused me is that gravitational waves cannot be both “just like” gravity waves “but smaller” and “totally different.”

They are totally different, and confusing. Gravitational waves are as described above. Gravity waves have to do with buoyancy and the wave motion at the interface between fluids as gravity induces movement towards equilibrium.

Most of us have seen those clear plastic containers filled with alcohol & oil, with food color added — you turn them upside-down and they slosh back into equilibrium. While that’s settling, you can see a wave at the boundary between them, as the two fluids move up & down due to buoyant forces — caused by gravity. That wave motion of the boundary line between the two fluids as they settle is an example of a gravity wave.

Totally different.

14. Brainstorms says:

BTW, this phenomena, of two black holes coalescing, as was detected, is a VERY energetic thing. It’s estimated that the two black holes were each 30-35 solar masses (where 1 solar mass = the mass of our Sun).

The combination of the two was “missing” 3 solar masses. Matter the size of 3 of our Suns “instantly disappeared” at the moment they merged — converted, at E=mc^2, to the energy that belched out the gravity waves. That’s a level of energy beyond fathomable, and given the very short period of time it was released, that translates into a mind-boggling level of power.

Power is the transfer of energy per unit of time, so for large amounts of energy being released in a very small interval, you get a humongous power output: This black hole merger is estimated to have out-powered the entire universe by a factor of 50! (But of course, the universe is more or less steady-state, not lasting only a few nanoseconds.)

15. Obstreperous Applesauce says:

Ah. Good, clear explanation. Thank you, Brainstorms.

16. Brainstorms says:

Another way to look at #16:

Our Sun has been converting mass to energy for roughly 5000 million years. It will continue to do so for another 5000 million years.

When “its time has come”, it will swell up, last a bit longer, then throw off its outer layers and scrunch down to a small, hot ball.

If you add up the weight of the cinder (a white dwarf) plus the gases it will expel (a planetary nebula), and compare that to the mass it started with 10 billion years earlier, you’ll find that most nearly all of the original mass will still be accounted for: 99.9%. (It will destroy roughly 200-300x the mass of the Earth during its lifetime.)

Now imagine how many more billions of years the sun could shine, at the intense, hot, energetic level it’s at now, if it were to convert 99% of its mass into energy, not just 0.1% (i.e., 1000x more) –> 10,000 billion years of shining out energy hot enough to give you a sunburn standing 100 million miles away…

Now triple that.

Now release it all in 20 milliseconds, not “dribbled out” over 30 trillion years (a reduction of 5×10^22, or a multiplier factor of nearly 50,000 million million million).

A power output that’s 50x the entire visible universe’s output.

17. Elias says:

Can gravitational waves affect future space travel? Future space ships traveling at extreme speeds, could they tear apart when hitting a “strong” gravitational wave? There must be some logic to this, remember the Challenger disaster when it hit this weather phenomena breaking o-ring on one of the two external fuel tanks, which in turn exploded the main fuel tank and the ferry, killing all astronauts!

18. Brainstorms says:

Potentially, if one’s craft were close enough to a coalescing black hole pair, but in that case the crew would know a priori about the upcoming cataclysm and would stay far enough away. If a craft were far enough away to not know what was about to happen, they wouldn’t be close enough to be affected either.

There are other astronomical hazards out there for future deep space travelers. For example, gamma ray bursts from collapsing stars (another exemplar of mind-boggling energy release) will shoot out beams of x-rays and gamma rays strong enough and long enough to fry anything in its stellar neighborhood — including potentially sterilizing entire planets that might have the misfortune of lying in the beam path.

Those kinds of events would not be predictable in the way that star mergers would be. (Stars don’t telegraph their imminent collapse, an event which progresses rapidly, whereas we can observe and predict orbit decay.)

Fortunately, GRB’s are fairly rare, as stellar events go. “Outer space” is full of dangerous stuff, and “the safety of Earth” is relative…

19. outeast says:

Brainstorm, kudos and thanks for some seriously good explanations there. I’d say you should write a book, but you probably have!

20. Dr. Ligo Skeptic says:

[Link to science denying site removed]

Why didn’t LIGO simply wait for another signal? Would not waiting for a second signal detract from the chance of this being seen as scientific fraud perpetuated on the taxpayer?

Well, perhaps the best answer as to why LIGO did not wait for at least a second signal is that they were beginning to think they’d never see another one! And they panicked! They knew had to act fast so as to maximize their chances for optimal hype, funding, and hiring and tenure decisions. They couldn’t let any more time going by without seeing a signal, or otherwise it would look more and more like an unrepeatable anomaly, misinterpretation, noise, hoax, or prank. And once the multimillionaire funders decided on the press conference, what lowly postdoc is going to speak out against the regime’s decision, let alone the senior scientists who devoted their life to it?

The first supposed LIGO detection of two merging black holes happened just before they officially turned the LIGO machine on, on September 15th.

Since then, they have seen no other signals.

Imagine that you are on the LIGO team.

You “see” a supposed signal of a “binary black hole merger” before you officially turn the LIGO machine on on September 15th 2015.
You are elated as this means the Nobel Prize, tenure, billions in funding!
You then turn on the machine.
You see no other detections nor signals.
Months go by and no other signals are seen.
You begin to panic.
You realize that your window for major publicity and funding is closing fast.
Five months go by and no other signals are seen.
Panic sets in.
You pull the trigger to get massive funding and hopefully billions more in funding.
You hold a massive press conference on February 11th, hyping only one tiny event.
The world’s unquestioning useful idiot science bloggers all fall in line, hyping and regurgitating your spurious press releases as the gospel truth.
Kip Thorne solidifies it all stating that now we will finally be able to observe the pseudoscience of giant cosmic strings born of the pseudoscience of inflation.
Hundreds of millions of dollars of granting and hiring deciscions are made while hundreds of LIGO members receive tenure and promotions for merely falling in line on a project seeking evidence for a 100-year–old theory which already appears quite correct.
Another signal is never seen.
All the faulty blogs and wikipedia pages and tenure and funding deciscions remain, as once done, they are “impossible” to undo, according to the elite.
The unquestioning regime of billion-dollar corporate science is advanced, giving the middle finger to the lone patent examiner and humble physicists who did far, far more with a pencil, paper, logic, reason, and honor, sans the corporate state’s hype and lies.
So LIGO, when, if ever, will we see another “signal”?

Dr. Skeptic, the project waited months before reporting the finding in order to be sure it was correct. There was no panic causing them to report early.

I’m just going to add that beyond that, your comment is a poorly informed and delusional rant. You need to get some help. I am not joking.

I really enjoyed this blog and your explanation of this phenomenon and how it was detected. As our advancing technology is able to detect smaller and smaller phenomenon there will always be questions of the validity of the discovery.
Think back to the late 1800’s when they first discovered DNA, I’m sure there was a lot of skepticism back then. As scientists we are suppose to question everything we see, including the work of other scientists. And I’m sure the LIGO scientists welcome the skepticism they are getting. That is how scientific discovery works.
I for one am excited about what this means for the future of science.

23. amjc says:

The first scientific who predicted the gravitational waves was not Albert Einstein (in 1916). The French mathematician Jules-Henri Poincaré mentioned these waves in June 1905, for the first time, and he called them “ondes gravifiques”. See page 1507 of the paper titled “Sur la dynamique de l’électron”, in http://web.archive.org/web/20050127151248/http://home.tiscali.nl/physis/HistoricPaper/Poincare/Poincare1905.pdf

Poincaré had created the special theory of relativity before Einstein, during the period 1895-1905, and he also investigated some other concepts that can be found in general relativity. This theory was generally known as the “Relativity of Poincaré and Lorentz” in those years. See http://www.brera.unimi.it/sisfa/atti/1998/Giannetto.pdf

24. Yaniv Stern says:

W against T, reference required. #conspiracyofscience

25. Arthur Pilbeam says:

Henri Poincaré predicted gravitational waves 11 years before Einstein made his prediction.

http://www.science20.com/relativity_and_beyond_it/henri_poincare_predicted_the_existence_of_gravitational_waves_as_early_as_june_5_1905-165539

“In 1905, Poincaré first predicted gravitational waves (ondes gravifiques) emanating from a body and propagating at the speed of light as being required by the formalism of spacetime” from –
https://en.wikipedia.org/wiki/Henri_Poincar%C3%A9