The coalescence of two black holes—a very violent and exotic event—is one of the most sought-after observations of modern astronomy. But, as these mergers emit no light of any kind, finding such elusive events has been impossible so far.

Colliding black holes do, however, release a phenomenal amount of energy as gravitational waves. The first observatories capable of directly detecting these 'gravity signals'—ripples in the fabric of spacetime first predicted by Albert Einstein 100 years ago—will begin observing the universe later this year.

When the gravitational waves rolling in from space are detected on Earth for the first time, a team of Northwestern University astrophysicists predicts astronomers will "hear," through these waves, five times more colliding black holes than previously expected. Direct observations of these mergers will open a new window into the universe.

"This information will allow astrophysicists to better understand the nature of black holes and Einstein's theory of gravity," said Frederic A. Rasio, a theoretical astrophysicist and senior author of the study. "Our study indicates the observatories will detect more of these energetic events than previously thought, which is exciting."

Rasio is the Joseph Cummings Professor in the department of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences.

Rasio's team, utilizing observations from our own galaxy, report in a new modeling study two significant findings about black holes:

- Globular clusters (spherical collections of up to a million densely packed stars found in galactic haloes) could be factories of binary black holes (two black holes in close orbit around each other); and
- The sensitive new observatories potentially could detect 100 merging binary black holes per year forged in the cores of these dense star clusters. (A burst of gravitational waves is emitted whenever two black holes merge.) This number is more than five times what previous studies predicted.

The study has been accepted for publication by the journal *Physical Review Letters* and is scheduled to be published today (July 29).

"Gravitational waves will let us hear the universe for the first time, through the ripples made by astronomical events in spacetime," said Carl L. Rodriguez, lead author of the paper. He is a Ph.D. student in Rasio's research group.

"Up until now, all of our observations have been from telescopes, literally looking out at the universe. Detecting gravitational waves will change that. And the cool part is we can hear things we could never see, such as binary black hole mergers, the subject of our study," he said.

Rodriguez and colleagues used detailed computer models to demonstrate how a globular cluster acts as a dominant source of binary black holes, producing hundreds of black hole mergers over a cluster's 12-billion-year lifetime.

By comparing the models to recent observations of clusters in the Milky Way galaxy and beyond, the results show that the next generation of gravitational-wave observatories could see more than 100 binary black hole mergers per year.

Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) is one of the new gravitational-wave observatories. Slated to begin operation later this year, Advanced LIGO is a large-scale physics experiment designed to directly detect gravitational waves of cosmic origin. Laser interferometers detect gravitational waves from the minute oscillations of suspended mirrors set into motion as the waves pass through the Earth.

For the study, the research team used a parallel computing code for modeling star clusters developed through a CIERA-supported interdisciplinary collaboration between Northwestern's physics and astronomy department and electrical engineering and computer science department.

**Explore further:**
New insights found in black hole collisions

## docile

Jul 29, 2015## carlo_piantini

From my entire understanding of GR, "spacetime" is a geometrical object, a mathematical symbol used to represent space - it *isn't* a mechanical medium, in the same way that the aether was supposed to be a physical, substantial medium. So exactly how are waves supposed to propagate through it in the first place?

That's an open question to literally anyone, I'd love a genuine answer on this.

## Tuxford

SubQuantum Kinetics provides the answer: The propagating medium is physical, but just too small to be directly observed by the larger structures of our universe.

## docile

Jul 29, 2015## Protoplasmix

## docile

Jul 30, 2015## docile

Jul 30, 2015## docile

Jul 30, 2015## docile

Jul 30, 2015## docile

Jul 30, 2015## shavera

More accurately, a gravitational wave is a variation in how local observers measure time and space that varies over time. We know energy, especially in the form of mass, but all energy more broadly, causes how one measures lengths and times to vary with one's proximity to that energy.

One way to precisely describe this curvature is to assign every point in space(-time) around the object a "definition" of how the rulers and clocks change, as compared to some other observer (usually infinitely far away). When you put all these "definitions" into a "dictionary" you create what is called a "field." Some mathematical expression that allows you to calculate the "definition" anywhere

## shavera

One of the mathematical extensions of this "dictionary" is the fact that the dictionary is not static. If you're a massive body, you not only form part of the definition, but you must also move according to the definition of when and where you happen to be. Which means that the "definitions" change over time.

And obviously one of the simplest (for mathematical reasons) solutions for us to solve for in these intensely complicated problems are "wave-like" solutions.

## shavera

Let me repeat. A wave is a thing that obeys a specific mathematical form, NOT a physical thing moving through a physical medium.

So how do we, to borrow bschott's misunderstanding, use EM to measure variations in space-time? Well we put two rulers in an L shape. If one of the legs of the L changes (in a certain way) and not the other (or, you can mathematically 'rotate' the L to get the same effect), then you know that how one measures distance is different in one direction than another. If you eliminate all the other sources of error, you are directly measuring the local "definition" of how you measure space-time.

But the only rulers we know of that are precise enough to make this measurement are "laser interferometers." So we use those as our rulers.

## docile

Jul 30, 2015## docile

Jul 30, 2015## docile

Jul 30, 2015## carlo_piantini

I have to say though, that more and more I learn about GR, the more I genuinely dislike it as a model for gravitation - despite how well-tested and accurate it is. That isn't to say it's *wrong*, which is an opinion I used to have before I learned more about it and how well-tested it in fact is. But I simply cannot develop a love for a theory that is so completely devoid of rational mechanics. It's all math. It's literally nothing but geometry. For me, if gravitation has a physical effect on real-world objects, then its source must have a physical, mechanical nature, like Faraday's lines of force.

## docile

Jul 30, 2015## carlo_piantini

I like Faraday's intuition that gravitation can also be treated via lines of force, and I also like Lodge's original consideration that gravitation manifests mechanically out of the atom's internal structure. These are the lines of thought that I want to follow up with in trying to construct a mechanical model of gravitation. I think it can be done, and I think it needs to be done.

I don't like that we have non-classical, non-mechanical descriptions for gravitation and quantum theory, despite the fact that they work.

## docile

Jul 30, 2015## docile

Jul 30, 2015## shavera

But to understand that "length" and "(time) duration" are not fixed, immutable concepts, is a thing we have very little intuition for. What use does an ape have for knowing these things? Eppur si muove. (Gallileo's famous quote "and yet it moves")

## shavera

## carlo_piantini

I'll start with Hartle's "Gravity," it's been added to my exceptionally long booklist lol. Thanks for addition!

@docile: If you pour iron filings over a magnet, they structure themselves along the lines of force...that's really all I need to consider them physical objects. If they have an *effect* on real-world objects, they themselves must be physical, albeit, intangible.

## Protoplasmix

/*shakes my head*

Attn Physicists: Please redo physics. Without so much icky math.

## carlo_piantini

## docile

Jul 30, 2015## docile

Jul 30, 2015## shavera

We know, from other experimental data (Pound-Rebka experiment) that general relativity predicts very accurately the shift in wavelength given some height (ie, that light at some height above the ground will blueshift closer to the ground, or redshift vice versa). We have performed this experiment to a measurement of 0.01% sensitivity. So yes, GR definitely tells us something about how light shifts in the presence of space-time curvature.

The thing you're mocking as "purely mathematical" predicts very precisely measurable results. How is that any different from Newtonian physics predicting the arc of a cannonball? The maths are only as good as their ability to predict experimental outcomes.

## shavera

Again, abusing the ambiguities in language. *Some* waves are physical, in that they follow the mathematical description of a wave (see the classic example of a stringed instrument). But not all waves are physically observable.

What would be more precise is for scientists to say "we intend to observe anisotropic variations in measures of length that vary in such a way that the second derivative of the variation in spacetime is zero." But that's a huge long sentence. Easier to say "a wave is anything that behaves where the second derivative in spacetime is zero (the d'Alembertian squared of some description is zero)", and with that description of a wave "we intend to observe anisotropic variations in measures of length that are wave-like"

## shavera

False dichotomy. Some things are gravity, some are EM. Of freaking course atoms are formed by EM interactions. But some structures, larger than atoms, are driven by gravitation. Or gravitation at least brings them close enough for new EM interactions to create new molecules and so on; molecules colliding and freezing into small "grains" that collide and agglomerate into objects of varying size. The binding of these objects electromagnetic in nature, but, since each object is neutral, has very limited range of electromagnetic interactions (namely through multi-pole moments of the material)

So once you have some neutral things put together, we can't account for them accumulating electromagnetically, but we can when we include gravity. It would be foolishness in the greatest to pretend that you can only have one or the other, and not both.

## docile

Jul 30, 2015## Protoplasmix

I "injected" myself with a helpful post that included a link to the (well referenced, peer reviewed) basics, in response to your "open question for literally anyone" on why ("exactly how") gravitational waves propagate through spacetime.

Apparently not, the link's loaded with mathematics, so nevermind, carlo_p, keep guessing...

## my2cts

I agree that it is hard to see how there could be GHz gravitational waves, although they could be part of a shock wave. But then you call the CMB "natural gravitational waves". A class 1 mistake.

## my2cts

One hundred years ago some physicist may have entertained such ideas, but those days are long, long gone.

## Benni

Tsk, tsk, tsk, <$.02 worth. Your knowledge of "physics" shines through ever so dimly.

The "lines of force" to which F,M,& T were making reference are what we call (electro) magnetic. And of course they are real, they are the purveyors of energy throughout the universe. Even the lines of magnetic flux between magnetic poles are photons albeit very low frequency ones.

Lines of EM flux have an inherent field gravity that is equal to the proportion of the flux field if it were transformed to mass. Gravity fields do not flip on/off based on whether energy/mass inversion occurs, it is conserved. If half of a mass is transformed, the inversion takes half of the gravity of the original mass with it & exerts that gravity field in the same manner as if it were mass.

## carlo_piantini

Right, which I made no comment on whatsoever, because shavera very concisely explained the difference between mechanical waves and gravitational waves. I then chose to express my own *personal* distaste for GR due to the fact that it is not, as Feynman said, a mechanical theory of gravitation, which is what I'm interested in - but is rather significantly mathematical and abstract in nature.

You then chose to jump in and add *nothing* to the conversation but snark, for the sake of being condescending and rude. Faraday laid the entire foundation of EM theory and the beginning of field theory without in over ~1000 pages without a single equation. There's nothing wrong with me taking issue with GR, and the fact that you need a mathematics PhD to understand it. It's my own opinion.

## carlo_piantini

Well, that's very well and good that the majority of physicists have a consensus. I happen to agree with the electrical scientists and engineers who *did* consider them physical objects - Faraday, Maxwell, Tesla, JJ Thompson, and I'm pretty sure Steinmetz (haven't read too much of his stuff yet). It's my *own* opinion, one shared with the aforementioned, and it worked pretty well for them.

## Benni

@c_p: There is no such consensus among the majority of physicists. Just read my response a couple posts above yours, seems to comport closely with your own. I'm an Electrical/Nuclear Engineer with 6 years of Engineering School education & from that background in science it is so obvious most of the stuff my2cts is posting is totally from the plantations of funny farm science.

## Protoplasmix

It's irrational to take issue with something you admittedly don't even understand. Why espouse an opinion borne by ignorance?

## carlo_piantini

Because my inability to understand it fully is due to its abstraction from mechanics and the extremely complex mathematics used to describe the theory - which was my *entire* issue expressed with the theory.

I, personally, would like a model of gravitation that could be expressed in physics mechanics that is more intuitive than GR appears to be, from everything I've learned about it *thus far.* I don't find that irrational at all. I don't think GR is the best model of gravitation we can construct, and I'm interested in at least *attempting* to build a better one.

I already, very politely, agreed with shavera that I should learn about GR in its entirety before doing so, so forgive me, but I'm not sure what your issue is here. Your second comment was nasty, and uncalled for.

## docile

Jul 31, 2015## Reg Mundy

Good Heavens, c_p, you are actually thinking for yourself! Don't you realise that it is anathema to the acolytes of mainstream academe to do that! No, you must accept their dogma, and refrain from logic and common sense, otherwise they will attack you and accuse you of being an idiot or crank --- oh, dear, too late, they have already started!

Do you want to know how "gravity" really works, without gravity waves, gravitinos, gravitons, etc. etc.?