## Using Gravity to Peer into the Most Violent Places in the Universe: Colliding Black Holes

by Fraser CainSimulation of colliding black holes

Nothing matches the destructive power of a black hole; a singularity of dense matter with a gravitational pull so strong that nothing, not even light can escape. What goes in, doesn’t come back out. And so you can imagine how difficult it would be to probe the region inside a black hole’s event horizon. And yet, there’s a catastrophic event that should give scientists a momentary glance into the maelstrom, to partly understand what’s going on “in there.” That event would be the *collision between two black holes*.

As you probably know, there’s a supermassive black hole lurking at the heart of every galaxy. As these galaxies merge, these black holes encounter one another too. Sometimes a black hole is violently kicked into deep space, and other times they merge together into an even more super-supermassive black hole. The collision happens out of sight, beneath the shared event horizon. So, there’s no way to see what’s going on … and live to tell about it.

By looking at the gravity, however, astronomers might be able to peer right into the collision zone. One of the predictions made by Albert Einstein, as part of his famous General Theory of Relativity, is that dramatic gravitational events in the Universe, like the formation or collision of black holes should be detectable by the gravitational waves they generate. As these waves wash over us, the ripples in spacetime should be detectable by extremely sensitive instruments or spacecraft flying in formation.

A team of researchers from Cardiff University, Ioannis Kamaretsos, Mark Hannam and B. Sathyaprakash, have used a powerful supercomputer to simulate what kinds of gravitational waves might be generated by merging black holes. Two black holes orbiting one another should be emitting gravitational waves and gradually losing energy. This causes them to spiral inward, collide, and create a black hole which is highly deformed.

According to their simulation, the gravitational waves from this deformed black hole will give off a distinctive “tone”, like a ringing bell. In fact, by measuring only this tone, astronomers will be able to deduce both the mass of the black hole and the speed of its spin. Furthermore, the distortion of the gravitational waves should allow researchers to “see” what’s going on within the black hole’s event horizon; to understand what happened to the merging monsters after they disappeared beneath the shared event horizon.

“By comparing the strengths of the different tones, it is possible not only to learn about the final black hole, but also the properties of the original two black holes that took part in the collision,” Ioannis Kamaretsos said in a news release.

Of course, it’s important to note that gravitational waves themselves are still purely theoretical. Even though there are multiple ground-based detectors already built, and even more sensitive space-based detectors on the way, there hasn’t been a direct detection of a gravitational wave yet, only indirect detections. However, I wouldn’t bet against Einstein. He’s had a pretty good track record.

Original Source: Cardiff News Release

Here are two relevant (PDF) papers:

* Is black-hole ringdown a memory of its progenitor?

* Black-hole hair loss: learning about binary progenitors from ringdown signals.

Oh thanks!

The ringing does not escape the event

horizon. The ringing is spacetime

curvature or waves which are emitted close to the event horizon. If you have two black holes in a close orbit

there is dynamical spacetime close to the two event horizons. By dynamical it

means the space in the spacetime is being twisted around in such a way that

these twists or distortions form waves that can escape the system. The intense gravity means that null

directions, or the paths taken by massless particles such as light or gravity

waves, remains close to the black holes and slowly peels away. These waves can be present near the black

hole after the merger. They reach “infinity”

some time later and carry information about the original black holes which

formed the merger. From the perspective

of an observer the data about the final state of the two initial black holes

appears later than the apparent merger.

LC

imagine a different type of event: a rare linear collision of two somewhat equal masses. would not the mass/volume relation be distorted above the limit required to maintain collapse and give rise to an explosive relaxation?

The merger of two black holes has different appearances for an exterior observer and an interior observer who decides to enter. For the exterior observer the merger of two black hole horizons is similar to the merging to two pant legs leading up to the waist. If one thinks of the vertical direction as time the two horizons fuse into a single event horizon. For that exterior observer there will be gravity waves produced from the rapidly changing geometry of space in time. Once this observer sees a single black surface for the merged black hole there will continue to be gravity waves present which are due to the complicated spacetime configuration before the merger. This is what this group has computed. Now of course this will damp out with time and escape to infinity.

For the infalling observer who enters one of the black holes things are stranger. The observer who falls through a single black hole observes no change in the horizon, or what is the apparent horizon. As one gets closer to the black hole the event horizon appears as a black spherical surface that increases in size and becomes a nearly flat plane. This persists after the observer has passed the actual event horizon. This means there is still a region closer to the singularity that is causally disconnected from the observer. This persists up to the point the observer reaches the singularity.

What happens with a black hole merger according to the interior observer is a bit more complicated. First one has to take off the pair of pants and put on a skirt. The legs coming out of the skirt are the apparent horizons of the two black holes inside the event horizon. The skirt is the appearance of a new apparent event horizon according to an interior observer not too close to either of the two apparent horizons of the black hole. An observer not too close to either black hole apparent horizon witnesses the sudden appearance of a larger apparent horizon. An observer close to one of the apparent horizons in effect goes “up the skirt” and does not see it.

So an interior observer will see the large horizon of one black hole they are in and the other black hole horizon approaching, appearing as a black sphere. If the observer is not too close to the first apparent horizon then the two apparent horizons abruptly become one. If this observer is close to the apparent horizon of the black hole they entered then the two apparent horizons remain distinct up to the point they reach singularity.

The different between these two perspectives is that the exterior observer witnesses physics that is covariant, or not frame dependent. The interior observer witnesses events that depends upon the frame they are on. Depending on their frame they either witness the appearance of the “skirt,” the new apparent horizon, or they do not. This is a rather interesting development for it suggests some topology. However, the exterior world is such that observables are covariant and so anything actually inside the event horizon is not observable.

LC

so a distant observer wouldn’t see much. no energy released.

Energy is a funny thing in general relativity. Energy is only defined properly if the symmetry of the spacetime is of a certain type.

In the end nothing that is observed passes across the event horizon. This solution here finds there is a delay of signal from before the coalescence of the black holes which gives data concerning the two black holes which made up the final black hole. So external observers who collect data about the final black hole shortly after the two input black holes coalesce can measure data concerning the initial two black holes. However, this data has been “time delayed” by the curvature of spacetime.

LC

I’m fascinated by what kind of frame dragging goes on in such a system.

The math surrounding this phenomena has got to be staggering… I’d love to hear Leonard Susskind talk about this somewhere…

When two black holes near each other, doesn’t the gravity on facing sides cancel out? One black hole is pulling one way, the other the other way – there should be low-to-zero gravity between. I imagine that would get ugly as the compressed matter between the two releases – though it’s all in the event horizon so we’d never see it.