## What Happens When Supermassive Black Holes Merge?

by Jason Major

Frame from a simulation of the merger of two black holes and the resulting emission of gravitational radiation (NASA/C. Henze)

The short answer? You get one super-SUPERmassive black hole. The longer answer?

Well, watch the video below for an idea.

This animation, created with supercomputers at the University of Colorado, Boulder, show for the first time what happens to the magnetized gas clouds that surround supermassive black holes when two of them collide.

The simulation shows the magnetic fields intensifying as they contort and twist turbulently, at one point forming a towering vortex that extends high above the center of the accretion disk.

This funnel-like structure may be partly responsible for the jets that are sometimes seen erupting from actively feeding supermassive black holes.

The simulation was created to study what sort of “flash” might be made by the merging of such incredibly massive objects, so that astronomers hunting for evidence of gravitational waves — a phenomenon first proposed by Einstein in 1916 — will be able to better identify their potential source.

**Read: Effects of Einstein’s Elusive Gravity Waves Observed**

Gravitational waves are often described as “ripples” in the fabric of space-time, infinitesimal perturbations created by supermassive, rapidly rotating objects like orbiting black holes. Detecting them directly has proven to be a challenge but researchers expect that the technology will be available within several years’ time, and knowing how to spot colliding black holes will be the first step in identifying any gravitational waves that result from the impact.

In fact, it’s the gravitational waves that rob energy from the black holes’ orbits, causing them to spiral into each other in the first place.

“The black holes orbit each other and lose orbital energy by emitting strong gravitational waves, and this causes their orbits to shrink. The black holes spiral toward each other and eventually merge,” said astrophysicist John Baker, a research team member from NASA’s Goddard Space Flight Center. “We need gravitational waves to confirm that a black hole merger has occurred, but if we can understand the electromagnetic signatures from mergers well enough, perhaps we can search for candidate events even before we have a space-based gravitational wave observatory.”

The video below shows the expanding gravitational wave structure that would be expected to result from such a merger:

If ground-based telescopes can pinpoint the radio and x-ray flash created by the mergers, future space telescopes — like ESA’s eLISA/NGO — can then be used to try and detect the waves.

Read more on the NASA Goddard new release here.

*First animation credit: NASA’s Goddard Space Flight Center/P. Cowperthwaite, Univ. of Maryland. Second animation: NASA/C. Henze.*

Black Holes, Einstein, eLISA, Goddard, Gravitational Waves, NASA, Physics, video

The amount of data connected to black hole mergers is an interesting problem. The Einstein field equation is nonlinear and not an elliptic complex that is readily solved. Solutions to the Einstein field equations are only possible if there is some high degree of symmetry exists in the spacetime. This has an interesting result. It is commonly known that Newtonian mechanics only solves the two body problem exactly, and for n > 2 bodies Newtonian mechanics does not give solutions in closed form. For n > 2 one must use perturbation theory and other approximation techniques. General relativity can’t solve the two body problem. A two body problem is only solved if one of the two bodies has a very tiny mass, such as Mercury orbiting the sun. In that case one can compute orbits very precisely. For two bodies of comparable mass in a close orbital configuration exact solutions are not possible. One must use perturbation techniques.

A standard approach to perturbation theory in general relativity is to write the metric in a series

g_{ab} = ?_{ab} + h_{ab} + h’_{ab} + …

where ?_{ab} is a flat spacetime metric and h_{ab} is the first perturbation term. The transverse traceless part of this can be shown from the Einstein field equations to satisfy

(?^2 – ?^2/?t^2)h_{ab} = 0

which is a linear wave equation. This would describe weak gravity waves measured by a distant detector or what would be produced by two black holes orbiting each other at some distance. If you bring those black holes together, which will happen as gravitational radiation bleeds off energy, then one needs to work with the h’_{ab} in this series. This will produce a nonlinear wave equation. In addition if one is to predict the dynamics of the two orbiting black holes one needs to do more work with this series in addition. Then as the orbit tightens one would need to invoke the h”_{ab} and so forth. The solutions one gets with each series becomes more complex, and there are in principle an infinite number of terms.

So how much information or data is available from this collapse? It is clearly not infinite. In fact it can’t exceed the entropy of the black holes with S = k A/4L_p^2, for k = Boltzmann constant, A = 4?M^2 the area of a black hole with mass M, and L_p the Planck unit of length L_p = sqrt{G?/c^3} ~ 10^{-33}cm. The ratio A/4L_p^2 is the number of Planck units of area on the black hole horizon. The amount of information produced by the coalescence of two black holes can only amount to about 30% of the mutual information contained in the two black holes. So the infinite sequence in the perturbation series does not tell us “everything,” and in fact there simply can’t be an infinite amount of information.

LC

While it is nice to speculate about universe-gobbling black holes, I wonder more about the size of black hole that science says is too small to accumulate. What happens to these pin size black holes? How does the energy dissipate? In what form of energy? Since science speculates these too small to grow and just big enough to start growing holes are everywhere, how they get big is important. And are they sourced from nova explosions?

Kathleen wrote: “How does the [black hole] energy dissipate?”

Do you mean other than Hawking radiation?

There is a range of black hole sizes. A black hole the size of an atom is such that mass-energy in the interior with a wave length can tunnel out. This is Hawking radiation, which causes the black hole to evaporate. There are questions about how black holes at galaxy centers became as large as a billion solar masses. It is rather hard in a way to get something in a black hole. If you throw something at a black hole that is not right on target it will go into an orbit around the black hole rather than pass into it. There are questions about how a star sized black hole can grow into a multi-million solar mass to billion solar mass black hole.

LC

http://www.mmdtkw.org/VMoreBlack.html

Disapate into what?