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The Question

(Submitted December 07, 1996)

In my study of Black Holes I have been taught that nothing can escape the gravity well of this object because the escape velocity is = c. However, Science magazine, vol. 270, dated 10 Nov. 1995, page 943, states that during the theoretical collision of two spherical Black Holes in the equilibrium state, upon coming into close proximity to each other, " the instant these two tidally distorted black holes then merge and in the process convert 3 x 10-4 of the system's mass into energy in the form of gravitational waves radiating outward."

My question then is : Is gravity a form of energy which can escape from the infinite density singularity or is it really escaping itself once it (the black hole) has achieved equilibrium. If gravity is a form of energy which radiates outward from this object then it cannot radiate very far before it is bent back upon itself by the force of its' own influence.

This is a fascinating subject to me and I understand it from an overall view point. I have only three years college under my belt and have not the math background to understand the physics behind it. So don't go into it very deeply. It's just that I seem to have painted myself into a corner with this aspect of gravity.

Thank you for your time in answering this question.

The Answer

Thank you for your question about gravitational radiation from black holes. It is clear that you have given this quite a bit of thought! Your question has touched upon a number of other very interesting questions concerning gravity, such as what is meant by the term gravitational energy and where is it located. However, in a system of two colliding black holes the issues are not necessarily difficult to understand, at least at a basic level. As you can probably guess, a complete treatment can rapidly become incomprehensible and can be left to those people who do this kind of work for living. The short answer therefore, hopefully appropriate for Science readers, is the following:

The gravitational radiation associated with this system comes from the time dependent nature of spacetime outside the black holes as these objects move towards each other and accelerate. This is, to some extent, equivalent to the electromagnetic radiation of two charges attracted to each other. You need not know the precise structure of the charges or whatever singularities they enclose. All you care is that they accelerate as they approach each other, and that produces time dependent electromagnetic fields. At large distances these time dependent electromagnetic fields appear as radiation. The just replace the charges with black holes and electromagnetic radiation with gravitational radiation.

There are qualitative differences of course, the charges have to have opposite signs to attract while the black holes attract each other anyway. There is also some of the energy (whatever the term means in this case) which is lost down the black holes while this is not in the charge case. There are additional issues which the experts worry about and presumably treat correctly.

Steve Snowden, Laura Whitlock, and Demos Kazanas

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