# Can a trapped surface be formed by a mass configuration outside of that trapped surface?

Can a trapped surface be formed without any massive bodies inside that trapped surface, but only by a configuration of massive bodies surrounding the trapped volume?

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It seems obvious that the answer is no, if you interpret photons and gravitons as massive bodies, but it is not immediately obvious how to prove it, because of the problem of localization of stress energy in GR. –  Ron Maimon Aug 31 '11 at 21:52

If I understand your question correctly, the answer is yes, by virtue of the fact that trapped surfaces can form without there being any massive bodies in the universe whatsoever. In other words, trapped surfaces can form purely from the focusing of gravitational waves. Now take some massive bodies all very far apart from each other, and shake them vigorously. If the shaking is hard enough, the profile of the gravitational waves would be sharp enough that the results in the linked paper would (in principle) apply.

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I think the proper interpretation of the question is to include gravitational waves as bodies, at least, this makes it more interesting. But your answer is 100% correct, of course, +1. –  Ron Maimon Aug 31 '11 at 21:51