How do gravitons escape black holes to tell the universe about their gravity? (Advanced)

In 1991 I posed the following question to David Schramm while we were on a solar eclipse trip. He did not blink an eye in answering but I really could not follow his rationale.

If a photon is the carrier of the electromagnetic force and a graviton is supposedly the carrier of the gravitational force, how can the gradient (force) of the gravitational well of a black hole be sensed if neither photons nor gravitons can escape? If the 'information' of how massive the black hole is gone forever, how does a body know how strongly to be attracted to the hole? If I cannot feel any gravitons from the black hole why would I be pulled into orbit around it if I approached from a safe distance? Why would spacetime warp if it does not know a massive body exists next door?

These questions have bothered me for 19 years now. I hypothesized that there would be such a thing as a "Fermi age/length" (not its value) when a teenager so I know that I am both very smart and very dumb but this one perplexes me.

This is a very insightful question. I'd be interested to know what Dr. Schramm told you about it. In any case, the answer is somewhat subtle. I'll treat the gravitational case here, but the same general ideas apply to electromagnetism. In General Relativistic terms, the force of gravity is represented solely by the curvature of space-time around the black hole. The black hole is just a very curved portion of space-time. There is no need to "communicate" this information to the rest of the universe, because it is already there, lying in wait for a passing test particle. The field can also be thought of (in classical mechanics) as a "fossil field," generated by the continuously collapsing star which, to an outside observer, never actually crosses the event horizon due to time dilation.

Quantum mechanically, the situation is a bit different. Note that at present no quantum theory of gravity exists, but we do know some of the properties that it must have. In the quantum universe, forces are mediated by virtual bosons, like photons and gravitons. The key here is that these particles are virtual, not real. They exist of course, so in that strict sense they are real, and en masse can be detected and measured. However, they are the physical manefestations of the Uncertainty Principle, and as such live for only a very short time and cannot be detected individually.

You may have heard about how space is a "frothing sea" of elementary particles, with particles and antiparticles continuously popping into and out of existance. That would be another manifestation of virtual particles. Virtual particles are essentially allowed to do anything at all short of violating causality during their lifetimes. They necessarily violate conservation of energy just by existing, and they're also allowed to violate many other physical laws before they disappear. One of these is the restriction on travelling slower than the speed of light. So the virtual gravitons and photons speed away from the singularity faster than light until they're beyond the event horizon, from whence they spread out into the universe to carry on their virtual business. Various physical processes conspire to insure that no actual information is transmitted superluminally; that is, if the black hole were to instantly disappear, its gravity would still be felt outside a radius ct from the singularity.

For further reading, a very good technical (but understandable) essay on this subject can be found in QED by Richard Feynman.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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