From Neil Wickens
I was very interested to read the account by Ed Fomalont and Sergei Kopeikin of their work on the measurement of the speed of propagation of gravity (11 January, p 32).
If their results can be verified, it would seem that gravity does indeed obey the fundamental laws of the Universe. However, this gives rise to the question: how does gravity then escape from a black hole?
The escape velocity within the event horizon is greater than that of light (and, it would appear, gravity). This ought therefore to make it impossible to detect a black hole by its gravitational effects on other bodies. Moreover, it would surely be impossible for a black hole to “grow” any further once an event horizon had been produced.
Is there any way out of this paradox?
Advertisement
Eugenie Samuel writes:
• It would indeed be a paradox if gravity could escape from a black hole. But it can’t! The gravitational field around a black hole is actually that of the collapsing star a split second before the black hole formed. Read more at math. ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html.
Carey Gully, South Australia
