Friday, September 06, 2013

Equivalence

Sabine H. of Backreaction wrote about Special Relativity. Mostly she complained about certain violence popular science writer inflict on relativity, but one theme was that old question of whether one needs the General theory of Relativity for accelerated reference frames. Like all sensible persons she says no, but plenty of people who should know better don't agree. This led in turn to the equivalence principle, where plenty of more people who really really should know better (like authors of textbooks on GR) don't seem to grasp the notion that the equivalence principle refers to local equivalence of gravitation to an accelerated reference frame.

In a minute I will get to my point.

This led in turn to a debate in which Philip Helbig said, if equivalence holds, why doesn't a charge fixed in a gravitational field (say on the surface of a planet) radiate? I couldn't resist throwing in an ordinary mass on a planetary surface - would it feel Unruh radiation (the radiation felt by an accelerated object)? I was foolish enough to say that I thought the answer to the questions was yes, but I no longer believe that.

It seems that there are some subtleties here. One of them is that physicists can't even agree whether a uniformly accelerated charge radiates. See, e.g., Physics stack exchange here and Feynman's point of view (NO!) at Math Pages here. In any case, most don't believe that a charge on the surface of a gravitating body radiates, but, in any case, the energy flux is too small to measure - except maybe for neutron stars and black holes. It also seems likely that Hawking and Unruh radiation is only experienced when a horizon (or perhaps some sort of psuedo horizon) is present. Lumo makes a statement here, but provides few details.

Other arguments or opinions? Anybody? Bueller?