Paradox of radiation of charged particles in a gravitational field electric field has an accompanying magnetic field, and the whole oscillating electromagnetic radiation field propagates independently of the accelerated charge, carrying away momentum and energy. The energy in the radiation is provided by the work that accelerates the charge. The theory of general relativity is built on the equivalence principle of gravitation and inertia. This principle states that it is impossible to distinguish through any local measurement whether one is in a gravitational field or being accelerated. An elevator out in deep space, far from any planet, could mimic a gravitational field to its occupants if it could be accelerated
Direction, position, or indication sign sign category into direction signs, which are only those giving distances or directions to a given location; road identification signs (also known as "reassurance signs"), which repeat the name or number of the road, and place identification signs, which give the name of a landmark, such as a town, river or border. Unlike the other classes of sign, direction signs remain broadly undefined by the convention; the only restrictions given are that direction signs must be either a rectangle or an arrow shaped pentagon, and that they may not contain placenames in more than two languages. Additionally, direction signs on