Oddly enough, the clearest statement of this insight came only as an afterthought, appearing in Einstein’s second paper on relativity in 1905, in which he explicitly concluded that “radiation carries inertia between emitting and absorbing bodies”. The point is that light conveys not only momentum, but inertia. For example, after a body has absorbed an elementary pulse of light, it has not only received a “kick” from the momentum of the light, but the internal inertia (i.e., the inertial mass) of the body has actually increased.

Once it is posited that light is inertial, Galileo’s principle of relativity automatically implies that light propagates isotropically from the source, regardless of the source’s state of uniform motion.

— 1.6  A More Practical Arrangement

— Reflections on Relativity

— mathpages

2013.05.06 Monday ACHK

To summarize, black holes are the most universal localized objects in any theory that includes general relativity. They’re not just random siblings of comets, planets, stars, and quasars: they’re special. They maximize the density of many things. Despite the large matter densities that are needed to produce them, black holes are connected with “apparently empty” space, space without matter. Their properties therefore have something to do with the properties of the empty space itself.

— Event horizons and thermodynamics: more than an analogy

— Lubos Motl

2013.05.04 Saturday ACHK

The black hole entropy is a fundamental property of quantized space. The information may be generally attributed to surfaces – the event horizons. In Jacobson’s derivation of Einstein’s equations via Rindler spaces – much like in the case of the “cosmic horizon” of the anti de Sitter space – these event horizons depend on the observer. But there are also event horizons that are shared by pretty much everyone who lives in the Universe – the event horizons of localized black holes that causally separate their finite internal volume from the rest of the Universe.

— Event horizons and thermodynamics: more than an analogy

— Lubos Motl

2013.04.30 Tuesday ACHK

In early 1995, Ted Jacobson found an intriguing way to “localize” the relationship between the temperature and entropy on one side, and the acceleration and areas on the other side. In fact, he derived Einstein’s equations from a well-known thermodynamic equation. See also Einstein’s equations as equations of state.

— Event horizons and thermodynamics: more than an analogy

— Lubos Motl

2013.04.29 Monday ACHK

There is another consequence of the UV/IR connection: this connection makes the black hole microstates simultaneously composite as well as elementary.

They’re elementary because they can be thought of as some excitations of a collection of strings and branes. Even if you had a different theory, it would have to be able to calculate the black hole entropy from the number of excited states of “something”. This “something” would play the very same role as strings in perturbative string theory. And differently excited strings must be interpreted as different fields in spacetime – in an effective field theory. The same has to hold for “something” whatever it is.

On the other hand, the number of black hole microstates is so large that there must exist many ways to excite the “something”, some internal degrees of freedom.

— Event horizons and thermodynamics: more than an analogy

— Lubos Motl

2013.04.27 Saturday ACHK