Eddington limit (Sir A. Eddington)

The theoretical limit at which the photon pressure would exceed the gravitational attraction of a light-emitting body. That is, a body emitting radiation at greater than the Eddington limit would break up from its own photon pressure.

Edwards-Casimir quantum vacuum drive

A hypothetical drive exploiting the peculiarities of quantum mechanics by restricting allowed wavelengths of virtual photons on one side of the drive (the bow of the ship); the pressure generated from the unrestricted virtual photons toward the aft generates a net force and propels the drive.

Ehrenfest paradox (Ehernfest, 1909)

The special relativistic “paradox” involving a rapidly rotating disc. Since any radial segment of the disc is perpendicular to the direction of motion, there should be no length contraction of the radius; however, since the circumference of the disc is parallel to the direction of motion, it should contract.

The cornerstone of Einstein’s general theory of relativity, relating the gravitational tensor G to the stress-energy tensor T by the simple equation

G = 8 pi T.

Einstein-Podolsky-Rosen effect; EPR effect

Consider the following quantum mechanical thought-experiment: Take a particle which is at rest and has spin zero. It spontaneously decays into two fermions (spin 1/2 particles), which stream away in opposite directions at high speed. Due to the law of conservation of spin, we know that one is a spin +1/2 and the other is spin -1/2. Which one is which? According to quantum mechanics, neither takes on a definite state until it is observed (the wavefunction is collapsed).

The EPR effect demonstrates that if one of the particles is detected, and its spin is then measured, then the other particle — no matter where it is in the Universe — instantaneously is forced to choose as well and take on the role of the other particle. This illustrates that certain kinds of quantum information travel instantaneously; not everything is limited by the speed of light.

However, it can be easily demonstrated that this effect does not make faster-than-light communication or travel possible.

See permeability of free space.

Eotvos law of capillarity (Baron L. von Eotvos; c. 1870)

The surface tension gamma of a liquid is related to its temperature T, the liquid’s critical temperature, T*, and its density rho by

gamma ~= 2.12 (T* – T)/rho3/2.

See Einstein-Podolsky-Rosen effect.

epsilon_0

See permittivity of free space.

The basic postulate of A. Einstein’s general theory of relativity, which posits that an acceleration is fundamentally indistinguishable from a gravitational field. In other words, if you are in an elevator which is utterly sealed and protected from the outside, so that you cannot “peek outside,” then if you feel a force (weight), it is fundamentally impossible for you to say whether the elevator is present in a gravitational field, or whether the elevator has rockets attached to it and is accelerating “upward.”

Although that in practical situations — say, sitting in a closed room — it would be possible to determine whether the acceleration felt was due to uniform thrust or due to gravitation (say, by measuring the gradient of the field; if nonzero, it would indicate a gravitational field rather than thrust); however, such differences could be made arbitrarily small. The idea behind the equivalence principle is that it acts around the vicinity of a point, rather than over macroscopic distances. It would be impossible to say whether or not a given (arbitrary) acceleration field was caused by thrust or gravitation by the use of physics alone.

The equivalence principle predicts interesting general relativistic effects because not only are the two indistinguishable to human observers, but also to the Universe as well — any effect that takes place when an observer is accelerating should also take place in a gravitational field, and vice versa.

See weak equivalence principle.

The region around a rotating black hole, between the event horizon and the static limit, where rotational energy can be extracted from the black hole.

The radius that a spherical mass must be compressed to in order to transform it into a black hole, or the radius at which time and space switch responsibilities. Once inside the event horizon, it is fundamentally impossible to escape to the outside. Furthermore, nothing can prevent a particle from hitting the singularity in a very short amount of proper time once it has entered the horizon. In this sense, the event horizon is a “point of no return.”

The radius of the event horizon, r, for generalized black holes (in geometrized units) is

r = m + (m2 – q2 – s/m2)1/2,

where m is the mass of the hole, q is its electric charge, and s is its angular momentum.

See Schwarzschild radius.