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photon radius

In a certain distance from a spherically symmetric black hole, the deflection of light because of the black hole’s gravity is so great that light can move on closed circular orbits – photons (light particles) can, at this distance, orbit the black hole like a planet the sun. This particular distance is called the photon radius.

An observer at rest at this distance can see the back of his or her own head (or at least a small region thereof), as the photons emitted by the back of the head travel once around the black hole and fly directly into his or her eyes.

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sun

The central (and most massive) body of our solar system; the star closest to us; a ball of gas with a radius of ca. 700000 km (for comparison the radius of the earth: 12756 km) and a mass of 1.989·10³⁰ kilograms [see exponential notation]. In the interior of the sun, nuclear fusion processes run their course; they are responsible for the sun's impressive brightness.

photon

Synonym: light particle, light quantum. In quantum theory, light is not a continuous electromagnetic wave, but a steady stream of tiny energy packets - photons.

photons (photon)

Synonym: light particle, light quantum. In quantum theory, light is not a continuous electromagnetic wave, but a steady stream of tiny energy packets - photons.

photon radius

In a certain distance from a spherically symmetric black hole, the deflection of light because of the black hole's gravity is so great that light can move on closed circular orbits - photons (light particles) can, at this distance, orbit the black hole like a planet the sun. This particular distance is called the photon radius.

observer

In the context of relativity, "observer" can mean two different things. Often, observer is synonymous with reference frame or (spacetime-)coordinate system: An observer in this sense is someone who assigns coordinates to everything that happens around him. In particular, all events are assigned space coordinate values and a time coordinate value. In the context of special relativity, it is often the case that when one talks about an observer, what is really meant is an inertial observer, corresponding to a special type of reference frame. On other occasions, the term is used in a more narrow sense - in those cases, an observer is someone sitting at a certain point in space and using the light signals reaching that location to construct an image of his surroundings. In the context of optical effects in relativity, for instance gravitational lensing, observer is usually meant in this way.

black hole

Region in space where a sufficient amount of mass is concentrated so that it forms a gravitational prison - a region into which matter or light can enter from the outside, but from which nothing that has once entered can ever leave. Basic information on this key phenomenon of Einstein's general theory of relativity can be found in the chapter Black holes & Co. of Elementary Einstein. Selected aspects of the physics of black holes and neutron stars are described in the category Black holes & Co. of our Spotlights on relativity. In Einstein's theory, black holes are truly black, due to the fact that no radiation or light can ever escape them. Once quantum theory is taken into account, this assumption no longer holds - on the contrary, it seems as if black holes should emit so-called Hawking radiation. However, for astrophysical black holes (that typically have more or even much more than one solar mass), that radiation would be undetectable, even if we could transport today's most sensitive sensors into the immediate vicinity of the black hole.

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photon radius

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