Sonic black hole

A sonic black hole (sometimes called a dumb hole) is a phenomenon in which phonons (sound perturbations) are unable to escape from a fluid that is flowing more quickly than the local speed of sound. They are called sonic, or acoustic, black holes because these trapped phonons are analogous to light in astrophysical (gravitational) black holes. Physicists are interested in them because they have many properties similar to astrophysical black holes and, in particular, are predicted to emit a phononic version of Hawking radiation. The border of a sonic black hole, at which the flow speed changes from being greater than the speed of sound to less than the speed of sound, is called the event horizon. At this point the frequency of phonons approaches zero.

Sonic black holes are possible because phonons in perfect fluids exhibit the same properties of motion as fields, such as gravity, in space and time. For this reason, a system in which a sonic black hole can be created is called a gravity analogue. Nearly any fluid can be used to create an acoustic event horizon, but the viscosity of most fluids creates random motion that makes features like Hawking radiation nearly impossible to detect. The complexity of such a system would make it very difficult to gain any knowledge about such features even if they could be detected. Many nearly perfect fluids have been suggested for use in creating sonic black holes, such as superfluid helium, one–dimensional degenerate Fermi gases, and Bose–Einstein condensate. Gravity analogues other than phonons in a fluid, such as slow light and a system of ions, have also been proposed for studying black hole analogues. The fact that so many systems mimic gravity is sometimes used as evidence for the theory of emergent gravity, which could help reconcile relativity and quantum mechanics.

Black hole

A black hole is a geometrically defined region of spacetime exhibiting such strong gravitational effects that nothing—including particles and electromagnetic radiation such as light—can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.

Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; it was during the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black hole (networking)

In networking, black holes refer to places in the network where incoming or outgoing traffic is silently discarded (or "dropped"), without informing the source that the data did not reach its intended recipient.

When examining the topology of the network, the black holes themselves are invisible, and can only be detected by monitoring the lost traffic; hence the name.

Dead addresses

The most common form of black hole is simply an IP address that specifies a host machine that is not running or an address to which no host has been assigned.

Even though TCP/IP provides means of communicating the delivery failure back to the sender via ICMP, traffic destined for such addresses is often just dropped.

Note that a dead address will be undetectable only to protocols that are both connectionless and unreliable (e.g., UDP). Connection-oriented or reliable protocols (TCP, RUDP) will either fail to connect to a dead address or will fail to receive expected acknowledgements.

Firewalls and "stealth" ports

Black hole (disambiguation)

A black hole is an object with sufficient density that the force of gravity prevents anything from escaping from it except through quantum tunneling behavior.

Black hole may also refer to:

Computing

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