In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The IEEE Transactions on Signal Processing includes audio, video, speech, image, sonar, and radar as examples of signal. A signal may also be defined as any observable change in a quantity over space or time, even if it does not carry information. In nature, signals can be actions done by an organism to alert other organisms, ranging from the release of plant chemicals to warn nearby plants of a predator, to sounds or motions made by animals to alert other animals of food. Signaling occurs in all organisms even at cellular levels, with cell signaling. Signaling theory, in evolutionary biology, proposes that a substantial driver for evolution is the ability of animals to communicate with each other by developing ways of signaling.

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Modeling Solar Winds: Simulations Reproduce Complex Fluctuations in Soft X-Ray Signal Detected by Satellites

Researchers from Montreal and India detect radio signal from galaxy 8.8 billion light years away

India's GMRT receives first-ever signal from distant galaxy, 880 million light years away

Radio signal from 9 billion light-years away from Earth captured

Radio signal from 9 billion light-years away from Earth captured

Radio signal detected from galaxy 8 billion light years away; breaks previous records

Astronomers capture radio signal from ancient galaxy at record-breaking distance

Uncovering Secrets of the Early Universe: Astronomers Capture Radio Signal From Distant Galaxy

Astronomers Detect Radio Signal from Strongly Gravitationally Lensed Galaxy

Record-Breaking Signal From Distant Galaxy Is Furthest of Its Kind Ever Detected

Astronomers Use Ripple In Space-Time To Capture Most Distant Signal Ever From A Galaxy

The First Galaxies Unveiled: Non-Detection of Key Signal Reveals Key Information Regarding the Early Universe

It may be possible to traverse a wormhole and then send a signal home

Signal processing algorithms improved turbulence in free-space optic tests

Sun-like star thought to be a likely source of the famous WOW! signal ruled out after follow-up observations.

Maybe we don’t see Aliens Because They’re Waiting to Hear a Signal From us First

Mysterious Astronomical Signal Is Black Hole Jet Pointing Straight Toward Earth

Non-detection of key signal allows astronomers to determine what the first galaxies were -- and weren't -- like

Double telescope study of zone where Wow! signal originated comes up empty

If We’re Going to Get Under the Ice on Europa, How Will We Send a Signal Back to the Surface?