A detectable physical quantity that carries information about an event or measurement.In physics, a signal is any detectable physical quantity that carries information about an event, measurement, or process. It is the fundamental link between the physical world and our ability to observe it. Without signals, there would be no way to know what happens in a system, from the spin of an electron to the collision of black holes billions of light-years away.
A signal arises when a physical quantity—such as electromagnetic radiation, sound waves, or electrical voltage—changes in a way that encodes information. The information is extracted by measuring the signal's properties, like amplitude, frequency, or phase, and comparing them to a known baseline. In practice, signals are often embedded in noise, so detection involves filtering and statistical analysis to distinguish the true signal from random fluctuations.
Signal processing is central to all modern technology, including telecommunications, medical imaging, and astronomical observation, where faint signals from distant objects are amplified and analyzed.
The word 'signal' comes from the Latin 'signum,' meaning a mark or sign, and in physics it often refers to a measurable change that stands out from background noise.
| W | variable | Wavelet transform coefficient — wavelet transform coefficient |
| s | variable | Scale parameter — scale parameter |
| τ | variable | Translation parameter — translation parameter second |
| x | variable | Input signal — signal |
| t | variable | Time variable — time second |
| ψ* | variable | Complex conjugate of wavelet function — wavelet function |
| ∫ | operator | Integration over time — integral operator |
| z | variable | Analytic signal — analytic signal |
| x | variable | Real signal — signal |
| a | variable | Instantaneous amplitude — instantaneous amplitude |
| φ | variable | Instantaneous phase — instantaneous phase radian |
| t | variable | Time variable — time second |
| H | operator | Hilbert transform, shifts phase by -90° — hilbert transform operator |
| i | operator | Imaginary unit, √(-1) — imaginary unit |