infrared
Electromagnetic radiation in the frequency region between a hundred billion and a trillion oscillations per second, corresponding to wave-lengths between 0.8 micrometres and 1 millimetre. The thermal radiation associated with everyday ...
inflation
Hypothetical phase in the earliest universe during which the cosmos underwent exponentially growing expansion. ...
inertial observer
An inertial reference frame is a reference frame in which the first law of classical mechanics holds: A body on which no external forces act either remains at rest or moves with constant speed along a straight path. An inertial observer is ...
inertial forces
In classical mechanics or special relativity: Whenever an observer who is not an inertial observer wants to explain the movements of bodies using the law "force equals mass times acceleration", that observer has to assume the existence of additional forces; ...
inertia
Basic law of classical mechanics and special relativity: bodies on which no external forces act move with constant speed on straight paths. In the geometrical language of special relativity, this can be reformulated as: bodies on which no external forces act ...
imaginary time
Certain quantum calculations (notably the calculation of path integrals as a way to find quantum mechanical probabilities) involve an algebraic manipulation of the following kind: Wherever the time coordinate t occurs, it is replaced by i·t, where i is the ...
hydrogen
The lightest (and, in our universe, the most abundant) chemical element. The atomic nucleus of an ordinary hydrogen atom is a single proton. If the atomic nucleus contains an additional neutron, the atom is called heavy hydrogen or ...
Hubble space telescope
Cooperative project of NASA and ESA: Space telescope that was put into orbit in 1990. Orbiting 600 kilometres above the earth, it leaves behind the densest parts of the earth's atmosphere, allowing an unrivalled, undisturbed view into space. ...
