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en.wikipedia.org/wiki/Speed_of_light

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its exact value is 7008299792458000000♤299,792,458 metres per second It is exact because the unit of length, the metre, ...

www.emc2-explained.info/The-Constant-Speed-of-Light

The speed of light is certainly very impressive, but it has another quality that is even more remarkable. In free space (i.e. a vacuum) its speed is constant. At first this doesn't seem all that odd, but, as we shall see, it's actually one of the strangest things in the universe. Before we can appreciate the significance of the speed of ...

www.desy.de/user/projects/Physics/Relativity/SpeedOfLight/speed_of_light.html

For example, they assume that the particle of light, the photon, is massless. If the photon had a small rest mass, the SI definition of the metre would become meaningless because the speed of light would change as a function of its wavelength. They could not just define it to be constant. They would have to fix the definition of ...

www.livescience.com/29111-speed-of-light-not-constant.html

Apr 27, 2013 ... The speed of light may not be constant, a possibility that could have broad implications for fields of cosmology and even astronomy, say physicists.

www.sciencenews.org/article/speed-light-not-so-constant-after-all

Jan 17, 2015 ... Even in vacuum conditions, light can move slower than its maximum speed depending on the structure of its pulses.

math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html

For example, it assumes that the particle of light, the photon, is massless. If the photon had a small rest mass, the SI definition of the metre would become meaningless because the speed of light would change as a function of its wavelength. The SI Committee could not just define it to be constant; instead, they would have to ...

math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html

The origins of the letter c being used for the speed of light can be traced back to a paper of 1856 by Weber and Kohlrausch [2]. They defined and measured a quantity denoted by c that they used in an electrodynamics force law equation. It became known as Weber's constant and was later shown to have a theoretical value ...

www.space.com/15830-light-speed.html

Mar 6, 2018 ... Thanks to the work of Albert Einstein and others, we now understand light speed to be a theoretical limit: light speed — a constant called "c" — is thought to be not achievable by anything with mass, for reasons explained below. That doesn't stop sci-fi writers, and even some very serious scientists, from ...

www.vcalc.com/wiki/vCalc/c+%28speed+of+light%29+with+units+m%2Fs

Jan 18, 2018 ... The Speed of Light (c) in a vacuum is 299792458.0 meters per second (m/s). This constant is provided in meters per second. However, this can be automatically converted to other velocity units via the pull-down menu.