en.wikipedia.org/wiki/Speed_of_light

The **speed of light** in vacuum, commonly denoted c, is a universal physical
**constant** ...

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**.

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 ...

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

If the mass weren't zero, the **speed of light** would not be **constant**; but from a
theoretical point of view we would then take c to be the upper limit of the **speed of**
...

www.physicsoftheuniverse.com/topics_relativity_light.html

It was the young Einstein's genius to explain just WHY the **speed of light** is
**constant** and does not depend on the speed of its source or its observer. In 1905
...

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

This defines the **speed of light** in vacuum to be exactly 299,792,458 m/s. This
provides a very short answer to the question "Is c **constant**": Yes, c is **constant** by
...

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

Nov 14, 2014 **...** The **speed of light** in a vacuum, usually denoted c, is a fundamental **constant**
central to much of physics, particularly Einstein's theory of relativity ...

www.theguardian.com/science/2016/nov/28/theory-challenging-einsteins-view-on-speed-of-light-could-soon-be-tested

Nov 28, 2016 **...** New paper describes for first time how scientists can test controversial idea that
**speed of light** is not a **constant**.

newt.phys.unsw.edu.au/einsteinlight/jw/module6_constant.htm

... of **light constant**? The fine structure **constant** α ... One could say that they
determined the ratio of the **speed**(s) of **light** in the two directions. For example, the
...