The phrase "speed of light" is commonly used for the constant c =3E8 m/s, a feature that's "hardcoded" into the structure of spacetime. All massless waves and particles move at this speed, and it's a key concept in all fundamental theories in physics.

Light, in vacuum, is one fine example. But light in air or glass, or microwaves in a waveguide, move slower than c. The speed of actual light isn't the "speed of light". And of course there are plenty of other things besides electromagnetic quanta whose behavior is described by equations involving c.

The letter "c" alone isn't good for general writing and conversation, especially with non-scientists. It is fine as a mathematical symbol in equations, but isn't catchy or self-descriptive (albeit misleading) as "the speed of light".

EDIT: I have an impression I'm not asking this question in quite the right way. It's not at all about light, or phase or group velocities of light, but about our spacetime geometry having a fundamental constant speed "built in" - what to call it?

Historically it was discovered in connection with light, but c isn't specific to electromagnetism. It's important in all the fundamental forces, and in all situations with high energy particles flying about.

"Universal spacetime speed constant" would be an answer, but for that it's long and clumsy to repeat often in writings about physics. I'm looking for some name or phrase shorter and easier than that, but not so brief as just "c" which is okay in equations and tables of physical contants.

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    $\begingroup$ I don't think you've made a case for this question being much more than a statement of your own opinion about a term. Besides, questions that are just about terminology and not about the underlying physics are borderline off-topic. $\endgroup$
    – David Z
    Commented Mar 5, 2011 at 21:55
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    $\begingroup$ In natural units we just say "1". $\endgroup$ Commented Mar 5, 2011 at 21:55
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    $\begingroup$ I know this question is closed, but I'll put in a word of justification for it. It's logically possible (although extremely unlikely) that the photon has mass. In that case, $c$ would still be an important constant in the theory but would not be the speed of light. What should we call it when we're considering such theories? $\endgroup$
    – Ted Bunn
    Commented Mar 5, 2011 at 22:52
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    $\begingroup$ I like "maximal physical speed of transmission of any sort of information" better than "speed of light" since it's not tied to electromagnetism, but it's, er, kinda looooonnnnng... $\endgroup$
    – DarenW
    Commented Mar 6, 2011 at 6:28
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    $\begingroup$ You can call it the vacuum light speed to be precise. If you don't want to mention light, it is just the proportionality constant between space and time. We need $c$ because we measure distances in meters, and not in seconds (30 cm $\approx$ 1 lightnanosecond). $\endgroup$
    – jdm
    Commented Sep 27, 2011 at 13:51

1 Answer 1


The speed of light, c, actually is the speed of light. The other thing you mentioned is the phase-speed of light, which should not be confused with c.
So there are better words for all these thing you can confuse with the speed of light, c.

But not for the speed of light.

  • $\begingroup$ Yes, c is the speed of light, but it's just as special for gravity, neutrinos, stuff spewed from neutron stars, string theory, etc. Not that massive particles have to travel at c, but c is important in equations describing such things. As for the phase or group speeds, that's the kind of detail which confuses some non-science semi-technical people trying to understand technology and physics. $\endgroup$
    – DarenW
    Commented Mar 6, 2011 at 6:37

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