Why is c the symbol for the speed of light? (1997) | Hacker News Reader

Why is c the symbol for the speed of light? (1997)

math.ucr.edu

139 pointsthe-mitr45 comments

> Apart from in relativity, the most pervasive use of c to represent a speed today is in the wave equation

This gives credence to what I was taught since high school, in French physics class: v=vitesse (speed), c=célérité (celerity).

Speed is used when talking about physical object movement, celerity is used when talking about propagation of a phenomenon (wave speed).

https://fr.wikipedia.org/wiki/C%C3%A9l%C3%A9rit%C3%A9

Though since c has come to specifically mean the speed of light is a vacuum as a notation, I see it used less often for other wave speeds when the constant is needed. Both speed and celerity are still used when writing and talking, with their respective meanings.

I always remember c as the constant speed of light [in a vacuum], but never pondered the actual origin beyond the perhaps rudimentary algebra “x, y, z are variables and a, b, c are constants”.

I don’t think I thought c came from “constant”, because that’s a very English-centric view of science, but the annoying thing about reading a smart article like this is that it’s impossible to explore the depths of how your mind worked before acquiring the knowledge!

I think it would have been 'k' if it stood for "constant" since most of them were German-speakers or collaborating with them.

k was already in use as Boltzmann's constant by that time, I think:

https://en.wikipedia.org/wiki/Boltzmann_constant

As a tongue in cheek remark, it is English-centric to think that using c for constants is an English-centric view of science. French, Spanish, Italian, Portuguese, Romanian, all use the same word of Latin origin.

I did not know about that Latin term however I thought this came from the french since in the old time, many mathematicians and physicists used "celerite" instead of "vitesse" to talk about speed.

> The same Latin root is found in more familiar words such as acceleration and even celebrity, a word used when fame comes quickly.

According to the OED, celebrity comes from the Latin “celebritās, [the] state of being busy or crowded, festival, games or other celebration characterized by crowded conditions, reputation, renown, fame, frequency or commonness.”

https://www.oed.com/dictionary/celebrity_n?tab=etymology#991...

I was going to make the same point!

Wiktionary notes that celer and celeber could be related but are likely not. (I'm not sure whether there's more recent debate on this point.)

https://en.wiktionary.org/wiki/celeber#Latin

Which in turn may come from the same root.

> From Proto-Italic kelizris, perhaps root cognate with clueo, from Proto-Indo-European ḱlew-; alternatively (if the rare meaning of "swift, in rapid succession" is to be taken as primary) connected with celer (with Greek κέλλω from a root *kel-). Jackson An Etymological Dictionary of the Latin Language (1828:77).

Four citations given: https://en.wiktionary.org/wiki/celeber#References

Yeah, a celebrity is someone who is celebrated. The “quick rise to fame” explanation seem like a folk-etymology.

It is so, so refreshing to actually get an answer straight up instead of 6 paragraphs about the context, and how the author came to learn it, and what confusion might be, and how other constants have other names etc.

Right! Most articles I stumble upon nowadays is filled with everything besides the answer until I scroll to the bottom.

Thank the SEO practices for that :(

It is often understood that in order to rank on Google you have to have a certain length to the content.

So then what do people do when they want to write about something that has a simple answer? Why, pad it with heaps of unnecessary paragraphs of garbage text of course. All in the name of ranking.

And what’s more is, the longer the text is the more space is available for inserting more ads to bombard the reader with as well. Which further serves to incentivise adding filler text that is not really useful to the reader.

Yep, my understanding is that Google interprets you clicking then going back within a short period of time as “I didn’t find what I was looking for.”

I mean, if you go back I'd assume you didn't find the answer. If you close the tab, that's a different thing, and tells me you did, in fact, find what you were looking for.

And if you're the type to open 5 or 6 tabs and look through them?

I don't know, but I don't think SEO practices were that prevalent in 1997.

Modern web article on why c is used for speed of light:

"When Sonia returned to her apartment on a warm, New York summer evening, little did she expect that ... "

I scroll to 70% and by then the secret is given out.

Its from 1997.

2004

Yes. Although the beginning quote is immediately contradicted by the short answer. So that was confusing.

> Weber apparently meant c to stand for "constant" in his force law, but there is evidence that physicists such as Lorentz and Einstein were accustomed to a common convention that c could be used as a variable for velocity. This usage can be traced back to the classic Latin texts in which c stood for "celeritas" meaning "speed". The uncommon English word "celerity" is still used when referring to the speed of wave propagation in fluids. The same Latin root is found in more familiar words such as acceleration and even celebrity, a word used when fame comes quickly.

Although the c symbol was adapted from Weber's constant, it was probably thought appropriate for it to represent the velocity of light later on because of this Latin interpretation. So history provides an ambiguous answer to the question "Why is c the symbol for the speed of light?", and it is reasonable to think of c as standing for either "constant" or "celeritas".

All hail Weber’s Electrodynamics!

> Although the c symbol was adapted from Weber's constant, it was probably thought appropriate for it to represent the velocity of light later on because of this Latin interpretation.

Why do you say that?

> So history provides an ambiguous answer to the question ...

You provide an ambiguous answer, with no evidence (unless I misunderstand). That's not history.

The use of "c" as the symbol for the speed of light can be traced back to the pioneering work of the French physicist Augustin-Jean Fresnel in the early 19th century.

In his research on light and optics, Fresnel used the letter "c" to represent the speed of light in his equations and formulas. The choice of "c" was likely influenced by the Latin word "celeritas" and its association with speed.

And here I was thinking it stood for Causality. Good to know.

I always just assumed that it was Constant. I wonder what other inaccurate assumptions I have.

Perhaps your only other inaccurate assumption is to assume you have another inaccurate assumption.

Something related which I've not been able to find the root of - in some older British mathematics textbooks, F=ma is written instead p=mf (and this is definitely p meaning force and f acceleration and not an equivalent formula in terms of impulse or similar). I would love to know why these letters were used.

The p is probably Latin potentia or French puissance, but I'm not sure about the f!

The p stands for impetus; i was already used for inertia, and m is in the same equation for mass, so p was the next best letter to use.

https://mathoverflow.net/a/23258 claims it is from Latin “festinatio.”

Are you sure you didn't read p=mg instead ?

poids = masse * gravité

weight = mass * gravity

Also related to “celery”, known as the fastest of all vegetables.

(Joke of course, celery comes from Greek selinon)

First letter from the Latin for Speed.

The article is a good read and adds some nuance to this answer.

I remember seeing a cartoon of Einstein at the blackboard on which the equation E=ma^2 (crossed out) then E=mb^2 (crossed out) and finally E=mc^2 with a beaming Einstein.

Then C++ is a overflow because nothing can travel faster than the speed of light.

In software, anything is possible.

It's warp speed

Light is how we “see”

Sí, sí.

2/3 of sunlight on earth hits the sea

You name C things which are fast.

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