Why is C the symbol for the speed of light? (2004)
120 comments
·March 25, 2025xenadu02
layer8
Causation is a bit of a troublesome concept in physics: https://plato.stanford.edu/entries/causation-physics/
I wouldn’t use causality as a foundational concept ontologically.
What’s true is that c doesn’t just apply to the electromagnetic field, but to all fundamental physical fields.
auntienomen
"Photons (being massless) have 100% of their velocity in the spatial dimensions and no velocity in the time dimension"
I kinda see what you're trying to say, but these words aren't a particularly good match for the math in special relativity. To an observer, any photon's velocity 4-vector looks like V = (c,c,0,0). That's a "c" in the spatial direction the photon's moving, and a "c" in the time direction. So, plenty of velocity in the time direction.
What is zero is the _proper time_ along a photon's trajectory through space & time. An observer who's co-moving with the photon (call me if you ever meet one, we can write a paper together) would see a) the photon holding still and b) no time passing.
phire
And when you think about c as the speed of causality, it becomes a lot more obvious why FTL travel/communicate is (almost certainty) impossible without breaking causality....
You are going faster than causality, of course it breaks.
The other option is to break relativity, which is what most science fiction media does, often accidentally.
eggn00dles
special relativity is undefined at v = c. so no from a photons point of view the trip doesn’t happen in an instant nor does the length of the universe shrink to zero. there is no definition for a photons point of view. time dilation and length contraction only apply when comparing a moving frame of reference to one that is at rest and photons have no rest frame.
stared
It sounds like a really nice (and the shortest possible) backronym.
aatd86
Isn't it supposed to be c for celerity? Edit.. Oh I see. Nvm
xarope
is that a highly technical way of saying WE think c is the speed of light, because that's all WE can measure?
chrisweekly
That sounds like an interesting theory; is it yours, or do you have any citations / references you could share?
ameetgaitonde
He's just describing Einstein's Theory of Special Relativity.
chrisweekly
Huh. I'm admittedly no astrophysicist, but I don't recall encountering this "photons have no velocity in the time dimension" before. Maybe I'm one of today's lucky 10,000[1].
smokel
If you think it is implied that c stands for the first letter of the word causality, then reading the article should be enough to convince you otherwise.
Einstein published his seminal works in German, and we'd be more likely to have E=mk².
grues-dinner
It is, in my opinion, not extremely obvious from the usual descriptions of relativity, but basically when you move faster and faster, you trade your velocity through time for velocity through space. Rather then a funny result, that is the theory.
If you're at rest, you have maximum time velocity (1 you-second per frame-second). If you're at the speed of light, it's zero you-seconds per frame-second.
This is described by the Minkowski space, which is a metric that puts two events the same distance apart in spacetime regardless of reference frame.
Greg Egan's series "Orthogonal" looks into what the universe would look like if time didn't have the opposite sign (so that time is another dimension just like x, y, z). The effects of that one sign change are very wierd.
xenadu02
I can't claim credit for any of this. As others have noted Einstein (and other much smarter people) are responsible.
xeonmc
I like to put it as: Special Relativity is just the Pythagorean Theorem.
marcellus23
It's Einstein's :)
zachooz
An introduction to general relativity spacetime and geometry by Sean Carrol
wetpaws
[dead]
throwawayk7h
Please fix the title, it's _c_, not _C_.
idoubtit
I never understood the appeal of title case. It brings nothing valuable, but sometimes leads to stupid mistakes like this, or raises ambiguities that hamper my reading.
I hope the American influence won't make this practice more common in British English. But it's not just about the country: The Washington Post has sane titles, like every British newspaper I've read, while The New York Times Has Elite Titles With Many Big Letters.
9rx
Technically it is the character with Unicode code point U+1D450. But HN arbitrarily removes it from the string upon submission, so, since we're approximating, C will do.
fsckboy
>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
not to mention the more common "acceleration"
munificent
Yes, the very next sentence in the article:
The same Latin root is found in more familiar words such as acceleration and even celebrity, a word used when fame comes quickly.
null
froh
yes! and that is composite
* "a-/ad-" towards
* "celeritas" speed
the second derivative, of sorts:
towards+speed
DiogenesKynikos
Indeed, and c_s ("c subscript s") is commonly used to denote the speed of sound.
tonymet
C is for "Cochranes", named after Zefram Cochrane
AnonymousPlanet
I thought it was because C is fast (compared to most other languages)
bee_rider
But light is fast compared to all other things, not just most other things. It is the fastest thing. So, this can’t be it, otherwise Fortran would be traveling back in time… wait, is Fortran traveling back in time?
awesome_dude
C is for cookies, and that's good enough for me!
hangonhn
I was always taught that it stood for "constant", which is what the speed of light is in every frame of reference and I've never stopped to question it because it made sense. But it seems that usage actually predates Einstein according to the above article. It's interesting how a good story can be used to sell something that's not entirely true and I never stopped to question it.
wnissen
My immediate guess based on no specific knowledge was “arbitrary constant while they were figuring things out” and it sounds like that’s not far from the truth. The process of discovery is often far more protracted than it seems when one is reading about it decades after the fact.
pyuser583
It wasn't the symbol for the speed of light, it was the symbol for the Lorenz Constant.
rakoo
I've always learned that "speed" is something that can be used to describe an object aka something with mass. When something without mass travels (basically information) you use "celerity", because it's not talking about the "same" thing
psd1
A little too neat. It has the distinct aroma of lies told to children. (Not that I would have detected it myself, as a child.)
deadbabe
So c = celeration, the derivative of acceleration.
gnarlynarwhal42
Cool to see a local school on here.
Off-topic but their botanical gardens and Cactus/Desert garden is a really enjoyable afternoon.
JdeBP
I wonder whether this is the Philip Gibbs that set up viXra.
simonjanssen
Somewhat off-topic, but here's "A joke about measurement": https://www.youtube.com/watch?v=KmfdeWd0RMk
madcaptenor
Okay, now why is m slope?
I'd also wondered why r is the correlation coefficient but it turns out it's the "regression" coefficient, as in how strong the regression to the mean is.
JdeBP
A lot of these conventions just develop according to what notation sticks, like how π caught on almost 3 millennia after the ratio was treated as a constant.
And not all of them are even as universal as one might think. mx+c is not.
Charles Hutton used y = ax + b for the equation of a "right line" in xyr 1811 A Course of Mathematics, for example.
Keyframe
Okay, now why is m slope?
the great divide between US and Europe :D should be a - a, b, c, d... why m though, indeed?
anthk
For a Basque speaker as me, m was ideal because malda in Basque means... slope :)
kqr
Wait, is m slope? I learned it with k (for coefficient) for the slope, and m for the intercept (I don't know why m in that case though).
fuzzfactor
Kind of reminds me of a mountain you're going to climb :)
madcaptenor
From a bit of googling I see some people say it's from French "monter", meaning to climb, which does come from the same root as English "mountain".
DarkNova6
Because C++ wasn't invented yet.
c is technically the speed of causality. The speed at which perturbations in the fields that make up the universe propagate.
The speed of light is a consequence of this, not the cause. Calling c the "speed of light" is putting the cart before the horse.
Photons (being massless) have 100% of their velocity in the spatial dimensions and no velocity in the time dimension. They move at the maximum speed that any change in the electrostrongweak force can propagate in our universe because they are not free to do anything else.
From a photon's POV a trip across the entire universe happens instantaneously - taking no time whatsoever.