[HN Gopher] Why is C the symbol for the speed of light? (2004)
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Why is C the symbol for the speed of light? (2004)
Author : fanf2
Score : 103 points
Date : 2025-03-25 15:42 UTC (7 hours ago)
(HTM) web link (math.ucr.edu)
(TXT) w3m dump (math.ucr.edu)
| madcaptenor wrote:
| 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.
| fuzzfactor wrote:
| Kind of reminds me of a mountain you're going to climb :)
| madcaptenor wrote:
| 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".
| JdeBP wrote:
| A lot of these conventions just develop according to what
| notation sticks, like how p 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.
| anthk wrote:
| For a Basque speaker as me, m was ideal because malda in Basque
| means... slope :)
| Keyframe wrote:
| _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?
| kqr wrote:
| 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).
| xeonmc wrote:
| Could also stand for Speed of Causality.
| matt3210 wrote:
| Because it's the fastest programming language
| silisili wrote:
| Yep but some are claiming this makes the speed of light
| incredibly unsafe and unpredictable.
|
| A small but loud group of scientists are working to replace c
| with a crab emoji in most literature going forward.
| anthk wrote:
| >unsafe and unpredictable.
|
| Just llike the Electron.
| prerok wrote:
| Travelling at c is UB.
| benob wrote:
| I definitely though of the programming language at first.
| Although the speed of light is a bit presumptuous to qualify
| the speed of C, I now wonder why every benchmark is against C,
| and not some other language.
| Someone wrote:
| Partly because it is fairly fast, partly because, whatever
| you're running _your_ language on, chances are better that
| you can run a C compiler on it than that you can run most
| other languages.
| mike_ivanov wrote:
| Wrong answers only
| bregma wrote:
| C is the Roman numeral representing 100. You can't go faster than
| 100% of the speed of light. QED.
| racl101 wrote:
| Quod Erat Demonstrandum.
| jihadjihad wrote:
| That's celerious
| Dwedit wrote:
| You need "C" to be the symbol for the speed of light to make the
| joke from Animanics work.
| hiccuphippo wrote:
| The one where they spell acme backwards?
| racl101 wrote:
| e m c a
| travisgriggs wrote:
| Link? I was a big animaniacs fan, but not recalling this one.
| Dwedit wrote:
| https://www.youtube.com/watch?v=rsyJX3sESjs
| nullhole wrote:
| "How can I eat cookies when the theory of relativity has
| escaped me?!" is a pretty good line
| havaloc wrote:
| C you later?
| gnarlynarwhal42 wrote:
| Cool to see a local school on here.
|
| Off-topic but their botanical gardens and Cactus/Desert garden is
| a really enjoyable afternoon.
| JdeBP wrote:
| I wonder whether this is the Philip Gibbs that set up viXra.
| zabzonk wrote:
| A and B were already used. Somewhere.
| raverbashing wrote:
| B non-ironically was (I guess) - it means magnetic field.
|
| A, no idea.
|
| Also might I add the speed of light is (lower-case) c, not C
| annodomini2019 wrote:
| Wouldn't a have been used for acceleration?
| hehbot wrote:
| Magnetic Vector Potential?
| pyuser583 wrote:
| It wasn't the symbol for the speed of light, it was the symbol
| for the Lorenz Constant.
| simonjanssen wrote:
| Somewhat off-topic, but here's "A joke about measurement":
| https://www.youtube.com/watch?v=KmfdeWd0RMk
| wnissen wrote:
| 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.
| DrNosferatu wrote:
| _C_elerity?
| 762236 wrote:
| The 'short version' is very well written and to the point. (I
| didn't read the long version, so my lack of comment on that isn't
| a criticism of it)
| Vox_Leone wrote:
| "As for c, that is the speed of light in vacuum, and if you ask
| why c, the answer is that it is the initial letter of celeritas,
| the Latin word meaning speed."
|
| Isaac Asimov in "C for Celeritas (1959)"
|
| 'https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLig...
| antirez wrote:
| Because of the language
| hangonhn wrote:
| 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.
| throw-qqqqq wrote:
| > Why is C the symbol for the speed of light?
|
| Because C++ and Rust wasn't invented when they formalized it! ...
| I'll see myself out
| fsckboy wrote:
| > _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"
| froh wrote:
| yes! and that is composite
|
| * "a-/ad-" towards
|
| * "celeritas" speed
|
| the second derivative, of sorts:
|
| towards+speed
| DiogenesKynikos wrote:
| Indeed, and c_s ("c subscript s") is commonly used to denote
| the speed of sound.
| munificent wrote:
| 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._
| tonymet wrote:
| C is for "Cochranes", named after Zefram Cochrane
| AnonymousPlanet wrote:
| I thought it was because C is fast (compared to most other
| languages)
| bee_rider wrote:
| 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 wrote:
| C is for cookies, and that's good enough for me!
| ck2 wrote:
| I always thought it was because it's not the speed of LIGHT
|
| It's the speed of causality
|
| But it still blows my mind the universe expands faster than C
|
| https://en.wikipedia.org/wiki/Cosmological_constant#/media/F...
| formerly_proven wrote:
| 'cause you don't c it coming
| throwawayk7h wrote:
| Please fix the title, it's _c_, not _C_.
| idoubtit wrote:
| 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 wrote:
| 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.
| xenadu02 wrote:
| _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.
| belter wrote:
| > c is technically the speed of causality.
|
| How does this notion reconcile with seemingly instantaneous
| quantum phenomena like spooky action at a distance?
| jlawson wrote:
| Quantum entanglement ("spooky action at a distance") doesn't
| allow for communication, so there's no causality between the
| particles.
| belter wrote:
| Thanks for the answer. I had forgotten about the No-
| communication theorem.
|
| This led me into another rabbit hole :-) of why c is
| 299,792,458 m/s and for example not 499,792,458 m/s or some
| other value, and the fundamental constants that bring this
| value. [1]
|
| Is there a current theory that tries to justify those
| constants that bring the current value of c ? Are those
| values the ones that must be, for the current Universe to
| be feasible?
|
| [1] https://arxiv.org/abs/hep-ph/0205340
| pixl97 wrote:
| I'm not sure if there is a good theory on why it ends up
| being this speed, but if any of the constants were even
| slightly different we wouldn't be in this universe to
| talk about it.
|
| https://en.wikipedia.org/wiki/Fine-tuned_universe
| dcow wrote:
| Veritasium has a few recent videos on Action. In one of
| them I believe he shows speed of light expressed as one
| unit of action, which implies that it's the highest
| "resolution" any propagation can resolve at based on the
| fundamentally smallest path integral any _thing_ can
| traverse, or something.
| xenadu02 wrote:
| That touches on a long-running very complicated debate around
| quantum mechanics, hidden variables, spooky action at a
| distance, no-communication, etc.
|
| For various quantum effects that seem to be paradoxes by
| classical physics what is happening "under the covers"? Does
| the delayed choice experiment _really_ send information
| backwards in time? Even if it appears to do so if we can 't
| at least send _information_ back in time with that mechanism
| isn 't it just sophist philosophy at that point?
|
| For my part I'm not smart enough to claim to have answers to
| anything but my intuition is there are no quantum paradoxes.
| Delayed choice does not send anything back in time. We don't
| experience quantum phenomena at the macro scale so our
| intuition and reasoning are ill-suited to thinking about it.
| That easily leads us to incorrect conclusions.
| chrisweekly wrote:
| That sounds like an interesting theory; is it yours, or do you
| have any citations / references you could share?
| ameetgaitonde wrote:
| He's just describing Einstein's Theory of Special Relativity.
| chrisweekly wrote:
| 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].
|
| 1. https://xkcd.com/1053/
| bee_rider wrote:
| But, it is sort of common to observe the somewhat trippy
| fact that, unlike matter, photons don't experience time
| passing, right? (Although, I will be honest, I could not
| ever really grok relativity like a physicist. However, I
| can enjoy the Lorentz factor going to infinity as v goes
| to c).
| da_chicken wrote:
| From my understanding, it's more correct to say that time
| is undefined for massless objects.
|
| Through special relativity, our understanding of mass,
| gravity, and spacetime are linked. If something has no
| mass, then special relativity can't describe how gravity
| affects it's spacetime reference.
|
| Remember, however, that this explanation is based on the
| mathematics that explain the observations we've made or
| theorized. Just as the map is not the territory, the math
| is not the universe.
| salviati wrote:
| I enjoyed this video on this topic a lot. Maybe you will
| too: https://youtu.be/fB8eatgkOyM?si=D9s01MY8jByWREPX
| nh23423fefe wrote:
| It's not true. It's a mashup of a few ideas that don't
| belong together.
|
| 1. an object at rest has a world line which points (has
| tangent vectors) in the time dimension
|
| 2. light has proper time (dtau) = 0, so clocks moving at
| the speed of light dont tick
|
| 3. the magnitude^2 of an objects 4-velocity is c^2
| (objects move through spacetime at c)
|
| 4. light has no 4-velocity (because dtau=0, you cant
| divide by zero)
|
| You can't say (3) means objects have 4-velocity c in
| spacetime and light has 3-velocity c in space and so that
| means the time component of 4-velocity for light is zero.
|
| Because light has no 4-velocity.
| marcellus23 wrote:
| It's Einstein's :)
| xenadu02 wrote:
| I can't claim credit for any of this. As others have noted
| Einstein (and other much smarter people) are responsible.
| smokel wrote:
| 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.
| zachooz wrote:
| An introduction to general relativity spacetime and geometry
| by Sean Carrol
| grues-dinner wrote:
| 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.
| 762236 wrote:
| I downvoted you simply because that's the way HN works.
| stared wrote:
| It sounds like a really nice (and the shortest possible)
| backronym.
|
| https://en.wikipedia.org/wiki/Backronym
| aatd86 wrote:
| Isn't it supposed to be c for celerity? Edit.. Oh I see. Nvm
| layer8 wrote:
| 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.
| kqr wrote:
| I realise now I had probably only thought of c as a natural
| companion to v because of Qwerty keyboards...
| rakoo wrote:
| 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
| deadbabe wrote:
| So c = celeration, the derivative of acceleration.
| psd1 wrote:
| 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.)
| themaninthedark wrote:
| I thought c stood for clock speed and that is how we know we are
| in a simulation.
| TZubiri wrote:
| Because god coded it on C
| Aardwolf wrote:
| Thought something similar, it's all because they wrote C
| instead of the actual c in the title
| borntoolate wrote:
| Because C++ is object oriented.
| DarkNova6 wrote:
| Because C++ wasn't invented yet.
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