alt.hn

3/25/2025 at 3:42:03 PM

Why is C the symbol for the speed of light? (2004)

 https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/c.html

by fanf2

3/25/2025 at 6:50:46 PM

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.

by xenadu02

3/25/2025 at 10:45:47 PM

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.

by layer8

3/26/2025 at 6:56:05 AM

There is a more precise concept of causality in physics that is very much fundamental part of all modern theories.

by uecker

3/26/2025 at 12:26:07 PM

And what concept would that be?

by layer8

3/26/2025 at 3:53:06 PM

Einstein causality, i.e that no physical effect propagates faster than light. In QFT it it is modeled by the rule that space-like separated measurements commute.

by uecker

3/26/2025 at 4:26:21 PM

That merely equates the term "causality" to propagating waves. It doesn't introduce any new concept. Ontologically, there are only quantum fields in space-time. The physical laws governing them imply from their mathematics that they cannot contain any propagations faster than c. But attaching the term "causality" to it doesn't really add anything here.

by layer8

3/26/2025 at 4:37:50 PM

This does not equate causality with propagating waves. I also did not claim it is a new concept. Instead I said physics has a precise and fundamental concept of causality.

by uecker

3/26/2025 at 4:40:40 PM

How is it fundamental? You can do all the physics without reference to it. It is a consequence of the equations, not a premise.

Edit: It is not actually a complete consequence, because cause and effect imply a directonality that the equations do not have, being time-symmetric. At the same time, this directionality or asymmetry has no derivable consequences for the practical physics, meaning having the asymmetry doesn't add anything. This is one of the conceptual troubles that introducing "causality" here brings.

by layer8

3/26/2025 at 5:44:14 PM

It is used in theories that discuss particles with imaginary mass, such as tachyons, that travel backwards in time.

Theories that violate causality are considered unphysical, whereas theories in which measuring such time traveling particle, cannot be distinguished from creating a particle that moves forward in time are not.

by niemandhier

3/26/2025 at 5:07:09 PM

Physicists put this into the theories as an assumption at a fundamental level, e.g. they put it as an axiom into axiomatic QFT. This is why it is fundamental. Also arrow of time is something else than Einstein causality.

by uecker

3/25/2025 at 10:16:07 PM

It sounds like a really nice (and the shortest possible) backronym.

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

by stared

3/25/2025 at 10:24:06 PM

Isn't it supposed to be c for celerity? Edit.. Oh I see. Nvm

by aatd86

3/26/2025 at 1:02:12 AM

"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.

by auntienomen

3/26/2025 at 6:49:13 AM

I can't recommend such a co-author. Doesn't get anything done.

by uecker

3/26/2025 at 2:40:51 AM

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.

by eggn00dles

3/26/2025 at 2:12:58 AM

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.

by phire

3/25/2025 at 6:55:59 PM

That sounds like an interesting theory; is it yours, or do you have any citations / references you could share?

by chrisweekly

3/25/2025 at 8:54:43 PM

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.

by smokel

3/25/2025 at 7:14:16 PM

He's just describing Einstein's Theory of Special Relativity.

by ameetgaitonde

3/25/2025 at 7:24:08 PM

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/

by chrisweekly

3/25/2025 at 10:51:27 PM

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.

by nh23423fefe

3/25/2025 at 7:30:57 PM

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).

by bee_rider

3/25/2025 at 8:39:50 PM

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.

by da_chicken

3/25/2025 at 10:48:47 PM

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.

by grues-dinner

3/27/2025 at 2:38:48 AM

I'd say it's extremely non-obvious, given "velocity" no longer means distance / time

by chrisweekly

3/25/2025 at 8:11:15 PM

I can't claim credit for any of this. As others have noted Einstein (and other much smarter people) are responsible.

by xenadu02

3/26/2025 at 12:01:36 AM

I like to put it as: Special Relativity is just the Pythagorean Theorem.

by xeonmc

3/25/2025 at 7:21:30 PM

It's Einstein's :)

by marcellus23

3/25/2025 at 8:58:10 PM

An introduction to general relativity spacetime and geometry by Sean Carrol

by zachooz

3/25/2025 at 7:21:35 PM

[dead]

by wetpaws

3/26/2025 at 2:31:58 PM

No, there is no causality anymore in quantum physics, since the EPR paradoxon was disproved.

c stands for the latin for for speed of course. causality is immediate, a higher speed than c. because it's logical, not measurable.

by rurban

3/25/2025 at 6:53:35 PM

> c is technically the speed of causality.

How does this notion reconcile with seemingly instantaneous quantum phenomena like spooky action at a distance?

by belter

3/25/2025 at 7:17:49 PM

Quantum entanglement ("spooky action at a distance") doesn't allow for communication, so there's no causality between the particles.

by jlawson

3/25/2025 at 8:10:45 PM

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

by belter

3/25/2025 at 9:59:04 PM

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 anything can traverse, or something.

by dcow

3/25/2025 at 8:41:47 PM

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

by pixl97

3/26/2025 at 1:45:22 AM

The speed of light is 1c and is a fundamental constant.

Humans choose to express it in metres and seconds.

A meter is huge if you compare it to the Planck length. Humans are pretty big creatures compared to fundamental particles, so we have a big basic unit of length. But seconds are gargantuan, because humans are absolutely glacial if you compare them to the time it takes light to travel a Planck length.

It's like a continental plate asking why humans zip around so fast.

by wisty

3/26/2025 at 11:25:24 AM

> The speed of light is 1c and is a fundamental constant.

Yes, but why is the value that it is and not higher or lower? From my (basic) research it seems, it could be plus or minus 20% a different value, and the current Universe would still be feasible.

by belter

3/26/2025 at 2:52:52 PM

Well, it has to be _some_ value, right? I mean, if we were in a universe where it was 20% greater, you would be asking the same questions.

by marcellus23

3/27/2025 at 11:01:04 AM

No, the point is that if it was 20% bigger we would not be here to ask those questions :-) Stars fusion or weak atomic interactions would not work. Maybe Hari Seldon decided....

by belter

3/27/2025 at 6:26:51 PM

But you said in your comment that "it could be plus or minus 20% a different value, and the current Universe would still be feasible"

by marcellus23

3/25/2025 at 8:18:53 PM

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.

by xenadu02

3/25/2025 at 11:09:35 PM

Sure, but why did Einstein call the speed of light c?

by phkahler

3/26/2025 at 12:07:58 AM

I think the article explains that.

by layer8

3/26/2025 at 2:49:55 AM

is that a highly technical way of saying WE think c is the speed of light, because that's all WE can measure?

by xarope

3/25/2025 at 7:29:59 PM

[flagged]

by 762236

3/25/2025 at 5:58:20 PM

Please fix the title, it's _c_, not _C_.

by throwawayk7h

3/25/2025 at 8:54:40 PM

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.

by idoubtit

3/26/2025 at 7:31:57 AM

I am glad to see that I am not the only one who thinks the same. My mother language doesn't use title case, maybe it's why I find it ugly.

Another thing that I also find really annoying is using commas instead of "and" in headlines. It just makes reading them much harder for no obvious benefit.

by pezezin

3/26/2025 at 12:19:22 AM

While I prefer the aesthetics of only an initial capital in the title, title case in prose helps us distinguish the title from the rest of the sentence.

by mmooss

3/26/2025 at 1:33:32 AM

There are more suitable typographical techniques of accomplishing the same result.

by pepa65

3/25/2025 at 9:12:19 PM

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.

by 9rx

3/25/2025 at 5:33:23 PM

>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"

by fsckboy

3/25/2025 at 7:35:07 PM

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.

by munificent

3/25/2025 at 6:07:13 PM

yes! and that is composite

* "a-/ad-" towards

* "celeritas" speed

the second derivative, of sorts:

towards+speed

by froh

3/25/2025 at 6:33:30 PM

Indeed, and c_s ("c subscript s") is commonly used to denote the speed of sound.

by DiogenesKynikos

3/25/2025 at 5:24:31 PM

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.

by hangonhn

3/25/2025 at 5:36:16 PM

C is for "Cochranes", named after Zefram Cochrane

by tonymet

3/25/2025 at 6:15:16 PM

I thought it was because C is fast (compared to most other languages)

by AnonymousPlanet

3/25/2025 at 7:24:06 PM

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?

by bee_rider

3/25/2025 at 7:10:33 PM

C is for cookies, and that's good enough for me!

by awesome_dude

3/25/2025 at 4:47:41 PM

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.

by wnissen

3/25/2025 at 4:40:45 PM

It wasn't the symbol for the speed of light, it was the symbol for the Lorenz Constant.

by pyuser583

3/25/2025 at 4:24:01 PM

Cool to see a local school on here.

Off-topic but their botanical gardens and Cactus/Desert garden is a really enjoyable afternoon.

by gnarlynarwhal42

3/25/2025 at 5:13:18 PM

I wonder whether this is the Philip Gibbs that set up viXra.

by JdeBP

3/25/2025 at 7:57:28 PM

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

by rakoo

3/25/2025 at 8:00:41 PM

So c = celeration, the derivative of acceleration.

by deadbabe

3/25/2025 at 8:59:24 PM

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.)

by psd1

3/26/2025 at 7:26:28 PM

> Why is C the symbol for the speed of light?

I'm not sure why that would be an interesting question. Sure it's probably constant or causality or something else, but really most mathematical symbols exist because someone wrote a paper using that symbol and other people adopted it.

by Suppafly

3/25/2025 at 4:09:07 PM

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.

by madcaptenor

3/25/2025 at 4:48:52 PM

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.

by JdeBP

3/26/2025 at 7:23:01 AM

I used y = ax + b all the time in school. This is the first time I encounter mx + c.

by seba_dos1

3/25/2025 at 4:59:41 PM

For a Basque speaker as me, m was ideal because malda in Basque means... slope :)

by anthk

3/25/2025 at 5:22:20 PM

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?

by Keyframe

3/25/2025 at 7:04:56 PM

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).

by kqr

3/25/2025 at 4:32:45 PM

Kind of reminds me of a mountain you're going to climb :)

by fuzzfactor

3/25/2025 at 5:10:23 PM

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".

by madcaptenor

3/25/2025 at 10:21:08 PM

Because C++ wasn't invented yet.

by DarkNova6

3/26/2025 at 3:10:04 PM

It is because light is the only part of the radiation spectrum you c.

by smallmouth

3/25/2025 at 4:53:57 PM

_C_elerity?

by DrNosferatu

3/26/2025 at 6:11:24 AM

Celerity is the new FLASH villian? Haha.

by CHB0403085482

3/26/2025 at 12:15:38 PM

as a programmer I would answer: well c ist the fastes way to construct a complete bogus programm :)

by Surac

3/25/2025 at 4:09:10 PM

Could also stand for Speed of Causality.

by xeonmc

3/25/2025 at 4:33:45 PM

A and B were already used. Somewhere.

by zabzonk

3/25/2025 at 4:35:57 PM

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

by raverbashing

3/25/2025 at 4:45:18 PM

Magnetic Vector Potential?

by hehbot

3/25/2025 at 4:42:20 PM

Wouldn't a have been used for acceleration?

by annodomini2019

3/25/2025 at 9:42:03 PM

Because C++ is object oriented.

by borntoolate

3/26/2025 at 12:57:30 PM

An interesting question : )

by WaNumberLookup

3/25/2025 at 9:00:34 PM

Because god coded it on C

by TZubiri

3/25/2025 at 9:02:28 PM

Thought something similar, it's all because they wrote C instead of the actual c in the title

by Aardwolf

3/25/2025 at 7:03:32 PM

I realise now I had probably only thought of c as a natural companion to v because of Qwerty keyboards...

by kqr

3/26/2025 at 3:44:34 AM

c. Not C

by anacrolix

3/26/2025 at 4:50:53 AM

Neither. It should be https://www.compart.com/en/unicode/U+1D450

But HN is broken and the character disappears upon submission, so we have to approximate it with a symbol that looks similar. C is similar enough.

by 9rx

3/25/2025 at 4:12:36 PM

Because it’s the fastest programming language

by matt3210

3/25/2025 at 4:33:18 PM

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.

by silisili

3/25/2025 at 7:08:26 PM

Travelling at c is UB.

by prerok

3/25/2025 at 4:35:08 PM

>unsafe and unpredictable.

Just llike the Electron.

by anthk

3/25/2025 at 4:57:23 PM

[dead]

by levzzz

3/25/2025 at 4:44:05 PM

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.

by benob

3/25/2025 at 6:25:41 PM

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.

by Someone

3/25/2025 at 5:11:58 PM

Wrong answers only

by mike_ivanov

3/25/2025 at 4:20:44 PM

You need "C" to be the symbol for the speed of light to make the joke from Animanics work.

by Dwedit

3/25/2025 at 4:24:31 PM

Link? I was a big animaniacs fan, but not recalling this one.

by travisgriggs

3/25/2025 at 4:23:46 PM

The one where they spell acme backwards?

by hiccuphippo

3/25/2025 at 4:38:24 PM

e m c α

by racl101

3/25/2025 at 4:10:08 PM

[flagged]

by Wistar

3/25/2025 at 4:35:02 PM

[flagged]

by N7lo4nl34akaoSN

3/25/2025 at 4:35:57 PM

They used lowercase on the actual page, the HN title is what's wrong.

by connicpu

3/25/2025 at 4:40:02 PM

Oh man, maybe HN should not allow commenting until at least the link is clicked.

by orphea

3/25/2025 at 4:16:32 PM

That's celerious

by jihadjihad

3/25/2025 at 4:41:28 PM

[flagged]

by poulpy123

3/25/2025 at 5:56:41 PM

'cause you don't c it coming

by formerly_proven

3/25/2025 at 5:22:04 PM

Because of the language

by antirez

3/25/2025 at 4:21:00 PM

C you later?

by havaloc

3/25/2025 at 7:57:31 PM

I thought c stood for clock speed and that is how we know we are in a simulation.

by themaninthedark

3/25/2025 at 5:29:13 PM

> 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

by throw-qqqqq

3/25/2025 at 4:13:46 PM

C is the Roman numeral representing 100. You can't go faster than 100% of the speed of light. QED.

by bregma

3/25/2025 at 4:35:50 PM

Quod Erat Demonstrandum.

by racl101

3/25/2025 at 4:58:02 PM

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)

by 762236