# Why is light used as the unique descriptor for the universal speed limit? **URL:** \ **Category:** Factual Questions\ **Created:** [March 11, 2012, 5:39am UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445 "2012-03-11T05:39:09Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [March 11, 2012, 8:54pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/21 "2012-03-11T20:54:39Z")
> [@njtt](#): > > Whether or not that is true (and frankly, I remain skeptical that a physicist studying, say, radioactive decay, would habitually refer to a nucleus as emitting light rather than gamma radiation) Context is important. If a physicist is studying processes involving only gamma radiation specifically, he might refer to it as “gamma radiation.” Or he might just say “emits a photon”, or “light of wavelength X”, or any number of ways. It may depend on his individual style, the intended audience, the putting on or off of an informal or formal affect, and just, well, how it sounds in the context of the rest of the paper. Either way, as a physicist I can assure you that “light” is understood to mean “electromagnetic radiation.” That is why there is such terms as “visible light,” and “infrared light.” The word “light” requires a modifier if one wants specifically to to refer to the visible part of the electromagnetic spectrum. The reasons some other forms of light are referred to as “waves” or “rays” is because, in addition to being a type of light, they have other properties that are good for lending names. After all, it’s nice to have a name for something. Microwaves and radiowaves are very large “waves” of light, for example. > [@njtt](#): > > the question remains as to why “light” rather that one of the several other words referring to particular parts of the electromagnetic spectrum is preferentially used, in popular accounts of relativity and the “speed limit of the universe” (I expect it is true, as **Candyman74** says, that physicists these days mostly say “c”). The answer lies in the history of the matter, not in the physical facts _per se_. The reason is that “light” was the first part of the electromagnetic spectrum to come into popular usage. Next came the discovery of infrared light, and then ultraviolet light. They were thought of as “invisible” types of light that extended just beyond the rainbow you might see when you send light through a prism. So at the time both were referred to as types of “light” because of the ways in which they were discovered: as extensions to the rainbow of colors of visible light. Infrared (just before red) and ultra violet (just past violet). So already at the time of their discovery, different parts of the electromagnetic spectrum were understood to be part of the same underlying phenomena, and were referred to collectively as “light.” You say this is “history of the matter, not in the physical facts” but in this case it was precisely the physical facts that determined the history. Next came the discovery of Maxwell’s equations, which put all forms of electromagnetic radiation under one umbrella. It predicted things like radio and microwaves, which were looked for, and found. Historically these were, and still are, often referred to as “waves” because that is exactly what they are, macroscopic waves of light. For a similar reason, parts of the other end of the spectrum are often referred to as “rays.” So each type of radiation has a history associated with it and a unique name that it was given shortly after its discovery, but each were understood to be an extension of the spectrum initially referred to as “light.” Since “visible light” was not “discovered”, no new name was given to it. It had always been known as “light.” And its colloquial use remained, but in scientific circles the solution, at the time, was to start referring, in the case of ambiguity, to the visible spectrum as “visible light,” and extended electromagnetic spectrum simply as extended frequencies of “light.” > [@njtt](#): > > I know that some physicists find it hard to accept that not all questions, not even all questions concerning the language and practice of physics, are to be answered entirely in terms of physical theory, but it is, in fact, the case. Historically, the popular terms come from the terms physicists use. I am explaining to you that physicists use the term “light” out of convenience. I’ve described some more of the history above, in case the OP is interested, although I disagree with you that that is what the OP is really asking. He references information and gravity, which are not any kind of “light” at all, and yet the speed c is obeyed by both. I think the OP is asking whether light is physically special or not. The answer is that it is not special. Light just happens to be massless, and in great quantity. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 12, 2012, 1:52am UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/22 "2012-03-12T01:52:55Z")
> [@Cheshire\_Human](#): > > The AC power you get out of the electrical outlet is modeled in physics using imaginary numbers. Nothing impossible or incorrect about my toaster’s functioning. Yes, but when you talk about observable quantities — the “physical” voltage or current in your toaster — that’s always a real number. Same goes for quantum mechanics: complex numbers are used “under the hood”, but the quantities you can actually observe via experiment are all real. A particle’s mass is also observable via experiment, and so if your model predicts an imaginary number for this, I will have to raise a skeptical eyebrow towards your model. ---
**Author:** ![Indistinguishable](https://avatars.discourse-cdn.com/v4/letter/i/90ced4/32.png) [@Indistinguishable](https://boards.straightdope.com/u/Indistinguishable)\ **Post date:** [March 12, 2012, 2:05am UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/23 "2012-03-12T02:05:59Z")
> [@MikeS](#): > > Yes, but when you talk about observable quantities — the “physical” voltage or current in your toaster — that’s always a real number. This is just semantics, as they say. I could just as well say observable quantities are always positive integers, or always binary digits, or such things, and interpret every other kind of measurement as just some aggregate formed from these. But certainly, there are measurements in the physical world which are readily and naturally interpreted as complex numbers: the ratio between two vectors in a given plane, say. [Yes, one can interpret this as an aggregate of two constituent real quantities: a size ratio and an angle, say, or separate parallel and perpendicular size ratios. But in the same sense, one might always interpret a real quantity as actually a semipositive real quantity and a sign bit, and say there’s not really any such thing as a negative measurement, per se. And so on. It’s just a matter of perspective.] It depends on what kind of measurement one is doing; that’s all. If the kinds of experiments described as “measuring mass” are such that their results are always interpreted as real numbers, then, yes, mass measurements can only be real numbers. But this has to do with mass specifically, then; not the general natural of measurements and complex numbers. ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [March 12, 2012, 6:29am UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/24 "2012-03-12T06:29:00Z")
> [@MikeS](#): > > Yes, but when you talk about observable quantities — the “physical” voltage or current in your toaster — that’s always a real number. Same goes for quantum mechanics: complex numbers are used “under the hood”, but the quantities you can actually observe via experiment are all real. A particle’s mass is also observable via experiment, and so if your model predicts an imaginary number for this, I will have to raise a skeptical eyebrow towards your model. Skeptical is one thing. I was objecting to the statement that imaginary numbers indicated “impossible” and “incorrect model”. Those are quite a bit beyond “skeptical”. ---
**Author:** ![petew83](https://avatars.discourse-cdn.com/v4/letter/p/7bcc69/32.png) [@petew83](https://boards.straightdope.com/u/petew83)\ **Post date:** [March 12, 2012, 8:22pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/25 "2012-03-12T20:22:56Z")
The responses I received to this question from everyone were exceedingly insightful, thank you. Special thanks goes out to njtt and iamnotbatman, who ironically seemed to be the most at-odds pair in the discussion pool (lending credence to the dialectic process). To provide clarification, I was interested in _both_ the historical reasons _and_ the physical reasons for why light is the preferred descriptor for c. I don’t believe that I made that abundantly clear in my question, so it is natural that one would read his or her field of expertise into it. Nevertheless, after some coopetition from njtt on the historical side of things, iamnotbatman ended up providing superb explanations for both lines of inquiry. Well done. Most insightful for me were these two bits: On the historical end: > [@njtt](#): > > It is a contingent historical fact that, when Einstein came up with the idea of c as the universal speed limit, he was primarily thinking of actual visible light (although he was aware that it is the same type of phenomenon as other electromagnetic radiation), and it was actual visible light whose speed had recently been measured. If Michelson and Morely had measured the speed of gamma radiation and Einstein had imagined surfing on a gamma wave, we would probably tend to refer to c as the speed of gamma. On the physical end: > [@Jiamnotbatman](#): > > Light is the only massless particle we can easily measure. That is all. All massless particles move at speed c. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [March 12, 2012, 8:26pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/26 "2012-03-12T20:26:20Z")
> [@Cheshire\_Human](#): > > The AC power you get out of the electrical outlet is modeled in physics using imaginary numbers. Nothing impossible or incorrect about my toaster’s functioning. If it’s the power that makes your toaster function, it’s not modeled with imaginary numbers. That is, of course, unless your toaster has an induction motor in it. ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [March 12, 2012, 8:32pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/27 "2012-03-12T20:32:22Z")
> [@iamnotbatman](#): > > Light is the only massless particle we can easily measure. That is all. All massless particles move at speed c. Does a photon accelerate? Is it going c as soon as it is produced or does it take take some time to get up to c? Is speed related to mass? If a particle has the tiniest bit of mass, will it be able to be at rest? Or is it like a photon, always moving at some speed, although less than c? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 12, 2012, 8:52pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/28 "2012-03-12T20:52:16Z")
> [@](#): > > In any case, would it not be just as true to say that light is gamma, just gamma that our eyes happen to be able to detect? In fact, the general symbol for a photon, of any energy, is the letter gamma. **filmore** , a massless particle can’t move through space at any speed other than c. For a particle with mass, it will depend on how much energy it has: If its energy is much, much greater than its mass, its speed through space will be very close to c, and if its energy is much, much less than its mass, its speed through space will be very close to 0. Another way to look at this, though, is to look at each particle’s entire motion through space_time_, not just the portion of its motion that’s through space. In that case, you find that _everything_ is moving through spacetime at c, and it’s just a matter of how much of the motion is through time, and how much is through space. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [March 12, 2012, 9:05pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/29 "2012-03-12T21:05:44Z")
> [@Napier](#): > > If it’s the power that makes your toaster function, it’s not modeled with imaginary numbers. That is, of course, unless your toaster has an induction motor in it. The coils in your toaster do have a sizable self-inductance, actually. That’s why you sometimes get a spark at when you unplug something like a toaster, an iron or a space heater when it’s running — the self-inductance causes a large back EMF, which is enough to cause a spark in the air. ---
**Author:** ![Astroboy14](https://avatars.discourse-cdn.com/v4/letter/a/df705f/32.png) [@Astroboy14](https://boards.straightdope.com/u/Astroboy14)\ **Post date:** [March 12, 2012, 9:24pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/30 "2012-03-12T21:24:56Z")
Any faster would be dangerous. [Cite.](http://www.coldbacon.com/pics/kliban/bklight.gif) ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [March 12, 2012, 10:06pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/31 "2012-03-12T22:06:19Z")
> [@Napier](#): > > If it’s the power that makes your toaster function, it’s not modeled with imaginary numbers. That is, of course, unless your toaster has an induction motor in it. I’m aware of that. Toaster was the first AC powered appliance I thought of off the top of my head. I realized it was a bad example too late to edit, but most people aren’t familiar with the radio coils and antennas I’m most familiar with the math from. I didn’t think the matter was worth a clarification post. ETA: And **MikeS** points out I was close enough, anyway. ---
**Author:** ![petew83](https://avatars.discourse-cdn.com/v4/letter/p/7bcc69/32.png) [@petew83](https://boards.straightdope.com/u/petew83)\ **Post date:** [March 12, 2012, 10:17pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/32 "2012-03-12T22:17:45Z")
> [@Chronos](#): > > **filmore** , a massless particle can’t move through space at any speed other than c. For a particle with mass, it will depend on how much energy it has: If its energy is much, much greater than its mass, its speed through space will be very close to c, and if its energy is much, much less than its mass, its speed through space will be very close to 0. > > Another way to look at this, though, is to look at each particle’s entire motion through space_time_, not just the portion of its motion that’s through space. In that case, you find that _everything_ is moving through spacetime at c, and it’s just a matter of how much of the motion is through time, and how much is through space. Energy has Syndrome X (Brooke Greenberg), while matter has progeria. The former only moves, while the latter only ages. Normally citing a difference implies a higher order similarity (eg “tall” vs. “short” implies “height”). What do space travel and time travel have in common? Well, since these are basically the two most fundamental things in the universe, we can only say that they are united by both moving through “spacetime”, the most abstract physical concept we have. “Space travel is \_\_\_\_er than time travel.” ← I’m open to suggestions for filing in that blank. ---
**Author:** ![Sailboat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sailboat/32/461_2.png) [@Sailboat](https://boards.straightdope.com/u/Sailboat)\ **Post date:** [March 13, 2012, 12:40am UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/33 "2012-03-13T00:40:19Z")
I’ve read something recently that implies information can propagate faster than light, but I don’t remember the details – something about a light wave propagating. Then there’s [this](http://www.nature.com/news/2011/110922/full/news.2011.554.html). ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [March 13, 2012, 3:57am UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/34 "2012-03-13T03:57:03Z")
> [@filmore](#): > > Does a photon accelerate? Is it going c as soon as it is produced or does it take take some time to get up to c? . . . It is “born” going c… It doesn’t accelerate per se… But, if I understand it correctly, the process of, say, an electron dropping an energy level and producing a photon takes time. The process takes an amount of time greater than zero. It’s a doggone small amount of time, but it can be calculated. (Can it be observed/measured?) ---
**Author:** ![GoodOmens](https://avatars.discourse-cdn.com/v4/letter/g/d26b3c/32.png) [@GoodOmens](https://boards.straightdope.com/u/GoodOmens)\ **Post date:** [March 13, 2012, 2:18pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/35 "2012-03-13T14:18:44Z")
You could say the “speed of electromagnetic radiation” but “light” is shorter. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [March 13, 2012, 2:54pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/36 "2012-03-13T14:54:58Z")
> [@iamnotbatman](#): > > I wouldn’t take the idea of tachyons very seriously. They are logically inconsistent if they are able to interact with non-tachyons in any way; there is no way for a particle to “become tachyonic” (ie cross the light barrier). I realize that this is a hijack, but can you explain more about the inconsistency of interaction? I’m not talking about ordinary particles ‘becoming tachyonic’, but more about the possibility of tachyons interacting with photons and vice versa. This is a purely hypothetical notion that I’ve played around with as a basis for ansibles or FTL radio in science fiction, and I’ve never been able to find out much about how consistent it is with ‘real physics.’ ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [March 13, 2012, 3:55pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/37 "2012-03-13T15:55:00Z")
> [@filmore](#): > > Does a photon accelerate? Is it going c as soon as it is produced or does it take take some time to get up to c? From the photon’s perspective it doesn’t even MOVE. Its emission and absorption are a single event and its origin and destination are contiguous in spacetime. ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [March 14, 2012, 2:44pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/38 "2012-03-14T14:44:22Z")
> [@chrisk](#): > > I realize that this is a hijack, but can you explain more about the inconsistency of interaction? I’m not talking about ordinary particles ‘becoming tachyonic’, but more about the possibility of tachyons interacting with photons and vice versa. > > This is a purely hypothetical notion that I’ve played around with as a basis for ansibles or FTL radio in science fiction, and I’ve never been able to find out much about how consistent it is with ‘real physics.’ If a tachyon can interact with ordinary matter, then we can create (ie emit) and absorb tachyons. But if we can emit and absorb tachyons, then we can send information backwards in time, leading to causal paradoxes. Yes, there are some very contrived loopholes involving space-time [geometries](http://en.wikipedia.org/wiki/G%C3%B6del_metric) that allow [closed time-like curves](http://en.wikipedia.org/wiki/Closed_timelike_curve), but these unique examples do not correspond to nature. But in general, the causal [paradoxes](http://en.wikipedia.org/wiki/Tachyonic_antitelephone) are severe (basically the same as the [grandfather paradox](http://en.wikipedia.org/wiki/Grandfather_paradox)) and cannot be avoided without without wildly unmotivated new laws of physics that would magically somehow stop the paradoxes from happening (ie, whenever you try to send a signal back in time to destroy the machine that sent the signal (threatening a logical paradox), physics will always invent some way of annoyingly preventing the past from being changed. This is considered preposterous by most physicists, but there is just enough wiggle room to keep some sci-fi writers in business 😉 ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [March 14, 2012, 3:55pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/39 "2012-03-14T15:55:40Z")
> [@njtt](#): > > Information is only limited to light speed because it has to be carried by something physical, so it cannot move any faster than the fastest possible physical thing (i.e., light or other electromagnetic waves). The limitation on the speed of information is not an independent issue. I think this is misleading though. There are certainly effects where “something” can propagate faster than light; e.g. entangled particles’ state. Obviously you don’t consider such effects to be something _physical_, but physical is one of those terms that’s weakly defined and seems to creep over time (I think Physicalism is a virtually meaningless philosophy). > [@Dano83860](#): > > From a practical stand point it’s a very easy way to talk about large distances. How would you describe 85 light years in miles? In SI units 85 light years is roughly 850 petametres (850 Pm). We’d get used to it. But it is handy that our current unit of distance also tells us how long it would optimally take to get there / send comms. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [March 15, 2012, 12:41pm UTC](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445/40 "2012-03-15T12:41:20Z")
> [@iamnotbatman](#): > > If a tachyon can interact with ordinary matter, then we can create (ie emit) and absorb tachyons. But if we can emit and absorb tachyons, then we can send information backwards in time, leading to causal paradoxes. Thanks, okay, that helps. I’ve heard of the ‘faster than light is equivalent to back in time’ bit, (though I’d be lying if I said I really understood it,) and I’m familiar with the logistical difficulties of constructing a consistent universe in which time travel is possible. And yet, I think that’s all reasonable grounds for speculation. I just wanted to make sure that I wasn’t speculating something that was a much more heinous crime against the laws of physics, on the level of violating conservation of spin, or dodging laser beams. 😉 [Previous page](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445.md?page=1)