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in reverse order, clarifications:

- had a job working as a research assistant doing something that would count as mathematics, for a guy employed in a philosophy department, a couple summers during college. have been employed as a TA or grader for a couple of CS courses.

- more 'vague' or 'polysemous' than 'ambiguous', it's not like the 'bank' (side of river) vs. 'bank' (financial institution), where there are distinct words that happen to be pronounced the same. it's just that the term is kinda fuzzy.

-for 'bigger than' it doesn't matter which of volume, radius, and diameter you use, since if it's bigger in one it's bigger in the other two, so there's no real way to distinguish.

-when people talk about 'size', either they're using it for comparison, in which case it's the same as 'bigger than' and it doesn't matter (unless they're talking about ratios), or they're probably going to name a specific size and otherwise give context in which case the way they're talking (and possibly just the units) will narrow it down. when the distinction matters and i can't tell from context what they mean (e.g. they say one planet is 'twice the size of' another), i probably assume they're talking 'volume', but without much confidence, and i ask for clarification if it's at all important.

-what does 'physical size' mean here, anyway? all the size concepts mentioned are in some sense 'physical', so defining 'size' as 'physical size' seems kinda circular and not especially elucidatory. i assume this is some kind of technical term in astronomy? does it mean something 2D like 'great circle area'? (this is a guess based on the fact that you've got 1D and 3D but not 2D size concepts in there.)

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"Physical size" is a vague term which means any/all of radius/diameter/volume/surface area/cross-sectional area, since (for a sphere) these are all dependent upon each other. I included it b/c the first person I asked this question of in person responded "physical size" before I gave zim options of radius or mass.

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will you eventually tell us more about the context that provoked the question?

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There was a worksheet someone else wrote that talked about how the surface conditions of terrestrial planets were affected by two factors: distance from Sun, and size of planet. The author said somewhere in there that it was really the mass of the planet which affected the atmosphere, but that size was a proxy for mass, implying (but not explicitly stating) that he meant physical size when he said "size". It just seemed an unnecessary complication and/or poor wording that could throw students for a loop.

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the vagueness here is something that's bothered me for a long time. like, i think most people would say that a 2 cup measure is twice the size of a 1 cup measure, which suggests volume, but on the other hand somebody would describe a 1/12 (linear dimension) scale model as being 'one twelfth the size' of the thing it's a model of, rather than 'one 1728th the size', which is what you'd get if you were talking volume. so it seems like we're just not consistent about this.

i'm trying to think of a case where it's area of a single surface, rather than volume or linear dimension, that matters, but the cases that come to mind are all things like sizes of different film formats (e.g. 35mm motion picture film frame is about half the size of 35mm still photography film frame), where it's intutively area that matters, but where volume scales with area (film thickness doesn't change with size of film frame), so there's no clear way decisively tell whether area or volume is the real thing that's being compared.

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Cross-sectional area and angular size are often used in astronomy. For example, the angular size of the Moon and the Sun (as seen from Earth) are essentially the same (which is why we can have total solar eclipses). This may also be why when you ask a "person on the street" whether the Moon or the Sun is bigger, lots of them don't know or give the wrong answer. I'm sure you already know, but in case you forgot or someone else is reading this, for angular size we mean how big something looks in two (angular) dimensions, it matters where you're observing from, and the SI units are steradians; for cross-sectional area we're talking more about something impacting upon something else (stellar wind impacting on dust grains, radiation impacting on a planet), we're talking about two physical size measurements (as if you cut an object down the middle), and SI units are m^2. Oh yeah, and surface area (also in units of m^2) is important when an object is radiating energy to space, like a star or planet radiating light.

Yeah, the ambiguity/vagueness (what's the difference anyway?) of terms relating to size bothers me. In my teaching, I try to be clear about size-related words, so students know how many dimensions I'm referring to. Even worse, there *are* circumstances when an astronomer actually means mass when ze says something related to "size" - i.e., "a blue giant star is bigger than a red dwarf". While it's true that a blue giant star has a larger radius, the mass is what's more important when comparing stars so in saying a blue giant is bigger, what we really care about is that it's got more mass.

I brought this up b/c the thesis I'm reading had a worksheet having to do with how planet size affects the surface appearance.

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as a native English speaker with no particular astronomical training, if somebody used size comparison terms to talk about mass, i'd assume they were, at best, being really sloppy.

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Well, words like "bigger" and "larger" in common English seem to imply physical size, so if you want to explicitly use mass terms you'd have to say "more massive", which is unwieldy. There's also the convenient fact that in most (but not all*) cases, objects with a greater physical size also have a greater mass, so size words can serve as a proxy for mass words.

*There's some really complex diagrams out there in astronomy where lines for same mass and same radius can be plotted alongside other quantities. The HR-diagram for example plots luminosity (inherent brightness) as a function of temperature (with each data point being an individual star), and you can then put on top of that lines for stars of the same radius, and lines for stars of the same mass. If you're google-imaging this, start with "hr diagram" and add either/both of "radius line" or "evolution line". For stars on the main sequence only, radius is a proxy for mass. Mass is the inherently more important quantity of the two though.

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well, in most everyday English, we just say 'heavier', but i suppose that wouldn't work so well in an astronomy setting.

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Dur, yeah, "heavier" never even occurred to me. (Precisely because it's not a good choice b/c in physics that really refers to weight, not mass.)

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Sometimes mass is the only well-defined, size related quantity of an object. If you say a top quark is bigger than an electron, the only unambiguous quantity you could be talking about is mass. For that matter, if you say a proton is bigger than an electron, yeah, it's bigger in radius, but they're both so small you don't care--the only thing you could mean is mass.

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interesting. i wonder what the criteria for using size talk vs. weight talk are here.

like, it seems weird to say an atom of U238 is 'bigger' than an atom of U235 (unless it actually has a larger radius? i'm assuming it doesn't, but i honestly wouldn't know.), but it's perfectly natural to say it's 'heavier'.

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"Bigger" is a shorter, easier-to-say, more common word than "heavier" and so is more likely to be used in lab-speak, particularly when context makes it obvious.

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ambiguity/vagueness (what's the difference anyway?)

(I checked Wikipedia for example sentences because I couldn't be bothered to make up my own.)

An example of an ambiguous sentence that isn't vague: "I saw the man with the binoculars."

And a vague sentence can be completely unambiguous if there is context. For example, my mom might say something like, "Go to the store. You know, the good one." Which is incredibly, frustratingly vague and ambiguous if I am unfamiliar with the stores in the area, but at the same time, without losing its vagueness, would be completely unambiguous to my father who would be familiar with my mother's store preferences.

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the terms 'ambiguity' and 'vagueness' are not used with great consistency (appropriate, eh?), but here's what i mean, which accords roughly with practice in my field:

the term 'bald' is vague. Patrick Stewart had a little hair on his head in 1994, but still pretty unquestionably counted as bald. a person with a small bald spot or hair line that's just starting to recede generally doesn't. there are people in between who we might call 'bald' in some cases, but decline to call 'bald' in others, or about whom we'd generally be unsure. this is not because 'bald' has multiple different meanings (although perhaps it does in other contexts) - it's because in these cases there's one meaning at work that just happens to be sort of fuzzy around the edges (as most meanings are, to one degree or another).

the sound (and spelling) 'bank' is ambiguous between the 'financial institution' meaning and the 'edge of river' meaning. there are just two different meanings that get pronounced the same way. similarly, the sentence 'i saw the professor with the telescope' is ambiguous between a meaning where the professor had the telescope with them when i saw them, and the meaning where the telescope was the instrument with which is saw the professor.

we sometimes distinguish things more finely the 'bank' case is a case of homonymy - two different words that happen to have the same sound (homophony) and spelling (homography). the example about the professor and the telescope is an example of structural ambiguity - all the words involved in both have basically the same meanings individually (with the possible exception of 'with'), but there are, in some sense, two ways of 'putting them together' that give different meanings, depending (roughly) on whether 'with the telescope' is understood as modifying 'professor' or 'see'.

getting back to homonymy - sometimes we distinguish homonymy from polysemy (multiplicity of senses). we want to say that 'bank' (financial institution) and 'bank' (side of the river) are totally different words. but what about the sense of 'bank' in which a particular building with tellers and a vault and operating hours is a 'bank', and the sense of 'bank' in which a particular corporation that engages in certain kinds of financial business is a 'bank', even though it may control many such buildings (or, if it's in the process of starting up or shutting down, may not have any). these two uses of the word 'bank' are in some sense different inter-related meanings of the same word, so this is an example of polysemy.

as should not be surprising, the dividing line between homonymy and polysemy is sort of vague.

'ambiguity' is usually understood as encompassing structural ambiguity and homonymy (and homophony for spoken cases and homography for written cases), and excluding vagueness. where polysemy fits in is not always immediately clear.

that was probably more answer than you needed. sorry.

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Cross sectional area (σ) is often used as the relevant size in physics, because that's the size that determines collision/interaction rates. Sometimes the scattering length (a) is used instead, though. And then there's the complication that usually σ = 4π a², but occasionally it's σ = 8π a² if all the objects interacting are indistinguishable.

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Note that this is particularly useful where an object doesn't have a well-defined radius/volume in the first place (or, at least, what is meant by radius/volume is context dependent), so scattering length can be used as a substitute for radius.

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I did an REU in astronomy the summer after my freshman year of college. It mostly involved modeling photoionization of neon (for the purpose of spectroscopy of stars), so on a couple different levels it only barely/technically counts as having worked in astronomy.

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hrafn.livejournal.com

I realized looking at the lists of options that I do not think about this, generally, in that kind of specificity. I mean, if someone talks about diameter that also implies radius, and vice versa, right?

What does "physical size" even mean? Given how volume doesn't imply mass and vice versa. D: That /was/ my pick, for what I think, but now I realize I maybe don't know what -I- mean by that. >.<

I confess I rarely think about -mass- when people talk about size/bigness of planets. And so now I'm not sure how to mark the survey. Especially in the "one is bigger" category, since a planet can be bigger in volume but smaller in mass. *cries*

Ambiguous words are ambiguous.

I also haven't finished eating breakfast yet.

ETA: Having now read the other comments, I feel better about being confused/frustrated. I've definitely noticed that if people are talking -mass- (or weight) for an object, generally they make it clear, since in my experience, the only people talking about mass are talking within a scientific context, and so they are specific about that.

Edited Date: 2013-08-17 04:40 pm (UTC)

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