[HN Gopher] In 1870, Lord Rayleigh used oil and water to calcula...
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In 1870, Lord Rayleigh used oil and water to calculate the size of
molecules
Author : mailyk
Score : 132 points
Date : 2024-09-23 19:08 UTC (3 hours ago)
(HTM) web link (www.atomsonly.news)
(TXT) w3m dump (www.atomsonly.news)
| hn_throwaway_99 wrote:
| I love articles like this. I feel like too often in science
| education (at least my science education) that laws and theories
| are presented as just something that you need to memorize, when
| in my opinion the stories of how things were originally
| discovered and figured out is eminently more fascinating and
| inspiring. Like I remember having to learn all of these
| biochemical pathways, but I left school with nary a clue as to
| how these pathways were uncovered in the first place.
|
| Thanks for submitting! Would welcome suggestions for any other
| publications on how scientific theories were first discovered.
| stevenwoo wrote:
| Did you get your physics education in high school or
| university? I only had to take one physics class in the USA at
| college for my major, quantum electrodynamics for electrical
| engineering but my professor wrote the textbook and I recall he
| went over each experiment starting from the fundamentals of our
| understanding of the basics of the atom, Newton's understanding
| of light at the time, double slit experiment, to Maxwell's
| equations, the Michelson Morley ether experiment, to deriving
| then proving experimentally proving general relativity and
| decomposing GR into Newtonian physics/other laws of
| electromagnetism, I am still in awe at the people just figuring
| this stuff out from first principles.
|
| Anyways, I haven't read this (have it on hold at my library)
| but someone recommended this book on reddit How to Make an
| Apple Pie from Scratch: In Search of the Recipe for Our
| Universe, from the Origins of Atoms to the Big Bang
| https://www.publishersweekly.com/9780385545655
| layer8 wrote:
| > experimentally proving general relativity
|
| Can you elaborate on that? What experiments did the professor
| perform?
| augustusseizure wrote:
| What's the name of that textbook? It sounds really
| interesting.
|
| Isaac Asimov wrote a couple books that follow the narrative
| of science from the beginnings up until the 80s or so, which
| I highly recommend. One is called Atom and is more focused on
| how we got to our "present" understanding of particles.
| There's also one that takes a broader view, it's something
| like History of Science (? not at my bookshelf right now).
|
| There's several books in this genre for math as well. IMO
| it's a much better structure for pedagogy since we can piggy
| back the education on our natural wiring to care about
| narrative and mystery/puzzles.
| nick238 wrote:
| The page is timing out for me, but is it the inverse problem of
| the time when Steve Mould/Matt Parker measured the unknown
| quantity p, but already assuming a size of the molecules?
| Presumably Lord Rayleigh already had a at least a good order-of-
| magnitude approximation of pi...
|
| https://www.youtube.com/watch?v=lmgCgzjlWO4
| thirdhaf wrote:
| By 1870 pi was known to several hundred decimal digits, for
| something like this calculation where you have other large
| sources of error Archimedes approximation from 2 millennia
| earlier would probably be fine. (<1% error)
|
| https://en.m.wikipedia.org/wiki/Chronology_of_computation_of...
| jjk166 wrote:
| Note that pi to 40 digits is sufficient to calculate the
| circumference of the observable universe to subatomic
| precision.
| bqmjjx0kac wrote:
| > Assuming that the oil formed a single layer of molecules -- a
| monolayer -- then the thickness of the oil film is the same thing
| as the length of one oil molecule.
|
| How did he know that the film of oil was one molecule thick?
|
| It feels like a huge assumption to me, but maybe this blog post
| left something out.
| munchler wrote:
| Agreed. The experiment actually gives an upper limit on the
| size of a molecule in one particular dimension. Still a very
| useful result.
| layer8 wrote:
| It isn't necessarily an upper bound. The molecules might
| spread out more distant than their size.
| happytoexplain wrote:
| It feels intuitive that a thin fluid on a low-friction surface
| (like water) would spread out "as much as possible" given
| enough time. There certainly may be confounding factors, but it
| seems like a reasonable thing to pin as an "assumption" in a
| hypothesis. I.e. he didn't have to "know" - assumptions are OK,
| and I don't feel like this one is _huge_.
| stolen_biscuit wrote:
| > How did he know that the film of oil was one molecule thick?
|
| He didn't. It was an assumption
| tech_ken wrote:
| Blog post seems to have elided this point, but it did link the
| original paper which was quite short:
| https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lite...
|
| Rayleigh's experiment was actually trying to solve for the
| minimum thickness of oil required to stop some camphor shavings
| from moving around on the water. He never states it explicitly,
| but I think the assumption is that the minimum thickness
| required to stop the shavings' movement would be such that the
| oil volume 'just' covers the surface, ie. is 1 molecule thick
| everywhere and hence the shavings never touch water. I think
| he's specifically making a slightly more clever point about
| surface tension, but that's a little beyond me.
| jjk166 wrote:
| If there were multiple layers of molecules then the film would
| spread out over a wider area. With repeated experiments it
| would be clear that films are always an integer multiple of
| this thickness and never thinner.
| bangonkeyboard wrote:
| I would have loved to have had a course in school about "The
| Design of Scientific Experiments." One that described the
| processes of landmark historical experiments from antiquity
| onward, and challenged students throughout: "Given this set of
| constraints, how would you design and execute an experiment to
| estimate the size of the Earth? Disprove phlogiston and
| luminiferous aether? Measure the speed of light?"
| dekhn wrote:
| I don't think many people today would be able to propose the
| Michelson Morley experiment and then actually do it. It was
| truly heoric (and Michelson was a genius).
|
| We did this oil/water experiment in freshman physics or
| chemistry lab. It was rushed, everybody just did the minimum,
| the teachers barely explained any of it, and then we moved on.
| _dark_matter_ wrote:
| I agree. The Michelson Morley experiment reminds me of some
| difficult algorithms: simple only in hindsight, and
| implementation is _hard_ to do correctly.
| bsder wrote:
| Experiments are _HARD_. There is a joke among physicists that
| theoreticians are washed up by 35 but experimentalists don 't
| even get started until 45.
|
| To make a physics experiment work you have to be ridiculous
| about recording details and have a _strong_ intuition. You have
| to design the experiment such that you can differentiate
| between "hypothesis wrong" and "equipment doesn't work"
| because _you don 't know the answer_.
|
| (For example: When they turned on LIGO for the first time, they
| almost immediately caught a great event. Huge victory party,
| right? Nope. They promptly ignored it assuming that something
| was wrong with the machine. And it was only after significant
| post analysis and correlation that they decided that it was a
| real event.)
| CountHackulus wrote:
| We recreated this experiment in one of my university physics
| classes. It was a lot of work, and our results weren't nearly as
| good, but it was instructive and interesting. The equipment
| requirements were completely reasonable for an undergrad physics
| lab. I highly recommend giving it a try if you can.
| metadat wrote:
| How is the measurement for the area the oil has spread over
| performed? Visually or some other way?
| opencl wrote:
| The actual manuscript from Rayleigh [1] explains it better: the
| area is the entire area of the vessel the oil was placed in,
| and the thing actually being measured was how much oil was
| required for it cover the whole area.
|
| [1]
| https://www.damtp.cam.ac.uk/user/gold/pdfs/teaching/old_lite...
| dekhn wrote:
| Some powder is added to the water, which covers the surface of
| the water but not the oil patch (which is circular). Then the
| oil patch diameter is measured.
| misnome wrote:
| This was how we did this when we replicated this experiment
| in high school. I guess from the other responses here that
| this wasn't common?
| dekhn wrote:
| The original way was to cover the surface of a round bowl
| with oil. It certainly makes a lot more sense to me than
| trying to measure a floating disk of oil.
| ummonk wrote:
| He used a fixed area (a 33 inch diameter bowl) and measured the
| weight of oil required to just about calm the entire water.
| That turned out to be 0.81 milligrams.
| alnwlsn wrote:
| These are the best kind of posts, where there's something I've
| never even heard of before. I never knew 'oiling the seas' was a
| thing, or that it (apparently?) works.
| dr_dshiv wrote:
| I won't trust this until I myself can calm an acre of water with
| a teaspoon of oil. (Or at least see a YouTube video of someone
| doing it)
| isp wrote:
| YouTube video:
| https://www.youtube.com/watch?v=RST_ylwVrUw&t=1m27s
| Terr_ wrote:
| That reminds me of the Millikan & Fletcher oil-drop experiment
| [0], which measured the charge of the electron.
|
| In short, microscopic atomized oil droplets had their fall-time
| through air measured to figure out their volume, and then a known
| electric field was used to levitate them. The calculated charge-
| per-molecule clustered around multiples of a smaller value, which
| would be the charge of an individual electron.
|
| [0] https://en.wikipedia.org/wiki/Oil_drop_experiment
| n_plus_1_acc wrote:
| How can you make sure you don't end up with 2e as a result? (Or
| any other multiple)
| Terr_ wrote:
| In 1909 the results results were couched in some "elementary
| electric charge" quantity, since the now-familiar subatomic
| particle model (and the "electron") was still gaining
| acceptance.
|
| I expect that the greater the number of trials, it becomes
| easier it is to detect a distinction between closer-
| multiples, and if at some point more trials stops changing
| the answer then you've likely converged on e, unless there's
| some new principle like "X-ray exposure only affects charge
| in in multiples of e greater than one."
| dekhn wrote:
| He did- he selected the lowest value, ignoring all the
| multiples.
| ceejayoz wrote:
| Not ignoring the multiples; the multiples verify the
| result.
|
| If you calculate the charge of one at 1e and you measure
| 2.5e, something went wrong. All values must be a multiple
| of the lowest.
| nkmnz wrote:
| You do. Thae size of the steps between the results is the
| "quantum" of a single transferable charge.
| SyzygyRhythm wrote:
| For that to happen, you would have to be very unlucky: all of
| your measurements would have to be 2e, 4e, 6e, etc. If a 3e
| or 5e sneaked in there, you'd realize that the charge was e,
| not 2e. With enough measurements, you can be confident that
| you've hit all the expected multiples of the quantum.
| xenocratus wrote:
| Luckily it wasn't my grade that got this experiment as the
| practical exam in one of the National Physics Olympiads I went
| to... :) poor souls, most got answers orders of magnitude away.
| IncreasePosts wrote:
| A few days ago, there was a HN post about surface acoustic wave
| filters, and a commenter mentions how inspired the inventor of it
| must have been(https://news.ycombinator.com/item?id=41604937).
|
| That was this same fella!
| arvindh-manian wrote:
| Related: Agnes Pockels' experiments [0]
|
| [0]: https://en.wikipedia.org/wiki/Agnes_Pockels
| RachelF wrote:
| Semi off topic:
|
| Interesting to look at picture of the text of the 1890 paper.
| That typesetting is almost the same as modern scientific papers.
|
| Maybe Rayleigh had an early copy of LaTeX? ;-)
| wwarner wrote:
| In 1676 Roemer estimated the speed of light by timing the orbit
| of Jupiter's moon Io, noting that as the Earth approached
| Jupiter, Io emerged from behind Jupiter a little earlier every
| day, and as the Earth traveled away from Jupiter it appeared a
| little later every day, with the time of day varying by 22
| minutes over a year. Knowing the difference between the two
| distances, he reckoned that light travels that distance in 22
| minutes, or 227 thousand km/s. The actual speed is about 300
| thousand km/s. Not bad!
| thimkerbell wrote:
| "Rayleigh divided the volume of the oil by the area it covered,
| thus estimating the thickness of the oil film. Assuming that the
| oil formed a single layer of molecules -- a monolayer -- then the
| thickness of the oil film is the same thing as the length of one
| oil molecule.
|
| This is how Lord Rayleigh became the first person to figure out a
| single molecule's dimensions, many years before anyone could see
| such molecules."
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(page generated 2024-09-23 23:00 UTC)