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In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

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Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#101

Post by CamperBob2 »

bryanlarsen wrote: post #62
ummonk wrote: post #37

Earlier quoted context omitted.

He used olive oil though, not a soap oil.

Olive oil is a soap oil.

If that's true, then the ChatGPT answer isn't wrong, at least for the specific case of olive oil.

I love how a few self-appointed guardians of hackerhood are able to declare certain resources as haram, even when they propose a plausible answer to a question that went unanswered more than once in the thread. Instead of embarking on a discussion that leaves us all a bit better informed, the literal messenger is attacked. Does that gratify intellectual curiosity? Really?

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#102

Post by andreareina »

bsder wrote: post #49
bangonkeyboard wrote: post #5

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?"

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 .…

The lengths they're going to fix the "loopholes" in the Bell Inequality tests are amazing.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#103

Post by physicsguy »

NegativeLatency wrote: post #57
hn_throwaway_99 wrote: post #2

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 a…

Discovering the quantization of the charge of electrons sounds like something you'd be interested in: https://en.wikipedia.org/wiki/Oil_drop_experiment We did it with several hundred volts (DC, scary) in college and it was pretty fun collecting the data and watching the numbers fall out in excel doing the analysis.

I remember doing this one and the equipment leaking oil all over me! Not long after that I decided to go more Theoretical…

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#104

Post by imp0cat »

ziofill wrote: post #69

>"not more than a Tea Spoonful," according to his diary — Franklin poured it onto the agitated water. The oil spread rapidly across the surface, covering "perhaps half an Acre" of the pond and rendering its waters "as smooth as a Looking Glass." What??

Here's a video from another post: https://news.ycombinator.com/item?id=41630637

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#105

Post by mda »

bqmjjx0kac wrote: post #4

> 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.

I have also immediately thought the same question. This is probably the most crucial part of the whole estimation and indeed left out in the article.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#106

Post by lIl-IIIl »

owyn wrote: post #84
ghastmaster wrote: post #78

Earlier quoted context omitted.

You're referring to The History of Physics. An excellent read for a budding mind. Asimov was incredibly talented.

I was looking at my parent's bookshelf and saw a book on Shakespeare and I recognized the author's name: Asimov! https://en.wikipedia.org/wiki/Asimov's_Guide_to_Shakespeare It's like 800 pages, I haven't read it but I think I'll keep that one. Seems like it might be hard to find another physical copy. He was definitely prolific on a number of topics.

Not surprising!

"Asimov was so prolific and diverse in his writing that his books span all major categories of the Dewey Decimal Classification except for category 100, philosophy and psychology" - from his Wikipedia page.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#107

Post by lIl-IIIl »

augustusseizure wrote: post #53
stevenwoo wrote: post #31

Earlier quoted context omitted.

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 Ma…

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).…

His book "Understanding Physics" is amazing. Similar in spirit to Petzold's "Code" that is often praised on HN.

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#108

Post by bloak »

bqmjjx0kac wrote: post #4

> 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.

If you try the experiment lots of times with drops of different sizes you find the oil layer always has roughly the same thickness. That's an interesting observation that calls for some kind of explanation, and the hypothesisis that the thickness of the oil layer is the length of one molecule is perhaps the most obvious and plausible explanation. Then one would look for confirmation, of course. (What was the next thing to confirm this, historically?)

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#109

Post by lIl-IIIl »

nielsbot wrote: post #99
rkagerer wrote: post #89

This is fascinating, but wasn't it still a bit of a conjecture to assume the oil would spread to a minimum thickness of one molecule? Did he have any doubts, like that surface tension might keep it thicker? Or other clues hinting it was indeed a monolayer?

My question exactly.. I hope someone can chime in :) EDIT: Thinking a bit more... I suppose it's a reasonable assumption that the molecules (mostly) wouldn't stack on top of each other. They all want to get lower and perhaps the resistance to the oil spreading out is much lower proportionately that the gravitational force encouraging the oil to flatten

But if I spill some oil on my plate it doesn't look a molecule thin to me. Why is it different with water?

Re: In 1870, Lord Rayleigh used oil and water to calculate the size of molecules

#110

Post by db48x »

SyzygyRhythm wrote: post #39
n_plus_1_acc wrote: post #28

Earlier quoted context omitted.

How can you make sure you don't end up with 2e as a result? (Or any other multiple)

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.

Not quite so. They did end up measuring a multiple of the fundamental electric charge. The experiment really measured 3e, 6e, 9e, etc. It turns out that the electron and proton have an electric charge 3× bigger than that of a quark. Since the experiment didn’t generate any free quarks, nobody noticed for years. Even today the mistake persists and school children are taught, unironically, that down quarks have ±⅓ of the fundamental indivisible unit of electric charge and that up quarks have ±⅔e.
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