Crude Monte Carlo Simulation of Light-Bulb Physics

Last week, I did a post for Forbes on the surprisingly complicated physics of a light bulb. Incandescent light bulbs produce a spectrum that’s basically blackbody radiation, but if you think about it, that’s kind of amazing given that the atoms making up the filament have quantized states, and can absorb and emit only discrete frequencies of light. The transition from the line spectra characteristic of particular atoms to the broad and smooth spectrum of black-body radiation is kind of amazing.

The way you get from the one to the other is through repeated off-resonant scattering. The probability of any particular atom emitting a photon with a wavelength drastically different from its characteristic spectral lines is extremely low, but it can happen. And once it does, those new photons are unlikely to scatter again. So, in a sufficiently large sample of atoms, where an emitted photon is guaranteed to scatter many times on the way out, the light that finally escapes is pushed to longer wavelengths.

That post is based on this old blog post thinking out loud about this stuff, and more specifically on this AJP article that I was pointed to in the comments to that post. The article includes a simple model showing how you can take a bunch of two-level “hydrogen” atoms that absorb and emit in the deep ultraviolet (Lyman-alpha light, in spectroscopic terms) and end up with a black-body spectrum once the sample gets sufficiently optically thick.

It’s a very nice paper, and I recommend reading it, but it does have one aspect that might feel like a cheat. The spectra they generate are based on solving the equations Einstein used in his famous 1917 paper on the statistical behavior of light quanta, assuming a particular temperature for the atoms. They do some work to get this in terms of the optical thickness of the sample, but they’re always dealing with continuous functions, not discrete probabilistic absorption.

On some level, what you’d really like to see is a Monte Carlo simulation of this sort of thing, where you look at absorption and emission events by individual atoms, and see how that builds up light at lower frequencies. So I wrote one.

Well,sort of. My numerical simulation skills remain pretty rudimentary, but I cobbled together a crude thing in VPython. I put the code on Gist (GlowScript didn’t like it), so you can point and laugh:

So, what is this doing? Well, I imagined a collection of “atoms” with a single transition frequency that it likes to absorb or emit, but two lower-frequency modes of possible light. Why three? Because that lets me store the state of the light as a vector object in VPython, saving me from having to figure out how to do arrays with more components. I imagined a single photon of light at the main transition frequency coming into a large array of these atoms, which could absorb it and then re-emit something.

A photon at the main transition frequency is represented by a vector (1,0,0), and is assumed to be absorbed (you could imagine lots of additional atoms that get skipped over). It can then emit a photon of one of three frequencies: the main transition (in which case the state vector goes back to (1,0,0)), a somewhat lower frequency (changing the vector to (0,1,0) with a 1% probability) or a really low frequency (changing the vector to (0,0,1) with a 0.01% probability).

A medium-frequency photon has a 1% chance of being absorbed at the next step, and a low-frequency photon has a 0.01% chance of being absorbed. If either of these get absorbed, they preferentially emit back to the state– so if the state is (0,0,1) and it gets absorbed, there’s a 1% chance of going to (0,1,0) and a 0.01% chance of going to (1,0,0).

Then this process gets repeated a whole bunch of times (ranging from 10 to 1,000,000), and we record the final state. That process then gets repeated 1000 times to give a probability of ending up in each of the three states. All this repetition leads to the following graph:

Results of the crude Monte Carlo simulation in the text.
Results of the crude Monte Carlo simulation in the text.

The graph is a log-log plot (so each division increases by a factor of 10) showing the probability of ending up in each of the photon states– the strongly absorbing resonant state, the medium-frequency state, and the weakly absorbing low-frequency state– as a function of the number of absorption steps in the simulation.

A small number of steps corresponds to an optically thin sample– the initial photon gets absorbed and re-emitted at most 10 times on the way through. As you would expect, this mostly leaves you with light at the resonant frequency, with a low probability of finding either of the other two– about a 10% chance of the medium-frequency photon, and 0.1% of the low-frequency state. Those are about what you would expect– with a 1% probability of emitting into the medium state, you you would expect about a 10% chance that this would happen in a set of 10 scattering steps.

As you increase the number of scattering steps, though, the chance of finding the lower frequency increases dramatically. By 200 absorptions, the probability of ending up in the medium state has shot up to about 90%, and after 1,000,000 absorptions there’s a 60% chance of ending up in the weakly absorbing low-frequency state. This would presumably increase further for even more steps, but running through the loop a billion times took most of a lecture class period, and I don’t have the patience to push it even further.

What’s going on here? Well, it’s the process described in the AJP paper: there’s a very low probability of scattering into an off-resonant state, and once that happens, those photons are unlikely to scatter again. Which means that, over a long cycle of scattering, these highly improbable events inevitably push the state of the light toward the low frequency.

Now, there’s a bunch of work to get from here to a black-body state (doing this right would need to start with a Maxwell-Boltzmann distribution of energies corresponding to some temperature, and include a lot more frequencies), but this illustrates the basic idea. Well enough for a blog post, anyway.

————

(One technical note here: I dithered about what to use for the emission probability when an off-resonant photon scatters; this version of the simulation has it preferentially emit back to the same state that was originally absorbed (so (0,0,1) goes back to (0,0,1) most of the time, and only rarely to (1,0,0)). This is basically mapping a classical sort of frequency response onto the photon picture. The other thing you could do would be to treat it as a true absorption to the imaginary excited state, which would then emit with the same probability as the original state (so when a (0,0,1) state gets absorbed, it usually emits to (1,0,0) and only rarely back to (0,0,1). In that case, you see a similar sort of behavior, but the end result is rough parity between the three states.

(For completeness, that graph looks like this:

Results of the Monte Carlo simulation described in the test, with a different emission probability.)
Results of the Monte Carlo simulation described in the test, with a different emission probability.

(As I said, I dithered about this for a while; I think the first graph is the right way to do it, and it’s easier to see a path from that to the expected result of a black-body spectrum. I am not 100% confident in this, though; there might be some way to get from rough parity to a black-body spectrum through the Maxwell-Boltzmann distribution of energies. Which is why this is a blog post, and not a paper written up to submit to AJP.)

(Though if anybody with genuine computational skills wanted to collaborate on doing a better job of this, I’d be happy to talk about that…)

Amazing Blackbody Radiation and LHC Basics

I was proctoring an exam yesterday in two different sections of the same class, so I had a lot of quite time. Which means I wrote not one but two new posts for Forbes…

The first continues a loose series of posts about the exotic physics behind everyday objects (something I’m toying with as a possible theme for a new book…), looking at the surprisingly complicated physics of an incandescent light bulb. A light bulb filament emits (to a reasonable approximation) black-body radiation, which is historically important as the starting point for quantum physics. But when you think about it, it’s kind of amazing that you get a black-body spectrum from a large collection of atoms that absorb and emit at discrete frequencies…

(As I type this, I have a crude Monte Carlo simulation running in VPython, so there will be more on this subject later…)

The second post was prompted by the news that the LHC is now colliding protons at 13TeV, and offers answers to some really basic questions about the LHC.

So, you know, if you’d like some physics-y stuff to read as you wait for the official start of the weekend, well, there you go…

Hyperactive Dogs and Fancy Motorcycles

I’m still in the late stages of an awful cold, but shook it off a bit to write a new conversation with Emmy, the Queen of Niskayuna over at Forbes:

“HEY! YOU POODLES! STAY OFFA MY LAWN!”

“Emmy! Stop barking!” I sit up. She’s at the gap between the fences, where she can see into the front yard.

“But, those poodles..”

“We’ve had this conversation. It’s a public street, other dogs are allowed to walk on it. No barking.” She comes over, sheepishly. “Why can’t you just lie down and enjoy the nice day, hmm?”

“Well, I would. But, you know… Quantum.”

“What?”

“I would love to just lie in the sun, but I can’t. You should understand– it’s quantum physics.”

I look at her. She looks back.

I rub my temples. She wags her tail cheerfully.

I’m going to regret this. “Oh, fine. How does quantum physics explain your inability to quietly bask in the sun without getting up every two minutes?”

“I’m glad you asked. See, it’s all about confinement…”

There’s also a human-centered explanation, after the conversation with Emmy, that brings in Richard Thompson:

So, if that combination sounds interesting, well, head over to Forbes and read the whole thing

My Quantum Alarm Clock

One of the things I struggle with a bit when it comes to writing about cool modern physics is how much to play up the weirdness. On the one hand, people just can’t get enough of “spooky action at a distance,” but on the other hand, talking too much about that sort of thing makes quantum physics seem like a completely bizarre theory with no applications.

Which is unfortunate, because quantum physics is essential for all manner of everyday technology. For example, as I try to explain in a new post at Forbes, quantum physics is essential to the cheap alarm clock that wakes me up in the morning.

So, you know, go over there and check it out. Also, you can see a carefully curated selection from the giant pile of books that usually lives on my nightstand with the alarm clock…

How Not to Control the Weather for Your Dog

I’m rooting around in my bag for a pen, and pull out a laser pointer by mistake. Since I’d really prefer not to be grading, I flip it on and shine it on the floor next to the spot where Emmy is half-dozing. She immediately leaps up (she’s pretty spry for a dog of 12…), and pounces on it. Or tries to, as I flick the spot across the room.

“Get the dot! Get the dot! Getthedotgetthedotgetthedot!” she mutters as I lead her on a lively chase around the room. After a few minutes, I click the laser off, and put it down. Emmy comes over, panting, and I scratch behind her ears.

“That was fun, eh, girl?” I say.

“Important, too!” she says brightly.

“Important… how?”

“I’m helping fix the weather!”

“Ummm… How does chasing my laser pointer spot around the room help fix the weather?”

“Well, you remember how cold it was on our walk this morning, right? When I got that nasty squeaky snow between my toes, and you had to rub it out? And you said ‘I am so sick of this stupid winter…’ Only, you know, with some extra bad words mixed in.”

“Yeah…”

“Well, you talk about all these physicists who do laser cooling, and how cold they make stuff, and it’s super cold outside, so I figure it must be all those lasers. And if I can take a few of them out, maybe it will warm up enough for the bunnies to come back out in the yard so I can chase them!” She wags her tail proudly.

I sigh. “OK, I don’t quite know where to start with this… First of all, the amount of stuff you can cool with lasers is really tiny.”

“Sure, but even a small amount of stuff makes a difference if it’s cold enough. I mean, when you drop an ice cube into my water dish, that cools off all the water. And these laser things are a whole lot colder than an ice cube, right?”

“Well, yeah– an ice cube is usually right around freezing, or 273 Kelvin, while laser-cooled atoms are at a temperature of a few millionths of a Kelvin. But a really big laser-cooled sample might run to a hundred billion atoms.”

“And that’s a lot!”

“Not if you’re talking about atoms. An ice cube contains, let’s see… probably about 1023 molecules of water. That’s a trillion times as many atoms as a really big laser-cooled sample.”

“Oh. But there are lots of people doing laser cooling these days, so maybe all of them working together…”

“No. There aren’t a trillion people in the world, let alone a trillion research groups doing laser cooling.”

“Oh.”

“And anyway, laser cooled samples are always contained in ultra-high vacuum chambers. You remember the big metal chamber from the time we visited my lab for that photo shoot, right? They’re inside that, not out where they could cool anything else.”

Me and Emmy with some of the laser cooling apparatus in my lab.
Me and Emmy with some of the laser cooling apparatus in my lab.

“Why is that?”

“Well, because laser cooling only works for very specific atoms and molecules. You need to have a laser that’s tuned close to but a little bit below one of the frequencies of light that the atoms like to absorb. Then atoms moving toward the laser will see it Doppler shifted–”

“Eeeeeeee-ooooowwwwwww!” Emmy makes a race car noise. I kind of wish I had never used that example with her.

“Yes, exactly. The atoms moving toward the laser see the frequency shifted up, closer to what they want to absorb. And when they absorb a photon from the laser, they get a kick in the direction it was headed, which makes them slow down.”

“Like bouncing a little ball off a big ball!”

“Right.”

“And then you get to chase both balls!”

“Try to focus, please. Atoms moving toward the laser slow down, but atoms moving away see the laser shift even farther from their natural resonance frequency, and so aren’t affected at all. Which is why you can use laser beams to slow specific atoms. But it only works for those atoms, which is why we do laser cooling experiments in the middle of ultra-high-vacuum chambers, so they don’t collide with other atoms and heat back up.”

“OK, but couldn’t you just, like, keep cooling them down? So, you know, when the other atoms from the air hit the laser-cooled atoms, they get a little bit colder, and if you keep doing that, eventually the air gets cold even though it doesn’t interact with the lasers?”

“That’s a good idea,” I say, and she wags her tail. I scratch her favorite spot just behind her ears. “That’s a real thing that people do, called ‘sympathetic cooling,’ and it’s a big part of some experiments. Sometimes, you can’t effectively cool one type of atoms with convenient lasers, so instead you mix those together with another kind of atom that you can laser cool, and cool your target atoms indirectly.”

“Which lets you use laser cooling to control the weather!”

“Um, no. Even sympathetic cooling experiments are done under vacuum. It’s the same problem as with the ice cube– the number of atoms you can effectively laser cool is too small to affect the vastly greater number of atoms in air.”

“Well, what if you got a really big laser?”

“You could cool more atoms with more laser power, but you’re not going to get a trillion times bigger that way. And, anyway, if you tried to do that, the laser power supply would generate so much waste heat that you’d end up making the weather warmer, not colder.”

“Oh, right. The second law of thermodynamics.”

“Exactly.” (I know better than to ask how she knows about that…)

“Stupid entropy…”

“Um, yeah. Anyway, laser cooling is not responsible for the frigid weather, so you’re not actually doing important work by chasing the laser-pointer spot around the room.”

“OK, maybe it’s not fixing the weather, but it is important.”

“Important, how?”

“Well, you’re getting a blog post out of this, right?” She looks insufferably smug.

“You’re a very clever dog,” I say.

“So can I chase the spot some more?”

“After I type this up, sure.”

“Yippee!!!”

———–

(If you’re new here, and enjoy this, let me note that I have two books full of talking-dog physics: How to Teach [Quantum] Physics to Your Dog and How to Teach Relativity to Your Dog; there are also more talking-dog physics blog posts. I’ve also got a new book, albeit not with Emmy, about how you think like a scientist without even realizing it, which isn’t directly relevant to this post, but is awesome in its own right…)

Three More Things Every Human Should Know About Light

Rhett Allain has a list of 5 Things Every Human Should Know About Light, to tie in with the International Year of Light, and it’s a good list with lots of .gifs. Of course, there are some gaps, so let me offer some additional things that everyone ought to know about light:

Light Is a Particle

Rhett and I have a long-running argument about the use of photons in introductory physics; he’s against them for reasons that make no sense to me. To my mind, it is unquestionably true that light has particle-like properties (and here’s a follow-up with some math), and that’s a thing that everybody ought to know.

Quantum Physics Starts with Light

So, Rhett says light is a wave, and I say it’s a particle. which is it? Well, both, really, and that discovery drove the development of quantum physics, as explained in this video:

(Or you can read the approximate text here)

The particle nature of light was introduced by Planck, and refined by Einstein, and the wave nature of matter starts with Bohr’s quantum model of hydrogen, which was needed to explain the light absorbed and emitted by atoms. So, quantum physics is all about light.

And quantum physics is essential to, well, everything. Understanding the quantum physics of light and matter lets us build lasers and semiconductors, which are essential for modern telecommunications and computing. You wouldn’t be able to read this if not for light leading to quantum physics.

(More in this vein: Four Things Everybody should Know About Quantum Physics and Seven Essential Elements of Quantum Physics. And if you prefer video, here’s a TED-Ed animation about Schrödinger’s cat and computers.)

Everything We Know About the Universe Comes From Light

This isn’t just a statement about our heavy reliance on our eyes for getting around the world. Pretty much everything we know about the origin and fate of the universe comes from studying light. We can even use light to detect the existence of stuff that doesn’t emit any light:

See also this Tor.com article I wrote about spectroscopy, jumping off from Gandalf’s assertion that splitting white light to study it departs from the path of wisdom. Far from it– splitting light is behind some of the most amazing discoveries in the history of science.

So, in summary, light is awesome. The more cool things you can know about it, the better.

(“Featured image” from Russel Dickerson’s site, because it amused me.)

The Sun Is Red Because The Sky Is Blue

SteelyKid missed the bus this morning– she was dressed and ready, but I was talking to Kate, and if there isn’t a person at the end of the driveway when the bus comes around the corner, they won’t stop. So I drove her over to school myself (which is faster, anyway). The GE research lab complex is behind her school, so there’s a nice view from the parking lot to the eastern horizon, where the sun was just poking over a big band of clouds.

“Hey, look at that cool sunset!” she said as we were walking from the car to the building.

“That’s not a sunset, honey, it’s a sunrise. It’s morning.”

“Oh, right. I don’t usually see the sunrise, because I’m usually on the bus on the opposite side.”

“Mm-hmm.”

“It looks pretty orange. The sun’s usually yellow.”

“Right, that’s because it’s sunrise. When the sun’s near the horizon, it looks more red.”

“Yeah, at sunset it’s red, too.”

“Do you know why that is?”

“Why?”

“Because the sky is blue.”

“What?”

“You see the blue sky above us? That looks blue because light from the sun that’s headed that way” (big westward hand gestures) “hits stuff in the air and gets bounced down toward us. And that works better for blue light than for red light.”

“So the sky looks blue.”

“Right. And when the sun is low on the horizon, its light passes through a lot of air to get to us, and hits a lot of stuff. So all the blue light that ought to be there gets bounced down, to make blue skies for people who live over that way,” (big eastward hand gestures). “That leaves red light, so the sun looks red to us because the sky looks blue to them.”

“Oh. That’s pretty cool.”

“Isn’t it?”

“How do you know that?”

“Well, it’s physics. The kind of science I do looks at how light interacts with stuff, and that lets us understand blue sky and red sun. It’s Rayleigh scattering.

“Oh. I was wondering, because I didn’t think you, like, went up into the sky to look.”

“No, but that would be kind of cool… Anyway, here we are at the door. Have a good day, honey.”

“Thanks, Daddy.”

Advent Calendar of Science Stories 22: Hazing

One of the very best books I ran across in the process of doing research for Eureka is The Second Creation: Makers of the Revolution in Twentieth-Century Physics by Robert P. Crease and Charles C. Mann. It’s an extremely detailed treatment of the development of quantum theory, and includes anecdotes that I haven’t seen elsewhere. It also does a fantastic job of showing the essential interplay of experiment and theory through the difficult process of developing quantum field theory, which is often underplayed in popular treatments (which tend to be written by theorists, and often treat experiments as a sort of afterthought). Anyway, I got so much good stuff from Crease and Mann that I can’t let this series go by without using at least one of the new things I learned from their book for the first time.

One of the critical discoveries along the path to QED was what is now known as the “Lamb shift.” Its announcement at the famous Shelter Island conference kick-started the work of both Richard Feynman and Julian Schwinger, and when news of the Lamb shift reached Japan via the popular press, it did the same for Sin-Itiro Tomonaga, who would later share the Nobel with Feynman and Schwinger for developing the theory.

The curious thing about this is that Lamb is mostly known as a theoretical physicist. But the Lamb shift was an experimental discovery, so it couldn’t be named after him because of a prediction of the shift (which was mostly explained by Hans Bethe on the train home from Shelter Island, and the last piece was nailed by the development of QED). So, how did Lamb get his name on this? It turns out to be a by-product of physicists hazing each other.

Lamb was a student of Robert Oppenheimer’s, but didn’t particularly get along with Oppenheimer (according to Crease and Mann). He wasn’t in the first round of scientists invited to join the Manhattan projects for security reasons, but as the effort expanded, Oppenheimer called him in 1943 and asked him to join. Lamb declined the opportunity to work on the bomb, but did join up with the effort at Columbia University to improve radar systems.

The radar project was headed by the great experimental physicist I. I. Rabi, a colorful character in his own right. When Cold War paranoia led to hearings on whether Oppenheimer himself was a security risk, Rabi’s testimony included the line “We have an A-bomb and a whole series of it, and what more do you want, mermaids?” He was a great physicist, an outstanding scientific administrator, and didn’t have much patience for fools.

Anyway, Lamb joined Rabi’s project to make better radar sources, the microwaves for which were generated by magnetrons. Lamb’s primary job was to do calculations about how to improve these, but before he could do that, he had to do pass a test. Crease and Mann quote Rabi saying “We had a rule there that everyone who worked there had to make a magnetron. So that’s how Lamb got introduced to experimental work.”

Making a magnetron helped Lamb realize the potential of the war-driven improvements in microwave technology. Which, in turn, led to the realization that microwave spectroscopy offered a possible way of measuring the energy between the two lowest levels in hydrogen atoms– the first excited state of hydrogen has an exceedingly long lifetime, so you can’t just wait for atoms to decay. But if you prepare a bunch of atoms in that state, you can use microwaves to drive them to another nearby state that decays very quickly, and the energy difference between the emitted photon and the microwaves lets you figure out the energy of the lowest excited state. This was, at the time, a source of argument, as some experiments in the 1930’s had claimed to see a splitting of the two lowest excited states, something that both theory and other experiments claimed shouldn’t be there.

Lamb realized that the magnetrons being built for use in radar had nearly the right frequency to drive the transition in question, and were sufficiently high quality to settle the question once and for all. So, after the war, he recruited a grad student at Columbia, Robert Retherford, to help him do the experiment. And when they finally got results, the microwave frequency they needed had the “wrong” value, by about 10%. This indicated that the splitting suggested by earlier experiments was real, and would demand new physics to explain it. Lamb was so excited by this that he returned to the lab after Retherford had gone home for the night, to make sure he could reproduce the result. According to Crease and Mann, he ended up recruiting his wife as an assistant when it turned out he couldn’t do everything by himself. Happily, the result turned out to be very robust, and physics was never the same after that.

So, Lamb’s story is another reminder of the great things that can come out of stepping a little out of your comfort zone. (And can maybe also be cast as an example of the importance of phenomenology, as in Friday’s post…) One of the greatest revolutions in theoretical physics can be traced in part to the time when a new theorist was forced to act like an experimentalist to gain entry into a wartime radar project.

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(Part of a series promoting Eureka: Discovering Your Inner Scientist, available from Amazon, Barnes and Noble, IndieBound, Powell’s, and anywhere else books are sold.)

(Image of Lamb in a lab taken from this AIP history page.)

(For a more detailed but non-mathematical description of what the Lamb shift is, I highly recommend this blog post.)

Advent Calendar of Science Stories 16: Undergraduate Research

“You wanted to see me, Herr Professor?”

“Hans! Yes, come in, come in. Just going over the account books. Frightful amount of money going out of this place.”

“Well, radium is expensive…”

“Ha! Oh, and speaking of which– here’s one of the sources. Absent-mindedly dropped the fool thing in my pocket last night when I locked up. Terrible habit, I really must work on that. Had a drawer full of the things in Montreal…”

“Thank you. And you wanted to see me about…?”

“Oh, yes. We have a new student, Hans, and I’d like you to put him to work on the gold foil project.”

“Shouldn’t he have his own project, sir?”

“Not yet, Hans, this one’s an undergraduate. Very keen fellow, though. Seems to show some promise.”

“Very well. What shall we have him do?”

“Well, first see if he has any aptitude for counting scintillations. If he does, you might have him check for background sources. Check for stray alphas on the same side of the foil as the source, that sort of thing.”

“Isn’t that futile? We know there won’t be any backscattered…”

“True, true. But then, it’s a valuable introduction to the frustrations of research, no? Anyway, you never know until you look.”

“I suppose.”

“Right, then, that’s settled. He’s in the second-floor common room, or was the last I knew. Come along, and I’ll introduce you to young Mr. Marsden…”

I’m running short of days in which to complete all the stories I’d like to tell in this series, so I’m jumping forward almost a hundred years, though staying in England. This totally imaginary scene is set in Manchester around 1908. The talkative fellow in the above bit is Ernest Rutherford, carrying on an imaginary conversation with his post-doc, Hans Geiger.

Rutherford had recently won a Nobel Prize for his work on radioactivity (in Chemistry, ironically, given his famous dismissal of sciences other than physics), and was studying the interactions between the alpha particles and other stuff, specifically a thin foil of gold. They were looking at the deflection of alphas as they passed through the gold, in hopes of learning about the structure of matter. This turned out to be one of the most important experiments in the history of physics, in a very unexpected way. Rutherford hired on an undergraduate student named Ernest Marsden, then 20 years old, and assigned him to the gold foil project.

The detector they were using consisted of a piece of glass coated with zinc sulfide, that would make faint flashes of light when an alpha particle struck. These were viewed through a telescope in a dark room, and it was a very taxing measurement, requiring a long period to allow the eyes to adjust to the dark, and the focus needed to count these scintillation flashes effectively was difficult to maintain. Rutherford himself famously had no patience for it, and the tedium of the counting probably helped inspire Geiger to invent to famous counter that bears his name, as a less irritating means of measuring particle flux. The ability to effectively use these detectors was highly prized, and during Rutherford’s time running the Cavendish laboratory, entering students were rigorously screened for scintillation-counting aptitude.

Either because he was good at it, or because they wanted to get him some practice, Rutherford and Geiger set Marsden to looking for flashes from alpha particles on the “wrong” side of the foil. According to the atomic models of the time, there shouldn’t’ve been any to see, as the high-energy alpha particles should’ve blasted right through the gold. To everyone’s shock–Rutherford famously compared it to an artillery shell bouncing off tissue paper– Marsden saw alpha particles. Lots of them.

The only way this can happen is if the vast majority of the mass of the atom is concentrated in a tiny nucleus at the center, something Rutherford quickly realized, and he introduced the solar-system sort of atomic picture that is the standard cartoon image these days. Of course, such an atom is utterly impossible according to the rules of classical physics, and fixing that problem led Niels Bohr to introduce his quantum model, and physics changed forever.

And it all came out of an undergraduate research project.

Marsden went on to have a very successful career in science, as a professor and administrator in Rutherford’s home of New Zealand (some sort of conservation of Ernests involved, perhaps…). Unlike a fair number of other scientists who made revolutionary discoveries at a very young age, he didn’t go nuts and start advocating wacky pseudoscience. Possibly because nobody ever won a Nobel for the discovery of the nucleus, something that seems kind of incredible, but comes down to weird Nobel politics.

Anyway, the lesson to take from this is that great discoveries sometimes come from unlikely places. Some project may seem unpromising based on the best models you have, but you never really know until you try it…

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(Part of a series promoting Eureka: Discovering Your Inner Scientist, available from Amazon, Barnes and Noble, IndieBound, Powell’s, and anywhere else books are sold.)

(Rutherford was a fascinating character, and I recommend Richard Reeves’s short biography, and also Brian Cathcart’s The Fly in the Cathedral about the Cavendish laboratory under Rutherford’s direction.)

High Precision, Not High Energy: Video

Back in August, I gave a talk in Stockholm at the Nordita workshop for science writers, about precision measurement searches for physics beyond the Standard Model. There’s now video of this online:

The video quality isn’t great, but if you’d like a clearer look at the slides, I’ve posted them on SlideShare. The talk was divided into two parts, though the video is not:

Part 1:

Part 2:

As always, watching this is a slightly weird experience. I appear a little more low-key in the video than I remember feeling as a speaker, but this is probably because I’ve been doing a lot of editing of promotional videos for the new book, and those are necessarily a bit more hyper. But anyway, if you want to know what my speaking style is like for more technical sorts of things, well, here’s your chance.

(If you want to see either style of talk live, shoot me an email; I enjoy going places and giving talks, and we can probably work something out…)

Several of the other talks are also online now, part of a playlist associated with that video, if you want to see more of what went on. I can’t say anything about the quality of the other video, because I’m very self-centered, and only watched my own…