Coffee vs. neutrinos

Anaïs took this photo while I was writing this post. I am drinking *tea* here because, after the reading I’ve been doing (see below), I just can’t handle the question of whether I should be drinking coffee. Like, on a philosophical level. It’s right up there with “what is truth” and “does free will exist”.

Let’s start with some numbers. According to the International Coffee Organization, the USA bought enough coffee beans in 2014 to brew something like 100 billion cups of coffee. That’s roughly one cup of coffee per day for every single US resident. Including babies. Coffee is everywhere, all the time.

So here’s a perfectly reasonable question: is coffee good for you? Humans have been drinking coffee for at least 600 years — plenty of time to come up with an answer.

The answer is a resounding “maybe”. Consider:

  • A 1985 article in the New York Times suggested that 5 or more cups a day increases risk of heart disease. Specifically, it described a study which concluded that drinking that much coffee will triple the risk of heart disease, relative to people who drink none. (Coffee bad.)
  • A 2012 article in the New England Journal of Medicine reported a correlation between increased coffee consumption and a decrease in “all-cause mortality” rates, essentially a measure of the likelihood of death. (Coffee good!)
  • A 2013 study by the Mayo Clinic concluded that 4 cups per day increases the likelihood of all-cause mortality. (Coffee bad.) They recommend that young people limit their consumption of coffee to less than 4 cups per day.
  • A 2014 study in The Annals of Internal Medicine concluded that “Regular coffee consumption was not associated with an increased mortality rate in either men or women. The possibility of a modest benefit of coffee consumption on all-cause and CVD mortality needs to be further investigated.” (Coffee good?)

I could go on. And on. And on. There’s an overwhelming supply of studies that will support either side of this argument. We haven’t even started talking about coffee’s effect on specific medical conditions; there are studies out there addressing coffee’s effect on the incidence and/or severity of Parkinson’s, liver disease, diabetes, Alzheimer’s, anemia, depression, dementia, athletic performance …

Superficially, this is crazy. Coffee is everywhere. If you are not currently drinking coffee, you probably could be five minutes from now if you put your mind to it. I’m writing this post while sitting in a coffee shop. How can we still have so much uncertainty about something so ubiquitous?

The answer is typically science-y, of course. Coffee’s effect on the human body is complex because the human body is complex. These confusing and contradictory experimental results can motivate scientists to seek deeper, more complete answers to difficult questions. In the case of coffee, some very recent work suggests that your genes determine whether “coffee good” or “coffee bad”. Brave readers with lots of free time might want to stick around for the epilogue.

Anyway, I assert that coffee is hard to understand, despite its ubiquity. Do you know what else is ubiquitous and hard to understand?

Did you just yell “NEUTRINOS!” at the top of your lungs? Yeah, that’s the answer I was going for.

Subatomic particles, generally, are so, so far outside our everyday experience as human beings. Chances are, you’ve never had any reason to care about muons in your daily life. The subject just doesn’t come up, right? What about cosmic ray flux? Or neutrino oscillation rates? Not as often as you drink coffee, amirite?

I refuse to apologize for my puns. This is no exception.

I’m willing to bet that this is the only Quark that you have any substantial experience with.

So here you are, minding your own business, when a physicist starts blogging at you about neutrinos. They’re all around you, he says. Trillions of them pass through your body every second, he says. What are you supposed to do with that? To be blunt, how are you supposed to believe something so far outside your daily experience, when you don’t even know whether “coffee bad”?

Here, I don’t mean “believe” in the sense of truth vs. lies. I mean, how can you know that your body is permeated by neutrinos in the same way you know that gravity pulls you down to Earth, or that snow is made of frozen water, or that Daniel is handsome? You have direct, personal experience, through your senses, that these things are true. There are no intermediate steps. You don’t have to consult a scientific instrument to know that things fall down — you can feel the pull of gravity and you can see its effects on everything around you. Likewise, you don’t need to read a book in a library to know that coffee tastes amazing at 8 am.

But are your senses the only reliable source of truth? Are you skeptical about neutrinos because you can’t see them? You haven’t seen live dinosaurs either, and your day-to-day experience suggests that the Earth is flat, not round. Maybe you should limit your appreciation of truth to what you can sense for yourself.

More than two thousand years ago, the ancient Greeks kicked that idea right in its butt.This is a long discussion and I can’t do it justice in an already-long blog post. Essentially, your perception can change depending on circumstances. For example, maybe a fig tastes sweet to you. But if you eat honey before you eat figs, maybe those figs won’t seem so sweet anymore. What can you say that you know (like, really really know) about the taste of figs?

Your perceptions can be unreliable. Just think about the last time you got hangry. You skipped breakfast, maybe, and then right around 11 am the world started to suck, right? The line at the coffee shop started to seem unreasonably long, or the barista’s haircut seemed unreasonably annoying, or the guy behind you in line was talking unreasonably loud on his phone. In that moment, are you really perceiving an objective reality? Do you have well-deserved, righteous indignation about the barista’s haircut? Maybe you should get a muffin with that coffee.

Our senses, by themselves, are not the sole arbiters of truth. They are vital, beautiful, and useful, but they are not the whole story. Humans reach for truth in ways besides immediate sensory experience. One of those ways is called science. We have built tools and systems of thought in order to help us reliably, repeatably demonstrate complex and obscure phenomena.

Leon Lederman is a Nobel laureate, a former director of Fermilab, and the co-author of a truly enjoyable book with an admittedly silly name: The God Particle: If the Universe is the Answer, What Is the Question? (1993, Bantam Press). Here’s a particularly relevant excerpt. (Note for young people: TVs used to be bulky vacuum tubes with electron beams inside.)

The lady in the audience was stubborn. “Have you ever seen an atom?” she insisted.  … My attempts to answer this thorny question always begin with trying to generalize the word “see”. Do you “see” this page if you are wearing glasses? … If you are reading the text on a computer screen? Finally, in desperation, I ask, “Have you ever seen the pope?”

“Well, of course,” is the usual response. “I saw him on television.” Oh, really? What she saw was an electron beam striking phosphorous painted on the inside of a glass screen. My evidence for the atom, or the quark, is just as good.

Sometimes, you need to reach for truth through a pair of glasses. Or a television. Or a particle accelerator.

I’ll be talking about these ideas in more depth on December 6th at an event called Ask A Scientist. It should be fun! Hope to see you there.


Epilogue for Sticklers

For those of you still reading, I should admit to being a little glib for rhetorical reasons. As I said before, the human body is an incredibly complex system. To pose a binary question about whether coffee is categorically good or bad is to be ridiculously reductive.

Sometimes, the questions worth asking have complex answers. The questions we ask should allow for answers complex enough to be correct. As they say on Twitter, you should want better for yourself.

Arguments about all-cause mortality are statistical in nature and difficult to apply to a specific individual with her own specific physiology, metabolism, gut flora, lifestyle, etc. And in fact, there are a couple studies I’ve seen recently that bear this out.

  • Does coffee increase your risk of heart disease? Well, you’ve got a gene called CYP1A2 that tells your liver how to make enzymes that help to metabolize caffeine. If you’ve got the CYP1A2*1A allele, your liver will make enzymes that help you to metabolize caffeine quickly; in that case, “coffee good”. But if you’ve got the CYP1A2*1F allele instead, you metabolize caffeine slowly and coffee might increase your risk of a heart attack. (Coffee bad.) This is hard to summarize in a paragraph-friendly way. Check out the article for better information.
  • Likewise, there seem to be genetic factors that influence the effect of coffee on the risk of Parkinson’s disease.

Probably, then, the question “is coffee healthy” is a bad question to ask since the answer depends so much on individual factors. Perhaps a better question would be, “will I personally benefit from drinking coffee?” And perhaps you can’t answer that question without doing some of your own research, listening to your body, … I’ve even heard of people ordering genetic tests for themselves so that they can have some certainty about this.

Tip your baristas, ladies & gentlemen.

On the craziness of neutrinos, or: Why is there stuff?

I’m going to try to blow your mind two separate times in this post. Stay with me while things get weird, ok?


Today’s photos are of MINOS, a neutrino experiment at Fermilab.

Let’s have some fun with neutrinos. For this very-sophisticated physics demonstration you’ll need your hand and one second of time. I’ll wait while you collect those supplies.


Ok, first hold out your hand, palm up. Now wait for one second. Are you done? Did you notice anything freaky happening?

What if I told you that about a trillion neutrinos passed through your hand in that one second? Not figuratively, the same way you might say “I’m so hungry I could eat, like, a trillion pizzas”. I mean that several thousand billion particles — called neutrinos — pass through your body every single second of every day and night.

Don’t freak out! Well, go ahead and freak out a little if you want. A trillion is a freaky-big number. But the thing about neutrinos is that they’re guaranteed not to bother you. They basically never interact with other matter. You could shoot a neutrino through a brick of lead one light-year long, and that neutrino would only have a 50% chance of colliding with one of the atoms in that lead brick. They’re like the ghosts of the particle physics world.

Wait wait, come back! I’m sorry I made you think about trillions of ghosts whooshing silently through your body. I promise they won’t hurt you.

All those neutrinos are coming from the sun, by the way. The sun is so hot and massive that individual solar protons will squish together and fuse into helium. One of the by-products of that solar fusion is some neutrinos. And by “some” I mean “a number so big it hurts to think about it”.


Neutrino experiments tend to happen in underground caves. Burying your experiment under a hundred meters of rock and dirt is a great way to keep unwanted radio waves, cosmic rays, and other surface dreck from confusing your data. And the neutrinos don’t care where they are, of course.

Now let’s change gears slightly. Remember the Big Bang? When the universe was a crazy-hot pinprick of horrendous energy? As the infant universe expanded and cooled, little globs of matter — particles! — started to condense out of that energy. That happened the way Einstein said: some energy E would turn into some mc2, and then all of a sudden there’d be a particle (with mass m) where before there was just a wad of energy.

The thing is, Einstein’s E = mc2 doesn’t say anything about particles vs. anti-particles. A wad of Big Bang energy should be just as likely to make an anti-electron as to make an electron. (Or whatever, pick your favorite particle.) Statistically, then, you would expect equal amounts of matter and anti-matter to form after the Big Bang.

But! Any Trekkie will tell you that when matter and anti-matter collide, they annihilate in a puff of energy. (That’s how the Enterprise runs!) So in an early, hot universe with particles zooming around helter-skelter, you’d expect matter to collide with anti-matter, leaving nothing. That’s right folks, if physics was simple and things generally made sense, the universe would be filled with nothing. Instead of which, we have not-nothing! There is stuff, and all of the stuff we’ve ever found in the universe is made of matter. Nobody has ever seen an anti-matter galaxy full of anti-matter stars.

If I’ve done my job right, you’re freaking out again right about now.


The experiment was RUNNING when I took this tour! That guy is sticking his hand into AN ACTIVE NEUTRINO BEAM. No problem! The white spot is where the neutrinos leave the experimental hall and pass through 450 miles of rock. They emerge at the bottom of a mineshaft in Minnesota, where another set of particle detectors are situated. All of what you just read is real.

Physicists have worked out a theoretical model that explains this matter/anti-matter asymmetry. Theoretically, there’s some mechanism that biased the early universe in favor of matter. In order to test that model, we need to study some pretty esoteric things about neutrinos. That brings us to the final question of this post: If neutrinos are so insubstantial, how can you possibly study them?

The answer has to do with statistics. Let’s say you build a particle detector and then you start throwing neutrinos at it. If you only throw one neutrino at a time, there’s basically no chance of that particular neutrino interacting with your detector. What if you throw a million trillion neutrinos at once? Each individual neutrino still has a vanishingly small chance of interacting with your detector, but now statistics is starting to work in your favor. This is kind of like the lottery. If you buy one lottery ticket, I promise you won’t win the million-dollar jackpot. But if you buy a million lottery tickets, you might have a decent chance of winning.


Computer hardware for data acquisition. If you like things that look cool, you might like to tour a particle physics laboratory.

There are some experiments that manage to use only solar neutrinos to answer very specific questions. The Ice Cube experiment in Antarctica is a notably, amazingly hardcore example of this. But really big questions (“Why is there stuff?”) require way more neutrinos than the sun alone can provide. Another experimental approach is to make your own neutrinos in way, way larger quantities, and to throw them all at your particle detector as fast as you possibly can. (This is equivalent to buying all the lottery tickets.) And what’s the best way to do that? Why, with a particle accelerator of course! This is just what I was talking about in last week’s post.

I want to build a neutrino factory. Don’t you?

Makin’ those Big Decisions


cloudsI want to be able to tell myself a story about the future. If I have no idea what the next month will bring me (like when I was applying for jobs this past spring) I can get a little stressed. If I have a story to tell myself, then I have a goal I can work towards. But if I don’t know enough to put together a story, then in my mind every future is equally likely. I could get my dream job or I could get no job. We could move to Illinois or we could get sucked through a rogue cosmic wormhole and end up on Planet Squizznonks. I have a good imagination! But my imagination needs some structure or it will freak out, like a middle school student who’s too smart for his own good.

Right now, I can tell myself a pretty convincing story about the next year or two. Anaïs might graduate and start looking for jobs. I might be working like crazy on my new experimental program. Maybe we’ll make it through a couple Chicago winters and they won’t seem so insane anymore. Story: check. No freak-outs: check.

But if I look a little farther out, there’s some pretty big stuff I just don’t know about yet. Are we going to buy a house like real grown-ups? Are we going to start having kids? Can I plan on staying at my new job that I love, or will we need to solve the “two-body problem” again when people start showering Anaïs with super-amazing job offers? (Anaïs will certainly get showered with job offers because she is brilliant and hard-working and beautiful.)

So for now, I’m trying to focus on the present. The present is pretty good. And I haven’t noticed any rogue cosmic wormholes in my neighborhood yet, so that’s something.

Now let’s peer into our copper cauldron to ponder the phuture of particle physics…

cauldronSame story, different characters. When you talk about the future of particle physics, there’s some near-term stuff that’s easier to plan and talk about, and some long-term stuff that’s hazy and hard to imagine.

In the near-term, there’s a lot of exciting questions about neutrinos that we can answer with today’s technology. For example, some people think there might be new, weird flavors of neutrino we haven’t observed yet. Also, a careful study of neutrinos could help us answer this question: “why is there stuff?” Don’t you want to know why there’s stuff?

Let’s leave that as a teaser for the next blog post: the mystery of the existence of stuff. But for now we’re talking about the future. In my artful and clever allegory, all this neutrino business is in the easy-to-imagine, anxiety-mitigating near-future. It’s good to know we have some important work ready to be done right now.

Beyond that neutrino stuff, though, it’s harder to tell ourselves the story of the next big accelerator. The problem is that right now, we don’t know enough about the next big questions. Is there only one Higgs boson, or are there a bunch of them? And what about supersymmetry? Is that a thing, or what?

Those are big, big questions in physics that will be answered by building a big, big accelerator. And until the LHC generates more data, we really don’t know what kind of accelerator we’ll need. Should we even build a new, giant accelerator? (Yes.) Should it collide protons or muons, should it be circular or linear, and who should build it? Right now we just don’t have enough data. It’s hard to say what the next big machine will be like because we’re not quite sure — yet — how to ask the next round of big questions.

That uncertainty about the future of particle physics is fuel today for a lot of meetings and powerpoint slides and general hand-wringing. My physics pals and I are working just as fast as we can to put together the next big story in a way that makes sense to us all. And that’s what I’ll spend my next few blog posts talking about. Stay tuned!

I made you a li’l movie!

Hi! Hello. I made a small movie about a small particle accelerator.

But first. A public apology to the woman who cut my hair the other day. (And who is definitely reading this right now?) I’m sorry I made you talk about physics.

For the 100% of you who are not the lady who cut my hair, I will explain. Briefly. During a lull in standard-issue haircut conversation, Haircut Lady asked me what I did for work. My internal dialogue went like this:

  1. I should just tell her I’m a scientist and leave it at that. Supplying more information is an implicit assumption that she wants to hear me say a lot of science stuff.
  2. On the other hand, I’m going to be sitting in this chair for a long time and physics is fun to talk about.
  3. On the other hand, plenty of people have told me that they hated physics in high school and they don’t know anything about it now.  She might be one of those people.
  4. While I’m sitting here thinking, an awkward pause is stretching out into weird, uncomfortable seconds. I should just say what I do for a living.  I shouldn’t be so hung up about this.  Daniel! Have a conversation with a stranger! Go!

Anyway, so I tried a little experiment.  I told her what I do, and then I asked her what she thought of when I said “particle accelerator”. Not like a pop quiz! I wasn’t looking for a specific answer. (I said that, too.) Instead, I think scientists have a responsibility to clearly communicate their work to the general public. If the general public knows what we do, then we’re doing a good job of communicating. And if they don’t, we obviously aren’t doing a good enough job.

Your homework for tonight: What do you think of when you think about particle accelerators? What would you draw if you had to draw one? Describe it in the comments below!

Dear Haircut Lady, I’m sorry that I put you on the spot because I was curious about abstract ideas. You handled it very gracefully.

For the curious: she had the general idea that accelerators are sort of like a laser beam, but wasn’t clear on what they might be useful for. I think this is where most people are at.

Which brings me to this video I made! A colleague at work had this little science demo that she let me play with, and I had so much fun I wanted to share it.  Here we go!

Friday Physics Photos: A Scientific Conference

One of the most important things a scientist can do is to share her work with other scientists.  It gives experts the opportunity to ask critical, helpful questions; it lets scientists find areas of common interest; it gives us new perspectives on our work; and it help us to avoid stepping on each others’ toes.  You can publish your work in a scholarly journal or, if you want a more face-to-face interaction with your colleagues, you can present your work at a scientific conference.

Most areas of research have a yearly conference or two, where everybody meets to compare notes and share their most recent results.  Maybe you noticed that I wasn’t posting for a few weeks?  That’s partly because I was at the International Particle Accelerator Conference, hosted this year in New Orleans, LA.

The conferences I go to usually have two parts.  In the morning, we have a bunch of talks.

And in the afternoon, we have a poster session.

The poster session is a bit like your high school science fair.  (Yes, there really is a reason for those things!)  Everybody goes into a big room, you put up posters about your work, and then you stand around answering questions about your work.  Or, you walk around and check out everybody else’s poster.  Also, coffee.

Here's my buddy Ryan in front of his poster. Q: What's with the t-shirt? A: Scientists don't tend to wear suits. Even at conferences.

I’ve given talks and I’ve presented posters.  I actually feel like the poster session is more fun.  The people who are most interested in your research have a chance to speak with you face-to-face.  You end up having some very interesting conversations with some very interesting people.  Giving a talk is considered more high-profile, but it’s hard to have a stimulating dialogue with a dark room full of sleepy people.

I bet you’re wondering what else I got up to in New Orleans.  Well, in addition to talks and posters, the third thing to do at conferences is to have conversations.  For a week, you’re staying within a mile radius of all your field’s experts.  It’s the perfect opportunity to knock around your ideas with some smart people, to start new collaborations, and to brainstorm about the future.  I’d say I was working after hours, over dinner, just as often as I was sitting and listening to talks.

Of course, I couldn’t go to New Orleans and only work.  After talks on the last day, I ran off to the bayou with some friends and met some gators.

No, that is NOT my hand! That is the hand of a trained professional, petting a nine-foot gator.


Are you a regular reader of this blog? Are you really so brave and generous with your free time? If you are a regular reader, you’re likely scratching your head over a pretty significant question I’ve been ignoring.


I’ve told you that accelerators are big, that they’re hard to build, and that they can get very complex.  Why go to all that trouble?  What are these behemoths good for?  Well, here’s a list.  The internet likes lists, right?

    • Fundamental particle physics.  This is the one everybody thinks of first, especially since the term “atom smasher” is so popular.  If you collide two highly energetic beams together (like at CERN, for example), the resulting Einsteinian cataclysm is interesting in all sorts of ways.  You can search for new particles, test fundamental physical theories, and study exotic environments.  You can ask questions like “why is there matter in the universe?” with a straight face and the expectation of some sort of answer.  This all gets very awesome very quickly.
    • Basic physics, materials science, biochemistry, etc.  We can talk about this more later, but with an accelerator you can create ultra-bright, ultra-fast light pulses.  And then you can use those light pulses to study very small, very fast processes.  You can look at how proteins behave to study diseases and create new drugs.  You can design catalytic processes to enable artificial photosynthesis, because if plants can make fuel from sunlight, why can’t we?  There’s a long, long list of things you can do with these “light sources”. In fact, accelerator-based light sources are a HUGE field of research.  More accelerators around the world do this kind of thing now than do the above-mentioned fundamental physics research.
    • Medicine. I know people who have gotten radiation treatment for various forms of cancer.  Typically, doctors will attack a tumor with x-rays.  But the problem with x-rays is that they’ll also attack the healthy cells around a tumor.  A fascinating alternative is hadron therapy. It turns out that you can “tune” a beam of protons (or neutrons, or both) so that they deliver energy to a very specific target volume.  You can use beams of protons (or neutrons, or carbon atoms, or whatever) to attack tumors without so much damage to the surrounding, healthy tissue.  And where do those hadron beams come from?  Accelerators, of course!  I assert that this is cool. 
    • Safer nuclear power.  Nuclear power is generally pretty safe, but last year we all got an object lesson in its potential problems.  I know people working on accelerator-driven nuclear reactors that overcome some of these problems.  If you build a reactor right, you can control the fission process with an accelerator.  No beam, no fission!  This makes things much safer — you don’t have to worry anymore about meltdown.  “Aha”, you’re saying, “but what about nuclear waste?”  Well, it turns out that you can “burn” (really, transmute) nuclear waste with accelerators too.  Really.  I’m not kidding.  Here’s a video, shot at Fermilab:

There’s another hugely important use for accelerators that’s harder to talk about in concrete terms.  They’re hard to build, right?  And they require cutting-edge technology?  The technology that’s developed for accelerators often makes its way into the private sector, and from there into everyday life.  Do you know anybody who has had an MRI scan?  The magnet technology in MRI machines was first developed for bending particle beams in accelerators.

Not good enough?  Maybe you’ve never had an MRI?  Well, have you ever used this “world wide web” thing?

Seriously.  The vast architecture of the internet was developed by many people over a long time.  But the internet that you interact with daily is based on the work of a few people who worked at CERN.  They tried to figure out a better way to share their data, and the result was the World Wide Web.  You know how you type into your browser in order to look at my site?  That’s the result of some people at an accelerator laboratory, trying to solve an interesting problem.

A long post today, I know.  I tried to pad it out with youtube videos, so it wouldn’t just be a vast ocean of text.  But hopefully, at the end, you have more questions now than you did when you started.  I encourage you to ask these questions in the comments section!  At the very least, stay tuned.  I plan to dig down deeper into these ideas (and talk about other uses for accelerators that I didn’t have room for here) in future posts.

Friday Physics Photos: Art or plumbing?

Here’s another Friday Physics Photo in which the physics is arguably absent.  (Although I’d argue that anything in an accelerator enclosure counts as physics.)

I was touring a machine a couple months ago, and this giant weird thing surprised me as I came around a corner.  It turned out to be a sump pump, but it reminds me way more of a sculpture installation at a modern art museum.  I think it’s quite striking.

Friday Physics Photos: A cryomodule that’s all plugged in and ready to go

Remember how I talked before about cryomodules?  I showed you photos of what they look like as they’re being assembled.  But I also told you (a) they’re complicated, and (b) they have a lot of different jobs to do.  They handle the plumbing of liquid and gaseous helium, they deal with high-power electrical connections, they house diagnostic equipment … it’s a lot of stuff.

Here’s a photo of a cryomodule that’s part of an actual accelerator, with all those connections made:

This one in particular is part of the ILCTA at Fermilab.

Friday Physics Photos: Welding refractory metals.

I’ve mentioned in previous posts that accelerators employ things called cryomodules and that cryomodules are complicated.  Let’s talk about one of the many, many ways that cryomodules are complicated.

Cryomodules have titanium components.  How do you build things out of titanium?  Specifically, how do you weld titanium?

Welding requires hot, hot heat, and when you make titanium (or niobium, or molybdenum, or tantalum …) hot, it starts to suck oxygen and nitrogen out of the air.  If you’re welding titanium and you’re not careful, all of a sudden you’ll have titanium oxide all around your weld.  And titanium oxide is brittle.  It’s weak.  You don’t want a load-bearing joint in your accelerator made of titanium oxide.

Instead, you have to build a box around the thing you want to weld and then blow argon past your weld joint.  The argon keeps oxygen and other atmospheric gasses away from your weld.  (Argon is a noble gas and is too snobbish to react with something as déclassé as titanium.)  Here’s a photo of that thing I just described:

Admittedly, there’s not a lot of physics going on in this photo.  I just like this thing because it looks like a UFO.  Actually, it reminds me a whole lot of these little plastic space ship toys I used to get at the National Air & Space Museum.

I have a couple secret goals with all these photos.  (1) I want to give you a sense of how complicated it can get to build a particle acccelerator.  (2) I want you to understand in rough terms how an accelerator gets built.  How am I doing so far?  Leave your questions and suggestions in the comments section!

Friday Physics Photos: Accelerating cavities.

Quite a few posts ago, I gave a super-brief overview of accelerators. I said they were the distant cousins of tube televisions, yes?  I acknowledge to you that this analogy is pretty silly, even if it’s apt.  But now let’s talk about how things get accelerated in the first place.  You do this with a cavity.

A cavity is just a hollow hunk of metal.  If you run electric current through the walls of the metal hunk, you’ll get electric fields in the hollow part.  And if you shape the metal hunk just right, those fields will have a certain shape.  An electric field applies a force to a charged particle in a specific direction.  So now do you see where this is going?  If you build a cavity right, you can send particles through the cavity in such a way that the cavity fields give those particles a push.  Do this enough times, and you’ll have some pretty fast particles.

How about some numbers?  Let’s say that high-tension power lines support roughly 100 kilovolts.  A cavity – much like the ones I’m about to show you – can apply 100 times that voltage to an electron.  How about some cooler, more rock-n-roll numbers?  Lightning is complicated, but Wikipedia says – roughly – that an accelerating cavity supports electric fields 10-100 times larger than what’s required to initiate a lightning strike.


Here’s a photo of an accelerating cavity:

Some notes about what you’re looking at:

  • The cavity itself is that five-lobed-hourglass thing.  Everything else is support, dressing, and power couplings.
  • This is a display model that’s standing on end, so that the hypothetical particle path goes up-and-down rather than the customary and more reasonable side-to-side.
  • It’s made of niobium, which is a superconductor at low temperatures. Remember I mentioned that accelerators use superconducting gizmos sometimes?

It can be hard to look at a structure like this and visualize exactly how this could be useful in accelerating particles.  Lucky for you I did some computer simulations of cavity fields at work that I haven’t ever used, so I can recycle them in this blog.

Here’s a cartoon of the electric field lines in an accelerating cavity.  You can see the wavy hourglass-type outline of the cavity structure.  The arrows represent the electric field, color-coded according to strength.

Properly speaking, this is the top half of a cross-sectional view of a cavity.  Anyway, the important part is down at the bottom, which represents the beamline.  That’s where the particles go: across the bottom of the picture from left to right.  You can see that the electric field lines all point in a straight line, pushing the beam from left to right.  That’s how it’s done, son!

PS, If you look closely, maybe you’ll notice some weird things about the shape of the electric field lines in this picture.  I assure you, those weird things are normal.  But if you’re feeling curious, then by all means ask a question in the comments section!