Great Moments in Pedantry: James and the Giant Peach needs moar seagulls

Children's literature is about the wonder of discovering new worlds, the power of imagination, and the all the little triumphs and defeats that make up a life.

It's also an excellent place to find hypothetical questions that test the laws of physics.

For instance, presupposing that one could grow a peach to the size of a house, could one also really sail that peach across an ocean? And then, presupposing that one could harness the power of 501 seagulls, would that number of seagulls be sufficient to carry said peach through the air?

These are the questions posed in "James' Giant Peach Transport Across the Atlantic", a paper published last fall in the Journal of Physics Special Topics.

The paper was written and researched by four physics master's students from the University of Leicester in the UK — Emily Watkinson, Daniel Staab, Maria-Theresia Walach, and Zach Rogerson. Based on the events in Roald Dahl's James and the Giant Peach, the four set out to discover whether the book's fictional account of an adventure could stand up to serious scientific inquiry.

Turns out, it can. Mostly.

The team's analysis relied on two sets of equations — one to test the buoyancy of the peach and another to determine whether the 501 seagulls could produce enough lift force to overcome the weight of the peach and get it airborne.

In order for something to float, it needs to be less dense than the liquid it's meant to float on. Say you have the same amount — the same volume — of water and of rubber. That rubber will only float if it weighs less than the water, volume-to-volume. That's true whether you're talking about a rubber duckie or a giant peach. Or, for that matter, a boat. And this is where Roald Dahl made a very smart plot choice.

Turns out, the flesh of a peach is actually more dense than seawater. Technically, it should sink. But Dahl took advantage of the same trick that allows steel boats to float even though the metal they're made of is more dense than water — you just hollow out the inside. That's because air is less dense than water, and a boat is just a shell of steel surrounding a pocket of air. Taken together, air and steel are a lot less dense than just the steel by itself. So the boat floats. And, as the Leicester team found, so does the peach — provided that, for a peach with a radius of 6 meters, the flesh was no thicker than 1.24 meters.

Next, the team turned their attention to the skies. Could 501 seagulls really airlift a peach that large? The answer: No. To make it work, James would have needed approximately 2,425,907 seagulls. (Assuming we're talking about Common Gulls. Different birds, different numbers.)

That's because of lift force — a calculation that compares the pressure over and under each bird's wings with the area of those wings and the density of the air. (You might also know it as Bernoulli's Principle) To get off the ground, just by themselves, each seagull has to have a lift force greater than their own weight. Subtract the weight from the lift force and what you have left over is the amount of lift each bird can put towards carrying other things. The sad fact is, 501 Common Gulls don't have enough leftover lift force to get that peach to rise. Two and a half million gulls, though? That'll do just fine.

So why does science care?

What's the point of all this? That's the really interesting part.

You're right in thinking that the plausibility of fictional scenarios isn't exactly a great problem of our time. But nobody ever said it was. That wasn't the point of this paper, or any of the other eight papers Watkinson, Staab, Walach, and Rogerson published last year.

Instead, this was about teaching them how to be better scientists.

The Journal of Physics Special Topics is, itself, a pretty special journal. It's written, edited, peer reviewed, and published by students of physics professor Mervyn Roy. Throughout the course of a semester, teams of students come up with problems they can use physics to solve. They get a week to research and write each two-page paper. Then they hand those papers off to their peers, who put them through a rigorous peer review process — critiquing the physics, demanding edits in grammar and style, and sending the students back over and over until they've polished up something that is worthy of publication.

It's a microcosm of the way academic publishing is done in the real world and it gives the students a chance to learn through trial and error both how to write a paper AND how to peer review one. That's important. Remember the story I wrote here a couple years ago, explaining how the peer review process works? One of the big critiques that scientists have of that system is that nobody is really taught how to peer review. You're just kind of tossed into it. Some people figure out how to do it well. Others ... not so much. The Journal of Physics Special Topics is an attempt to solve that problem by having physicists learn peer review before they actually have to do it for real.

They also learn how to handle the social fallout of peer review. "There were some awkward moments. One of my housemates was in a different group and when I was writing reviews of what he'd written it was sometimes a bit difficult," Daniel Staab told me.

The research also forced the team to learn how to research subjects far outside their own specific field. For instance, the density of peach flesh. As you learned earlier in this article, knowing that number is a pretty important part of knowing whether a giant peach would float or sink. But it's also not a number that your average physics student in England has easy access to. At one point, Staab said, they thought they might actually have to do their own study, opening and analyzing a bunch of peaches. But in the end, they found a reference — a 1948 paper by researchers at the Georgia Institute of Technology.

So, in the end, we learn the physics of giant peaches, students learn how to be better scientists, and it's pretty much a happy ending for everybody. Except, you know, magic and wonder and suspended disbelief. But Emily Watkinson isn't too worried about that.

"When we were writing this paper, some people wondered whether it would take away from the magic of the story," she said. "But, for me, Roald Dahl keeps the magic. We just wanted to see whether you could actually implement his ideas, because if you could it would be even more fascinating."

Read the fully study on the physics of James and the Giant Peach

Read all the papers from the fall 2012 edition of the Journal of Physics Special Topics — including papers on the physics of "Breaking Bad", Spiderman, lightsabers, and Katrina and the Waves' 1983 song "Walking on Sunshine".

Image: Lighthouse 0.12, a Creative Commons Attribution No-Derivative-Works (2.0) image from benimnetz's photostream


  1. The larger barrier to seagull-hoisted transport than their carrying capacity is the fact that seagulls don’t form coherent directional flocks like migrating birds. Even if they were able to lift the giant peach out of the water, the lines would become hopelessly tangled almost immediately and the peach would go approximately nowhere.

    1. Plus the logistics of connecting all the seagulls up before getting started, the presumably not inconsiderable weight of the spider silk cables (and how you’d mount them, even assuming there was room for them all, if you could get the spiders to make them), and the impact of wind on the performance of the peach. Plenty of opportunity for further study…

    2. …seagulls don’t form coherent directional flocks like migrating birds.

      I have a small remnant of corn dog that says you’re wrong about that.

  2. A friend of mine works in that department!

    James and the Giant Peach wasn’t written in 1995, though. That citation is missing an edition number, at least. And an ISBN?

      1. Ah. Missed it before. Thanks.
        (Though google’s search-by-image did return some lovely, if unhelpful, results for me in the interim.)

  3. I always found it more suspicious than anything else that the seagulls went for the Earthworm. Yes, I believe seagulls will eat earthworms, but we’re talking about an earthworm that’s six feet long and wears a hat and tie. 

  4. That is a great idea for grad students. Most grad students do something similar (and, well, I had a professor who gave us peer review assignments as an undergrad) but the way it’s set up there is far better than anything I’ve heard of.

    Of course, it’d be tough to do that outside of physics, which is inherently capable of analyzing random situations in what is essentially a back-of-the-envelope way (taken to something of an extreme here). Hence the xkcd what if? section, which is all physics, and the lack of anything similar in any other field.

    Perhaps, though, that’s missing the point – it still would have been exceedingly useful for me to do this as a grad student in geology even though there are almost no topics that can be approached in this way in geology (not even within geophysics). We could have just done physics since the point is learning the research methods and peer review and so on, and I would love a chance to dig in and brush up on physics with purpose anyway.

    I’m hoping to begin grad school again this fall, but maybe… imagine a social network of sorts, consisting of grad students across all disciplines and all around the world, set up to do essentially what these students do – collaborate on relatively silly research papers, peer review them, and everything. Add in some competitive aspects and so on. Could be fun.

  5. Am I the only one who feels certain that a magically enlarged peach is bound to have different peach flesh density than a traditional peach?  To me this creates a massive flaw in their math.  Additionally, I doubt traditional peach flesh would be able to provide the necessary structural integrity to support the size and shape described.  I think the more interesting physics question is to work backwards from the known size and 501 seagulls or measured structural integrity in order to determine the necessary peach flesh density after (or while continuously) being magically enlarged.

  6. I think projects like this – both the paper and the journal – are awesome.

    Another really interesting journal, along similar lines but maybe even a bit more ambitious in scope is the Canadian Your Scientist Journal , which takes the very best high school students work across the world – think the very best in national science fairs, that sort of thing – and puts them through the same process (rather than having the project just end and then never be recorded anywhere). It actually has a paper rejection rate almost as high as Nature or Science, and attracts papers from all across the world (largely because we think it’s the only journal of its kind) and is sent to every secondary school across Canada for high-school “journal clubs”. It has lost its main source of funding, but the hope is it can keep going.

  7. I wonder how much the added wight of all the silk needed to harness the birds would change the bird count.

  8. Inertial Dampeners — How Do They Work? 

    Reactionless Thrusters — How Do They Work?

    Structural Integrity Fields — How Do They Work?

    Children’s Stories. How Do They Work?

  9. I would have loved this in my undergrad physics degree! We had some similar units that got us to come at physics from a different direction, one of which involved us producing a treatise on the state of propulsion systems with a speculation on whether interstellar travel was likely (sadly it wasn’t, but we hadn’t heard about NASA’s warp drive then!).

    All the kudos to Leicester and Mervyn Roy, all physics departments should encourage some back-of-the-envelope speculation about the world. Physics can get pretty dry once you’re a couple of years in, it’s a good idea to remind students why they’re studying it!

  10. The gulls were lassoed during flight, would this affect the lift needed? Another point to consider, how thick in diameter is a strand of spider silk and/or silkworm silk (silkworm was left out of the movie btw) and would that be strong enough? 

    Also, James should probably go into the rodeo or something if he can lasso 501 flying seagulls in such a short amount of time.

    I love Roald Dahl’s work.

  11. Since pedantry is in the article title, I think it should be pointed out that “moar” should be spelled “more”.
    I can’t fathom how that misspelling occurred.

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