What if

No doubt anyone who’s reading this already knows about the web comic xkcd. (If not, drop what you’re doing and start reading it.) But you may not know that the author, Randall Munroe, has started a new feature in which he answers a weekly “what if” question about physics (broadly construed). There have been three entries so far:

I actually don’t think his answer to the first one is quite right. He says there’d be a lot of nuclear fusion reactions, but I don’t see any reason to think there would be. The collisions are certainly energetic enough for fusion to happen, but I think the probabilities (cross sections) for other reactions, mostly simple scattering, are much higher than for fusion. At first I thought that this made a big qualitative difference in the expected behavior, but I don’t actually think it does: the ball’s kinetic energy is so large that the extra energy released via fusion wouldn’t make much difference anyway.

Anyway, that’s a quibble. The main point is to entertain (while conveying some scientific information), and as you’d expect Munroe is awesome at that. A little excerpt:

Next, we need to know how fast it was rising. I went over footage of the scene and timed the X-Wing’s rate of ascent as it was emerging from the water.


The front landing strut rises out of the water in about three and a half seconds, and I estimated the strut to be 1.4 meters long (based on a scene in A New Hope where a crew member squeezes past it), which tells us the X-Wing was rising at 0.39 m/s.

Lastly, we need to know the strength of gravity on Dagobah. Here, I figure I’m stuck, because while sci-fi fans are obsessive, it’s not like there’s gonna be a catalog of minor geophysical characteristics for every planet visited in Star Wars. Right?

Nope. I’ve underestimated the fandom. Wookieepeedia has just such a catalog, and informs us that the surface gravity on Dagobah is 0.9g.















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Ted Bunn

I am chair of the physics department at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!

2 thoughts on “What if”

  1. He’s wrong on the second one too (but probably not qualitatively). On most SATs you can miss 4-5 questions on the verbal and 1-2 on the math and still receive a “perfect” score. Since the number you can miss is tied partially to the distribution that year, and the distribution will be quite low if everyone guessed, the number of missed problems allowed would be on the high end of the above range, or quite possible above it. So multiply his probabilities by at least a factor of 16000, though you still get 0.

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