Sailing Stones: Mystery No More

My first research project, my first research paper, was on a perplexing phenomenon: the sliding rocks of Death Valley’s Racetrack playa. Racetrack playa is a large desolate dry lake bed that has one distinguishing feature above and beyond its amazing flatness. At the south end of the playa are a large number of large rocks (one man size and smaller), and behind these rocks, if you visit in the summer, are long tracks caked into the dried earth of the playa. Apparently these rocks, during the winter months, move and leave these long tracks. I say apparently, because, for many many years, no one had ever seen these rocks move. Until now! The following video makes me extremely happy

This is a shot of one of the playa stones actually moving! This is the end result of a large study that sought to understand the mechanism behind the sliding stones, published recently in PloS one:

• Norris, Richard D., et al. “Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion.” PloS one 9.8 (2014): e105948.

In 1993, fresh out of Yreka High School, I found myself surrounded by 200+ geniuses taking Caltech’s first year physics class, Physics 1 (med schools sometimes ask students at Caltech to verify that they know Calculus because the transcripts have just these low numerical course indicators on them, and of course Physics 1 couldn’t actually be physics with calculus, could it?) It would be a lie to say that this wasn’t intimidating: some of the TAs in the class where full physics professors! I remember a test where the average score was 0.5 out of 10 and perhaps it didn’t help that my roommate studied with a Nobel prize winner as a high school student. Or that another freshman in my class was just finishing a paper with his parents on black holes (or that his dad is one of the founders of the theory of inflation!) At times I considered transferring, because that is what all Caltech students do when they realized how hard Caltech is going to be, and also because it wasn’t clear to me what being a physics major got you.

One day in Physics 1 it was announced that there was a class that you could gain entrance to that was structured to teach you not physics, but how to do creative research. Creativity: now this was something I truly valued! It was called Physics 11 and it was run by one Professor Tom Tombrello (I’d later see his schedule on the whiteboard with the abbreviation T2). The only catch was that you had to get accepted into the class and to do this you had to do you best at solving a toy research problem, what the class termed a “hurdle”. The students from the previous class then helped select the new Physics 11 students based upon their performance on the hurdles. The first hurdle also caught my eye: it was a problem based upon the old song Mairzy Doats which my father had weekly sung while showering in the morning. So I set about working on the problem. I don’t remember much of my solution, except that it was long and involved lots of differential equations of increasing complexity. Did I mention that it was long? Really long. I handed in the hurdle, then promptly ran out of time to work on the second hurdle.

Because I’d not handed in the second hurdle, I sort of expected that I’d not get selected into the class. Plus I wasn’t even in the advanced section of physics 1 (the one TAed by the professors, now those kids were well prepared and smart!) But one late night I went to my mailbox, opened it, and found…nothing. I closed it, and then, for some strange reason, thought: hey maybe there is something stuck in there. So I returned and opened the box, dug deep, and pulled out an invitation to join physics 11! This story doesn’t mean much to you, but I can still smell, feel, and hear Caltech when I think of this event. Also I’ve always been under the impression that being accepted to this class was a mistake and really the invitation I got was meant for another student in a mailbox next to mine. But that’s a story for another session on the couch.

So I enrolled in Physics 11. It’s not much of a stretch to say that it was the inspiration for me to go to graduate school, to do a postdoc, and to become a pseudo-professor. Creative research is an amazing drug, and also, I believe, one of the great endeavors of humanity. My small contribution to the racetrack playa story was published in the Journal of Geology:

• Bacon, D., T. Cahill, and T. A. Tombrello. “Sailing stones on Racetrack playa.” The Journal of Geology (1996): 121-125.

The basic mystery was what caused these rocks to move. Was it the wind? It seemed hard to get enough force to move the rocks. Was it ice? When you placed stakes around the rocks, some of the rocks moved out of the stakes and some did not. In the above paper we pointed out that a moving layer of water would mean that there was more wind down low that one would normally get because the boundary layer was moving. We also looked for the effect of said boundary layer on the rocks motion and found a small effect.

The answer, however, as to why the rocks moved, turned out to be even more wonderful. Ice sheets dislodged and bashing the rocks forward. A sort of combination of the two competing previous hypothesis! This short documentary explains it nicely

So, another mystery solved! We know more about how the world works, not on a level of fundamental physics, but on a level of, “because it is interesting”, and “because it is fun”, and isn’t that enough? Arthur C. Clarke, who famously gave airtime to these rocks, would, I think, have been very please with this turn of events