C Compilers Disprove Fermat’s Last Theorem

[Update: I wrote another post on this topic that may explain the underlying issues more clearly.]

Obviously I’m not serious: compilers are bad at solving high-level math problems and also there is good reason to believe this theorem cannot be disproved. But I’m getting ahead of myself. Recently — for reasons that do not matter here — I wanted to write C code for an infinite loop, but where the compiler was not to understand that the loop was infinite.  In contrast, if I had merely written

  while (1) { }

or

  for (;;) { }

most optimizing compilers would see the loop’s inability to exit and generate code accordingly. For example, given this C code:

  void foo (void)
  {
    for (;;) { }
    open_pod_bay_doors();
  }

Most compilers will emit something like this:

  foo:
    L2: jmp L2

In this case the compiler emits neither the call to open_pod_bay_doors() nor the final return instruction because both are provably not executed.

Perhaps interestingly, LLVM/Clang recognizes that this slightly obfuscated infinite loop never exits:

  unsigned int i = 0;
  do {
    i+=2;
  } while (0==(i&1));

Faced with a loop optimizer that has some brains, I decided to stop messing around and wrote a loop that should thwart any compiler’s termination analysis:

  const int MAX = 1000;
  int a=1,b=1,c=1;
  while ((c*c*c) != ((a*a*a)+(b*b*b))) {
    a++;
    if (a>MAX) {
      a=1;
      b++;
    }
    if (b>MAX) {
      b=1;
      c++;
    }
    if (c>MAX) {
      c=1;
    }
  }

This loop only terminates if it finds a counterexample to a special case of Fermat’s Last Theorem.  Fermat’s Last Theorem, of course, states that no solution exists for the equation an + bn = cn for positive integers a, b, and c and for integer n>2. Here n=3 and a,b,c are in the range [1..1000]. On a platform with 32-bit integers 1000 is a reasonable maximum because 2*10003 is not much less than 231.

It turns out that when optimizations are enabled, several compilers (LLVM/Clang 2.7, Open64-x86 4.2.3, Sun CC 5.10, and Intel CC 11.1.072) go ahead and permit this loop to terminate.  Specifically, when the loop is enclosed in a function, the compilers emit x86 assembly which looks something like this:

  fermat:
    ret

The implication, of course, is that the compiler has disproved Fermat’s Last Theorem.  Faced with this incredible mathematical discovery, I held my breath and added a line of code at the end of the function to print the counterexample: the values of a, b, and c.  Unfortunately, with their bluffs called in this fashion, all of the compilers emitted code that actually performed the requested computation, which of course does not terminate.  I got the feeling that these tools — like Fermat himself — had not enough room in the margin to explain their reasoning.

What is really going on here is that compiler optimizations and termination analysis have long been at odds.  In other words, if compilers were obligated to preserve the termination properties of the code they translate, they would be unable to perform (or would have more difficulty performing) some of the optimizations that they use to create efficient code.  A choice is being made by compiler developers — probably consciously, though it’s hard to be sure — to prefer speed over correctness. The news, however, is not all bad: Microsoft’s C compiler, the Wind River Diab C compiler, and several versions of GCC all did the right thing, changing the termination properties of none of the examples I tried.

Update from Sat 5/1: It turns out the LLVM folks have been working on this problem lately and their latest SVN now does not contain this bug.  Very nice!

Update from Sat 5/1: Someone on Reddit noticed (and one of my students confirmed) that the Microsoft compilers do have termination bugs.  The compilers in both Visual Studio 2008 and 2010 generate code for the Fermat function, but then calls to this function are dropped because it is believed to be free of side effects (this was exactly what LLVM did before they fixed the problem).

Update from Friday 4/30

I’ll try to clarify a few of the questions that have come up on Reddit and in the comments here.  Also I fixed a mistake in the statement of Fermat’s Last Theorem that someone on Reddit pointed out.  Thanks!

Q: Does this actually matter at all?

A: Yes, but in very specialized situations that usually only come up when developing embedded software.  One example is described here: the poster wants the program being simulated to hang when main() exits, but LLVM deletes the loop that was intended to hang up the processor.  The workaround was to compile the code with optimizations turned off.  Another example happens when an embedded system has updated its firmware and wants to do nothing until the watchdog timer reboots the processor into the new version.  It’s no coincidence that gcc and the Wind River C compiler — both of which are heavily used in the embedded world — get termination right.

Q: Since infinite loops are bad style, isn’t it OK for the compiler to terminate them?  Shouldn’t people be putting the CPU to sleep, blocking the running thread, or whatever?

A: First, not all programs have an operating system or even a threading system to call out to. Embedded software commonly runs on the bare metal.  Second, the meaning of a program is defined by the language standard and style has nothing to do with it.  See my earlier post The Compiler Doesn’t Care About Your Intent.

Q: Does the C standard permit/forbid the compiler to terminate infinite loops?

A: The compiler is given considerable freedom in how it implements the C program, but its output must have the same externally visible behavior that the program would have when interpreted by the “C abstract machine” that is described in the standard.  Many knowledgeable people (including me) read this as saying that the termination behavior of a program must not be changed.  Obviously some compiler writers disagree, or else don’t believe that it matters.  The fact that reasonable people disagree on the interpretation would seem to indicate that the C standard is flawed.  In contrast, the Java language definition is quite clear that infinite loops may not be terminated by the JVM.

Q: Are you saying the compiler should do termination analysis?  That’s impossible by trivial reduction to the halting problem.

A: Termination analysis does not need to be part of the compiler at all. However, I (and others) would claim that the compiler should perform a termination analysis of any useless loop before deleting it.  Although the general problem is not computable, many specific instances can be easily solved.

Q: Does the Fermat code in this post execute any signed integer overflows or other undefined behaviors?

A: I don’t believe so.

Update from Saturday 5/1

Q: Didn’t you know Fermat’s Last Theorem was proved in 1995?

A: I did know that.  Since I got my math degree in 1995, it would have been very difficult for me to miss this event :). I was making a weak joke and also referring to the fact that proofs, especially complicated ones, can contain errors. In fact, as someone noted in the comments, Wiles’ initial proof was wrong. Also note that the n=3 special case was proved much earlier, in 1770.

Q: What’s the best workaround if you really want an infinite loop in C?

A: As several people have pointed out, looping on a volatile-qualified variable is probably the best choice. But keep in mind that compilers don’t always respect volatile….

One more update from Saturday 5/1

Here’s a fun complete program that is more compelling than the code above because it explicitly uses a return value from the “theorem disproved” branch of the code:

int fermat (void)
{
  const int MAX = 1000;
  int a=1,b=1,c=1;
  while (1) {
    if (((a*a*a) == ((b*b*b)+(c*c*c)))) return 1;
    a++;
    if (a>MAX) {
      a=1;
      b++;
    }
    if (b>MAX) {
      b=1;
      c++;
    }      
    if (c>MAX) {
      c=1;
    }
  }
  return 0;
}

#include <stdio.h>

int main (void)
{
  if (fermat()) {
    printf ("Fermat's Last Theorem has been disproved.\n");
  } else {
    printf ("Fermat's Last Theorem has not been disproved.\n");
  }
  return 0;
}

Here’s what the Intel and Sun compilers have to say:

regehr@john-home:~$ icc fermat2.c -o fermat2
regehr@john-home:~$ ./fermat2
Fermat's Last Theorem has been disproved.
regehr@john-home:~$ suncc -O fermat2.c -o fermat2
"fermat2.c", line 20: warning: statement not reached
regehr@john-home:~$ ./fermat2
Fermat's Last Theorem has been disproved.

Open64-x86 and LLVM/Clang 2.7 have the same behavior.  Although plenty of folks in the peanut gallery disagree, it seems perfectly clear to me that this is a serious error in these compilers.  I mean, why return 1?  Is that worse or better than returning 0?  Neither result makes any sense.

Belated Introduction

When I started this blog I didn’t write an introductory post because, of course, I had no readers.  Lately Google Reader indicates that I have a bit over 100 subscribers so I thought it might be time for a quick introduction.

For a long time I was uninterested in blogging because I thought the only good blogs were highly focused ones, and I wasn’t interested in writing a blog like that.  Also, I wasn’t interested in taking on a new long-term time commitment since I already have enough of these.  Finally, the nature of my job (and life) means that I’ll likely post in a bursty fashion that would seem annoying to readers.

Obviously I eventually decided to start a blog.  The main triggering event was that I became extremely bored with the FSP blog (reading it is like going to a faculty meeting) which used to be perhaps my favorite blog and was the original source of my observation that a good blog should be focused.  In other words I decided that posting about random topics was OK and perhaps even interesting. Also, I realized that a blog doesn’t need to a be a long-term commitment: if I get tired of blogging (and I expect this will happen in some number of years) I’ll just drop it until it seems interesting again.  The advent of good RSS readers means that bursty posting is not a problem (my view of the web solidified in roughly 1996 so I was effectively the last person to become a user of these readers).  Finally, I’m on sabbatical this year and have been taking a bit more time to think about the big picture than I used to.  Thinking, for me, usually leads to the desire to write.  It’s a common flaw among academics.

Anyway, back to the intro.  I’m an associate processor of Computer Science at the University of Utah, coming up on the end of my first sabbatical.  The sabbatical arrived at about the right time: the tenure runup left me feeling a little burned out and ready to think about what kind of directions I’d like to take my career in, in the long run. I teach courses in computer systems, such as operating systems, embedded systems, and advanced compilers.  I enjoy teaching a lot and it only becomes a drag when students perform poorly or are apathetic. My research is broadly in computer systems and is specifically about compilers and embedded systems.  My work is usually centered on software tools, which are based on the idea that we can use software to create better software.  It’s a lot of fun and it’s a great thing to be doing from academia since in industry they’re generally very focused on the next product: they don’t take the time to create tools that can add a lot of value in the longer run. I supervise a group that currently has six students.  Running a group is fun for multiple reasons.  First, it’s neat to watch students change from their early, tentative selves into mature and confident researchers.  Second, the students (on a good day) act as amplifiers, getting far more work done than I could ever do alone. Most of the time I have the best job in the world.  The thing I like most is that I can choose the problems I work on.  If I want to focus for multiple years on something that is of little interest to most people (e.g. compiler bugs), I can do that.  If I wish to bounce among projects in a flighty way, I can do that too.  It’s liberating.

I’m extremely fortunate to have healthy and inquisitive children, Jonas is 5 and Isaac is 3.  It’s hard to imagine life without them.  My wife Sarah is also a professor at Utah.  She, I think, slacked off on her work a bit when the kids were little (out of necessity — Utah’s parental leave policy only solidified after the boys were born) but lately she has been making up for lost time and is working on tons of projects; her career seems to be progressing well.  She’ll be lucky if she can spend another 10-15 years doing good research before (I predict) being roped into some sort of administrative job.

This summer we’ll have lived in Utah for 10 years, and it has been great.  Salt Lake City itself is OK, probably not much different from Indianapolis or Kansas City, but the setting is amazing.  Mountains can be found in all directions, and in fact the state contains entire mountain ranges that few people have heard of (The Wah Wahs?  The Deep Creeks?  The Silver Island Range?).  Just east of the city, and right outside my front door, is the Wasatch Range, climbing 7000′ above the city.  A bit further away are the red rock deserts including the incredible San Rafael Swell which is barely close enough to visit on a day trip, but really much better suited to 3-4 day car camping trips. Utah’s vast West Desert fills around a third of the state, in this area you can camp out for days and drive hundreds of miles on dirt roads without seeing anyone.  My list of trips to take in Utah never gets shorter since every time I take a trip, I see a half dozen more places to go.  My hiking buddy (and local guidebook author) John Veranth has spent hugely more time outdoors in Utah than I have, and says the same thing is true for him too.

Anyway, to sum up: this blog is probably going to be active for a finite time.  Posting will be bursty and on totally random topics :).

Into the Brooks Range, Part 3

[Continued from Part 1 and Part 2.]

August 6 — We See Bears

Finally we were back to walking a wide river valley not unlike our first day hiking. To stay in the river bed, we had to pass through some dense thickets of willow brush. Since it’s very bad to surprise a brown bear, we made lots of noise. Later, while walking along the riverbank, Eric heard something and motioned for us to stop. Down below on the gravel, a big female bear was standing up on hind legs, making warning noises: we had violated her personal space, which in the arctic encompasses a much larger area than for example in southeast Alaska where the bear density is higher. She calmed down after we stopped coming nearer, and we saw that she had three cubs nearby. Eventually, they wandered off into the willow scrub and we moved on. Later, we camped in a really pretty site up on the river bank.

I hadn’t spent time with brown bears before this trip, and it was interesting to do so. Most of the time, of course, we were managing bear risk as opposed to dealing with actual bears. For example, we carried a separate cook tent and never stored food in our tents overnight. We each had a can of pepper spray at hand basically at all times. Statistically speaking, this stuff is supposedly more effective than carrying a firearm, and certainly it poses less risk to the carrier (though there always seemed to be the possibility of being forced to discharge it into the wind). Even Eric, who has extensive bear experience, was hard-pressed to explain how one might distinguish a mock charge from a real charge before it was too late. A few times we joked that if the bears had their act together, they’d deploy one in front while another snuck up behind us. However, fundamentally, this kind of tactic is unnecessary since a single large bear could easily kill all of a five-person group like ours. However, the Brooks Range bears are not at all habituated to humans; their suspicion about the new shapes and smells causes them to back off more often than not, and attacks are rare (though not unheard of).

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All photos © William B. Thompson or John Regehr 2009.

August 7 — A Warm Day and a Swim

This was a sunny, warm day with generally easy walking. The Ivishak was finally deep enough to contain plausible fishing holes — Ben had carried his fly rod the whole trip waiting for this. But no luck, it was too early for the arctic char sea run. One excellent location had deep, clear water and Ben, Eric, and I couldn’t resist a quick dip to wash off a week’s worth of grime and sweat. I’d guess the water was around 50-55 degrees: cold enough to trigger the gasp and hyperventilation reflexes, but not producing a strong feeling of impending cardiac arrest.

In the evening we found a gorgeous campsite on the river bank and Ben fished again. Around 11:30 Eric started yelling for Ben to come up to camp: a bear was prowling around on the opposite bank. We watched it foraging for a while: it was acting natural and hadn’t heard us over the river noise. Before turning in we banged some pots and pans to make sure it knew we were there: this got its attention right away and it stood on hind legs to try to figure us out. It lost interest quickly and wandered off, but even so most of us too a pot or pan to bed that night as a noise-maker in case he came back to investigate further. As far as we know, he didn’t come back.

Throughout the trip, everyone else did a better job than I did in spotting animals; my vision is about 20/50 and I decided not to wear corrective glasses most of the time. Also, as Sarah enjoys pointing out, I’m not the most observant person in the world. Eric on the other hand has 20/15 vision and his job depends on spotting wildlife in difficult conditions. Throughout the trip we were seeing plenty of caribou and raptors plus a single moose; these sightings quickly became routine and I’m only mentioning the more interesting ones.

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All photos © William B. Thompson or John Regehr 2009.

August 8 — Last Day Walking and a Wedding

Our last walking day was cloudy and cool. The steep valley walls made it best to stick to the gravel bars and we spent most of the day in sandals. The frequent river crossings were uncomfortably cold. Also, as more side drainages added water to the Ivishak, and as it rained around us, the crossings got deeper. They weren’t scary but certainly we had to focus on maintaining our footing in the current. By the end of the day, crossing the main channel would have been dicey.

Finally we arrived at the big alluvial fan containing the takeout air strip. Although we were certain the location was correct (Shannon had been there before, as the starting point of a rafting trip) we had no luck finding any wheel tracks. Shannon went out and put a makeshift windsock on the part of the fan where she thought Kirk would land.

In the evening we had a fun surprise: Shannon and Ben had decided to get married. They asked Eric if he would marry them, and he was happy to (an adult Alaska resident can officiate at a wedding in the state). It was a nice ceremony in the most beautiful possible setting. Afterwards, we had drinks — sort of. Ben had stashed a mini bottle of gin that we mixed up with fizzy electrolyte drink tablets.

Shannon and Ben are a neat couple. They live in a cabin near Denali NP. They do various kinds of work such as guiding in the summer and working in Antarctica in winter. It sounds like an interesting life and I like to secretly think that in some alternate universe I’d have done this kind of thing instead of, or at least prior to, becoming a professional academic.

Overnight, a front rolled through and we had hours of high winds mixed with rain and sleet. We were fortunate to have set up camp in the lee of a small rock outcrop, but even so the biggest gusts brought my tent ceiling more than halfway down to my head. For a while I was pretty sure the tent would collapse or else go airborne. However, it did not, perhaps because I had added three extra guy lines. Nobody slept much and in fact around midnight we found ourselves all outdoors in the miserable driving rain putting extra-large rocks on our tent stakes. Ben and Shannon’s tent had partially blown down and they had to realign it; Bill and Eric had pretty solid tents and I — having probably the least weather-worthy tent — was very lucky to have set it up the right way.

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All photos © William B. Thompson or John Regehr 2009.

August 9 — In a Snowstorm and Not Getting Out

In the morning the winds had died and we found the snow line barely above camp. The cloud level was only a few hundred feet higher. Still, the weather was improving and we hoped the plane could make it in. Eric and I took a short hike, but we didn’t want to wander far in case Kirk arrived.

As the day progressed the weather deteriorated and we realized we were almost certainly in for an extra night. We moved the tents into a slightly more sheltered configuration in case the winds picked up. In the afternoon it began to snow pretty hard and we spent the rest of the day chatting in the cook tent and napping. We had little reserve food and had an extremely light dinner before going to bed hungry.

During the night it kept snowing. My light tent let in every little gust of wind and I started to get cold. As part of a weight-saving plan I brought only a 30 degree sleeping bag, knowing that it would make hiking easier but that I would suffer if things went badly. So I shivered, wearing basically every single piece of clothing I had brought along, including the fleece top that had been serving as a pillow.

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All photos © William B. Thompson or John Regehr 2009.

August 10 — Snow and Sun and Out

We woke to perhaps six inches of snow, which represented a new obstacle to getting out: the bush pilot can’t land if he can’t see the terrain. Someone told a story of a bush pilot who overflew his clients after a snowstorm and dropped them a shovel, circling while they cleared the strip. With the threat of a second extra night out, we rationed food pretty severely and stayed hungry.

As the day progressed it partially cleared and the snow began to burn off. It was incredibly pretty, definitely worth the discomfort and inconvenience. Sometime in the morning we heard a plane and rushed to take down tents — but the plane passed overhead. The rest of the day we read and napped, not wanting to stray far from the air strip. By late afternoon we were resigned to another night, but then around 6:30 Kirk showed up. We packed up and ran for the plane, not wanting to keep him there any longer than we had to. The flight out to Arctic Village was spectacular, with clear air this time.

It turned out Kirk had tried hard to get us out the previous day, but had been turned back by severe turbulence. His brother had also tried, from a different direction, also unsuccessfully. This was something interesting to learn about bush pilots: their clients’ lives are in their hands and they take this responsibility very seriously. This, in combination with the levels of skill and experience of the best pilots, helped put the cost of bush plane travel into perspective (it constituted one of the major parts of the total trip expense).

At Arctic Village it was clear that we weren’t going any further. An old guy with an ATV gave Eric and me a ride into town, where by some stroke a luck the store was still open. We stocked up on high-calorie foods and walked back to the air strip to wait for Ben and Shannon. When they arrived, we ate a huge amount of spaghetti and candy bars. Unfortunately, the little visitor center was locked up, so we slept on its covered porch. I burned a bit of sat phone time to tell Sarah all was well, luckily she had been adequately briefed on the possibility we’d be stranded out.

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All photos © William B. Thompson or John Regehr 2009.

August 11 — Heading Home

Wright Air Service was aware of our situation and sent an extra plane up to Arctic Village, putting us in Fairbanks by noon. Now we were two days late and Bill had missed his flights home. My parents were in Fairbanks with a car and Eric and I planned to ride down to Anchorage with them. However, Eric was stressed about work and stuff and flew home instead. I hadn’t yet missed my scheduled flight, a redeye late on the 11th, so we had a leisurely drive south. Luckily I had taken a shower in Bill’s hotel room or else we’d probably have driven with the windows open the whole way.

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All photos © William B. Thompson or John Regehr 2009.

Notes

All pictures are by me or Bill Thompson. If you care, Bill’s pictures all have filenames starting with “dsc” whereas mine all start with “picture.” I shot a number of panoramas too.  We used identical equipment: Nikon D90 with the kit lens, an 18-105 zoom. Bill’s landscape pictures often look better than mine (at least partly) because he shot in raw format and then used a good converter, whereas I let the camera do everything.

Most of my gear performed admirably; the fact is that today’s mid-range equipment is overkill for almost any normal use. My favorite items were the Smartwool midweight wool long underwear top and bottom that became more or less a second skin for the cold parts of this trip. Completely comfortable, and not stinky like synthetic. My puffy synthetic Patagonia jacket was really nice as well, and way too warm for anything except sitting around or sleeping. The Arcteryx Bora 95 pack was awesome: big and bombproof. I have no idea when mine was made, I picked it up used from a guy who was moving away from SLC. Like all Arcteryx products, these are more or less prohibitively expensive to buy new. The La Sportiva Trango Trek GTX boots I took were about perfect: decent support, good waterproofness, and not unbearably heavy. After over a week in very tough country they look and work about like new. To carry water I took a single 1-liter platypus bottle, these are super light and can be collapsed to pocket-size when empty. Probably the main drawbacks are the easily-dropped cap and the narrow opening which makes it slow to fill. My old Cascade Designs Z-Rest performed about as well as it ever does, which is to say that it’s light and respectably comfortable, but really wants to be placed on soft ground.

A few items I was less happy with included the 30 degree Marmot synthetic sleeping bag that I got for cheap, which had very little loft. It weighed about the same as Shannon’s 5 degree bag from Western Mountaineering, which had so much loft it looked inflated. Seriously, you can bounce a quarter off that kind of bag. My Sierra Designs Sirius 2 tent was decent overall, but the open vestibules were a major drawback. First, they provided very little shelter for keeping items dry outside the tent. Second, they acted like wings to catch the wind: not good. Also, this tent is pretty short; I’m six feet and had to lie diagonally to keep from pressing head and feet against the tent ends.

Although I spend a lot of time outdoors and car camp as frequently as possible, my previous backpacking experience was minimal — probably no more than two weeks total, prior to this trip. So it was fun to refine my techniques and learn new tricks. One of the most useful was rigging a clothesline inside the tent so that socks and such could be dried overnight. Another good one was putting clothing and other items at the head and foot of my sleeping bag to keep it from getting wet from condensation due to touching the outside of the tent. A great pair of camp shoes can be improvised out of a pair of tevas and a pair of neoprene socks.

Into the Brooks Range, Part 2

[Continued from Part 1, also see Part 3.]

August 3 — Over the Arctic Divide

Our third hiking day took us over a 5700′ mountain pass where the Wind, Ivishak, and Ribdon river drainages converge. Since the creek-bed of our side drainage was totally impassable, we climbed steep talus slopes, leaving the last tundra behind. Eventually the rocks leveled out and we came to a high bowl containing the Seefar glacier, which appears to be dead. We stopped and climbed its moraines a while: the basin was really spectacular and Bill and I were bummed that the thick smoke eliminated the possibility of good photographs.

To exit the Seefar bowl and get to the pass, we had to bypass a small waterfall on angle-of-repose scree. It was doable, but dicey with the big packs; we went one-by-one to avoid kicking rocks down on each other. After a bit more walking we reached the pass itself, which might as well have been on Mars, it’s as desolate a location as I’ve ever seen. It would have been nice to stick around for a while and climb the unnamed 7500′ peak nearby, but we wanted to get down to a suitable campsite. The mountains in this area have no doubt been climbed before, but definitely not very many times.

The talus slog down from the pass on the Ivishak side was steep, loose, and not much fun. It contained, however, a memorable site: the remains of a Grumman Goose that crashed in 1958, killing Clarence Rhode, the regional director of the US Fish and Wildlife Service, and two others, triggering Alaska’s then-biggest-ever search and rescue operation. The search was unsuccessful and the fate of the plane and its three occupants was a mystery until 1979 when the wreck was found by hikers (Debbie Miller’s book Midnight Wilderness describes this). The bent propellers showed that the plane was powered when it hit the mountain. The arctic climate preserves things well: a can of coffee near the wreck still contained recognizable coffee grounds, and a typed “permit to take wolves and/or coyotes from an airplane” was perfectly legible after 50 years outdoors. The site was disconcerting, probably especially so for Eric — a current employee of the US Fish and Wildlife service and a heavy user of small aircraft in the arctic.

We dropped down to a confluence of small sub-drainages that would have been a very small, rocky campsite, then continued until being stopped by a waterfall. We bypassed this and went to the next confluence, which contained a beautiful meadow where we stopped, exhausted after a very long day.

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All photos © William B. Thompson or John Regehr 2009.

August 4 — Layover

Our schedule had some slack built into it for weather and other difficulties. However, now that we were over the pass most of the risk had disappeared so we decided to take a rest day. Unfortunately, we had descended so far from the pass that nobody had energy to hike back up to it to climb some peaks. We poked around, read books, and generally enjoyed a gorgeous sunny day outdoors. This confluence was some sort of caribou highway and small herds walked past our campsite all day. I poked back upstream to the waterfall that had blocked us the evening before; it was gorgeous. Bill and I hiked around the next bend in the river and saw a group of Dall sheep.

On this trip Eric read much of The Brothers Karamazov, which seemed like a fine choice. I brought along Little, Big, which I’d read before and loved. However, Shannon brought perhaps the best book: A Naturalist’s Guide to the Arctic. It is targeted at the interested layperson, and is packed with information about the kinds of things one wonders about while walking around this part of the world. What is the difference between hibernation and torpor? What is a tussock, exactly? What is the relationship between the moon’s phases and the “moon stays up” periods that correspond to the midnight sun? Among three PhDs, it is possible to speculate endlessly without any actual information, so this book was a godsend.

Although we missed the midnight sun, we didn’t have any real darkness on this trip. I’m used to fall camping trips in Utah where the nights are quite long; it felt really strange to not pack any light source at all for an extended backpacking trip, but that’s what we did. I’ve never had trouble sleeping in the light, luckily. I almost didn’t take a wristwatch on this trip, but I was really glad I did: lacking daylight-based cues about the time, I often had no idea at all what time it was.  Each day shortly after midnight, the sun rose in the southeast, made a near-complete circuit of the sky, and then set in the southwest around midnight. I took a very small tube of sunblock on this trip, guessing that the low sun angle plus likely clouds and rain would make sunburns unlikely; this wasn’t a very good decision.

Hiking in Utah, a person gets used to always filtering, purifying, or boiling water. Each of these is a pain, and one of the things I loved about the Brooks Range is that water is clean enough to drink straight from any moving source.

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All photos © William B. Thompson or John Regehr 2009.

August 5 — Out of the Mountains

After the rest day, the going was easier. Our bodies were getting used to the packs, the packs were getting lighter as we ate food and burned fuel, and we were hiking downhill. However, in the morning we were still in a seriously mountainous area and the stream would often constrict to a narrow gorge. Luckily the tundra benches were wide and fairly level, so we stayed on them most of the day. By the time we made camp, we were back into a fairly wide river valley.

It was claimed, by people on this trip, that a certain kind of tundra moss makes a passable substitute for toilet paper. Not the dry, rough top side of the moss, but rather the soft, damp underside. I just wanted to mention this and won’t bother the reader with details.

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All photos © William B. Thompson or John Regehr 2009.

[Continued in Part 3.]

Into the Brooks Range, Part 1

[Also see Part 2 and Part 3.]

In Summer 2009 I went on a 1.5-week backpacking trip in the Alaskan arctic with my brother Eric, my colleague and hiking buddy Bill, and our guides Shannon and Ben from Arctic Treks. It was an amazing trip through a very rugged part of the world. Not only did we not see any other people, but most days we saw no signs that people had ever been there. If the civilized world had ended, we wouldn’t have found out until nobody came to pick us up.

July 31 — Getting There

It took most of two days to get from Salt Lake City to our starting point: the highest airstrip on the Wind River, on the South Slope of the Brooks Range’s Philip Smith Mountains. Bill and I first flew up to Fairbanks, through Anchorage. Descending into Fairbanks, we couldn’t see anything at all due to smoke from wildfires, and it was lucky that we even got there — earlier in the day they were turning planes back. After dinner we did some last minute gear-sorting and met up with Ben and Shannon, who gave us our share of the group gear. We’d been aiming for 50 pounds but with a full load of fuel and food, mine was around 60; Ben took more than his share of gear and had 70 pounds. Most everything we took ended up being useful or necessary, our main luxury was a tent apiece for Eric, Bill, and me. As Eric puts it, “If weight is so much of a concern that I have to share a tent with a dude, I’m not going.” I hadn’t managed to shake a bad cold, and decided to leave behind a half-bottle of bourbon. I went to bed early; Eric, who lives in Anchorage, had to work late and didn’t show up until early morning.

Next morning we went to the air taxi company to take a single-engine Cessna turboprop to Arctic Village, a town of less than 200 people next to the Chandalar River at the foot of the Brooks Range. The flight could have been spectacular but we hardly saw anything, again due to smoke. The big gravel air strip is maybe a half mile out of town; we dropped packs and slapped bugs waiting for the bush plane. Bill, who has done a number of trips in the arctic, said it could be 15 minutes, could be all day. It wasn’t too long until Kirk Sweetsir and his little Cessna arrived, but we still had to wait — he first had to drop off a couple who had flown out of Fairbanks with us, who were hiking over the continental divide and then packrafting all the way the Arctic Ocean, ending at Kaktovik, a pretty serious trip. When our turn came, Kirk flew us over (or rather through) a rugged and forbidding patch of mountains instead of heading up the Junjik river valley; it was spectacular, thought still very hazy. Before landing, Kirk had spotted a moose and a brown bear. As the plane flew off it began to drizzle and we pitched tents on boggy ground near the airstrip. A little later, Kirk came back with Eric and Ben and then we were left alone in the wide river valley, probably 45 miles from the nearest people. Before Kirk left for the second time, Shannon double checked with him regarding the pickup day and location. This seemed like a fine idea.

The Alaska definition of “airstrip” is something like “someone landed there once.” Guys like Kirk have an amazing job but the level of flying skill, rapid risk assessment, and luck required to grow old in that line of work must be fantastic.

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All photos © William B. Thompson or John Regehr 2009.

August 1 — Up the Wind River Valley

Our first walking day was in the miles-wide Wind River valley. Lacking trails, the main tension in this part of the world is between walking in the river channel, which is easy when on gravel bars but involves lots of river crossings and may be very brushy, and walking the bank, which is generally brushy, hilly, tussocky, and boggy. Tussocks are pillar- or mushroom-shaped tufts of grass that are raised about a foot above the surrounding terrain. If you try to step on them, they tend to flip over or otherwise give way, creating risk of injury. If you try to step between them, you also get unsure footing — the tussocks are so close together you can’t clearly see the gaps. Additionally, the gaps are usually filled with water or mud. You might think (at least, I certainly thought) that a sloped river bank would be well-drained, but somehow in this part of Alaska that is not the case. Although a tussock field looks inviting from afar, since the surface grass is nearly flat, it can be an amazingly effective obstacle to progress. Even a strong hiker wouldn’t expect to make more than about a half mile per hour in a nice, flat tussock field. Happily, on this trip we were mostly in the high country and didn’t get killed by tussock travel.

We ended up spending the first day mainly on the gravel bars, and except for Ben we stayed in sandals since otherwise the braided river would have forced multiple changes of footwear per hour. Ben had gaiters that appeared to keep his boots dry if he crossed quickly. This was the first of August and the river was low, so the crossings were inconvenient instead of scary.

We camped close to where the valley forked three ways and watched a snowstorm hitting the high mountains. Eric and I walked up a big alluvial fan and found a nice place to sit where we could be above the mosquitoes and scan the valley for bears. We didn’t see any, but had a good talk. This was one of the things I had been hoping would happen on this trip; Eric and I aren’t particularly close and in fact I’m not sure I have much of a handle on what kind of adult he’s become. Of course I still think of him as my little brother, though he is 35.

Eric is a field researcher for the US Fish and Wildlife Service, studying polar bears. As far as I can tell, it is the perfect job for him because he enjoys organization (capture work has hellish logistical requirements), is talented at statistics and modeling, and also has a strong commitment to field work. The field work is most often flying over arctic sea ice in a helicopter, darting a bear, and then landing to weigh it, take blood samples, and whatever else. His videos from bear-darting operations are amazing: a white bear is zig-zagging around on white ice under a white sky. Anyway, Eric has a ton of polar bear stories including some that are scary. It’s a neat job.

Throughout this trip, the mosquitoes were a constant low-grade annoyance. After too many years in Utah’s deserts my pain threshold for these pests is pretty low, and I ended up wearing my mosquito net for a while this second evening. Luckily I never felt the need to put it on after that, and in fact only seldom applied any DEET. In early August the mosquito season is just about over and by Alaska standards they were not at all bad. The Brooks Range mosquito adopts a slightly different strategy than those I’m used to: it wants to spend a bit of time hovering about a foot above your head before diving in to strike. I eventually learned that if I found a location where they couldn’t hover above me — such as sitting in my tent with the doors wide open — they wouldn’t bother me at all. Something nice about the arctic is that mosquitoes are the only pest: there are no chiggers, ticks, biting flies, or any of the other little critters that can make life difficult.

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All photos © William B. Thompson or John Regehr 2009.

August 2 — Into the Mountains

On our second moving day the air was clear of smoke: the only really clear day on this whole trip. We walked up the middle fork of the three-way split, which rapidly narrowed as we made progress. Walking was easy on the benches, and the river crossings were at most calf-deep. We saw a wolverine pretty close, which was fun: it was chasing something on opposite side of the creek and didn’t pay much attention to us. By the end of the day the valley had become a canyon and we were walking on talus, having gained enough elevation to leave most vegetation behind. We camped next to a beautiful but imposing waterfall that emerged from the side-drainage we had to hike into the next day.

You might wonder why three strong hikers who can read a map and have plenty of wilderness experience, arctic experience, and even hands-on bear experience would take a guided trip instead of rolling it ourselves. One reason is logistics: we’re busy people who live far from Alaska (well, Eric lives up there but he wasn’t actively involved with planning this trip) and having people on-location getting the gear together and setting up the bush plane was super handy. As our trip progressed I learned that it is pretty damn nice having someone else cook and clean up. In the mornings we generally slept quite late (arctic summer trips seem to tend in this direction due to the near-constant daylight) and Shannon or Ben always had coffee going when we got up. It is not a bad life to get out of the tent long after the morning chill has burned away and then sit around for an hour looking at maps, drinking two pots of coffee, and chatting. Shannon and Ben turned out to be great company, and certainly Bill and I, and Eric and I, would have gotten on each others nerves if it had been just the three of us. Finally, adding people lightens loads (early on, we hadn’t been sure Eric could come) and greatly increases bear safety.

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All photos © William B. Thompson or John Regehr 2009.

[Continued in Part 2.]