“Surely You’re Joking, Mr. Feynman”: Adventures of a Curious Character

thing.

To demonstrate that the locks meant nothing, whenever I wanted somebody’s report and they weren’t around, I’d just go in their office, open the filing cabinet, and take it out. When I was finished I would give it back to the guy: “Thanks for your report.”

“Where’d you get it?”

“Out of your filing cabinet.”

“But I locked it!”

“I know you locked it. The locks are no good.”

Finally some filing cabinets came which had combination locks on them made by the Mosler Safe Company. They had three drawers. Pulling the top drawer out would release the other drawers by a catch. The top drawer was opened by turning a combination wheel to the left, right, and left for the combination, and then right to number ten, which would draw back a bolt inside. The whole filing cabinet could be locked by closing the bottom drawers first, then the top drawer, and spinning the combination wheel away from number ten, which pushed up the bolt.

These new filing cabinets were an immediate challenge, naturally. I love puzzles. One guy tries to make something to keep another guy out; there must be a way to beat it!

I had first to understand how the lock worked, so I took apart the one in my office. The way it worked is this: There are three discs on a single shaft, one behind the other; each has a notch in a different place. The idea is to line up the notches so that when you turn the wheel to ten, the little friction drive will draw the bolt down into the slot generated by the notches of the three discs.

Now, to turn the discs, there’s a pin sticking out from the back of the combination wheel, and a pin sticking up from the first disc at the same radius. Within one turn of the combination wheel, you’ve picked up the first disc.

On the back of the first disc there’s a pin at the same radius as a pin on the front of the second disc, so by the time you’ve spun the combination wheel around twice, you’ve picked up the second disc as well.

Keep turning the wheel, and a pin on the back of the second disc will catch a pin on the front of the third disc, which you now set into the proper position with the first number of the combination.

Now you have to turn the combination wheel the other way one full turn to catch the second disc from the other side, and then continue to the second number of the combination to set the second disc.

Again you reverse direction and set the first disc to its proper place. Now the notches are lined up, and by turning the wheel to ten, you open the cabinet.

Well, I struggled, and I couldn’t get anywhere. I bought a couple of safecracker books, but they were all the same. In the beginning of the book there are some stories of the fantastic achievements of the safecracker, such as the woman caught in a meat refrigerator who is freezing to death, but the safecracker, hanging upside down, opens it in two minutes. Or there are some precious furs or gold bullion under water, down in the sea, and the safecracker dives down and opens the chest.

In the second part of the book, they tell you how to crack a safe. There are all kinds of ninny-pinny, dopey things, like “It might be a good idea to try a date for the combination, because lots of people like to use dates.” Or “Think of the psychology of the owner of the safe, and what he might use for the combination.” And “The secretary is often worried that she might forget the combination of the safe, so she might write it down in one of the following places—along the edge of her desk drawer, on a list of names and addresses … and so on.

They did tell me something sensible about how to open ordinary safes, and it’s easy to understand. Ordinary safes have an extra handle, so if you push down on the handle while you’re turning the combination wheel, things being unequal (as with locks), the force of the handle trying to push the bolt down into the notches (which are not lined up) is held up more by one disc than another. When the notch on that disc comes under the bolt, there’s a tiny click that you can hear with a stethoscope, or a slight decrease in friction that you can feel (you don’t have to sandpaper your fingertips), and you know, “There’s a number!”

You don’t know whether it’s the first, second, or third number, but you can get a pretty good idea of that by finding out how many times you have to turn the wheel the other way to hear the same click again. If it’s a little less than once, it’s the first disc; if it’s a little less than twice, it’s the second disc (you have to make a correction for the thickness of the pins).

This useful trick only works on ordinary safes, which have the extra handle, so I was stymied.

I tried all kinds of subsidiary tricks with the cabinets, such as finding out how to release the latches on the lower drawers, without opening the top drawer, by taking off a screw in front and poking around with a piece of hanger wire.

I tried spinning the combination wheel very rapidly and then going to ten, thus putting a little friction on, which I hoped would stop a disc at the right point in some manner. I tried all kinds of things. I was desperate.

I also did a certain amount of systematic study. For instance, a typical combination was 69-32-21. How far off could a number be when you’re opening the safe? If the number was 69, would 68 work? Would 67 work? On the particular locks we had, the answer was yes for both, but 66 wouldn’t work. You could be off by two in either direction. That meant you only had to try one out of five numbers, so you could try zero, five, ten, fifteen, and so on. With twenty such numbers on a wheel of 100, that was 8000 possibilities instead of the 1,000,000 you would get if you had to try every single number.

Now the question was, how long would it take me to try the 8000 combinations? Suppose I’ve got the first two numbers right of a combination I’m trying to get. Say the numbers are 69-32, but I don’t know it—I’ve got them as 70-30. Now I can try the twenty possible third numbers without having to set up the first two numbers each time. Now let’s suppose I have only the first number of the combination right. After trying the twenty numbers on the third disc, I move the second wheel only a little bit, and then do another twenty numbers on the third wheel.

I practiced all the time on my own safe so I could do this process as fast as I could and not get lost in my mind as to which number I was pushing and mess up the first number. Like a guy who practices sleight of hand, I got it down to an absolute rhythm so I could try the 400 possible back numbers in less than half an hour. That meant I could open a safe in a maximum of eight hours—with an average time of four hours.

There was another guy there at Los Alamos named Staley who was also interested in locks. We talked about it from time to time, but we weren’t getting anywhere much. After I got this idea how to open a safe in an average time of four hours, I wanted to show Staley how to do it, so I went into a guy’s office over in the computing department and asked, “Do you mind if I use your safe? I’d like to show Staley something.”

Meanwhile some guys in the computing department came around and one of them said, “Hey, everybody; Feynman’s gonna show Staley how to open a safe, ha, ha, ha!” I wasn’t going to actually open the safe; I was just going to show Staley this way of quickly trying the back two numbers without losing your place and having to set up the first number again.

I began. “Let’s suppose that the first number is forty, and we’re trying fifteen for the second number. We go back and forth, ten; back five more and forth, ten; and so on. Now we’ve tried all the possible third numbers. Now we try twenty for the second number: we go back and forth, ten; back five more and forth, ten; back five more and forth, CLICK!” My jaw dropped: the first and second numbers happened to be right!

Nobody saw my expression because my back was towards them. Staley looked very surprised, but both of us caught on Very quickly as to what happened, so I pulled the top drawer out with a flourish and said, “And there you are!”

Staley said, “I see what you mean; it’s a very good scheme”—and we walked out. Everybody was amazed. It was complete luck. Now I really had a reputation for opening safes.

It took me about a year and a half to get that far (of course, I was working on the bomb, too!) but I figured that I had the safes beaten, in the sense that if there was a real difficulty—if somebody was lost, or dead, and nobody else knew the combination but the stuff in the filing cabinet was needed—I could open it. After reading what preposterous things the safecrackers claimed, I thought that was a rather respectable accomplishment.

We had no entertainment there at Los Alamos, and we had to amuse ourselves somehow, so fiddling with the Mosler lock on my filing cabinet was one of my entertainments. One day I made an interesting observation: When the lock is opened and the drawer has been pulled out and the wheel is left on ten (which is what people do when they’ve opened their filing cabinet and are taking papers out of it), the bolt is still down. Now what does that