Tuesday, January 17, 2017

A brief disruption, and a small explanation.

The other morning I went to the beach to walk for a few miles, as per usual, with the minimum accompaniment of my wife, our dog, and my Rubik’s brand cube that only goes to the beach. It’s a newer cube (2013 or later) with plastic tiles and the newer mechanism. This particular cube is rather loose as a fair amount of sand has gotten in it over the past couple of years and worn away at the plastic. Periodically I rinse the cube out, and then re-lubricate it, and it’s OK again.

We meet a lot of people at the beach. Most of them are tourists, but I suppose that there are a fair number of locals. Every once in a while I get to talk to people about the cube, but usually I have to already be stopped by dog socialization or people who need to ask my wife about some dog-related thing. This particular morning I got an unusual request – a kid, I can only assume that he's 10 or so - asked me if he could attempt to solve it. Since I run into other solvers so infrequently, I immediately hand the kid the cube, waiting to see what his ability level is. Disappointingly, he immediately grabbed a corner piece and twisted it in place, which resulted in me asking for the cube back a second later. I untwisted the corner, and ran through a rather clumsy corners-first solve while he watched. When I was down to three pieces left to solve, he asked me if I was still attempting to solve it, and seemed genuinely surprised four moves later (R2 E R2 E’) when I was done.

Our little interchange over, I felt bad as I walked off. Certainly, I could have let him keep going. It’s not like twisting one corner was going to make a big difference in a solution being attempted by a person that decides that twisting a corner is a viable move. Unfortunately, it struck me as a rather familiar scenario – when some people are presented with something complex they may not understand, often disruptive behavior can help them make breakthroughs when a more conventional strategy seems hopeless and time-consuming or analysis seems unnecessary or unwanted. Many a Street Fighter match or a football game has been won by a hare-brained strategy that a reasonable opponent wouldn’t even think to defend against. Our very history as a country in America has been defined, and is still being defined in new ways, by disruptive behavior.

On a small closed group, however, every unconventional disruption has to be undone for the group to return to its natural order. Had the kid managed to get closer to solving the cube, that single corner twist that he did would have to have been reversed at some point in the process. I don't know if he felt like I forced convention on him, or if he even understood the implication of what he did at all. At that moment, I felt like I had to show him that there was a right way, and it didn't take shortcuts. Unfortunately (whether for me or for him will be left as an exercise for the reader) I only could show him by solving the cube, instead of offering up some sort of explanation.
For a moment, let's imagine a rather simple cube. Since we're only concerned about the corner pieces for the purpose of this thought experiment, let's imagine a 2x2x2 cube that only has stickers on the U and D layers. When we talk about orientation, we will say that if it's correct - we'll assign a numerical value of 0 to that. If it's counterclockwise from the correct position, we'll call that -1/3, and if it's clockwise from the correct position, we'll call that 1/3.  If you turn the U or the D layer, no change in orientation has occurred. If you turn F, B, L, or R 90 degrees, what happens is that you get two of the pieces changed by -1/3, and two of the pieces changed by 1/3. Add the numerical values of each piece up and you get 0.

That's not to say that you always get 0 - if you started from the solved state and did the move R' D R you will have three pieces with an orientation of -1/3 and all of the rest of them correct. Add those up and you get -1. If you had done R' D' R you get three pieces with orientation 1/3 which add up to 1. As it turns out, no matter how many moves you do, it will always add up to an integer. 

This lends itself to a basic of cube behavior. Normally when I explain it, I would tell people that a single corner cannot be out of orientation by itself. They can be in opposite pairs (1/3 and -1/3) or all three in the same direction, like in our R' D R example.

If you study the edge pieces on a 3x3x3 in the same way, you discover that edges cannot be individually out of orientation for a similar reason, and you will only find an even number of edges can be out of orientation. If you try the thought experiment for edges out fully, remember an incorrectly oriented edge would have a numerical value of 1/2, since an edge only has two possible positions.

Luckily it is rather difficult to disrupt the orientation of an edge cube by hand due to the way it sits in the mechanism, but it is a concern if someone were to reassemble a cube at random. There is also a possible problem with the parity of the pieces if a 3x3x3 cube is reassembled at random, where you could possibly get to the end of the cube and only have two pieces out of place, which is also not normally possible. You can have a minimum of three pieces out of place, like three corners or three edges, or you can have two corners and two edges for a total of four pieces out of place, but no less.

At a certain point, the only solution for disruption is disassembly and careful reassembly. 

(Don't even get me started on what to do with an older cube whose stickers have been moved around.)