Posts tagged ‘physics’

## Remembering all the things I forgot

Yesterday evening, John, Jacob (though not Jingleheimer-Schmidt) and I had a bull shoot for an hour and a half. We started out with John proposing a memory caching system which was intended to make garbage collection and blocking irrelevant. From there, the conversation natuarally progressed to online algorithms, cache-oblivious algorithms, religion, free will, consciousness, Searle’s Chinese Room argument, Chalmers’ Zombie Planet argument, qualia, logic-based representations of object-oriented programming, the lambda calculus, machine learning, finite discrete universes, quines, the GÃ¶del processor, complexity theory, the No Free Lunch theorems, and probably some other topics that I’m forgetting. It was fantastic.

However, this, along with a couple other conversations this week, have showed me just how much I have forgotten since college. I couldn’t remember the phrase “competitive analysis” when we discussed online algorithms. I couldn’t come up with a representation of linked lists that supported the head and tail functions in the lambda calculus, and worse, I mixed up the S and K combinators and couldn’t even give the definition for what turned out to be the S combinator. Last week I mixed up pipelining and instruction-level parallelism, and couldn’t come up with the phrase, “speculative execution.” Last week at lunch I couldn’t come up with the formal definition of temperature (something about the kinetic energy of the molecules relative to each other and maybe the amplitude of the interatomic vibrations within the molecules, but what do you do if you have a singe atom?). Today I couldn’t explain why (if?) bending a piece of metal makes it harder but more brittle, or why bending a polymer makes it more maleable (though in my defense, I could explain the difference between stress and strain, and what a polymer is as well as a bunch of examples of polymers). I remember why the X combinator is important, but can’t give the definition for it or the Y combinator. Yesterday I realized that something I was working with was a metric space, but couldn’t remember anything useful about metric spaces (I got as far as knowing that the triangle inequality holds, but I didn’t remember anything else). I can no longer use LaGrange multipliers to optimize a quantity subject to a constraint (you need the two to have parallel tangents at the extrema, but I don’t remember how to find the slopes of these tangents). Last week I forgot the name of Topological Sort, let alone how it works (something related to DFS, but that’s all I came up with).

Now that I’m writing this, I’m noticing all sorts of other things that I used to know but can’t quite remember (how Java does garbage collection, queen-asking continuations in Roman Keycard Blackwood, how the telephone lab in Baby Stems worked). It’s scary to realize how much I’ve already forgotten, and know that I’m only going to forget more things from here on out. Does anyone want to have a refresher session? If so, post your questions (or answers) here!

## An idea for a video game

Take a classic game that makes heavy use of a simple physics engine (such as Asteroids), and add in general relativity to the physics engine (and set the speed of light so that the game has very noticeable relativistic effects). For the Asteroids example, asteroids traveling at high speeds relative to you would exhibit Lorenz contraction, asteroids coming towards you would be bluish and those going away would be reddish, firing your gun would propel you backwards a little, etc. I’m not sure if accelerating would have different behavior than in the usual version (sure, the asteroids would age faster, but that wouldn’t be noticeable in-game). Stuff displayed on the screen would be what the spaceship pilot would perceive “now.” I suspect it would be an interesting twist on a classic game, and give people a better intuition for relativity (assuming people like it and play it a bunch). I can’t find such a game already created on the internet, but I haven’t looked too hard.

Any thoughts?

## Questions for a physicist and/or chemist

1. My can of soup says to put in a bowl, cover loosely, microwave, and then let stand for a minute. I do all of this in a plastic bowl with a flimsy plastic lid that can be sealed shut (but I don’t seal it). When I take the soup out of the microwave, the lid is not only sealed on, but bowed in from lack of pressure (lack of atmosphere?) inside the bowl. The soup is still hot (I don’t think it has cooled much). What’s going on?
2. My shower has a smooth, clear, glass door. When I shower, the water covers it with a smooth, even layer, so I can still see out pretty clearly. When I touch my finger to the glass, the water “runs away” from my finger and leaves the glass nearly dry in the vicinity. This is so strong that the water will actually flow up to “escape” my finger’s vicinity. New drops of water that hit the glass while my finger is touching it also move away (though at a slower pace, since they can’t flow as easily without more water around). When I remove my finger, the film of water returns to the way it was before. This isn’t dependent on something I’ve put on my finger; it works with all 10 fingers/thumbs, both elbows, my tongue, and at least one toe. I don’t have a water softener. What is happening?
3. Galileo showed that all objects fall at the same rate, no matter how much they weigh. I have a bowl full of Cheerios and put some raisins on top. The Cheerios and raisins are about the same size, but the raisins are denser. I put my hand over the top of the bowl and shake it. I would expect all objects in the bowl to fall at the same rate when I shake, and keep the raisins on top. Instead, they gradually migrate towards the bottom. What’s going on?
4. There is a doorway with the sun shining through it onto a wall (with the shadow of the doorframe on the wall). I stand several feet in front of the door so that the side of my shadow nearly touches the side of the door’s shadow. The part of my shadow closest to the door’s is mirrored, so it appears that the side of my arm is coming out of the doorframe. This occurs even if I move around slightly (it appears as though the door’s arm is moving in the opposite direction). The shadows need to be within a couple inches of each other to get this to work. What is going on?

The world is a strange and fascinating place.