wondering
I’m daily reminded of four things:
- How little I know
- How much I know that is wrong
- How much there is to know
- How fun it is to figure out
These reminders almost always start with a question, usually prompted by some dissonance between the way I expect the world to work and the way it does. I’ve been asking questions that I’ve found extremely difficult to answer to my satisfaction with increasing frequency. 1 I’m at the point where I’m having trouble keeping track of my questions and the progress I’ve made on them. To that end, here are my questions (accompanied by a short description),2 and here’s the progress I’ve made, as documented by Google Notebook. Whenever I “answer” a question, I’ll post it with a polished explanation, saving future askers the trouble. That means that the standard for success is not just my understanding, but the right assortment of visualizations and metaphors to make the answer intelligible to an eight-year old kid.
And of course, if you have any insight or suggestions to any of these questions, don’t hesitate to contact me. I pay finder’s fees and rewards for hard-to-find answers.
How does aerodynamic lift work?
Unanswered: So, I was told the so-called equal transit time explanation of how a wing creates lift. And you know what? It’s a myth! You know what else? Lift is really, really hard.
How does a Benham’s disk work?
Unanswered:
What causes the sensation of “cool” in a stream of air?
Unanswered:
Why the lack of symmetry in handedness?
Unanswered:
How does the frequency of light change upon reflection?
Unanswered:
How do we localize sound?
Unanswered:
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5 responses to 'wondering'
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So, I made up my own answers to two of these, sort of. I picture a stream of air as a river of spheres, like we’d model most fluids. So as billiards teaches us, when one sphere collides with another, occasionally the first sphere will impact the second and “steal” the second’s kinetic energy/momentum - for example, if the two are moving towards each other, and the first one bounces backwards while the second is stopped in its tracks. The same goes for photons from lasers hitting objects to make them colder in Bose/Einstein condensate/superconductor experiments. Molecularly, getting slowed down translates into coldness. The neat part about this is that your body then sends an army of blood cells up to circulate super-fast through your now-cold skin, so if you take your hand out of the stream it will be red and feel hot.
Localizing sound is neat. Sound hits one ear before the other, or, why it’s hard to tell when someone is standing right behind you, or, why elephants have four feet and communicate on low frequencies. Birds have ears that are close together, and communicate on high frequencies - the best explanation I got was about wavefronts hitting these two (or four, for the pachyderms) points at different times, and localizing based on that subconscious information.
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What causes the sensation of “cool” in a stream of air?
Lets first look into what allows us to feel “cool.” It appears that inside and on the surface of our skin cells are transient receptor potential ion channels that open their gates under a number of conditions. For instance TRPV3 lets it signal flow to your head between local conditions of 22 to 40 degrees C and TRPV1 opens at temperatures greater than 43 degrees C. TRPM8 is a signifant constributor specifically to feeling cool.
So these funny little structures in our skin change shape and let ions flow to produce action potentials when certain environmental conditions around them change. How does a stream of air cause that change?
My immediate thought was to look at the water. Water has a higher rate of evaporation under lower static pressure. It takes a lot of energy to evaporate water. We have a lot of water in us. It seems like it may be the culprit. So we need to think about the water on the surface of our skin, which luckily is very neighborly with those TRPs. So with little time to spare. surface water has a certain amount evaporating from it all the time. Your body is warm all the time so that amount isn’t insignificant. The stream of air passes over at high speed lowering the pressure at the surface and increasing the rate of evaporation. Those little bits of gas bombarding the liquid let more and more little water molecules escape those molecular forces binding them together in a liquid. So when does it get cold? well the stream of air helped the water escape, but it needed more momentum than the average bit of air to make its voyage. Some other little water molecules must’ve given a bit here and there in some collision for it to be sent on its way. Then once its out of there, like a squirrel you caught in a laundry hamper, it isn’t coming back. That energy got taken and the constant flow of collisions made the loss almost unnoticed. But with a whole stream of air, there is a shload of water ready to jump and ton of high momentum air particles to help them along so we’re bound to get some local cold spots that will let our TRPs send some cold signals and stop sending some hot signals.
re: sound localization: there’s more to the story than inter-aural time differences! If you think about it, that only gets you one dimension, but we can hear in 3D (though not with uniform accuracy all around). Basically, your brain can tell the angle a sound is coming from by taking the differences between the spectra of sounds coming into the two ears. You head and ears act as filters, so depending on exactly how much head the sound has passed through on the way to each ear, a filter gets applied, and you are actually inferring the direction the sound came from using a model of the acoustic characteristics of the head. When this is simulated (e.g. to synthesize “3D” sound played back over headphones), the model is called the HRTF, or “head related transfer function.”
benham’s disk is awesome. My brilliant old friend Peter Cariani makes simulations of neural nets that represent information using temporal codes (this is different from the usual model). Temporal codes may be subject to certain types of illusions arising from time-structured sensory input that can mimic the code… at least, I think that was his hypothesis for how the rapid black and white flashing pattern of the top can stimulate color perception. I don’t know if there’s a way to test this hypothesis, either. Here’s Peter’s website: http://homepage.mac.com/cariani/CarianiWebsite/CarianiHomePage.html