3 ms·

This is a mean free path computation and everything is spheres (except the skier who is probably a point source). What you've missed out is that a physicist wo

by joshvm 8y ago

This is a mean free path computation and everything is spheres (except the skier who is probably a point source). What you've missed out is that a physicist would need to derive a collision cross section for a typical tree-sphere. That may be as complex or as simple as you like. The simple limit would take into account the trunk diameter and tree density within the forest. But perhaps the skier is particularly agile and can avoid most trees, so the cross section is smaller. You could add other particles and cross sections, eg the bear in ski free. >First principles apply very well to our nice toy models, but the sheer vastness of existence usually out-scales our toy models very quickly. That said, this is how a lot of simulations in astronomy work. You take a bunch of physical laws and apply them at scale on millions of particles.

pas 8y ago
the bear for anyone curious: https://imgur.com/r/nostalgia/qi8Is1q https://imgur.com/r/nostalgia/qi8Is1q :) And to add a bit to how physicists simulate stuff. They build a model. Make it as simple as possible, while still approximating the natural phenomena under investigation. So the mean free path model would have a lot of parameters as the parent commenter noted (collisional cross-section, tree density, trunk diameter, etc.). And it's also important to keep in mind that there are theory builders in physics (theorists), "applied physicists" (phenomenologists, who try to use the fancy theories, match it with real data) and derive future predictions (like climatologists, cosmologists doing large scale universe evolution simulation from the big bang, and so on), the experiment designers, who basically try to weed out the useless models/theories through very targeted experiments. All of these are about the same natural world, but different aspects, different levels of details, different concerns. (Eg. experimental physicists place enormous care on quantifying the systemic and particular uncertainties of the experiment, whereas theoretical physicists instead focus on the fundamental workings of the theory - they test it through computation. All of those are math, math, math, just different.)

by joshvm 8y ago

pas 8y ago

the bear for anyone curious: https://imgur.com/r/nostalgia/qi8Is1q https://imgur.com/r/nostalgia/qi8Is1q :) And to add a bit to how physicists simulate stuff. They build a model. Make it as simple as possible, while still approximating the natural phenomena under investigation. So the mean free path model would have a lot of parameters as the parent commenter noted (collisional cross-section, tree density, trunk diameter, etc.). And it's also important to keep in mind that there are theory builders in physics (theorists), "applied physicists" (phenomenologists, who try to use the fancy theories, match it with real data) and derive future predictions (like climatologists, cosmologists doing large scale universe evolution simulation from the big bang, and so on), the experiment designers, who basically try to weed out the useless models/theories through very targeted experiments. All of these are about the same natural world, but different aspects, different levels of details, different concerns. (Eg. experimental physicists place enormous care on quantifying the systemic and particular uncertainties of the experiment, whereas theoretical physicists instead focus on the fundamental workings of the theory - they test it through computation. All of those are math, math, math, just different.)