4 ms·

There's a lot of pieces of the physics of that story I don't quite understand. The main one is that (IIUC) one of the reasons the damage wasn't as bad as it cou

There's a lot of pieces of the physics of that story I don't quite understand. The main one is that (IIUC) one of the reasons the damage wasn't as bad as it could have been is that at the relativistic speeds the protons were travelling at, his head was extremely space-dilated, so they were basically blasting through tissue paper. But I would expect that tissue paper to have all the mass of a human head dilated into a thin disc, so the density would be far, far higher... Is density not a factor in proton-beam interactions with the material its interacting with (or is it more "it is, but total distance is a much larger factor")?

whatshisface 1y ago
The fact that cross sections become smaller at high velocities is very intuitive: force applies its kick over time, and there is a lot of distance between the parts of an atom. If a particle is going fast enough it whizzes by without pushing on the other atoms for very long.
- shadowgovt 1y ago
  Yes, thank you, this was the intuition I was missing! Protons almost never collide directly with a nucleus because most of an atom is empty space; a proton beam disrupts the matter it's passing through mostly by electrostatic effect, and that's a function of position and time!
cma 1y ago
Thinner makes it get through faster if it doesn't collide. If it is a charged particle dwell time is important as it usually doesn't collide unless its charge interacts with other charged particles enough to slow itself down giving it more time to interact with charged particles. This is how I understand ion beam deposition in silicon to work at least, it goes to a predictable depth and if the silicon is thinner than that it goes on through most of the time. Thinner and denser then makes it interact more per unit time but any induced charge imbalances have closer neighboring material to rebalance charge shifts with, maybe you have a better chance of getting through the same number of particles over a shorter distance than at higher distance. You definitely have a better chance faster than slower. its when it slows to a critical speed its non-collision charge based interactions build.

whatshisface 1y ago

The fact that cross sections become smaller at high velocities is very intuitive: force applies its kick over time, and there is a lot of distance between the parts of an atom. If a particle is going fast enough it whizzes by without pushing on the other atoms for very long.

shadowgovt 1y ago
Yes, thank you, this was the intuition I was missing! Protons almost never collide directly with a nucleus because most of an atom is empty space; a proton beam disrupts the matter it's passing through mostly by electrostatic effect, and that's a function of position and time!

cma 1y ago

Thinner makes it get through faster if it doesn't collide. If it is a charged particle dwell time is important as it usually doesn't collide unless its charge interacts with other charged particles enough to slow itself down giving it more time to interact with charged particles. This is how I understand ion beam deposition in silicon to work at least, it goes to a predictable depth and if the silicon is thinner than that it goes on through most of the time. Thinner and denser then makes it interact more per unit time but any induced charge imbalances have closer neighboring material to rebalance charge shifts with, maybe you have a better chance of getting through the same number of particles over a shorter distance than at higher distance. You definitely have a better chance faster than slower. its when it slows to a critical speed its non-collision charge based interactions build.