4 ms·

> 270 mW/m^2 would still be way more than any known biological system could sustain, right? It's negligible compared to our sun. All this from the top of my he

by yaantc 5y ago

> 270 mW/m^2 would still be way more than any known biological system could sustain, right? It's negligible compared to our sun. All this from the top of my head (so double check if you really care ;) but our sun in winter at surface level is roughly about 100W/m^2 and in summer about 1kW/m^2.

infogulch 5y ago
Yes, but the point is that these are wildly different types of energy delivery. The sun delivers energy by gamma radiation ("plain old light") but here we're talking about alpha particles ("relativistic protons") which has a very different effect on biological systems. The fact that we are able to compare these two things by their total energy content at all is shocking to me. To make this comparison more concrete, maybe we could consider how much alpha particle energy is present inside of a nuclear reactor running at peak energy output. I'd be surprised if it was as much as 1 W/m^2.
- Retric 5y ago
  Alpha particles don’t penetrate even the thin atmosphere of Mars. It’s just a question of total heat, which in cosmic voids would be negligible. https://en.wikipedia.org/wiki/Void_(astronomy) https://en.wikipedia.org/wiki/Void_(astronomy)
  - pfdietz 5y ago
    Protons at 120 MeV will deposit a significant amount of energy via nuclear collisions. The binding energy of a nucleon in a nucleus is about 8 MeV, so plenty of free neutrons will be produced, which will then create 14C by the (n,p) reaction on 14N. Back of the envelope gives this creating about 10 tonnes per second of 14C in the planet's atmosphere (assuming the proton density of 1/cm^3, and assuming the planet is Earth-like and all the protons struck the atmosphere). This is about 11 orders of magnitude higher than the production on Earth, and would result in such a high equilibrium concentration of 14C that the biosphere would likely be sterilized, even ignoring thermal effects.
    - Retric 5y ago
      We are talking a proton density of ~10^-8/cm^3 or lower in cosmic voids. At 1/cm heat alone is going to cook everything long before radiation has time to do anything. Next solar wind actually becomes significant protection at those densities. It varies significantly and simulating what happens gets complicated, but it shouldn’t be ignored at those densities. Anyway, the outer fringes of earths atmosphere for example is almost exclusively Hydrogen and Helium as it gets sorted by atomic weight. Free neutrons decay in a matter of minutes and therefore would almost entirely end up as more hydrogen. What’s a much larger risk is stripping the atmosphere off of any planet.
      - pfdietz 5y ago
        These planets would be accelerated at SMBH mergers, which would occur in galaxies. Yes, once the planet gets to a void it's hitting less gas, but it has to get out of a galaxy and galaxy cluster to get there, and that will take many thousands, perhaps millions, of years. The outer fringes of the Earth's atmosphere are hydrogen, but the density there is so low that these cosmic ray particles would be unlikely to interact there. Also, the neutrons here are FAST neutrons (the (n,p) reaction on 14N is a fast neutron reaction) so they are traveling at very high, if not relativistic, speed, and would not have time to decay before they react (or are thermalized and become irrelevant to 14C generation from 14N).
        Retric 5y ago
        Planets could be accelerated by SMBH mergers, but they wouldn’t stay orbiting a star through the process. It’s slightly more plausible for a one to form afterwards. As to the density of earths hydrogen, that’s a function of earths atmosphere. Under sustained bombardment, assuming the planet kept an atmosphere, it’s going to have significantly less nitrogen in the upper atmosphere as that’s destroyed by collisions with relativistic hydrogen/helium.

by yaantc 5y ago

infogulch 5y ago

Yes, but the point is that these are wildly different types of energy delivery. The sun delivers energy by gamma radiation ("plain old light") but here we're talking about alpha particles ("relativistic protons") which has a very different effect on biological systems. The fact that we are able to compare these two things by their total energy content at all is shocking to me. To make this comparison more concrete, maybe we could consider how much alpha particle energy is present inside of a nuclear reactor running at peak energy output. I'd be surprised if it was as much as 1 W/m^2.

Retric 5y ago
Alpha particles don’t penetrate even the thin atmosphere of Mars. It’s just a question of total heat, which in cosmic voids would be negligible. https://en.wikipedia.org/wiki/Void_(astronomy) https://en.wikipedia.org/wiki/Void_(astronomy)
- pfdietz 5y ago
  Protons at 120 MeV will deposit a significant amount of energy via nuclear collisions. The binding energy of a nucleon in a nucleus is about 8 MeV, so plenty of free neutrons will be produced, which will then create 14C by the (n,p) reaction on 14N. Back of the envelope gives this creating about 10 tonnes per second of 14C in the planet's atmosphere (assuming the proton density of 1/cm^3, and assuming the planet is Earth-like and all the protons struck the atmosphere). This is about 11 orders of magnitude higher than the production on Earth, and would result in such a high equilibrium concentration of 14C that the biosphere would likely be sterilized, even ignoring thermal effects.
  - Retric 5y ago
    We are talking a proton density of ~10^-8/cm^3 or lower in cosmic voids. At 1/cm heat alone is going to cook everything long before radiation has time to do anything. Next solar wind actually becomes significant protection at those densities. It varies significantly and simulating what happens gets complicated, but it shouldn’t be ignored at those densities. Anyway, the outer fringes of earths atmosphere for example is almost exclusively Hydrogen and Helium as it gets sorted by atomic weight. Free neutrons decay in a matter of minutes and therefore would almost entirely end up as more hydrogen. What’s a much larger risk is stripping the atmosphere off of any planet.
    - pfdietz 5y ago
      These planets would be accelerated at SMBH mergers, which would occur in galaxies. Yes, once the planet gets to a void it's hitting less gas, but it has to get out of a galaxy and galaxy cluster to get there, and that will take many thousands, perhaps millions, of years. The outer fringes of the Earth's atmosphere are hydrogen, but the density there is so low that these cosmic ray particles would be unlikely to interact there. Also, the neutrons here are FAST neutrons (the (n,p) reaction on 14N is a fast neutron reaction) so they are traveling at very high, if not relativistic, speed, and would not have time to decay before they react (or are thermalized and become irrelevant to 14C generation from 14N).
      - Retric 5y ago
        Planets could be accelerated by SMBH mergers, but they wouldn’t stay orbiting a star through the process. It’s slightly more plausible for a one to form afterwards. As to the density of earths hydrogen, that’s a function of earths atmosphere. Under sustained bombardment, assuming the planet kept an atmosphere, it’s going to have significantly less nitrogen in the upper atmosphere as that’s destroyed by collisions with relativistic hydrogen/helium.