4 ms·

The radiation propagates at light speed, but the harmless neutrinos reach us a few hours before the potentially dangerous X-rays and gamma rays. A supernova wou

The radiation propagates at light speed, but the harmless neutrinos reach us a few hours before the potentially dangerous X-rays and gamma rays. A supernova would have to be within roughly 160 light years to be damaging to Earth. https://news.fnal.gov/2019/03/waiting-for-neutrinos/ https://news.fnal.gov/2019/03/waiting-for-neutrinos/ https://earthsky.org/astronomy-essentials/safe-distance-from-a-supernova-earth/ https://earthsky.org/astronomy-essentials/safe-distance-from...

denton-scratch 3y ago
> but the harmless neutrinos reach us a few hours before the potentially dangerous X-rays and gamma rays. Huh? Presumably the electromagnetic radiation travels at the speed of light, and neutrinos travel more slowly; so the neutrinos arrive after the dangerous rays, not before.
- Scaevolus 3y ago
  Current measurements of neutrinos demonstrate them traveling nearly exactly the speed of light. https://en.wikipedia.org/wiki/Measurements_of_neutrino_speed https://en.wikipedia.org/wiki/Measurements_of_neutrino_speed
- chongli 3y ago
  You are correct that the radiation travels at the speed of light and the neutrinos less than that. The detail you’re missing is that the neutrinos depart the inner core of the star during the initial stages of the supernova and pass right through all of that material unimpeded. Though they travel at less than the speed of light, it’s not very much less, so the head start the neutrinos have allows them to arrive first. Since the light emitted from the inner core during the supernova is instantly absorbed by the matter collapsing inward, it must be re-emitted and re-absorbed many times before it can reach open space and begin its journey to earth. With such an incredibly high density in the core of the collapsing star, the mean free path [1] of the light is extremely short, so this process can take a very long time. [1] https://en.wikipedia.org/wiki/Mean_free_path https://en.wikipedia.org/wiki/Mean_free_path
  - denton-scratch 3y ago
    OK, thanks. So the early arrival of the neutrinos is entirely the result of EM radiation being absorbed within the star and re-emitted, while the neutrinos can pass straight through. I wondered if that was the explanation.
    - chongli 3y ago
      Yes. This same phenomenon is how we study gamma ray bursts. The neutrinos are detected early enough that the gamma ray detecting satellites have time to be pointed at the source. Otherwise they would be far too brief to observe!

denton-scratch 3y ago

> but the harmless neutrinos reach us a few hours before the potentially dangerous X-rays and gamma rays. Huh? Presumably the electromagnetic radiation travels at the speed of light, and neutrinos travel more slowly; so the neutrinos arrive after the dangerous rays, not before.

Scaevolus 3y ago
Current measurements of neutrinos demonstrate them traveling nearly exactly the speed of light. https://en.wikipedia.org/wiki/Measurements_of_neutrino_speed https://en.wikipedia.org/wiki/Measurements_of_neutrino_speed
chongli 3y ago
You are correct that the radiation travels at the speed of light and the neutrinos less than that. The detail you’re missing is that the neutrinos depart the inner core of the star during the initial stages of the supernova and pass right through all of that material unimpeded. Though they travel at less than the speed of light, it’s not very much less, so the head start the neutrinos have allows them to arrive first. Since the light emitted from the inner core during the supernova is instantly absorbed by the matter collapsing inward, it must be re-emitted and re-absorbed many times before it can reach open space and begin its journey to earth. With such an incredibly high density in the core of the collapsing star, the mean free path [1] of the light is extremely short, so this process can take a very long time. [1] https://en.wikipedia.org/wiki/Mean_free_path https://en.wikipedia.org/wiki/Mean_free_path
- denton-scratch 3y ago
  OK, thanks. So the early arrival of the neutrinos is entirely the result of EM radiation being absorbed within the star and re-emitted, while the neutrinos can pass straight through. I wondered if that was the explanation.
  - chongli 3y ago
    Yes. This same phenomenon is how we study gamma ray bursts. The neutrinos are detected early enough that the gamma ray detecting satellites have time to be pointed at the source. Otherwise they would be far too brief to observe!