5 ms·

“And just to get this out of the way, we’re in no danger from this explosion. It’s far too far away.” Also, it happened 12 million years ago, so I think we are

by nmc 13y ago

“And just to get this out of the way, we’re in no danger from this explosion. It’s far too far away.” Also, it happened 12 million years ago, so I think we are pretty safe.

gabipurcaru 13y ago
How is this an argument? If I shoot a bullet from a very distant planet and it will reach Earth after 12M years, will you be safe if it came directly towards you?
- sliverstorm 13y ago
  Surely after 12M years, the bullet will have rusted to dust in-flight? :)
  - throwaway_yy2Di 13y ago
    No, it would have been perfectly preserved, hanging motionless in a dry vacuum. Archivists would be envious. Speaking of archivists: http://sci.esa.int/rosetta/31242-rosetta-disk-goes-back-to-the-future/ http://sci.esa.int/rosetta/31242-rosetta-disk-goes-back-to-t... http://blog.longnow.org/02009/05/21/what-13500-pages-micro-etched-into-nickel-looks-like/ http://blog.longnow.org/02009/05/21/what-13500-pages-micro-e... (this is the object of yesterday's HN story https://news.ycombinator.com/item?id=7092313 https://news.ycombinator.com/item?id=7092313)
    - sliverstorm 13y ago
      Oh I know, just kidding :)
  - pavel_lishin 13y ago
    Not enough oxygen for it to rust. And even if it were, a collection of dust traveling at bullet speeds would still probably kill you. But worry not! A bullet fired from 12 million light years away would probably be ablated into nothing by the interstellar dust.
    - btilly 13y ago
      Nope. Stop and think about the fact that starlight can travel that distance. Therefore the accumulated dust along that path is not enough to block light. That little dust won't stop a bullet either.
      - pavel_lishin 13y ago
        But it's not just starlight - it's the light of a whole nova (and the collective light of a whole galaxy.) I can see starlight from the bottom of a pool, but the water in the pool will still stop a bullet. (I am not a scientist, I could be entirely wrong, but it seems like the collective dust of 12 million light years would surely ground down a bullet, while being intermittent enough to let starlight pass through in aggregate.)
        btilly 13y ago
        Well let's calculate it. According to http://www.universetoday.com/30280/intergalactic-space/ http://www.universetoday.com/30280/intergalactic-space/ there is about 1 hydrogen atom per cubic meter. According to https://www.google.com/search?q=light+year+in+meters&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=fflb https://www.google.com/search?q=light+year+in+meters&ie=utf-... there are about 10^16 meters in a light year. So 12 million light years is about 10^23 meters. Avagadro's number is about 6 * 10^23. So a tube a square meter across is a bit under 0.2 moles of hydrogen. According to http://answers.yahoo.com/question/index?qid=20080619205711AAuBH5O http://answers.yahoo.com/question/index?qid=20080619205711AA... at standard air pressure a mole of hydrogen would be about 22 liters, so we're talking about 4 liters of gas at standard pressure. Which would be about 1 square meter across, and 4 millimeters tall. The collective dust of 12 million light years of intergalactic space does not seem likely to grind down a bullet very much. That said, the whole calculation is flawed because first you have to get out of our galaxy, which is much higher density. Though that also wouldn't stop a bullet. But then you have our atmosphere, and that most definitely would destroy a bullet that tried to pass through it! But the moral remains. Space is empty. Really empty. Unimaginably so.
    - 13y ago
  - lutorm 13y ago
    More likely, it will have reached its apoapsis and fallen back on your planet, unless you have one hell of a gun...
- vignesh_vs_in 13y ago
  An explosion is no way similar to a bullet, like comparing laser to a light bulb. The energy or rays gets dispersed over distance.
  - rosser 13y ago
    Lasers or light bulbs, photon density (flux) follows the inverse square law. EDIT: For the follow-up questioning the correctness of my comment: https://en.wikipedia.org/wiki/Inverse_square_law#Light_and_other_electromagnetic_radiation https://en.wikipedia.org/wiki/Inverse_square_law#Light_and_o... https://en.wikipedia.org/wiki/Radiative_flux https://en.wikipedia.org/wiki/Radiative_flux
    - cjfont 13y ago
      I believe this is incorrect, anyone can confirm?
  - scott_s 13y ago
    I think you misunderstand the point of gabipurcaru's analogy. nmc's confusion was about time, not energy dispersion. That is, nmc indicated that we would somehow be safe because the explosion happened 12 million years ago. But gabipurcaru pointed out how long ago something happened is irrelevant if the energy that reaches you is high enough. You are correct that we are safe because of energy dispersion - or, put another way, because it is 12 million light-years away. But nmc thought we were safe because it was 12 millions years ago, and gabipurcaru tried to show by analogy why that is not true.
ChuckMcM 13y ago
Unfortunately there is no inverse square law for time, only for distance. So if it is large enough, regardless of how long it takes to get to us, we'd be just as toasted. You get sun burns when you are out too long, even though that was so like 8 minutes ago. :-) That said, it also means we can probably skip that half of galaxy M82 in our search of extraterrestrial intelligence. For the planets that were within a few light years of the supernova, they probably lost anything that wasn't bolted down in terms of atmospheric gases :-).
jswhitten 13y ago
How long ago it happened is irrelevant. It really is just the distance that is keeping us safe.

by nmc 13y ago

“And just to get this out of the way, we’re in no danger from this explosion. It’s far too far away.” Also, it happened 12 million years ago, so I think we are pretty safe.

gabipurcaru 13y ago

How is this an argument? If I shoot a bullet from a very distant planet and it will reach Earth after 12M years, will you be safe if it came directly towards you?

sliverstorm 13y ago
Surely after 12M years, the bullet will have rusted to dust in-flight? :)
- throwaway_yy2Di 13y ago
  No, it would have been perfectly preserved, hanging motionless in a dry vacuum. Archivists would be envious. Speaking of archivists: http://sci.esa.int/rosetta/31242-rosetta-disk-goes-back-to-the-future/ http://sci.esa.int/rosetta/31242-rosetta-disk-goes-back-to-t... http://blog.longnow.org/02009/05/21/what-13500-pages-micro-etched-into-nickel-looks-like/ http://blog.longnow.org/02009/05/21/what-13500-pages-micro-e... (this is the object of yesterday's HN story https://news.ycombinator.com/item?id=7092313 https://news.ycombinator.com/item?id=7092313)
  - sliverstorm 13y ago
    Oh I know, just kidding :)
- pavel_lishin 13y ago
  Not enough oxygen for it to rust. And even if it were, a collection of dust traveling at bullet speeds would still probably kill you. But worry not! A bullet fired from 12 million light years away would probably be ablated into nothing by the interstellar dust.
  - btilly 13y ago
    Nope. Stop and think about the fact that starlight can travel that distance. Therefore the accumulated dust along that path is not enough to block light. That little dust won't stop a bullet either.
    - pavel_lishin 13y ago
      But it's not just starlight - it's the light of a whole nova (and the collective light of a whole galaxy.) I can see starlight from the bottom of a pool, but the water in the pool will still stop a bullet. (I am not a scientist, I could be entirely wrong, but it seems like the collective dust of 12 million light years would surely ground down a bullet, while being intermittent enough to let starlight pass through in aggregate.)
      - btilly 13y ago
        Well let's calculate it. According to http://www.universetoday.com/30280/intergalactic-space/ http://www.universetoday.com/30280/intergalactic-space/ there is about 1 hydrogen atom per cubic meter. According to https://www.google.com/search?q=light+year+in+meters&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a&channel=fflb https://www.google.com/search?q=light+year+in+meters&ie=utf-... there are about 10^16 meters in a light year. So 12 million light years is about 10^23 meters. Avagadro's number is about 6 * 10^23. So a tube a square meter across is a bit under 0.2 moles of hydrogen. According to http://answers.yahoo.com/question/index?qid=20080619205711AAuBH5O http://answers.yahoo.com/question/index?qid=20080619205711AA... at standard air pressure a mole of hydrogen would be about 22 liters, so we're talking about 4 liters of gas at standard pressure. Which would be about 1 square meter across, and 4 millimeters tall. The collective dust of 12 million light years of intergalactic space does not seem likely to grind down a bullet very much. That said, the whole calculation is flawed because first you have to get out of our galaxy, which is much higher density. Though that also wouldn't stop a bullet. But then you have our atmosphere, and that most definitely would destroy a bullet that tried to pass through it! But the moral remains. Space is empty. Really empty. Unimaginably so.
  - 13y ago
- lutorm 13y ago
  More likely, it will have reached its apoapsis and fallen back on your planet, unless you have one hell of a gun...
vignesh_vs_in 13y ago
An explosion is no way similar to a bullet, like comparing laser to a light bulb. The energy or rays gets dispersed over distance.
- rosser 13y ago
  Lasers or light bulbs, photon density (flux) follows the inverse square law. EDIT: For the follow-up questioning the correctness of my comment: https://en.wikipedia.org/wiki/Inverse_square_law#Light_and_other_electromagnetic_radiation https://en.wikipedia.org/wiki/Inverse_square_law#Light_and_o... https://en.wikipedia.org/wiki/Radiative_flux https://en.wikipedia.org/wiki/Radiative_flux
  - cjfont 13y ago
    I believe this is incorrect, anyone can confirm?
- scott_s 13y ago
  I think you misunderstand the point of gabipurcaru's analogy. nmc's confusion was about time, not energy dispersion. That is, nmc indicated that we would somehow be safe because the explosion happened 12 million years ago. But gabipurcaru pointed out how long ago something happened is irrelevant if the energy that reaches you is high enough. You are correct that we are safe because of energy dispersion - or, put another way, because it is 12 million light-years away. But nmc thought we were safe because it was 12 millions years ago, and gabipurcaru tried to show by analogy why that is not true.

ChuckMcM 13y ago

Unfortunately there is no inverse square law for time, only for distance. So if it is large enough, regardless of how long it takes to get to us, we'd be just as toasted. You get sun burns when you are out too long, even though that was so like 8 minutes ago. :-) That said, it also means we can probably skip that half of galaxy M82 in our search of extraterrestrial intelligence. For the planets that were within a few light years of the supernova, they probably lost anything that wasn't bolted down in terms of atmospheric gases :-).

jswhitten 13y ago

How long ago it happened is irrelevant. It really is just the distance that is keeping us safe.