The thing about proof-of-work is that, while performing the work, you have to know about which block you’re creating. You aren’t doing generic work. You’re doing work that’s specific to the block data. If you wanted to double spend, you would have to re-do the work, because the work you did is only applicable to the exact block you mined.
And therein lies the problem with using a globally observable event like you’ve described: It is not tied to any particular block. I can use the radiation data from the fifth of March to mine a block, then use the same data to mine a conflicting block. “Nothing at stake” problem is related.
The thing about proof-of-work is that, while performing the work, you have to know about which block you’re creating. You aren’t doing generic work. You’re doing work that’s specific to the block data. If you wanted to double spend, you would have to re-do the work, because the work you did is only applicable to the exact block you mined.
And therein lies the problem with using a globally observable event like you’ve described: It is not tied to any particular block. I can use the radiation data from the fifth of March to mine a block, then use the same data to mine a conflicting block. “Nothing at stake” problem is related.