It's interesting that you can justify this investment in at least four separate ways, independently:
1) Even if there is a low chance of success, it would be wildly profitable if successful, and investing early gets you a good share if it's successful.
2) Fusion power would make the world a better place; investing in this way, when you already have huge returns and it's someone else's money, is actually rational even if you think it's not the best financial investment.
3) This looks like an amazingly smart team; even if they fail at fusion, they might find something worthwhile in the process. Just handling magnetics and high power well could be a useful toolkit for other problems.
4) "Halo effect" -- both because it's awesome science/engineering and because it shows a willingness to take extreme risks -- boosts YC (which probably doesn't need it) and Mithril (which is maybe even more awesome than YC, but nowhere near as widely known). If it loses $1.5mm but makes it more likely the next Facebook comes to either of these funds, it's a win.
1) Even if there is a low chance of success, it would be wildly profitable if successful, and investing early gets you a good share if it's successful.
2) Fusion power would make the world a better place; investing in this way, when you already have huge returns and it's someone else's money, is actually rational even if you think it's not the best financial investment.
3) This looks like an amazingly smart team; even if they fail at fusion, they might find something worthwhile in the process. Just handling magnetics and high power well could be a useful toolkit for other problems.
4) "Halo effect" -- both because it's awesome science/engineering and because it shows a willingness to take extreme risks -- boosts YC (which probably doesn't need it) and Mithril (which is maybe even more awesome than YC, but nowhere near as widely known). If it loses $1.5mm but makes it more likely the next Facebook comes to either of these funds, it's a win.