[FairlyInvolved]

Several people have answered this from the angle of why the cost is large, but missed the other (necessary) angle - the benefits are also massive and well understood. Plenty of potential projects have huge costs but they don't exist because the benefits aren't big enough, highways exhibit a sort of survivor bias in the mega project world.

Highway systems create massive economic benefits that are relatively simple to calculate, just in the raw # hours saved which massively increases productivity. This can be well captured by the state through taxes or (less frequently) by private industry through tolls - i.e. the people that direct their construction.

Other mega projects have much less certain benefits, or the benefits are harder to capture by those with the ability to allocate resources because incentives aren't sufficiently aligned.

[leeter]

It's reasons like this that I don't necessarily see LHC or whatever rev of it as a problem per se. If the technologies developed to get there pay back the cost of the collider then it could end up being a very expensive toy and I wouldn't care. To me it's about the journey and getting there, because for big science in many case the real science is not the tool produced at the end but the science and engineering that has to go into making that tool.

Thus the question I have to the scientists pushing for a new LHC is: "What would we get from the journey?" If they truly believe in the project they'll have a dozen technologies that need to be developed to make their project happen, all of which have practical applications beyond their experiment. If they don't they'll just be pushing for "same but larger" which is IMO a waste of resources. Because why shouldn't we be investing in the technology to do "Same but smaller" in that case so we can do more with less?

[marcosdumay]

> If they truly believe in the project they'll have a dozen technologies that need to be developed to make their project happen, all of which have practical applications beyond their experiment.

I don't think one should even have to demonstrate "practical applications beyond the experiment" beforehand. Just that it is a complete problem that can be solved independently from the entire project.

If there are a lot of those, it's much better to fund those and get the results that will push the cost of the full thing down. Even if the total costs turn up to be higher than focusing on the collider from the start.