51% attacks work on arbitrarily long timescales. Though obviously you can still do short double spend attacks at some probability with less hashing power.

An attempt to rewrite the ledger with a 51% attack would be detected almost immediately. The Bitcoin in the rewritten ledger would quickly become worthless, including that belonging to the keys owned by the attacker. All that mining gear used to perform the attack would then be worthless too. This doesn't seem to me like a profitable attack vector once you consider the complete picture.

Shorting Bitcoin doesn’t require you to ever own any Bitcoin. It does require a counter party(s) that would pay out.

The value of the gear is just a question of the future profits it can generate. If that future revenue is less than what you get from the great short then it’s profitable. And that’s assuming you can’t reuse the same equipment for some other purpose.

> doesn’t require you to ever own any Bitcoin

It requires you to borrow bitcoins from someone -> sell them -> do the attack -> buy them back -> and then return them.

If you could borrow enough bitcoin, and gain enough hash, and crash the price enough for it to be profitable, it could possibly kill Bitcoin. Seems like an insane gamble to me though.

I agree.

That said if I where writing a story I would have the protagonist gain the hashing power first, something strands their assets such as increased electricity prices making mining unprofitable or a next generation of hardware coming out etc. Then they come up with the idea of a big short.

The cool thing about that also is the hash power can be accumulated secretly and dumped on the network all at once

If you can maintain it, sure.

But you can only double spend coins you already control, so at this point you would control 51% of global SHA2 hash and have a bunch of bitcoin that you would be double spending?

Who would accept that as payment? What incentive would you have to do something like that since you would be taking a massive loss?

Maybe as a government attack?

The assumption was the point of the attack would be to destroy Bitcoin’s value to make money from a short position not to sell the same coins twice.