Yes but in practice you can't convert that much BTC into USD that quickly.

>Yes but in practice you can't convert that much BTC into USD that quickly.

Would they even need to convert BTC into USD? Even ignoring the obvious opportunities for shorting an entire economy?

I mean, I'm kinda curious to know what effects they could cause by e.g. choosing to mine zero-block transactions for a few days.

I thought miners already try to mine zero block transactions first, then add in fee transactions? apparently you can chew through the nonce really fast..

Not exactly. Some Chinese mining firms maintain a stratum connection to pools in the US so they can get just the new block header when a new block is found on the network. They then mine on that header until the full block is downloaded and validated by their local bitcoind (which can take 20 seconds, to several minutes in the worst case) This has been referred to as "SPV Mining".

If they find the next block during the time they are mining only against the block header, that block will contain no transactions. This doesn't happen very often, and there are some other issues that can arise from the practice (a multi-block fork happened once), but miners believe that it increases their profitability so (afaik) they continue to do it. More info here: http://bitcoin.stackexchange.com/questions/38437/what-is-spv...

But if that was a predictable consequence, they can't turn $200million into $400million now, because other would anticipate the future state of things and the price would plummet before a couple years. Not that I think that is at all what is going to happen.

>because other would anticipate the future state of things and the price would plummet before a couple years.

Why would the price plummeting matter to them if they've already made their money?

The only way they make money (cash out) is if trust remains in the bitcoin platform. By doing 51%, they tank the platform itself. No liquidity. Provable and transparent majority attack is MAD in the strictest sense.

>The only way they make money (cash out) is if trust remains in the bitcoin platform.

Not if their cash isn't - somewhat ironically - tied up in BTC.

Really, I'm sure between the two of us, we could imagine a thousand and one viable ways of making a tonne of cash very, very quickly that wouldn't involve transacting in BTC at any point!

Not sure I follow you here. How would they make money from controlling BTC other than by selling it? ...which is impossible if they break it, as argued above.