some questions about 51% attack and hash power

[Gol]

Hello all!

i have some questions about 51% attack and hash power, any professional info will be highly appreciated.

1. if some entity will built a machine with grater hash power then the Bitcoin network to date
(i know it's almost make no sense, but theoretically :
http://www.forbes.com/sites/reuvencohen/2013/11/28/global-bitcoin-computing-power-now-256-times-faster-than-top-500-supercomputers-combined/)
how log until the rest of the (legit) network will figure that something is wrong?

2. what could the "legit network" can do about it?

3. is there a formula to calculate hash power / $?

thank you!
Gol

[1538088614]

1538088614

[mustyoshi]

There's nothing explicitly wrong with having 51% of the hashpower, you could use it to mine every block for the duration of your majority and that would be the end of it.

But, you could also use it to remine old blocks and mess with the transaction history and screw people over for the fun of it.

But at the most you'd have an advantage for just a little while, more miners would join and your majority would be removed.

[Foxpup]

how log until the rest of the (legit) network will figure that something is wrong?

Probably within a few hours or so. In about 20 minutes, legitimate miners will start to notice their blocks being orphaned (that's how a 51% attack works, and it's not possible for this to go unnoticed), though it will likely take several more blocks for other causes of orphaned blocks to be ruled out.

what could the "legit network" can do about it?

An alert will be issued, telling users to stop creating and accepting transactions until the attack has ceased. This will cause massive disruption to the Bitcoin economy, though it will be temporary.

is there a formula to calculate hash power / $?

No. All such formulae have no way to account for changes to the price of and demand for mining equipment if an entity attempts to purchase a significant fraction of the available supply.

[Gol]

^^^
Thank you for the info!
Gol