And if half of the miners turn off their hardwares ?

[Anon136]

if you leave the biggest pool alive and pull the plug on three smaller pools at the same time then you pretty much gonna have 51% attack...

thats true but its important to understand the limitations of a temporary 51% attack. You cant just say "mowa ha ha ha" and write yourself 1 trillion bitcoins. Now a sustained 51% attack could be a more serious problem but there is no reason to expect the above outlined situation to lead to a sustained 51% attack.

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[algorista]

if you leave the biggest pool alive and pull the plug on three smaller pools at the same time then you pretty much gonna have 51% attack...

Exactly, and this is a serious thing to think about.

[algorista]

...
As for the original question re a week, that's exp(-7*1440/10) = 1.7 E-438.

Thanks for the additional information.
This is certainly a very interesting subject.

[razorfishsl]

A point I made months ago…..

Also consider if two or three pool operators decided to work together……. with a high speed link between the 3….

Think about a very big country where cheap electronics comes from…….

[btcusr]

This is also a way to inflate bitcoins..

1. Shutdown 50% miners; half of usual blocks found; (25 * 3 * 24 * 14) for two weeks; >> 25200 BTC
2. Difficulty down to half of usual
3. Bring those 50% miners back, (25 * 12 * 24 * 14)  for another two weeks; >> 100800 BTC
4. Usual for a month (25 * 6 * 24 * 28); >> 100800; excess, 25200 BTC

May be someone is already doing this. Right?

[2_Thumbs_Up]

This is also a way to inflate bitcoins..

1. Shutdown 50% miners; half of usual blocks found; (25 * 3 * 24 * 14) for two weeks; >> 25200 BTC
2. Difficulty down to half of usual
3. Bring those 50% miners back, (25 * 12 * 24 * 14)  for another two weeks; >> 100800 BTC
4. Usual for a month (25 * 6 * 24 * 28); >> 100800; excess, 25200 BTC

May be someone is already doing this. Right?

Difficulty adjustment is based on block count, not time. With half the miners gone, difficulty will take 4 weeks to adjust. And when they double the hashing power again, it will take 1 week. So essentially, the result will be that it takes 5 weeks to mine the blocks that usually takes 4 weeks.