if one entity somehow manages to control over half to the total ETH tokens, does this enable an attack analogous to bitcoin's 51% problem (which happens when one miner controls over half of the network's raw cpu power)?
Also, see this tweet from Vitalik: https://twitter.com/VitalikButerin/status/130129808602782105...
So even if there was a successful first attack from some organised body that's all the time they get.
I'm not sure that's correct (the tweet doesn't get into any details so I went to ethereum's website https://ethereum.org/en/developers/docs/consensus-mechanisms...)
> The threat of a 51% attack still exists in proof-of-stake but it's even more risky for the attackers. To do so, you'd need to control 51% of the staked ETH. Not only is this a lot of money but it would probably cause ETH's value to drop. There's very little incentive to destroy the value of a currency you have a majority stake in. There are stronger incentives to keep the network secure and healthy.
The keypoint seems to be that if your attack fails your stake gets destroyed so besides the positive incentives (a good stable network working for all) this system also relies on punishing failed attacks.
A 51% staker who just censored transactions could hold out longer. If the problem were severe, the community would have to decide whether they want to manually fork off the attacker. The equivalent for PoW would be changing the hash function.
Gaining 51% can be more expensive to do on PoS than on PoW. If 10% of the tokens are staked, you need to accrue another 10% of the total market cap. On PoW, if the annual inflation rate is 2%, the hardware is good for two years, and half the mining cost is electricity, then the total value of mining equipment is only 2% of the market cap, and that's how much you'll have to spend to get 51%. (If miners are rentable, then much less for a brief attack.)
The community will notice and can decide to do a hard fork of the network where they "delete" the attackers coins.
So the network would have experience a hickup, but the hacker has lost billions of dollars worth of ETH and can't attack anymore.
This is different from Proof of Work & Bitcoin. If an attacker gets 51% of the "mining power" (physical hardware), there's nothing the community can do to "delete" their hardware.