I assume this is a bad-faith question, given how much it begs the question, but: there is no known solution to the double-spend problem that achieves the same security properties as proof-of-work without consuming energy.

I don't understand how this viewpoint can persist now that we have quite a few PoS chains humming along in production: Cosmos, Tezos, Polkadot, Mina, Solana, Avalanche, Cardano, Eth2.

Edit: I suppose what you said is a fair statement; PoS security is different in that it relies on weak subjectivity. In particular, users can maintain security by syncing every X days, where X is based on the staker bonding period, and not accepting long forks beyond X. It's not equivalent to PoS security, but there's nothing really problematic about it IMO. Several years ago, one could make the case that this security model was untested, but not so much today.

To add to that, Algorand has the ability to prevent double spending, and is decentralized proof of stake.

Dumb out-of-the-loop question: then what's this talk of Ethereum moving to proof of stake? Whenever someone brings up that PoW is bad, people say Ethereum; whenever someone says why PoW, someone says because PoS isn't safe.

Don't want to duck your question but: the reason this is hard to get a handle on is that it's really complicated.

It's not that the premises and protocols themselves are complicated — sure, they're technically sophisticated, but they can be explained simply. The problem lies in that it is very hard to predict the economic consequences and security guarantees of the various consensus mechanisms. Frankly, if Bitcoin hadn't been working flawlessly for the past 12 years very few people would guess that it would work.

There are two problems here: The first is that a lot of ethereum talk is just that; hot air with little substance.

The second is that PoS has significantly worse security properties than PoW.

The core reason is that "changing your mind" (rolling back the blockchain) is free with PoS, except for possible loss of value of the cryptocurrency, whereas it's extremely expensive with PoW. There are all sorts of band-aids you can slap on like ignoring block reorgs of a certain depth, but then you lose properties like network partition tolerance.

We shipped ethereum proof of stake December 1st, 2020. There are over 100,000 validators in the network, securing over $6 billion worth of value. This isn't hot air with little substance.

EDIT: adding source https://beaconcha.in

Short answer is it doesn't work now and is not known to be able to work.

Much easier to make promises than keep them.

Your knowledge is outdated, see the other responses.

Sorry, "butcoin" was a typo...