How Bitcoin Spacechains Work – Bitcoin Magazine

The idea of sidechains as a scaling and feature extension mechanism for Bitcoin is a very old concept. A kind of basic “ancestor” idea of sidechains, merge mined chains, even goes back to before Satoshi disappeared.

That proposal was simply the idea of two completely separate and unrelated chains being mined by the same group of miners, with no ability to move anything between chains. The original sidechain proposal was made in 2014 by many of the people who went on to found Blockstream literally a week or so after the paper was published. The basic idea was to be able to have coins move back and forth between the main Bitcoin blockchain and other sidechains, with simple payment verification (SPV) proofs being used to prove things are valid when you send coins from one chain to the other. This never came to fruition due to complexities in implementation around chain reorganizations, the potential for theft and risks of mining centralization (all of which can be read about in section four of the Bitcoin white paper).