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My understanding of light nodes is they need PoW in the block headers. That PoW proves that $x amount of energy has gone into securing that chain.

With PoS, the proof that $y is securing the chain requires knowledge of blockchain state, namely account balances. Does this mean SPV nodes cannot function with Casper?

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My impression is that SPV nodes cannot be validator nodes for the reason you stated (proof that ETH is securing the chain requires knowledge of blockchain state). With Bitcoin and node can convert to or from a SPV node to a full node (or vise vera) at any time.

SPV nodes can exist with Casper but they cannot be validating PoS nodes. SPV peers essentially have to trust validator peers. SPV peers also cannot independently switch from an SPV peer to validator peers on demand (they must first gain approval from the P2P validator network to do so).

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  • how can a light node determine the weight of a blockheader without access to state?
    – A F
    May 21, 2016 at 21:26
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    github.com/ethereum/wiki/wiki/Block-Protocol-2.0 In Ethereum, a ''light node'' can be defined as a node that accepts block headers, and performs the verifications in (2) with the exception of the transaction and stacktrace trie hash verifications but does not perform the verifications in (4) and (6), similar to the headers-only verification that light nodes do in Bitcoin. Light nodes would thus store the state roots, and perhaps some portion of the state, but not the entire state.
    – studycrypto
    May 21, 2016 at 21:58
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    "If a light node wants to know the balance or contract state of a given account, it can request the value from other nodes in the network alongside the minimal subset of Patricia tree nodes that prove that the given key/value pair is actually in the state."
    – studycrypto
    May 21, 2016 at 22:01
  • true, but without PoW, its trivially easy to create a new block with an invalid state change. Thus I could create a a block where I send all the money from an exchange's wallet to my staked account, thus becoming the largest voter in the staked pool.
    – A F
    Feb 24, 2017 at 16:03

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