[SilasX]

It doesn’t sound like you’re disagreeing with my claim that forks under PoW (at least of a specific kind) can make the branches vulnerable to attack as soon as the hash power sloshes away; your comment is only speaking to PoS and a different kind of Bitcoin fork that doesn’t match the BCH one.

[turkeytotal]

No, I am disagreeing with you. You are claiming that the hashpower oscillations between BTC and BCH contradict the GP's comment, when in fact you have you missed exactly what I and the GP have said:

>the miners must select one chain to mine

I am pointing out that the "sloshing" is in fact perfect evidence that they are only mining one chain at a time, or splitting their hashpower to varying proportions. This is completely normal and healthy miner behavior, they are simply greedy actors looking for the most profit. They cannot however mine both chains with their full hashpower for free (as you can with PoS), and this is an essential understanding when analyzing the security model of a consensus algorithm.

Also, the oscillations are most commonly exploitable only when the difficulty adjustments happen very quickly (see: BCH's EDA), so if 70% of the hashpower left, even if 16% of the remaining hashpower was malicious, the efficacy of a 51% will be handicapped by the disproportionately high difficulty.

[SilasX]

Sorry if I’m being dense here, but I don’t actually see how you’re disputing that the threshold for attacking a single fork has (in the example) fallen from 51% of all bitcoin-capable miners, to 16%.

I don’t care whether sloshing is “normal healthy behavior”. I care whether a fork has become easy to attack. You seem to focus on every topic but that one.

[turkeytotal]

I guess you are missing the entire point of this conversation. It is about the marginal cost of participating on multiple chains with PoW. You can point out oscillations as much as you like, it doesn't change the marginal cost. You are pointing out something that is entirely orthogonal.

[SilasX]

Your point was different from my point. I entered the thread to dispute that PoW schemes aren't weakened by forks. Here's the exchange where I came in[1]:

Original comment: "Also, with Proof of [Work] you get protection against an explosion of too many hard forks..."

My reply: "That's not what happened with the Bitcoin Cash fork though ... any given time, one of the networks is easy[er than 51%] to attack."

If you're not disputing that point, then you're not refuting my objection to that original comment.

[1] https://news.ycombinator.com/item?id=15606486