This was actually one of the reasons Bitcoin Core devs were opposed to raising the block limit. Bitcoin requires very little bandwidth (around 1.5mb/10minutes).

You can actually get the Bitcoin blockchain from Satellite for free: https://blockstream.com/satellite/ However, as I understand it, it only allows receiving and not transmitting data.

>However, as I understand it, it only allows receiving and not transmitting data.

what about something like shortwave radio? You probably can't have everyone transmitting at the same time, but a few stations around to world can broadcast/relay transactions across pockets of connectivity.

Very interesting thought. I believe that chains would initially split, effectively being forks of each other. However, the consensus algorithm of Bitcoin dictates that the chain with the most work is the valid one, meaning that in the event of a "reconnection", one of the chains would be abandoned (and everyone would agree which one to keep, namely the one with the most work).

In the case of a total disconnect between countries, there is no other way than splitting the network. If partial connectivity exists (using satellites for example), transactions could still be broadcasted from one country to another. However, the same coins could be double-spent in your own isolated network as well... until a reorg happens upon reconnect.

Philosophical question: At what point do divergent chains become irreconcilably and separately valid?

Is this a property we would ever want in a should-be decentralized currency?

One way this can happen today is with intentional forks.

That is actually a good and very practical question.

I think it reduces to the chains will always have to be reconciled because you would otherwise open yourself up to N% attacks where N is whatever threshold you decided was "safe.'

I'm guessing that when they all reconnect again the longest chain would win and basically all the other forks would roll back, so essentially only the transactions of the continent with the most hash power would stay.

IIRC bitcoin does not have any conflict resolution besides "most work wins".

But won't each continent adjust its difficulty to account for its local hashrate? And would a high difficulty fork accept that a low difficulty longer fork is valid? Or would it reject it for having invalid blocks?

The algorithm is tuned so that a block is mined about every 10min no matter what the hashing power available is. In the case of a serious and persistent network partition, both “sides” would continue to mine blocks at roughly the same rate as before, minus the first blocks that would take a while as the algorithm tried to catch up to the drastically reduced local hashing power.

The real issue is what happens once the partition is healed, and it’s not a technical problem. The technical solution is straightforward: longest chain wins. The real issue is a social one: how do you deal with finding out that the last N months worth of transactions are just gone?

> The technical solution is straightforward: longest chain wins.

One caveat here is that "longest chain" is measured not in blocks but rather in cumulative difficulty. So the fork with more hashing power, having a higher difficulty per block, will mine a "longer" chain over the same time period.

> The real issue is a social one: how do you deal with finding out that the last N months worth of transactions are just gone?

If the transactions are still valid (i.e. the inputs haven't been spent in diverging transactions on both chains) then they can be reintegrated into new blocks on the winning fork after the partition heals.

The real problems come from double-spends, whether deliberate or accidental; if funds were moved from TxA to InB in one chain and from TxA to InC in the other then only one of these can be retained, and all transactions downstream from the losing version will be nullified.

In general, to create such a situation a client would need to be aware of both forks and sign conflicting transactions for the same unspent output. There is one major exception, however, which is that the coinbase transactions introducing new block rewards on the losing fork can never be valid inputs for transactions on the winning fork. Ergo, any transaction which depends on recently mined coins is ineligible for reintegration.

If I'm stranded in Antarctica, and decide to compute hashes with my pocket calculator to pass the time, and drop the difficulty to zero (because I can do one hash every 10 min, so they all have to be winners), does the bitcoin protocol say that my chain of empty blocks is a valid contender for longest chain? Or does "longest chain" also take into account difficulty? (Setting aside the obvious fact that in reality everyone would obviously just ignore me)

This isn’t quite how the algorithm works.

To make a valid block, you have to start with another valid block. The difficulty is set (edit: I was wrong, and I fixed this) every 2016 blocks based on how long they took to mine. This works out to about 2 weeks at the expected rate of mining. The average time of those 2016 blocks is then used to calculate the difficulty of the next 2016 blocks, and the difficulty is marked down in the chain itself.

So in your hypothetical you’d have to solve around two weeks worth of blocks at the current difficulty level, which isn’t possible in a human lifespan. After that the difficulty for your chain would rapidly fall, but your sub-chain would already be very far behind and would never catch up.

That makes sense, I remembered that it adjusted every 2 weeks, but I forgot that the 2 weeks was actually measured in blocks, not wall time.

In practice you wouldn’t be able to mine at all, because the difficulty updates infrequently, so you would have to mine several blocks at the old difficulty. Obviously this is impossible on a pocket calculator.

I’ve always assumed that the notion of longest chain takes difficulty into account, although I haven’t really been able to find confirmation of this.

> It is well known that major military powers have the ability to cut essentially every ocean-floor optical fiber bundle handling the backbone of the internet at a moment’s notice.

I'm not doubting your statement, but I would love to hear where you got that point from, since it seems really interesting.

I hope mesh networks and satellite internet connections are robust enough to counter this.

I think starlink is up and would provide the secondary line.