Let's imagine this hypothetical scenario: cryptocurrencies and, more specifically, Bitcoin, see massive adoption and become an integral part of society. At the same time, we become an interplanetary species that extend their reach to the Moon, Mars, and beyond. How do we manage our digital currencies across such vast distances? This scenario is discussed in the short, but thought-provoking paper Hypothetical problems concerning the theory of relativity on cryptographic currency implementations. Could a cryptocurrency like Bitcoin function universally, given the constraints of the laws of physics?. Let's have a look!
Bitcoin relies on a global consensus system where every transaction can be validated by any miner worldwide. However, information transmitted through the internet can't travel faster than the speed of light. The almost instantaneous validation of transactions on blockchains on Earth is possible since they propagate in seconds. Consider that it takes light about 0.13 seconds to complete one trip around the Earth. For a light signal to travel to the moon and back, it is approximately 2.56 seconds, still feasible! But between Earth and Mars, the delay could be up to 24 minutes, depending on their relative positions. For interstellar distances, it could take years. Not very convenient!
This delay would create massive problems. For instance, there would be forks everywhere, since competing versions of the blockchain could emerge as miners in different locations unknowingly would validate conflicting transactions. In spacetime regions that are causally disconnected, that is, that they can't communicate to each other via light rays, there would be double spending, as malicious actors could exploit the delay to spend the same coins twice.
Einstein's general relativity makes things worse due to the effect of time dilation: miners on planets with different gravitational fields or moving at high speeds would experience time differently, which would disrupt Bitcoin's block timing, which relies on synchronized clocks. Moreover, the larger the network, the longer it takes for blocks to propagate. This would increase the likelihood of "orphan chains", i.e. blocks that are rejected because they arrive too late.
Given all these physical constraints, universal cryptocurrencies might not be feasible, and blockchains would be meant to be localized. We can imagine that each planet or star system could have its own blockchain, with periodic synchronization between them. Whenever we travel among systems with different cryptocurrencies, we could have interstellar exchanges that allow currency trade among different regions. However, in the far future we can imagine relativity-aware protocols, which consist of advanced algorithms that could account for time dilation and propagation delays, but this would require significant innovation!
This thought experiment might not only apply to Blockchains, but to any network dealing with extreme latency or synchronization issues. Today this might look like science fiction, but for all of those crypto-enthusiasts that are also immortalists, this might be a real issue to take care in the future. Tech moguls as Elon Musk might not be able to take his beloved Doge to Mars, after all!
