According to Wikipedia, solipsism is the philosophical idea that only one's own mind is sure to exist. In 1993, Cynthia Dwork and Moni Naor proposed the idea that one could use proof-of-work to combat spam e-mail. The same concept was later adopted by Bitcoin.
The idea behind proof-of-work is simple. Using a cryptographic hash function (in Bitcoin's case SHA-256) we have to find an input that produces an SHA-256 hash with some agreed special property. Since SHA-256 is presumed to be a one-way function there's no better way to find such an input than to use brute force to generate inputs and see if their SHA-256 hash satisfies the given property. The expected amount of trial-and-error required to produce a hash with the desired property is inversely proportional to the proportion of hashes satisfying the property.
In Bitcoin's case the desired property is that the calculated hash, interpreted as an integer in little-endian byte order, is below some given threshold. In the case of Bitcoin the number of SHA-256 calculations required to generate such hashes has been more than 10^26 in some cases. For example, if you take the number 98468625188936598445757615969686194678278100228635623545838523999870405803634, represent it in big-endian binary format, calculate the SHA-256 and reverse the bytes, you get the 256-bit hexadecimal number 00000000000000000000011246f099d94f91628d71c9d75ad2f9a06e2beb7e92, which has 87 leading zero bits. With a high probability, finding this number has taken more than 2^87 ≈ 1.5*10^26 SHA-256 computations.
You can find a Python script and a few more examples here.
What has all this to do with solipsism? The fact that we can name such numbers shows that there are computational resources capable of producing them (i.e. calculating more than 10^26 SHA-256 hashes). On the other hand such extreme amounts of computation are beyond the capability of human mind by about 20 orders of magnitude. Even if all 7.5 billion humans on Earth were to compute SHA-256 hashes at the rate of 1 per second (1 per day is more realistic), it would still have taken more than 400 million years to produce the number above. Thus we can conclude that vast computational resources must exist outside one's mind and that solipsism is false.
About assumptions: It is not rigorously known that SHA-256 is computationally hard to invert. In particular, proving that one-way functions exists would show that P!=NP.