Here's why wavefunction collapse matters, or might. A few months ago, we were debating the "End of the Golden age of crypto", and that went into a discussion of the inherent (or not!) difficulty wrt factoring large numbers consisting of big primes. And the question that was raised (and is still on the table), is whether 1) is it possible that one day factoring will be 'cracked' and become much easier, 2) whether factoring will always remain dificult, or 3) will we be able to prove 2 above or will we have to live with the uncertainty forever? Quantum Cryptography is a possibility that could allow 'us' (ie, whoever will have access to it) to 'know' for sure that our transmission is secure, based on physical law. This security only exists, however, if we believe current quantum mechanical theory, which says that... 1) Wavefunction collapse happens at measurement time 2) There are no hidden variables 3) ANY attempt to read what the photon states 'really' are will collapse the wavefunction 4) An eavesdropper of a Quantum-encrypted code must necessarily reveal their presence, due tpo the fundamental QM laws of the universe.(4 logically follows from 1,2 and 3 above.) In other words, the use of Quantum Cryptography is an attempt to eliminate the uncertainty associated with difficult (but classical) cryptosystems. But it only eliminates the uncertainty if we believe reality really works this way. TD (#1 and #2)