BLOCKCHAIN & WEB3 FUNDAMENTALS - Chapter 1, Exercise 3 Solution ========================================================== Why a Plain File Can't Be Money PROBLEM ------- Explain, in your own words, why a plain digital file (like a Word document representing "£10") cannot function as money on its own, and identify specifically which property of physical cash that file is missing. SOLUTION -------- A plain digital file - any file, on any computer - can be copied perfectly and instantly, as many times as you like, with absolutely no way to distinguish the "original" from a copy. If you emailed a file called ten_pounds.docx to one friend, there is nothing stopping you from emailing that exact same file to a second friend five minutes later. Both files would be byte-for-byte identical; neither would be more "real" than the other. Physical cash doesn't have this problem, because it isn't information at all in the same sense - it's a specific physical object. A real £10 note exists in exactly one place at a time. Handing it to someone physically removes it from your own possession, so the very act of paying someone is also the act of no longer having the money yourself. There's no way to "copy" a banknote and end up with two genuinely spendable notes (counterfeiting aside, which is a different, physical-forgery problem, not a copying problem). The property physical cash has that a plain digital file lacks is therefore: the physical impossibility of being in two places, or two people's possession, at the same time. This property is sometimes called "non-duplicability" or, in economics, part of what gives a physical token real scarcity. A digital currency has to somehow recreate that same property - guaranteeing that a given unit of money can only be "possessed" and spent by one party at a time - using nothing but data, which is inherently, trivially copyable. That's precisely the double-spending problem this chapter opens with, and it's why every serious attempt at digital money (going all the way back to David Chaum's work in the 1980s) has had to build some mechanism specifically to prevent duplication being usable as currency, rather than treating "make a digital token" as the hard part. ANSWER: A plain file can be copied with zero cost and zero distinguishability between the copy and the original, so nothing stops the same "money" being spent twice. Physical cash instead relies on being a single physical object that can only be in one place, and therefore in one person's possession, at any given moment - a property plain digital information simply doesn't have on its own. ---- WHY THIS WORKS AS AN ANSWER This isolates the exact property (physical non-duplicability) that makes cash resistant to double-spending "for free," and shows why digital information needs a deliberately engineered mechanism to recreate that same guarantee - directly setting up why the rest of this course exists at all.