Hi everybody. I'm beginning my first course on quantum physics, and our professor introduced the box normalization for plane waves. My question is: why do we need to impose conditions on the borders? I haven't been able to find any explanation on the internet, as every text I found just introduce these conditions (which leads to a discretization of the spectrum) without explaining the physical motivation behind them. I asked my professor, he replied in quite elusive way: he said that such condition is there because it should stand for any kind of wave function, even the constant one (which would fail to satisfy any other kind of bonduary condition). This is rather a reply to "why this particular condition (ψ(x, y, z)=ψ(x+L, y, z)=ψ(x, y+L, z)=ψ(x, y, z+L))?" instead of my question, that is "why impose any condition at all?" Thanks in advance.