Because everything is composed of quanta and when quanta interact their wavefunctions form a superposition.
This is arguable. Von Neuman held that because everything is composed of quanta, and wavefunction collapse requires something that is not quantum based, then he held that that was human consciousness.
This doesn’t work for me though since you can have a mechanical device doing the detection and mess the interference pattern up. A consciousness is a step removed.
It seems the universe despises any kind of record keeping (e.g. anything that could record the photon going through one or the other slit). As long as we cannot know then the universe feels free to mess with our heads.
But how could the universe know that?
And, could you explain how you know that without making an observation?
But if human consciousness is not ‘made out of quanta’, how could it interact with something that is?
The ‘explanation’ that consciousness causes collapse was never anything other than subbing in one mystery for another – we don’t understand how a classical world emerges from the quantum level, and we don’t understand how consciousness emerges from non-conscious processes, so hey, let’s just tape those two together! The problem is that explaining one mystery in terms of another mystery amounts to explaining nothing at all.
Thankfully, we’ve since made great progress on both issues (in quantum mechanics mainly through the development of the decoherence picture, which produces the appearance of wave-function collapse while featuring nothing but unitary quantum evolution), so that it no longer seems necessary to wallpaper over our ignorance like that.
The current understanding in quantum mechanics of wavefunction collapse is that consciousness is not a necessary condition. Sufficient, but not necessary. Any physical interaction between a superposition and an external object will collapse the wavefunction. This is what wbeaty was talking about with the shadow coin interacting with air and ambient radiation.
I’d like to see an actual quantum mechanics equation that has “consciousness” in it somewhere.
Well, i * h-bar * d(|consciousness>)/dt = H |consciousness>.
But I suspect that’s not what you were after — and, I suppose, assumes that consciousness is in fact describable in terms of a quantum-mechanical state. (Which I do think is the case, but I realize that there’s plenty of debate about this question.)