Same difference 

I'd prefer "basis we just so happen to be measuring in". Or "measurement basis" for short.

You could use "pointer variable", but this would commit you to writing several more paragraphs to unpack what it means (which I encourage you to do, maybe in a later post).

Your use of "pure state" is totally different to the standard definition (namely rank(rho)=1). I suggest using a different term.

The QM state space has a preferred inner product, which we can use to e.g. dualize a (0,2) tensor (i.e. a thing that eats takes two vectors and gives a number) into a (1,1) tensor (i.e. an operator). So we can think of it either way.

Oops, good spot! I meant to write 1 minus that quantity. I've edited the OP.

This seems very interesting, but I think your post could do with a lot more detail. How were the correlations computed? How strongly do they support PRH? How was the OOD data generated? I'm sure the answers could be pieced together from the notebook, but most people won't click through and read the code.

Ah, I think I understand. Let me write it out to double-check, and in case it helps others.

Say $\delta =0$, for simplicity. Then $A^l={\sum }_t{E}_t{a}_t^l$ . This sum has $k$ nonzero terms. 

In your construction, $W^{l,in}={\sum }_t{V}_t^l{W}_t^{l,in}{E}_t^T$. Focussing on a single neuron, labelled by $i$, we have $(W^{l,in}{)}_i={\sum }_t(V_t^l{)}_i{W}_t^{l,in}{E}_t^T$. This sum has $\sim pT$ nonzero terms.

So the preactivation of an MLP hidden neuron in the big network is $p_i^l={\sum }_{t,t^{\prime }}(V_t^l{)}_i{W}_t^{l,in}{E}_t^T{E}_{t^{\prime }}{a}_{t^{\prime }}^l$ . This sum has $\sim kpT$ nonzero terms. 

We only "want" the terms where $t=t^{\prime }$; the rest (i.e. the majority) are noise. Each noise term in the sum is a random vector, so each of the $\sim kpT$ different noise terms are roughly orthogonal, and so the norm of the noise is $O\left(\sqrt{kpT}\right)$ (times some other factors, but this captures the $T$-dependence, which is what I was confused about).

I'm confused by the read-in bound:

${ϵ}_t^{l,in}=O\left(wa\sqrt{kT\frac{md}{MD}\log M}\right)$

Sure, each neuron reads from $T\frac{n\log M}{M}$of the random subspaces. But in all but $k$ of those subspaces, the big network's activations are smaller than $\delta$, right? So I was expecting a tighter bound - something like:

${ϵ}_t^{l,in}=O\left(wa\sqrt{(k+T\delta )\frac{md}{MD}\log M}\right)$

Ah, so I think you're saying "You've explained to me the precise reason why energy and momentum (i.e. time and space) are different at the fundamental level, but why does this lead to the differences we observe between energy and momentum (time and space) at the macro-level?

This is a great question, and as with any question of the form "why does this property emerge from these basic rules", there's unlikely to be a short answer. E.g. if you said "given our understanding of the standard model, explain how a cell works", I'd have to reply "uhh, get out a pen and paper and get ready to churn through equations for several decades".

In this case, one might be able to point to a few key points that tell the rough story. You'd want to look at properties of solutions PDEs on manifolds with metric of signature (1,3) (which means "one direction on the manifold is different to the other three, in that it carries a minus sign in the metric compared to the others in the metric"). I imagine that, generically, these solutions behave differently with respect to the "1" direction and the "3" directions. These differences will lead to the rest of the emergent differences between space and time. Sorry I can't be more specific!

> could one replace the energy-first formulations of quantum mechanics with momentum-first formulations?

Momentum is to space what energy is to time. Precisely, energy generates (in the Lie group sense) time-translations, whereas momentum generates spatial translations. So any question about ways in which energy and momentum differ is really a question about how time and space differ.

In ordinary quantum mechanics, time and space are treated very differently: $t$ is a coordinate whereas $x$ is a dynamical variable (which happens to be operator-valued). The equations of QM tell us how $x$ evolves as a function of $t$.

But ordinary QM was long-ago replaced by quantum field theory, in which time and space are on a much more even footing: they are both coordinates, and the equations of QFT tell us how a third thing (the field $\varphi (x,t)$) evolves as a function of $x$ and $t$. Now, the only difference between time and space is that there is only one dimension of the former but three of the latter (there may be some other very subtle differences I'm glossing over here, but I wouldn't be surprised if they ultimately stem from this one).

All of this is to say: our best theory of how nature works (QFT), is neither formulated as "energy-first" nor as "momentum-first". Instead, energy and momentum are on fairly equal footing.