Why the collisionless reconnection rate is close to 0.1
In plain words
Computer simulations and spacecraft find that reconnection without particle collisions always proceeds at about one tenth of a natural speed limit. Nobody has a derivation that explains why this number is so universal.
Precise statement
In collisionless electron-ion reconnection (realistic $m_i/m_e$, guide field $B_g/B_0$ from 0 to above 1, plasma $\beta\ 0.01\ \text{to}\ 1$, symmetric and asymmetric inflows), the normalized rate $E_{\mathrm{rec}} c/(B_0 v_A)$ is about 0.1 in kinetic simulations and MMS data, nearly independent of the dissipation mechanism. Derive from first principles the mechanism that selects this value and its dependence on $B_g/B_0,\beta$ and inflow asymmetry in 2D and 3D. An answer is a derived formula that matches fully kinetic simulations to within 20 percent across these parameters.
What would settle it
A first-principles theory whose predicted rate and parameter dependences agree with large 3D particle-in-cell scans and MMS event statistics.
Status in the literature
A 2017 geometric argument (Liu et al., PRL 118, 085101) bounds the local rate near 0.1, but whether it is the selection mechanism, especially in 3D, is still debated.