Follow the momentum across the boundary
For light in vacuum, optical momentum flow is power divided by c in the propagation direction. In steady state, the optical force on an assembly is the incoming momentum flow minus all outgoing optical momentum flows. A complete accounting includes specular reflection, diffraction, diffuse scattering and transmission. Thermal emission adds another outgoing contribution.
The laser and beam dump must be placed explicitly relative to the measured system. If they are inside it, their recoil and intercepted light belong in the same accounting. Physical cables can also transmit force across the boundary.
Read the visual transcript
An external beam travels toward a surface. The reflected direction changes from 0 to 30 to 45 degrees and back to 30 degrees, measured from the backward normal. The recoil arrow changes with the outgoing momentum. This shows a single reflected channel, not all optical loss channels. No measured force is plotted.
k denotes a unit propagation vector.
The reference force is deliberately ordinary
At normal incidence, an ideal stationary mirror intercepting 2 W receives 2P/c = 13.343 nN. A perfect absorber receives P/c = 6.671 nN before thermal recoil is included. These values are theoretical reference forces, not reported Metric Flight measurements.
For an opaque device with reflected power fraction R concentrated into one beam at angle θ from the backward normal, the components are Fn = P(1 + R cos θ)/c and |Ft| = RP|sin θ|/c. Real devices require the power in every outgoing channel. A steering efficiency is not automatically the total reflectance.
Control is useful only after loss is counted
A programmable surface could change the direction of the outgoing optical momentum. The design question is how much useful control remains after absorption, unwanted diffraction, heating and supporting hardware are included. A large phase shift alone does not answer that question.
Our immediate optical gate is a traceable comparison with a published device response. Source geometry, material data, bias conditions, polarization and extraction uncertainty must be resolved before a numerical model can be called a reproduction. Hardware purchasing should follow that comparison, not substitute for it.
Questions worth asking
Does a resonant cavity multiply spacecraft thrust?
It can increase internal optical forces, but net force on the complete assembly still follows momentum crossing its boundary.
Is the 13.343 nN force already measured by Metric Flight?
No. It is the ideal 2 W mirror reference used to define the proposed benchmark.
SOURCES & SCOPE / REVIEWED 23 OCTOBER 2026
- Relativistic spacecraft propelled by directed energy — Kulkarni, Lubin & Zhang ↗Published theoretical treatment; not Metric Flight hardware.
- Metric Flight: current MF-01 research package ↗Proposed measurement program, source records and open requirements.
External research is not Metric Flight performance. Concept images and diagrams are illustrative. This project introduces no new company experimental result.
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