Educational calculation · inputs are assumptions, not device specifications
TRY THE MOMENTUM BUDGET
Turn the beam. Read the force.
Normal incidence in vacuum. One outgoing order; all remaining light has zero net lateral momentum. These inputs are illustrative, not measured graphene-device performance.
Fx = −ηP sin(θ) / c
c = 299,792,458 m/s. η is the fraction of incident power in the selected outgoing order, not the absorption. Total moving mass includes the complete assembly. Changing the sign of θ describes ideal symmetric states.
LATERAL RECOIL−13.343 nN
Acceleration magnitude1.3343e−7 m/s²Ideal reversal difference26.685 nN
05
Interception, absorption and mass all count.
The calculator uses optical power reaching the surface. It does not assume the full output of a distant laser reaches a spacecraft, and it does not include launch, pointing, trajectory or relativistic dynamics.
Why a light membrane is interesting
A lightweight optical surface can respond appreciably to a small force, but the mission carries a payload and control hardware too. A proposed sail should report total areal mass and total system mass. Comparing the graphene sheet alone with an entire conventional sail would conceal the main engineering trade.
What has been tested
Gaudenzi, Stefani and Cartamil-Bueno studied graphene-on-copper-grid sails in vacuum and microgravity in 2020. Their supported samples moved under laser illumination. The authors reported thrust above their radiation-pressure calculation, leaving a mechanism question to investigate. This is useful experimental context, not permission to assume an unexplained thrust multiplier.
Bare graphene is not a perfect reflector
A suspended monolayer absorbs roughly 2.3% of visible light and transmits most of the remainder. That absorption heats an extremely thin material. A reflective or diffractive hybrid can behave very differently. Optical measurements of the whole stack determine which model applies; a monolayer absorption value cannot be copied into every graphene design.
The route from a sample to a mission
After a bench result, the next questions are beam interception, spectral response, thermal equilibrium, sail deformation and stable control. Strong material properties do not settle those questions. A useful first application may involve precise orientation of a small spacecraft rather than rapid transport of a large vehicle. The mission requirement must set the needed force.
QUESTIONS THAT MATTER
Does a light sail use propellant?
A photon sail can exchange momentum with incoming and outgoing radiation without expelling onboard material propellant. It still requires an illumination source and a complete energy budget.
Why can’t a pattern provide unlimited photon thrust?
The force follows the change in photon momentum. In vacuum, an ideal passive single-pass reflector at normal incidence receives 2P/c; geometry does not eliminate this budget.
FOLLOW THE EVIDENCE
Go to the original work.
These are independent publications. They are not Metric Flight results or endorsements.