Sound is the hands. Light is the fingertips.
Having both carriers is not redundancy — it is the only way to get real force and high bandwidth in the same instrument. Per watt, sound pushes matter 10⁵–10⁶ times harder than light, because a phonon carries its energy at the speed of sound and momentum is energy divided by speed. Light gives that up, and gets everything else: nanosecond propagation, six decades of frequency, and sensing precision limited by signal quality rather than wavelength.
One boundary, three roles.
Light
Every voxel gets its own signature; every build instant is also a measurement. The optical stem in the bore is a full two-way holographic port — patterns out, state readings back.
Sound + induction
Acoustic holograms trap and carry grains; induction lifts and melts conductive droplets. All the momentum, delivered from the boundary, spent only where the pattern says.
Electrons
Never information. Copper exists at the boundary to power the field — the field does everything else. This split is derived, not preferred: the physics itself forbids light from being muscle.
The tempo follows the same split: acoustic builds, EM verifies. Electromagnetic chords ring up about ten thousand times faster than the acoustic chords they check — so verification is effectively continuous, running inside every acoustic cycle.
Same frequency, five to six orders finer.
At any given frequency, sound's wavelength is about a hundred thousand times shorter than light's — which means sound sculpts fine spatial structure with electronics that light would need terahertz hardware to match. The machine spends that gap deliberately: sound shapes sub-millimetre structure from hundreds of kilohertz, light brings its speed and its six decades of spectrum to addressing and verification.
Hands bring the part to the fingertips.
Each carrier owns the scale where its physics wins. Sound delivers the payload into the optical working zone; light does the final registration. Every hand-off is chosen by the same rule: momentum where mass needs moving, wavelength where precision needs placing.
Computed holograms — in each other's medium.
The currency of the crossings is not power — it is addressing, verification, and measurement diversity. Each carrier can serve as the other's editor and the other's witness.
Light writes sound
A structured light pattern absorbed on any visible surface launches a shaped acoustic wave — including on the workpiece itself, which becomes a programmable acoustic source with optically defined elements.
Light reads sound
The drive field bends light measurably, so the acoustic hologram stops being inferred from its ports and becomes photographed — a second, independent witness on every build.
Sound shapes light
Pressure modulates the refractive index, making the acoustic field itself an optical element — a precision trim channel in air that strengthens by orders of magnitude in melt, where the machine does its densest work.
The Two Carriers paper is the machine's vocabulary: every degree of freedom each carrier offers, which ones the design spends today, and which are held in reserve — headroom the architecture can convert into performance next.