Computation Is Physical
This closing lesson unites the physics and the computing halves of the course with one idea: computation is a physical process. Every bit stored, every gate switched, and every message sent obeys the laws of physics — which both enable computing and limit it.
Signals carry information physically. A bit is represented by a physical quantity — a voltage level in a wire (Unit 6's 0 and 1), a charge in a memory cell, a pulse of light in a fiber, a magnetic orientation on a disk. Real signals are analog (continuous voltages) that we interpret digitally by thresholding — above a level is 1, below is 0. This digital abstraction gives noise immunity: small analog disturbances do not flip the interpreted bit, which is why digital systems are so reliable. Converting between the continuous physical world and discrete bits (analog-to-digital conversion) is itself governed by physics (the sampling theorem).
Physics sets hard limits. Switching a transistor takes energy and produces heat (Unit 3's power, ) — so a chip's performance is bounded by how fast heat can be removed, the reason clock speeds plateaued and computing went parallel. The speed of light limits how fast a signal crosses a chip, bounding the clock (Unit 6's critical path). And thermodynamics sets a theoretical floor on the energy to erase a bit (Landauer's principle).
Physics enables new computing. Semiconductors gave us the transistor; light gives us fiber-optic communication; and quantum mechanics — where a bit becomes a qubit in superposition — promises a fundamentally new model of computation. The abstract world of algorithms and the physical world of electrons, photons, and heat are inseparable: mastering computing ultimately means understanding that information is always embodied in something physical.
Common pitfall: treating computation as purely abstract, divorced from physics. Every bit is embodied — as a voltage, charge, or magnetic state — and every operation costs energy and time bounded by physical law (heat dissipation, the speed of light, thermodynamic limits). Ignoring this makes real constraints (why chips run hot, why clocks stopped getting faster, why signals need thresholds) inexplicable; computation is physical, not free-floating.