Illustrated Physics
physics, drawn
Concept

Potential well — where it appears

A region in which the energy of a system has a minimum, so that a displaced body is pushed back toward it and can be bound rather than escape. Its depth decides what can escape and its curvature decides how fast anything trapped oscillates, and the two are independent properties.

Named by 5 essays across 3 fields — each of them below, with the objects they name alongside it.

One parabola, several wells. Unlike potential wells, each divided by its own curvature at the bottom, against the single parabola ½x² drawn through all of them. They agree near the minimum because a function with a minimum has no linear term there, so the quadratic term is the first thing it has. The labels give where each well departs from the parabola by more than 1% of the parabola's own value there: a pendulum at 0.35, a chemical bond at 0.01, a pair of atoms at 0.0015. A symmetric well has no cubic term and stays close for a long way; a well that is steeper on one side than the other has one, and leaves the parabola almost at once — which is why those numbers differ by factors of hundreds and not by a few per cent.

Every minimum is a parabola

A pendulum, a bond between two atoms and a ship rolling in a swell obey the same equation, and the reason is not that they are alike. It is that a function with a minimum has no linear term there, so the first thing every potential well looks like is the same well.

mechanics · Harmonic approximation The term that abolishes the inner orbits. The effective potential of the Schwarzschild geometry per unit mass, in units of c², at 4 angular momenta, against radius in Schwarzschild radii. Newton's version has a minimum — a stable circular orbit — at L̃²/GM for every angular momentum there is, however small, and it is the dashed curve at the same L̃, here with its minima at 10.13, 7.61, 6.00, 5.12 rs. General relativity adds one term, −GM L̃²/c²r³, and that minimum stops existing below a definite angular momentum. At L̃ = 4.50 GM/c the barrier and the well are still separate, at 1.83 and 8.29 rs. The two merge at L̃ = √12 GM/c, at 3 rs = 6GM/c² — the innermost stable circular orbit, where the curve drawn here has neither a maximum nor a minimum but a single inflection. Below that momentum — the curve at L̃ = 3.20 GM/c — there is no stationary point anywhere outside the horizon, so no circular orbit exists at all, at any angular momentum whatever. None of that is a property of matter; it is a statement about the geometry, and it fixes the energy of the innermost orbit at √(8/9) = 0.94281 of mc², so 5.719% of the rest mass has been radiated by anything that reached it. Every well drawn here was located on the emitted curve by golden section and checked against the closed form.

The orbit that cannot be made smaller

Newtonian gravity allows a stable circular path at every radius, however tight, and there is no innermost one. General relativity adds a single term to the expression that says so, and below 6GM/c² no stable circular path exists at any angular momentum whatever. Anything arriving there has radiated 5.72% of its rest mass, which is eight times what hydrogen fusion converts.

astrophysics · Orbit stability The average position of something that is only shaking. The mean displacement of an oscillator against temperature, taken as a Boltzmann average over each well rather than from any expansion of it, with the temperature measured against each well's own depth so that unlike bonds can share an axis. A symmetric well gives exactly zero at every temperature — heating a harmonic solid makes it vibrate harder and does not make it larger. The others drift outward, because the outward side is the shallower one, and the measured slopes are a pendulum 0.000, a chemical bond 0.766, a pair of atoms 0.150. Thermal expansion is not a property a spring has; it is one a spring lacks.

Why heating a perfect spring changes nothing

A harmonic solid vibrates harder when heated and does not get any longer. Thermal expansion lives entirely in the term that the harmonic approximation throws away — and so does the fact that a solid conducts heat at a finite rate, which is the same discarded term doing a second job nobody would have connected to the first.

thermodynamics · Harmonic approximation A bound state that lives outside its well. The radial probability density of the one bound state of a square well of radius R, against distance in units of R, for four depths just past the one that first binds a state. In units of ε = ħ²/2μR², a state bound by 0.00589 ε has 93 per cent of its probability outside the well and a mean radius of 7.0 R; a state bound by 0.0397 ε has 82 per cent of its probability outside the well and a mean radius of 3.0 R; a state bound by 0.245 ε has 62 per cent of its probability outside the well and a mean radius of 1.5 R; a state bound by 1.4 ε has 35 per cent of its probability outside the well and a mean radius of 0.9 R. The more weakly the state is bound, the further its exponential tail reaches, and the less of it is where the force is.

The molecule that lives outside its own well

The harmonic picture describes the bottom of a well, where a molecule vibrates near the length of its bond. The last bound level lives at the top, and when it is bound by almost nothing it is not near the bond at all. Two helium atoms whose potential well bottoms out at three ångströms form a molecule whose atoms sit on average about fifty ångströms apart, most of the time where the force between them is negligible. Such a state forgets the shape of the well that made it. Its size, its binding and the way it scatters are all set by one number, and nothing inside the well can be read from them.

mechanics · Harmonic approximation The shift a member galaxy shows against the centre. The predicted gravitational shift of a cluster member's light relative to the central galaxy's, in km/s, against the member's projected distance from the centre, averaged along the line of sight with the members assumed to follow the mass out to twice R200, for clusters of 1 × 10¹⁴ M☉, 3 × 10¹⁴ M☉, 1 × 10¹⁵ M☉. Every member is blueshifted relative to the centre, because its light climbs out of a shallower part of the well. For the heaviest the shift is −13.2 km/s at half a megaparsec and −21.6 at two; for the lightest, whose members end at 1.9 Mpc, −4.4 and −5.7 at one and a half. Projection blunts the profile: members seen near the centre include many that lie far in front or behind it.

The light that climbs out of a cluster of galaxies

A cluster of galaxies is a well a few megaparsecs wide, and its galaxies sit at different depths in it. Light from the one at the bottom climbs furthest, so every other member should look slightly blue beside it — by ten kilometres a second, against galaxies that rush about at a thousand. No single cluster can show it. Thousands stacked together can, but only once a second effect of exactly the same size, the slowing of the galaxies' own moving clocks, has been taken out — and it cannot be separated by choosing heavier clusters, because both are set by the same depth.

astrophysics · Gravitational redshift

Named alongside it

The objects these essays reach for when they reach for this one.

Harmonic approximationAnharmonicityAngular momentumApproximationBinding energyThe Boltzmann factorBound stateCentrifugal forceCircular motionCurvatureEffective potentialEfficiency

All concepts