Category: Distinctions routinely treated as synonyms

The list below is believed complete, though membership – like everything here – is subject to revision.

• Circle – The set of points at one fixed distance from one point. A curve, not the region it encloses, and the definition several doctrines here decline to read.
• The CMB dipole – The warm side of the sky: a 3.4 millikelvin asymmetry caused entirely by our own motion, and used to calibrate the instruments that measure it.
• The double-slit experiment – The experiment that settled whether light is a wave, and is now chiefly cited for a claim about consciousness resting on one word being read as ordinary English.
• Flat – Of no curvature anywhere – a plane. A stronger condition than 'level' or 'horizontal': a tilted floor is flat but not level, and still water is level but not flat.
• Foucault's pendulum – A weight on a wire that reports the Earth's rotation without reference to the sky, at a rate that goes exactly as the sine of the latitude.
• General relativity – A theory of gravitation containing no force of gravity, built out of the fact that a falling man feels nothing at all.
• GPS – The most thoroughly relativistic machine in ordinary civilian use, whose clocks were detuned before launch, and which is regularly offered as evidence that relativity is unnecessary.
• Gravity – The weakest of the four forces and the worst-measured quantity in physics: sixteen laboratories have determined its constant and they do not agree.
• Grimble's razor – A principle of economy in argument: where a proposition entails something that cannot be entertained, the entailment is set aside and the proposition retained.
• Horizontal – The plane perpendicular to the plumb line – flat, and tangent to the level surface, but true only where it is taken; two distant horizontals are not parallel.
• The isotropic antenna – The reference that antenna gain is measured against. None has been built, and the standard defining it calls it hypothetical in the definition.
• Kinematic and dynamic equivalence – Two claims about reference frames: the first true and never disputed, the second contested, the two offered as one.
• Level – The geodetic sense of the word: a surface everywhere perpendicular to gravity, and therefore – over any distance – curved. Still water is the demonstration.
• Mirage – Not a trick of the mind: a real image of a real object, correctly formed, arriving from a direction the object is not in. It photographs, which settles it.
• Newton's bucket – The last of five arguments for absolute space, now quoted alone and in support of the position it opposed.
• Occam's razor – The principle of parsimony, named for a man who did not write the words, did not originate the idea, and was not the person the phrase was coined about.
• Orbits – A fall that keeps missing. The space station drops 4.32 metres a second, and over the 7.66 km it covers meanwhile, its own orbit drops 4.32 metres.
• The Sagnac effect – The fringe shift from sending light both ways round a rotating loop, offered as the disproof of relativity, and in fact a direct measurement of the relativity of simultaneity.
• Schrödinger's cat – A thought experiment devised in 1935 to show a reading of quantum mechanics absurd, now the emblem of the reading it was built to ridicule.
• A. Sterling Sill – Victorian gentleman (1821–1879) of independent means and a single proposition – that the horizontal does not slope – which is locally true, globally fatal, and as far as he ever got.
• Special relativity – Two postulates and their consequences, named after the wrong half of itself: not that everything is relative, but that the speed of light is not.
• Sphere – The set of points at one fixed distance from one point in space. A surface, not the solid it bounds, and no part of it will lie flat.
• Wave–particle duality – The finding that light and matter behave in ways the words wave and particle were built to keep apart. The difficulty is in the words.

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