Greg Egan
SF writer / computer programmer
Latest novel: MORPHOTROPHIC
Latest collection: SLEEP AND THE SOUL
Web site: http://gregegan.net
Also: @gregeganSF@mathstodon.xyz
- In special relativity, observers in relative motion will disagree about various measurements. But if Alice draws any planar figure, and fires a simultaneous pulse of light from each point on it, perpendicular to the plane, any observer will agree on the figure’s shape and size.
- What? Simultaneity is an observer-dependent notion! But if Alice uses *her* definition of simultaneity, she will encode her chosen planar figure in the collection of light pulses in a frame-invariant way.
- What happens if Bob, flying past in a spaceship, uses *his* definition of simultaneity to pick different events on the worldlines of the same light pulses? Suppose A is a unit-length spacelike vector orthogonal to Alice's worldline, and C is a lightlike vector orthogonal to A.
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View full threadSo whatever events on those two worldlines Bob considers to be simultaneous, he still ends up measuring exactly the same distance between them as Alice does.
- I’m sure the writers of “Death By Lightning” know exactly what they’re doing with the anachronistic dialog: endless F-words, “quality time”, “crash and burn” etc. It’s fun, in a West Wing With Beards way. But it still makes me nervous about trusting them on any of the history.
- Today in the supermarket I scanned 2 packs of flour … and one of them was listed on the receipt as frozen raspberries. So that must have been a genuine barcode misread, rather than just incorrect data in a product database. The barcodes looked fine, so maybe the reader glitched.
- Two pairs of ants set off from the same starting point, walking side-by-side. One pair takes the red path, the other pair takes the blue path. The red and blue paths are geodesics (the straightest possible paths, like great circles on a sphere) …
- … and each ant in a pair follows a geodesic that lies a small distance to either side of the marked one, initially pointing in the same direction. If this ellipsoid has semi-axes of 1 and 3, the ants taking the blue path will bump into each other at the “pole” …
- … where all meridians converge, after travelling a distance of 3.34122. How far will the ants following the red path travel before they bump into each other? More, less, the same?
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View full threadThese are the paths of the individual ants, from the starting point until they bump into each other.
- Just rewatched “The Wages of Fear”, which suited the ambience on a 37C day. I last saw it 40 years ago, and I’d forgotten everything but the overall premise, so it was every bit as jaw-dropping as if I was watching for the first time. 4.9/5 stars, with -0.1 for the very end.
- “Can a time capsule outlast geology? A ridiculous but instructive thought experiment involving deep time, plate tectonics, erosion and the slow death of the sun” This is hugely enjoyable! The subheading is exactly right: don’t take the premise seriously, just go along for the ride.
