Einstein's universe

What Hacker News says

1. fennecfoxenJun 2020
   When you send a rocket into space, you burn chemicals (releasing energy located in electrons in the chemical bonds holding the atoms together) gaining thermal energy (the energy of moving atoms in hot rocket exhaust) which is then redirected into kinetic energy (still moving atoms, but they're all moving in the same direction, and more orderly about it).

   When the spaceship moves upwards relative to a large mass (Earth), this kinetic energy is traded for gravitational potential energy. It's still associated with the atoms and other particles, just like the kinetic energy was.

   This does mean that objects in distant space have more of this energy than objects at the bottom of a gravity well, and are therefore more massive (because of mass-energy equivalence). Falling into an intense gravity well is actually one of the more efficient ways to convert mass into energy, which is why the area around black holes is often particularly energetic, with the matter falling in emitting energetic X-rays. This is why you can see quasars even though they are billions of light-years away (the radiation heats the surrounding material and it is very bright.)

   Besides general physics reading, I can recommend the book "Einstein's Universe" to help build a general understanding of both special and general relativity, with particular regard to matters such as mass-energy equivalence.

   Show morethread
2. eggyApr 2026
   I have loved math since I was a child, and I think it depends on when you grew up and how steeped you are in reality vs. the virtual or the computer world, and how much of an abstract vs. concrete thinker you are. I was always making things in modeling clay, that greasy grey-green stuff, and so my scale was what I could make out of one brick of such stuff. I bought my first computer in 1977 (Commodore PET 2001), and the CBM ASCII set had some graphics, but nothing compared with today's graphics. My first encounter with visualization and scale was writing a program to let me know which of the four moons of Jupiter I was seeing in the sky that night. Io, Ganymede, Callisto, and Europa's orbits are almost edge-on to our view from earth, so I made Jupiter a capital O, and the moons were lowercase letters. I printed this out on a thermal printer (like a wide receipt). Cosmos was the rage on TV and I had read Einstein's Universe by Nigel Calder. I had a telescope and a microscope, so the micro and macro were very real to me. I suspect if you grew up on tablets and only built things on a 3D printer scale, you don't have that unbridled sense of the small and large except on very abstract terms. However, not a donut, not a universe-scale torus, but rather a pool donut comes to mind when I first hear torus! I built an XYZ router table in the early 2000s out of old stepper motors. It was 8'x4', and I built stitch-and-glue wooden kayaks from the panels I cut on it. These would wind up being 16 to 22 foot long kayaks to go into the real world and have fun!
   Show morethread