DavidE

A comment on Arthur's episode 'Parallel Universes'. A hundred years ago, HG Wells wrote a novel: Men Like Gods, and in chapter 4, Section3, he suggested a way of looking at parallel universes. Imagine the pages in a book, each page being a two dimensional universe in a three dimensional higher universe. Each universe is next to two other universes. The inhabitants of each universe know nothing about the other universes.

Now move up one dimension, so a stack of three dimensional universes are stacked next to each other in a four dimensional space, that is each three dimensional universe is a 'surface' in 4 space.

This ties in with the idea of branes, short for membranes, stacked next to each other.

Men Like Gods is not one of Wells' best novels. In the novel the hero, and a few other people, are diving along a road in southern England when the road suddenly changes, as does the surrounding countryside. They had driven through a gap between two universes.

Why does hydrogen added to the Venusian atmosphere make it cooler? the 90 Earth atmospheric pressure of CO2 is the problem. How would you liquefy hydrogen from Jupiter while in orbit?

FJOyarzum. Excellent post. Absence of evidence is not evidence of absence.

www.scientificamerican.com/article/alone-in-a-crowded-milky-way/

About 1950, during the flying saucer era, the physicist Enrico Fermi realised that the Milky Way Galaxy is so old that you only need one spacefaring race to start spreading across our galaxy, and they will occupy all habitable worlds in a tiny fraction of the life of the galaxy. In conversation with friends he famously asked: ‘where is everybody?’ Why do we not meet aliens in space vehicles?

The article above considers this paradox from a different perspective. The authors assume that aliens are just as limited by the speed of light as we are, and can only travel at, say, 10% of the speed of light. This restricts each step in interstellar travel. Aliens may occupy a planet, but other stars are too far away to travel further. The authors regard all stars in the galaxy as moving effectively in a random fashion. Our nearest star is 4 light years away, but will be closer in a few thousand years, but further in the future, another star will be our nearest star. The authors assume that alien cultures do not last forever, so an alien culture that occupied Earth during the time of the dinosaurs may have died out for reasons unknown, and signs of their existence have been obliterated by the passage of deep time.

The authors point out that Pacific islands are an analogy for stars in our galaxy. Pitcairn Island was occupied by Polynesians until about the year 1400, then was unoccupied until the mutineers from the Royal Navy ship Bounty settled there late eighteenth century. Easter Island was also occupied, then unoccupied, then occupied again in modern times.

The authors suggest that there may well be space faring aliens, but they spread across the galaxy in a patchy manner.

Thank you Yvonm for the Youtube links you posted 7 months ago.

It seems to be the assumption that an O’Neill cylinder will have 1g gravity once spun up. This is absurd. I am 75 and feel 1g, and even when I was 18 and did much more walking, it was tiresome to climb hills. Even young people can hurt themselves if they fall over.

I suggest 0.3g. I know that astronauts in the ISS need to exercise a lot each day to avoid bone and muscle loss, before they return to Earth. I remember whenever a Russian capsule returned from the ISS, the astronauts were guided out of the capsule and onto armchairs set out for them.

At 0.3g in an O’Neil cylinder, people can cycle long distances easily. It would be absurd to live under 1g, and move about the cylinder by battery golf carts. Little or no rain, light breezes at the most and good weather. Bicycles work well, and the ease of cycling under 0.3g means people would exercise a lot without thinking much about it.
Ceilings could be another metre higher, allowing more storage higher up. Jump off the floor, rise to the ceiling, grab a grab handle and remove something from an upper cupboard, then let go and float down to the floor. A 75kg person weighs only 25 kg under 0.3g. There is little or no rain in an O’Neill cylinder, so expect flat roofs used as lounges. You may climb stairs to the roof, giving exercise, but you might come down by fireman’s pole. You would prefer not to use a fireman’s pole under 1g.

Wikipedia tells me that O’Neill’s proposed cylinder would be 8 km in diameter, or about 25 km in circumference. With 0.3g, the centrifugal force (artificial gravity) will be one third that at 1g, so the diameter could be larger. Not 3 times larger, because centrifugal force depends on angular velocity squared. Coriolis forces, such as they are, will be less.
Air pressure need not be sea level Earth pressure, which would exert a force of 14psi on the outer shell of the cylinder. Wikipedia tells me that air pressure on Earth halves with every 5500 metres of altitude, or every 18,000 feet. Air pressure on Earth is 90% of sea level at 1000 metres altitude, 80% at 2000m, and 70% at 3000m. Some people on Earth live above 3000m, or 10,000 feet, and have to move about under 1g. La Paz, Bolivia, Lhasa, Tibet, and Cuzco, Peru are all above 3000m. I suggest air pressure in an O’Neill cylinder be 70% sea level Earth pressure. People will adapt.

At 0.3g, structures will experience less force and can be lighter. I suggest aerated concrete as a building material. Not much compressive strength, but at 0.3g, no problem.
I live in Australia, and our early settlers tried to reproduce English buildings in a different climate. For one thing, the sun in Australia comes from the north, not from the south. We have, of course, adjusted, and I expect a similar adjustment on O’Neill cylinders.

One of Isaac's videos was about augmentation of people, including mind augmentation. A large part of education is just memorizing facts, definitions and relationships between facts. I envisage solid state memories embedded inside the scalp, and interacting with flesh and blood brain, so that once installed, basic facts of a subject are just there, without having to learn them. You still have to think to marshal facts to solve a particular problem. How do solid state memories interact with the flesh and blood brain? I do not know.