So the small end of the universe was in view at the start of the 20th century, except, as I said, for that zoo of featureless fundamentals, but what of that large end?
Is there a universe?
Such a question!
But seriously, what we are asking is whether there is a meaningful totality of interacting material reality — or is there not? If the universe is infinite, then it cannot be fully interactive: Olbers light paradox and Bentley’s gravity paradox made so much clear. An infinite supply of interacting material would burn up or collapse.
So, instead, people toyed with the idea that the universe was infinite but not fully interacting. If that doesn’t make sense to you, don’t worry; you are in very good company. “Infinite” and “material” are themselves incompatible.
But if it was not infinite, how could it have an edge?
At the end of the 19th century, with several other star parallaxes in hand, and some other ways of estimating star distances, the great physicist Boltzman suggested that there were multiple universes suspended fully 10100 meters apart. That would mean that the galaxies were separated by several times the diameter of our own universe as we presently know it.
What could such a distance possibly mean?
Clearly Boltzman intended to solve the gravity and light paradoxes by placing things at such a distance that the sources of light and gravity would burn out and disperse before their effects were infinitely stacked up. But what would keep these sub-universes apart?
And were they really there?
The larger universe was definitely known only out to the farthest reaches of our Milky Way Galaxy; beyond that, all was theory. The thinking at the beginning of the 20th century was that the Milky Way was the universe.
If there was “a” universe.
In 1917, things changed abruptly. Albert Einstein very simply addressed the need to stop talking about space as a void in which anything could be imagined, including infinity. Infinity and the void do not have properties that can be talked about in a scientific way, because they cannot be measured and weighed, any more than God can. Einstein defined space as:
the network of all possible paths of motion.
Paths of motion… possible paths… Such a simple definition! But it changed everything, because it answered Bruno’s riddle. Yes, the arrow will go up for a while as long as you aren’t standing on a planet like Jupiter where intense gravity deprives you of the strength to bend your bow; but anyway the arrow will return – because of gravity. Even light responds to gravity. So “the network of all possible paths of motion” does not extend to infinity, and Einstein certainly did not think the universe infinite. It does not have a fence at the edge, such as the crystalline sphere of fixed stars, but the limit is just as surely set by gravity.
Meantime, a different argument had come to a head about whether the little nebulae all over the Milky Way were inside it or, in fact, outside. Some were clearly inside; others seemed to be outside – and if so, very far away.
Edwin Hubble resolved this question in 1925, showing that many of the nebulae were entire galaxies having their own proper motions; furthermore, they were receding from our galaxy, already having achieved distances of millions of light years — nothing approaching Boltzman’s suggestion, but very far removed. Once again, the universe was stupendously larger than had been conceived.
1027 * 1024 * 1021 = galaxy size * 1018 * 1015 *
1012 * 109 sun size * 106 * 103 * 100 =1 people size
10-3 * 10–6 * 10-9 molecules * 10-12 * 10-15
(You may think that infinity is larger, but, being non-specific, it does not challenge the imagination. “Larger” has no meaning before infinity.)