The things we think of as "observers" are extremely complex - so we're proposing something very restrictive, when we propose that we might understand physics. We're saying that there are no artifacts of this relationship that blind us.

It's important to be aware of the known mechanics of electrons; they don't move at all like classical objects; but it is their motion from which classical motion emerges, in some sense, too

It is almost arbitrary, whether you discuss things as a space populated with stuff, or if you just discuss the stuff and its relationships with itself; the nature of space is one of unreality.

I was downvoted a bit, but the top answers are acting like atoms just "end" at some point; I mean, the answer they give, I'm guessing, would be just about right - but my point is that it is a sort of a "functional" definition... they are, in some sense, processes - atoms. Or events.

u/DudeWithAnAxeToGrind is saying good things; it's about when you decide it no longer looks like a "quark-gluon plasma" to you, and when it looks more like atoms. we don't even understand how water works, if you start looking at it close, like that

I guess you found PBS Spacetime already, if you're on YT. And I like David Kipping's work; it looks nice and helps me fall asleep. If lectures and discussions are your thing, I could maybe name a couple more.

A good thing to consider is when they formed; look at what the big bang people say the size of the universe was when the CMB formed, and divide. Otherwise I'm not sure the question will ultimately be easy to well-define. (But I don't know this stuff)

Because it was a quantum annealing machine?

If it is infinitely spatially extended, it either repeats simply, or it "never repeats"- but you can always find a repetition for any finite region. Since our awareness is finite, this means yes - it "repeats".

But we don't know if it is infinitely spatially extensive. We can only see a tiny part of it, as far as we can surmise. It "looks like it keeps going", outside the 50 billion light year sphere we're looking at right now.

No; no - it's a much milder effect than that in all the relevant ways. Basically we're going as fast as possible - light speed, straight in to the future, relative to nearby, stationary, objects. When you provide a velocity, you need to specify what observer is measuring it; that is something we see immediately from relativity.

Everything moves at the same speed, in a certain sense - "c", we call it. That seems counterintuitive, but it isn't, if along with the rotations you can picture there is another dimension in which one might steer - or rather, steer away from. We could take some of our speed through time and exchange it for some speed through space. This is time dilation. It is one piece more complex than that, as we are really talking about imaginary time, here. But "imaginary" in the complex plane sense. Flipping our perspective so that linear motion in one direction represents translation in complex time is called a Wick rotation.

You're thinking in terms of gravity, and I'm answering with geometry; but they are equivalent descriptions.

If you could sit somewhere near our observational horizon - say, 13 bly away, or somehow protected and hauling around a black hole, then we would appear to slow greatly - but also we would be very redshifted, and it would be very hard to get back. So practically you can kind of ignore it; but there is something there, if you really really want to run the world ahead into the future a little faster, and you are allergic to anesthetics.

We are pretty confident we understand black holes of a range of sizes, but we still can't say for sure that some fundamental particles are not actually themselves little black holes, or if the whole universe is swarming with little black holes somehow. This idea of white holes isn't discussed much. You wouldn't expect to see one in space, unless you mean by viewing the big bang as a white hole - it is, after all, a region of space that light and matter cannot enter. And it is thought to gravitate; we initially thought expansion would slow from all the matter pulling on itself. A white hole would gravitate, just like a black hole, so you could 'orbit' it, assuming it wasn't spitting out light or matter. And we're kind of orbiting the big bang; only it happened everywhere at once, so that involves a lot of sitting around.

I'm not sure how else one might mean opposite; there is the unruh radiation... like, you could look at the furthest reaches we can see, and think about how we relate to a singularity. There might be some interesting symmetries. It's above my head.

The wave mechanics of gravity are complex; regions of space can focus gravitational waves, or spread them. Space is warped by 'frame-dragging' around a spinning black hole. And we don't quite get either dark matter or dark energy.

In short, the easiest opposite is the time reversal. Black holes are closed-off regions of space; they have properties that seem bizarre to us. Their opposite is either not bizarre, or just bizarre after having been put through a Wick rotation.

since i'm right here - "spin" is this other thing; it can only be "up" or "down"... electrons have it