Mr Feder doesn’t let go of a topic. He’s still stewing about Time. “Moire or somebody said the Big Bang is the Bottom of Time because there wasn’t any time before then. I guess I gotta buy that, but bottoms gotta have tops. What’s the Top of Time?”

“Whoa, Mr Feder, that’s a fuzzy question with a lot of answers, most of which are guesses.”

“No theories?”

“Not really, A few used to be called theories but people started muttering about testability so the theories got downgraded to hypotheses and now they’re guesses except for the ones that’ve been dropped altogether.”

“Like what?”

“Steady State, for one — the idea that Time has no end. That used to be popular, mostly because it was simple. Problem was, Edwin Hubble showed that other galaxies are separate from the Milky Way and in fact they’re receding from us. That clashed with the Cosmological Principle, the idea that on a large scale things are pretty much the same everywhere. Galaxies moving away from each other leave behind empty space that isn’t ‘pretty much the same.’ For the Steady State model to work, new matter would have to spring into existence between the departing galaxies.”

“Nature hates a vacuum, eh?”

“Apparently she doesn’t. Evidence has piled up against the Steady State model and in favor of the Big Bang. We still think the Cosmological Principle is a good assumption, but only on scales bigger than a few hundred million lightyears.”

“So Time has a Top, then.”

“Depends on how you define ‘Top.’ We’re now into Metaphysics territory, where theories come cheap and flimsy. It’s conceivable, for instance, that the Universe curves back onto itself along one or more of its dimensions. If it loops back along the time dimension then we’d be in an oscillating universe that cycles from Big Bang to Big Crunch and back out again. Time would have no Top or Bottom. Crosswise to time, some thinkers like the idea that the Universe circles back along a space dimension. If that’s true and we could see far enough we could inspect the back of our heads.”

“Wait, we’ve got lots of black holes. If their singularities are in the infinite future like you said, that’d stymie the circling.”

“Good point, Vinnie. As I understand the math, connectivity like that is possible if our 4D spacetime is embedded in a ‘bulk‘ with five or more dimensions. But that’s more complicated than I’m willing to accept without at least some evidence which no-one’s shown me yet. The endings of the 2001 and Interstellar movies don’t count.”

“What else you got?”

“What other theories, Mr Feder? How about block universes? Maybe the space dimensions are solid but only part of the time dimension is real. Some people opine that the only reality is ‘NOW,’ an infinitely thin slice of time evolving towards the future. A memory would only be a surviving imprint of things that stopped existing when Time was done with them.”

“I don’t like that one. For one thing, it doesn’t jibe with the ‘everyone’s got their own NOW‘ thing from relativity.”

“Einstein didn’t like it either. The easiest way to reconcile all those different versions of NOW is to assume that they all co‑exist permanently. I call that notion the closed block model. The idea is that all reality — past, present and future — is real and rigid. We perceive time as flowing only because consciousness floats upward along the time coordinate. The Top of Time is way up there, just waiting for us to arrive.”

“Why no sinking downward?”

“Good question, no good answer that I’ve seen. Besides, the closed block model doesn’t allow for free will. I like having free will.”

“Me, too. OK, if there’s closed block, what’s open block?”

“The future doesn’t exist yet. Picture the open block model as our 4D spacetime being a bowl with the Big Bang at the bottom. Time progressively fills the bowl like water. NOW is the Top of Time. Those relativity‑shifted NOWs only show up when we compare records of past observations.”

“Cheap and flimsy, but a pretty picture.”

~~ Rich Olcott