The fringe idea is that a heat-death universe looks homogeneous and the Big Bang also looks homogeneous and so maybe we could potentially hook up the t=0 of one universe with the t=∞ of another and get rid of the “well but it takes infinite time to get from one universe to another” by handwaving of “yes but no particle in that universe actually experiences infinite time because they all become massless eventually, so it effectively takes finite time.”
Like, the principle that massless particles cease experiencing time itself is fine, albeit kind of hard to think about (it is a negation of an idea; because they don't experience time they cannot “experience” anything, so I could tell you that “they also see the entire universe except for the thing that emitted them as one point/ray straight ahead of them” but I mean they don’t actually “see” anything so I am describing a limiting behavior here).
Whether you can stitch universes together like this is very very fringe but props to him for being a physicist and looking for some sort of observable consequence of a theory that could have otherwise just been a “yeah the math could go either way on this, think of it that way if you want to but there is no pressure.”
I don't think passing through "infinite time" is a fringe concept in modern physics. It is well accepted that passing the event horizon of a black hole involves traveling through infinite coordinate time from the perspective of an outside observer; however if you change your coordinates to that of the object falling into the black hole, you will see that it takes finite time to pass the horizon.
I think that’s almost it, but not quite (big grains of salt here, I’m trying to remember a talk from a decade ago). I went to a talk of his, and iirc it wasn’t stitching t=0 to t=infinity. It’s was stitching t=0 to t=big enough to be homogeneous enough, but there are still itty bitty remnants of patterns left that might be detectable today.
He reuses a lot of his slides, the one that I have a picture of speaks about taking the boundless expansion and performing an “∞-compression” which I am taking to be a conformal rescaling of the state of the universe at infinity, but I must cede to you that this may be a misunderstanding because he also says on this slide that the Big Bang involves an “∞-expansion” and he certainly doesn't believe that the Big Bang was an infinitely long time ago. In any case what I remember beyond this was that he figured that the expansion at the end of a universe could be used to locate the “cosmic inflation” of a universe _before_ its Big Bang, in the heat death of the prior universe, and it was my impression that it was this “evolution to infinity” of the prior universe but I might be totally mistaken.
Curious. Would you think that if Penrose is correct, that each subsequent universe's timeline would be deterministically exact replays of the former universe, or would they have some degrees of freedom to develop differently?
I mean, whether it’s deterministic is really hard. We’re talking about extensions of quantum field theory and there have existed both deterministic and nondeterministic accounts of quantum mechanics, the deterministic ones generally lagging the nondeterministic ones substantially.
But whether it’s _exactly the same_, I think Penrose would say that they are _almost never_ the same. The question would be whether the Big Bang of this universe is perfectly mirrored in its final state and I think Penrose would say “well all the stuff that happened in the middle probably had some sort of effect on how the thing ended up.” So if we are seeing the black holes of some other universe’s death writ large on our cosmic microwave background, then I can’t imagine our black holes will eventually look exactly the same, will they? (Keep in mind, I’ve only met Penrose once and I embarrassed myself thoroughly at the time, so my guesses about what he would say are terribly terribly weak predictions.)
But maybe they will be the same. I talk a lot on HN about the impact of fixed-point theorems on US politics, but they would also apply here. If the universe has an infinite number of sequences one might expect that it starts to converge on a situation where the black-hole-patterns in are the same as the black-hole-patterns out. Now if that happened one might imagine that, if that fixed point supports life, then there is an infinite replay of nearly-identical life experiences across the many many eons of universe, and then if we treat our existence as “choose a conscious person across the eons to be” we would almost surely randomly choose someone in the oft-repeated universe. And so yes, we would be very likely to say that our universe's end would lead to another universe wherein the exact same people living out the exact same lives having the exact same discussion on Hacker News, haha.
I am a Christian and so these sorts of things are a sort of idle speculation, a joke to laugh at rather than something to inform today’s experience, a slacking-off between meetings rather than a contribution of deep value. But it’s certainly fun and funny to think about.
Maybe the previous universe's black hole evaporation ripples cause enough chaos effect for the new universe to be different so even though the whole system is deterministic, it could always be different because of that effect. We live in a PRNG.
Edit: OOh I have an idea. What if the quantum effect is deterministic but appears random we experience are in fact ripple effects from the previous universes? Has anyone ever tried to measure quantum behavior in multiple simultaneous experiments in a kind of geometrical matrix system to see if there are shapes of correlation?
Like, the principle that massless particles cease experiencing time itself is fine, albeit kind of hard to think about (it is a negation of an idea; because they don't experience time they cannot “experience” anything, so I could tell you that “they also see the entire universe except for the thing that emitted them as one point/ray straight ahead of them” but I mean they don’t actually “see” anything so I am describing a limiting behavior here).
Whether you can stitch universes together like this is very very fringe but props to him for being a physicist and looking for some sort of observable consequence of a theory that could have otherwise just been a “yeah the math could go either way on this, think of it that way if you want to but there is no pressure.”