Sabine Hossenfelder
1 HN recommender · recommended in 3 months, 2022–2023
16:07 · 7.5M views · Jul 23, 2022
Sabine Hossenfelder explains how special relativity leads to the block universe, in which past, present and future all exist equally, and asks what that means for whether the past still exists.
HN commenters share it as a clear explanation of the block universe and relativity of simultaneity.
3 comments from Hacker News
I haven’t heard anyone express this before, but when it comes to my death and the deaths of others, I take comfort in the idea of the Block Universe. On the Block Universe concept, time isn’t real in the way we think about it—everything that ever happened or will happen exists in a timeless, eternal block of spacetime. (This video by Sabine Hossenfelder explains it nicely: this video .) If this is true, then our lives are eternal—not in the way envisioned by religious people, but eternal in a very real sense. It’s true that I am finite in time in exactly the same way that I’m finite in space, but just as I don’t lament the fact that my body isn’t infinitely large, I shouldn’t lament the fact that my life doesn’t extend through all of time. (I wouldn’t mind living for a lot longer , but that’s a different matter :-D.) I realize this conception of immortality will be too abstract to provide any consolation to many people. But ideas of an afterlife or reincarnation are abstract, too. This one at least has the advantage, to my mind, of being plausible. It’s not that I once was not, then I was, and after I die I will be no more. My life, my presence, this experience—it’s eternal and ever-present.
I feel like I must be missing something but in the part about the relativity of simultaneity [0] I feel like this is more about the relativity of the observers and the limitations of speed of light than it is about the ontology of time itself? The definition of "now" as when the light hits the mirror on each side is a "constructed" now, a false now if you may. It takes the light longer on the right side than the left side because she is moving through space, all that tells me is that there is no way to communicate a "now" between different observers because there can't be instantaneous communication between observers, not that time itself is relative or physical stuff itself doesn't have a constant time? She says in the video that we have to find a way to operationalize how to find a now, to measure it empirically but maybe that's not possible without running a complete simulation after the fact. But to me it seems not surprising that without instantaneous communication there is no way to create a "now" empirically but that doesn't mean it extends to the ontology of physical stuff itself or time itself. There would still be a time in there (you should be able to see it in a simulation?) Also for example, if you use sound instead of light, if I'm far away from a sound source (farther than the speed of sound), and another observer is closer to it, we would not be able to use the sound source as a source "now", because I would hear it later. Is this the same type of situation or
To be fair there is some disagreement over what exactly "the past" _is_. this video discusses this a bit.