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Stupid layman question here. Apologies if my question "isn't even wrong". I have always had trouble reconciling the "tidal stresses rip everything apart" with

• layer8 3y ago
  The issue with the singularity, and why it is a problem, is that the mathematics break down when a particle reaches the singularity, which it does after finite (proper) time, so we don’t know what happens to the particle after it hits the singularity. Physical objects falling towards the singularity get ripped apart, so their constituents reach the singularity at different times. We don’t know what happens at the respective moment they individually reach the singularity, but since there’s nowhere else they can go, we kind of assume that they’ll necessarily accumulate at the singularity, which therefore would be infinitely dense. But really the problem is that we have no functioning theory that tells us what actually happens at the singularity. In quantum mechanics, particles aren’t singular points, and instead have an extent corresponding to their wavelength. In that picture, it’s not possible for a particle to hit a singularity even as a limit.
• monkeydreams 3y ago
  > I have always had trouble reconciling the "tidal stresses rip everything apart" with the idea that "everything hits the center in finite time, thus infinite density and singularity". From whose perspective? Two astronauts, a watcher and a faller, have two very different experiences. > Do the photons nearer the center pull away from the photons further out? I believe their wavelengths increase toward infinity. They don't move more quickly away from one another but, rather, become red shifted. But I'm just a layman so my guess is as good as yours.
  • lordfrito 3y ago
    > From whose perspective? Two astronauts, a watcher and a faller, have two very different experiences. Right, that's sort of my point. If the experiences differ outside the event horizon, they must differ even more inside the horizon. Imagine Astronaut "A" who is closer to the horizon gets pulled in first, and is accelerating faster than Astronaut "B" outside the horizon. Once A is inside the horizon, B can never interact with A again. I would assume this is true even after B crosses the horizon later on -- they both accelerate but tidal forces keep them moving further and further apart. Even though both are inside the horizon and heading in the same direction (center), at no point do they ever slow down or "catch up" to each other. B still can't interact with A, as A was infinitely redshifted already. Would seem they're always cutoff from each other. > I believe their wavelengths increase toward infinity. They don't move more quickly away from one another but, rather, become red shifted. Red shifted from the point of an observer looking towards the center of the black hole (anyone looking away from the center sees the outside universe blue shifted). Red shifted light from far away stars implies an expanding universe. Wouldn't red shifted light looking at the center of a black hole imply that the space between the observer and the center of the hole be increasing as well?? So increasing wavelength = increasing curvature = space expansion? I guess my naïve interpretation is that, while you can never get out of the black hole (you can't outrun the pull towards the center), at the same time you can never hit the center as things closer to it get further and further away from you. Would seem to me that there is expansion of space inside a black hole, maybe it's this that prevents a true singularity from forming?
    • deleted 3y ago
• lmm 3y ago
  > This would imply that the "space" between matter increases indefinitely as you approach the center, right? > Also, wouldn't this apply to the original star that's collapsing? Meaning the particles inside the star are getting ripped apart as the black hole forms and they're drawn toward the center at faster and faster speeds. Is it possible that the particles in the star are also ripped apart by tidal forces, never reaching each other? In that model, yes. If you take GR literally (and personally I wouldn't), space itself is distorted and matter falling in falls infinitely far. (There is an argument that this itself prevents a true singularity from ever forming: there is an eternally collapsing pseudosingularity that is getting smaller and smaller and denser and denser, but the denser it is the more time-dilation it undergoes and so the collapse can never finish in finite time, at least for an outside observer). > Also, does this also apply to photons in say a light ray? Do the photons nearer the center pull away from the photons further out? If not, why not? And if so, then how can this be possible if all photons travel at same speed C? Is it that space is expanding inside the event horizon, faster than the speed of light? You can't really talk about photons in the context of GR, but yes, from the perspective of an infalling observer, light from an entity behind you gets increasingly redshifted and eventually the two of you are accelerating away from each other faster than light can reach, like a "big rip" in minature. > But the idea of a singularity is that "everything is in a single point of infinite density". > So how can matter be getting ripped apart at the same time it's getting squooshed together? From the outside it's a point. But from the inside it's an infinitely large volume of space. (Again, if you take GR literally)
  • lordfrito 3y ago
    > there is an eternally collapsing pseudosingularity that is getting smaller and smaller and denser and denser, but the denser it is the more time-dilation it undergoes and so the collapse can never finish in finite time, at least for an outside observer) > From the outside it's a point. If I understand what you're saying, it's that from the outside it wants to be a point, but can never quite get there in (our) finite time. So we just see a big red shifted "frozen in time" black hole event horizon. Evaporates maybe in the future, but point is we never actually get a singularity on our side of the horizon. > From the inside, it's an infinitely large volume. This makes sense to me, more than a singularity existing. On the inside everything is ripped apart as space infinitely expands. So doesn't this imply there is no singularity on either side of the event horizon then? It's as if the "wanting" to be a singularity on our side of the horizon is countered/balanced by frozen time. And on the other side of the horizon, from the particles point of view you can't freeze time. So instead the singularity is countered by infinitely expanding space to prevent just this sort of thing from happening. Is this some sort of relativistic time/space conservation law? It's like the extreme gravity trying to create a particle singularity just ends up expanding the space between the particles instead. Hmmm. We know that at relativistic speeds, trying to accelerate a particle to C leads to time slowing down. In this case too much gravity trying to create a singularity leads to space expansion instead? Are these just two sides to the same coin. Frozen time = infinite space depending on your relativistic point of view?? Sort of like the cosmic censorship hypothesis, just extended from "no naked singularities exist" on our side of the universe, to "no singularities exist" on the other side as well. Did I get that right, or did I butcher it?