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Rethinking the origins of the universe

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Re: Rethinking the origins of the universe

#71

Post by lutusp »

dhimes wrote: post #50
sramsay wrote: post #28

I'm not a physicist. I imagine that if I were, this would be exciting news (that is, an exciting possibility). I am a literary critic, though, and I have to say that it's bittersweet news at best. Black holes are so much a part of the space-age imagination. The article mentions Hollywood, but black holes are woven deeply into our everyday metaphors and have been for decades. They have stood among the most wondrous th…

I imagine that the metaphor "black hole" will be here for a while. After all, we still use the term "big bang" for the 'first moment' of the creation of the universe, even though the "Big Bang Theory" isn't generally believed any longer.

> even though the "Big Bang Theory" isn't generally believed any longer.

Really? Given that science doesn't turn on belief but evidence, still, the Big Bang remains the best explanation for existing evidence. It's the prevailing cosmological theory of the universe' beginnings. Like all scientific theories, it's subject to replacement as new evidence appears and as new theories are crafted, but it's a good match at the moment.

Re: Rethinking the origins of the universe

#72

Post by lutusp »

davesque wrote: post #66

From the article: "Laura Mersini-Houghton has mathematically proven that quantum effects are strong enough to stop the formation of black holes..." If I'm not mistaken, Mersini-Houghton's work has just hit the scene. It still has to be vetted by the larger community before we could say that anything's been "proven."

Not to mention what I'm about to -- that scientific theories are never proven, only disproven. You can always tell when a journalist is speaking about a topic for which a scientist might be a better source.

As to "mathematically proven", it's an unfortunate juxtaposition of math (where things really can be conclusively proven) and physics (where they cannot be).

Re: Rethinking the origins of the universe

#76

Post by dragonwriter »

lutusp wrote: post #61
trhway wrote: post #33

Ok. To start she addresses only "Gravitationally Collapsing Star"s. Interesting result, may be many star lifecycles don't end in a black hole. The "before-black-hole" state she describes still would involve significant time dilation and thus what we see as Hawking radiation from black hole may just be Hawking radiation from "before-black-hole" star. Looks the same :) The result doesn't say that black hole is impossib…

> A huge black hole may have pretty weak gravitation at its horizon. Not really. The amount of spacetime curvature at the event horizon is fixed in the theory -- it's always the same. It's how the event horizon is defined. Near the event horizon, light orbits endlessly (in principle), and (again in principle) if you were located at an event horizon and there was sufficient illumination, anywhere you turned you would…

> The amount of spacetime curvature at the event horizon is fixed in the theory -- it's always the same.

No, its greater the smaller the horizon is (so, equivalently, less the bigger the horizon is).

> Near the event horizon, light orbits endlessly (in principle)

Sure, at the event horizon, light orbits endlessly. But the amount of curvature needed to do that is less the further across the horizon is.

Re: Rethinking the origins of the universe

#77

Post by lutusp »

guscost wrote: post #65
lutusp wrote: post #59

Earlier quoted context omitted.

> What is an example of this positive evidence? Are you asking what observational evidence exists for black holes? There are very massive, very dense objects at the center of most galaxies, objects that by their mass and density fall well inside the theoretical limits for black holes. At the center of our galaxy, there is a very massive, compact object, around which many stars are orbiting, in orbits that reveal the…

I don't disagree with any of that, but the evidence in question still depends on several big assumptions.

It's the other way around. The evidence doesn't depend on the assumptions, the assumptions depend on the evidence.

The observations are very good and offer little latitude for interpretation -- there is a very massive, very dense object at the focus of multiple stellar orbits near the center of our galaxy, and both the mass and the density of the object are easily and unambiguously derived from the orbits.

It's the same with other galaxies -- a massive, dense central object dominates the orbital dynamics of the galaxy near its center. We're obviously not free to say that means black holes exist, it's just another piece of evidence. But it's a way to exclude certain alternatives.

Re: Rethinking the origins of the universe

#78

Post by lutusp »

dragonwriter wrote: post #76
lutusp wrote: post #61

Earlier quoted context omitted.

> A huge black hole may have pretty weak gravitation at its horizon. Not really. The amount of spacetime curvature at the event horizon is fixed in the theory -- it's always the same. It's how the event horizon is defined. Near the event horizon, light orbits endlessly (in principle), and (again in principle) if you were located at an event horizon and there was sufficient illumination, anywhere you turned you would…

> The amount of spacetime curvature at the event horizon is fixed in the theory -- it's always the same. No, its greater the smaller the horizon is (so, equivalently, less the bigger the horizon is). > Near the event horizon, light orbits endlessly (in principle) Sure, at the event horizon, light orbits endlessly. But the amount of curvature needed to do that is less the further across the horizon is.

>> The amount of spacetime curvature at the event horizon is fixed in the theory -- it's always the same.

> No, its greater the smaller the horizon is (so, equivalently, less the bigger the horizon is).

We're using different meanings of "curvature". For all black holes regardless of their properties, the event horizon has the same spacetime curvature -- that required to produce orbiting photons that cannot escape. Outside the event horizon, photons can escape. Inside, they cannot (and those photons don't orbit either, but cross the horizon). All the same.

> But the amount of curvature needed to do that is less the further across the horizon is.

No, it's the same. Same curvature, different large-scale geometry. For a sufficiently small zone near the event horizon, the conditions are identical.

Again, this is about the meaning of "curvature". The circumference of the horizon is greater for a large mass than a small one, but the spacetime curvature at the horizon is the same -- it's the specific value that causes photons to orbit perpetually, and interestingly from a local perspective, the entire horizon surface appears as a plane of infinite extent, sort of like two facing mirrors but with more dimensions.

From the perspective of a hypothetical observer at the horizon, he wouldn't be able to judge the size of the black hole using local observations -- there would be a plane of infinite extent for any black hole regardless of size.

Re: Rethinking the origins of the universe

#79

Post by pluma »

dllthomas wrote: post #27

"from Star Trek to Hollywood" So... Hollywood? Star Trek wasn't filmed on location - Desilu and Paramount were both located in Hollywood. Not that this, y'know, matters - just found it amusing.

Filming Star Trek on location would have been pretty difficult, I'd imagine.

Re: Rethinking the origins of the universe

#80

Post by lutusp »

guscost wrote: post #70
lutusp wrote: post #69

Earlier quoted context omitted.

> I will say that "pretty good" evidence is not consistent with the language we usually see in the media[1] but that's really another issue. That's the difference between science and journalism. In journalism, some things are proven true, while others are cast into doubt. In science, some things are proven false, while others are less doubtful than they once were, but never become true. The tl;dr: in science, things…

So the region within the event horizon is where we see the sufficiently large/infinite curvatures and the imaginary terms and GR can be said to break down? I've heard that from a frame of reference outside the event horizon the time dilates and light would seemingly never get there, hence the whole idea of a black hole. Mind-boggling stuff, thanks again for writing out these explanations.

> So the region within the event horizon is where we see the sufficiently large/infinite curvatures and the imaginary terms and GR can be said to break down?

Yes. At the event horizon, general relativity still predicts the outcome. Below it, no more conventional physics.

> I've heard that from a frame of reference outside the event horizon the time dilates and light would seemingly never get there, hence the whole idea of a black hole.

I've read that too, but in fact, because of the energies of accretion disks that are vacuuming up light and matter from the neighborhood, and the fact that some of the photons tend to orbit the horizon endlessly, it's actually a very hot place in most cases.

> ... thanks again for writing out these explanations.

You're most welcome.

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