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Rethinking the origins of the universe

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Re: Rethinking the origins of the universe

#61

Post by lutusp »

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 see the back of your own head.

> Thus no-escape is valid only in the sense that escaping object just would never reach "a point at infinity" from the black hole and the light would get red-shifted into full oblivion.

From a more distant frame of reference, yes, but not at the event horizon itself.

Re: Rethinking the origins of the universe

#62

Post by lutusp »

AnimalMuppet wrote: post #9

If I understand correctly, black holes do not play a large role in our model of the origins of the universe. So this doesn't (directly) lead to "rethinking the origins of the universe". Does this work call the big bang into question (or even our current models of the big bang)? Or is it just details like galaxy formation that it makes us rethink? Another question: Something very massive and very small is at the cente…

> If I understand correctly, black holes do not play a large role in our model of the origins of the universe. So this doesn't (directly) lead to "rethinking the origins of the universe".

But it does, because one hypothesis about the Big Bang is that it commenced from conditions identical to a black hole's singularity. So our understanding of black holes is critical to at least one idea about the universe's beginnings.

> Or does this just make us question whether that massive thing is in fact a black hole?

The answer is that there is something like a black hole at the center of our galaxy, but people are free to theorize about what exactly it is.

Re: Rethinking the origins of the universe

#63

Post by xefer »

If I read the abstract and introduction correctly, it doesn't say that black holes per se don't exist, only that they can't form as the result of a collapsing star or supernova. The linked-to article seems to be jumping to conclusions (e.g., rethinking the origin of the universe; impossibility of black holes at all, etc.) that the paper isn't even making.

Re: Rethinking the origins of the universe

#64

Post by guscost »

lutusp wrote: post #58
guscost wrote: post #49

Another member of the peanut gallery here. I've not been bold enough to make any claim in writing (it would probably be full of misconceptions), but the black hole hypothesis always seemed a bit half-baked, and I've definitely said words to that effect in conversation. In the hypothesis Einstein's field equations obviously break down, and instead of acknowledging that any explanation will be enormously difficult to t…

> The other thing that sets off my hand-wave-o-meter is the pretending that we are somehow already observing these things "new telescope detects hints of black holes" or have concluded that they exist without any positive evidence. Actually, the evidence is pretty good. We have orbital velocities around the mass at the center of our galaxy that cannot exist unless there is a very massive, very dense object at one foc…

This is a very good answer, thanks for taking the time to write it. 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.

Can't SR only model photons at c because some term disappears with zero mass which prevents infinite energies? Isn't it a whole different question whether GR is still reliable in the analogous case with massive objects?

[1] http://www.phdcomics.com/comics/archive.php?comicid=1174

Re: Rethinking the origins of the universe

#65

Post by guscost »

lutusp wrote: post #59
guscost wrote: post #53

Earlier quoted context omitted.

What is an example of this positive evidence? Also is it at all plausible that the center of the galaxy is evaporating slightly outside its event horizon in this way? Anyway I'm not sure that the two of us are going to answer any of these questions without some help.

> 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.

Re: Rethinking the origins of the universe

#66

Post by davesque »

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."

Re: Rethinking the origins of the universe

#67

Post by ihuman »

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."

Also from the article:

"The paper, which was recently submitted to ArXiv, an online repository of physics papers that is not peer-reviewed..."

I'd wait until something like this is peer-reviewed too.

Re: Rethinking the origins of the universe

#68

Post by Ono-Sendai »

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.

Erm, the big bang theory is still the best theory for the history of the universe.

Re: Rethinking the origins of the universe

#69

Post by lutusp »

guscost wrote: post #64
lutusp wrote: post #58

Earlier quoted context omitted.

> The other thing that sets off my hand-wave-o-meter is the pretending that we are somehow already observing these things "new telescope detects hints of black holes" or have concluded that they exist without any positive evidence. Actually, the evidence is pretty good. We have orbital velocities around the mass at the center of our galaxy that cannot exist unless there is a very massive, very dense object at one foc…

This is a very good answer, thanks for taking the time to write it. 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. Can't SR only model photons at c because some term disappears with zero mass which prevents infinite energies? Isn't it a whole different question whether GR is still reliable in the analogous case with massive ob…

> 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 can only ever be proven false, never true.

As philosopher David Hume famously put it, "No amount of observations of white swans can allow the inference that all swans are white, but the observation of a single black swan is sufficient to refute that conclusion."

> Can't photons only travel at c because some term disappears with zero mass which prevents infinite energies?

I'm not sure I've successfully decoded your question, but only massless particles can travel at c. This is why, when it was established that solar neutrinos were changing their identities while traveling from the sun to our detectors, that meant they were experiencing time, which meant they had mass. All confirmed in later experiments.

> Isn't it a whole different question whether GR is still reliable at c when massive objects are involved?

From a mathematical standpoint GR is perfectly reliable from v = 0 to v = c, including places where large masses are present. Any velocity past c (or a sufficiently large amount of spacetime curvature) and GR is no longer able to provide reliable predictions. The reason is that the mathematical results include imaginary terms. As has been said by many, the relationship between mathematics and reality is much closer than we once imagined.

Re: Rethinking the origins of the universe

#70

Post by guscost »

lutusp wrote: post #69
guscost wrote: post #64

Earlier quoted context omitted.

This is a very good answer, thanks for taking the time to write it. 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. Can't SR only model photons at c because some term disappears with zero mass which prevents infinite energies? Isn't it a whole different question whether GR is still reliable in the analogous case with massive ob…

> 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.

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