Tuesday, September 30, 2014

Some Very Informed Feedback on that Startling Mersini-Houghton Paper -OR- Why Black Holes Almost Certainly Do Exist

This entry features a brief overview of the controversial -- and not-yet-peer-reviewed -- findings of two physicists who claim that their mathematical calculations "prove" that black holes do not exist. It also includes my friend Kristof's take on the matter -- sent to me as his friend seeking his general view of the matter.

For starters, the story actually appeared last week when the study first appeared on the arXiv electronic preprints website for scientific articles site. However, here is the story as it appeared on Huffington Post yesterday (Sept. 29th).



The article is by Laura Mersini-Houghton of the University of North Carolina at Chapel Hill, and Harald P. Pfeiffer of the Canadian Institute for Theoretical Astrophysics located at the University of Toronto.

UNC - Chapel Hill -- perhaps foolishly and certainly prematurely -- touted the findings in a press release dated Sept. 23, 2014.


The key finding (as stated in the press release) is as follows:

"But now Mersini-Houghton describes an entirely new scenario. She and Hawking both agree that as a star collapses under its own gravity, it produces Hawking radiation. However, in her new work, Mersini-Houghton shows that by giving off this radiation, the star also sheds mass. So much so that as it shrinks it no longer has the density to become a black hole.

"Before a black hole can form, the dying star swells one last time and then explodes. A singularity never forms and neither does an event horizon. The take home message of her work is clear: there is no such thing as a black hole."

*******

For starters, this is apparently not the first time that someone has mathematically "disproven" black holes. Secondly, I asked my friend Kristof to comment on this. Kristof is among the top scientists in his field of tropospheric - stratospheric chemistry transport and dynamics, but he also has a Ph.D. in mathematics and is very knowledgeable about general relativity.

Here is his reply -- reprinted with permission (with the text broken up to accommodate some artist conception images and schematics of black holes, accretion disks, and gamma ray bursts):

Hi Richard

I looked at the Mersini-Houghton paper. There's too much math there to check within a short time for someone who is not highly 'edumucated' in General Relativity. However, I suspect the problem may be in the way she sets up the stress energy tensor. Specifically, the Hawking radiation component of it. The stress energy tensor is the right hand side of Einstein's field equation and it represents matter (the left hand side represents space-time geometry). It is a generalization of mass density in the classical theory of gravity.

Why do I think that? On the face of it, the whole thing doesn't add up: her calculations yield a two-fold mass loss of a collapsing star due to the Hawking radiation (HR) during the collapse!


But there exist very precise calculations that show that the HR is energetically extremely inefficient.

The half-life of an evaporating stellar mass black hole ('the Page time', after Don Page) is orders of magnitude greater than the age of the Universe. Many orders of magnitude. Now, projecting things like the temperature of the HR as 'measured' far away to the vicinity of the horizon is very tricky, as virtually everything that involves gravity is. You have to be a quantum physics equivalent of a brain surgeon to do it correctly.

Can't blame anyone for getting it wrong.

You may want to read this short piece on the IFLS website. The woman who runs the website actually approached William Unruh and asked for his opinion. He was kinda harsh, actually. I'm leaning towards accepting his view on the matter.

If anyone understands the Hawking radiation it's him (and Hawking). Back in the 70's he showed that an accelerating frame of reference radiates photons. This 'Unruh effect' is equivalent to Hawking radiation, as acceleration is locally indistinguishable from gravity by Einstein's equivalence principle. He probably knows what he's talking about.


Why has this particular paper gone viral on the internets is beyond me. Maybe it's because of the statement that Hawking made last year ('There are no black holes'). I'm sorry, but Hawking's paper (if you can call it that) was just a collection of random musings on the 'firewall paradox' and the information paradox. No math, no nothing.

If it were published by anybody else no one would pay any attention to it - but being an attention whore he said what he said, bless his heart (actually, he's kind of irresponsible in that way). So now someone says the same thing and there is some actual math in the paper so it gets attention. My gut tells me the result is wrong.

Must have a glass of wine now:-)

K

*******

If I can add my own 1-1/2 cents as a conclusion, and not knowing anything about Einstein's field equations or stress-energy tensors, I also suspect that the two scientists are probably wrong in their conclusions -- it simply flies in the face of too much carefully developed theory that matches so well to so many indirect astrophysical and cosmological observations. Their findings are just a mathematical curiosity based on a set of assumptions that are not "realistic" in the truly bizarre physics that exist near (and in) a black hole.


*No, not the upscale department store.


Finally, here is a more elaborate explanation of what you would theoretically experience falling into the Schwarzschild black hole -- that is, a non-rotating, uncharged one, the "simplest" (and most unrealistic) of all black holes.

OK, that's all for now.

My next planned update will be on Wednesday night (unless the computer issue I mentioned in this entry comes to pass, in which case my blogging will be severely curtailed until late October).

--Regulus

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