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Black Holes Tutorial for lay people;


beecee

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OK, I have my 59th Old Boys reunion coming up shortly.....a right royal piss up it looks like being. As the only one interested in cosmology to the extent that I obviously am, three of the more interested old farts have asked me to properly describe BH's. So here it is..........Any errors, alterations and/or corrections recommended by our on line professionals is welcome?

 BH's of sorts, were hypothesised to exist in 1789 by John Michell. This Newtonian variety were called "Dark Stars"  and did have a surface just below where the density of the relevant matter, had an escape velocity equal to "c". His work was not taken seriously and  was pigeon holed until many years later when Neutron stars were discovered and questions were asked about further possible collapse of stellar objects. Arthur Eddington called them a cosmological absurdity.
GR and its equations predicted their outcome, yet Einstein originally thought that there would always be a physical barrier to prevent such catastrophic collapse.

When a star uses up its available fuel for fusion, it has one of three paths to take, based on the original mass of that star. It can end up a White Dwarf as our own Sun will eventually see out its life. These WD's are held up from further collapse by EDP [Electron Degeneracy Pressure]
Larger mass stars finish their lives as Neutron/Pulsar stars, held up from further collapse by NDP. [Neutron Degeneracy Pressure]
Inevitably even larger mass stars are monstrous enough to overcome both EDP and NDP, and forming what was to be known as Black Holes. First known as Gravitationally Completely Collapsed Objects, John Wheeler coined the BH.

Simply put a BH's EH [Event Horizon] is where the escape velocity equals "c" the speed of light in a vacuum.
Beyond this point nothing can escape, including light. 

A BH's size can vary from those predicted to have occurred at the BB, microscopic quantum sized BH's, to stellar collapse BH's a few 10s of kms in diameter, and the monsters that lurk at the centers of galaxies ranging from millions to billions of solar masses. These are termed Super Massive BH's.
BH's can only have three properties of mass, charge and angular momentum. Charge would obviously be quickly negated, leaving mass and angular momentum.
Angular momentum also would gradually be negated, but over much slower rates.
This leaves the end state of all BH's in the distant future, as the Schwarzchild variety, or a BH with no spin or charge, also the most mathematically convenient to work with.
If we were to categorise all the BH's we have in our Universe today, the most common would be the Kerr BH, or the one with angular momentum, first suggested by Roy Kerr.

GR tells us that once any massive object is forced to undergo collapse, once it reaches a point called the Schwarzchild radius [which for a BH is the EH] then further collapse is compulsory. This means that if we squeeze the Sun inside a volume of around 5 kms diameter, it would reach its Schwarzchild radius, further collapse would continue and it would become a BH.

Once a stellar remnantcollapses to or below this radius, light cannot escape and the object is no longer directly visible, thereby forming a BH. It is a characteristic radius associated with every quantity of mass. The name itself results from Karl Schwarzchild a German astronomer, who calculated the exact solution during the first world war in 1916.

This compulsory collapse leads us to what we refer to as the Singularity. Mathematically speaking, the Singularity is where all our known laws, including GR break down, or are not applicable. Most cosmologists today worth his salt, accept that this mathematical singularity and associated infinities, does not really exist. Rather a surface of sorts just at or below the quantum/Planck level should exist. This is where GR fails to tell us anything.

Many other seemingly weird aspects of BH's are known, but as weird as they are, they are aligned with the facts borne out by SR/GR that space and time are not absolute and that all frames of references are as valid as each other. An example of one of these seemingly paradoxical scenarios is as follows......
If myself and a companion travel towards a BH and I stay a safe distance away, while my curious friend travels on towards the BH and its EH, due to gravitational time dilation, I will from my position in space, never see him cross the EH to his doom, only slowly and gradually just  redshifted beyond my instrumental viewable ranges.
From my friends point of view though, he proceeds towards the EH, and crosses it with no changes from his perspective in time, ignoring tidal gravitational effects which depend on the BH's size...the smaller the BH, the more critical and sooner will be the effect of spaghettification as it has become known. Both views, mine and my intrepid friend's are as valid as each other.

Physically speaking the Singularity lies at the heart of the BH, and is where all its mass is concentrated in a form that as yet we have no description for.
It lies at the quantum/Planck level of spacetime, and may in the future have more light shed upon it by a validated QGT [ Quantum Gravity Theory]

A quantum effect called Hawking radiation which theorises on particle pair creation at near a BH's EH, where one escapes and the other succumbs to the BH, tells us that over the lifetime of the Universe, BH's should evaporate.
Hawking radiation though, just like BH's have never been observed.
Though we still have some that will claim that BH's do not exist, none can ever describe the effects on matter/energy and spacetime, that are observed in any other logical scientific way, the effects that can only ever be attributed to BH's.
BH's for all intents and purposes, and based on the current laws of physics and GR, and as evidenced by recent gravitational waves discoveries,  most certainly do exist. If not then some other even more incredible unbelievable object is present.
Likewise Hawking radiation, and what we know about the quantum world, tells us that logically it is a real concept, but in most cases, would be only evident over the lifetime of the Universe, and we are talking time frames of many hundreds and even trillions of years.


Stephen Hawking once used a phrase from a poem by Dante, about the gates of Hell.
"Abandon all hope ye, that enter here" probably aptly describes a BH.

 

Would a bunch of old 74 year old farts accept that science? :P:D

 
 
Edited by beecee
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I don't see anything that jumps out at me as being incorrect, you could add that Hawking radiation will only occur when a BH blackbody temperature is colder than the BB of the CMB background. The evaporation itself being a thermodynamic process.

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You've linked to a description of stellar remnants, and covered ending the fusion cycle by depleting fuel, but I wonder if your group wouldn't appreciate just a bit more about the formation of a BH from that point on, and what energies are at work overcoming degeneracy pressure? Is this being presented orally? Either way, mentioning supernovas should wake up anyone who nods off. 

Sounds like a fun group, beecee. Enjoy!

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5 hours ago, Phi for All said:

You've linked to a description of stellar remnants, and covered ending the fusion cycle by depleting fuel, but I wonder if your group wouldn't appreciate just a bit more about the formation of a BH from that point on, and what energies are at work overcoming degeneracy pressure? Is this being presented orally? Either way, mentioning supernovas should wake up anyone who nods off. 

Sounds like a fun group, beecee. Enjoy!

Thanks! Yes it will be presented orally by  your's truly, and you have raised a good point re BH formation and S/novas. A fun group it is! Still 11 of us left...three have passed on over the last 6 years or so, so the rest of us have contributed $100 each into a fund, with the last one standing, collecting the money! :P

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