Are black holes black afterall? I remember reading in Hawking's book that black holes aren't black.
Are you referring to Hawking radiation?
They are extremely black (since light cannot escape them) but they are not absolutely black due to Hawking radiation (which unaided human eyes wouldn't be able to detect anyway).
Unless and until they get very small in which case the Hawking radiation (HR) would "climb" up the EM spectrum into visible light and all the way to gamma ray frequencies. There is also the "escape" of pairs of real particles taking their energy from the magnetic field. Similar to HR but a different process.
so Hawking radiation is basically composed of the particles that fell into the blackhole?
No. Hawking radiation is created outside the black hole (outside its "event horizon", the sphere from which nothing can escape that surrounds the singularity). Basically it is composed of quantum virtual particles which have aquired enough energy from the stressed spacetime near the black hole to become real. Since the release of the stress by this production reduces the black hole's gravity, it has in effect reduced the black hole's mass.
so if there was enough of those particles radiating out, eventually the black hole's mass would become zero?
Yes, but it takes a very long time.
If you have time and inclination, please take a look at the arguments I have been making in the thread "First Stars - how big- Black Holes now." Your text above is a perfect example of what I call "empty words" or "feel good" words that do not really explain the mechanistic process anymore than "Morphine makes you sleepy because it contains a narcoleptic agent" does.
I case of BH mass loss, two entirely different sets of empty words exist:
(1) The BH has swallowed so much entropy that it's event horizon, EH, is very very hot and radiating mainly gamma rays - this is why it is losing mass.
(2) The production of virtual particle near the EH occasionally has one swallowed and one escaping to live much longer that the uncertainty principle permits a violation of the conservation of energy. - this is why it is losing mass.
Neither "explanation" addresses the real mystery: In both explanations, the causal effect (gamma or VP escape) occurs outside the EH and the mass loss is at the point singularity. How does this "spooky" (Einstine's term) action at a distance occur?
I also have a lesser quibble with your post. I do not think the "stress," what ever that is, creates the VPs. The Casimir effect, measured in the lab, and quantum theory show that VP pairs are easily and copiously produced without any need for a BH near by. The function of the BH hole (in "empty word," set 2) is that its gradient attracts one member of the VP pair more strongly than the other and tends to separate them, permittting only one to be captured by the BH. The gradient at the EH of a small BH is much stronger than at the EH of a large one. - This is why only small BHs "evaporate" at any significant rate.
Many people have assumed that I am challenging the fact that small BHs evaporate (I do not) when I challenge the "empty word" about how they do so. Some have even use the word "crackpot" - I in turn am amazed so many people swallow these "feel good" words with so little thought, especially because two entirely different sets of these "empty words" are offered as the"explanation.
Are you not the least bit troubled by two different "explanations," neither of which says anything about the mechanism of how something outside of the EH produces a change at the point singularity? Have people forgotten how to think independently and now are capable of only paroting back the "explanations" they are given?
How does "release of stress" reduce mass? Is not your "stress" just a gravity gradient? If yes, is it not just field near a mass by another name - very much like "Narcoleptic" is another name for "sleepy"? No offense intended. I only want some "feel good" words that are explanitory and certainly not circular words that are just renaming of the same effect.
Different thread so I'm not breaking my promise to not post again on the other one. There is no sense in you visiting PF since you refute any explanations that include what you call "feel good words". But, how is any sane PF member to ever post a reply without knowing what you will or will not call "feel good words"? Anything we could post you could deny just by saying that we used "feel good" words, so any post would, by definition, be self-conflicting. :surprised
How about "atom" or "Nucleus" or "Event Horizon" or "Gravity" or "Einstein" (which you misspelled)? Are these "feel good words" if their use doesn't fit or agree with some point you have in mind??. You have too often mentioned Virtual Particles and the Casimir effect in one, single context as to make it obvious that you are totally unaware of the fact that "vacuum fluctuations" (feel good?) are only one of at least four ways that VPs are produced in the "real universe" (feel good?). There is no way that I will spell them out for you because your response is all too easy to predict.. :zzz:
The ony accurate thing I have seen from you comes from your own site where you state:
Now THAT "feels good"!
.. :rofl: .. :rofl: .. :rofl: .. :rofl: .. :rofl: .. :rofl: .. :rofl: .. :rofl: .. :rofl:.. :rofl: .. :rofl: .. :rofl: .... :yuck:
I am not refuting any widely accepted physics. I would appreciate your indication of which of the two different "explanations" of why black holes do evaporate is more satisfying to you. (gamma rays or VP particles escaping as cause.)
I also would appreciate some words about how something outside the EH, causes a reduction in mass at the point singularity of the BH. (This seems to be action at a distance with no stated mechanism - Einstein's "spooky" - sorry if my dyslexic scrambled "ein" into "ine" in last typing of his name.)
IMHO, it is reasonable to ask which of the different "explanations" is to be preferred and how the "action at a distance" is achieved. If you will answer these two concerns, I will stop calling the current words you and others have offered "feel good words."
Asking such questions does not make one a "crackpot." BTW I do not recognize the quote you gave as mine. Can you give the thread and post from which your are quoting? I think you must have miss read me or it was related to perception, not physics.
I might be properly be called a "crackpot" in the area of cognition, but not in physics. I do disagree with almost all cognitive scientists (Despite spending a year as guest scholar in Johns Hopkins U's cognitive science department.) The first post to philosophy thread "What Price Free Will" gives three independent proofs that the standard theory of how perception of 3D world is achieved from the 2D image on our retinas is simply wrong - but that does not prove my alternative ideas are correct.
To me, one must dispute widely accepted theory to be a crackpot, hence in physics I am not one, as I dispute nothing that is widely accepted in physics, including the evaporation of BHs. Surely one can ask questions and express a desire for a more a mechanistic understanding. I have even admitted that the loss of mass from a BH may be something, like quantum entanglement, that no human is ever going to really feel like it is understandable. (my "feel good") I am quite prepared to accept this about mass loss from BHs, as I have about quantum entanglement, but not willing to accept two different "explanations" as to why BHs evaporate, nor do I like (any more than Einstein did) action at a distance. I would much prefer to admit that this is a part of physics that humans must accept without pretending to have any set of words that explain the predictions that fall out of the math. To have two entirely different sets of "explanatory" words is embarrassing.
Billy T - You are welcome to ask for (or offer) clarifications on any point, but please keep the discussion on-topic. I prefer not to see a debate about semantics or rehashing previous debates in every topic. Your first post in this topic includes rebuttles to statements not made in this topic.
If you prefer to see PhD level discussions on every point, then you can find some of that here at PF (or at least links to external sources), but there's nothing wrong with offering a simple explanation to a simple questions. When someone is first learning about topic, you don't necessarily start them off at the most complicated level. "Feel good words" may be short hand for a lot of complicated mathematics.
Just a quick point in case there's any confusion. Most black holes we see are visible because of radiation from matter falling into them, not from Hawking radiation. Whether or not you include the accretion disk in the "black hole" is a matter of semantics, but you should know that we sometimes detect them by observing other matter radiating near the event horizon.
No. Time of evaporation depends on BH mass.
It does, but I think he was referring to the observed black holes. They're well above the evaporation limit.
Despite what I have said against "feel good" words, I tend to agree with you PROVIDED that when "explanations," that are not actually true are offered, it is made clear that it is just a convenient way to think about a more complex subject. I, for example, have no objection to Faraday's suggestion that magnetic field lines are a convenient aid to thought, if one also states that they do not actually exist. etc. Their cutting thru conductors is an aid in predicting when a current will be induced in a closed circuit (or a voltage difference will exist between the ends of an open circuit conductor) in both transformers and generators. The image of field lines concentration near the poles also helps understand why the Earth's magnetic field is stronger and nearly vertical at the N an S magnetic poles, or why electrons are trapped in the ionosphere, or why lighting strikes on the other side of the Earth can make static in your radio etc.
Einstein said it well (not exact quote): "Everything should be made as simple as possible, but not more so."
Is there an evaporation limit? It is my understanding that black holes of any mass can lose mass through Hawking radiation as long as they are not accreting more mass than they are losing through the evaporative process.
Of course, it may be that there is molecular hydrogen and other hard-to-see stuff in much of "empty" space and a large black hole will always be able to accrete more mass than it can lose through Hawking radiation... Does anybody have links to good papers on this?
By "evaporation limit", I'm referring to the mass at which it would take a Hubble Time for the black hole to evaporate away. We usually assume that black holes less massive than that will be long gone. It's not a limit on which black holes radiate, nor is it a strict limit on which can exist. A black hole created yesterday could be well below that limit.
Ah, a time-based limit. Ok.
In a former incarnation, I was a Process Chemist in a very large and very new pulp mill and much of my time was spent doing mass/energy balances around large systems and subsystems and identifying areas where inefficiencies could be identified and imbalances could be exploited to help make the mill more efficient. When I consider systems (something I can draw a border around in any meaningful way) I need to see some kind of balance, or I'm uncomfortable. I can accept the concept that very small BHs created in a Big Bang might have evaporated by present time, but whenever I think about primordial BHs, I imagine them existing in a very dense environment where they can gleefully accrete at rates that would be impossible for us to imagine in present physics, and this makes me wonder if true evaporation is possible, even for very tiny black holes.
A completely reasonable thought, I'd say. In fact, you're in good company, Jerry Ostriker has been exploring this possibility of late. Unfortunately, it looks like the radiation created by these accreting black holes would produce observable things which are not observed. They can exist in small numbers, though, and their current mass would likely be above the "limit" anyway.
A few generic examples:
How can you possibly object to "feel good words" Isn't that your specility? I though you bragged about your book full of explanations that are feel good but have no content.
No. I did not mean to "brag." I hesitate to mention my book, unless asked about it or something in it is directly relevant to a post others have made.
If you visit site, www.DarkVisitor.com you will find a list of all the physics and climate related information in it. Perhaps you are confused by the fact that I have included some very plausible, but false, statements in the book. (I call them "Easter Eggs," not "feel good words" after those that computer programmers often hide in commercial code they write.) With the exception of these eggs, all of the science in book is correct. I can't claim to be a great writer. - You can judge for yourself, without even taking the time to down load book for free, as sample text is provided at the site.
In the post script, I give hints about these "easter eggs." I also give text that directly contradicts the "false statements eggs." For example, one of the easter eggs is the statement that the small black hole approaching our solar system "will never be seen in telescopes because Black Holes do not reflect sunlight." The astronomer character, Jack, is (as stated later in the book) too busy looking for "wiggling stars" along the trajectory he has inferred from his observations of perturbations of Pluto (to refine his trajectory data) to write the book. (Why he asked his historian friend , Billy T, to write most of it.)
I will now give, for first time, another reason why a black hole will be detected in telescopes before it significantly affects Earth's orbit. Some of the solar wind will fall into the black hole. When it is closer to our solar system, the convergent solar wind particles will make some collisions outside the event horizon and this should give rise to some radiation visible in telescopes.
Jack is not sure it is a black hole. It could be the very old core of a neutron star, that either no longer has magnetic field trapped or if it was once a pulsar, it is now so slowly spinning or has spin axis roughly perpendicular to its trajectory, so that its approach has not been detected by radio telescopes.
The book is a recruiting tool for physics students - bright people with no current interest in science. Thus I am trying to scare them enough that they may want to learn some, to see if it is really true that life in the Northern hemisphere will become impossible by 2010, due to thick ice coverage. It (I hope) resembles Orsen Wells' radio broadcast:
in this regard, but unlike Wells' science fiction, my story is based on physical facts, and probably will happen to Earth some distant day. There is even a hint, related to the discovery of Pluto and Pluto's tilted orbit, as to why it could be soon.
Because the approaching object could be any one of several dark cosmic masses, stellar evolution is explained by Jack. Because of the orbital effects, including gravity gradients that may rip Saturn apart etc, Jack explains these things to Billy T. Because of the climate change, a mutual friend, George who is brother of Jack's wife and former NOAA researcher, explains climate mechanisms (much more than just Coriolis effect) to Billy T.
In summary Dark Visitor is a vehicle to teach a lot of science, without being obvious about doing so, because my target reader would never be interested in a book that taught science. Every time I mention the book here, I always tell that at site you will learn how to read for free.
My object is to try, in my small way, to reverse the decline of the US (actually the western world) in science. Home page of site clearly states this purpose, but see also page 30 of March 2005 issue of Physics Today for some really disturbing data. For example: "US share of worldwide high-tech exports fell from 31% to 18%.) while "emerging Asian economies increased from 7% to 25%." (most recent year included in study period was 2001 - it is worse now.)
Soon, if trend continues, scientific leadership will be lost to Asian nations as technological leadership already has been. In a generation, US will have mainly non-exportable jobs, like cutting someone else's hair and be unable to compete / afford to import oil. I only wish others were 10% as concerned as I am, and I live in Brazil! (But all my grandchildren are living in the US and will be very badly hurt, if there is not a change in student course preferences - that is why I wrote Dark Visitor)
Added by edit later: Another reason why an approaching neutron star might not have been detected by radio telescopes long ago is that the magnetic axis and spin axis could be nearly the same. It is the rotation of the magnetic field around the spin axis that makes (like a generator) the electric field that accelerates the charged particles that produce the continuous "microwave beam" we might observe as a "pulsar" if it happens to sweep through the part of the heavens that Earth is in.
There is a calculation in Dark Visitor that estimates how close an approaching neutron star would need to be before it is seen in reflected sunlight, by larger telescopes. This calculation is admittedly (in the book) crude as "Jack" could only guess at the coeficient of reflectivity and, more importantly, if the surface is "optically smooth", or not. It very likely is, and then only an extremely small area will reflect light back towards Earth. That is, even if the reflectivity were unity, an optically-smooth, old and now cold, neutron star could get "quite close" to the solar system before being detected by telescopes. "quite close" means that its effects on the visible planets' orbits would probably be easily observed first. (no calculation - just a quick guess.) "now cold" means the contiuning radiation is moderate to long wavelength IR.
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