[Howard] Does anyone remember a letter or article In the Mensa Bulletin about the non-existence of black holes? I think the author's point was that it takes forever (as viewed from the outside) for an object to fall through the event horizon of a black hole, therefore nothing has ever passed the event horizon so there are no black holes. I'd like to read it again. I think it was in a 1992 issue. Stephen Hawking has pointed out that black holes emit particles and are therefore unstable ( in 10^66 years or so). They will not be around forever (as seen from the outside). So, it seems that the letter writer is on to something. Black holes may still exist and something may fall onto one. Getting very close to the event horizon in a finite amount of time is possible, but we don't have to worry about what happens inside the event horizon since the black hole will completely evaporate before anything that is now outside ever gets through the event horizon. Or am I all wet? Are there any gravitational experts out there? ----------------------------------------------- [Howard to Rich] >> and after I jump off the bridge, the fact that I'm still falling proves that there is no river? Of course not. But what if you can never get wet and then the river goes away? Gravity slows time, and I think that time stops at the event horizon. An object that falls into a black hole APPEARS to a observer to take an infinite time to reach the Schwarzchild radius. However, the situation is supposed to be different for an observer falling with the object. The observer's sense of time would also slow down and the observer would reach the Schwarzchild radius in finite subjective time. Theory predicts that the observer would then continue to fall toward the singularity and be crushed to infinite density shortly thereafter. There is also speculation that if the black hole is rotating, then it might be possible to go through a wormhole and come out again. I'm not sure how important the word APPEARS is in the statement above. If black holes did last forever, one could argue that the falling observer will reach the center of the hole in sometime after forever. We would never see it happen but the falling observer would. However, if black holes evaporate in finite time then forever never happens. An outside observer merely has to wait 10^66 years. The falling observer will not yet appear to have reached the Schwarzchild radius and the black hole, along with its Schwarzchild radius and the falling observer, will disappear. I'm not saying that black holes don't exist. If a star collapses to form one, there would be matter inside what will become the Schwarzchild radius even before the black hole forms. The thing that we would observe in our universe would be the region of space just outside the hole and the gravitational effect of the mass in and on the black hold. However, if nothing has ever fallen from outside the black hole through the event horizon and if all black holes will evaporate before anything ever will, then it makes no sense to talk about what would happen to an observer falling into a singularity at the center of a black hole. The observer must evaporate along with the event horizon while still just outside. So, there is no worm hole. The black hole could still exist, but it would be a surface rather than a volume. The inside of the surface would not be part of the universe. -------------------------------------------- [Cindy to Howard] I kinda think that article was FOS; I don't remember the details, but I do remember thinking that the writer didn't know what he was talking about. The "Hawking" black hole is not the typical stellar object -- a massive star that implodes to create one -- but rather, the *primordial* black hole. Those contain only the mass of asteroids, or even less; they lose mass due to the quantum-physics "tunneling effect." If they lose mass faster than they accrete it, then they'll eventually evaporate. When they get to a certain tiny size, they lose *all* their remaining mass at once (ie, no longer waiting for one particle at a time) and explode in a blinding flash, possibly brighter than a star or nova or supernova (that little detail, the amount of brightness, I seem to have forgotten). We've never observed such a phenomenon, in all our centuries of watching the sky -- that's taken to be strong evidence against the existence of *primordial* black holes. They're called primordial because they were created during the Big Bang itself. According to Hawking. Anyway, the *other* kind of black hole is created during a supernova. It *won't* evaporate, because it's accreting far more mass from the interstellar medium than it could ever lose thru the tunneling effect -- so it's not the class of black holes that you're discussing. Lessee, what else do I know about 'em? That thing about the event horizon, and nothing ever crossing it -- I don't think that's quite how it works, but I'm not quite knowledgeable enough to expound on that one. I do know it applies to stuff falling *into* the BH, not to the mass that *caused* the BH during its creation -- ie, the mass inside the event horizon. I think you'd be better off reading any of the tons of layman-level astronomy books (William Kauffmann has some really good ones) than what non-professionals write in the Bulletin. Those tend to be more than a little off-base. (Hi. I remember you from the good old days in the Astronomy SIG. Do you still belong???) --------------------------------------------- [Howard] Hi Cindy. I remember you from the old days too. I'm no longer in the astronomy sig. I believe that a black hole is a black hole. The only differences between the primordial ones and the much larger stellar ones are quantitative and due to mass differences. According to Hawking in "A Brief History of Time", a primordial black hole with an initial mass of a thousand million tons would have a lifetime roughly equal to the age of the universe. Since a black hole with a mass a few times that of the sun would have a temperature of only one ten millionth of degree K, you are correct that it would be gaining mass, from the cosmic microwave background radiation if nothing else. Buts that's only the case temporarily. In the chapter "Black Holes Aint So Black", Hawking writes, "If the universe is destined to go on expanding forever, the temperature of the microwave radiation will eventually decrease to less than that of such a black hole, which will then begin to loose mass. But, even then, its temperature would be so low that it would take about [1*10^66] years to evaporate completely." So, large black holes would eventually become small ones and then explode. I've read that the Hawking's evaporation theory is unimportant to large black holes. And that's probably true for time periods of billions or trillions of years, but without it, black holes could last forever. And forever is longer than 10^66 years. If the universe isn't destined to expand forever, then large black holes won't have time to evaporate, but the falling observer won't have time to reach the Schwarzchild radius either. There's an interesting book "Deep Time" that extrapolates 10^100 years into the future of an ever expanding universe. Suns burn out in 10^11 years, protons decay in 10^33 years, stellar black holes evaporate in 10^66 years. Galactic black holes take longer, but even they eventually decay. All that's left after 10^100 years are very dilute atoms made of positrons and electrons. The distance between the positron and the electron in these atoms would be billions of light years! As far as seing a primordial black hole explode, it would be far dimmer than a supernova. My Scientific American Cosmology +1 book estimates that the final explosion would have the energy equivalent to about 10 million one-megaton hydrogen bombs. However, since this energy would be emitted mostly as gamma radiation, it would be impossible to see without instruments.