Black Holes Disputed 296
JScarpace writes: "Researchers at the Los Alamos National Laboratory in New Mexico and at the University of South Carolina in Columbia have proposed the existence of "gravastars" which are bubbles of superdense matter. If they are correct, the idea of a black hole with a singularity at the center may be just a fantasy."
But if there are no black holes... (Score:5, Funny)
Re:But if there are no black holes... (Score:2, Funny)
You can still get sucked in (Score:3, Insightful)
Re:You can still get sucked in (Score:2, Funny)
Re:You can still get sucked in (Score:3, Insightful)
Actually, it is rather the contrary. These "gravastarts" are like giant hollow spheres, so technically, they would have a (huge) hole in the center.
"Real" black holes on the other hand, are solid (full) spheres, and thus have no hole, at least not in the classical sense (they do deform space-time continuum in very interesting ways though)
Re:You can still get sucked in (Score:3, Informative)
Re:You can still get sucked in (Score:2, Informative)
That is the problem with black holes, they are singularities. Mathematical points, infinitely dense. It even says so in the article.
That is one of the problems with black holes, and why these guys are looking for something different.
Particle physicists have the same problems with electrons (they appear point like with no structure)
Feynman had the similar problems that he couldn't get rid of.
Re:You can still get sucked in (Score:3, Interesting)
True, but I never claimed that (at least not for black holes)
there is no matter inside the horizon
That depends on your definition of matter. It's not matter in the classical sense, but rather severely degenerated (collapsed) matter. And concentrated in one point (the singularity). Then of course, there's also the philosophical question about whether it is legitimate to talk about what goes on inside the event horizon. After all, there is no way that we could possibly see what's inside (no light and no information gets out).
(Incidentally, gravistars, as described, aren't really hollow. Inside the "outer shell of gravitational energy" is a dense condensate.)
Maybe, I misread the article, but I understood it to mean that the condensate was on the shell.
So what is this springy, weird space? Some special kind of "vacuum"? The condenstate? But if all matter is expelled towards the shell, wouldn't we end up with a "hollow" sphere? Hollow, except for the weird field, that is.I hate "+5 Insightful" ratings for wrong answers...
And I hate self-righteous ACs.
Re:You can still get sucked in (Score:2)
Big bang (Score:3, Interesting)
Again? (Score:2)
It's not as if it needs another one, is it? (-:
On a more serious note, the theory will simply get re-engineered and tweaked and plastered over until a new unproveable conjecture happens along. But it must be one which doesn't smack of Young Earth Creationism, or otherwise - as Richard Lewontin wrote - ``allow a Divine Foot in the door [ic.net]''.
My guess is: when the time comes to admit that the new idea is much sexier (it does have a springy foundation, after all), the small differences between a Cosmic Egg and a First Cosmic Balloon won't get anyone but theorists too excited.
If it involves balloons, though, MacDonalds will want it attributed to Ronald.
Re:Big bang (Score:2)
This might actually be one interesting way to test the theory: It would seem that if there were this phase transition in black-hole-like conditions, these conditions would certainly have existed in the early universe. This means there would have been some sort of an inflation caused by the same forces that keep these gravstars hollow. This should leave traces that are observable to this day. If we don't find those traces, the theory is false.
Re:Big Bang, Black Holes, Quantum Computing, etc.. (Score:2)
Modern physics is anything but "voodoo". In fact, the whole point of it is that it is open to independent verification. You don't HAVE to be a "priest" to weigh in. What you DO have to do is put in the intellectual effort to understand what has already been done; learn some math, learn some physics, and nuclear chemistry - then you can examine the evidence for yourself.
Big Bang: take a simple spectrograph and a telescope (e.g. the Mt. Wilson 100-inch, which you can rent time on). Look at a sample of distant galaxies. Measure the wavelength of the hydrogen emission line (which is doppler shifted according to the relative velocity between the telescope and the galaxy); you'll see that the more distant the galaxy is, the faster it is receeding from you. Think about what that implies about the past. Certainly the simplest (if not the only) answer is that in the past the galaxies were closer to each other than they are now - ultimately at some point they would have to come from a single point.
It goes on and on - but the point I'm trying to make is that you shouln't treat modern physics as some kind of revealed religion. Instead, understand the evidence for and against the various theories - but also give the honest effort to learn the tools of modern science (above all math, but also physics, chemistry & astronomy).
If you don't then you're just spouting an uninformed opinion - and there are too many of those on /. already!
Re:Big bang (Score:2)
Nah. Those are far too similar to describe this comparison. We're talking apples and orangutans.
Researchers (Score:2)
Here's their paper (Score:5, Informative)
Re:Here's their paper (Score:5, Informative)
Also, folks, don't slashdot the site unless you know a bit about cosmology (if you don't know what I'm talking about when I say "line element" forget it) - this is a site that is very important for physicists in their daily work.
Re:Here's their paper (Score:2)
That said, I have read this paper, and I think it would probably be comprehensible to anyone who has made it through Kolb & Turner or equivalent, and good undergraduate GR and QM textbooks. Alternately, you could read something like Shapiro & Teukolsky on black holes, but I wouldn't recommend buying it out of curiosity because it's $130 for a not-very-large paperback.
If you really want to read the stuff in the preprint archive, feel free by all means. Just wait until its not getting lots of hits from a story on the front page of
Re:Here's their paper (Score:2)
still the same (Score:3, Interesting)
I've never heard of this site, but I must admit that was an extremely well-written article; they shoved a lot of physics in it but maintained a really high level of clarity (though it seems to based on a New Scientist article, so they may have just lifted passages from there).
Re:Creationists (Score:2)
Maybe that 'light' people see is your 'soul'[?] escaping from the gravastar or what-not.
Although, I've seen plenty of reports that say that the 'light' and euphoria is nothing more than a LSD trip your brain makes up to mask the horrible pain of dying.
Re:Creationists (Score:2)
I saw a HUGE thing long ago about near death experiences, and some scientists said the brain goes through a LSD trip. While there is no LSD [of course] there is the 'trip'.
Re:Creationists (Score:2)
Not LSD, ketamine [lycaeum.org]. Obviously in the typical NDE there is no ketamine. but there are several other mechanisms in the brain that may produce the effect.
Re:still the same (Score:2)
actualy you're being a little generous, there only has to be enough for it to become steady-state. I all ways thought that the exsistance of Hubbles constant is proof that our universe is a event-horizon limited object, i.e when hubble's constant forces your volocity to equal the speed of light, your at the edge. Interestingly your always at the center, you could move past what appeared to be the edge of the universe, and still be in the center of it, your friend who didn't move would become imaginary, as you passed his event horizon.
Also FYI the equation is E^4 = m^4C^4p^2q^2, not E= MC^2.
no singularity... (Score:5, Insightful)
A "singularity" is a point at which the gravitational force is infinite. This logically doesn't even make sense, so it's no wonder that it's disputed.
Cryptnotic
Re:no singularity... (Score:4, Informative)
Re:no singularity... (Score:5, Insightful)
Indeed. This prediction is a result of trying to integrate a function through points where the value is infinite, and then dividing that result by another infinite number.
The end is result is, to use the beautiful terminology of a Mathematician, "indeterminate."
The conventional interpretation of this results is that the theory cannot reliably predict the behaviour of the universe at points like this. Given the total lack of experimental evidence in regards to these phenomenon, I'd say that's a safe bet.
The same sort of thing happens when you try to calculate the total energy in a cavity (due to heat) using a classical theory. It tells you "infinity." What that means is "you should have used Quantum Mechanics."
Re:no singularity... (Score:2)
Re:no singularity... (Score:2)
I've never understood this. Escape velocity is the velocity you need to get out frem a given point in a gravity well with no thrust. Can anyone give a good intuitive explanation of why a rocket could not get out of a black hole?
Re:no singularity... (Score:2)
Re:no singularity... (Score:2)
Try again.
Re:no singularity... (Score:4, Insightful)
Question: what happens to this object?
Answer: it gets arbitrarily far away from earth. After a year, for example, it is about as far away as the moon.
What this demonstrates: you don't need to reach escape velocity to get out of a gravity well.
Re:no singularity... (Score:2)
Question: what happens to this object?
Answer: it gets arbitrarily far away from earth. After a year, for example, it is about as far away as the moon.
What this demonstrates: you don't need to reach escape velocity to get out of a gravity well.
These thought experiments always work better after a session with the SpaceBong 4000.
Re:no singularity... (Score:2)
Whoa, dude! That is totally intense my man!
Now, pass the SpaceBong 4000 this way, hombre.
Re:no singularity... (Score:2)
You will always need an infinite amount of energy to get towards light speed. But if even light, moving as fast as it does, gets dragged into the blackhole, you will need an infinite amount energy to escape. You can't!
Re:no singularity... (Score:2)
I don't know mi/hr, so I'm going to present you with something in m/s.
Gravity on the earth's surface is 9.8m/s.
If you have a payload on the ground of arbitrary weight and a motor that can provide enough thrust to lift it at 10m/s, in second 1 it will have
A rocket that would provide 9.8m/s of thrust would provide constant upward velocity, so it would hover, or if you threw it straight up at 1m/s it would maintain that 1m/s until it fell sideways.
Let's say for arguement's sake that our payload makes it into orbit, and it's a stable orbit. Once it is there, it is traveling around the earth at a very high speed in relation to the ground.
The space shuttle does about 17,500 miles an hour in low earth orbit. Last time I checked it doesn't fly off into the sun when it reaches that speed.
Why? Because earth escape velocity at sea level is 11.2 kilometers per second. That's somewhere around 40,000 km/h, or around 25,000 miles an hour. Now, escape velocity for the shuttle in a LEO would be less than it would be at sea level, but it would still need to reach it to leave orbit. In fact, by firing their engines and accelerating, they could achieve a higher orbit. They would also be going faster. Keep in mind that the moon is in orbit - So an object 'about as far away as the moon' is still in orbit of the earth, and will remain that way until it reaches a velocity that will remove it from orbit.
That aside, back to the inital arguement.
Nothing can go faster than the speed of light.
The gravity around what has been described as a 'black hole' is so intense that there is a point around it at which light can not escape.
What this means to you is that the acceleration at this point around the center of the 'black hole' is equal to the negative value of the speed of light - translation? You need a rocket that can produce 299,792,458 m/s of thrust, and not only produce that amount of thrust, but to simply maintain its position on the event horizon, it must continue to maintain that thrust FOREVER.
At any point, if the rocket stops producing that thrust, it will be pulled farther into the 'black hole' and will will need an even higher amount of thrust to stop 'falling in.'
If, by some magic trick you could go FASTER than the speed of light, then you could escape, but seeing as we can't even reach that speed yet, let alone exceed it, I don't see how your 10 mi/h rocket is going to do the same trick.
Here's a link for you that might help explain escape velocity for you, but you'll need a calculator. What is escape velocity? [physlink.com]
Oh, and stop wasting my time.
Re:no singularity... (Score:3)
Re:no singularity... (Score:4, Insightful)
Re:no singularity... (Score:5, Insightful)
This reminds me of the explanations in books for the general public on how airplanes work. The usual explanation is that because of the curve on top of the wing, when the airflow splits at the leading edge, the part that goes over the top has to go farther to meet up with the corresponding part that goes under the wing, and since it has to go farther in the same time, it has to go faster. We know from Bernoulli that a faster airflow has lower pressure, so we get a pressure differential, and the wing rises.
Anyone see the problem with that? The first problem is that no reason is given for the airstream over the top to have to meet up with the airstreem under the bottom. Why can't it just flow straight back?
The real reason wings work is that they cause vortices to be generated that, because of the shape of the wing, go down, and since vortices have momentum, by the usual laws of elementary physics, there is force upward on the wing.
There is no simple explanation of why wings cause these vortices, but no one likes to say in their books for laymen that it takes a PhD in aerodynamics to understand how airplanes work, so we get the totally irrelevant Bernoulli explanation.
It would be interesting to compile a list of various areas in science and engineering where there are explanations for the general public like this, that are basically wrong, and are so widespread that even scientists and engineers use them without thinking about them when writing for the general public.
It's called the Kutta condition (Score:5, Insightful)
>Anyone see the problem with that? The first
>problem is that no reason is given for the
>airstream over the top to have to meet up
>with the airstreem under the bottom. Why
>can't it just flow straight back?
See here [unige.it] for one of many explanations of the Kutta condition, one of the foundational principles of aerodynamics. This has nothing to do with an explanation for the layman. Basically, it states that the air MUST meet smoothly at the back of the wing.
Logically, if you spend some time thinking about the flow, you cannot possibly construct a situation where the air above the wing somehow slips past the air below. Remember that a jet moves so fast that its wing is only passing through a portion of the air for fractions of a second - it's simply not possible to make the air move fast enough to slip like this.
This principle has been demonstrated NUMEROUS times. You can demonstrate it very easily with a line of smoke through which a wing passes, among a zillion other simple experiments.
Your own reference seems to contradict you (Score:4, Informative)
Hey, it even says so, in bold: Stating that the fluid flowing above the airfoil is accelerated with respect to the fluid flowing below it ``because it must travel for a longer route in the same time'' is then definitely wrong. Betrayed but your own reference texts, eh?
As harlows_monkey says, in order to understand why the streams do meet if there is a correct angle of attack, you do need deeper insight into aerodynamics than is spelled out in the simple "lay-man's" explanation.
Re:Your own reference seems to contradict you (Score:2, Funny)
Also, think for a moment that you (YANAAE) are disputing the word of an aerodynamics engineer who works with this stuff every day. That's like disputing Alan Cox's idea of how the Linux kernel works.
Re:Your own reference seems to contradict you (Score:3, Interesting)
Of course, if the traditional explanation for how a wing works were correct, planes would not be able to fly upside down.
But they can...
Re:Your own reference seems to contradict you (Score:2)
Re:Your own reference seems to contradict you (Score:2)
Your glider can''t fly upside down because you have no engine and therefore can't sustain a high angle of attack.
Non-aerobatic planes can't do it because the fuel is gravity-feed. Also I don't think a conventional engine likes it's oil pan being turned upside down.
Aerobatic planes can and do regularly fly upside-down.
Re:Your own reference seems to contradict you (Score:2)
I'm afraid being an engineer doesn't give you the key to absolute truth. Engineers are given equations and taught the traditional explanation for lift. Since the equations do in fact work, the engineer can do his job even though his understanding of the mechanism is a little flawed.
Having a curved upper surface doesn't magically exempt you from the laws of physics. The basic physics behind lift are the same as everything else. The upward force of lift must be generated by something moving downwards. The only candidate when you're flying is 'air', so the wing must somehow be pushing air downwards in order to generate lift. This is acheived using the vortices the earlier poster mentioned and, at high angles of attack, by deflecting the air downwards.
For more information, check out this interesting article [washington.edu] (unfortunately it's a PDF).
Re:no singularity... (Score:3, Informative)
Now for black holes:
The equation for escape velocity is: v = sqrt(2*G*mP/rP), where mP is mass of the body, and rP is its radius. For 'black holes', the radii would be insanely small (maybe even zero, but insanely small is good enough) and the mass is very big as well, which would push the velocity to well above the speed of light.
Also, the equation for gravitational acceleration of a uniformly dense sphere (I think black bodies are small enough for this to be accurate) in newtonian physics is: a = G*mP/r^2, with the same variables as above, and r representing the distance from center of the body. This means that with sufficiant mass, and small enough distance, the acceleration would be so large that it would take more energy to accelerate you than is contained in your spacecraft's mass, meaning that you couldn't make that constant acceleration even if you had a perfectly efficiant engine.
So in order to get out from behind the event horizon, you would need a spacecraft with a more-than-perfectly efficant engine that could generate energy faster than a perpetual motion machine, and also be able to accelerate you to faster-than-light velocities, which would probably requrire a second engine and also some way to prevent relativistic physics from affecting you.
Often hilarious, too, methinks. (Score:2)
Some of the serious expose material should be more than side-splitting enough by itself, however...
I think there'd have to be a Balderdash edition as well, where people put in realistic-sounding explanations that are either total and deliberate frauds and/or proposed by people who really have no idea (like my ex-wide, who asked me (and this is a literal quote) ``I've always wondered, how do they get the batteries into battery chickens?'').
Dawkin's weasel would have to feature in the Balderdash book, together with Haeckel's ontogeny recapitulating phylogeny and that wonderful theory of electricity involving different coloured electricity for the different colours of traffic lights and so on.
Ex-wide (Score:2)
Hah! Well-picked... but actually, she's skinnier (and always has been) than her less-broken more-honest replacement.
Re:no singularity... (Score:3, Insightful)
It has got something to do with it, because it is true; right? Whether that is the easiest way to explain it to layman- probably not.
To my mind a good analogy of falling into a black hole is falling over a waterfall. If your boat can go at 10 mph, and the water goes at 9 mph, you can escape, but it will take a while. If the water is going at 11 mph, you are doomed. Right?
Space near a black hole is just like the water, IT MOVES and speed of light is like the boat's speed. The water is moving, and at the event horizon it exceeds light speed. I mean sure, you can max out your speed, but that doesn't matter because you're going down.
>(2) there is no simple intuitive explanation of why you can't get out of a black hole (where "simple intuitive" means comparable to the escape velocity explanation).
Your boat can't go fast enough due to the laws of physics.
Re:no singularity... (Score:2)
Perhaps, but that's pretty much what happens. Any object in the space around a gravitationally attracting mass finds that its inertial frame heads off towards the mass.
That's one effect. Another is that if the gravitating mass is rotating, it actually makes things spiral in, rather than fall straight in. I think that the idea that space moves, is a pretty reasonable approximation to what the GR equations are telling us.
You're water slide doesn't capture the second effect.
Re:no singularity... (Score:2)
To a first approximation it does, or they are trivial to construct. Draw the light cone of a body at rest relative to a gravitating body. You can define that as a motion of space. It's not normally described that way.
The curvature of space is a bit similar to the curvature of the water surface in a waterfall or a whirlpool, but as you note the analogy is not exact. (NO analogy is exact, by definition).
Re:no singularity... (Score:2)
Cool, you have a non-existence proof? Excellent. I'll get the pop-corn. This should be good.
Re:no singularity... (Score:2)
In ordinary environments like the one you're familiar with, it doesn't make sense to say that "time moves" either. "Moves with respect to what?" But it moves inexorably forward to the future, and everyone has a good intuitive sense of what this means even if they have trouble defining it logically.
This is analogous with the situation inside an event horizon, where space attains this property owing to the extreme space-time curvature. All futures take you in the direction of the singularity.
Re:no singularity... (Score:2)
Yes, that is a better verb for describing what time does in ordinary circumstances and space can sometimes do in extreme ones. Although the English language is poorly equipped for describing things that happen in a curved four dimensional space. It lacks the appropriate words and you can't talk about things without getting subtle things wrong.
Even trying to come up with definitions for "time" and "space" is difficult. Relativity attracts lots of philosopher types (so does quantum mechanics) who like to construct definitions that usually sound good but turn out to be logically inconsistent. The best definitions anyone has come up with for time and space are passive ones like "what a clock measures" and "what a ruler measures".
In any case I think you and I are in agreement on the subtleties of GR, but "space moves" is still a helpful notion when you're trying to explain to someone why rockets and rope ladders are useless for escaping from a hole. Making an analogy with the passing of time appeals to people's intuition.
Vorticity and speed difference (Score:3, Informative)
Re:no singularity... (Score:4, Interesting)
The "escape velocity" explanation is basically expressing an energy requirement. It is not insightful, like the light cone explanation becuase the light cone (when visuallized) shows you exactly which trajectories are possible. They all head towards the blackhole. The whole idea is that you cannot overtake the speed of light.
So why is the answer not satisfying? Being a physics TA, I have to understand the misunderstandings of students. It would be very helpful to me to understand why the answer is not satisfying.
Re:no singularity... (Score:2)
As a non-physicist, I feel fully qualified to answer this. The problem is gettin your head around bended space--most people's conception of space is described by a graph with three axises at 90 degree angles. No normal person ever doubts Euclid's fifth postulate.
The way I get my head around it is to imagine a globe. I can go East or West all I want; but if I'm also going North (forward in time), I will hit the North Pole--it is a point I cannot avoid. Why this is the case is not obvious if I express it in Cartesian coordinates, and most of us never get beyond them.
Re:no singularity... (Score:2)
I've always been great at analogies and explaining things to non-technical people, so I'll give this a try.
First off, I have to make you understand something. What the common person thinks of as "common sense" or "natural" is solely defined by the things they've gotten used to observing through their daily lives.
Science is NOT what we think should happen, or what makes "common sense". It is completely defined by what is observed.
Yes, this means that there are hundreds of things that young physicists encounter which *make no common sense*, but yea, there they are. The universe clearly demonstrates time after time that yes, the world works like that, which is completely counter to what you, a big huge blob of carbon and water, normally sees.
Now I'm going to draw a parallel that will hopefully bridge the gap between what you know and believe, and what the universe has taught us.
You've just jumped off a cliff on earth. You're an average human being, you have no strange artificial tools strapped to you. You have some cannonballs and rocks. Bo matter what you do, you will *not* be able to prevent falling down to the bottom. You could madly throw cannonballs and rocks downward, in an effort to use the common-sense principle of action/reaction to "thrust" yourself upwards. But it will not prevent you from falling down, merely slow you. You could have jumped upwards from the cliff, or tried running before jumping off, but either way you're going to fall to the bottom.
Why?
Because this close to the earth, on the edge of a cliff, gravity is quite simply stronger than you could ever be. And that's the way it is.
Maybe if you were a million miles away from the earth and jumping off a satelite, it'd work. You wouldn't need much thrust from a few thrown rocks to get away. But not down here.
Similarly the laws of physics (as we've observed them over a hundred years) tell us that for normal every-day matter that gets that close to a "black hole" (enourmous amount of matter all in one place), there is nothing that can be done to prevent that matter from falling in to the bottom. Even if you converted all but one atom of your mass to thrust, in any conceivable form, that last remaining atom would not be able to get out. It would only delay it's descent. The gravity has simply become *that* strong. Maybe if you were a little further away. But you're not. You're within the "event-horizon", the line which, once stepped over, there is no possibility of escape.
Even if you were a bit of light, the fastest moving, lightest thing in the universe, the gravity will drag you in. And if you're a photon of light, there's no way for you to go any faster, no bit of you that you can "cast off".
Re:no singularity... (Score:3, Insightful)
You have a flashlight.
You shine the light ahead of you.
If the light can't get out then you sure can't get out.
You can think of a photon as something with zero mass and infinite thrust with a finite momentum.
This stuff is mean enough so that relativistic effects dominate. The 3-dimensional space is NOT Euclidian 3-dimensional space, so things like distance depend on who's carrying the yardstick.
You can keep making progress, but it's like adding 1 + 1/2 + 1/4 + 1/8 +
Somebody that actually knows this stuff can maybe give you a better explanation. It all comes from the speed of light is constant for all observers. And messes up most everything else to keep that fact.
Re:no singularity... (Score:2)
You have a super-duper rocket ship and want to reach a star that is 13 light years away.
This means that it takes light 13 years to go from where you are to where you want to go.
Light travels faster than you will ever travel.
Now, how long does it take you to reach the star?
Over 13 years, seems like.
But this rocket ship can be goosed up to half the speed of light.
Goose it to half the speed of light. Goose it again.
Goose it a total of 24 times.
Wait 13 months and reverse the process.
The thirteen year trip took 13 months.
Make the return trip.
You've been gone 26 months.
Everybody else thinks you've been gone 26 years.
The gravity well is an attempt to describe what flat really is. Wierd stuff and you cannot trust your instincts.
Re:no singularity... (Score:2, Funny)
Re:no singularity... (Score:2)
Significantly, the singularity theorems demonstrate that GR is not a self-consistent theory, and contains the seeds of its own destruction.
The natural question which remained was : does stellar collapse of massive stars produce such trapped spheres, and hence black holes? Or is there a "proper graveyard" of stable, dead stars once a star ends its lifetime as a supernova?
The answer astrophysicists have provided is : no, there is no stellar graveyard, above a certain mass. The only possible outcome is a neutron star. Why? Because during collapse, electrons and protons fuse together to form neutrons (and neutrinos), leaving behind only neutrons. Neutrons are fermions and dislike sitting together. They exert a pressure amongst themselves, which allows a certain class of stable stellar remnants known as neutron stars to exist. However, as Chandrasekhar first demonstrated for white dwarfs, fermionic matter has a natural maximum mass -- for neutron stars, this limit is certainly no more than a few times the mass of the sun, regardless of how much the nuclear matter resists gravity.
Now, along come Mazur and Motolla (whose paper, incidentally, has been submitted, though not accepted, by Physical Review Letters) who claim that that just prior to the formation of a black hole, the matter forms a stable kind of Bose-Einstein condensate, and that something weird happens at the surface which sends the accreting matter around in a "u-turn". This all sounds extremely far-fetched to me, though I haven't read their paper in detail. However, I can make two comments :
1) They have yet to show how actual stellar collapse proceeds to their proposed gravitar, and not instead to neutron star or black hole. ("Robert Wald of Chicago University adds that Mottola and Mazur have put forward no arguments about how gravastars could form in the devastating collapse of a massive star.")
2) If indeed there is no horizon, and accreting matter gets turned around in a "u-turn", that should manifest itself as extremely powerful outflows. We already know of such jets and winds from black hole/accretion disk models, and from observation, so it should be possible to formulate a comparison between the gravitar prediction and black hole models. It seems quite likely that firm constraints on the gravitar model could be placed by examining the known observations.
Bob
Re:no singularity... (Score:2)
Cryptnotic
in case it's slashdotted (Score:3, Informative)
January 17, 2002 08:00 CDT
Two U.S. scientists have questioned the existence of black holes and suggested, in their place, the existence of an exotic bubble of superdense matter, an object they call a gravastar. The two are pointing out that physicists have swept some "humiliating" problems with black holes under the carpet. By confronting these problems, they claim to have found an alternative fate for a collapsing star.
Emil Mottola of the Los Alamos National Laboratory in New Mexico and Pawel Mazur of the University of South Carolina in Columbia think gravastars are cold, dense shells supported by a springy, weird space inside. They'd look like black holes, lit only by the material raining down onto them from outside. In fact, they seem to fit all the observational evidence for the existence of black holes.
So far, however, physicists have mixed feelings about the idea of gravastars. Their verdicts range from "outstandingly brilliant" to "unlikely." What's certain is that gravastars will rekindle a great debate of the early 20th century: are black holes fact or fantasy?
The idea of black holes dates back to the First World War, when German astronomer Karl Schwarzschild solved the equations of Einstein's newborn theory of gravity while serving on the Russian front. He showed that space-time around any massive star would be curved. Squeeze a large enough star into a tiny enough space and its density would become infinite and the curvature of space-time would spiral out of control. The gravity near one of these objects would be so strong that nothing -- not even photons -- could escape its grasp.
Einstein shared the view of most physicists of that time that such objects, later dubbed black holes, were too outrageous to exist. He argued that it was all academic anyway, since stars never shrink this small. But scientists gradually became convinced that they do. If a star is very massive, it will blast apart in a supernova explosion at the end of its life; and if a core twice as heavy as the Sun remains, no known force can prevent gravity squeezing it to a point.
The result is a "singularity" with infinite density, where the known laws of physics break down. The singularity's gravity would be so powerful it would be cloaked in an "event horizon", a boundary beyond which matter or light couldn't escape.
The dramatic idea of a black hole, which would rip to shreds anyone caught inside it, fired the imaginations of scientists, artists and writers alike. But no one has ever rooted the drama in fact.
"So far, there is no direct observational evidence to show that any of the things astronomers call black holes have event horizons or central singularities," says Neil Cornish, an astrophysicist at the University of Montana in Bozeman.
We know there are compact objects millions of times as heavy as the Sun that hog the centers of galaxies. These black hole candidates give themselves away because hot stars, gas and dust spiraling toward them emit bright X-rays. But that doesn't mean there's a cataclysmic black hole in the vicinity; it could simply be a very massive object. The debate petered out decades ago but there's still no ironclad proof that black holes exist.
There are enough problems in black-hole theory itself to make their existence seem implausible to say the least. These problems stem from the fact that our Universe is actually very different from the one that Schwarzschild considered. If we're to produce a proper description of the Universe we live in, Einstein's classical theories need to be meshed together with what we know about the quantum laws governing the behavior of fundamental particles and fields.
Mazur and Mottola have been thinking about quantum gravity for nearly a decade. They began by examining the nature of "quantum fluctuations" in space, time and even in energy fields. Empty space, for example, is never really empty.
On the tiniest scales, little particles are popping in and out of existence all the time, creating a seething, fluctuating fluid. "Like a fish in a calm pond, who is not aware of all the incessant jiggling of the water molecules, we are usually not aware of the quantum medium we are immersed in," Mottola said.
And they have found that quantum fluctuations in the electromagnetic fields that describe tiny things like photons can influence gravitational phenomena on the large scale-such as black holes. So, they reasoned, when early black hole theorists ignored quantum effects they were creating an unreal space-time.
This traditional approach to black holes has produced strange anomalies anyway, and these have remained unresolved, Mazur and Mottola claim. There are problems, for instance, with a black hole's entropy -- a measure of the amount of information it holds. An object that contains many possible states has high entropy, in the same way that a computer with more bits of memory can store more information.
When a star forms a black hole, all the unique information about the star -- its chemical composition, for instance -- appears to be squashed out of existence. Yet current theory suggests black holes have enormous entropy -- a billion, billion times that of the star that formed them. No one can fathom where all this extra entropy comes from or where it resides. "Where are all these zillions of states hiding in a black hole?" says Mottola. "It is quite literally incomprehensible."
Another seemingly impossible feature is that photons falling into a black hole would gain an infinite amount of energy by the time they reach the event horizon. But the gravitational effects of this enormous energy are ignored in the classical theory. Mottola says these problems have forced physicists to dream up far-fetched excuses. They say, for example, that some of the black hole's entropy might be hidden in other universes.
Mottola doesn't buy these "esoteric assumptions" and concludes that black holes are a bag of contradictions that don't make a good case for their own existence at all.
But is there an alternative? Could it be that when a star collapses, something happens to prevent a black hole forming? Mazur and Mottola think so. They have shown that quantum effects can make space-time change into a new and curious state that would lead to the formation of a strange new object.
That change is a phase transition, like liquid water turning into a solid block of ice. They believe that in the extreme conditions of a collapsing star, space-time undergoes a quantum version of a phase transition. The phenomenon is nothing new. The Nobel Prize for Physics in 2001 was awarded for the observation of just such an event in the lab: the transformation of a cloud of atoms into one huge "super-atom," a Bose-Einstein Condensate (BEC). This clump of atoms, which all share the same quantum state, forms at temperatures within a whisker of absolute zero.
When an event horizon is about to form around a collapsing star, Mazur and Mottola believe that the huge gravitational field distorts the quantum fluctuations in space-time. These fluctuations would become so huge they would trigger a radical change in space-time, very similar to the formation of a BEC.
This would create a condensate bubble. It would be surrounded by a thin spherical shell composed of gravitational energy, a kind of stationary shock wave in space-time sitting exactly where the event horizon of a black hole would traditionally be. The formation of this condensate would radically alter the space-time inside the shell.
According to Mazur and Mottola's calculations, it would exert an outward pressure. Because of this, infalling matter inside the shell would do a U-turn and head back out to the shell, while matter outside the shell would still rain down on it.
In a paper submitted to Physical Review Letters, Mazur and Mottola have shown that, like classical black holes, gravastars are a stable solution of Einstein's equations. What's exciting, they say, is that gravastars don't suffer any of the mathematical ailments of black holes.
There's no riotous singularity where the laws of physics break down. There's no event horizon to imprison light and matter. And the entropy of a gravastar would be much lower than that of any star that might collapse to form it, dodging the problem of excessive entropy that plagues black holes.
Take a gravastar with a mass 50 times that of the Sun, for example. Like the event horizon of a black hole with the same mass, the shell would be roughly 300 kilometers in diameter. But it would be around just 10-35 meters thick. Just a teaspoonful of the material would weigh about 100 million tons. But Mazur and Mottola have shown it would have a temperature of only about 10 billionths of a degree above absolute zero. And it wouldn't emit any radiation, making it as black as any black hole would be.
Gravastars would be just as much fun for sci-fi buffs -- in fact, they'd be even more ruthless. Imagine a black hole of a million solar masses, like the one thought to sit in the center of our Galaxy. You could cross its event horizon without feeling a thing: it's only as you approached the singularity that you'd be torn apart by the huge gravity gradient. But if you were drifting toward a gravastar of the same size, you'd never get anywhere near its center. As soon as you hit the shell you'd explode into pure gravitational energy.
Marek Abramowicz, an expert on black holes at Gothenburg University in Sweden, calls the idea of gravastars "outstandingly brilliant. Their unique and remarkable properties could explain several high-energy astrophysical phenomena that now are puzzling." He thinks they might explain gamma-ray bursts -- ultra-intense flashes of gamma radiation from a distant source that appear somewhere in the sky about once a day.
Astronomers aren't certain what causes gamma-ray bursts. It might be the formation of a black hole in a supernova explosion, but this process would struggle to muster enough energy. The birth of a gravastar, on the other hand, would be extraordinarily violent and might shed enough energy to account for gamma-ray bursts.
Mottola points to another possible connection between gravastars and astronomical observations. Three years ago, data from distant stellar explosions suggested that the expansion of the Universe is getting faster all the time (New Scientist, 11 April 1998, p 26). Many physicists ascribe this acceleration to a mysterious "dark energy" that gives space an outward pressure.
Mottola says that if you scale the size of a gravastar up to around the size of the visible Universe, the pressure of the vacuum inside roughly matches the pressure that seems to be accelerating the expansion of the Universe. So our Universe might be one big cosmic gravastar: a giant shell trapping the Milky Way and all the other galaxies we see. "We might be able to entertain the really radical notion that we -- and everything we see in the Universe -- could be inside such an object," Mottola speculates.
It's a bold claim, and he and Mazur are still working out whether it's justifiable. Unlike their hypothetical gravastar, the Universe contains copious ordinary matter and its visible edge is always ballooning outward. But they're keen to see what happens when they modify their gravastar model to include these complications. "It is certainly premature at this point, but the seeds of a possible new cosmological model are contained in the gravastar solution," says Mottola.
In the meantime, they are trying to figure out how they could tell ordinary-sized black holes and gravastars apart. The differences might be subtle -- after all, in isolation, they're both dark and the gravitational fields outside a black hole event horizon and the gravastar shell would be the same. But a good guess would be that gravastars would shine more brightly, since matter falling onto one would be turned into radiation. Black holes would gobble all the matter, but a gravastar would let its energy escape.
The next step is to identify the telltale signs of a gravastar, Mottola said. "It is the only way to convince the skeptical-including ourselves-that nature really behaves this way." Yet physicists aren't even sure what black holes look like.
In October last year, they reported seeing what appeared to be a heavyweight black hole, but material falling onto it is emitting far brighter X-rays than theories predict. The excess energy is roughly equivalent to the output of 10 billion Suns. If it is a black hole, it's not clear why it's so bright.
The object may be whirling round and dragging magnetic fields at the event horizon with it, and these could generate the extra energy by whipping up and heating nearby gases. But Mazur thinks there's a better explanation for that extra energy. The "black hole" could be a gravastar, he says. Stars, gas and dust raining down onto its shell would violently dissolve into pure gravitational energy that might emerge as bright X-rays.
To try to resolve this issue, Mazur is working out what a rotating gravastar might look like. Like every other compact object in the Universe, a gravastar would almost certainly be spinning rapidly.
Not all astronomers are as enthusiastic about gravastars. Cornish questions whether an exploding star could really lose enough entropy to form a gravastar, given that the second law of thermodynamics says that the entropy of an isolated object will always tend to increase.
"In other words, a cup can break into a thousand pieces, but it is highly unlikely that a thousand shards of pottery will spontaneously come together to form a cup," says Cornish. "Mazur and Mottola talk about a star shedding entropy in some way to make the formation of a gravastar possible, but I don't think that is a likely scenario." But Mottola points out that when exploding stars form other remnants, such as neutron stars, they do shed entropy.
And although Cornish admits that black hole singularities are mathematically troublesome, he also believes that a satisfactory quantum theory of gravity will cure this problem. Then there'll be no need for gravastars, he says. Robert Wald of Chicago University adds that Mottola and Mazur have put forward no arguments about how gravastars could form in the devastating collapse of a massive star.
Even if they did form, how would they survive the onslaught of matter raining down on them? "What happens if a gravastar has accreting matter showered upon it? Won't it collapse to a black hole?" he says.
"The gravastar is stable," counters Mottola. He says that matter falling onto the shell could make it wiggle and radiate away energy, but because the gravitational pull of the shell balances the force of the springy vacuum inside, it couldn't actually collapse. Any matter that fell onto the shell would simply become part of it, he says.
All the same, Mottola and Mazur admit there are still unsolved issues with the formation of gravastars. "We must have a better idea of how this phase transition actually occurs in the gravitational collapse process," says Mottola.
The exact nature of the exotic stuff inside the gravastar shell is still open to debate, and they hope to find out whether gravastars can really form in the mayhem of a star's violent death -- and whether gravastars could merge to form the heavyweight objects that sit at the center of galaxies. They are encouraging others to join the investigation. "There are many unanswered questions and we are really just opening a new direction for future research," says Mottola.
But if gravastars can weather the controversy, then maybe there'll no longer be any need for black holes -- maybe they really are pure fantasy. It wouldn't be the first time that Einstein's dazzling intuition has been proved correct.
Source: New Scientist
Cosmiverse Staff Writer
Need to shine a little experimental light. (Score:4, Interesting)
What I do find interesting is that this gravastar model, like the black hole model, implies that the universe and black holes/gravastars are similar in nature: that they belong to the same class of objects. It is a wonderful puzzle to look into a black hole wondering "what's IN there", when the answer might be something that has qualities similar to the life-cycle of our own cosmos.
Until we get some solid predictions about ways to differentiate one from the other, this is going to be a purely theoretical debate. Hopefully someone can advance the debate into the experimental realm soon. Maybe the new gravitational observatories can "shed some light" on this shadowy subject. ;)
Re:Need to shine a little experimental light. (Score:2)
"the first time that Einstein's dazzling" (Score:2, Funny)
Source: Old Scientist
You can't have both.. (Score:3, Interesting)
As the article mentions - you just CANT go around violating the second law of thermodynamics like they do (i.e. for a gravstar to form it must 'lose' entropy).
According to these guys the spherical outer shell (a standing gravitational wave) would balance out with the incoming matter. Now waiiit just a minute. Eventually the matter on the shell would exceed the force of the inner substance supporting it - then what do you have? They say that it would cause the sphere to wiggle and radiate away energy - well every struture has it's limits, what would happen when, say, a more massive gravstar impacts a less massive gravstar? Or two gravstars of equal mass impact each other?
Just b/c our understanding of physics breaks down at the singularity doesn't mean it does not exist (remember we can't describe in physical terms just what the first few picoseconds of the big bang where like - the physics just can't cope with the amount of matter/energy involved).
Now, we can *never* actually observe a black hole (God Abhors a Naked Singularity) which doesn't mean they don't exist.
"infalling matter inside the shell would do a U-turn and head back out to the shell, while matter outside the shell would still rain down on it." TO do so the matter woudl have to exceed the speed of light. Right.
Re:You can't have both.. (Score:3, Insightful)
But in the same way, just because our understanding of physics breaks down at the singularity / start of the big bang level, we cant be sure that the "laws" of thermodynaics are "violated" at the scale and energies involved at this theoretical gravastar state.
TO do so the matter woudl have to exceed the speed of light. Right.
Why not? how many times does everyone have to be surprised by physics before its finally conceded that there are no "LAWS" of physics.
As far as we are concerned, there is only our theoretical understanding and our observations, and thats all. Anything else that exists iin the universe (whatever that is), continues to exist whether we observe it and theorize about it or not.
If you approach science in this way, it becomes an open persuit instead of a closed one, which makes it ultimately more fruitful.
Re:You can't have both.. (Score:3, Informative)
As the article mentions - you just CANT go around violating the second law of thermodynamics like they do (i.e. for a gravstar to form it must 'lose' entropy).
Well, my physics is getting rusty too, but I think I might have some idea what's going on here...
The second law of thermodynamics says that entropy always increases in a closed system. But, during the formation of the gravistar (they've realy got to work on that name), a lot of the original star's mass would get blasted off into space. That's a huge arseload of entropy that's gone somewhere else.
At least, it works that way for a neutron star, and I can't see why a gravistar would be any different.
The question is, does it take away enough entropy?
A few points... (Score:2)
That point in itself would be against the gravistar theory. Because, they themselves have admitted that there have been mathematical shortcomings. The implication of this is that quantum behaviour _can_ stabilise, in which case, we would have had BEC occuring naturally, which is not the case.
"The gravastar is stable," counters Mottola. He says that matter falling onto the shell could make it wiggle and radiate away energy, but because the gravitational pull of the shell balances the force of the springy vacuum inside, it couldn't actually collapse. Any matter that fell onto the shell would simply become part of it, he says.
It is easier to accept the Black Hole theory. Just consider the Chandrashekar limit for example - if you are withing range, you are absorbed, else you are pulled closer. And since photons themselves have been proved to be absorbed by these, there is evidence of even horizon. But in this, we would be having an evergrowing event horizon. Given the age of the universe, if the horizon _does_ grow, just imagine for a moment what this means. Heck, there would absolutely no chance of survival for giant stars, which is not the case. Agreed, could be anamolies, but nevertheless worth a thought, right?
IMHO, as someone with experience in particle physics (I've worked on SQUIDS), I feel that this theory has a lot of points which need to be ironed out.
Re:A few points... (Score:2)
Again, then, as the article states, to get to a neutron star state, entropy must be lost. Why is it unreasonable to call for a proportionately higher increas is entropy loss for a more massive object's formation (a gravastar) vs formation of a neutron star? For me, Occam's razor would mean that the gravastar idea is MORE likely than the blackhole idea. It fits into already factual reality...neutron stars DO form, they lose entropy in the process (the star that produces them, that is). Thus, a more massive star, too big to form a simple neutron star, produces a gravastar by essentially the same mechanism - entropy loss but with the greater mass/energy involved, vacuum fluctuation energy becomes prominent.
Re:A few points... (Score:2)
However, both the theories face the same problem - how do they account for the lost entropy? These guys suggest a BEC formation which sheds, and Black Hole theorists have been having equally
bizarre explanations.
Notice their explanation for the region within -
Which could be a layer of BECs? Or some undefined nature? But in a blackhole, the entropy exists within, saying we have a parallel universe interacting to preserve entropy. Big deal, both are equally bizarre.
And blackholes can work well with dark matter. Not these.
The novel assumption required for this solution to exist is that low energy gravity can undergo a vacuum rearrangement phase transition in the vicinity of r = RS, in which the energy density and eq. of state change.
Knock, knock? Very great mass/energy ratio, but a really high mass. Would it still exist? Or would it somehow disintegrate? Would the phase transition undergo reversal?
It'd be interesting to work on that one
Re:Why is Occam's Razor always invoked . (Score:2)
From their paper -
Further, when a massless field such as that of the pho-ton is quantized in the fixed Schwarzschild background, one finds that the black hole radiates these quanta witha thermal spectrum at the asymptotic Hawking temperature TH = _h/8*kBGM [1]. The inverse dependence of TH on M implies that a black hole in thermal equilibriumwith its own Hawking radiation has negative specific heat.
In fact, that point in itself is a strong OR against BHs. But anyways, coming back....
Well, I reasoned Occam's Razor for the following reasons -
From the paper:
Energy conservation plus a thermal radiation spec-
trum imply that the black hole has an enormous entropy, S 10 77 k (M=M ) 2 [3]
Laws of thermodynamics clearly define that Entropy is on the rise. Given the amount of matter in a BH, this is acceptable (although theories on where these are a little wierd).
The novel assumption required for this solution to exist is that low energy gravity can undergo a vacuum rearrangement phase transition in the vicinity of r = RS, in which the energy density and eq. of state change.
Occam's Razor. Now just imagine a handful of dark matter in the vicinity, it no longer is _low_ gravity. I'd like to argue more technically, but I'd prefer it over e-mail than here.
Hence essentially all the mass of the
object comes from the energy density of the vacuum con-densate in the interior, even though the shell is responsi-ble for all of its entropy.
Different entropy concentrations? It's as wierd as BH causing Hawking Radiation and a high entropy of S ~10^77 k (M=M )^2
Since theentropy of these objects is some 20 orders of magnitude smaller than that of a typical stellar progenitor, a violentprocess of entropy shedding, as in a supernova, is needed to produce a cold gravastar remnant.Explosive bursts in which a finite fraction of atoms are ejected have been observed in attractive BEC's in the laboratory
A laboratory is one thing, a naturally occuring phenomenon is another. I do not mean to be derisive, but I find entropy shedding a little too hard to believe.
Agreed, BECs can be formed at high density too, besides low temperatures, but just apply Occam's Razor here and you will see that this means.
These are presumably characterized by a finite density of vortices of normal phasepenetrating the condensate core. Such rotating gravitational vacuum condensate stars, dark `grava(c)stars' arecandidates for the stable remnants of stellar evolution for stars exceeding the Chandrasekhar limit.
....
The remnants are left in an excited oscillatory state afterwards.
Have these been observed outside the lab? There is evidence of what seem to be black holes. Occam's Razor.
Hot Off the Presses! (Score:3, Funny)
SEATTLE - Bill Gates, richest man in the world, announced late Friday evening, that in keeping with his company's new policy of "Discover Your World" he would be putting 30 billion dollars into funding a trip to the far reaches of outer space to finally put an answer to the question, " What is a black hole?"
"We must strive to stretch our horizons" Mr. Gates said while unveiling a mock up spacecraft. "The Qube is singularly revolutionary - it is controlled via neural networks which interface between the ship with these nifty lasers that go over one eye."
However skeptics maintain that information will not be able to travel back from the ship even if it does collect scientific data about the nature of black holes. Microsoft, however, seems unfazed; "We realize at the current time, that issue may cause us problems, though we aren't worried - we work best under deadlines - take our release of windows 97 . . . well that's a bad example, umm. . . Microsoft has a great team that helps us get out of tight situations - take the DOJ trials - no one expected us to come out of that scot-free."
Top Enron executives also expressed interest in coming along for the trip, and plan on funding their portion selling Enron Ethics Handbooks on Ebay [ebay.com], with a source close to the vice president mentioning that "Anyplace would be better than here when our employees find us." - a view shared by Garth Wayne Johnson, Ken Lay's future cellmate in New York's "Ban Gurahz" prison, "Ah no dat da eron beetches ah gonna be ah hoes, so dey nee' packtis an' shit!"
The universe is expanding yes? What is it expanding into?
More wierd stuff... (Score:3, Interesting)
Ofcourse. And that is what the Unified Field Theory is all about. In fact, if only gravitons could be proved to exist, then there is a very high probability of the existence of the UFT. In fact, there are just 6 universal constants which need to be meshed in with their corresponding DEs to get the UFT up and running. Which, I'd say, seems simpler than what these guys may have to offer. Are these guys trying imply that UFT does not exist?
They believe that in the extreme conditions of a collapsing star, space-time undergoes a quantum version of a phase transition. The phenomenon is nothing new. The Nobel Prize for Physics in 2001 was awarded for the observation of just such an event in the lab: the transformation of a cloud of atoms into one huge "super-atom," a Bose-Einstein Condensate (BEC). This clump of atoms, which all share the same quantum state, forms at temperatures within a whisker of absolute zero.
In this context, are these people trying to say that the gravistar behaves as a BEC? That makes it a hell lot more complex because you will need really low temperatures, and adding more particles rushing to you at the speed of light increases the temperature and the entropy, both of which their theory goes against. Also Bosons (which are carrier particles, having an integer spin measured in the units of h-bar) would all possess exactly the same quantum state. So considering the existence of identical entities elsewhere, we could jump to any of these thingys just like that. Or any matter trapped in even one of these, could be spread across multiple copies of these entities.
The implications are really wierd, I somehow feel that Black Hole theories were a lot more plausible.
Re:More wierd stuff... (Score:3, Insightful)
Secondly, black hole theory is a mess and only looks acceptable to modern eyes due to familiarity. The singularity in the system is a BIG clue that it's wrong.
I'm not saying that the gravastar idea is right, and the temperature issue is a big question mark, but no one has given any reason in the last 30 years as to why we should accept the current BH theory other than it looks good on paper and the "problems" will be solved one day. Soon. Not yet, no, but I'm sure someone will clear it up. Probably.
TWW
Re:More wierd stuff... (Score:3, Insightful)
But the reason these constants _are_ of prime importance is because as a solution to certain tensor calculus equations in relativity, and these constants have been observed to be unattainable, but have been observed. Ofcourse, it is entirely possible like how we once thought that the speed of sound could not be exceeded, we may still be wrong about the speed of light and absolute zero, but that is a remote possibility because no particle in the world has been observed to have c nor have absolute zero (now don't get me started on photons.... as I read somewhere, your guess is as good as mine on what they are).
Good point. But you are forgetting one important point - there has been _some_ evidence showcasing possible black hole like behaviour, which cannot be explained by gravistar theory, atleast not yet. Example - Event horizon, dense areas which are surrounded by matter with an invisible core, and so on. In fact, Chandra has observed the existence of an Event Horizon in M82 [nasa.gov].
If you have done any amount of tensor calculus & quantum physics related mathematics (which I'm assuming you have), you'll know that Black Holes can be described with considerable ease in a Riemann plane, than gravistars.
Think of the implications these guys are suggesting --
1. You have submanifolds which would overlap as more matter gets in, and so the relativistic frame would in itself be a function carrying many frames. Assuming a standard rate of expansion for each of these frames, you can imagine the number of frames which would be in existence by now.
2. The gravitational effects caused by a tending mass are described in general relativity. These use a mere 16 coupled hyperbolic-elliptic nonlinear partial differential equations, called the Einstein field equations. Now, you have a solution for these called Bertotti-Robinson Solution, and when these are applied to a Black Hole, they work out just fine. This, despite assuming a uniform magnetic field.
However, you will realise owing to the submanifolds, you may not be able to apply the same to a gravistar. It'd be way too complex. And Bertotti-Robinson have been proved
3. Despite what the say about the Schwarzschild Black Hole, the exterior solution for such a black holes _has_ been proved [wolfram.com], and it conforms to the field equations proposed by Einstein.
Now these are independent results to the same set of equations. I think I'd rather trust Einstein than these guys
Anyways, it would be interesting to watch how this would get on. I'm not against this theory, just that there _seem to be_ far too many unanswered questions.
Re:More wierd stuff... (Score:2)
My mistake. I meant Cauchy-Riemann manifolds. Sorry
What the heck are you talking about? "Overlapping submanifolds"?? The authors aren't describing anything like that. "Frames carrying many frames?" You're babbling.
From the article:
Even if they did form, how would they survive the onslaught of matter raining down on them? "What happens if a gravastar has accreting matter showered upon it? Won't it collapse to a black hole?" he says.
"The gravastar is stable," counters Mottola. He says that matter falling onto the shell could make it wiggle and radiate away energy, but because the gravitational pull of the shell balances the force of the springy vacuum inside, it couldn't actually collapse. Any matter that fell onto the shell would simply become part of it, he says.
If the initial reference frame is defined by a predefined set, then the frame would forever grow. Whilst in a BH it would forever be there.
Think about it. A gravistar is forever growing. That way you will have Riemann manifolds for each previous state overlapping with the new ones.
The stability of the gstar would be defined by a very delicate balance indeed. So it kind of becomes very very recursive.
Dear Mr.photon, do you want to go inside. Fine, let me measure you and balance myself and then decide. Oh ok, now you are free to go! Get in.. Uh oh.. what about that one over there? Where does he come in? Well, he was defined by the previous state. Why? Since his distance is different, he is in a different ref frame. You mean for each of these guys my balance makes it madatory to apply different inertial frames? It appears to be a logical extension, doesn't it?
Also, consider a light cone into a gravstar. Wouldn't you have to take discrete functions for fixed intervals since a gravstar does not plainly absorb, it also grows.
What does the Bertotti-Robinson solution have to do with anything being discussed here? It's just a universe with a uniform magnetic field.
Intrapolation of br-eqn is done to determine individual gravitational patterns. When a gravistar keeps growing, it would in effect be an increasing gravitational function. If you consider a reasonable amount of gravistars distributed then you will have a universe forever varying in the mag field.
As each system has more and more matter flowing in, the force exerted would literally be really really be big.
Assuming that when two of these come together it would create a mega field. A BH does not grow beyond the event horizon. A combination of even 3 such things distributed widely is enough to cancel out the validity of BR eqn.
What is the assumption in BR eqn? The universe has a uniform field. I maybe wrong, but wouldn't a series of immense gravstars forever growing in gravity, distributed around, kind of counter this? Please clarify.
Thanks.
Re:More wierd stuff... (Score:3, Informative)
He proved that the radius itself (after some modifications) could be used as a limiting factor, i.e your Event Horizon.
There has been evidence from galaxies about the existence of EHs as observed by Chandra and Hubble, independently. In fact, there is also evidence of tunneling in EHs which have been photographed.
Given so many facts, it is really really very hard to just throw the Black Hole theory out of the window. It's just not that simple.
Re:More wierd stuff... (Score:2)
Actually, all the singularity tells us is that we don't know shit about quantum gravity. Which, well, we already knew.
Re:More wierd stuff... (Score:2, Interesting)
The last thing I removed from my thesis...: (Score:2)
The reason wasn't that I'm particulary skeptical about black hole theory, but that I figured I really didn't need to assume anything about it, as long as all the other assumptions I make hold (which they won't, but that's an entirely different matter :-) ). The central engine of quasars can be whatever it likes... :-)
My knee-jerk reaction to the article posted was that it seems like the gravastar isn't allowed to grow, and it has to grow, right? However, skimming the researcher's preprint, it seems like they are addressing the issue, so it is probably just something I've missed. Besides, there'll be enough knee-jerk postings here... :-)
Re:The last thing I removed from my thesis...: (Score:2)
My 0.2! That is one point against it, in fact. Also, if everything within the shell border limit of rho=p would be in the fluid state, which could be BEC. Just imagine these thingys floating around.
In fact, the way the paper considers the region in the shell is interesting. Their proof shows that r has to be a constant for the shell.
One approximation in the paper which scares me is when they say that f & h are approx. constant in the shell. It is not. The difference would mean that the gravastar _would_ grow, albeit a little by little, atleast by cosmic accounts.
Cosmic junk food (Score:2, Funny)
Black holes are really Moon Pies?
My question is... (Score:2)
We've already proved the existence of black holes (Score:4, Funny)
A bit wrong conclusion.. (Score:2, Informative)
So the stae of matter below the horizon has NOTHING to do with properties of the such a compact object above it. This is because of causality - there is no information flow from behind the horizon..
Re:A bit wrong conclusion.. (Score:2)
From the point of view of matter not falling into the black whole, matter falling in never crosses the event horizon. For matter falling in itself not to notice its crossing, relativity would have to say that the faster you go the slower you go; it obviously does not say this.
Also, ignoring quantum effects, it would stand to reason that anything below the event horizon would collapse into a singularity: it must become as small as is possible. Matter could not survive such pressure, and the energy at that point would appear to be infinite. Of course, since this all assumes that energy occupies a particular place and time, the concept of a singularity might not be appropriate in quantum mechanics.
Nifty (Score:2)
Let's hope their paper doesn't include words like "zillions." Also:
But if gravastars can weather the controversy, then maybe there'll no longer be any need for black holes -- maybe they really are pure fantasy. It wouldn't be the first time that Einstein's dazzling intuition has been proved correct.
It also wouldn't be the first time his dazzling intuition has been proved wrong. Remember, Einstein didn't believe quantum mechanics was possible (at first, anyway).
-Legion
How to form such a beast? (Score:4, Informative)
Anyway, and I quote from their own article, "Here we forgo any discussion of the details of the quantum phase transition and present only the solution of Einstein's eqs." Mazur and Mottola have no clue how to make such a beast, either. If nothing else, the energy density wouldn't approach that needed for a phase transition until long after the entire assemblage was well within its own event horizon, again giving -- you guessed it! -- a Schwartzchild black hole. Recall, when a solid mass reaches the density required to fall within its own event horizon, the total density isn't much above nuclear densitites. During big bang baryosynthesis, densities are easily this large and inflation obviously didn't occur then (or else we'd have no protons in the universe).
Flying through a gravastar (Score:2)
Re:Geniuses abound! (Score:3, Insightful)
But isn't that the point of slashdot?
I mean, this is a discussion forum, and most people here want to read and post.
Perhaps if that bugs you, you shouldn't read
Michael
Re:Geniuses abound! (Score:2)
Of course, to observe that this accounts for much of the traffic on Slashdot is itself not terribly insightful.
Re:gravastar? (Score:5, Funny)
From now on they will be referred to as Doom Spheres.
Re:gravastar? (Score:2)
How about Sinistar?
Re:Mathematical models were based on... (Score:3, Funny)
Re:What is the difference? (Score:5, Insightful)
Escape velocity not only depends on the mass, but also on the distance from the center. In a black hole, escape velocity only exceeds the speed of light if you get closer than the event horizon.
Now, if for some reason the necessary mass would not fit into the event horizon, no black hole could occur. This new theory stipulates that if you have such a huge mass, it will actually form a hollow sphere where much of the mass is actually concentrated outside of the event horizon. Now, a hollow sphere has the following "interesting" properties (or would have, in classical mechanics):
- outside the spere it is the same as for a point mass: mM/r^2
- within the shell it would be approximately linear in r: mM*(r-r_inner)/r_outer^2/(r_outer-r_inner)
- inside, it would be zero
Probably this is not 100% exact, as we're in a relativistic context here, rather than a classical one, but we can still presume that gravity inside the sphere would be much weaker than outside.(approximative formula, for "thin" shells)
This would basically mean that you would not have an escape velocity exceeding the speed of light anywhere:
Re:Of course it's a fantasy! (Score:3, Insightful)
Nobody ever said quantum mechanics made sense, either (and if they did, they're lying). But not making sense doesn't mean it's not right.