General Relativity predicts massive stars which eventually collapse under their own gravity become Black Holes, an object of such intense gravity where nothing can escape, not even light. Its core is considered to be a ‘singularity,’ a point in space with infinite density and zero volume. It is thought the laws of physics break down within a Black Hole.
Recently, a group of scientists have come up with another theory for a collapsed star; a Gravastar. Gra-gravitational, va-vacuum, -star. It has also been suggested our universe is one huge Gravastar.
However, if one were to compare the two theories using Occams Razor, Black Hole theory would be supported and it would seem unlikely our universe is a giant Black Hole.
Occam’s Razor is a fairly weak argument, and should be invoked only when there is no evidence to support one theory or the other. I don’t think that will turn out to be the case here. An objective scientist would support whichever theory holds up best in light of observations.
There are some interesting arguments for or against each one. The effect of a black hole is observed but we really have no idea what one really is. I would go against a BH because it is pretty well defined - too well for something we only have a faint idea of. I’m not saying that a gravastar is it but in some ways makes a little more sense. a gravistar is just another form of matter while a bh is akmost sucking the matter out of the universe
Who says the matter is out of the Universe? It’s just behind a curtain that no one can peer through. Given that the gravitational effect of matter being added to a black hole can still be measured (the diameter of the event horizon increases for one thing) would lead me to believe the matter is still in there in some fashion.
Occam’s Razor is a fairly weak argument, and should be invoked only when there is no evidence to support one theory or the other.
Occams razor is not really an argument but more a ‘rule of thumb,’ and is used to decide how to evaluate competing theoretical explanations prior to testing them against the real world. It states that the simplest explanation for a phenomenon is the one preferred by nature. Observations may support multiple theories however an objective scientist will use the razor to rule out all but the simplest of those theories.
In the case of BH and Gravastars, the razor would eliminate the Gravastar.
K2dave
The effect of a black hole is observed but we really have no idea what one really is.
Actually we do. Objects we observe to be candidates for BH are not only predicted by General Relativity, they are also ‘detected’ using GR, due to their gravitational influence upon matter.
X-ray astronomers have been studying binary systems in which a neutron star is literally ‘ripping’ matter from its visible companion star, and since the neutron star is below its theoretical limit for stable neutron stars, it is assumed that is what they’re seeing. However, upon weighing an unseen companion, it is observed that the masses are well in excess of the neutron star mass limit.
The energy of the X-rays emitted changes over time which require the presence of a small compact object less than a few hundred kilometers across. Upon examining the doppler shift of the X-ray emmission, one discovers it matches to within experimental error. The cores of many galaxies are in excess of several million to a few billions solar masses which cannot be accounted for by the starlight observed. Spectroscopic studies reveal the speed of stars and gas increase rapidly near these cores in which the matter forms discs.
All of these observations, and others, pinpoint to an invisible object of very high mass with powerful gravitational fields which displays all the relativistic effects astronomers are able to measure. They all match an object predicted by General Relativity, a Black Hole.
I admit that I do not know any of the Physics behind Gravastars; the way you’re talking about them, I’d say that you think you do. If that’s the case, then please explain to me why they should be shot down with Occam’s Razor. Occam’s Razor is used to rule out perpetual motion machines which utilize “Newton’s Fourth Law of Motion”. It’s used to rule out physics books which open, “I had to publish this by myself, because all physicists are idiots and nobody believes me.” It’s used to rule out outdated geocentric theories. It is not used to rule out ideas which are published in a peer-reviewed journal in a paper beginning, “We propose an experiment to test the validity of…”
The laws of Physics do not breakdown in a black hole - we just don’t yet know what they are. There’s gotta be some sub-quark entity, string?, thay we just don’t have a handle on. If we can’t figure out dark matter laying around all over the place, it’ll be a while before we get black hole power generators in the garage.
Qeue I agree with what you said but those also hold true for a gravastar (except for being able to see it but that’s not a sure bet either and if you could they would be very dark). From what I’ve see/read about gravastars they also are a solution to GR.
Occam’s Razor is used to rule out perpetual motion machines which utilize “Newton’s Fourth Law of Motion”. It’s used to rule out physics books which open, “I had to publish this by myself, because all physicists are idiots and nobody believes me.” It’s used to rule out outdated geocentric theories. It is not used to rule out ideas which are published in a peer-reviewed journal in a paper beginning, “We propose an experiment to test the validity of…”
I think you may be referring to John Baez’s, “Crackpot Index.”
please explain to me why they should be shot down with Occam’s Razor.
Although both theories, BH and Gravastars, are at this time limited to conflicting mathematic models, and that neither have been directly observed or sampled, BH are the simplest of the two theories. Gravastars are theorized to have an ultra thin shell in which matter is obliterated upon reaching its surface and is also transformed to a new state of matter. The interior is theorized as a Bose-Einstein condensate appearing as a vacuum with very low entropy. They go on to speculate that the vacuum pressure is inside roughly matches the pressure that seems to be accelerating the expansion of the universe.
IMO, I think these guys are simply trying to tackle the problem of BH entropy and have come up with a complex model in an attempt to solve the problem. It appears to be too complex a solution for what we have observed.
Black Hole’s are simple. Matter is simply crushed by its own gravitational influence.
On the other hand, one, the other, neither, or both may actually exist. Frankly, the uncertainty is enjoyable.
Okay I see what you’re saying now. As of now, there is no evidence one way or the other. In that case, Occam’s Razor may be applied. But as I understand the situation, a scientist would be silly to draw a conclusion at this point, because the theory is testable, but has not been given the time required to test it. Science isn’t about just coming to conclusions. At least, not when the evidence doesn’t yet exist.
Does a Gravastar have an event horizon? If so, then there is no real observational difference between a Gravastar and a black hole. On the other hand if a Gravastar does not have an event horizon then it would have a hard surface, and the matter impacting this surface would create a radiation signature totally different from that of a BH.
I think a gravastar’s event horizon is its surface. If you touch the surface there is no coming back out again o you won’t get weird radiation emanations. With a sufficiently large black hole you can pass the event horizon and not even know it (or at least suffer no ill effects). No matter how large a gravastar is if you hit the event horizon (its surface) you’re done for on the spot. Your done for in the black hole too but it’ll just take a little longer as you travel to the singularity.
That’s what they’re trying to do, except that there is no “black hole entropy problem”. The “problem” that the proponents of the gravastar point out with black holes is that a black hole has an entropy many orders of magnitude greater than that of the progenitor star (the entropy of a black hole turns out to be surprisingly well-defined, and is proportional to the area of the event horizon). A little more thought, though, will show that this isn’t a big deal: One would expect, based on the Second Law, that the final state of a system would have an entropy higher than the initial state, and balck holes are pretty much the ultimate in final states. By contrast, a gravastar has an entropy many orders of magnitude less than the progenitor star, which means that a collapsing star must shed essentially all of its entropy, and then some, during its collapse.
To make things worse: A black hole’s entropy is proportional to M[sup]2[/sup], which means, for instance, that when two black holes merge, the entropy increases. This guarantees that black hole merger is an irreversible process. A gravastar’s entropy, by contrast, is proportional to M, which would suggest that gravastar merger is a reversible process, and gravastars could spontaneously split, which does not bode well for actually detecting the things.
Another issue that gravastars are claimed to address is that they don’t have a troubling singularity in the center. However, before you reach the center, you get to extremes of curvature which make one suspect that new physics come into play, presumably some sort of quantum gravity. It is expected that quantum gravity, in the extreme conditions near the center of a black hole, will eliminate the need for a central singularity. With gravastars, however, one must invoke quantum gravitational effects just outside what would be the event horizon, and for a sufficiently massive object, space is perfectly ordinary in the vicinity of the event horizon (the curvature can be arbitrarily small). It’s OK to invoke quantum gravity under extreme circumstances, and behind a horizon, but gravastars require us to invoke it in a region which is both ordinary and visible, and require that the quantum effects take a particular form, of which we cannot yet be certain.
I would add one more spin. Occam’s Razor is less about eliminating possibilities, and more about how to distribute scarse resources. When grant money is being earmarked, when Hubble telescope or particle accelerator time is being allocated, even when grad students are being assigned to major professors, the most likely theories are given the lions share. If those theories eventually fail to produce results, and the competing “less likely” (and less supported) theories gain ground, the balance of power can quickly change.
It wasn’t all that many years ago that the Big Bang was a minority theory. Same with Quantum Mechanics. The wonderful thing about science (when it is done right) is that today’s “crackpot” idea really does have the opportunity to become tomorrow’s accepted theory.
As for which theory we might support, the Los Alamos press release you cited doesn’t give the likes of us nearly enough information to make a intelligent decision. Those things are mostly intended to give the general public something to “gee whiz” about. For example, the suggestion that our entire universe might be inside a gravastar sounds exactly like the kind of speculation an otherwise respected scientist might proclaim just to give the article more sex appeal. Chances are that what he really means is, “there’s nothing in our theory so far that would eliminate the possibility.” The same could be said for the interior being filled with discarded Peter Max artwork.
But as I understand the situation, a scientist would be silly to draw a conclusion at this point, because the theory is testable, but has not been given the time required to test it.
That’s the problem in a nutshell. How does one go about testing theories for BH or Gravastars ? All we can do at this point is continue to make observations from a distance. Perhaps we can make a BH/Gravastar in the Lab.
Maybe not.
Ring
Does a Gravastar have an event horizon?
Apparently, the event horizon, in the case of a Gravastar, is the ultra-thin, super-hard layer that separates the pressurized condensate from the impacting matter. In the case of a BH, it is simply an imaginary point where light no longer is able to escape. I believe most consider the Event Horizon as the BH. What’s beyond that, no one knows for sure.
Singularity ?
Whack-a-mole (great name)
With a sufficiently large black hole you can pass the event horizon and not even know it (or at least suffer no ill effects).
Not really. As you get closer to the BH, the tidal forces begin to take effect. You begin to stretch out because the part of you that is closer to the BH feels the effects more strongly and sooner than the part furthest away. As you get even closer, the tidal effects intensify such that you are actually torn apart long before you ever reach the Event Horizon.
The whole process takes under ten minutes to reach the Event Horizon and then another few seconds for you hit the singularity. That is, whatever pieces are left of you.
Not true. Chronos alludes to this in his post earlier in this thread…"…for a sufficiently massive object, space is perfectly ordinary in the vicinity of the event horizon (the curvature can be arbitrarily small)."
What you detail is true for a black hole formed from a single star. However, if you take a black hole on the scale of those found in galactic cores (several million solar masses) you can quite happily pass the event horizon with no ill effect. The ill effect will, however, be meeting you shortly as you will without doubt meet the singularity at the black hole’s center (and, AFAIK, you will be torn apart by tidal forces before meeting the singularity anyway…you just have to be closer than the event horizon in a really big black hole).
If a collapsing star were to shed all of it’s entropy, and then some, would that not mean it would also decrease in temperature to the point of almost zero ? If so, then it would seem the condensate inside the Gravastar would not be “pressurized” and therefore it should collapse.
Is not the area of black hole the entropy and the temperature corresponds to the surface gravity?
Whack-a-mole
Not true. Chronos alludes to this in his post earlier in this thread…"…for a sufficiently massive object, space is perfectly ordinary in the vicinity of the event horizon (the curvature can be arbitrarily small)."
Yes, but I think he is alluding to Gravastars and not BH.
But the star isn’t shedding its entropy if you are assuming the black hole model. The entropy is increased once the black hole has formed.
I realize that. Chronos suggested that stars would have to shed their entropy, and then some, in order to form a Gravastar. In other words, they need to shed their entropy before forming anything, Gravastar, BH or any other potential object.
In regards to a BH model, the entropy is determined by the area and the temperature corresponds to the surface gravity. Therefore if we use these methods to determine the entropy and temperature of a Gravastar, then the temperature of the Gravastar should be zero. If so, there would be no “pressure” inside the Gravastar and hence it would collapse.
How is it the Gravastar can maintain its structure if the condensates temperature is near or at zero ?
