The feeling I get from reading the article is the theory is highly speculative. Could be just this line: "‘loop quantum gravity’ — a theoretical attempt that has yet to find experimental support"
Could be just this line: "‘loop quantum gravity’ — a theoretical attempt that has yet to find experimental support"
As @jerf points out here as well, LCG has as much experimental support as any of the other competing GUT hypotheses... which is to say, none. This whole "thing" about the lack of experimental support and lack of testable / falsifiable assertions in physics (especially String Theory) is explored at length in two pretty interesting books: Not Even Wrong[1] by Peter Woit and The Trouble With Physics[2] by Lee Smolin.
Considering that the Higgs Boson was theorized about in 1964 and finally proven in an experiment in 2012: this seems to be the way of theoretical physics. ;)
The difference is that the Higgs was a necessary logical consequence of the Standard Model. Quantum loop gravity is purely speculative, much like string theory.
In the case of QM versus GR we have the very weird situation where we have two theories that are logically incompatible with each other, so we know at least one of them must be wrong. And yet neither one has ever made a prediction that has ever been falsified by experiment, so we have no guidance from nature regarding which theory is wrong or how. That's the reason that the theoretical physicists seem to be grasping at straws, and everyone is a little disappointed that the LHC hasn't done anything surprising.
While I agree with what you are saying, I'd like to add a bunch of addenda (caveats?).
The Higgs mechanism was one possible model which the standard model required, the others have now obviously been disproven thanks to the LHC. If no one had proposed this particular model, we'd still be wondering how fundamental particles gained mass and have another unexplained experimental result with the peak at 125 GeV from the data at the LHC.
Similarly, GR and QM are both extremely well tested, however GR has dark matter and energy to deal with, and we still have no real idea what that is. This could potentially point to GR being wrong, however none of us are able to come up with another theory of gravity that could incorporate this and be better than GR at this stage. If a theory of quantum gravity (or just gravity) came along that could explain dark matter quite effectively then it would be hailed as a better theory than GR.
The reason string theory and loop quantum gravity are considered completely speculative is because their predictions are so far beyond our grasp that even in the relatively far future we still have no way of confirming or denying either of them. Had they predicted things that could've been confirmed within the next century or so, I'm sure we'd all be rallying to try and confirm them. They aren't speculative so much so because we don't know which of GR or QM is wrong, but because their testability is quite precarious at this point.
I think it's not so much that the Higgs was logical; the difference with Higgs is that the Higgs was conceived as something testable from the start (I'm assuming, but it doesn't seem like a big leap, considering that particle physics has always been driven by experiments). The really big problem with untestable ideas is that they are not even science. Untestable ideas about physics are more like a peculiarly abstruse art form practiced by physicists.
>In the case of QM versus GR we have the very weird situation where we have two theories that are logically incompatible with each other, so we know at least one of them must be wrong
nope. Incompatible are only stretches of the theories into the areas where conditions contradict basic assumptions of the theories.
QM assumes fixed space-time which we know isn't true, so the QM can be true only where the difference between fixed space-time and real one is negligeable, ie. small scale/local effects. (And even at the small scale, assumption of the fixed space-time may be very limiting - who knows, may be powerful forces at local level do "bad" things to space-time too and this would be an explanation for the things like non-separability of quarks or a for a lot of others "strange" effects observed at quantum level).
GR is about real, non-fixed space-time and disregards any forces/interactions other than gravitation. Thus it is not correct when applied at local scales where other interactions overpower gravitation.
Thus stretching GR into small scale just makes a wrong physical theory the same way as stretching QM into GR space makes for a wrong theory too.
What you say is true, but it is also true that they are logically incompatible. GR assumes that matter is precisely localized and QM assumes that it is not. They can't both be right (and in fact we know that GR is wrong about this). So, for example, GR predicts that the interior of a black hole contains a singularity at a precise location, while QM (via the Heisenberg uncertainty principle) says that this is impossible.
Theoretical physics encompasses far more than speculative hypotheses with no experimental support (and in some cases without a way to even describe accurately any experiment which might lend support). A majority of theoretical physics (including some areas mistaken to be "purely theoretical") consists of work where there exists the means to experimentally support (or falsify) hypotheses right now, or at least construct experiments which could do so.
They tend to be less glamorous than string theory or loop quantum gravity, though, and so they don't get as much attention.
Loop quantum gravity is in competition with the much more popular String Theory, which is also "a theoretical attempt that has yet to find experimental support". The Grand Unified Theories all fit in that bucket. It's important to keep that in mind, but since there isn't a GUT that does have "experimental support" we can't really demand that at this point....
I understand that the shrinking of the black hole will cease, if underlying space/time is discrete. And I understand that that will happen quickly from POV inside the black hole, and slowly outside.
What I don't get is why once the newly formed white hole begins to spit out stuff, it will continue to do so. Why won't gravity pull it together agian?
> What I don't get is why once the newly formed white hole begins to spit out stuff, it will continue to do so. Why won't gravity pull it together agian?
It will, but not fast enough. Basically, the theory (at least as I understand it) says that the "quantum bounce" reverses the collapse process, so a massive object collapsing very fast turns into a massive object expanding very fast. The expansion decelerates because of the gravity of the hole, but the deceleration is not enough to stop it from exploding outward and redistributing all the matter that collapsed back out into the universe.
If the "bounce" is perfectly "elastic" then the explosion should not eject matter fast enough to reach escape velocity from the local system. The explosion would then collapses back onto itself into a black hole, and the whole thing would repeat over and over. Kind of like the old theories of the eternally recurring Big Crunch --> Big Bang.
I'm can buy that the bounce it's not a classic bounce, but that doesn't answer the parent comment. If the ejected matter has sufficient energy to reach escape velocity, where does the additional energy come from? Does the theory hypothesize that some of the matter is converted to energy?
I cannot speak to the theory. But, if one starts out with the classical "bounce" intution, one then arrives at the escape velocity issue you and the OP are noting. I was trying to point out that the escape velocity argument may not be relevant, because of the assumption/intuition which went into framing the problem, as stated by the OP.
In general, yes, the escape velocity would need to be overcome. But because it would need to be a quantum and general relativistic phenomenon, there are quantum and curved spacetime effects which make framing the problem as simply "escape velocity", a potentially problematic approach.
I don't think QM or GR really change anything here. Matter and energy in must equal matter and energy out. So if the "bounce" produces the same amount of matter that went in, then it seems in general to lack the energy to reach escape velocity. If some (significant?) amount of matter is converted to energy, then maybe it works.
I glanced over the paper and I don't see anything that specifically addresses this, but I might be missing it, because I only skimmed, and I'm not particularly knowledgeable about physics.
I think we are mostly agreeing with each other. I was not saying energy and momentum conservation are no longer important. Rather, I was merely noting one should use caution when trying to intuit how energy and momentum conservation manifest themselves in non-classical systems such as was proposed in the Nature article.
Or maybe it's all one universe and the big bang was just a localized event within it, but since we can't see that far away it seems like everything. (Not a physicist, this idea may seem silly to the more well-versed in this topic.)
The book "The Mathematical Universe" is an interesting read. The author theorizes that there are 4 types of infinite universes. The one we're in contains an infinite number of "universes" that are so far away from each other, that light hasn't had the chance to reach others. These universes all abide by the same physics.
the other "universes" are in a kind of different dimension, where each one has different laws of physics.
This theory is basically the end-all of just how insignificant we are, starting from the theory that earth was the center of the universe. Not only are we not the center of the universe, but our universe itself is an insignificant part of the whole.
It does seem like natural thought - that at some point even a black hole reaches it's maximum density.
However, I do seem to recall (some) black holes shoot a stream of particles from either pole (eg. an Astrophysical Jet (http://en.wikipedia.org/wiki/Astrophysical_jet). In that case, it's plausible that it doesn't take on additional mass and thus never reaches a maximum stress point, thus doesn't explode.
Of course there's another theory that they're dumping that matter elsewhere.. but who knows. I imagine much of what we currently believe will be disproved in the decades and centuries to come.
> It does seem like natural thought - that at some point even a black hole reaches it's maximum density.
There is no compelling reason, neither theoretical nor observational, that black holes should have a maximum mass. In fact, we've observed some extraordinarily heavy black holes and so far no upper bound has become visible other than factors stemming from the amount of time active and the environment in their vicinity that allowed those holes to grow.
> However, I do seem to recall (some) black holes shoot a stream of particles from either pole
That happens whenever a lot of matter accretes around a source of gravity - a hot disk is formed and matter is accelerated out of the poles. It happens with heavy objects other than black holes as well. The ejected matter comes from the accreted stuff, not from the black hole itself.
> In that case, it's plausible that it doesn't take on additional mass and thus never reaches a maximum stress point, thus doesn't explode
No, only a small part of the accreted matter is ejected.
> Of course there's another theory that they're dumping that matter elsewhere..
There is no plausible theory at this point that suggests anything of that nature. Matter as we know it ceases to exist in a black hole, it becomes compressed in a way that its usual properties are lost - so much so that in fact, how and if information loss happens in a black hole is still a matter of debate and has led to some credible theories on how information and entropy might be preserved at the border to ordinary space.
There is also nothing to suggest that black holes are losing mass at any appreciable rate, other than possibly through Hawking radiation. Combined with the fact that the known cosmos does not exhibit any regions where measurable amounts of matter simply come into existence, the idea that opposite of every black hole is a white hole spewing out the stuff both ignores the fact that black holes get bigger by ingestion and the lack of any evidence for matter spewers.
Of course that does not prove it's not happening, but there is zero reason at the moment to believe this is going on. And that pertains to this article as well, the content being presented here is not supported by any evidence or current theoretical modeling.
Not strictly speaking. Black holes are just ordinary gravity wells. They're strong, and stuff falls into them, but they don't actively suck stuff in anymore than planets or stars do. However, over very long time spans, orbiting systems should lose energy by radiating gravitational waves. That means after an inordinately long period of time, and after ejecting a lot of its mass, each galaxy will likely end up just one big black hole. But there is always stuff traveling through the universe that is not gravitationally bound to anything, and since space itself is expanding, a lot of the matter in the universe will just keep on drifting without ever colliding with anything ever again.
Not that it matters from a practical perspective. Once the last stars have gone cold, that's pretty much as good a definition of "the end" as any other, and that will happen long before maximum entropy is reached.
There might not be a definite end state to the universe itself, but there are certainly some thresholds past which everything will become so boring that we may just as well consider them to be final.
Is it no longer a valid concept that 'cold' universes are just another part of a cyclical 'era' in our universe, awaiting the quantum interactions to start the 'Big Bang' anew?
With expansion, and the fact that there are objects in space without a practical gravitational bounds, does that imply a form of finality which may inhibit the QM interactions that took place during the initial 'Big Bang'? I've been of the understanding for some time now that the 'Big Bang' could not have happened without the vacuous void which was to exist preceding it, is my understanding incorrect?
Your understanding is indeed incorrect. According to the big bang theory, there is no vacuous void preceding the big bang. I know that it's hard to wrap your head around that, but there is nothing before the big bang. Not even time. Time itself begins with the big bang. The idea of "before the big bang" has no meaning in the big bang theory as we understand it.
I've been thinking about why this idea of cyclicality is so powerful and persistent in the absence of any evidence, and it's probably not only inspired by notions of "spirituality" but also because everything on Earth seems to be cyclical during the normal course of human experience. Maybe we're tuned to expect things to always work that way, even though not even the conditions on our home planet are really cyclical when you look at longer time spans.
Still, I find it interesting. Recently a hypothesis was floating around that after the universe reaches maximum entropy this would somehow reproduce the conditions where a quantum fluctuation could produce another universe. It's irrational on several levels to just postulate that without any concrete reason, and all the more astonishing since even if it were plausible, no living thing would be in existence at that point, and not even the building blocks of matter would "survive" such an event.
Even if the universe worked that way, there would be absolutely no reason to feel comforted by it.
I get that people are looking for cosmic harmony or maybe a sense of meaning when they postulate these cyclicalities, but in fact over superhuman time spans almost nothing in the universe is actually recurring. We're all just ephemeral patterns helplessly sliding down the big hill of entropy. Our universe is incompatible with the notion of permanence, even if it's introduced through the back door in the form of eternal cycles.
Well that's not necessarily true in the sense that we have no idea what was before the big bang. Time as we know it did not exist before the big bang, however there could've been almost anything before and it doesn't affect our universe. (Except in certain theories where brane collisions could possibly have driven the big bang, in which case the specific type of collision does matter etc.)
> In that case, it's plausible that it doesn't take on additional mass and thus never reaches a maximum stress point, thus doesn't explode.
In the current understanding of jets from black holes, the jets originate from outside the black hole. So the jets are not reducing the mass of the black hole. The amount of material launched in a jet is smaller than the amount of material which falls into the black hole. So black holes with jets would gain mass, rather than losing mass or staying the same mass.
Is it a natural thought? I'm not an expert but I thought it would just bigger and bigger.
For the dumping matter part, they talk in the article of the hawking radiation, I think the astrophysical jet wouldn't reduce the mass of a black hole but only slow its rotation, as the matter haven't reached the event horizon it would not be considered part of it.
Again, I'm no expert, so you shouldn't trust me.
