In Einstein's theory of general relativity, Riemann tensor is assumed to be symmetric, due to great simplification of the theory compared to the one with non-symmetric Riemann tensor, and an absolute lack of experimental evidence to the contrary for 100 years since it was discoveried. Torsion, which is the cause of asymmetry of the Riemann tensor is set to zero. I do not know why this author is so excited about torsion tensor, the idea that torsion could be non-zero has been around for at least 90 years, but got zero traction. Disclaimer : I have a phd in string theory.
I don't have a PhD in string theory, but would like to make the point that many times in the history of science ideas that were originally regarded as uninteresting lie dormant for a long time only to usefully surface much later in the context of new evidence.
In this case, the researcher seems to be excited specifically because of the potential of torsion to explain dark energy -- a recently discovered phenomena (although of course, oddly presaged by Einstein's cosmological constant hack).
The classical theory of gravity with torsion certainly does not explain dark matter. The author is trying to build some quantum theory of gravity, where torsion is required, speculating that it would explain dark matter. I should add here that the attempts to build a quantum theory of gravity without torsion failed in the last 100 years (string theory is the best candidate), and the author is trying to add even more complexity by adding torsion tensor.
I'm trying to follow you as a (very interested) layman, so excuse me if I'm being stupid. Couldn't the failure to build quantum gravity be BECAUSE torsion has been excluded (which is I believe what the author suggests)? Maybe this is something that has to be looked into.
I'm thinking that a marriage of physics, mathematics and CS might be necessary to overcome our limits in understanding these structures. Something like an IBM Watson for physicists, where a computer is fed with all informations we have and solves an optimisation problem to come up with a unified theory explaining all the phenomenons with the least complex solution (i.e. the least universal constants). Another requirement would be to have 42 as an error code for all possible failures in the calculation ;).
The author paraphrases "spacetime tells matter how to move, and matter tells spacetime how to curve". The torsion axiom extends this to "...and matter tells spacetime how to curve and twist". The way he explains it makes it sound like an elegant way to complete the General Relativity hypothesis, which Einstein described as "the happiest thought of his life". If an axiom has a certain elegance about it, then perhaps a quantum theory of gravity without torsion is adding the complexity, not the other way around.
Mind you, string theory itself includes torsion: the Riemann tensor that arises in (e.g.) curved backgrounds includes contributions from the NS-NS B-field that correspond precisely to the effects of torsion. I don't think that non-zero torsion in itself is an outlandish idea, but I completely agree that it's fair to be skeptical of claims that torsion has significant observable effects. Disclaimer: I also have a PhD in string theory.
I'm not sure I understand. In Riemannian geometry it might be convenient to use a connection with non-vanishing torsion, but it is not required. What do you mean by 'includes'?
Well, to give an explicit example (sorry, non-specialists!), consider the nonlinear sigma model action for superstring theory in an arbitrary background geometry (that is, arbitrary spacetime metric G and NS-NS B-field; we'll ignore the dilaton). The coefficient of the 4-fermion term is the Riemann tensor. But if you construct that Riemann tensor just from the metric (as one does in general relativity without torsion), you'll omit essential pieces: you also need to include terms in the Riemann tensor resulting from a non-zero torsion tensor T=-dB (in differential form notation). A similar situation holds for the covariant derivatives in the fermion kinetic terms.
Now, it's entirely up to you whether you write that 4-fermion term coefficient as "Riemann tensor (including torsion)" or as "Riemann tensor (torsion free) + (lots of weird, arbitrary-looking interaction terms involving derivatives of B)". So on some level, there's no reason that you must use a connection with non-vanishing torsion. But I would claim that the equations are much more elegant (and give deeper insight) when expressed with torsion "built-in". [Fun fact: So does Polchinski, but he's not talkative about it. If you look up "torsion" in the index of Vol. 2, the first reference is to the page with the equations I've referenced above... but the word "torsion" doesn't appear anywhere in the text of the page!]
Aside: If anyone out there is interested in how this torsion stuff fits into the mathematics of Relativity, you might have a look at my notes on how it would be incorporated into Bob Wald's textbook: http://www.slimy.com/~steuard/teaching/tutorials/GRtorsion.p...
Of course headlines have to be catchy, but this universe-in-a-black-hole concept is hardly a new one. Rather, the novel part here seems to be the idea that the connection in GR might might possess non-zero torsion, contrary to what is normally assumed.
In the whole black-holes-create-universes vein, last century the theoretical physicist Lee Smolin presented the quite delicious idea (described in his book The Life Of the Cosmos) that entire universes might be subject to the process of natural selection.
This would work if singularity formation involves the transmission of physical constants to the 'daughter' universe with slight modification, quite a thing to suppose given we don't really know what physical constants 'mean'.
If it occurred, this natural selection would optimize these constants for the production of black holes, which luckily for us co-incides with the production of life-bearing stars. Of course, all very unfalsifiable, but kinda epic.
One SF short story takes this further. The best form of interstellar travel generates black holes, so the universe is optimized for producing the type of life which does interstellar travel via that process. No need for coincidences here, and even more un-falsifiable.
I remember trying to compute the Swartzschild radius of the universe during my GR class and thinking it was compatible with the whole universe being inside a black hole.
According to wikipedia [1] it turns out it's a bit smaller than that, but the same order of magnitude.
Wow! That's a revelation to me! So, in effect, that implies that information cannot escape the "boundary" of the universe as nothing can surpass the speed of light, ie, the universe is a black hole. This would certainly explain some things - like why energy is conserved, why (some) physical constants have the values that they have.
I am not a physicist but I have a question about this. If matter is being sucked into a new universe from an old one, I can understand that the mass of the new universe would count on the old universe's balance sheet if you will, showing that the sum of matter and energy is still constant, but wouldn't the new universe be only partially logically self-contained in this regard? Wouldn't the sum of matter and energy be increasing?
And if our universe is on the inside of a black hole, shouldn't the amount of energy and matter be increasing here as well? If not, why not? Why don't we get to suck in fading stars from our parent universe and get an increasing amount of m or e?
Inside a black hole, one dimension of space and time are transposed; this is one explanation of why you can't get out, because doing so would require going backwards in time, since the radial spatial dimension as seen from outside the black hole has been transposed with time inside the black hole. You wouldn't actually see a singularity that you're falling towards, rather the singularity exists in your future, with all matter that ever falls into the black hole collapsing together. So I would suppose that, in the timeline inside the black hole, the initial universe-creating singularity already contains all matter that ever falls into the black hole in the parent universe.
I'm not sure that the extra matter ever "enters" the black hole in a traditional sense...
As matter approaches it (pass the Schwarzschild radius) and increases in velocity, time slows down more and more for it.
When velocity approaches C, time approaches zero.
Though in another thought, I'd guess that due to the forces involved, the matter is ripped apart on a sub-quantum scale to the point that it literally disintegrates into its energy component and basically adds energy to the black hole (which can be interpreted as mass). But again, it never enters it.
This is a well-understood problem in GR. To an outside observer, things never appear to cross the Schwarzschild radius (R_S), giving the impression that time stops for in-falling matter when it gets to R_S. This, however, is an illusion forced on a distant observer.
The stuff that is actually falling into the black hole never notices R_S because the velocity they measure is not greater than c, and crossing the event horizon is a completely benign event. It's the later extreme tidal forces / spaghettification that destroys in-falling matter.
So it is fair to say that stuff enters a black hole.
Interpreting various equations from my limited knowledge of such a discussed area, I've come up with this:
Hawking radiation makes a black hole lose mass, hence diminishing its Schwarzchild radius. The Schwarzchild radius came up because it's a singularity (in the mathematical sense) in a solution to Einstein filed equations. The same way matter collapses onto itself up to creating such a singularity and being encompassed into the now existing event horizon, as soon as there is not enough mass to sustain the singularity, the event horizon vanishes, hence the remaining matter "pops" back into our sight.
You could view it otherwise with a thought experiment: take particles out of the black hole one by one. Each time it will reduce the mass and maybe the radius. At some point the mass/radius ratio may not be small enough to hold light and the bubble bursts in plain sight as the event horizon breaks down. In such a thought experiment, the worst case would be that the black hole would require every single particle but one to evaporate (unless you assume a single particle could be a black hole in itself).
An interesting back of the envelope calculation is computing the mass contained in a Schwartzchild radius of Planck length (the size at which quantum effects take over).
l_P = sqrt(hG/c^3)
r_s = 2Gm/c^2
hence m = sqrt(hc/4G) = 1.0882546265651108e-08 kg which is a bit more than 1e22 electrons sitting at a position 1e-15 smaller than a single one of them.
Is there anything to preclude there being an ultra-massive particle that might be able to cause a singularity by itself?
The particles we observe today exist in a relatively low-energy environment. Whatever's going on in the furious intensity that is the inside of a neutron star that's on the edge of becoming a black-hole could be quite spectacular in comparison.
Indeed, doesn't this violate all sorts of conservation laws when you consider hawking radiation?
We've spent decades trying to sort out the problems of black holes and information/entropy, surely to put a whole new universe inside every one is going to bugger it right up!
"And if we're in a black hole, what evidence, if any, do we have for the effects of evaporation?"
Unexplained weight loss? ;-)
I am still wondering how to square the idea that we are in a black hole with the basic principle that aside from the law of conservation of matter and energy.
I'm a complete noob when it comes to this but aren't black holes always sucking up "stuff". If we're inside a black hole does that mean that new "stuff" is added to our universe?
The Law of Conservation of Matter and Energy suggests that E + mc^2 should remain constant over time. However if new stuff is being added to the universe because we are inside a black hole, E + mc^2 should be a function of time and not a constant value.
It seems there is a testable hypothesis for someone.
If it is true that stuff is sucked inside our universe from the parent universe, I imagine the same must be true for "our" black holes: they suck matter from our universe to the child universe inside them...
And having the sum of stuff sucked in and sucked out to be zero, there should be a limit to the number (or dimension) of black holes our universe can have (?)
But the mass of the black holes from the outside universe count on what I call the energy/mass balance sheet, don't they? In other words, for the parent universe I would think you get to count all mass and all energy in all child universes. But in a child universe I don't see a way to count the parent's mass and energy.
What if there's a symmetric relationship - I.e. our "parent" looks to us like just another black hole that we, not knowing better, would consider a "child" of our universe.
Hawking radiation in one is from stuff falling into the black hole from the other.
Then everyone is on everyone's balance sheet, and e+mc^2 = constant.
It's said that other non-gravitationaly-locked galaxies are moving away from us at an ever increasing rate. Over time, distant galaxies will no longer be observable from our galaxy because of how fast they are moving away.
The other galaxies don't really evaporate, but from our perspective the result is the same.
I've considered that connection, and given that A: it is blindingly obvious to me, a layman and B: I've never heard it from a cosmologist, I'm running on the theory there must be something very very wrong with the idea, because I'm just about 100% confident it would not have escaped their notice if it had even a faint trace of plausibility. (Second most-likely theory is they have talked about it and I've somehow not noticed, as a layman.)
Ok, so if our universe is inside a black hole which is in a "parent" universe and so on, doesn't there have to be some kind of "root" universe? So where does that "root" universe come from?
Since there's no concept of space and time "inside" a blackhole, I think it would be better to think of blackholes as doors to different rooms in a huge mansion rather than each room having a smaller room contained inside it.
Space and time still are well defined inside of a black hole. They only break down at one point, termed the singularity. But nothing mystical happens and neither space nor time break down when you pass the event horizon. So there is indeed a well-formed notion of "inside", as long as you're not in the very center, where the singularity breaks everything.
A black hole is technically a single point. The part of the hole that we colloquially refer to is the event horizon. The headline speaks of universes "inside" a black hole, not an event horizon.
There's a brilliant explanation on reddit by RobotRollCall on this topic. I'll update my post when I find the link.
So, what happens when black holes collide, and merge into a single one? Serious question, btw - would we experience it, or does our universe contain all matter to ever enter our black hole, therefore all it has merged with?
If the latter, how big can black holes get? Can they consume a universe? That would be a neat end-result - the whole universe goes down the drain, to cause a new one.
Our Universe has disproportion between (visible) matter and antimatter. Black holes my contain missed anti-matter. So when black hole will consume all matter of our Universe, it will bang again, without help of any parent Universe.
I said "other 'side'", not 'other side'! (dimensions are silly :) )
However, I'm not a physicist and really have no idea what I'm talking about :( reading about cosmology is only a hobby/daydream of mine.
What the article doesn't mention is that the universe inside the black hole is exactly the same as this one. So if you head over to a black hole, dive in, and travel to earth, everyone welcomes you "back" and asks how it went.
The esact mechanism of 'Torsion' may not be right, but there have been indications for a long time that there may be another universe inside a black hole -- because if you look at the solution to Einstein's equations, there's a place on 'the other side' that is flat.
If there is some process like cosmic inflation triggered by high energy densities (like Andre Linde's chaotic inflation) it's pretty believable that some kind of 'bang' happens when a black hole formss and the output of that bang fills the 'other side'.
On a related note, I find it very intriguing that the three properties that distinguish black holes from one another (mass, charge and spin) are the same ones that distinguish quarks. That's a beautiful symmetry between some of the smallest and largest things in existence.
Perhaps this helps explain Fermi's paradox (at least partly) ... some of the many, many aliens we should have found by now might be living either out in the parent universe, or inside other black holes in our universe. I'm way out on a limb here, but it's an interesting speculation. Whadda you think?
I don't think so because all the elements being plugged into the Drake Equation (for example) exist within our universe and not in any parental or child universes. In fact, the Drake Equation only speaks of potential life in our galaxy rather than the next step beyond that.
The estimates relating to the paradox all come from what we've understood to be around us - galaxies, stars, planets, etc.
(IANA anything of any consequence to this discussion - just an eager reader!)
Yeah, but there could be a new parenthetical, "likelihood of an alien living in another universe, multiplied by all the effects of Fermi's Paradox itself, and multiplied by the likelihood of that alien's ability to traverse universes."
I think that would be a very small extra factor, especially if you also add the likelihood of that alien's interest in traveling to specifically this part of this universe.
And of course, we have no reasonable basis for estimating any of those numbers. We have a sample size of one universe and we don't even know the rate of occurrence of life in that one. Still, it can be fun to speculate.
I don't think it helps much to explain Fermi's paradox since the estimation there is based on the size of the universe. If it turns out that the universe is massively bigger then so should the estimate of how many alien civilizations there are be.
He says "The arrow of time in our universe would therefore be inherited, through torsion, from the parent universe." I think that implies that any child universes, if they are parent->child and not a swirling cycle of interconnectivity via String Theory, would necessarily be younger.
So our children universes would inherently be younger than us, and that counts heavily against intelligent life, much less superior life the likes of which would have to come back through for us to meet. At least any time soon.
Consider other universes could be so different that they may not support life, at least not as we know it.
Actually, this is much to complicated for my feeble mind. I am sticking with the "Parallel Universe Box" from the Futurama series (http://futurama.wikia.com/wiki/Parallel_Universe_Box). It makes my head hurt less.
Black holes have been observed (insofar as anything in astrophysics is observed) at the center of galaxies and do not suffice to explain why they hold together. So they're no longer a postulate, and they do not serve the purpose described.
Dark matter and dark energy are postulated as such an explanation.
I hope your kidding, but if your both serious and not trolling:
Like charges repel and opposite charges attract which prevents large scale accumulation of charge in even a tiny area by cosmic scales. Gravity while locally weak does not have these issues and you can keep dumping more mass into the same area more or less indefinitely.
Dumping more and more mass into a black hole until it holds a whole universe is, apparently, not enough. But, hey, isn't it lucky that there is now also an unobservable dark matter and energy and just enough of it to make the 'gravity only' sums come out right? Hmmmm.
Does this really sound more plausible to you than that widely separated bodies in vacuum, large and small, can hold charge?
Keep reading your high-school textbook on physics and be sure to note the equations on calculating the force of an electrostatic charge over very large distances.
If you rubbed the entire galaxy with a cosmic-sized cat you still wouldn't have the effects you're postulating.
