Is there any another theory other than M-theory (supported by a portion of the scientific community) at the moment which proposes to unify GR and the Standard Model?
Is Quantum Loop Gravity out of the running, or still a viable alternative?
Order of Kilopi
Is there any another theory other than M-theory (supported by a portion of the scientific community) at the moment which proposes to unify GR and the Standard Model?
Is Quantum Loop Gravity out of the running, or still a viable alternative?
Order of Kilopi
I once heard that Lattice QCD was an alternative to string theory, which is a precursor of M-theory, but now that I look at it a little closer, it seems to be more focused on the strong interaction rather than unification. Of course, there are always unintended consequences....Originally Posted by gzhpcu
Everyone is entitled to his own opinion, but not his own facts.
Order of Kilopi
The last I heard Loop Quantum Gravity was very much in the running. It just doesn't have the following that String/M-theory has.
Banned
Here is a very comprehensive link on the differences and similarities.
http://www.edge.org/3rd_culture/smol...n03_index.html
Lee Smolin says somewhere in all of these page, that he thinks massive black holes will be determined to have a MUCH higher roll in the workings of our universe, and I couldn't agree more wholeheartedly!
He also says that the main problem with String/"M" Theory, is that it is background dependent, and since he has not been able to figure out how to fix that, and because he has found that there is a greater possibility of finding 'experimental evidence' in the LQG, he has switched over to that as his main focus.
Established Member
Press Gang
I think this is in fact backwards. String theory is much the minority idea, and loop quantum gravity is the majority theory. However, loop quantum gravity does not have the press agents that string theory does.
Order of Kilopi
Interesting about Loop Quantum Gravity is the idea that to unify the two theories, since GR is a classical theory, you need to quantize it as well, that is to say, space and time are granular. Again, it is Planck yardsticks that come into play: Planck time and the cube of the Planck length for the smallest volume.
Seems like the Planck figures are pretty essential, since the come to play in both theories as the lower limits. (which circumvents the problems QM has below these limits)
I wonder if any theory will ever be found that does not have Planck length as a limiting factor.
Order of Kilopi
Are you sure of this? You're not joking now, are you?
Banned
It would appear that your 'Granular' and 'Planck' terms for what space/time is made of is a correct assessment of the situation.
If this is the case, Tim and Celestial and gzhpcu, what is science to do?
Established Member
The Planck length is 10^-35 m and we observe the smallest structures at 10^-18 m. Is that because our instruments or just any particle is so complicated ?
In string theory the particles are moving in an empty space. How the virtual particles are created in an empty space from nothing ?
In LQG or Heim theory is there a structure of the space in which is some energy always.
The M-theory replace a previous simple string theory because it is not possible a zero volume in the space. This theory will unify with QLG in the future I hope.
Order of Kilopi
The whole thing is a consequence of the Heisenberg uncertainty principle. At the Planck length, according to Wheeler, we have "quantum foam". Virtual particles are allowed by the Heisenberg uncertainty principle. The Planck length is a QM problem, but we have no proof that quantum foam exists at the Planck level do we?
Since a unification theory has to embrace QM, it "inherits" the Planck length. What can science do? A better theory than QM?
Administrator
String Theory is not nailed down very tightly, and is actually a term for quite a few related ideas or models. I saw an interview with Brian Greene in which he said that he thought it was likely that when the mathematics and observations are sufficiently worked out a little further that Loop Quantum Gravity will be seen as one of the M-Theories.
Going back to the original poster's question "Is there any alternative...?" I expect that if a new alternative is developed that it will be embraced as a new flavor of String Theory.
Given everything we've seen, the idea of higher dimensions being required makes sense to me, but I am sympathetic to people who'd like an explanation that only requires three spatial dimensions plus time.
Forming opinions as we speak
Order of Kilopi
So M-theory stands for "many"-theories?
Does perhaps the current state of our technology limit our capability to come up with an alternative theory to M-theory (and perhaps both QM and GR)?
Order of Kilopi
Technology poses no barrier to our ability to create theories, only to our ability to test them.
Established Member
Sure
Actually, I don't think I am joking, and I do think that more people are working on loop quantum gravity, or at least some "classical" form of quantum gravity, than are working on string theory. Greene, Witten & Kaku are famous (but of course, both Greene & Kaku work at it, so it's no great surprise). Rovelli & Carlip are not famous, but they are just as "big" in loop quantum gravity, or "classical" quantum gravity (how's that for a twist of words?) as are the others in string theory. I'm not sure of loop quantum gravity in particular, but I do think that there are more people working in the non-string areas of quantum gravity than there are people working in the string theory areas. They just get more press.Are you sure of this? You're not joking now, are you?
I'm sure I do not understand where this question comes from. Science is to do what science always does, look for theories that are consistent with observation. There is no doubt that, "right" or "wrong", string theory is every bit as much a quantum theory of gravity as any of the others, so I don't see any motivation for the question in the context of string or M-theory.
But there is always another potentially bothersome thought. GR is surely a classical theory, and may in fact not be "quantizable" at all. It may well be that there is no theory of quantum gravity, and we will have to do without in the end. I doubt that's true, if only because we know that GR can be derived from string theory, even in it's present state. But one cannot say with assurance that it is impossible.
Order of Kilopi
Not sure if I agree. The ancient Greeks could never have come up with GR and QM. Their view of the world was quite different. I would think that as our technology advances, so does our understanding of the universe. Of course, I agree that the ability to test them is an extremely important aspect of technology.
Banned
All of these responses are exactly the point.
Through GR, QFT, String"M" Theory. we have already quantized the smallest possible 'base unit' of how the elements get their mass/gravity. This is the DM that makes up space. It is definitely at "Planck" scale!
So, if we can never measure anything down to this level, to satisfy the 'science', how do we determine how gravity works? What makes up 'space', How did/does this physical entity called DM come to exist?
And maybe, most importantly, since we know it is 'physical', how is 'more' of it being made, since it is quite apparent that the universe is expanding, and that expansion can be seen as occuring in and from the Voids!
What I am basically saying here, is when does all the accusations about hand waving and speculation stop and determinations of what actually 'fits' theory the best start?
Established Member
the facsinating thing people about the tying in of MP( macrophysics) and QM( quantum mechanics, microphysics) is this; on the Hubble site
http://hubblesite.org/
then go to gallery
then look up " Eta Carinae"
then compare this photo with
a diagram in the book I have, called;
The Picture Book Of QUANTUM MECHANICS( Siegmund Brandt, Hans Dieter Dahmen, ISBN # 0-387-95141-5 by the way do not be fooled by the books Title. it has all the mathematics you can handle.)
pg.267 , fig. 13.22( surfaces of constant probability density) , diagram #320 ( the diagram is three dimensional).
it is facsinating!!!
Banned
Tim Thompson;
This is really what I am refering to here.
http://www.bautforum.com/showpost.ph...4&postcount=66
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