About M-theory ?

[1123581321]

Hi, i was wondering what the property is which gives or allows 11 dimensions in m-theory..

 

(is it a natural property of the theory itself or something external)

[Pratt]

Actually 5 superstring theories combined to form a so called M theory.Now this also includes the 11-dimensional supergravity!The question is why gravity is so weak than the oder fundamental forces of nature.So scientists investigated it very deeply and rather found that gravity is not at all weak,but it APPEARS WEAK!!Now suppose theres a source of sound and d sound waves are coming out of it.A person is listening to that, sitting at a certain distance from it.He cant hear the sound xactly wid d maximum amplitude jus coz the sound waves spreads themselves in 3-dimensions in our view!Likewise, investigating mathematically,scientists found that the force of gravity spreads itself out into 11 such dimensions,out of which 10 are spatial dimensions,the oder being time!Now we can nly see the usual 3ds..and not more dan dat,coz anything which is in a smaller dimension,cant visualize d higher ones(none can put a 3d box inside a 2d sheet)!!So here comes the introduction of 11 dimensions!Actually if we view gravity in such 11 dimenions(jus for sake of imagination), den gravity wouldnt have been interpreted as weak.....!!...

Edited January 11, 2011 by Pratt

[tonee]

Hello,

How do the 11 dimensions tie into other universes ?

[ajb]

We know, thanks to Nahm that 11 dimensions is the largest possible for which we can have a consistent supergravity theory. In higher dimensional theories there would be particles of spin > 2 and then via the Weinberg–Witten theorem we know these to be incompatible with quantum theory.

 

Another hint comes from Witten who showed that the two 10 dimensional supergravity theories Type IIA and Type IIB (which are low energy limits of the Type IIA and Type IIB string theories) can be understood in terms of dimensional reduction of the unique 11 dimensional supergravity that Nahm classified.

 

This was the birth of the idea of M-theory.

[MigL]

Not very well informed on M-theory, but...

I was of the impression that, to account for the 'handedness' of the weak force, only an odd number of spatial dimensions will do. The max number of dimensions then becomes 9 spatial and one of time, for ten alltogether.

Eleven total dimensions implies ten spatial dimensions ( plus time ), and that throws a wrench into the weak force machinery.

Or am I wrong about the weak force.

[ajb]

Super string theories need to be 10 dimensional due to anomaly cancellation. That is free super strings are only well defined as a quantum theory in 10 dimensions.

[Realitycheck]

How do you utilize or delegate the other 6 dimensions? What do the other 6 variables stand for?

Edited August 24, 2011 by Realitycheck

[ajb]

They are extra spacial dimensions.

[Realitycheck]

Could you extrapolate? How do you translate 3d space into 9d space? I read something about space curling up into itself and a box inside a box representing 1 extra dimension, but the string materials I have looked at so far don't really explain that facet of the theory much.

 

Is this like a whole book I need to read to understand this concept?

 

I went up through Algebra 2 and Calculus 2, Trig, Stats, so it seems like it shouldn't be that hard to comprehend. And I was one of the ones keeping up in class.

Edited August 24, 2011 by Realitycheck

[IM Egdall]

Could you extrapolate? How do you translate 3d space into 9d space? I read something about space curling up into itself and a box inside a box representing 1 extra dimension, but the string materials I have looked at so far don't really explain that facet of the theory much.

 

Is this like a whole book I need to read to understand this concept?

 

I went up through Algebra 2 and Calculus 2, Trig, Stats, so it seems like it shouldn't be that hard to comprehend. And I was one of the ones keeping up in class.

 

May I suggest you read The Elegant Universe by Brian Greene. It explains string theory and multiple dimensions in everyday language.

[ajb]

Could you extrapolate? How do you translate 3d space into 9d space? I read something about space curling up into itself and a box inside a box representing 1 extra dimension, but the string materials I have looked at so far don't really explain that facet of the theory much.

 

Things get technical very quickly and one needs ideas from differential geometry and topology.

 

Roughly you are right, one can compactify the 6 of the 9 spacial dimensions to end up with a theory that is 3 +1 dimensional (space+time). This only works if you compactify in very special ways; you need either a Calabi-Yau or an orbifold. The reasons are to do with keeping the right number of supersymmetries in order to be phenomenologically viable.

 

The other option is to not compactify but use branes. Out Universe could be "stuck" on a 3-brane.

 

Or maybe even a mix of the two ideas above.

 

 

Is this like a whole book I need to read to understand this concept?

 

And more. String and M-theory compactifications are a whole very technical industry. Reading Brian Greene, as IM Egdall suggests is probably a good place to start.

[Realitycheck]

I've been reading up on Calabi-Yau and Green and it just seems unnecessarily complicated and hardly representative of reality, at least from my perspective. Green doesn't seem to talk with a lot of confidence about the theory saving the day. Since so much of it has QCD and the Standard Model incorporated into it, it's hard to really assign it much value, other than an essentially meaningless umbrella construct, don't mean to step on anybody's toes. The Standard Model makes sense. Even QCD makes sense, considering that all of these new parameters had to be devised and assigned. It doesn't make sense that it should all just be thrown away like a house of cards because you can't find evidence of a force that gives mass to matter. Gravity is your answer. Surely, someone thought of that. Surely, the issue isn't exactly as I have described. Is gravity inconsequential at the quantum level and that's what the issue is all about?

Edited August 25, 2011 by Realitycheck

[ajb]

I've been reading up on Calabi-Yau and Green and it just seems unnecessarily complicated and hardly representative of reality, at least from my perspective.

 

If you want pass from 10d to 4d then you need to do something with the extra 6 dimensions. Compactifying the dimensions needs to be done carefully and this is why Calabi-Yau manifolds or orbifolds are needed.

 

 

Green doesn't seem to talk with a lot of confidence about the theory saving the day.

 

Different compactifications will in general lead to different low energy physics. We know that string theory is "large enough" to contain the standard model, but understanding what compactifiction is the right one and why that one has proved to be very hard.

 

Since so much of it has QCD and the Standard Model incorporated into it, it's hard to really assign it much value, other than an essentially meaningless umbrella construct, don't mean to step on anybody's toes.

 

Right now this is how you should think about string theory, and M-theory. We have a very non-trivial generalisation of point-particle theories which have some novel and interesting features. Maybe this framework can be used to construct realistic theories of everything. There is reason to think why this maybe so, but it has proved much harder in practice to find the right theory. But the game is certainly not over yet.

 

The Standard Model makes sense. Even QCD makes sense, considering that all of these new parameters had to be devised and assigned. It doesn't make sense that it should all just be thrown away like a house of cards because you can't find evidence of a force that gives mass to matter.

 

So as you are alluding to, the Higgs field is believed to be the origin of mass and this is "bolted onto" the standard model. It is a rather ugly addition to the theory that does not really come from any deep mathematical principle. To date there is no real evidence that nature realises the Higgs. This may change fairly soon.

 

The Higgs-Kibble mechanism is not the only possible scenario. Technicolor is another possible way to generate mass.

 

Maybe nature is even more clever and uses some other as of now unknown mechanism. That would be very exciting, no Higgs and the LHC and no evidence of technicolor.

 

Gravity is your answer. Surely, someone thought of that. Surely, the issue isn't exactly as I have described. Is gravity inconsequential at the quantum level and that's what the issue is all about?

 

Gravity is inconsequential to quantum systems that are nowhere near the Planck scale. This is the energy or length scale in which one could not ignore quantum aspects of the gravitational field. Collider experiments are nowhere near this scale and so we would expect gravity not to play any important role. Unless there is more exotic physics going on like large extra dimensions.

 

Quantum gravity effects will be important in the (very) early universe and near black holes, and in particular the singularity.

 

I don't know how you would propose that gravity is responsible for mass, unless the graviton makes large contributions to the Green functions (mass is defined as a pole in a Green function). But then we know that gravity is weak and makes very small contributions to quantum theory, at least as long as we are not near the Planck scale. Then the equivalence principle may make it hard not to give mass to all particles. However gravity is highly non-linear and we could expect interesting dynamical effects...

 

 

...understanding mass in this way would be difficult, if not just impossible. In my opinion.

[Realitycheck]

Ahh... gravity is an acceleration, not a force. Does this make sense as the solution to everyones' problem?

 

Conclusion: One very large scaling error has jeopardized the entire standard model explanation of forces at the quantum level. The standard model has historically treated the gravity field as a force field, instead of an acceration field. They have thought that forces could be scaled by a straightforward comparison of numbers, whereas I have shown that it is the acceleration numbers which must be scaled, not the force numbers, and the acceleration cannot be scaled in a straightforward manner...

http://milesmathis.com/quantumg.html

Edited August 25, 2011 by Realitycheck

[ajb]

Ahh... gravity is an acceleration, not a force. Does this make sense as the solution to everyones' problem?

 

But F= m a right? So I don't quite see the distinction.

[Realitycheck]

I was thinking something along the same lines ... but,

 

The more massive something is, the more gravity it has. Around a gravity well, it is hidden and distorted, but matter gives itself mass via the gravity it imposes. The more mass it has, the more gravity it imposes. Therefore, gravity is responsible for giving mass to matter. I might propose that you're never going to find evidence of a Higgs except out in deep space because of gravity wells.

Edited August 25, 2011 by Realitycheck

[ajb]

But massless fields and particles can also act as sources of gravity.

 

The important point with gravity is that energy-momentum is the "thing" responsible for gravity and not just the mass.

 

I also think we are now straying off-topic here.

Edited August 25, 2011 by ajb

[MigL]

If I remember correctly, the idea of strings was introduced to get rid of the quantum effects that make a point-particle quantum gravity theory untractable, ie not renormalisable. And the string vibrational harmonics account for different elementary particles' masses ( almost, to a general approximation where very little=0 ).

 

But, why strings ??

 

At the Planck scale where string dimensions lie, we start losing distinction between spatial dimensions and time, so that ascribing a structure to strings is probably absurd and certainly unverifiable. Why not simple harmonic oscillators, or pulsing spheres or even something ( ?? ) which exsists in multiple or all times. All we really require is that a certain size ( Planck scale ) of space/time be allowed to vibrate/oscillate/pulse with differing harmonics.

 

Or is the particular structure needed to account for certain properties ??

[ajb]

If I remember correctly, the idea of strings was introduced to get rid of the quantum effects that make a point-particle quantum gravity theory untractable, ie not renormalisable. And the string vibrational harmonics account for different elementary particles' masses ( almost, to a general approximation where very little=0 ).

 

String theory was really discovered more by accident than anything else. Veneziano realised that the Euler Beta function satisfied all the properties needed of a scattering amplitude. String theory was then used as a theory of the strong force. This started in 1968. Shortly after it was realised that Veneziano's work corresponded to a theory of strings.

 

String theory proper was born in the 1970's. It was realised that string theory had a spin-2 massless boson in its spectrum. This at first was seen as a problem until it was realised that this boson had to correspond to gravity and thus the idea that string theory could be a theory of everything was born. This was the first sting revolution. Around 1984-1986 lots of work on string theory was done and how it could be used to describe elementary particles and forces. This is the departure from the initial idea of an effective theory of hardons.

 

 

But, why strings ??

 

Good question!

 

I will just say that modern string theory is no longer just about strings, or even strings at all. Higher dimensional objects, D-branes in string theory and M-branes in M-theory are now all important.

 

At the Planck scale where string dimensions lie, we start losing distinction between spatial dimensions and time, so that ascribing a structure to strings is probably absurd and certainly unverifiable. Why not simple harmonic oscillators, or pulsing spheres or even something ( ?? ) which exsists in multiple or all times. All we really require is that a certain size ( Planck scale ) of space/time be allowed to vibrate/oscillate/pulse with differing harmonics.

 

We lose any true meaningful distinction between space and time due to special relativity. This is not particular feature of being near the Planck scale.

 

Or is the particular structure needed to account for certain properties ??

 

String theory, although hard does have many attractive features. A few include

 

• Finite, so no renormalisation is required.
  
• Predicts gravity. ie. necessarily contains a gravition.
  
• "Large enough" to contain all the standard model physics.
  
• No anomalies if we fix d = 10. So it predicts the number of dimensions of the Universe!
  
• Superstrings accommodate fermions. (Not so easy in geometric based GUTs)
  
• No hierarchy problem (related to no remormalisation required).
  

 

The second string theory revolution in 1995 showed us how to go beyond perturbation theory and allows one to probe the non-perturbative aspects using "dualities". In short, superstring theories can be seen as perturbative expansions about different vacua of some larger theory, this theory is known as M-theory.

 

Now, M-theory is not a theory of strings, but rather M2 and M5 branes. This is a lot more complicated and even the fundamental degrees of freedom remain to be uncovered.

 

The "mini string revolution" occurred a few years ago with the work of Bagger, Lambert and independently Gustavsson who write down an action for a stack of M2 branes. There has been many papers exploring and generalising these initial works. However, it probably is fair to say that the initial hopes of the BLG-model and similar has not been realised. But papers still appear regularly on this topic.

 

Plenty of things are known about M-theory, but overall it remains quite a mystery. Passing from points to strings was hard, but very fruitful, passing from strings to M-branes is extremely difficult, but the rewards may come yet.

[DrRocket]

 

Plenty of things are known about M-theory, but overall it remains quite a mystery. Passing from points to strings was hard, but very fruitful, passing from strings to M-branes is extremely difficult, but the rewards may come yet.

 

 

 

A good start would be for someone to actully define what M-theory is, or even to rigorously define a string theory.

[khaled]

I think that multi-dimensions are true .. and humans can perceive the universe as a 4-dimensional projection of the universe,

 

I think that the most crazy physicist is dr.kaku, his writing are way out of mind

[ajb]

I think that the most crazy physicist is dr.kaku, his writing are way out of mind

 

You are reading or watching his popularisation pieces, these tend to be a little "out there" in order to catch the imagination of the general public. Some of his ideas about the future are quite extreme and fanciful, but one is free to speculate and do whatever to inspire the public's interest in science.

 

His real work in physics, or at least what he is best known for, is string field theory. Kaku is one of the originators of string field theory. His text books on quantum field theory and string theory are good, he is probably best known for these in the QFT/string community.

 

A good start would be for someone to actully define what M-theory is, or even to rigorously define a string theory.

 

My understanding of string theory is that classically it is well defined as a particular kind of sigma model. D-branes can also be understood in this way, classically. Quantisation of a string I believe is also well-defined, as much as any QFT is.

 

Interacting quantum branes are much harder and I think this is major challenge.

[MigL]

Again I need to preface this with the comment that I'm not very familiar with M-theory. I'm only just dipping my toes in it.

 

If we were 'stuck' on a 3-brane, everything i've read or heard implies that force carrier bosons are all open ended 'attached' strings, ie. their ends are attached to the brane and so are also stuck in it, they cannot pass to the other dimensions, wether compact or large. The only force carrier boson that is different is the graviton, which is not attached and can pass through all dimensions. This implies that at smallish distances, much smaller than the three large observed dimensions. the strength of gravity does not fall off with the square of the distance, but falls off with (n-1) of the distance, where n is the total number of spatial dimensions. This also implies that at progressively smaller distances, the force of gravity becomes progressively larger with respect to the other forces, ie we don't need to go to the Planck scale to see equivalent strengths of the forces, rather gravity would get to equivalence much quicker.

 

The LHC gets nowhere near Planck scale energy, but it does place sufficient energy in a small enough space that, if gravity was stronger than expected at small scales, microscopic black holes should have been formed. Since none were, does this mean gravity is not 'free' to pass through all the dimensions, and so the m-theory string model of the bosons is not valid. Another wrench in the works, if you will.

[DrRocket]

 

 

My understanding of string theory is that classically it is well defined as a particular kind of sigma model. D-branes can also be understood in this way, classically. Quantisation of a string I believe is also well-defined, as much as any QFT is.

 

Interacting quantum branes are much harder and I think this is major challenge.

 

My understanding is that no string theory has yet been clearly defined and that some theorists, Brian Green for instance, have suggested that Maldecena's AdS/CFT correspondence (itself an unproved conjecture since 1997) might eventually be used as a definition of string theory.

 

M-theory arose from a presentation and paper of Witten (in 1995) that offered a plasusibility argument for the unification of the then-competing string theories inder the umbrella of a single theory, M-theory. However the "dictionary" among those string theories has yet to be produced and what exists are conjectures regarding various symmetries that might produce the desired dictionary. So the most pressing problem in M-theory remains "What is M-theory ?"

 

Popularizations of string theory, IMO, present a badly distorted view of the maturity of the subject and wildly overstate the justifibility of conclusions regarding physics that can be drawn. So far as I know there have been zero new testable predictions from string theories, but a lot of hype about its implications for cosmology and particle physics. The exception to the overhyping that characterizes many authors is Witten, who generally presents a very objective view of the subject. Witten ought to know.

 

Without spectrographic data, I would not believe Kaku if he said the sky were blue .

[nalxhal]

A good start would be for someone to actully define what M-theory is, or even to rigorously define a string theory.

 

The great error of M-theory is the numbers of dimensions , 11 of space and 1 of time .The solution is 10 of space and two of tme in order to be in agreement with Stefan-Boltzman law .I thing a professor fix it 2006 who introduced two dimensions of time .

Edited August 28, 2011 by nalxhal