http://www.physics.rutgers.edu/~motl/

IMHO. Lisi paper has serious defects but it is still interesting and better than most papers on the theory archive. Peter Woit writes:

Garrett Lisi has a new paper on the arXiv, with the rather over-the-top title of An Exceptionally Simple Theory of Everything. Sabine Hossenfelder has a typically excellent posting about the paper, and Garrett has been discussing his work with people in the comment section there. Lubos Motl, has a typically, how shall I say, Lubosian posting on the topic.

...

Two of the ideas he is pursuing are general ones I’m also very fond of. One is well-known, and many people have also tried this, it’s the idea of bringing together the internal gauge symmetry and the symmetry of local frame rotations. The problems with this are also well-known, and some have been brought up by the commenters at Sabine’s blog. I don’t think Garrett has found the answer to this, or that he claims to. I’m still hopeful that this line of thinking will lead somewhere, but think some dramatically different new idea about this is still needed. The other idea he likes is that of trying to interpret the fermionic degrees of freedom of the BRST method for handling gauge invariance as providing the fermions of the Standard Model. I suspect there is something to this, but to get anywhere with it, a much deeper understanding of BRST will be required. I’ve been spending a lot of time trying to understand some of the mathematics related to BRST in recent years, and am in the middle of writing some of this up. It seems to me that there is a lot that is not understood yet about this topic even in much simpler lower-dimensional contexts, so we’re a long way from being able to really see whether something can be done with this idea in a realistic four-dimensional setting.

One idea Garrett is fond of that has generally left me cold is the idea of unification via a large simple Lie algebra like E8. While there may be some sort of ultimate truth to this, the problem is that, just as for GUTs and for superstring models, all you’re doing when you do this is changing the unification problem into the problem of what breaks the large symmetry. This change in the problem adds some new structure to it, but just doesn’t seem to help very much, with the bottom line being that you get few if any testable predictions out of it (one exception is with the simplest GUTs, where you do get a prediction, proton decay, which turns out to be wrong, falsifying the models).

http://www.math.columbia.edu/~woit/wordpress/?p=617

Sabine Hossenfelder writes:

This is without doubt cool: He has a theory that contains gravity as well as the other interactions of the SM. Given that he has to choose the action by hand to reproduce the SM, one can debate how natural this actually is. However, for me the question remains which problem he can address at this stage. He neither can say anything about the quantization of gravity, renormalizability, nor about the hierarchy problem. When it comes to the cosmological constant, it seems for his theory to work he needs it to be the size of about the Higgs vev, i.e. roughly 12 orders of magnitude too large. (And this is not the common problem with the too large quantum corrections, but actually the constant appearing in the Lagrangian.)

To make predictions with this model, one first needs to find a mechanism for symmetry breaking which is likely to become very involved. I think these two points, the cosmological constant and the symmetry breaking, are the biggest obstacles on the way to making actual predictions [4].

Bottomline

Now I find it hard to make up my mind on Garrett's model because the attractive and the unattractive features seem to balance each other. To me, the most attractive feature is the way he uses the exceptional Lie-groups to get the fermions together with the bosons. The most unattractive feature are the extra assumptions he needs to write down an action that gives the correct equations of motion. So, my opinion on Garrett's work has been flip-flopping since I learned of it.

So far, I admittedly can't hear what Lee referred to in his book as 'the ring of truth'.

http://backreaction.blogspot.com/2007/11/theoretically-simple-exception-of.html

On Nov 17, 2007, at 5:40 PM, Gary S. Bekkum / STARpod.org wrote:

Upon further review, surfer's new Theory of Everything severely

deficient

By Chris Lee

The /New Scientist/ must hire someone to trawl through the arXiv

This paper is actually a very impressive piece of work, though it is thoroughly overhyped and completely unintelligible to anyone who doesn't regularly do particle physics and group theory. The author, A. Garrett Lisi, has proposed a back-to-the-future approach to uniting quantum mechanics and gravity.

Over the course of the late 19th and 20th centuries, it was discovered that the electric force, the magnetic force, the weak force (responsible for radioactivity), and the strong force (responsible for holding the nucleus together) could all be described by a single theory. The different forces, their properties, and associated particles could all be obtained from different symmetry operations (think rotations and reflections) of an algebraic system. This very successful approach has withstood the test of time, with absolutely every experimental test falling within error bars of the calculated results. However, gravity stands apart as the force which does not get included in this set, so its inclusion (or a totally new theory) would constitute a theory of everything.

Much of the early work focused on exploring higher symmetry algebraic systems that might include gravity. Several were found, but none actually survived contact with reality. This approach has largely fallen out of favor because any object with sufficient symmetry operations can be made to unite gravity with everything else while still not agree with reality as we measure it. Lisi has revived this approach by looking at the shadows cast by an extremely complicated symmetry group (called E8). Unsurprisingly, if you choose (by hand) the right starting methodology, and ignore a large swathe of physical reality, a selection of symmetry operations will result in groups of symmetry operations that correspond to those from particle physics as we know it, something that might be the symmetry operations of gravity—and some other stuff.

The problem is that Lisi has ignored much of physics, where he adds normal numbers to vectors and other similar no-nos (imagine adding a speed to an energy and you have got exactly what Lisi has done). He has found that the chosen symmetry operations correspond to the symmetry groups of particles—not that surprising, considering the number of symmetry operations he has at his disposal—but he hasn't checked to see if the masses come out as found experimentally because he can't; once you put nonsense into a model, the only thing that comes out is nonsense.

this next part is propaganda. Lisi has a PhD in physics from UCSD. His work is interesting even if it is partially or wholly wrong compared to the voodoo cargo cult string physics most of which is "not even wrong" (W. Pauli) at a cost of ~ billion dollars of US tax money over 30 years supporting ~ 1000 theorists counting angels on pinheads. ;-)

In the Observatory thread

Many thanks to Geon and his post in the Observatory.

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