Theory of everything (for some universes)
... ... most likely not ours ... so far
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Every condensed matter system itself can be viewed as a model universe. Most
of those model universes are very different from our universe. The issue here
is to design a condensed matter system that resemble our universe.
The first rule of game is that only local
bosonic models with finite cutoff are allowed in the design.

Emergence of helicity +/ 2 modes (gravitons) from qubit models
(pdf)
ZhengCheng Gu and XiaoGang Wen
arXiv:0907.1203,
Nuclear Physics B online
.

Constructed two quantum spin models on a lattice which gives rise to emergent
gravitons (one with ω = k and the other with ω = k^3 dispersion).

After 6 years, many journals (Science, PRL, PRB, NJP, JHEP, NPB), and about
30 exchanges, one of our linear quantum gravity papers
finally get published in Nucl. Phys. B. Next problem: how nonlinear quantum
gravity can emerge from some lattice models.

The referee reports and our replies represent detailed discussions between
parties who have different points of view on quantum gravity: emergence point
of view vs geometry/gauge point of view. The exchange of opinions is important
and helpful for the development of the difficult field of quantum gravity.
So, I would like to share those exchanges with the researchers in the field.

Submit the short paper to PRL:
2006/5/25 Report from PRL,
2006/6/8 Reply to PRL,
2006/7/28 Report from PRL,
2006/11/7 Reply to PRL,
2007/1/3 Report from PRL,
2012/5/18 A late reply to PRL.

Resubmit the short paper to PRL and submit the
long paper to PRB:
2009/9/22 Report from PRL,
2009/9/25 Reply to PRL,
2009/9/25 Report from PRB,
2009/9/30 Reply to PRB,
2009/10/19 Report from PRB,
2009/10/25 Reply to PRB,
2009/10/30 Report from PRL,
2009/10/31 Reply to PRL.

Tried to appeal the rejection decision from PRB and PRL:
2009/11/4 Reply to PRB,
2009/11/5 Reply to PRL,
2009/12/10 Appeal report from PRB/PRL,
2010/2/8 Appeal reply to PRB/PRL.

Submit the long paper to NPB (and get rejected again):
2010/3/5 Report from NPB,
2010/3/6 Reply to NPB,
2010/3/8 Report from NPB,
2010/3/10 Report from NPB,
2010/3/15 Reply to NPB,
2010/4/16 Report from NPB,
2010/4/20 Reply to NPB.

2012/5:
Following Hong Liu's suggestion, we moved the long introduction of the long
paper to appendix and resubmited the long paper to
NPB. This time, it was accepted [:)

A lattice bosonic model as a quantum theory of gravity
(pdf)
ZhengCheng Gu and XiaoGang Wen
grqc/0606100

Constructed a quantum spin model on a lattice
which gives rise to emergent gravitons (linearized quantum gravity).

Photons and electrons as emergent phenomena
(pdf)
Michael A. Levin and XiaoGang Wen
Rev. Mod. Phys. 77, 871879 (2005),
condmat/0407140

Stringnet condensation provides a way to unify light and electrons.
Rejected by Science :(

Stringnet condensation: A physical mechanism for topological phases
(pdf)
Michael Levin and XiaoGang Wen
Phys. Rev. B71, 045110 (2005).
condmat/0404617

Pointed out that all the gauge theories and doubled ChernSimons theories
as well as all statistics (Fermi, fractional, and nonAbelian statistics)
can be realized in lattice spin models through different stringnet
condensations.

Quantum order from stringnet condensations
and origin of light and massless fermions
(pdf)
XiaoGang Wen
Phys. Rev. D68, 024501 (2003).
hepth/0302201

Quantum ordered states that produce and protect massless gauge
bosons and massless fermions are stringnet condensed states.

Different stringnet condensations are not characterized by
symmetries, but by projective symmetry group (PSG).
PSG describes the symmetry in the hopping
Hamiltonian for the end points of condensed strings.

PSG protects masslessness of Dirac fermions.
PSG leads to an emergent chiral symmetry.

Constructed an local boson model on cubic lattice that has emergent QED and
QCD.

Fermions, strings, and gauge fields in lattice spin models
(pdf)
Michael Levin and XiaoGang Wen
Phys. Rev. B67, 245316 (2003).
condmat/0302460

Pointed out that fermions can emerge in local bosonic models as
ends of open strings.

The string picture for fermions works in any dimensions, which is more general
than the fluxbinding picture in 2D.

Pointed out that emergent fermions always carry gauge charges.

Artificial light and quantum order in systems of screened dipoles
(pdf)
XiaoGang Wen
Phys. Rev. B68, 115413 (2003).
condmat/0210040

Constructed realistic screened dipole systems in 2D and 3D that contain
artificial photon as their low energy collective excitations.

Find that U(1) gauge bosons are collective motions
condensed stringnets.

According to the stringnet picture,
we can answer the following three
questions about light:
What is light?
Light is a fluctuation of closed strings of arbitrary sizes.
Where light comes from?
Light comes from the collective motions of ``things'' that fill our vacuum.
(Note that our vacuum is not empty. It is filled with ``things'' that form
the spacetime.)
Why light exists?
Light exists because our vacuum is a quantum liquid of stringnets.
Rejected by Nature :(

Origin of Light
(pdf)
Origin of Gauge Bosons from Strong Quantum Correlations
XiaoGang Wen
Phys. Rev. Lett. 88 11602 (2002)
hepth/0109120

Proposed that light and fermions originate from a certain
"pattern" of quantum entanglement in a manyboson system.
The "pattern" of quantum entanglement is called quantum order.

Proposed that gaplessness of photons and fermions is protected by
the quantum order.

Constructed a spin model (which can sit on your palm) that reproduces a
complete 1+3D QED (a cosmos with light, electrons, protons, atoms, ...).
Rejected by Science :(
At the PRL editor's request, the published version got a new and longer title.
An artistic summary of the result:
The following picture
has mountain, lake, trees, snow,... ..., and light.
If we look closer, we see
atoms, electrons, protons, and photons.
If we look even closer (I mean really really close), we see ... ...
boring spins on a lattice.
How can a simple boring spin system produce such
a nice scenery?
The answer is "more is different". When many quantum spins interact with
each other, the spins can get organized (or entangled). The pattern of
organization is called quantum order. It is this pattern
(ie the quantum order) of spins that makes wonders. The twists and defects in
the pattern correspond to "elementary" particles. In this way, quantum order
produces electrons, photons, atoms, ... ..., and the beautiful scenery.
(4/2002)
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