Emergent Particles?
How do we know that electrons and photons are fundamental and there is nothing beyond? No experiment can make us sure; anything we observe can be emergent and any understanding can be effective. Although we can not prove the contrary, we can one day find out that current theories is just a low energy or big scale description of something more profound. I know that low energy and big scale are used interchangeably in high energy physics, but I want to make a distinction for making my point more clear.
Understanding particles as a low energy configuration of a single entity is the current program of string theory. Everything we see around are very cooled strings and properties (particle spectrum) are determined by the specific the compactification of the extra dimensions which is a valid description only in low energies. But still one electron is made out of one string.
Another way of realizing the world would be seeing particles as a collective behavior of many small agents. They are emergent concepts in the big scale, but their degrees of freedom do not have any meaning at scale of fundamental building blocks. I am very sympathetic to this point of view because there is a particle-like object (that propagates, interacts and stays intact) but is actually just a collective motion of something smaller. This object is the phonon.
I think in the high energy physics community emergent concepts are highly underrepresented, compared to string theory. But certainly there were many people who tried it and failed and trust decayed over time. Simplest (best known) way of carrying this approach is the solitons. Most famous soliton particle is probably the Skyrmion, but it wasn’t very successful so far.
Today I want to talk about the most promising collective model of particles, I have seen so far. Idea is due to Prof. Xiao-Gang Wen of MIT. He was last year’s MIT Theory Retreat speaker and my interest in the subject is increasing since then. He started with the above motivations about the phonon. But no vacuum state we know (no matter how abstract) has the right gapless excitations of gauge bosons and fermions until the work of Wen. He studied a very interesting condensed matter system: condensation of strings. His strings are not (at least not necessarily) related to the superstrings. They are just excited states lying on curve in a lattice of ground state bosons with a certain simple interaction. Surprisingly these objects can give rise to fermions, gauge bosons of U(1) and SU(3) and probably many other things as a collective excitation. Main problem here is the chiral description. Theory does not naturally give chiral fermions therefore SU(2) couplings are problematic. If theory gives them in some limit or at some critical state approximately then it will be a big triumph. How about gravity? Theory does not have graviton excitations but it seems that loop quantum gravity “spin-networks” can be written in terms of these “spin-nets” and vice versa. Wouldn’t it be fun if loop quantum gravity has the standard model particles as collective phenomena!
If you want to read more about it start from cond-mat/0407140, cond-mat/0406441 and references therein. His latest book treating these issues in great detail is also recently published. Comments are welcome.
Understanding particles as a low energy configuration of a single entity is the current program of string theory. Everything we see around are very cooled strings and properties (particle spectrum) are determined by the specific the compactification of the extra dimensions which is a valid description only in low energies. But still one electron is made out of one string.
Another way of realizing the world would be seeing particles as a collective behavior of many small agents. They are emergent concepts in the big scale, but their degrees of freedom do not have any meaning at scale of fundamental building blocks. I am very sympathetic to this point of view because there is a particle-like object (that propagates, interacts and stays intact) but is actually just a collective motion of something smaller. This object is the phonon.
I think in the high energy physics community emergent concepts are highly underrepresented, compared to string theory. But certainly there were many people who tried it and failed and trust decayed over time. Simplest (best known) way of carrying this approach is the solitons. Most famous soliton particle is probably the Skyrmion, but it wasn’t very successful so far.
Today I want to talk about the most promising collective model of particles, I have seen so far. Idea is due to Prof. Xiao-Gang Wen of MIT. He was last year’s MIT Theory Retreat speaker and my interest in the subject is increasing since then. He started with the above motivations about the phonon. But no vacuum state we know (no matter how abstract) has the right gapless excitations of gauge bosons and fermions until the work of Wen. He studied a very interesting condensed matter system: condensation of strings. His strings are not (at least not necessarily) related to the superstrings. They are just excited states lying on curve in a lattice of ground state bosons with a certain simple interaction. Surprisingly these objects can give rise to fermions, gauge bosons of U(1) and SU(3) and probably many other things as a collective excitation. Main problem here is the chiral description. Theory does not naturally give chiral fermions therefore SU(2) couplings are problematic. If theory gives them in some limit or at some critical state approximately then it will be a big triumph. How about gravity? Theory does not have graviton excitations but it seems that loop quantum gravity “spin-networks” can be written in terms of these “spin-nets” and vice versa. Wouldn’t it be fun if loop quantum gravity has the standard model particles as collective phenomena!
If you want to read more about it start from cond-mat/0407140, cond-mat/0406441 and references therein. His latest book treating these issues in great detail is also recently published. Comments are welcome.
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