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Comment by erdevs | Hacker News Reader

Some commenters on the article discussed condensed matter physics as a potentially more viable path of inquiry for understanding underlying physics theory than high energy particle collision experiments.

I'm no expert here and don't understand the dichotomy, such as it were, here. Also, how are condensed matter physics contributing to fundamental theories of the universe today? I've heard a lot more of condensed matter physics in specific areas (eg superconductivity), but not often heard it discussed in relation to fundamental physics. But I'm pretty uninformed here, overall.

Does anyone with more expertise here have a quick breakdown of what these commenters mean?

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You're right that condensed matter (or other sections of physics outside high energy) is not a likely place to find new fundamental physical theories. The commenters are instead arguing that there is a lot of space for new discovery within our existing fundamental theories. That doesn't mean this work isn't exciting or valuable; this kind of work includes things like quantum computing (most of which falls within condensed matter).

If understanding the ground rules meant you understood everything that can arise within them, math would be boring by definition since we generally choose our ground rules in mathematics. So we're still finding interesting and unexpected things in other areas, including fundamental rules that are non-obvious features of existing theories (theoretical quantum information work finds new things here all the time). So for people in high energy that like finding new fundamental facts about the universe, this is a good place to go. Some methods developed by string theory and other unification theories are actually already used in condensed matter!

Condensed matter physics has provided important math and physical ideas for particle physics. The basic mathematical framework of particle physics - Quantum Field Theory - is also useful to describe many condensed matter phases.

Two extremely important ideas that developed first in condensed matter are symmetry breaking (invented to explain ferromagnetism and other phase transitions - ideas were borrowed for example in the Higgs) and renormalization. They are of fundamental importance in particle physics as well. Of course this was way back in the 50s and 60s.

I am aware of a couple of collaborations between condensed matter and particle/string theorists these days:

1. String theorists have discovered that their mathematical techniques are useful in solving certain Quantum Field Theories which are important in condensed matter. Google Ads/CMT, or strange metals.

2. String theorists believe that in quantum gravity entanglement and gravity are closely linked. They have used ideas from condensed matter studies of entanglement called tensor networks (MERA) to build toy-models of gravity.

When you have a crystal, a regular lattice of atoms, then you often describe it in a continuous approximation. So you interpolate for example the displacement of each atom from its resting position by a displacement field. When you then quantize the atoms, then you will also quantize the displacement, and you end up with a quantum field theory of phonons.

The nice thing about looking at such systems is, that we have experimental access at all scales, so we can study the breakdown of a quantum field theory in detail. The unfortunate thing is, that there may or may not be an analogy between a condensed matter system and fundamental physics, that is just as much conjecture as is naturalness.

Probably it is referring to the overlap in mathematics between condensed matter and particle physics. Both fields can be seen as explorations of the mathematical framework known as Quantum Field Theory (with somewhat different interpretation) - many discoveries of QFT in one field also turn out to apply in the other, such as the renormalization group. For example, the Higgs mechanism was first discovered by a condensed matter physicist (Phil Anderson).

I think the experiments in the different field highlight this. Particle physics experiments keep getting bigger and more expensive (LHC, Kamiokande, etc). For condensed matter, you can give an undergrad liquid helium for experiments with superconductors, which will show QFT phenomena like quasi-particles. Even looking at symmetry breaking in condensed matter doesn't require (comparatively) huge experimental setups.

I don't think it's a stretch to say that if more people are actively experimenting and thinking about problems in a field, the more likely new results are going to be found. (That's not to say particle physics experiments aren't useful.)

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