# Are the thermodynamical laws extra fundamental interactions beyond the 4 fundamental interactions?

The four fundamental forces or fundamental interactions are

1. Gravity
2. Electromagnetism
3. Weak interaction
4. Strong interaction

On Wikipedia, they are described as

the interactions that do not appear to be reducible to more basic interactions

This seems to imply that every other mechanism could be reduced to the fundamental interactions; or, put in another way, that everything is caused only by the four fundamental interactions.

However, I am a bit in doubt whether this is entirely true. Let's take for example a look at the tendency of systems to acquire the lowest possible energy state. The cause of this process is expleined here in more detail, but briefly said, it is due to the fact that every system "cycles" through every possible quantum or micro state. Since entropy is higher when thermal energy is increased, there are more such microstates with higher thermal energy (und less "other" energy types), which leads to the system giving away energy stored in heat and thus lowering internal energy.

To me, it seems like this process cannot be explained using the fundamental interactions. It rather seems to be pure mathematics which "sets" the entropy for different states.

So: Can really every process be explained only using the four fundamental interactions? Or are there exceptions, such as I explained above?

• Entropy relates to the behaviour of macroscopic systems: the bulk properties of media. But the material still interacts microscopically via one or more of the fundamental interactions of nature, usually electromagnetism. Thermodynamically, that systems settle into the lowest-energy configuration is a mathematical result, but it pre-supposes that the elements of that system are interacting in some way (otherwise there would be no relevant physics), and so the assumption is always there that one of the fundamental interactions is implicitly driving the evolution of the system. Commented Dec 16, 2020 at 15:06
• Emergent phenomena and properties such as entropy emerge as a combination of the above fundamental interactions. Since the exchange of energy among particles is electromagnetic, the underlying force here is electromagnetism. Commented Dec 16, 2020 at 15:14
• The OP mixed up "interaction" and "governing law". A governing law like Newton's second law $F = m a$, which defines a relations between force and acceleration (response). In 2nd law, the force may be any kind of fundamental interactions. Also, the law for thermal equilibrium: A system under a fixed tempeature will approach to its smallest Helmholtz free energy $F = U -TS$.. This is a governing law, which determines how the system will move. It doesn't mention about what kind of interaction the systme may be.
– ytlu
Commented Jan 25, 2021 at 17:07
• "The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations - then so much the worse for Maxwell's equations. If it is found to be contradicted by observation - well, these experimentalists do bungle things sometimes. But if your theory is found to be against the Second Law of Thermodynamics I can give you no hope; there is nothing for it to collapse in deepest humiliation." — Sir Arthur Eddington Commented Jan 25, 2021 at 17:19
• The original statement might be better worded as "Every process (to the best current understanding) is mediated by one of the four known forces." Otherwise one might overload the phrases "explained by" or "reducible" in unintended ways.
– Siva
Commented Jan 26, 2021 at 0:19

This entirely depends on your precise notions of what a "process" is, what it means to "explain" one of these, and what counts as "using the four fundamental interactions". You should probably not infer wide-reaching epistemological statements from a throwaway sentence in a Wikipedia article. Physics is excellent at answering questions that involve quantitative predictions of measureable values. It is much less excellent at answering the kinds of questions we humans consider "fundamental", but this is a failure of humanity, not of physics.

Let us consider a few examples in order to see why reductionism is tricky, but more for reasons of philosophy and language than reasons of physics:

1. A classical free particle. If you have a classical macroscopic particle moving freely through empty space, it continues on forever with constant velocity. Is this a "process"? If yes, is saying "it continues on forever because none of the four fundamental interactions say otherwise" really an "explanation"? If yes, is this explanation using "only the four fundamental forces", and would then not "because X doesn't say otherwise" become a universal explanation for anything "using only X"?

2. A coin flip of a classical weighted coin. A weighted coin is thrown by some machine that has been programmed to throw it with random variations in the initial linear and rotational velocities. A physicist uses a classical Newtonian model to compute the possible outcomes of the coin throw under the assumption that gravity is a constant force downward. They do not know anything about quantum mechanics, general relativity or anything else, they're just good at the math the Newtonian model needs of them (and/or have sufficient computing power available). In the end, the physicist presents a perfect prediction of the coin flip results for several different weightings of the coin.

Flipping a coin and noting whether it lands heads or tails is clearly a "process". Is the explanation by the physicist based on "the four fundamental interactions"? For all their Newtonian model cares, the downward force of gravity could be caused by magic fairies flapping their wings in the sky above the coin. They didn't need the Standard Model to arrive at their prediction, nor will learning about the fundamental forces improve their model for the flipping coin. Does the answer depend on whether or not the physicist believes in the fairies?

Furthermore, is the above an unphysical situation because the machine is doing "random" motions but we didn't specify how this randomness arises from the fundamental interactions? Do we have to design the machine to source its randomness from some quantum mechanical interaction in order for this to be physics?

3. Pauli exclusion. Two fermions cannot occupy the same state. This leads to various phenomena from the stability of matter (no stably higher occupied orbitals without exclusion!) to degeneracy pressure in dense stars and condensates. This is clearly not caused by any fundamental interaction, it works just with a bunch of fermions with no force-carrying boson in sight. Does that mean Pauli exclusion is a "fifth interaction"? This contentious philosophical discussion is dealt with in this question and its linked questions.

Maybe the question of whether certain interactions are "fundamental" is not that "fundamental", but rather ill-defined, after all. There is a technical meaning of "fundamental force" in the context of quantum field theory and the Standard Model, but once you try to extend this technical meaning to some sort of colloquial statement about physics and reality, it falls apart quickly.