All elementary constituents of matter (fermions) and all force carrying quanta (gauge bosons) have intrinsic spin which cannot be described as classical rotation but these elementary particles impart angular momentum when they interact with other particles and fields. The intrinsic spin can be called non-classical rotation.
It seems that rotation is a necessary feature of Nature at the macroscopic and microscopic scales. Higgs particle is an exception at the microscopic scales.
The newly discovered Higgs particle is very special. Higgs particle has spin=0, therefore it does not rotate at all, not even in a non-classical way. There may be other elementary particles with spin=0 (axions, for example) but they have not been discovered in experiments. The billionaire mathematician Jim Simons is funding projects searching for axions. One possibility of course is that the Higgs particle may be the only elementary particle with spin=0 in nature.
In general, composite objects do not necessarily rotate. Atoms for example – consisting of protons, neutrons and electrons – may or may not rotate. Similarly, molecules – consisting of many atoms – may or may not rotate.
Proton is a composite particle made of quarks and gluons. Do protons rotate? This is not an easy question to answer. Proton is a very complicated system. Quarks and gluons contribute to proton’s spin in a very complicated way. Experiments and theory are making progress but we are still not there to answer this question satisfactorily.
Physics is not supposed to answer the “why” questions but physicists try. Why is rotation a necessary feature of Nature at the macroscopic and microscopic scales?
I have seen answers invoking symmetries and conservation laws. I am not convinced by those arguments. We deduce the (abstract) rotational symmetries and conservation laws such as the law of the conservation of angular momentum from our empirical (concrete) observations of rotations. We then summarize our empirical observations in a mathematical form. So, if you argue that everything rotates because the mathematics of those symmetries and conservation laws dictate it then you are fooling yourself. You are effectively saying that “everything rotates because everything rotates.”
I have seen an explanation arguing as follows (para-phrasing): “when particles interact they don’t necessarily collide head-on therefore an angular momentum will be created.” For the composite particles and composite systems the argument based on the concept of off-centered interaction is a valid argument. For the elementary particles, however, it is a very naive argument because elementary particles have no extension as far as our experiments can tell. It is true that we can still speak of effective cross-sections of particles in practical calculations but the off-centered interaction concept cannot really explain the intrinsic angular momentum of fermions and bosons.
When we create an extreme energy density in a small volume of space, pairs of particle-antiparticle are created. The particle as well as the antiparticle of that pair has intrinsic spin. Where does the intrinsic spin come from in a pair creation? I hear people saying in unison “conservation of angular momentum.” Yes, of course, I know, conservation of angular momentum is the law but it does not explain why there is intrinsic angular momentum in the first place. Do you see my point? We are simply observing that angular momentum is there and it is conserved but we don’t know why it is there.
It seems to me that the rotation/spin is a built-in feature of Nature at the most fundamental level. In Prometheus and Chronos I tried to explain the idea that the intrinsic spin is an outcome of a special kind of symmetry breaking that I call twisting action. The consequence of ‘twisting action’ is the appearance of curvature on the primordial fabric.
I also claim that the primordial fabric has infinite number of threads and that there is an intrinsic flow in a given primordial thread. As a result of the ‘twisting action’ a multitude of curved forms of the fundamental thread are possible. The simplest form would be as shown below.
In Prometheus and Chronos I tried to build a conceptual model of particles based on the hypothesis of intrinsic flow in curved (open or closed) primordial threads. So, my answer to “why rotation” is the following:
- twisting action causes curvatures in the primordial threads
- when the intrinsic flow is forced to follow a curved path in a primordial thread it manifests rotation/spin as well as a virtual-center
- virtual-charge and virtual-time emanate from the virtual-center
- when the primordial thread closes on itself (closed-loop) a real-center is formed
- real-charge and real-time (physical time) emanate from the real-center
In other words, I am replacing the mystery of rotation/spin with the mystery of ‘twisting action’ and the intrinsic flow in the primordial thread. Mystery always remains but the progress is measured by the economy of our theory. The simplest and the most efficient (yielding to calculations) theory with the smallest number free parameters will be accepted as the most worthy. That’s what we are trying to do. It is clear that mystery will always remain.
I differentiate “physical time” from other kinds of time. The time factor or time effect that emerges from a closed-loop of a primordial thread is the physical time. In the article “Divine Time, Causal Time, Physical Time” I contrasted the physical time to the other kinds of time. In his latest book “Time Reborn” Lee Smolin argues that there are different kinds of time as well.
Rotation/spin is intimately related to physical time as I argue in the “My answer” section above. So, another way of answering the “why rotation” question would be this. Rotation/spin is necessary for the creation of physical time.
This is not because we measure the passage of time by observing the cyclical (rotation/spin) phenomena in objects but because rotation and time are intimately related at the most fundamental level of physical existence.