Meaning: The quote you provided is attributed to Roger Penrose, a prominent physicist known for his work in general relativity and cosmology. In this quote, Penrose is discussing the concept of spin and helicity in photons, which are fundamental particles of light.

To understand this quote, it's important to first grasp the concept of spin in quantum mechanics. In the context of quantum physics, "spin" refers to an intrinsic form of angular momentum that elementary particles possess. It is a fundamental property of particles, much like their mass or electric charge. However, it's important to note that the term "spin" is somewhat misleading, as it doesn't refer to an actual spinning motion of the particle, but rather to a quantum mechanical property that is analogous to the behavior of a spinning classical object.

Photons, as the carriers of the electromagnetic force and the basic units of light, are no exception to this rule. Despite having no mass, photons do possess spin. However, the nature of their spin is not quite the same as that of massive particles like electrons or protons. Rather than having an intrinsic angular momentum associated with their own rotation, photons exhibit a form of "spin" related to their polarization and direction of motion.

This brings us to the concept of helicity, which Penrose refers to in the quote. Helicity is a measure of the projection of a particle's spin onto its direction of motion. In the case of photons, their spin is directly related to their helicity. Specifically, a photon's helicity is determined by whether it is spinning in the same direction as its motion (right-handed) or in the opposite direction (left-handed). This distinction is significant in the context of particle physics and quantum field theory, as it influences the behavior of particles in various interactions.

Returning to Penrose's quote, when he mentions that "ordinary photons do have spin, they have a notion of helicity so they spin around their direction on motion," he is highlighting the fundamental nature of photons and their relationship between spin and helicity. It's important to note that while photons do not have the same type of spin as massive particles, their helicity serves as a crucial aspect of their quantum properties.

In the realm of physics, understanding the behavior of photons, including their spin and helicity, is essential for a wide range of applications, from quantum optics to high-energy particle physics. Moreover, the study of these fundamental properties contributes to our comprehension of the underlying principles of the universe at the most fundamental level.

In summary, the quote by Roger Penrose elucidates the intricate relationship between spin and helicity in photons, shedding light on the fundamental properties of these elementary particles and their behavior in the quantum realm. By delving into the nature of spin and helicity in photons, physicists continue to uncover the profound complexities of the quantum world, enriching our understanding of the universe at its most fundamental level.