Difference between Photons and Particles

In physics, the distinction between photons and particles can be a bit nuanced because photons are indeed particles—specifically, they are elementary particles. However, when we refer to "particles" in a general sense, we often mean particles with mass, such as electrons, protons, and neutrons. Photons, on the other hand, are massless particles that are the quantum of the electromagnetic field, including electromagnetic radiation such as light. To understand the differences, we need to delve into the properties and behaviors of photons compared to other particles.

Table of Differences

Property	Photons	Particles (with mass)
Mass	Massless	Have mass
Speed	Always travel at the speed of light (c)	Travel at speeds less than c
Energy	$E = h\nu$ (Planck's relation)	$E = mc^2$ (Rest energy)
Momentum	$p = \frac{E}{c} = \frac{h\nu}{c}$	$p = mv$ (Non-relativistic)
Wave-particle duality	Exhibited (e.g., in the double-slit experiment)	Exhibited, but less noticeable for larger particles
Force carriers	Photons are force carriers for electromagnetic interactions	Particles like W and Z bosons are force carriers for weak interaction; gluons for strong interaction
Charge	Electrically neutral	Can be charged (e.g., electrons) or neutral (e.g., neutrons)
Affected by gravity	Yes (through the curvature of spacetime)	Yes, through gravitational force

Formulas

Photons and particles with mass follow different formulas for energy and momentum due to their distinct properties:

• Photon Energy: The energy of a photon is given by Planck's relation: $$ E = h\nu $$ where $E$ is the energy, $h$ is Planck's constant, and $\nu$ is the frequency of the electromagnetic wave.

• Photon Momentum: The momentum of a photon can be derived from its energy: $$ p = \frac{E}{c} = \frac{h\nu}{c} $$ where $p$ is the momentum, and $c$ is the speed of light.

• Particle Energy: The total energy of a particle with mass is given by the relativistic energy-mass equivalence: $$ E = \sqrt{(mc^2)^2 + (pc)^2} $$ where $m$ is the rest mass of the particle, and $p$ is its momentum.

• Particle Momentum: The momentum of a non-relativistic particle is given by: $$ p = mv $$ where $m$ is the mass and $v$ is the velocity of the particle.

Examples

Double-Slit Experiment

The double-slit experiment is a classic demonstration of wave-particle duality. When photons are sent through two closely spaced slits, they produce an interference pattern on a screen, which is characteristic of waves. However, when the experiment is set up to detect which slit a photon passes through, the interference pattern disappears, and photons behave like particles.

For particles with mass, such as electrons, the double-slit experiment also shows an interference pattern, indicating their wave-like behavior. However, for larger particles, such as buckyballs (C60 molecules), the wave-like behavior is still present but becomes less pronounced.

Gravitational Bending of Light

Photons are affected by gravity, not by a force in the traditional sense, but because they travel along the curvature of spacetime caused by massive objects. This was famously confirmed during a solar eclipse in 1919, which provided evidence for Einstein's theory of general relativity.

Particles with mass are also affected by gravity, but the force of gravity acts on their mass, causing them to accelerate towards massive objects.

Photoelectric Effect

The photoelectric effect is another phenomenon that illustrates the particle nature of light. When photons with sufficient energy strike a metal surface, they can eject electrons from the metal. The energy of the ejected electrons depends on the frequency of the incident photons, not their intensity, which supports the idea that light consists of discrete energy packets (photons).

In summary, while photons are a type of particle, they are distinct from particles with mass in several key ways, including their masslessness, their constant speed (the speed of light), and their role as the force carriers of electromagnetic interactions. Understanding these differences is crucial for studying modern physics and quantum mechanics.