This conversation thread, especially with the images you've provided, delves into a very common point of confusion and discussion in physics, particularly around the nature of mass, energy, and photons. Let's break down the core argument being made in the screenshots and how it relates to our previous discussion: The Central Point of Confusion: Does a Photon Have Mass? The images, particularly the Stanford and Quora screenshots, directly address the question: "Do photons carry mass as suggested by the equation m = hf/c^2?" and "If E = mc^2 and E = hf, then hf = mc^2 which means photon could have a mass. But why is this wrong?" The key lies in understanding the distinction between different types of mass in relativity: * Rest Mass (m_0): This is the mass an object has when it is at rest. Photons, by definition, have zero rest mass (m_0 = 0). They are never at rest; they always travel at the speed of light in a vacuum. * Relativistic Mass (m_{rel}): Historically, some physicists used the concept of "relativistic mass," which increases with an object's velocity according to the formula m_{rel} = \gamma m_0, where \gamma is the Lorentz factor. For a photon, if you plug in m_0 = 0 into this, it would still yield zero. However, if you try to derive it from E=mc^2 and E=hf, as discussed, you get m = hf/c^2. This is where the confusion arises. The Modern Interpretation (and what the Stanford page explains): Modern physics, particularly in advanced treatments of special relativity and quantum field theory, has largely abandoned the concept of "relativistic mass" for anything other than clarity in certain contexts, precisely because it causes confusion when applied to massless particles. Instead, the focus is on: * Rest Mass (m_0 or just m): This is the invariant mass of a particle, a fundamental property. For a photon, m_0 = 0. * Total Energy (E): This is the total energy of a particle, which includes its rest energy and its kinetic energy. * Momentum (p): A particle's momentum. The more complete and fundamental relativistic energy-momentum relation is: E^2 = (m_0c^2)^2 + (pc)^2 Where: * E is the total energy. * m_0 is the rest mass. * p is the relativistic momentum. * c is the speed of light. Applying this to a Photon: For a photon, m_0 = 0. So, the equation simplifies to: E^2 = (0 \cdot c^2)^2 + (pc)^2 E^2 = (pc)^2 E = pc We also know from quantum mechanics that for a photon, E = hf and p = h/\lambda (where \lambda is wavelength) or p = hf/c. If we substitute p = hf/c into E = pc: E = (hf/c) \cdot c E = hf This is perfectly consistent. So, what about m = hf/c^2? The Stanford page (and similar explanations) clarifies this beautifully: "This equation states that the energy carried by a photon which has NO REST MASS, is equivalent to an amount of ordinary mass in grams, and that this 'effective mass' varies with the frequency of the photon." And then: "This effective mass can be acted upon by gravity which only cares how much mass a particle has; alternatively, gravity only cares about how much mass or EQUIVALENT ENERGY a particle has given by E = mc^2." Essentially, when you use m = hf/c^2 for a photon, you're calculating an "effective mass" or "equivalent mass" based on its energy. This isn't the photon's rest mass (which is zero). It's the mass that would have that amount of energy if it were at rest, or the mass equivalence of the photon's energy content. It's this "effective mass" that responds to gravity, explaining why light bends around massive objects. The Philosophical Implication - Revisit: Your original insight, "it's saying there's a non-materialist bases for reality. We're just calling it a particle a photon," still holds its philosophical weight. * If "matter" is strictly defined as "having rest mass," then photons are not matter. Yet, they are indisputably physical, carry energy and momentum, interact, and are fundamental constituents of the universe. * This forces us to broaden our definition of "physical reality" beyond just massive particles. It highlights that energy itself is a fundamental "stuff" of the universe, and mass is just one form it can take. * The "materialist universe" debate therefore often evolves into a "physicalist universe" debate, where "physical" encompasses fields, energy, and massless particles, not just massive ones. The equation hf=mc^2 directly points to this deeper unity of energy and mass, where energy can manifest in ways that don't conform to our intuitive, everyday understanding of "material" objects. The "absurdity" mentioned in one of the screenshots about hf=mc^2 is only an absurdity if one misinterprets the "m" to mean "rest mass" of the photon. When properly understood as an "effective mass" or "equivalent mass" for a photon's energy, it's a perfectly consistent and profound implication of relativity.