Final answer:
Einstein's equation E=mc² implies that mass can be converted into energy and vice versa, and that the speed of light is a constant in all inertial frames. It also establishes that mass has a profound impact on the possibility of reaching the speed of light. The correct answer is that c. Massless objects, like light, always move at the speed of light.
Step-by-step explanation:
Albert Einstein's equation E=mc² explores the profound relationship between energy (E), mass (m), and the speed of light (c). According to this relationship, energy is equal to mass multiplied by the square of the speed of light. In the context of special relativity, this demonstrates that mass and energy are interchangeable, and that the speed of light (approximately 3.00 × 10⁸ m/s) is a constant in all frames of reference, unaffected by the motion of the source or the observer.
Einstein's second postulate of relativity dictates that light in a vacuum travels at this constant speed, c, relative to any observer, no matter the observer's velocity. Therefore, an object with mass cannot move at the speed of light because, as it approaches c, its mass would become infinite and require infinite energy to move, which is not feasible according to the laws of physics.
Additionally, Einstein's theories contradicted classical Newtonian mechanics, which suggested that velocities add linearly like simple vectors. If this were true, two observers moving at different velocities would see light traveling at different speeds, which Einstein showed to be incorrect.
This implies that light and massless objects always move at a constant speed c in a vacuum, and that massless particles, such as photons, must always travel at this speed and no other. Therefore, the correct option to the question posed is: c. Massless objects, like light, always move at the speed of light.