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Why would accelerating a massive object to the speed of light require infinite energy?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

In the framework of special relativity, the mass of a moving object is not constant; it increases as the object's speed increases. This phenomenon is described by the concept of relativistic mass, where the mass $m$ of a body is given by $m = γ m_{0}$ , with $γ$ increasing as the velocity $v$ approaches the speed of light ( $c$ ).123

As an object’s speed approaches $c$ , its relativistic mass approaches infinity. Because the energy required to continue accelerating an object increases in proportion to its mass, providing the necessary force to reach the speed of light would require an infinite amount of energy. Consequently, it is physically impossible for any material object with rest mass to reach or exceed the speed of light, which serves as the universal speed limit. Only particles with zero rest mass, such as photons, can travel at this speed.13