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I’ve been trying to wrap my head around the concept of relativistic mass and why it seems to have fallen out of favor in modern physics textbooks. In my old college notes, it was presented as this key idea for understanding momentum at high velocities, but now everything just talks about invariant mass and energy-momentum. I’m confused about what this shift actually means for the physics—was it just a change in terminology, or is there a deeper conceptual reason for moving away from it?

In class we wrestled with relativistic mass, then the books shifted to invariant mass and four momentum. It felt like a terminology cleanup, but there is a real shift: you stop using a velocity dependent mass and describe everything with gamma factors and rest mass. p equals gamma m0 v and E equals gamma m0 c^2, and that makes the physics frame independent, though the gut intuition of a growing mass disappears in other frames.

I tried to teach it once by using m_rel = gamma m0 to keep momentum intuitive, but that led to questions like 'is my mass bigger now?' from students. I dropped it and stuck to energy momentum four-vectors and the idea that rest mass is fixed.

Honestly, I think the change is less about physics and more about teaching ergonomics. Having mass defined as a frame invariant data point feels less slippery, so instructors push that path. Still, you lose the gut feeling that mass grows with speed.

We did a quick project with cosmic ray muons and tried to explain momentum with relativistic mass; it felt cute until a student asked why they'd never use it in equations. We ended up writing the momentum as gamma m0 v and calling it a gamma factor, which looked odd to some.

Do you think the problem is really in the object or in how we measure it?

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