The Rev Dodgson said:

Alleged (from Quora):

“Everyone recognizes Einstein’s
E=mc2 But for decades, physicists have fiercely debated what the “m” actually means—and whether an object gets heavier as it approaches the speed of light.

Early in the 20th century, physicists observing high-speed particles noticed that as objects accelerate closer to the speed of light, they become increasingly resistant to further acceleration. To explain this intuitively, scientists and educators popularized the concept of “relativistic mass“—the idea that an object gets heavier as it moves faster. In this framework, the term “rest mass” was introduced to describe the intrinsic mass of an object when it is completely stationary. Because this model suggested an object’s mass changes with speed, scientists needed a baseline measurement.

However, modern particle physicists have largely abandoned the concept of relativistic mass. In contemporary physics, mass is treated as a fundamental, invariant property of a particle. An electron has the exact same mass whether it is sitting still in a laboratory or hurtling through a particle accelerator at 99.9% the speed of light. When an object speeds up, it is the object’s momentum and kinetic energy that increase, not its mass.

Because modern physicists view mass as an unchanging property, they consider the “rest” prefix completely redundant. To them, “rest mass” is simply “mass.”

The debate today is not about the underlying math—which both sides agree on—but about how physics should be taught. Proponents of using relativistic mass argue that it provides a brilliant, intuitive way to explain to beginners why nothing can travel faster than light: as an object approaches light speed, it becomes infinitely massive, requiring infinite energy to accelerate.

Conversely, most professional physicists argue that teaching relativistic mass creates unnecessary confusion later on. It complicates the math in general relativity and makes the concept of gravity more difficult to understand. As a result, the current consensus in advanced physics is to banish the idea of mass increasing with speed. In modern laboratories, mass is just mass, and momentum does all the heavy lifting.”

I don’t get this.

How does saying that mass is constant, so momentum no longer = mv, make things less confusing?

whatever, it’s effectively acting as mass in the equation, so you can choose to apply a correction to the variable or to the equation