I found myself in a dichotomy when I tried to explain someone why we start getting diminishing returns on speed vs the energy investment as the speed of light is approached.

Is it because of relativistic mass (which would indicate that the object becomes harder to accelerate)?

Or is it because of the way relative speeds work in the fabric of the universe, which, considering the Lorentz factor, makes it mathematically impossible to produce relative speeds higher than c?


1 Answer 1


Historically both explanation have been used, however the concept of relativistic mass has fallen out of favor recently and your second explanation is now preferred.

  • $\begingroup$ How did that happen exactly? Was it like how Pluto go demoted to a "dwarf planet"? Was there a vote or anything? Was the will to dump relativistic mass, and, seemingly, redefine mass to mean rest mass, and maybe some other things (I'm trying to find out at the moment whether energy is still considered to have mass or to be same thing as mass), coming from any particular group or nationality within physics? $\endgroup$ Dec 9, 2021 at 13:48
  • $\begingroup$ @MathewChristopherBartish It was simply that the distinction between rest mass and relativistic mass was a continuous cause of confusion. $\endgroup$ Dec 9, 2021 at 15:40
  • $\begingroup$ How was that? I find the conflation of the two confusing. If two things are different, they should have different names, I would say. Lewis Carroll Epstein's book "Relativity Visualized" says there is only one kind of mass, but he meant what some call "relativistic mass", with rest mass being the relativistic mass when the object was at rest. Others say there is only one kind of mass, meaning rest mass. But so few people have heard of the latter idea. Ask why you can't accelerate an object to the speed of light, and if they say anything, it is that the mass approaches infinity before that. $\endgroup$ Dec 9, 2021 at 16:34
  • $\begingroup$ @MathewChristopherBartsh Carroll's book was first published 40 years ago. Today mass is viewed as a scalar invariant. $\endgroup$ Dec 10, 2021 at 0:50

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