# For Einstein's equation $E= mc^2$ is it energy that increases or mass as the velocity increases? [duplicate]

I will be surprised if no one asked this before and I would be happy if it is answered already although I haven't located it just yet!

From Einstein's original paper "Does The Inertia of a Body Depend on it's Energy Content" 1905. I am wondering if maybe my original thinking is not right. I always thought that energy increased as velocity increased. End of story.

But as I hear videos and read books the mantra seems to be mass increases as velocity increases. So which is right I wonder. I know I left gamma out of the equation in the question. Assume it is there thank you.

• Try searching (using PSE’s search box) for “relativistic mass”. I got more than 3000 results. – G. Smith Jul 31 '20 at 21:25
• Does this answer your question? Why is there a controversy on whether mass increases with speed? – G. Smith Jul 31 '20 at 21:35
• What is complicating the search and the heart of the problem is that people may be including gamma in the relativistic mass definition whereby relativistic mass does increase but mass does not. It would be the gamma term increasing as velocity increases and as a result the energy increases. – user86411 Jul 31 '20 at 21:41
• The invariant mass doesn’t increase. The energy and the relativistic mass do increase. They are proportional to each other, so there is no need for the concept of relativistic mass. (There are also reasons why it is conceptually a bad idea.) – G. Smith Jul 31 '20 at 21:44
• But when physicists talk about mass they mean invariant mass. Much of the pop-sci world still means relativistic mass; they are decades behind current best practice. – G. Smith Jul 31 '20 at 21:51