# Why is the speed of light the speed of light? **URL:** \ **Category:** Factual Questions\ **Created:** [March 17, 2011, 6:46pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-the-speed-of-light/575051 "2011-03-17T18:46:16Z")\ **Posts on this page:** 1\ **Showing post:** 38
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 18, 2011, 9:47pm UTC](https://boards.straightdope.com/t/why-is-the-speed-of-light-the-speed-of-light/575051/38 "2011-03-18T21:47:53Z")
> [@Ring](#): > > **Exapno** I’m surprised at you. You know that statement is not true. Don’t you remember a previous thread where myself, **Stranger** and **Chronos** all explained why isn’t true. > > Mass and energy are not _things_ they’re properties of a system, and one cannot be converted into the other. OK, I just took another look at that thread. As I suspected you properly corrected me, but never were able to give an alternative way of expressing it purely in non-mathematical terms. That’s why I put in the “manifestations of the same underlying thing” clause. But if even that is not a good enough way to express it, can we try to come up with a phrase that satisfies you and still addresses “What are these units that can be manifested by either mass or energy?” Your version there was “a system that has energy of E and a mass of m.” and “Imo it’s much clearer to say that the mass of a system (if you’re consistent in defining the system) is absolutely conserved. However one form of the system’s energy can be converted to another, and the individual particles that make up the system can change resulting in a local mass defect.” That’s technical English but not popular science English. And one is convertible into the other. 🙂 --- _[View the full topic](https://boards.straightdope.com/t/why-is-the-speed-of-light-the-speed-of-light/575051)._