# Is (and how) e/m and atomic radius effected by relativity ?

**URL:** <https://boards.straightdope.com/t/is-and-how-e-m-and-atomic-radius-effected-by-relativity/707373>\
**Category:** Factual Questions\
**Created:** [December 16, 2014, 6:51am UTC](https://boards.straightdope.com/t/is-and-how-e-m-and-atomic-radius-effected-by-relativity/707373 "2014-12-16T06:51:08Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\
**Post date:** [December 16, 2014, 6:51am UTC](https://boards.straightdope.com/t/is-and-how-e-m-and-atomic-radius-effected-by-relativity/707373/1 "2014-12-16T06:51:08Z")

</div>

So my knowledge of physics is limited to freshman physics where relativity and quantum mechanics (or atomic structure) were taught as two different things.  
So, say a Hydrogen molecule travelling at half the speed of light - Is the e/m of the electrons the same ? Does the atomic radius change ? Does Lorentz transformation apply to atomic radii or is the change due to e/m or both ?

---

<div class="post-metadata">

**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [December 16, 2014, 4:05pm UTC](https://boards.straightdope.com/t/is-and-how-e-m-and-atomic-radius-effected-by-relativity/707373/2 "2014-12-16T16:05:28Z")

</div>

Neither charge nor mass is changed by motion at high speed, so neither is their ratio changed. Atomic radius isn’t as well-defined: An atom at rest is spherical, but one that’s moving past your at high speed will be squished flat in its direction of motion, so it won’t have a single “radius”. The circular cross-section will have the same radius, but it’ll be shorter than that from front to back.