# B Point of view and mass

1. May 13, 2017

### hsdrop

I know that when anything starts moving close to the speed of C (light speed) it ganes mass from an outside perspective. Does the same apply in the reverse? From the perspective of the thing that moving close to C does everything around it lose mass??

2. May 13, 2017

### Staff: Mentor

This concept of "relativistic mass" is actually not considered very useful in modern relativity. The modern view focuses on rest mass, better thought of as invariant mass since it is invariant under a change of reference frame.

If you are using the concept of relativistic mass, then from the perspective of the thing, everything around it is moving close to the speed of light (while it itself is at rest), and therefore the relativistic mass of everything around it is increased.

3. May 13, 2017

### hsdrop

Ok that makes sense, I just was not sure how to think of it.
If you don't mind, I would like to ask another quick question.
Do object traveling close to C true get thinner or is it just the way the light bounces off it at that speed?

4. May 13, 2017

### Staff: Mentor

If you're referring to length contraction, if you measure the length of an object traveling at close to c relative to you, it will be shorter in its direction of motion than it would be if it were at rest relative to you.

5. May 13, 2017

### hsdrop

Thank you peter for taking the time to answer my questions and phrasing it in such a way that everyone can understand no matter what level of education they might have.

6. May 14, 2017

### Staff: Mentor

You're welcome!