# Relativity and Observer Question. Help.

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1. Sep 24, 2015

### Holometer

Can someone or approve or disapprove my question.

I want to make sure I get this right.

I will use Bill & Nancy as observers and friends

For example, if Bob is going at the speed of light inside a spaceship, his mass will increase indefinite but he will not notice the effects. Bob will notice no changes of himself inside the spaceship. However Nancy is outside on a Asteroid outpost building and Notice her friend Bill traveling at the speed of light. Nancy could not believe her own eyes. Bill has grown way to huge of infinity amount. However Bill also notices Nancy. Bill also could not believe his eyes. Nancy has grown infinite in size when he was passing her up at the speed of light while Nancy was waving hi to him on the asteroid outpost. Nancy will also see herself as everything being normal with no increase in Mass.

Did I get this story right? Is it safe that I got this relatively rule right?

2. Sep 25, 2015

### Orodruin

Staff Emeritus
This is not possible. He is massive and can therefore not be travelling at the speed of light.

The relativistic mass is an archaic concept. It also has absolutely nothing to do with an increase in size, in fact, things are length contracted in the direction of motion and therefore smaller. This is a very common question and people usually have tons of misconceptions because they think the words means something they do not. I suggest you start by reading this: https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

Edit: Also note that the level tags (B, I, A) are intended to set the level of the thread. Based on your question, I have changed it to B sice it is quite apparent you are not looking for an answer at postgraduate physics level.

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