First post ever! Long time forum browser, but only registered yesterday! =D I'm in my final year of high school, and have therefore only properly been doing Physics for two years, but it's something I'd like to pursue in university. Sorry if my knowledge of relativity is lacking a little... I had this possibly stupid notion a while back... It's all hypothetical obviously: We, happily standing in our inert reference frame, see someone start to run. They get faster and faster... in fact, they soon approach the speed of light. Am I right in saying that we will observe them to gain mass? BUT... in their reference frame, they are stationary. So will they be aware they have gained mass? If they were to measure their own mass, would they find it to have increased? If anyone has any idea about this, please tell! Not sure if I'm wrong!