Sorry, I'm very new to this topic, this is going to annoy you I expect. How do we define velocity in relativity? There's no fixed aether to measure against. So in the lorentz transformation, when we say v2/c2... What does v mean? If we measure against a photon the photon will shoot past at c, so it's obviously not that. If we measure against something else then we get a velocity relative to something else. Am I right in thinking this is how we measure velocity? And that what we draw from this measured velocity when we use the lorentz transformation is how time dilates in our frame compared to how time dilates in the other frame which we see as "at rest"? And then the rest mass of something also confuses me... If there is no definite substance to measure against, how can we say that it has a certain rest mass when for all we know it is moving and therefore the rest mass we emasure is dilated? I suspect I'm being dumb and that this question is what relativity is based on... Apologies and thanks in advance.