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Message: <blockquote data-quote="Gidien" data-source="post: 2405234" data-attributes="member: 6372">Just to clear up some confusion... the Theory of Relativity works in all situations, if it is indeed true (which we have become pretty sure of due to many experiments). As you approach the speed of light parts of the equations of relativity approach zero, and if other numbers are being divided by those zeros they approach infinity. So, you can't really get to the speed of light using legal math, you can only take the limit as you approach it. Something flying at mach 4 is not lighter than something flying at mach 2... in fact, it has slightly (very, very slightly) more mass and more weight. Now, if said jet has used up a lot of fuel to get to mach 4 obviously it will have less mass, and if it is higher up gravity will have a smaller effect, though that will not affect the mass of the jet.And direction very much does matter in relativity, as anyone struggling with vector addition on a relativity test knows. If your spaceship is very close to the speed of light relative to an observer (having a stationary observer is the key to all of these questions, btw, so let's not leave it out), and you are walking around inside, you will walk very, very slowly, slow enough not to pass the speed of light. If your spaceship is at the speed of light (impossible) then you would not move at all. On the other hand, for you inside the spaceship, you can move freely, the spaceship is stationary, and everything else is moving very fast while slowing down in time. E=mc^2 is a cool equation that can be found by working the equations of relativity, but it is not the key equation in the theory.</blockquote>

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