Before Einstein came along, scientists believed that the speed of light (and indeed anything) could be measured relative to an invisible substance that existed throughout the universe called the "ether". Experiments confirmed Einstein's hypothesis, when it was discovered that the speed of light was always observed to be 300000 km/sec (in space) regardless of the observer's velocity. I.e. if you're travelling in the same direction parellel to a beam of light at 90% of the speed of light, you will still measure the speed of the light beam to be 300000 km/sec. This is because your perception of time is altered (another phenomenon introduced by the Theory of Relativity). My understanding is that the amount of energy needed to increase our velocity by amount x is proportional to the percentage of the speed of light equal to x. So if we were (and all the surrounding astronomical objects which we measure our velocity relative to) moving at 40 percent of the speed of light, our perception of time would be such that an increase in 1 percent of the speed of light would seem to require the same amount of energy as if we were only moving at 30% of the speed of light to begin with. Thus our velocity is and can only ever be measured relative to other objects. Someone correct me if I am wrong.
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