![]() ![]() Due to time dilation and the assumption that light travels at a constant speed relative to a given frame of reference, distances along the velocity axis between two frames of reference will be measured differently. The distance is different if we measure static objects from each frame of reference. The distance between internal objects, such as the length of the spaceship we're measuring from, will appear to be the same no matter the reference frame. The distance between external objects, such as stars in the galaxy, will appear to be different depending on our reference frame. Are we measuring the distance between objects that may be outside our frame of reference depending on our speed? Or are we measuring the distance between objects in one frame of reference, then re-measuring after changing all objects to a new frame of reference? The answer depends on the context of the question. If we both agree that the speed of light is the same, ie the ratio of the distance it has covered to the time it has taken, when we each think it has covered a different distance, then we must disagree about the elapsed time, ie time is relative. ![]() To you, however, that point is not 100 feet away, as you have been travelling relative to the platform, so it is some other distance. Take the classic set-up where you are on a railway carriage and I am on the platform of the station you are passing.Īs you pass me I flash a laser along the platform.Īfter what seems to me to be 100 nanoseconds, light has traveled to a certain point 100 feet along my platform (a foot is a light-nanosecond, hence a shorter version of a light-year). Indeed, the reason why times have to be relative is to allow observers who are moving relative to each other to agree on the speed of light. You can figure out for yourself almost all the effects predicted by relativity if you start with that assumption and think through its consequences. The distance light travels in a given period is the same for every observer. ![]()
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