<font color="yellow">Or is the effects of special relativity such as time dilation also relative (as in different frames of reference would get different mass or time measurements when looking at the same object)?</font><br /><br />Yes, they are. In special relativity any observer that is not accelerating is perfectly justified in claiming that they are at rest and everything else is moving. In a given observer's reference frame, her clocks run at the "correct" rate, while the clocks carried by *everyone else* that is moving with respect to that observer run slower. <br /><br />Just to clarify, when speedfreek mentioned that <font color="yellow">When an object moves away from an observer, it looks slowed down due to time dilation. When an object moves towards an observer it looks speeded up.</font>(s)he is not really talking about time dilation, but instead about the doppler shift. As an object moves toward an observer a clock attached to that object might appear to run fast since it takes less time for each successive photon emitted by the object to reach the observer, whereas if the object were moving away it would take more time for each successive photon emitted by the object to reach the observer and so a clock attached to the object would appear to run slow. This effect is not the result of special relativity and can happen in classical newtonian physics as well. The time dilation effect is not really about how a clock will appear to run to a given observer when she watches the light from the clock, but rather it is a statement that the clock will actually be running slower in the observer's rest frame. One way to think of this is to imagine that you line up a number of people with clocks, say you space them in a line 1 light second apart, the clocks are all synchronized and at rest with respect to each other. Now suppose another person carrying a clock comes toward you at 0.9 times the speed of light. Say he passes one of your fixe <div class="Discussion_UserSignature"> </div>