Can special realitivity be used to determine absolute rest?

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triclon

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I know that speed is normally considered relative but couldn't one use time dilation and changes in mass to determine when an object would be considered completely nonmoving in the universe. Mass increases as one approaches the speed of light, right? Well then couldn't one with really accurate measurements find when an object has the least amount of mass, no matter how it moved relative to the observer? 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)?
 
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SpeedFreek

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"Are 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)?"<br /><br />Well if I apply what I've recently learned, the answer to this is yes.<br /><br />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.<br /><br />So if you have an object travelling between 2 observers, it looks slowed down to one, and speeded up to the other, according to special relativity. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>
 
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nexium

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I recall one episode of original star trek where Captain Kirk order "All ahead stop" when they were near a nebula. My best guess is Scotty matched the vectorial average of the particles closest to them in the nebula. This however is no indication of absolute rest as the nebula and star ship had a sizable velocity with respect to almost everything in the Universe. Neil
 
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doubletruncation

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<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>
 
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alokmohan

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Plese explain time diatation point by point,it helps lay readers
 
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doubletruncation

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Without proper illustrations it may not be very intelligible. But, I can go through the basic argument for time dilation that is usually given.<br /><br />Some preliminaries:<br /><br />- We can take as an experimental fact that any non-accelerating observer sees light (or any massless particle) traveling at a constant speed c=3*10^10 cm/s. This has been known since the late 19th century and was first demonstrated in the famous Michaelson-Morely experiment.<br /><br />This is a very weird thing if you think about it, and is the basis for special relativity. What this means is, for example, that if you shine a flashlight at someone who is running away from you at 1.5*10^10 cm/s (0.5c) you would both measure the speed of the light to be 3*10^10 cm/s, the person running away from you would *NOT* measure the speed to be 1.5*10^10 cm/s like you might expect.<br /><br />- Now we will make the following definition: an event is an occurence at a specific point in space at a specific time - all observers will agree that an event happened. <br /><br />- We will also assume that there is no preferred reference frame. That is, the laws of physics are the same in all non-accelerating reference frames.<br /><br />Now for subtle but important point:<br /><br />- Suppose you were to hold up a ruler perpendicular to the velocity of another person who is moving with respect to you. Say the moving person holds up his own ruler so that it just overlaps yours as he passes you. You will both conclude that the ends of each ruler just touched each other since those are events, therefore you will both agree that the rulers are the same length. Therefore we can say that for two reference frames moving with respect to each other at velocity v, distances measured perpendicular to the direction of relative motion will be the same in both reference frames.<br /><br />Now to show how you get time dilation:<br /><br />Consider the following device: take photon source and counter at one end and attach them to a c <div class="Discussion_UserSignature"> </div>
 
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