# time dilatation wrongly interpreted ?

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#### killium

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<p>I read soooo many text and looked at soooo many animations about relativity.... I understand it, but i would like to clarify something. In all the examples (the train and embarquement, the clock etc...) they always state very clearly that the observer would SEE the time slow down. I don't refute this, in fact i agree 100%, a fast moving away clock would APPEAR to slow. And that is what tickles my neurons, all the text books i read use the words "appears to slow", "perceived time is slower", "see".....</p><p>&nbsp;</p><p>All the serious works on this says "time appears to slow", i never saw an explanation of the theory where they say "time does slow". Are we just mis-interpreting and taking what we see as the absolute reality ? Light takes time to travel, so all that we see is in the past. When we finally "see" the (fast moving away) clock, it is already farther in reality.</p><p>Let's do some common math:</p><p>Suppose we send a clock at 150,000 km/s (half light speed, let's round the numbers...). After 5 seconds, the clock shows 5 seconds and is at 750,000 km away.&nbsp;Light going at 300,000km/s will take (750,000/300,000) 2.5 seconds to come back to you. So 7.5 seconds after you sent the clock, you read it showing 5 seconds. time appeared to slow.</p><p>now faster:</p><p>Suppose we send a clock at 250,000 km/s (83% light speed, let's round the numbers...). After 5 seconds, the clock show 5 seconds and is at 1,250,000 km away.&nbsp;Light going at 300,000km/s will take (1,250,000/300,000)&nbsp;4.2 seconds to come back to you. So&nbsp;9.2 seconds after you sent the clock, you read it showing 5 seconds. time appeared to slow even more.</p><p>The faster it goes, the more&nbsp;you see it slowing, which is what the theory says, i'm not arguing this, and even agree that laboratories experiments showed that.... but does the clock really slow ?</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>

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#### BoJangles

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<p>we should join posts somewhere, because im sure ive asked the same thing. its funny when i read your&nbsp;post i feel like i should just point you in random directions of information, but i really do agree with you, its so anti-intuitive</p><p>ill try an link you to thread i started a while ago</p><p>http://www.space.com/common/community/forums/?plckForumPage=ForumDiscussion&plckDiscussionId=Cat%3ac7921f8b-94ec-454a-9715-3770aac6e2caForum%3ad148ee4c-9f4c-47f9-aa95-7a42941583c6Discussion%3af3d6d00c-fad2-41d1-b051-6f4fec822ce1&plckCategoryCurrentPage=0</p><p>Although this may not fully answer your question it will save some redundancy, i hope i can help, as i need a lot of it myself.</p> <div class="Discussion_UserSignature"> <p align="center"><font color="#808080">-------------- </font></p><p align="center"><font size="1" color="#808080"><em>Let me start out with the standard disclaimer ... I am an idiot, I know almost nothing, I haven’t taken calculus, I don’t work for NASA, and I am one-quarter Bulgarian sheep dog.  With that out of the way, I have several stupid questions... </em></font></p><p align="center"><font size="1" color="#808080"><em>*** A few months blogging can save a few hours in research ***</em></font></p> </div>

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#### DrRocket

##### Guest
<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I read soooo many text and looked at soooo many animations about relativity.... I understand it, but i would like to clarify something. In all the examples (the train and embarquement, the clock etc...) they always state very clearly that the observer would SEE the time slow down. I don't refute this, in fact i agree 100%, a fast moving away clock would APPEAR to slow. And that is what tickles my neurons, all the text books i read use the words "appears to slow", "perceived time is slower", "see".....&nbsp;All the serious works on this says "time appears to slow", i never saw an explanation of the theory where they say "time does slow". Are we just mis-interpreting and taking what we see as the absolute reality ? Light takes time to travel, so all that we see is in the past. When we finally "see" the (fast moving away) clock, it is already farther in reality.Let's do some common math:Suppose we send a clock at 150,000 km/s (half light speed, let's round the numbers...). After 5 seconds, the clock shows 5 seconds and is at 750,000 km away.&nbsp;Light going at 300,000km/s will take (750,000/300,000) 2.5 seconds to come back to you. So 7.5 seconds after you sent the clock, you read it showing 5 seconds. time appeared to slow.now faster:Suppose we send a clock at 250,000 km/s (83% light speed, let's round the numbers...). After 5 seconds, the clock show 5 seconds and is at 1,250,000 km away.&nbsp;Light going at 300,000km/s will take (1,250,000/300,000)&nbsp;4.2 seconds to come back to you. So&nbsp;9.2 seconds after you sent the clock, you read it showing 5 seconds. time appeared to slow even more.The faster it goes, the more&nbsp;you see it slowing, which is what the theory says, i'm not arguing this, and even agree that laboratories experiments showed that.... but does the clock really slow ?&nbsp; <br />Posted by killium</DIV></p><p>You should simply ignore the word "appears".&nbsp; They mean time slows, length contracts, etc.</p><p>If you look in some advanced books you can find some calculations on what one might actually "appear" to see and it is quite different.&nbsp; In fact some of the things that do happen, like length contraction, might not really "appear" to happen due to some optical effects that can cancel out the real relativistic effects.</p><p>But when you are trying to learn relativity, ignore the word "appears" as it is really just trying to emphasize the specific reference frame to which the author is referring.&nbsp; The "strange" effects on time, mass and length are truly real.<br /></p> <div class="Discussion_UserSignature"> </div>

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#### Mee_n_Mac

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>caveat: hopefully my maths is correct, but the principle remains, even if I got the numbers wrong.<br />Posted by <strong>SpeedFreek</strong></DIV><br /><br />The calcs seemed right to me.&nbsp; Now tell us what happens if I take the point of view that the Earth is moving away from the clock at 0.816C.&nbsp; It's an equally valid way of seeing the relative motion.&nbsp; <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The calcs seemed right to me.&nbsp; Now tell us what happens if I take the point of view that the Earth is moving away from the clock at 0.816C.&nbsp; It's an equally valid way of seeing the relative motion.&nbsp; <br />Posted by Mee_n_Mac</DIV></p><p>As long as you keep the clock moving away from the earth at constant speed, exactly the same thing happens.&nbsp; In that case both reference frames are inertial.&nbsp; But if you are heading for the twin paradox, where the clock stops and then returns to the earth, the reference frame of the clock in that case is not inertial and you cannot just plug in the equations of special relativity.&nbsp; Because in that case it is NOT true that both reference frames are equally valid -- one is an inertial reference frame and one is not.</p><p>Special relativity is formulated in an inertial reference frame, as is Newtonian mechanics.&nbsp; The basic equations do not apply in non-inertial frames.&nbsp; If one frame is accelerating with respect to another, then at least one of the frames is non-inertial.&nbsp; You don't get to pick which one it is.&nbsp; That is the basic problem with special relativity, it relies on the existence of an inertial reference frame.&nbsp; Unless you can find an inertial reference frame as a starting point, you cannot begin to apply the theory.&nbsp; Any reference frame in which one can detect acceleration, or in other words feel a force, or in which particles that do not feel a force travel in any manner other than a straight line, is not inertial.</p> <div class="Discussion_UserSignature"> </div>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The calcs seemed right to me.&nbsp; Now tell us what happens if I take the point of view that the Earth is moving away from the clock at 0.816C.&nbsp; It's an equally valid way of seeing the relative motion.&nbsp; <br /> Posted by Mee_n_Mac</DIV></p><p>I think the question becomes irrelavent if you can't physically determine which frame is the actual inertial frame.&nbsp; I can't think of any experiment where an inertial frame can not be determined other than a hypothetcial though experiment.&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### vandivx

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> In all the examples (the train and embarquement, the clock etc...) they always state very clearly that the observer would SEE the time slow down. I don't refute this, in fact i agree 100%, a fast moving away clock would APPEAR to slow. And that is what tickles my neurons, all the text books i read use the words "appears to slow", "perceived time is slower", "see".....&nbsp;All the serious works on this says "time appears to slow", i never saw an explanation of the theory where they say "time does slow". ... but does the clock really slow ?&nbsp; <br /> Posted by killium</DIV></p><p>&nbsp;</p><p>when you see the phrases regarding the time passage in moving frames of the kind of "<strong>appears </strong>to slow", "<strong>perceived </strong>time is slower", that means the idea is that we are talking only about relative, that is not real effect, not real in the sense that the time doesn't really slow down in the physical sense (with permanent effects such as aging differently) - same as in plain visual perspective when you see the house mile away as small and you say 'it appears or is perceived as small' and you rightly expect to find the house the usual size when you come near it, that's what the words 'appear' and 'perceived' mean IMO (that it is all in the eye of the beholder and not really happening out there) </p><p>&nbsp;</p><p>now, don't interpret the above as my position regarding time dilation, I just said how I understand such turn's of speech, nothing more and nothing less </p> <div class="Discussion_UserSignature"> </div>

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#### SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>now, don't interpret the above as my position regarding time dilation, I just said how I understand such turn's of speech, nothing more and nothing less <br /> Posted by vandivx</DIV></p><p>Yes, exactly. People like me, who use descriptive language to explain a scientific theory, have to be <strong>very careful</strong> how we word things (which is why I often have to re-read and revise what I have said, hopefully before someone else notices!). The same is true for subjects like the apparently superluminal recession velocities of distant galaxies. </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### vandivx

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'><font color="#008000">As long as you keep the clock moving away from the earth at constant speed, exactly the same thing happens.&nbsp; In that case both reference frames are inertial.&nbsp;</font> <font color="#800000">But if you are heading for the twin paradox, where the clock stops and then returns to the earth, the reference frame of the clock in that case is not inertial and you cannot just plug in the equations of special relativity.</font>&nbsp; <font color="#0000ff">Because in that case it is NOT true that both reference frames are equally valid -- one is an inertial reference frame and one is not.</font><br />&nbsp;</p><p>Special relativity is formulated in an inertial reference frame, as is Newtonian mechanics.&nbsp; The basic equations do not apply in non-inertial frames.&nbsp;<font color="#ff6600"> If one frame is accelerating with respect to another, then at least one of the frames is non-inertial.&nbsp; You don't get to pick which one it is.</font>&nbsp; That is the basic problem with special relativity, it relies on the existence of an inertial reference frame.&nbsp; Unless you can find an inertial reference frame as a starting point, you cannot begin to apply the theory.&nbsp; Any reference frame in which one can detect acceleration, or in other words feel a force, or in which particles that do not feel a force travel in any manner other than a straight line, is not inertial. <br /> Posted by DrRocket</DIV></p><p>&nbsp;</p><p>it seems that you are saying that even the parts of motion at uniform velocity do not count as happening in inertial frame given that the history of that frame is that it accelerated ["<font color="#800000">where the clock stops and then returns to the earth</font>"] - but you can't have relative motion at all without accelerating as one point or another and so it would seem that no frame could then be counted as inertial in the sense that SR would apply, I understand that during the acceleration the two frames are not equal, one is inertial and the other is not (the one that accelerated - assuming they both don't but only one of them) but when accelleration ceases the frame becomes inertial again, therefore SR is perfectly fine to calculate what happens in such frames during that time they move uniformly </p><p>also in the second part of the post which I highlighted orange you say <font color="#ff6600">You don't get to pick which one it is.</font> do you mean to say one can't tell which one of the two is accelerating? I believe it is perfectly possible to tell which one accelerates and which one is inertial or if both are accelerating </p> <div class="Discussion_UserSignature"> </div>

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#### SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>also in the second part of the post which I highlighted orange you say <font color="#ff6600">You don't get to pick which one it is</font>. do you mean to say one can't tell which one of the two is accelerating? I believe it is perfectly possible to tell which one accelerates and which one is inertial or if both are accelerating <br /> Posted by vandivx</DIV></p><p>If I may, I think DrRocket means exactly what you are saying. The frames are not equivalent, so you cannot simply pick either frame and expect to get the same results. If one frame is inertial and the other is non-inertial, <strong>you</strong> don't get to pick which is which. </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### vandivx

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Let's do some common math:Suppose we send a clock at 150,000 km/s (half light speed, let's round the numbers...). After 5 seconds, the clock shows 5 seconds and is at 750,000 km away.&nbsp;Light going at 300,000km/s will take (750,000/300,000) 2.5 seconds to come back to you. So 7.5 seconds after you sent the clock, you read it showing 5 seconds. time appeared to slow</p><p> but does the clock really slow ?&nbsp; <br /> Posted by killium</DIV></p><p>as you said, we don't care about precise figures, so <strong>on your terms</strong> suppose we send a clock out not at half but at full speed of light</p><p>after 5 seconds, the clock shows 5 seconds and is 1,500,000 km away (5*300,000)</p><p>light going at 300,000 km/s will take 5 seconds to come back to you</p><p>so 10 seconds after you sent the clock, you read it showing 5 seconds, time appeared to slow to half speed BUT it didn't stop completely as it should if the clock is moving out at speed of light at which speed it is known that the clock should stop completely!! </p> <div class="Discussion_UserSignature"> </div>

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#### SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>as you said, we don't care about precise figures, <strong>so on your terms</strong> suppose we send a clock out not at half but at full speed of light <br /> Posted by vandivx</DIV></p><p>Ok, so <em>you</em> are sending the clock at the speed of light, you are sending something with mass at an impossible speed for something with mass. Let's see what figures fall out of the Lorentz Transformation...</p><p>&nbsp;</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>after 5 seconds, the clock shows 5 seconds and is 1,500,000 km away (5*300,000) <br /> Posted by vandivx</DIV></p><p>Nope. If the clock were moving at the speed of light, after 5 seconds (Earth time) the clock moving at c would show that <strong>zero</strong> seconds had elapsed for the clock. The moving clock, travelling at the speed of light relative to clocks on Earth, would be subject to infinite time-dilation. </p><p>&nbsp;</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>light going at 300,000 km/s will take 5 seconds to come back to you so 10 seconds after you sent the clock, you read it showing 5 seconds, time appeared to slow to half speed BUT it didn't stop completely as it should if the clock is moving out at speed of light at which speed it is known that the clock should stop completely!! <br /> Posted by vandivx</DIV></p><p>The clock would indeed show that no time had elapsed. 10 seconds after sending the clock, you still read it showing <strong>zero</strong> seconds. Your post is confusing, but I can see how you are trying to illustrate the error in the original post. The error in the original post was neglecting to use Special Relativity. </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### vandivx

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Ok, so you are sending the clock at the speed of light, you are sending something with mass at an impossible speed for something with mass. Let's see what figures fall out of the Lorentz Transformation...&nbsp;Nope. If the clock were moving at the speed of light, after 5 seconds (Earth time) the clock moving at c would show that zero seconds had elapsed for the clock. The moving clock, travelling at the speed of light relative to clocks on Earth, would be subject to infinite time-dilation. &nbsp;The clock would indeed show that no time had elapsed. 10 seconds after sending the clock, you still read it showing zero seconds. Your post is confusing, but I can see how you are trying to illustrate the error in the original post. The error in the original post was neglecting to use Special Relativity. <br /> Posted by SpeedFreek</DIV></p><p>look, my post was intentionally composed by tracing the OP argument precisely, word for word except changing the figures and the purpose of it was to give him a hint that such argument as he presented has nothing to do with SR (namely with time dilation) and I wonder why nobody explained that to him so far </p><p>hint: the argument is based on <em>finite speed</em> of light while SR is based on the fact that its speed is <em>constant</em>, those are two completely different things - from this thread it is clear that while many can handle calculations of SR well, they don't have a good grasp of it in physical sense and that is typical, to be fair it must be said that that's where the inconsistencies of SR crop up and fully consistent picture is not possible and for that reason people typically shy off trying to get some physical interpretation and stick with the mathematical side of SR only, no wonder that many had big problems understanding SR when it came out because in those days they were used to understanding their physics and not just plugging figures into formulas</p><p>&nbsp;</p><p>also everybody knows that matter can't be accelerated to speed of light, however as the argument stands if I input some realistic figures there for .9 or .99 speed of light or less, it still wouldn't change anything on that argument - that it has nothing to do with SR time dilation and its not just a matter of precise calculations, the argument as done by him simply does not even roughly parallel the SR calculations that employ gama factor (as you approach speed of light) and that's what I wanted to show there by shifting the figures of the argument to the area where known results should result - but they don't by a long stretch by that argument, therefore that argument is fundamentally faulty (what I did there with his argument can perhaps be called 'reduction ad absurdum')<br /> </p><p>&nbsp;</p><p>in other words one doesn't do any SRelativistic approximation of time dilation when he takes into account the timeshift due to finite light speed such as astronomers routinely do</p> <div class="Discussion_UserSignature"> </div>

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#### killium

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<p>Thank you everyone. You're begining to "size" me It's true that i can plug the numbers in SR formulas and find the right results, i understand what SR says, i am not asking how it works but WHY it works And all the other questions i asked in the other of my threads here are related. And all your answers (and civil arguing) are exactly what i need!. Thank you all for that "food for neurons" ... i'm going back to woodsheding <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-innocent.gif" border="0" alt="Innocent" title="Innocent" /></p><p>&nbsp;</p><p>(edit: typos)</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>

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#### killium

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<p>btw, i think the most fundamental thing in all this is the CONSTANCY of the speed of light anyhow you measure it. In another thread i asked&nbsp;HOW it is measured. What is the fundamental proof we have that the light emitted by a receding object arrives here at c ?</p><p>&nbsp;</p> <div class="Discussion_UserSignature"> </div>

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#### SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>btw, i think the most fundamental thing in all this is the CONSTANCY of the speed of light anyhow you measure it. In another thread i asked&nbsp;HOW it is measured. What is the fundamental proof we have that the light emitted by a receding object arrives here at c ?&nbsp; <br /> Posted by killium</DIV></p><p>Your question covers a lot of ground, so for the moment I think the best thing I can do is give you a link to a website that describes the experiments used confirm the constancy of c.</p><h1><font size="3">What is the experimental basis of Special Relativity?</font></h1><p>&nbsp;</p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>that said I am not native English speaker<br /> Posted by vandivx</DIV></p><p>Well in that case I applaud you, as your English is actually very good, better than a few English people I know!<br /> </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

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#### lukman

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I read soooo many text and looked at soooo many animations about relativity.... I understand it, but i would like to clarify something. In all the examples (the train and embarquement, the clock etc...) they always state very clearly that the observer would SEE the time slow down. I don't refute this, in fact i agree 100%, a fast moving away clock would APPEAR to slow. And that is what tickles my neurons, all the text books i read use the words "appears to slow", "perceived time is slower", "see".....&nbsp;All the serious works on this says "time appears to slow", i never saw an explanation of the theory where they say "time does slow". Are we just mis-interpreting and taking what we see as the absolute reality ? Light takes time to travel, so all that we see is in the past. When we finally "see" the (fast moving away) clock, it is already farther in reality.Let's do some common math:Suppose we send a clock at 150,000 km/s (half light speed, let's round the numbers...). After 5 seconds, the clock shows 5 seconds and is at 750,000 km away.&nbsp;Light going at 300,000km/s will take (750,000/300,000) 2.5 seconds to come back to you. So 7.5 seconds after you sent the clock, you read it showing 5 seconds. time appeared to slow.now faster:Suppose we send a clock at 250,000 km/s (83% light speed, let's round the numbers...). After 5 seconds, the clock show 5 seconds and is at 1,250,000 km away.&nbsp;Light going at 300,000km/s will take (1,250,000/300,000)&nbsp;4.2 seconds to come back to you. So&nbsp;9.2 seconds after you sent the clock, you read it showing 5 seconds. time appeared to slow even more.The faster it goes, the more&nbsp;you see it slowing, which is what the theory says, i'm not arguing this, and even agree that laboratories experiments showed that.... but does the clock really slow ?&nbsp; <br /> Posted by killium</DIV></p><p>Object not need to travel away from you, time dilation also happens when you travel fast in circular orbit.</p> <div class="Discussion_UserSignature"> </div>

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