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

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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>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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SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>...but does the clock really slow ?&nbsp; <br /> Posted by killium</DIV></p><p>No, from the frame of reference of the clock, or somebody moving with the clock, the clock does <em>not</em> slow. All the clocks across the rest of the universe actually speed up (and don't just <em>appear</em> to).</p><p>But can we go back a little....?</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>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&nbsp; <br /> Posted by killium</DIV></p><p>Remember special relativity. The clock is moving away from Earth at half the speed of light. We use the Lorentz transformation to work out how much time will elapse back on Earth, after 5 seconds for the clock. We find that for every second the clock ticks, 1.154 seconds passes on Earth. So after 5 seconds for our moving clock, 5.77 seconds have passed on Earth. The clock is now 865500 km away (150000 x 5.77). Light from the clock will take 2.885 seconds to come back to us, so 8.655 seconds after you sent the clock, you read it showing 5 seconds.</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>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. <br /> Posted by killium</DIV></p><p>At 0.83c, for every second that ticks on the moving clock, 1.792 seconds pass back here on Earth. After 5 seconds for the clock, 8.96 have passed on Earth and the clock is now roughly (250,000 x 8.96) 2,240,000 km away. Light takes 7.4666 seconds to return. So 16.42666 seconds after you sent the clock, you read it showing 5 seconds.</p><p>If you know that the light took nearly 7.5 seconds to return, you realise that nearly 9 seconds had passed on Earth while only 5 had passed for the moving clock.</p><p>The clock <strong>has</strong> slowed relative to your frame of reference. It has not simply appeared to slow, due to "speed of light" lag. When you subtract out the light-lag (after correcting it for relativistic time-dilation), you find that the clock has <em>still</em> been running slower than your clock back on Earth.</p><p>And yet, from the frame of view of the moving clock, or those riding with the clock, it is ticking as fast as it ever was! But clocks on Earth are running faster than "normal". If your twin brother rides with the moving clock, he <em>will</em> age slower than you do, back here on Earth. If he returns quickly, his stubble will have grown at a slower rate, but if he travels at 0.83c, relative to you, for 45 years from your point of view, he would return looking 20 years younger than yourself (assuming he didn't get too much sun!).</p><p><em>(caveat: hopefully my maths is correct, but the principle remains, even if I got the numbers wrong)&nbsp;</em></p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </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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SpeedFreek

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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 cannot show you any math for this, but this is how I understand it works. Yes, motion is relative, so the pilot of the rocket ship might consider that the Earth is moving away from him at the same speed that an observer on Earth considers the rocket ship to be receding.</p><p>Time-dilation is <strong>symmetrical</strong> between observers in <em>inertial</em> frames of reference. If an object is moving at 83% of c, relative to another, observers on both the objects will see the <em>other</em> object slowed by the <strong>same amount</strong>, relative to themselves. Each observer can consider themselves to be at rest and that the other object is moving. </p><p>Let's change the figure we are using to 86.6% of c for this, as the relativistic change factor at that relative speed is a nice round number - 2. If an object is moving at 86.6% of c, relative to another object, observers on each object will calculate that the clock on the other object is ticking at half the rate of their own clock.</p><p>So, as the rocket ship recedes from Earth (at constant speed!), the people on Earth (if they had a good enough telescope) would see the clock on the rocket ship ticking away at half the speed of their own clock. But you asked for the point of view of the rocket ship, the moving clock... the people on the rocket ship would <em>also</em> see the clocks on Earth ticking away at half the speed of their own clock too! This is why the twins example is referred to as a paradox.</p><p>But how can we tell if there is actually a paradox or not? As far as each observer is concerned, time is passing for the other at half the rate it is passing for themselves.</p><p>If we change things so that the rocket ship is moving <em>towards</em> Earth at 86.6%c, what happens? Well now each observer sees the other clock ticking faster than their own, and again each sees the same change in time for the other. Again, time-dilation is symmetrical between observers in inertial frames of reference.</p><p>So how can the twin on the rocket ship age less than his twin back on Earth? Surely it is a paradox?</p><p>Well it seems not, because only one of the observers remained (pretty much) in an inertial frame of reference throughout - the observer on Earth. The observer on the ship however, had to accelerate. They had to accelerate away from the Earth, they had to accelerate when they turned around and they had to accelerate (in the other direction) when they returned to Earth.</p><p>When you accelerate you are changing your inertial frame of reference, and constant accelerations mean constant changes in your frame. According to Special Relativity, when you change your inertial frame of reference, there is a shift in simultaneity. The further away in space-time you are from another observer when you accelerate, the larger the shift in simultaneity. </p><p>This means that during the times that the rocket ship is accelerating, time-dilation is not symmetrical between the observers. Described from the view on the rocket ship (as that is what you asked), the changes in simultaneity are relatively small when it accelerates away from Earth and when it decelerates upon return, but during the turnaround phase of the journey, the observer on the rocket ship would see the clocks on Earth speed up tremendously, and these simultaneity shifts are responsible for the different ages of the twins upon the travelling twins return.</p><p>EDIT: I see DrRocket posted whilst I was composing this and he put it much more concisely than I ever could in my "enthusiastic amateur" way! He understands the maths required, whereas I have to rely on people like him to explain the implications of the theory to me. </p> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </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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Mee_n_Mac

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<p>Thanks all. My hope was to tease out what was said. </p><p>1) The time dilation observed by either party is symmetric. Both see the other clock as being the slow poke. What I see is that this gives people the impression that the twin <em>paradox</em> (yes DrR I was aiming in that direction&nbsp;<img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /> ) is a paradox because each twin "should" see the other staying young. </p><p>2) The paradox is resolved when you consider that the returning twin is not in an inertial frame and therefore you can't apply SR. </p><p>Now that all said I may wish to return to the other thread wherein I tried to use van_divx's refomulation of the twin paradox.</p><p>FWIW : When Saiph opened up his lectures in physics, this is the kind of discussion I had hoped for.&nbsp; Thx all !</p><p>&nbsp;</p> <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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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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SpeedFreek

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>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<br /> Posted by vandivx</DIV></p><p>The way I see it, the OP was asking about relativity and time-dilation and was suggesting that time-dilation was purely due to "speed of light" lag, and so the clock only "appeared" to slow. My <strong>first</strong> post in this thread pointed out that he needed to use SR to work out what the clock would really do. I used <strong>his</strong> figures to illustate this, rather than resorting to the infinite time-dilation of a relative speed of c, and I explained how time-dilation is not due to "speed of light" lag, but works as predicted by SR. We showed that "common math" is not up to this job. Why are you wondering why nobody explained this to him when we have done just that? </p><p>What <strong>you</strong> did is try to explain how, using the OPs rationale, a clock receding at the speed of light would read 5 seconds, rather than reading zero seconds as it should, but without explaining <strong>why</strong> it should read zero seconds - <em>"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!!"</em></p><p>The OP needs to know <strong>why</strong> the clock should stop completely, in your example. The answer requires special relativity, as we have pointed out, not "common math" as the OP suggested.</p><p>&nbsp;</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>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</DIV></p><p>Eh? "one" doesn't do any SR when he takes into account "speed of light" lag? Who doesn't? The OP didn't, that's for sure, but <strong>we</strong> have to use SR to get the correct answer to this particular problem. Again, your quote is confusing me. Maybe its just me.</p><p>As I said earlier, people who use descriptive language to explain a scientific theory, have to be <strong>very careful</strong> how we word things. </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'>The OP needs to know <strong>why</strong> the clock should stop completely, in your example. The answer requires special relativity, as we have pointed out, not "common math" as the OP suggested.<br /> Posted by SpeedFreek</DIV><br />you are <strong>difficult </strong><img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-foot-in-mouth.gif" border="0" alt="Foot in mouth" title="Foot in mouth" /></p><p>&nbsp;</p><p>I gave him a hint that something is wrong with his calculations and the idea was to make him rethink his approach to understanding SR time dilation via this kind of real world examples, the choice to explain or not is entirely my decision and pls leave that up to me, so what if he <strong>needs to know why</strong>, its not like getting answers here on forum&nbsp; is some life threatening affair is it, that one couldn't do without them, I am not obligated to answer, sometimes I judge that one does better by at least allowing the questioner to see his mistake and try to correct it by more thinking, like going back to blackboard, I believe in showing people when and why they have gone wrong, OP strikes me as asking good questions and he is (trying to) thinking physics through for himself by physical examples and he has a good chance one day to make a name in physics which won't happen if he should just accept the way the SRelativistic calculations are done, I suppose using the formulas of SR is not his problem, likely he could plug his example into those formulas and arrive at proper answers himself but that still would not give him real world grasp of SR which is what he is trying to do, those that can do the calculations are nowadays literaly dime a dozen but rarely do people understand what they are really doing </p><p>&nbsp;</p><p>actually I believe it is more important for him to see how his example doesn't apply at all, that what he is doing there is not estimation of time dilation as SRelativistic effect</p><p>that's because SR didn't and couldn't start (wasn't and couldn't be derived) from such considerations as his example given there - that the speed of light is finite and the consequences of that, rather it was born from dealing with the puzzling experimental fact of the constancy of speed of light and instead of explaining that fact, it was simply accepted as given and the choice was taken to build the transformation equations instead taking the fact of constancy of speed of light as basis and time dilation as well as length contration automatically followed out of those equations - themselves no less puzzling effects than the constancy of speed of light was, in effect one puzzle generated other puzzles (well, naturally, what else could be expected) </p><p>that means that one can't today understand (model) these effects (time dilation & length contraction) from such&nbsp; common sense examples no more than one can understand why speed of light is constant through them (by reverse derivation), in effect if one could model time dilation like that example and also the length contraction, then one would have a simple real life model explaining constancy of speed of light in those terms which would then be common sense fact which nobody would have difficulty understanding&nbsp; (same as nobody would have any conceptual problem with that example which started the thread)</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'>you are <strong>difficult&nbsp;</strong> <br /> Posted by vandivx</DIV></p><p>Sorry about that. My thinking is that in the physics forum we should be a little more scrupulous with our replies.&nbsp;</p><p>&nbsp;</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I gave him a hint that something is wrong with his calculations and the idea was to make him rethink his approach to understanding SR time dilation via this kind of real world examples, the choice to explain or not is entirely my decision and pls leave that up to me, so what if he <strong>needs to know why</strong>, its not like getting answers here on forum&nbsp; is some life threatening affair is it, that one couldn't do without them, I am not obligated to answer, sometimes I judge that one does better by at least allowing the questioner to see his mistake and try to correct it by more thinking, like going back to blackboard, I believe in showing people when and why they have gone wrong, OP strikes me as asking good questions and he is (trying to) thinking physics through for himself by physical examples and he has a good chance one day to make a name in physics which won't happen if he should just accept the way the SRelativistic calculations are done, I suppose using the formulas of SR is not his problem, likely he could plug his example into those formulas and arrive at proper answers himself but that still would not give him real world grasp of SR which is what he is trying to do, those that can do the calculations are nowadays literaly dime a dozen but rarely do people understand what they are really doing&nbsp; <br /> Posted by vandivx</DIV></p><p>Ahh, the Socratic method. I use it myself often, but the key is to use questions to point people in the right direction. The questions one poses are very important in themselves, when using the Socratic method of teaching.</p><p>But the point here is that, although the OP claimed to understand the basics of relativity - <em>"I read soooo many text and looked at soooo many animations about relativity.... I understand it, but i would like to clarify something."</em> - he then used an example that failed to apply relativity at all. This is not an attack on <strong>killium</strong>, who asks very well thought out questions and obviously wants to learn.</p><p>The OP was "If you take the travel time for the outward journey of the clock, and the light travel time for the return journey of the light reflected from the clock when the clock reads 5 seconds, this adds up to more than 5 seconds. So when the light returns after 7.5 seconds, the clock reads 5 seconds. Is that what they mean by time-dilation?"</p><p>To which the answer is no, that is not time-dilation, that is the observational lag caused by light travel time.&nbsp;</p><p>&nbsp;</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>actually I believe it is more important for him to see how his example doesn't apply at all, that what he is doing there is not estimation of time dilation as SRelativistic effect</p><p>that's because SR didn't and couldn't start (wasn't and couldn't be derived) from such considerations as his example given there - that the speed of light is finite and the consequences of that, rather it was born from dealing with the puzzling experimental fact of the constancy of speed of light and instead of explaining that fact, it was simply accepted as given and the choice was taken to build the transformation equations instead taking the fact of constancy of speed of light as basis and time dilation as well as length contration automatically followed out of those equations - themselves no less puzzling effects than the constancy of speed of light was, in effect one puzzle generated other puzzles (well, naturally, what else could be expected)</p><p>that means that one can't today understand (model) these effects (time dilation & length contraction) from such&nbsp; common sense examples no more than one can understand why speed of light is constant through them (by reverse derivation), in effect if one could model time dilation like that example and also the length contraction, then one would have a simple real life model explaining constancy of speed of light in those terms which would then be common sense fact which nobody would have difficulty understanding&nbsp; (same as nobody would have any conceptual problem with that example which started the thread)&nbsp; <br /> Posted by vandivx</DIV></p><p>I actually have no problem with any of that. Having re-read this thread, it does indeed seem that nobody (including yourself until you edited your recent post) thought to point out that, in a vacuum, light is always measured as travelling at the same speed relative to whoever is measuring it. Light always travels 300,000 km/s faster than yourself, whatever relative speed you have to other objects. This is probably due to the way the OP was worded, where <strong>killium</strong> said he understood relativity, but simply wanted clarification as to the "wording" of time-dilation.</p><p>As an aside (with friendliest intentions, I assure you), I feel I should point out that I struggled to read through your above quotes, as they have to be some of the longest run-on sentences I have worked through recently.</p><p><img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" /> </p><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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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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vandivx

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>As an aside (with friendliest intentions, I assure you), I feel I should point out that I struggled to read through your above quotes, as they have to be some of the longest run-on sentences I have worked through recently. &nbsp; <br /> Posted by SpeedFreek</DIV><br />I am working on it, I can be much worse I assure you&nbsp; </p><p>&nbsp;to myself I justify some of the more extended sentences after they were posted and I see them by believing that I am telling something worthwhile enough in them to make it worthwhile for those who care to read them to read them even several times over if need be to get out of them what I say in them LOL</p><p>I know that sounds arrogant but that is not my intention to be so, its just when I write I focus so much on composing what I write that I don't pay much heed to the form, I do reread and try to make it readable like inserting a comma to make meaning clear and often split paragraph into two but still that doesn't always cut it as you can see</p><p>this last post I did was no routine writing (in all humbleness I doubt you would find the argument I made there anywhere and I think that atypical approach also has to do with the difficulty of reading it), I was thinking it out on the fly trying to describe what was wrong with the guy's approach, I knew it only in rough outline but writing anything down is always more trying as you know, also like you I edit my posts quite a bit after posting them in the first five or ten minutes trying to make the meaning more clear and catch bad grammar mistakes as well as splitting it up a bit if its too much of a run, I love this internet writing style without capitals and dotting your sentences but there are certain limits to everything I suppose, that said I am not native English speaker and that makes my sentences a bit harder to read as they may not always follow quite proper word order or whole parts of sentences may be switched around despite my trying&nbsp; </p><p>&nbsp;</p><p>I understand Newton's Principia is notoriously hard to read <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-foot-in-mouth.gif" border="0" alt="Foot in mouth" title="Foot in mouth" /> </p> <div class="Discussion_UserSignature"> </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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