Centripetal force, gravity, and relativity

BoJangles · Aug 15, 2008

i wonder, with the case of muons, how do we know at which point they are interacting in the atmosphere to be able to judge how deeply they should penetrate. <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>

derekmcd · Aug 16, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>i wonder, with the case of muons, how do we know at which point they are interacting in the atmosphere to be able to judge how deeply they should penetrate. <br /> Posted by Manwh0re</DIV></p><p>High altitude balloons. </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>

vandivx · Aug 17, 2008

<p> </p><p>Replying to:</p><div class="Discussion_PostQuote"><em>... maybe the biggest change would come regarding the (in)famous space contraction in SR which never had any direct experimental basis and was just the result of the theoretical need to enable us to deal with the fact that the speed of light was found to be constant in all reference frames which it still would be in the reworked theory I hasten to say except that we would understand the physical reason why it stays constant and space contraction would be conceived in different way (it has to do with the reason behind constancy of light speed)...Posted by vandivx</em></div><p> </p><p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>There is experimental evidence for length contraction.  Length contraction and time dilation are two sides of the same coin.  The decay of the mu meson (in modern terminology called the muon) is a case in point.  These particles are formed high in the atmosphere by cosmic ray collisions and are found to penetrate our atmosphere farther than would be expected from the known decay time of these particles.  The explanation for this phenomena lies in the Lorentz transformatin of special relativity.  <strong>From the viewpoint of an earthbound observer, time dilation allows the muon to penetrate deeper into the atmosphere because it has more time to do so.</strong>  <strong>From the point of view of the muon length contraction [in the] permits the penetration during a </strong><strong>normal lifetime.</strong>If you accept only two premises: 1) that the laws of physics are the same in all inertial reference frames and 2) that the speed of light is the same in all inertial reference frames, then on the basis of mathematical logic alone you must accept the Lorentz transformation that is derivable from those two assumptions.  With the Lorentz transformation you get all of the features of special relativity including length contraction, time dilation, relativity of simultaneity, etc.  You cannot accept only part of the package, it either the whole thing or nothing.  <br /> Posted by DrRocket</DIV><br />I don't have much time for answering this weekend but I will say at least this: so you are saying it is all like some magic trick with the muon with nothing trully physicaly happening to it to make it have longer decay time but that it is like some sleight of hand - no real physical effect is involved in the muon penetration event except that perspective or the viewpoints of 'observers' have been altered due to muon speed involved, that from the point of view of the muon the atmosphere of the Earth has in effect become shallower making for shorter travel and thus deeper penetration towards Earth's surface during its standard lifetime while an onlooker on Earth 'understands' this deeper penetration as due to the time dilation in the moving frame of the muon which effect makes it live longer and so it penetrates deeper in the atmosphere</p><p> </p><p>but there is in SR such a thing as mass increase due to velocity and shouldn't the increased mass of the high speed muon have some effect on lengthening its lifetime? particle mass increase IF it it were 'real physical increase' should certainly affect the lifetime of muon as opposed to being just a 'perspective thing' - and due to our particle accelerators we know it is very real effect with real physical consequences - except that conventional view has relativistic mass increase conveniently only 'kick in' during acceleration and regarding it as unreal (unphysical, that is being just a matter of viewpoint or perspective of observers) while its motion is uniform but that is wrong - particle traveling at relativistic speeds has its mass increased in real physical terms (while it travels at uniform velocity) and that is why it puts up bigger inertial resistance when we try to accelerate it and has longer lifetime if it is a particle that decays such as muon </p><p> </p><p>an observer stationed on Earth then interprets the fast muon penetrating deeper in the atmosphere as due to its longer lifetime on account of its increased mass</p><p>however I am aware that SR holds this mass increase as being just a matter of perspective with no real physical consequences - that is unless acceleration comes into the picture, then it recognizes that real physical effects arise</p><p>I see this as a schism in the theory and hold that the mass increase is real even while the particles move at uniform (relativistic) speeds, problem with SR is that it regards space as relative as opposed to it being absolute - it can't afford to recognize any effects arising from motion at uniform velocity as being physically real, that is with real attending transient or lasting physical consequences, so it needs space contraction and or time dilation (depending on viewpoint) in the case of muon (time dilation is real effect though) and in the case of the 'twin time dilation experiment' it falls on its face and has to admit that uniform motion can have real physical lasting effect (differently aged twins) and it is justly attacked on this point and not only by myself</p><p> </p><p>to sum it up, fast muons are more massive and so have longer lifetimes - fast moving muons when further being accelerated are becoming more massive as they are picking up more speed as well as putting up increased inertial resistance due to the current level of increased mass and so have longer lifetimes, since gravitation is equivalent to acceleration in its effects, it has accordingly the same effect as acceleration on decaying particles making their lifetimes last longer - muon lasts longer if it gets born in stronger gravitational field than if the field was weaker (time dilation is part of it) </p><p>mass increase due to motion is of kinetic nature and represents real energetical increase of particles (it is not just a matter of perspective) and this has effect on their mass (are more difficult to accelerate) and naturally on their lifetimes if they are the kind of particles that decay </p> <div class="Discussion_UserSignature"> </div>

vandivx · Aug 17, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>you can not have length 'expansion' unless you have a negative velocity... impossible.  Or travelling greater than C... impossible. <br /> Posted by derekmcd</DIV><br />because I am talking here partly of new physics I'll say it with the qualifier - given current physics - both are impossible, that means that if one seemingly requires length expansion that something is wrong with current physics (SR) or with my analysis of the problem, you know where I stand </p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> The observer from the source, outside the astronauts reference frame, does not recognize the length contraction of the astronaut.  Everything appears normal to the outside observer. The astronaut will see the light travelling at C in his own reference frame and so will the outside observer, but they will disagree what time the light was emitted and how far it travelled. <br /> Posted by derekmcd</DIV><br /> I don't understand that, 'the observer from the source' must see moving ship and everything on it as contracted, moving objects are contracted</p><p> </p><p>don't try to find something complicated in my example, I am talking about explanations commonly used when explaning SR to laymen or to begining students, my point is everybody is always interested in dealing with oncoming ray of light because without SR we would have the astronaut measure higher than speed of light and everybody always want's to show that is not so, however catching up light is not so sexy becasue astronaut is in no danger of measuring higher than light speed and typically nobody takes it up or thinks about it BUT it is just as important, SR has to pass all such tests so that it is integrated theory, it can ill afford any hanging ends much less contradictory ones </p> <div class="Discussion_UserSignature"> </div>

DrRocket · Aug 17, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> ... I am aware that SR holds this mass increase as being just a matter of perspective with no real physical consequences - that is unless acceleration comes into the picture, ...Posted by vandivx</DIV></p><p>That is not true.  The mass increase increase in SR is real, but is dependent on the reference frame.  In the sense that it is dependent on the reference frame it is a matter of perspective, but it is absolutely real.  Special relativity has nothing to say about acceleration, because it is a theory of uniform motion between inertial reference frames.  But general relativity can handle acceleration and in general relativity the mass increase is also real.<br /></p> <div class="Discussion_UserSignature"> </div>

derekmcd · Aug 18, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>because I am talking here partly of new physics I'll say it with the qualifier - given current physics - both are impossible, that means that if one seemingly requires length expansion that something is wrong with current physics (SR) or with my analysis of the problem, you know where I stand</DIV></p><p>I don't understand how you conclude a length expansion is required by a photon chasing an astronaut.</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I don't understand that, 'the observer from the source' must see moving ship and everything on it as contracted, moving objects are contracted</DIV></p><p>The observer doesn't physically see anything contracting.  Only upon taking a measurement and comparing results with the astronaut could contraction be inferred.   Let me put it this way... Contraction can be measured and experienced.  It is a very real phenomena, but you can't visually see it or photograph the reality of it.</p><p>The speed of light is a measureable quantity.  Photons emitted simultaneously in a moving object's rest frame will not be recieved simultaneously.  In order for the eye or a camera to process the image of an object, it must essentially believe it is recieving the light simultaneously.  In the non-relativistic world it is interpreted as and appears to be simultaneous, but not at relativistics speeds.   </p><p>Take a meter stick and look at it.  Stationary, you recieve light from the front end before the back end and, with light being so fast, you interpret it as simultaneous and you recieve the full image.  Now, say it is approaching you at near light speed.  In order for you to see the entire meter stick, the far end of the stick would have to emit it's light <strong>before</strong> the front end to catch up with the moving front end.  By the time that light catches up to the front end so it can emit its light (to make the image simultaneous), the meter stick will have moved, thus elongating the image and negating the contraction.  The image will always be a distortion of reality.</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>don't try to find something complicated in my example</DIV></p><p>I'm not.  Special Relativity is complicated enough on its own.  I actually think you are over simplifying your though experiment without realizing the complications of SR. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I am talking about explanations commonly used when explaning SR to laymen or to begining students, my point is everybody is always interested in dealing with oncoming ray of light because without SR we would have the astronaut measure higher than speed of light and everybody always want's to show that is not so, however catching up light is not so sexy becasue astronaut is in no danger of measuring higher than light speed and typically nobody takes it up or thinks about it BUT it is just as important, SR has to pass all such tests so that it is integrated theory, it can ill afford any hanging ends much less contradictory ones <br /> Posted by vandivx</DIV><br /><br />I've fallen pray to some of the 'commonly used' explanations... they are not always correct or can be so simplified that you can lead yourself down the wrong road.  The Big Bang balloon and gravity rubber sheet analogies are 2 that are quite simple, yet often misleading.  I have no illusions of being well versed in SR, but I think I have a handle on some of the basics.  I even grasp some of the less complicated equations.</p><p>Let's go back to your light catching up concept:</p><p>Let's take an astronaut that leaves Earth travelling at .99C for 100 years.  After one year, I, here on Earth, emit a beam of light in the direction of the travelling astronaut.  What I will see is the light beam finally catch up to the astronaut right at 99 years.  I will see that the astronaut has been travelling for 100 years, covered .99 light years distance and the beam of light caught up to him after it has been travelling for 99 years (remember, i waited one year to shine my light).</p><p>What the astronaut sees.  At .99C will give a gamma factor of 7 (I think it's 7.1, but for ease of math, i just round).  Time will pass for him at 1/7th the pace and length contraction is 7 times what I experience in my rest frame.   So the length and time of his trip is ~14 light years distance at ~14 years time when the light reaches him.</p><p>Considering we synchronized our clocks prior to his departure, he now looks at his watch after 1 year (a time we agreed on), he turns around a looks at Earth, he will see that Earth is 7 light years distance away.  From this new (apparent) stationary reference frame, it would appear the earth is speeding away from him slowing the Earth's time down by a gamma factor of 7.  He is 7 light years distance away when, after my time dilated 1 year (according to him), I shine my light.  He looks at his clock and odometer and realizes that it is, in fact, he who has the gamma factor of 7, he realizes he can easily solve the twin paradox and understand the reality of the situation.</p><p>Essentially, from each of our perspectives, we will disagree (yet both be correct) what time I shone my light and how far away the astronaut was when I did.</p><p>The astronaut in his reference frame, still looking back at earth, will see the beam begin when he is 7 light years distance.  Given the constancy of photons in a vacuum, this beam of light will reach him in about 7 years travelling 7 light years distance (still his stationary reference frame looking backwards) and the Earth will have sped away an additional 7 light years distance.  Thus, the astronaut will measure that he is 14 light years distance from Earth when the light reaches him.  </p><p>Obviously, from this perspective, it still takes 14 years for the light to reach the astronaut.  His "real" reference frame looking forward, it also take 14 years.  Either way you look at it, it takes 14 years and light is travelling at a constant in which ever reference frames he chooses.</p><p>No paradox and no legnth expansion.  At least, that's how I see it... I could always be wrong. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /> </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>

vandivx · Aug 18, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That is not true.  The mass increase increase in SR is real, but is dependent on the reference frame.  In the sense that it is dependent on the reference frame it is a matter of perspective, but it is absolutely real.  Special relativity has nothing to say about acceleration, because it is a theory of uniform motion between inertial reference frames.  But general relativity can handle acceleration and in general relativity the mass increase is also real. <br /> Posted by DrRocket</DIV><br />in that case why is the muon example always given as you presented it in your post - only with reference to length contraction or time dilation, depending on reference frame view taken</p><p> </p><p>I suppose SR sees the mass increase as real but it doesn't seem to do anything with it except to say that when you accelerate you should feel extra resistance due to it, I don't know that anybody ever thought of taking this mass increase into account in any other way, such as in that muon example, surely such effect if real and I agree it is real should have some effect on 'aging', that is on time dilation and thus on particle decay time - particle mass increase due to relativistic velocity should definitely increase (prolong) its standard lifetime, that is decay time (I expect to be made co-author of paper if anybody should publish this idea, a work based on it to be precise, I also happen to know a 'mechanism' responsible for mass increase from which it is more plain how it all works and why particle decay time lengthens because of it, so I better be brought in on it) </p><p> </p><p>I have never seen in the muon example (which I think is considered as one of the first observations that experimentally confirmed relativistic physics as time dilation and length contraction are concerned) any reference to relativistic mass increase, it is as if such thing doesn't exist or perhaps it was taken to be of no (physical) consequence, one can only speculate if that was innocent oversight or if it was intentional, either way it doesn't put SR in good light I should think</p><p><br />I believe one can understand muon decay example without those unphysical 'perspective shenigans' of SR but only if one admits that space is real, absolute, that moving in it can result in real physical consequences but this view physicists try to avoid at all costs, even the cost of wilfull blindness - to be fair, the consequences of admiting the idea of absolute space are so revolutionary and far reaching that one naturally tends to shrink from it, the more so the more one realizes what fundamental changes its admiting would imply and that one has no idea how to implement them even if one dared to trod this high path which by definition is reserved for genius grade journeymen</p><p> </p><p>I know well that acceleration is the domain of GR but part of the traditional SR canon is the use of relativistic mass increase due to uniform velocity as reason that one can't accelerate matter to speed of light because  the mass (its inertial resistance to acceleration) would increase beyond all bounds as one approached the speed c and so one would have to overcome infinite inertial resistance... so in that area SR touches upon acceleration</p><p> </p><p>isn't it strange that the special relativistic mass increase due to uniform velocity seems to play no part in physics unless one talks about acceleration, reason is that relativity theory should be trully relativistic which means there shouldn't be any lasting physical changes happening due to uniform motion - it should all be trully a matter of perspective</p><p> </p><p>like in a perspective due to distance that we all know from daily life - on a grassy plain two people walk a mile appart and both then claim to see the other as small but both know that is just matter of perspective and know when they meet again at short distance that they will be both the same (original) height - SR theory should idealy be like that but it didn't end up being like that and it seems to me that those bettraying evidences (real lasting effects due to uniform motion) to relativity are downplayed or ignored and otherwise treated as some unwanted relatives (black sheep of family) by those who understand what is at stake here </p><p> </p><p>that twin experiment is for SR a whistle blowing event signalling something is fundamentally wrong here as it would be if the two men of similar body height on a plain agreed to test the visual perspective by not walking appart as before but one of them sitting down and waiting while the other walked a mile away and then returned to the sitting man and found himself really shorter upon return and the explanation would be put forward that the walking away and returning man was the one who really was moving and gaining distance and therefore it is no wonder he became shorter - that's the same like twin experiment is explained by saying that the younger twin was the one who was 'really moving' because he was the one who accelerated (when setting on the journey, turning around and then stopping upon return)</p><p>if space was not something physical, then it should make no difference who was moving and who was not and one couldn't get older or younger at different rate or whatever because it should all be perfectly symmetric like the two guys on that grass plain in a plain perspective experiment </p><p> </p><p>in SRelativity theory nobody or nothing should ever be said to be 'really moving' if it wants to live up to <strong>relativity</strong>, relativity means it should all be just a matter of perspective same as in the perspective effects that even ancient people knew, what Gallilean relativity was all about (that it didn't matter whether you were on shore or on a shipboat moving along the shore) </p><p> </p><p> </p><p> </p><p> </p> <div class="Discussion_UserSignature"> </div>

vandivx · Aug 18, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I don't understand how you conclude a length expansion is required by a photon chasing an astronaut. Posted by derekmcd</DIV></p><p>and I don't understand how you conclude I am talking about viewpoint of photon, I meant viewpoint of observer stationed on Earth for example in the classical setup typically used to explain basic principle of SR (how it is that one measures speed c in moving frame of reference, such as onboard of spaceship) to begining students in simple terms, what's so difficult about it - shrinken ruler and time dilation are used to explain why the astronaut measures speed c of oncoming light and what I am saying is that one would need precisely the opposite effects to explain why astronaut would measure speed c for light catching up with him from behind - which case is never talked about in the discussion of length contraction & time dilation</p><p> I suppose you could look up that thread in which I had long disccussion of this issue from all angles with Speedfreek (I think or somebody else), it was sometimes last year on old forum still, posts will likely have been chopped in length courtesy of our forum software, so results may vary </p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The observer doesn't physically see anything contracting.  Only upon taking a measurement and comparing results with the astronaut could contraction be inferred.   Let me put it this way... <strong>Contraction can be measured</strong> and experienced.  It is a very real phenomena, but you can't visually see it or photograph the reality of it. Posted by derekmcd</DIV></p><p><br />I am talking about thought experiments like when one says - If you could see in god's eye like setup spaceship in flight and astronaut and his measuring apparatus represented by a measuring stick, you would see they contract ... that kind of illustrative but unreal example that is being used to explain basic principles of relativity and when one doesn't take into account that nobody can ever see anything like that, nor take a photo of it, that's why I say you seriously misunderstand me to the point of evasion, I mean everybody who ever been around physics has heard of such thought examples at one point or another, for christ's sake, I mean if you want to be so realistic, we can drop all that talk about twins because on those terms which you seem to want to discuss physics that is all one huge nonsense of course, nobody can determine differential aging of people (their organism) due to traveling on rockets...so we can just drop any such discussion </p><p>space contraction has never been measured directly, it is always inferred, not sure what you mean by 'experienced', it is not real in the sense of being determined in positive fashion, it is only needed to explain SR effects because the theory relies on it, is built on it (in part) but there is no hard evidence for length contraction and if the theory should change so as not to require it, it would and could be dropped and that would be it, time dilation on the other hand couldn't be dropped as one example because we have direct experimental results of it, positive evidence for it in flying atomic clocks</p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>The speed of light is a measureable quantity.  Photons emitted simultaneously in a moving object's rest frame will not be recieved simultaneously.  In order for the eye or a camera to process the image of an object, it must essentially believe it is recieving the light simultaneously.  In the non-relativistic world it is interpreted as and appears to be simultaneous, but not at relativistics speeds.  Posted by derekmcd</DIV></p><p> I see why you talk about simultaneity, I never said the oncoming and catching up light beams should happen anywhere near the same time or anything like that, no correlation should be looked for between them, all I am saying that the SRelativistic machinery used to explain why an astronaut (an observer in moving frame of reference) measures the speed of oncoming light c (length contraction and time dilation) is not valid if used to measure another light beam, say a week later and which comes from behind - all I am saying is that in that case astronaut would need his ruler to expand and clock to speed up for him to measure speed c, which are both nonsense effects indeed (they are contradictory because the length can't be both contracted and expanded, ditto for clock speeds) and any theory that would require such is not a valid theory</p><p> </p><p>given I discussed this issue at length on this forum before and few people joined the discussion then as now, you'd think if I were making an error (not even some major error as this example would seem to be if I am not correct) they would all jump in to correct me and laugh me out, but that didn't happen then and is not happening now, why do you think all these people here who according to DrRocket know their physics so well are not jumping in to squash me and show how ridiculous I am and get rid of me once and for all, making me to crawl under nearest rock in shame </p><p> </p><p> </p><p>when I talk about this spaceship example, it has nothing to do with the twin argument, those I talk about entirely separately, also when I say astronaut measuring light speed I mean the guy was born some couple hundred years ago and was transported to our time and is still curious how light can have constant speed even when measured in a moving vehicle, like rocket say, benighted as he is he doesn't believe all that tripe from our professors about speeds not adding up simply at speeds near light speeds, when I say measuring light, I mean measuring how fast it moves, nothing more and nothing less </p> <div class="Discussion_UserSignature"> </div>

DrRocket · Aug 18, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>in that case why is the muon example always given as you presented it in your post - only with reference to length contraction or time dilation, depending on reference frame view taken I suppose SR sees the mass increase as real but it doesn't seem to do anything with it except to say that when you accelerate you should feel extra resistance due to it,[/QUITE]</p><p>The reason that mass increas is not discussed in the muon example is because it is not relevant to the issue of time dilation and lengtht contraction that are being illustrated.  SR does make use of mass increase.  It is reflected in energy via the famous equation E = mc^2.  It is the basis for determining kinetic energy in SR, as follows.  Fix an inertial reference frame.  The energy of a body at rest is m0c^2 where m0 is the rest mass.  Now put that same body in motion and the energy is mc^2 and here m is the relativistic mass that increases as velocity increases.  So the energy due to motion is mc^2 - m0c^2 and that is an experimentlaly determinable number.  In fact that is exactly what is involved in determining the energies that will be involved in the collisioins in the LHC experiments that everyone is anticipating.  </p><p> </p><p>Added in edit:  Here is link to a complete explanation with the calculations worked out.  http://en.wikipedia.org/wiki/Twin_paradox</p><p> </p><p> Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I don't know that anybody ever thought of taking this mass increase into account in any other way, such as in that muon example, surely such effect if real and I agree it is real should have some effect on 'aging', that is on time dilation and thus on particle decay time - particle mass increase due to relativistic velocity should definitely increase (prolong) its standard lifetime, that is decay time (I expect to be made co-author of paper if anybody should publish this idea, a work based on it to be precise,</DIV></p><p>The mass increase is not directly related to the issue ot time or of particle decay time, except that both are covered by the Lorentz transformation.</p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> I also happen to know a 'mechanism' responsible for mass increase from which it is more plain how it all works and why particle decay time lengthens because of it, so I better be brought in on it)  I have never seen in the muon example (which I think is considered as one of the first observations that experimentally confirmed relativistic physics as time dilation and length contraction are concerned) any reference to relativistic mass increase, it is as if such thing doesn't exist or perhaps it was taken to be of no (physical) consequence, one can only speculate if that was innocent oversight or if it was intentional, either way it doesn't put SR in good light I should thinkI believe one can understand muon decay example without those unphysical 'perspective shenigans' of SR but only if one admits that space is real, absolute, that moving in it can result in real physical consequences but this view physicists try to avoid at all costs, even the cost of wilfull blindness - to be fair, the consequences of admiting the idea of absolute space are so revolutionary and far reaching that one naturally tends to shrink from it, the more so the more one realizes what fundamental changes its admiting would imply and that one has no idea how to implement them even if one dared to trod this high path which by definition is reserved for genius grade journeymen</DIV></p><p>Absolute space and time were abandoned by Galileo.  Before that people believed in that concept.  Since that time it has been shown that considering motion to be relative provides a more accurate description of nature.  All attempts to identify a reference frame for absolute motion have resulted in utter failure and the conclusion that none exists.  You can still postulate such a frame, but the Lorentz transformation emasculates that concept and leaves that frame with nothing to do.  So, not only do most physicists consider that it does not exist, if there were to be suich a frame it would have no effect on physics given the manifest validity of the Lorentz transformation.  The Lorentz transformation forces the speed of light to be constant in all reference frames, even if there were a frame of absolute rest.  After that you simply get special relativity back again.</p><p> </p><p> Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I know well that acceleration is the domain of GR but part of the traditional SR canon is the use of relativistic mass increase due to uniform velocity as reason that one can't accelerate matter to speed of light because  the mass (its inertial resistance to acceleration) would increase beyond all bounds as one approached the speed c and so one would have to overcome infinite inertial resistance... so in that area SR touches upon acceleration isn't it strange that the special relativistic mass increase due to uniform velocity seems to play no part in physics unless one talks about acceleration, reason is that relativity theory should be trully relativistic which means there shouldn't be any lasting physical changes happening due to uniform motion - it should all be trully a matter of perspective like in a perspective due to distance that we all know from daily life - on a grassy plain two people walk a mile appart and both then claim to see the other as small but both know that is just matter of perspective and know when they meet again at short distance that they will be both the same (original) height - SR theory should idealy be like that but it didn't end up being like that and it seems to me that those bettraying evidences (real lasting effects due to uniform motion) to relativity are downplayed or ignored and otherwise treated as some unwanted relatives (black sheep of family) by those who understand what is at stake here  that twin experiment is for SR a whistle blowing event signalling something is fundamentally wrong here as it would be if the two men of similar body height on a plain agreed to test the visual perspective by not walking appart as before but one of them sitting down and waiting while the other walked a mile away and then returned to the sitting man and found himself really shorter upon return and the explanation would be put forward that the walking away and returning man was the one who really was moving and gaining distance and therefore it is no wonder he became shorter - that's the same like twin experiment is explained by saying that the younger twin was the one who was 'really moving' because he was the one who accelerated (when setting on the journey, turning around and then stopping upon return)if space was not something physical, then it should make no difference who was moving and who was not and one couldn't get older or younger at different rate or whatever because it should all be perfectly symmetric like the two guys on that grass plain in a plain perspective experiment </DIV></p><p>You are still misunderstanding the explanation. In the twin paradox, both twins are really moving.  But only one is in an inertial reference frame, the one left behind.  The twin on the trip is also moving, but he clearly experiences acceleration, most importantly at the point at which he stops, turns around, and begins the return trip.  So since special relativity only applies in an inertial reference frame, the only perspective in which you can hope to apply SR is the perspective of the twin who remained on earth.  To handle the problem properly you really need to use general relativity, and that supports the conclusion that the twin who took the  trip will be younger.</p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>in SRelativity theory nobody or nothing should ever be said to be 'really moving' if it wants to live up to relativity, relativity means it should all be just a matter of perspective same as in the perspective effects that even ancient people knew, what Gallilean relativity was all about (that it didn't matter whether you were on shore or on a shipboat moving along the shore)      <br />Posted by vandivx</DIV></p><p><br /> And in SR it is the case that you don't say that one reference frame is really moving and one is not.  That is correct.  But the twin paradox is not, strictly speaking, a problem in special relativity.  The explanation for what occurs is as explained earlier, but it in no way should be interpreted as saying that one twin is really moving and the other is not.</p> <div class="Discussion_UserSignature"> </div>

Mee_n_Mac · Aug 18, 2008

<p>Can we go back to Van_D's reformed twin paradox example ?  Jim and Bob in identical spaceships (powered by Toyota's Relativity Drive C102a engines) leave Earth at the same time and side-by-side accelerate to 0.866C.  They then coast.  Jim looks over towards Bob's ship and, through the portholes, watches the clock on the wall. He notes that the femto-second hand on Bob's clock moves at the same rate as the clock as his does.  Bob then decelerates while Jim coasts on.  Jim then decelerates the same a Bob did.  When Bob's clock strikes noon, Jim's still has a ways to go. (At least that what I think Van_D had in mind).  So now, to me, the question is back to one of simultaneity in 2 spatially different locations. </p><p> </p><p> </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>

DrRocket · Aug 18, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Can we go back to Van_D's reformed twin paradox example ?  Jim and Bob in identical spaceships (powered by Toyota's Relativity Drive C102a engines) leave Earth at the same time and side-by-side accelerate to 0.866C.  They then coast.  Jim looks over towards Bob's ship and, through the portholes, watches the clock on the wall. He notes that the femto-second hand on Bob's clock moves at the same rate as the clock as his does.  Bob then decelerates while Jim coasts on.  Jim then decelerates the same a Bob did.  When Bob's clock strikes noon, Jim's still has a ways to go. (At least that what I think Van_D had in mind).  So now, to me, the question is back to one of simultaneity in 2 spatially different locations.    <br />Posted by mee_n_mac</DIV></p><p>Now you have two reference frames that have clearly undergone acceleration.  Neither is an inertial reference frame.  I think at this stage that you need the full machinery of general relativity.  <br /></p> <div class="Discussion_UserSignature"> </div>

derekmcd · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>and I don't understand how you conclude I am talking about viewpoint of photon</DIV></p><p>I'm not talking about the view point of the photon.  Photon's don't really have a reference frame as they are travelling at C.  They experience no time.  My reference of "a photon chasing an astronaut" is all one reference frame.  I don't understand how that reference frame requires length "expansion". </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I meant viewpoint of observer stationed on Earth for example in the classical setup typically used to explain basic principle of SR (how it is that one measures speed c in moving frame of reference, such as onboard of spaceship) to begining students in simple terms, what's so difficult about it</DIV></p><p>I did explain it.  Maybe you missed it.  It's the rather lengthy post I submitted 5:26 am on 18 Aug 08. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>- shrinken ruler and time dilation are used to explain why the astronaut measures speed c of oncoming light and what I am saying is that one would need precisely the opposite effects to explain why astronaut would measure speed c for light catching up with him from behind</DIV></p><p>No, they are not.  Length contraction (time dilation is nothing more than a consequence of contraction) is used to describe the lack of simultaneity of events between two reference frames.  This can be seen in the mathematics of the Lorentz factor, spacetime diagrams, and real life physical comparative observations.  Nothing, whatsoever, to do with the constancy of the speed of light in different reference frames.  The constancy of C can be used to measure contraction, but it not a result of it.</p><p>This is why I don't understand you contention that length "expansion" is required to explain what you are describing. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> - which case is never talked about in the discussion of length contraction & time dilation I suppose you could look up that thread in which I had long disccussion of this issue from all angles with Speedfreek (I think or somebody else), it was sometimes last year on old forum still, posts will likely have been chopped in length courtesy of our forum software, so results may vary</DIV></p><p>It's never talked about because it make no sense.  I've discussed length contraction often on SDC and have never seen anything close to what you are postulating.</p><p>I have no intention of looking up an old thread... The debate is probably much the same as this one if I know SpeedFreek. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I am talking about thought experiments like when one says - If you could see in god's eye like setup spaceship in flight and astronaut and his measuring apparatus represented by a measuring stick, you would see they contract ... that kind of illustrative but unreal example that is being used to explain basic principles of relativity and when one doesn't take into account that nobody can ever see anything like that, nor take a photo of it, that's why I say you seriously misunderstand me to the point of evasion, I mean everybody who ever been around physics has heard of such thought examples at one point or another, for christ's sake</DIV></p><p>My apologies.  I took your "seeing contraction" literally.  I guess it is because Special Relativity that includes such things as length contraction and the twin paradox are no longer thought experiments.  They are a physical reality.</p><p>It certainly wasn't an evasion tactic as I believe I answered your supposed paradox quite thoroughly. </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I mean if you want to be so realistic, we can drop all that talk about twins because on those terms which you seem to want to discuss physics that is all one huge nonsense of course, nobody can determine differential aging of people (their organism) due to traveling on rockets...so we can just drop any such discussion</DIV> </p><p>Physics nonsense???  Can't determine differential aging (time dilation I assume you mean)?  Huh?  What the heck are you talking about? </p><p>http://en.wikipedia.org/wiki/Hafele-Keating_experiment</p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>space contraction has never been measured directly, it is always inferred, not sure what you mean by 'experienced', it is not real in the sense of being determined in positive fashion, it is only needed to explain SR effects because the theory relies on it, is built on it (in part) but there is no hard evidence for length contraction and if the theory should change so as not to require it, it would and could be dropped and that would be it, time dilation on the other hand couldn't be dropped as one example because we have direct experimental results of it, positive evidence for it in flying atomic clocks<br /> Posted by vandivx</DIV></p><p>You are really starting to lose me here, VanDivx.  While I agree that contraction has never been physically measure, it is mathematically sound and has been verified through time dilation experiments using the constancy of C.  If length contraction is wrong, then time dilation is wrong... They are interconnected.  Basically one and the same.  Timee  dilation is a direct result of contraction.  You can see this when applying the concept to spacetime diagrams.   </p><p>I just noticed you recognize the experimental test for time dilation that I posted a link to above.  However, in your quote above you claim no evidence for "differential aging (their organism)".  Are you suggesting that time slows down for the traveller, but their body continues to age at the same rate?  Please don't say that's so!!!  </p><p>If you actually believe that, you have just crossed over into the realm of crackpots.  How could one possibly suggest that atomic clocks can show a slowing down of time, but the atom within ones body are exempt?</p><p> <br />I skipped the rest of you post as I've already addressed it or is irrelavent to the discussion.</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>

derekmcd · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Can we go back to Van_D's reformed twin paradox example ?  Jim and Bob in identical spaceships (powered by Toyota's Relativity Drive C102a engines) leave Earth at the same time and side-by-side accelerate to 0.866C.  They then coast.  Jim looks over towards Bob's ship and, through the portholes, watches the clock on the wall. He notes that the femto-second hand on Bob's clock moves at the same rate as the clock as his does.  Bob then decelerates while Jim coasts on.  Jim then decelerates the same a Bob did.  When Bob's clock strikes noon, Jim's still has a ways to go. (At least that what I think Van_D had in mind).  So now, to me, the question is back to one of simultaneity in 2 spatially different locations.    <br /> Posted by mee_n_mac</DIV></p><p>I'm not sure what your question is.  Jim still having a way to go before reaching noon is an entirely consistant statement.  This really doesn't have much to do with simultaneity unless there is another event going on.</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>

derekmcd · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Now you have two reference frames that have clearly undergone acceleration.  Neither is an inertial reference frame.  I think at this stage that you need the full machinery of general relativity.  <br /> Posted by DrRocket</DIV></p><p>If I'm not mistaken, I think SR is entirely consistent when concerning time dilation.  The equivalence principle is an extension of SR used to describe time dilation in a accelerated reference frame.  I understand that GR is the only way to test it, but it was postulated to describe events within special relativity.  Accelerated reference frames under the equivalence principle are using a pseudo gravitational force and, technically, GR doesn't address this.  <br /> </p><p>And yes, I know gravity is also really a pseudo force, but you get my drift. <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /></p><p>I think even the relativistic doppler shift and gravitation time dilation are addressed as being within the framework of SR. </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>

vandivx · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>There is experimental evidence for length contraction.  Length contraction and time dilation are two sides of the same coin.  </p><p>If you accept only two premises: 1) that the laws of physics are the same in all inertial reference frames and 2) that the speed of light is the same in all inertial reference frames, then on the basis of mathematical logic alone you must accept the Lorentz transformation that is derivable from those two assumptions.  With the Lorentz transformation you get all of the features of special relativity including length contraction, time dilation, relativity of simultaneity, etc.  You cannot accept only part of the package, it either the whole thing or nothing.  <br /> Posted by DrRocket</DIV></p><p>there is no 'direct' experimental evidence for length contraction, it is all only inferred evidence, indirect evidence, direct evidence is totally beyond our capabilities, the length contraction in muon event is wholy theoretical thing, SR is simply built on it and so it interprets the muon event that way, length contraction is thus theory driven inferrence (interpretation) as opposed to direct experimental determination </p><p> of course experiments typicaly use some form of indirect measurement, it is fairly rare that something is determined directly, but that is still very much different from wholy theoretical inferrence which is the case here - one example of direct confirmation would be enacting the event called 'pole and barn paradox' for example but as I said anything like that is out of question and there is in nature no such similar setup that we could observe in which moving and 'stationary' length of some objects could be directly or even indirectly compared  </p><p><br />I will try to say something more regarding 'the whole package acceptance' since I understand only too well this integration of the theory you talk about, that's why I said that the theory will have to be reworked, clearly it can't 'give up' length contraction without some fundamental change</p><p>SR was built to deal with the unexplained and deeply puzzling experimental fact that light has the same speed in all reference frames and this fact was accepted as postulate and SR was built upon it, length contraction was one of the new features that were needed to deal with this fact </p><p>IF now it was found out why the speed of light is constant, one would expect SR theory to undergo some serious change but it would survive in some form, clearly some such theory is needed since we can't go back to Gallilean and Newtonian physics</p><p> </p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> 1) that the laws of physics are the same in all inertial reference frames </p><p> Posted by DrRocket</DIV></p><p> </p><p>I think this principle may need some expanding, in that Twin experiment the laws of physics are indeed the same in the frame of both twins in the sense of each of them regarding his own reference frame -  but the time in the frame of the traveling twin clearly passes at different rate !in absolute terms! relative to the rate in the frame of the stay at home twin</p><p>clearly the principle should change if the twin experiment is right (if motion at uniform velocity has effect on time rate) </p><p>absolute space means that laws of physics will depend on the motion in it, in each moving frame one should still be able to do the physical experiments in the same way and with the same results as in any other frame but only if one confined the experiments solely to his frame (such as ignoring measuring CMB radiation, that I think would reveal to him his motion in absolute space as this measurement would come out depending on the frame motion), taking it as rest frame, and not comparing his results with those coming from other frames moving relative to his own - then upon comparison with other frames moving relatively to his own he would find out for example that his clock must have run at different rate which fact would make his frame exclusive and different from other frames (unless they happened to move at the same speed in absolute space)</p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>http://en.wikipedia.org/wiki/Principle_of_relativity</p><p><strong>special principle of relativity</strong> states that physical laws should be the same in all inertial reference frames </p><p>The principle forces physical laws to be the same in any vehicle moving at constant velocity as they are in a vehicle at rest. A consequence is that <em>an observer in an inertial reference frame cannot determine an absolute speed or direction of their travel in space; they may only speak of their travel relative to some other object. </em></DIV><em><br /></em></p><p> italics are mine</p> <div class="Discussion_UserSignature"> </div>

DrRocket · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If I'm not mistaken, I think SR is entirely consistent when concerning time dilation.  The equivalence principle is an extension of SR used to describe time dilation in a accelerated reference frame.  I understand that GR is the only way to test it, but it was postulated to describe events within special relativity.  Accelerated reference frames under the equivalence principle are using a pseudo gravitational force and, technically, GR doesn't address this.  And yes, I know gravity is also really a pseudo force, but you get my drift. I think even the relativistic doppler shift and gravitation time dilation are addressed as being within the framework of SR. <br />Posted by derekmcd</DIV></p><p>If that is true then I would appreciate an explanation.  So far as I know SR is applicable only in the absence of either gravity or (equivalently) acceleration.  There are some approximations that one can make, as in the typical resolution and explanation of the twin paradox, but only when the effects of acceleration are negligible in at least one pertinent reference frame.  It SR can explain time dilation due to gravitation I have never seen such an explanation.</p> <div class="Discussion_UserSignature"> </div>

Mee_n_Mac · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I'm not sure what your question is.  Jim still having a way to go before reaching noon is an entirely consistant statement.  This really doesn't have much to do with simultaneity unless there is another event going on. <br />Posted by <strong>derekmcd</strong></DIV><br /><br />To be honest I didn't have a specific question but rather was trying to steer the discussion back to a point Van_D had raised earlier that I'd like to see hashed out more thoroughly.  Van_D was trying (I think) to even out any nonsymmetry between the twins by using this variation of the problem.  Let me put a detail into our gedankenexperiment.  Lets have Jim continue on for 1 hour (by his clock) after Bob starts his deceleration and then do his own, matching, deceleration.  When all is said and done both Jim and Bob are at rest wrt each other but displaced in space along the one dimension they traveled in.  When Bob's clock strikes noon he sends a photon towards Jim. What time does Jim's clock indicate when that photon arrives ?  Can we agree that any time dilation due to acceleration happened equally for both Jim and Bob and so GR doesn't factor into the answer ?</p><p> </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>

DrRocket · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>To be honest I didn't have a specific question but rather was trying to steer the discussion back to a point Van_D had raised earlier that I'd like to see hashed out more thoroughly.  Van_D was trying (I think) to even out any nonsymmetry between the twins by using this variation of the problem.  Let me put a detail into our gedankenexperiment.  Lets have Jim continue on for 1 hour (by his clock) after Bob starts his deceleration and then do his own, matching, deceleration.  When all is said and done both Jim and Bob are at rest wrt each other but displaced in space along the one dimension they traveled in.  When Bob's clock strikes noon he sends a photon towards Jim. What time does Jim's clock indicate when that photon arrives ?  Can we agree that any time dilation due to acceleration happened equally for both Jim and Bob and so GR doesn't factor into the answer ?  <br />Posted by mee_n_mac</DIV></p><p>If you start the clock after Bob has completed his deceleration then you can work the problem in Bob's reference since it will be inertial.  But it you do as you say and start it before he starts the deceleration, then neither Bob's nor Jim's reference frame is inertial, and you need to go to general relativity.  The problem is complex precisely because you are trying to use special relativity when you have no reference frame in which is applies.  And general relativity problems can look simple and still be complicated.  I'm not sure how to handle this one.  </p><p>That gets back to the problem that you alluded to earlier, simultaneity of events that are spatially separated.  That makes sense in special relativity, where space-time is flat and where time is the same at all spatial points within a single reference frame.  The spatial variation in time in SR requires relative velocity to be non-zero.  But in general relativity that is not true.    </p><p><br /><br /> </p> <div class="Discussion_UserSignature"> </div>

DrRocket · Aug 19, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>To be honest I didn't have a specific question but rather was trying to steer the discussion back to a point Van_D had raised earlier that I'd like to see hashed out more thoroughly.  Van_D was trying (I think) to even out any nonsymmetry between the twins by using this variation of the problem.  Let me put a detail into our gedankenexperiment.  Lets have Jim continue on for 1 hour (by his clock) after Bob starts his deceleration and then do his own, matching, deceleration.  When all is said and done both Jim and Bob are at rest wrt each other but displaced in space along the one dimension they traveled in.  When Bob's clock strikes noon he sends a photon towards Jim. What time does Jim's clock indicate when that photon arrives ?  Can we agree that any time dilation due to acceleration happened equally for both Jim and Bob and so GR doesn't factor into the answer ?  <br />Posted by mee_n_mac</DIV></p><p>Let's think about this.  Joe is sitting at Bob's landing site, and watches the whole thing. Joe is in an inertial reference frame, so we can apply SR.   So he sees Bob decelerating and landing and Jim continuing on for a while.  So relative to Joe, Jim spends longer at high speed and ages less, just like in the normal twin "paradox".  But once he has stopped, Bob and Jim see the same thing.  So Bob has aged less than Joe and Jim has aged less than either of them.  The key here is that Joe is in an inertial reference frame and either of the other two are, so as in the twin paradox, what appears to be a symmetry actually is not.  The issue is that special relativity requires that one single out a preferred class of reference frames in order to apply the theory at all.</p> <div class="Discussion_UserSignature"> </div>

DrRocket · Aug 19, 2008

<p>There is a key point here that I probably have not made clear, and I think it is worth making.  So here goes.</p><p>Newtonian mechanics and special relativity are both theories of mechanics that require an inertial reference frame.</p><p>In Newton's theory we have that objects in uniform motion stay in uniform motion unless acted upon by an outside force. And we also have F = ma.  So what is an interial reference frame?  First note that if we find one inertial reference frame, then any other reference frame that is in motion with respect to the first one at a constant velocity is also inertial.  But in fact any other reference frame that is not moving at constant with respect to the one known to be inertial is not inertial.  For if it were, a particle "at rest" in that frame would be accelerating in the first frame without benefit of a force, unless you simply invent a force.  That is what happens with  coriolis force and with centrifugal force.  So there is only one possible family of inertial reference frames and if can identify one you can identify all of them -- evrything in constant uniform motion relative to the first one, and hence relative to one another.</p><p>So then we as what is a Newtonian inertial reference frame?  Answer:  A reference frame in which Newton's laws of motions hold.  That, you have no doubt noticed, is not a very "physical" definition and doesn't help much to locate one.  But unfortunately that is the situation.  We typically assume that a reference frame tied to a point on the surface of the earth is inertial.  That works pretty well over small distances, and for objects at low velocities, but if you want to solve a problem involving say an artillery shell you have to consider coriolis which simply means that the chosen reference frame is not really inertial.  The typical answer to a question of what is inertial, given in an engineering dynamics class is a reference frame "in uniform motion relative to the fixed stars".  Now that works as a pretty good approximation in engineering calculations, but you can also see that it just doesn't work as an absolute definition.  There are no fixed stars.  So, from a completely rigorous perspective an inertial reference frame is one in which Newton's laws work.  There may not actually be any such reference frame, and there probably isn't.  But it is a good approximation in many cases and it is used successfully for many purposes.</p><p>Now what is an inertial reference frame for Special Relativity ?  It is an inertial reference frame in which Special Relativity holds,  Generally reference frames that are good approximations to the Newtonian ideal are also inertial for Special Relativity.  But the bottom line is that Special Relativity is a theory predicated on operating in an inertial reference frame.  You don't have ad hoc corrections like coriolis readily available to patch things up when your reference frame breaks down.  The patch is general relativity.</p><p>You can make approximations in Special Relativity, and that is what is done in the classical resolution of the twin paradox.  There you start with a reference frame that is known to be inertial, and you work the problem out in that reference frame.  And that is the resolution.</p><p>The important thing to recognize is that in Special Relativity, just as in Newtonian Mechanics, not all reference frames are created equal, and accelerated motion of one reference frame relative to another is NOT symmetric.  Only one of those frames can possibly be inertial.  The inertial reference frame does hold a preferred position.  So in the case of the twin paradox the twin that stays behind is, simpy by assumption, in an inertial reference frame and the twin on the trip cannot be.  The situation is not symmetric.  And it is not symmetric because one reference frame is painted red (for inertial) at the start of the problem and therefore the second one is not an allowable reference within Special Relativity.</p> <div class="Discussion_UserSignature"> </div>

vandivx · Aug 20, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>a reference frame "in uniform motion relative to the fixed stars".  Now that works as a pretty good approximation in engineering calculations, but you can also see that it just doesn't work as an absolute definition.  There are no fixed stars.  So, from a completely rigorous perspective an inertial reference frame is one in which Newton's laws work.  There may not actually be any such reference frame, and there probably isn't.  But it is a good approximation in many cases and it is used successfully for many purposes.<br /><br /> Posted by DrRocket</DIV></p><p>agreed, there are no fixed stars but the reason this star referrential system works in practical use is twofold: one is because for all practical purposes the stars don't move from our distant perspective (which is where engineers get satisfied) and second deeper reason which is of interest to physics is that stars in their motions typically deviate from the absolute rest in space only in terms of hundreds of mph with some exceptionally fast ones moving (I think) at thousands mph at most (which is nowhere near relativistic speeds and given the distance, such small speeds can be taken to be negligible) and if we took large enough star sample and took the average of their velocities we would find that overall the stars are at rest in absolute space and thus are valid choice of most fundamental inertial reference frame</p><p>of course for such view one has to accept the idea of absolute space, also there is good reason for star motion not deviating much from the mean speed which is rest in absolute space but argument for that would require presenting full theory which I didn't give and so this comment should be taken only as sampling of some of its results and I am not ready to argue the point here, I can only say that on this point (that stars may be the ideal inertial reference frame) physicists typically come close to the truth but are not able to arrive at rigorous definition because their physics are incomplete and or mistaken (as far as absolute space is concerned in this particular case)  </p><p> </p><p>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> ...</p><p> in the case of the twin paradox the twin that stays behind is, simply by assumption, in an inertial reference frame and the twin on the trip cannot be.  The situation is not symmetric.  And it is not symmetric because one reference frame is painted red (for inertial) at the start of the problem and therefore the second one is not an allowable reference within Special Relativity. <br /> Posted by DrRocket</DIV></p><p>isn't the 'twin on the trip' in the inertial reference frame during that part of the trip when he is coasting at uniform speed relative to the 'stay at home twin'? </p><p> </p><p> </p> <div class="Discussion_UserSignature"> </div>

vandivx · Aug 20, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> to steer the discussion back to a point Van_D had raised earlier that I'd like to see hashed out more thoroughly.  Van_D was trying (I think) to even out any nonsymmetry between the twins by using this variation of the problem.  Let me put a detail into our gedankenexperiment.  Lets have Jim continue on for 1 hour (by his clock) after Bob starts his deceleration and then do his own, matching, deceleration.  When all is said and done both Jim and Bob are at rest wrt each other but displaced in space along the one dimension they traveled in.  When Bob's clock strikes noon he sends a photon towards Jim. What time does Jim's clock indicate when that photon arrives ?  Can we agree that any time dilation due to acceleration happened equally for both Jim and Bob and so GR doesn't factor into the answer ?  <br /> Posted by mee_n_mac</DIV><br />yes, I believe that if one makes both twins undergo the same acceleration that it gets cancelled out (the effect of time dilation due to it will remain but will affect both in the same measure and so can be taken out of 'equation' and we are left only with bare time dilation due to motion at uniform velocity</p><p> </p><p>improved twin experiment (which gets rid of some further extraneous infuences) has them start somewhere in interplanetary or even intergalactic empty space and to 'mark' the starting 'home point' in empty space they can either take triangulation from stars and or galaxies or they can leave a buoy floating around to which they will eventually return</p><p>the twins accelerate each in his own rocket (alongside each other) for some time to reach a given traveling speed at which point one of them - the 'stay at home twin' - starts decelerating untill he comes to a stop (relative to the buoy at the starting point they left there), he then turns around and accelerates again up to traveling speed and then declerates for the same amount which brings him to rest back at the starting buoy</p><p>the stay at home twin (now back at the starting buoy) thus at this point underwent four equal acceleration legs of the trip and aged less due to them, in the meantime the traveling twin coasts along for some time having experienced only one acceleration leg of the four so far, at the end of some given time he decellerates to come to a stop (relative to the buoy left at the starting point) and turns around and accelerates back to 'traveling speed' at which he coasts back towards the starting point where the stay at home twin waits for him, when he has coasted along the same time he spent on coasting on the way out he starts on final decelleration which brings him to a full stop at the buoy starting point </p><p>he also has accumulated four legs of acceleration, same as his stay at home twin, and so on account of them he also aged less, in equal amount as his twin brother and so this time dilation due to acceleration can be disregarded and what we are left with is the one accumulated due to the time spent on travel while coasting at uniform traveling speed  </p><p> </p><p>----------</p><p>I should add for completness that the 'coasting' part of the trip should be done at relativistic speeds (speed relative to the starting point buoy) and that is not just so that some appreciable time difference accumulates between the twins - because one could simply make the coasting part of the trip immensely long and accumulate time difference in that way even at relatively low speeds that are normally called non-relativistic speeds - any speed is really 'relativistic' when it comes to it because time dilation happens in our daily lives except in such small amounts that it is beyond our capabilites to measure -  but if it was done that way (low speed travel for a long time) there would be potential pitfall in that as I explain below</p><p>for illustration suppose that the buoy starting point would be floating in space as some speed relative to absolute rest in space, say moving at 100 mph relative to absolute space in +x direction, now if the traveling twin chose the coasting part of trip to happen also at 100 mph and happened to be traveling in the direction -x from the buoy, he would in effect be at rest in absolute space during his trip and the result would be precisely opposite of what would be normally expected - he would end up aging more (not less) than the stay at home twin and would be older upon returning than the stay at home twin </p><p>that is because the time dilation is a function of motion in absolute space, such motion is the physical reason behind why it happens (I won't mention 'mechanism' how it works but that should intuitively be clear enough that if space is physically real something, that motion in it should have some such effects on the energy and thus on time passage of particles of matter)</p><p>because the experiments that were done so far to test time dilation happened in fact at relatively low speeds, some of them may have fallen pray to the pitfall I just mentioned, however I don't know of any experiments done that involve travel of some distance along a line but rather they always take on the form of travel in circle like those clocks flown around the earth on a plane and in that case one forgoes such pitfall due to nonrelativistic motion in absolute space </p><p>also because the twin time dilation experiment is just a thought experiment, such pitfalls that could affect practical experiment do not affect thought experiments - the stay at home twin is by default always assumed to be at rest in absolute space in effect even when physicists don't know about absolute space and or don't count on it in any way, simply because the experiment assumes the stay at home twin to be at 'rest' or 'not moving', the one who is 'moving' then has some value entry for velocity in mathematical equations describing his part of the journey and so it is he who suffers time dilation and ages less</p><p> </p><p>however if one really were able to enact such an experiment in real life, one would have to insure that the coasting velocity of the traveling twin was well above the potential drift velocity in absolute space which would always be the case for velocities in relativistic range simply because your typical planet, star or galaxy undergoes always only non-relativistic motion relative to absolute space and can be taken as reference point at absolute rest in space with only some minor error </p><p> </p><p> </p> <div class="Discussion_UserSignature"> </div>

Mee_n_Mac · Aug 20, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Let's think about this.  Joe is sitting at Bob's landing site, and watches the whole thing. Joe is in an inertial reference frame, so we can apply SR.   So he sees Bob decelerating and landing and Jim continuing on for a while.  So relative to Joe, Jim spends longer at high speed and ages less, just like in the normal twin "paradox".  But once he has stopped, Bob and Jim see the same thing.  So Bob has aged less than Joe and Jim has aged less than either of them.  The key here is that Joe is in an inertial reference frame and either of the other two are, so as in the twin paradox, what appears to be a symmetry actually is not.  The issue is that special relativity requires that one single out a preferred class of reference frames in order to apply the theory at all. <br />Posted by <strong>DrRocket</strong></DIV></p><p>Your answer above is the one I'd arrive at as well. That said I still have a nagging doubt that I can't quite get my finger on. I'll try to lay it out so it may get resolved.  </p><p>First I think we can agree that any time dilation as a result of acceleration(s) has happened equally for Jim and Bob. Joe may notice an aging difference (between himself and Jim & Bob) as a result of acceleration (as well as velocity) but Jim, when comparing himself to Bob, won't (as a result of acceleration).</p><p>Second I hope we can agree that while undergoing acceleration you can't apply the Lorenzt transforms to get time in 1 reference frame translated into the other, but <strong>while the coasting is going on</strong> the frames are inertial.  The time relations for the overall trip can't be computed w/o GR but during the coasting they should be able to be computed using only SR.  So during the coast won't both Jim and Bob, each looking at the other guys clock, see the same time dilation (a factor of 2). Bob "sees" Jim as moving and Jim "sees" Bob as moving. Time dilation appears in either frame, depending on which you <strong>consider</strong> to be the moving one.   This is the part that bugs me because now I can't see how Jim comes out less aged than Bob.</p><p>I've got a knock on my door to go do actual work but I hope I've laid it out enough to be easily punctured.  It's the usual objection raised (I think) that may now have validity because the accelerations have been equalled out (something not true in the normal twin paradox).  What is now different is that Jim and Bob aren't co-located, which is the case for the normal twin paradox, and talking about time/age in 1 location isn't the same as talking about it in location #2. </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>

derekmcd · Aug 20, 2008

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>If that is true then I would appreciate an explanation.  So far as I know SR is applicable only in the absence of either gravity or (equivalently) acceleration.  There are some approximations that one can make, as in the typical resolution and explanation of the twin paradox, but only when the effects of acceleration are negligible in at least one pertinent reference frame.  It SR can explain time dilation due to gravitation I have never seen such an explanation. <br /> Posted by DrRocket</DIV></p><p>I may have overstated my case concerning gravitational time dilation.  Obviously, in the presence of mass curving spacetime, General Relativity applies.  My point is that GR deal exclusively with curved spacetime and non of these thought experiments we are dealing with apply.</p><p>Can Special Relativity handle Acceleration: </p><p>http://math.ucr.edu/home/baez/physics/Relativity/SR/acceleration.html</p><p>Twin paradox: Equivalence Prinicpl Analysis: </p><p>http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gr.html</p><p> </p><p>I guess what I was trying to say is that SR can handle acceleration and time dilation in Minkowski space via redshift and the equivalence principle without relying on the metric tensors of GR.  Obviously, you have to apply the Schwarzschild metric to real gravitational fields.</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>

Aaupaaq · Aug 20, 2008

We, I mean the "Inuit Elders" believed in cycles.  We don't get specific like "Sunday" or "11:00".  We understand that it will come the time to harvest.  Cache your catch, and store for the upcoming winter, as winter marks the time of scarcity of living things.  There's a time for everything, a time to catch fish, a time to catch beluga, and time to catch caribou.  All of them has different time frames, but some are overlapping each other.  Nice design of way of life. <div class="Discussion_UserSignature"> We always walked on water, like skating! </div>

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