JohnFromAus

What do you mean by "a consideration"? It seems to me time has the same qualities as the other dimensions and can be measured with appropriate instruments. What's the difference between measuring a length with a ruler and a time with a clock? Length is not an energy but I assume you are not denying that length can be measured.So I am assuming you believe length to be a "consideration" which can be measured. So what's the big problem about time? Are you also denying the fact that there will be a difference in the length of an object measured in its rest frame to that measured in a frame in relative motion?

Fact or not .... he has said it all !!! John

I now am aware of the folllowing - I would be interested to see how I score. 1 - an OBSERVER is not a person seeing something. Here an observer is an infinity of synchronized clocks at rest at each point in the reference frame. 2 - Inertial frames in relative motion are all equivalent ( maybe not the right word) by that I mean when A sees Bs clock running slowly B sees As clock running slowly - when A sees Bs rod shortened B sees As rod shortened. 3 - "Moving clocks run slow" means A (single) clock moving relative to an inertial frame containing an observer ( see 1 above) will be found to run slow when timed by this observer. ie we compare 1 moving clock with 2 stationary synchronized clocks. 4 - Classical physics makes the assumption that signals can travel at infinite speed - SR proposes a limit - c . Surely SR is more believable! 5 - The twins paradox - well I am not 100% with this yet but can understand that 1 of the twins is not in an inertial frame all of the time - otherwise he could never return. Also I have a problem relating this situation to 3 above. So there is more to it than just "clocks running slow"! However I dont intend to argue the point with Mr E or others - I accept the fact and am doing what I can to understand it - perhaps I never will! The book I have has many problems I can work through to see if I can get the answers and thus confirm that I am on the right track. So far I have only studied the chapter "Relativistic Kinematics" - still have dynamics and electromagnetism to go. John

I have just read this whole thread and can see that the "non believers" have the same problem I had. They just cannot accept time dilation as a fact. I had trouble with this too and still have some work to do to get a full understanding but I am sensible enough to know when I am wrong and inquisitive enough to want to find out what Im missing and enhance my understanding of this place we live in. Thank you to the "dilationists" for clearing up some of my misunderstandings and to the "non believers" for pushing them to the point of frustration and eliciting such clear answers!! John

Hey - I have thought a bit more and come to realize that my problem with the twins paradox was really a problem I had accepting time dilation as a reality. Once you can accept that the paradox is understandable. I have to do some more reading/study to get this all straight in my mind but I am getting there. It seems to be that the Newtonian view assumed that the speed of all movement was relative to something or other and that time and space were fixed. It turned out that this was a workable assumption in the everyday world but is really only an approximation to reality. The MM experiment showed that c is constant for all observers which then leads to time and space having to be relative. - something like that anyway. So Im back to my books for a while. Thanks to everyone for the discussion. John

This is all very interesting and helpful to me although I am still puzzled. Is it that perhaps somethings need a "better" language than english to express them - mathematics perhaps. I am always boggled by the human races sense of its own importance and the belief that we can and will one day understand everything! Anyway back to the topic. My reference for Special Relativity is a 1968 book "Introduction to Special Relativity" by Robert Resnick. Plenty of maths and physics - not a general interest level. Anyone have a different undergrad level text to recommend? I am always a bit suspicious of stuff on the net unless I know who wrote it or am recommended to it by some trusted authority - after all I could easily put up some articles of what I think! 1 yes you can detect acceleration but you cannot distinguish this from the effect of gravity 2 What still puzzles me (among a mass of other things) is ... well a thought experiment.... Twins are anaethatized(? cant spell well either) and one is then accelerated. When the acceleration stops they are both aroused. They now have relative motion and presumably would see each others clock running slower than their own. ( I hope!) . They are then KO ed again and brought back together by appropriate accelerations and then woken up. Who is the younger - I suppose its the one who was accelerated BUT he saw his brothers clock running slowly so would expect he would be the elder not the younger! Or would their be two of each of them! So that A could see a younger B and B a younger A! I think what Im asking is ...is time dilation an effect of gravity (acceleration) or of relative motion? But its really like what I used to tell my students - If I knew the right question to ask I would also know the answer! I really must find that book and have another read!! John

OK - I understand all that. What I was trying to establish was whether it was the relative motion that causes the clocks to run slower or the acceleration. No no no thats not the point Im trying to sort out. Let me try again. 1 Is the twins paradox actually true - if that happened would the one who experience the accelerations be younger? 2 What does 'be younger" mean??? Measured by whose time?

In the twins paradox one twin is said to move away and come back and be younger. But how can they tell which one is moving? They are moving relative to each other - sure - but surely then things would be the same from either point of view. Is the fact that one (or both) of them must have accelerated to get some relative speed and must then decelerate/accelerate to turn around and then decelerate to stop - relevant? Ie if the speed changes that occurred happened without the application of a force (ie by magic) would things be the same? I first read the special theory back in the 1950s when a student and have never really understood this point. It seem to me that motion of a body can only be relative to something. Lots of us have had the experience of being on a train stopped along side another stationary train. Relative motion starts and you have no idea if its your train or the other that is moving until you look for a reference. I am probably missing something obvious here!!! John