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"measuring light with light" was indeed an oversimplification, even a pun (probably not the best idea for a clear explanation). I just wanted to indicate the similarity of propagation of light and of all other electromagnetic phenomena. A light clock is indeed the simplest illustration of time dilation. If we accept that a light clock physically slows due to increased light path: we should admit that "something analogue" happens in any co-moving clock, if both continue to tick at the same rate.

Yes, for you as a physicist, that's obvious. But you won't succeed to explain the relativistic effects that way to laymen. So, what's wrong with the mechanistic view? Of course it's incomplete (a complete description would be very complicated and involve QM) and approximative (but your analogy of the geometric description is so too). But I don't think it's contradictory, nor exclusive, to the geometric description, and it can give an idea of what's going on in a way that is comprehensible for everyone. Even for teaching Relativity on undergraduate level a constructive approach can be prefered at the beginning, see Miller A constructive approach to the special theory of relativity .

In the original frame, where the object was at rest at the beginning, there was a certain equilibrium on it's structure. When it was brought into movement, it's structure changed and came to a different equilibrium. In the co-moving frame, where the object stays at rest, it's structure didn't change and the observed equilibrium is still the same as that observed in the original frame when the object was at rest, and different from the new observed equilibrium in the original frame.

Isn't the underlined part a contradiction in terms? I don't see a contradiction. I'm speaking of the equilibrium of the internal structure of the object. Yes, of course, in the moving frame the observed equilibrium will be different (and in fact the same as that of the object when it was at rest in the original frame). I mean the forces which hold the atoms together in an object.

If by this you are referring to length contraction, then this is a resounding NO! Length contraction is not the result of forces acting on an object There are no more (or less) forces acting when an object is moving. It's an equilibrium. Of course there was once another force involved, the acceleration.

The reason is that, when we measure the speed of electromagnetic waves (such as light) we are forced to use instruments based on electromagnetic interactions. In fact, we measure the speed of light with light. Also, light is used to define the units of time and space. Matter is made mainly of emptiness between the particles that are joined by electromagnetic forces. These forces propagate like light. The structure of an object results from the balance between forces in all directions. When the object moves, a new balance gets settled between the transverse and longitudinal forces, what results in a contraction in the direction of the movement. In the same way, when a clock is in movement, the distance the interactions must pass between the atoms constituting the instrument increases, and that slows down the internal movement. To the limit, if the clock reached the speed of light it would stop, since the electromagnetic interactions could not go from one particle to the other (but at the same time, of course, the distance between the particles would become zero). It is evidently valid not only for the measuring tools, but for any object (including our own body, which thus ages less when it is in movement). In other words, in a moving frame of reference, time dilates and lengths decrease.

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Maybe I was unclear. I didn't mean that SR was based on an absolute frame of reference. The explanations of the twin paradox are. https://en.wikipedia.org/wiki/Twin_paradox#No_twin_paradox_in_an_absolute_frame_of_reference

Sure we have, but how compatible are they with Einstein's theory, as they are based on the idea of an absolute frame of reference, and on the fact that the two observers are not fully equivalent, even during the inertial periods?

I agree with that. The point is that the sentence (in post #32) A doesn't just "appear" to run fast, it runs fast. is confusing. It would be true if A felt in a real gravitational field. Light signals sent during that period would also prove that A's time really accelerated. It's not the case with the pseudo-gravity. At the end of the trip, B must accept that, as the light signals show, his calculation of A's time change was not in accordance with reality. Another confusing sentence is: The pseudo-gravity field is uniform and extends to infinity. If the pseudo-gravity is supposed to have any effect, the field can't fill the entire space instantaneously as this would contradict the theory. In our example, the effect would reach A when he aged 27 years. But instead, pseudo-gravity has no effect at all. IMO, that leaves us with no explanation of the twin paradox from the viewpoint of the traveller in terms of Relativity Theory.

I think pseudo-gravity can't be used in this context. Consider the following situation: B (blue line) is moving away from A at 0.8c during 9 (of his) years, and then comes back at he same speed. They both send a picture of their clocks every year. Red are A's signals, green are B's. B will receive 3 pictures during the first part of the journey, and one every 4 months during the second part. He won't see a sudden advance of A's time due to his own acceleration.

Thank you Tim88 for this excellent presentation. I think it would be helpful to add a little clarification to this paragraphe: IMO length and time measurement are not affected by Absolute Space as such, but by the effect of electro-magnetic propagation. When an EM wave is formed in a moving system, the propagation of the wave is independent of the motion of the system. That means that the centers of all the EM waves stay in one single frame wherein the waves propagate as growing spheres. But since the shape of all the objects depends on EM forces which thus propagate independently, the moving objects (including rulers and clocks) are affected in such a way that in there own frame, the propagation of the EM waves will always appear to be isotropic and their measured speed will always be the same (=c), despite of any change in velocity of the object's frame.

If frame A and frame B are moving relative to each other, you can say that according to A, B's time runs slow, and according to B, A's time runs slow. At the same time, of course, A's and B's times run differently according to all the other frames. That makes no sense for me, but with an abstract concept as time, you can say that. But if one considers that the cause of the time slowing could be a physical effect on the clocks (and all other processes), one can't say that all the clock's are affected in a different way at the same time. If that was your point, then I agree. But that doesn't exclude the possibility of a physical effect of the change of velocity in the own frame.

Sorry, but when I accelerate, you can say that I stay stationary and make move the rest of the universe, but I don't believe that. If after the acceleration something changed with my observations, I think the change happened in my frame.

Of course you won't affect someone else's clock. Changing your velocity will affect your own clock. That's the principle of the twin's story, isn't it?

Okay, there are no rigid bodies, only particles that are (simply speaking) "held together" by EM forces? But when the body is moving, those forces don't participate on the movement. So the body will physically contract. Or am I wrong? (I know I'm over-simplifying, I'm just indicating a basic principle.)

It seems to me that a (begin of a) "physical reasoning" for real-length-contraction was given by Tim88: Celeritas said: The ad-hoc qualification is surely right when speaking of the theories around 1900. At that time, matter was considered to be (ideally) totally rigid, so there was no possible explanation for the length-contraction. But is that still true, regarding the modern view on particles, atoms and molecules?

You got it correct, except for the last sentence: when d catches up with b, it doesn't show anything about a. After being synchronized with a, d lost time wrt b. a[0] -> <- b[0] ---------------------------------------- <- b[1] a[1] -> c[1] ->> <<- d[1] ---------------------------------------- <- b[3] a[3] -> <<- d[2] c[2] ->> Note that, according to b, a will also lose time, but b will have to calculate that, or send EM signals to verify. And according to a, b will lose time. You will never find a contradiction, but I understand your search. Those concepts are frustrating, and SR doesn't explain. It's more: "Don't think about. It works, so accept it."

You are right, that statement is quite imprecise. I meant that, at some time, the car accelerated wrt the landscape, and that in my opinion, that change in motion has a physical sense. Yes, I understand that, but the role of the atmosphere is unclear to me.

I think that the reason why Relativity is more questioned than other physics subjects, is because it handles about things we can relate to our every-day life, and confronts us with the way we imagine the world. For me, both models (3D space and spacetime) are complementary. I wonder why physicists so vehemently reject the first. I understand that physicists feel more confortable with the 4D mathematical representation. But as a layman, I don't have the impression that I live a 4D reality. Time, space and motion are physical notions for me. When I'm driving my car, I know I am in motion, not the landscape. And if I hit a tree, I don't think it's the kinetic energy of the tree that crashes my car. Relativistic effects are much more understandable by thinking of a theoretical rest frame (maybe the frame wherein the vectorial sum of the velocity of all the particles of the universe is zero?). No ether is needed. For me, nature is not weird at all (as far as relativity is concerned). c-invariance, time dilation, length contraction etc are all very understandable. But, of course, I also see that SR, with the 4D Minkowski spacetime, is a much more powerfull model and must be prefered for physics work.

If B records less time between both events because he is flying from A to X, isn't that the B experience per A?

But it's not a mathematical problem, it's a logical issue. You can't say "do the math, forget the logic". I am impressed, I don't see an error in robinpike's reasoning. Nobody doubts about that. The problem is here: After the de-acceleration, B runs at the same rate as A, but also slow wrt to C, wich travelled with him and still runs slow wrt to A.

This is how I understand Langevin: I refer to Langevin's example at [40] of two objects dropped thru a hole in the floor of a car. Let's now consider two holes at some distance. The observer in the car carefully synchronized two clocks in order to drop the objects at the same time. ______________________ | | | | | c1 c2 | / \--- -------- ---/ \ \_/ o o \_/ If the car is moving, the observer on the ground notices that the clocks are not synchronized to him. Therefore the object at c1 is dropped before the other, and the distance on the ground between the objects is longer than the distance between the holes in the car's floor. ______________________ | | | | | | ----> / \--- --------o---/ \ ____________\_/___o____________\_/_____________________________________ ______________________ | | | | | | -----> / \--- -------- ---/ \ __________________o__\_/____________o___\_/____________________________ But if the observer in the car defined the distance between the holes as 1m by mesuring the time for a light signal going on and back in 2/300000000 sec, the observer on the ground will say for him that's less than 1m. That's the ruler contraction.

You are right, there is a logical difficulty. I can illustrate it with a little story: Two space travellers are moving toward each other. They want to know whose ship is the longest. They decide to do the following experiment: they place a clock at each end of the ships and each of them synchronizes its two clocks. They take place in the middle of the ship. When they will cross the other, they 'll throw a paint spot to the other ship on each end (This moment can be calculated in advance, so the two shoots can be done at the same moment). After that they will compare the impacts. But they are astonished about the experiment and they decide to meet to compare the spaceships. The two ships have the same length. Where are the paint spots? These are the two versions of the experiment as viewed by A and B: (Note that each of them consider himself at rest) There are two important facts: - the synchronization is done by light signals. Therefor, each observer considers that his clocks indicate the same time, but that the other's clocks are not synchronized (the front clock runs after and the back one runs forward wrt the middle of the ship). - the bodies are contracted in the direction of the movement. VERSION A VERSION B 1. A is at rest. B is at rest B's clocks are not synchronized. A's clocks are not synchronized. B shoots first at the back and A shoots first at the back and misses A. misses B. v====B====| --> |=====B=====| |=====A=====| <-- |====A====^ 2. B is smaller than A, so A doesn't A is smaller than B, so B doesn't hit B. hit A. |====B====| --> v=====B=====v ^=====A=====^ <-- |====A====| 3. Now B shoots at the front and Now A shoots at the front and misses A. misses B. |====B====v --> |=====B=====| |=====A=====| <-- ^====A====| 4. When they meet, the lengths are the same and the result corresponds with both 's experience (they both missed the other). |=====B=====| |=====B=====| |=====A=====| |=====A=====| The crucial moment is of course point 2. At the same time, A pretends that B is smaller, but B states the contrary. Are they both right? Obviously, both versions are consistent, so both can be true. But logically, the two versions are mutually exclusive, so they can't be true together, at the event E, which is the moment A and B are at the same place (point 2). At that moment, the reason why the spots miss the target is - either because one of the ships is smaller than the other (1) - or because the clocks are not synchronous (2) (1) can not be right for both ships, but (2) implies that the ship in which the clocks are not synchronized is moving, so at least one of the observers has to accept that he is not really stationary.

I agree on that, but not in the context of SR. The theory is not difficult to learn. The difficulties are rather ontological, but quite neglected in physics lessons.