michel123456

To me, the one who keeps the rod in his hands knows better than the others. Does that mean that space contraction & time dilation do not actually happen?

I don't think it is correct. IIRC 25 is the age when you have top abilities and when you have the most neurons. But the brain has reached a full development at the age of 18. Even sooner . See diagrams of brain wheight at http://faculty.washington.edu/chudler/dev.html

Excellent. I don't expect you to argue that the small dots left & right are actually dancing like that just because an observer happened to travel accelerating rapidly? And I don't understand where is the answer to my question at the end of post #48.

Good question. At least you admit that WE are traveling. Time does nothing, WE do. It means that something happens inside us (us = anything material) and we don't understand anything of it, except that strange feeling of elapsing time. Thus, the question is not "what is time?" as if time was something mysterious out there, the question is "what is happening to us?".

If it is the only form of negative energy, and in order to cancel all other forms of positive energy, gravitational potential energy must be immense.

I guess Iggy's post was intended to make me change my mind... From the .pdf provided page 2: "The student in the study classified certain relativistic effects (including length contraction) as distortions of perception. Posner et al. report similar results in interviews with introductory students and their instructors." It seems I agree with the student. Woops, you are sliding from a subject to another. If we cannot even agree on what distance is, it is useless to jump into further considerations. That is something. O.K. distances in space are relative to reference frame. It means from some other FOR, the rod I have in my hand can be observed as contracted. Can you define some FOR where the rod in my hand is expanding?

I cannot accept this concept. You said before: Well, I am like "most people". To me, there is only one Reality, with a big "R". It is when the FOR of the observer is the same as the FOR of the object. The "right" length of the rod is the one measured by the observer who keeps the rod in his hand. All other observers, thousands of ligth years away, will make other measurements. The same goes with mass: the observer at rest with the rod will measure a specific, absolute rest mass, and another observer thousands of LY away will measure another mass. The difference between the 2 masses will be called "relativistic mass", and will be the result of a deformation of measurements together with length contraction & time dilation. As Spyman wrote, the rod does not contract, and as I say, its mass does not change. What change are measurements made by different observers from different FOR.

I have read several times from other forum members that length contraction "actually happens". I agree that it is all a question of measurements. But usually when I say that someone comes and hit my fingers. Discussion was about space & time. You talk about mass. (emphasis mine) Are you talking about rest mass? Because IMHO rest mass corresponds exactly to the "answer is right in some absolute sense".

I think it must be possible to show that the absence of anything is a physically impossible situation.

I agree with this description. This description corresponds to perspective, or optical deformation: the book intrisically does not change because someone somewhere is observing it. What is changing is the point of vue of the observer and his measurements of the phenomena "book". I suppose that some other people here will disagree, saying that following Relativity, the book "really" contracts. What happen with Relativity is that it is not an optical deformation (only), it is also a duration deformation, and a gravitational deformation. As a sum, it is a deformation of all measurements and as such, physicists may consider it is a physical deformation. But IMHO the book does not change and the rod does not contract.

On one side of the balance, reality, on the other side, dreams. Reality is somehow explainable, events happening in reality follow laws, mathematical, physical, cause & effect, etc. On the other side, the world of dreams is irrational, laws don't exist or are contradicted, bizarre cause strange effects, effects are produced without cause. 1. I really don't know why we should prefer something that is completely irrational (dreams) in order to explain something rational (reality). 2. When you dream, you still exist. Otherwise, as you asked, "what produces the dream?". Dreams have been studied. When you are asleep, you don't dream all the time. If you're interested, as a beginning there is a long wikipedia article. 3. In most case I can discern whether I am dreaming or not: when awake, it is not so difficult but when asleep there is some issue. When I am in a very strange situation, I usually says "it is a dream" and open my eyes. It happens when I want to run and my feet don't move, or when I want to push something and my arms are frozen. Or when I fly, or when I go back into the past, at school or at the army, or encountering some relative that has passed away a long time ago. In some way I have learned to insert rationality in my dreams and can escape at will. It does not work all the time though.

In order to get zero net energy, you need cancelation of positive energy with negative. Where is negative energy?

I agree on everything. I have the feeling that it is not the way Relativity is commonly understood. We should start a thread on this.

"An infinite number of mathematicians walk into a bar. The first orders one beer. The second orders half of a beer. The third orders a quarter of a beer. The fourth orders an eighth of a beer. The bartender says, Screw you! and pours two beers." thanks to Swansont's blog

Agree.

I am confused: Spyman wrote: (emphasis mine) And later: I don't understand anything. What do you mean? Is there only one reality, where " the rod does not actually change length", or a multiple "reality" where "the rod actually have a real different length in another frame". IMHO these 2 concepts of Reality are mutually exclusive.

Right. Never be afraid of mistakes and never feel embarassed. Just jump.

I wrote a lot of rubbish about how wonderful it is to learn languages, and I deleted everything. It is wonderful to know foreign languages, and very painful to learn. If you don't have any use of it, you will loose everything in a very short period of time. I know French (mother language), English (as an obligation), modern Greek (residence language), and I knew Dutch (12 years of learning) but I don't use it now than 25 years. Through French I almost understand Italian & Spanish. Through Dutch I should understand German but I don't. And I learned Latin for 6 long years that left nothing but painful memories. Too bad. My daughter knows fluent Greek, French, English, Spanish & basic German.

Let's take 3 bodies A B C _C moves away from A at 0,9 c. _C moves away from B at 0,1 c Observator 1 positioned on A looks at C and measures time dilation T1 & length contraction L1 for body C. Observator 2 positioned on B looks at C and measures time dilation T2 & length contraction L2 for body C Observator 3 positioned on C measures time dilation T3 & length contraction L3 for himself body C, where T3 & L3 equal to zero. Which of the 3 observators is right? What is "really" happening?

Mathematics do, but physics don't. Physics need distance to expand in order to explain observation.

This sum is equal to 1, isn't it?

If I am not mistaken, it is estimated that the Milky Way has made only 60 full rotations since the BB (and 59 since its birth). See Galactic year in wiki.

I expected another answer.

Somehow, although all evidences show it is a correct statement, logic shouts to me that it must be wrong. Indeed: we don't know that. From time to time I am surprised from your statements Spyman, and I really don't understand why we occasionaly disagree. I suspect I read in your text something that you didn't ment to write.

From the source of your images (Nasa site) (Emphasis mine) The number does not match your 26,000 years figure.