VandD

Correct, but the ground observer does not feel the two balls simultaneously dropping from the car. One hand will feel one ball falling. Then he has to wait some time, and then he will feel the other ball falling. For the length contraction of the car the scenario is different (see my previous sketches). The green observer will feel simultaneously front and rear of shorter car. Measuring a shorter car is between different events than Langevin's scenario. Below I add a sketch for the Langevin scenario. In Langevin's scenario the ground observer doesn't observe the car as it is at one moment in time in his ground frame. Langevin's ground observer does a wrong measurement to measure/observe the car present in his ground frame of simultaneously occuring events.

Let's hope this will help: If you cut a loaf of '3D' bread, all '2D' slices are equally 'real'. But depending on the angle you cut, the slice will contain a different set of grains, with spatial distances between those grains. If a frame makes a 3D section through 4D spacetime, all sections are equally 'real'. But depending on the angle you cut (relative speed) you will have a different set of simultaneous events, and spatial distances between those events. A slice of bread at one angle is not less or more real than a slice at another angle. And the shorter slice is not a 'contracted' one of a slice at another angle, because the slices consist of different set of grains. Same thing with the 3D sections of simultaneous events through a 4D train (or car, or ruler!) spread out in 4D spacetime.

There is no misunderstanding. Only for time intervals including acceleration/deceleration the symmetry is disrupted.

Block universe doesn't need 'causation'. An event exists in 4D spacetime until it is discovered/read by an observer's reference frame.

Einstein believed in Block Universe: Karl Popper about his encounter with Einstein: << The main topic of our conversation was indeterminism. I tried to persuade him to give up his determinism, which amounted to the view that the world was a four-dimensional Parmenidean block universe in which change was a human illusion, or very nearly so. He agreed that this had been his view, and while discussing it I called him "Parmenides".... >> (Karl Popper, Unended Quest: An Intellectual Autobiography.Routledge Classics. Routledge. pp.148–150). Einstein quotes: << From a "happening" in three-dimensional space, physics becomes, as it were, an "existence" in the four-dimensional "world". >> (Albert Einstein. "Relativity: The Special and the General Theory." 1916. Appendix II Minkowski's Four-Dimensional Space ("World") (supplementary to section 17 - last section of part 1 - Minkowski's Four-Dimensional Space). << Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence. >> (Albert Einstein, "Relativity", 1952). <<...for us convinced physicists the distinction between past, present, and future is only an illusion, although a persistent one." >> ( Letter to Michele Besso family, March 21, 1955. Einstein Archives 7-245).

Special relativity tells you exactly where, why and how -relative to the home clock- the travelling clock ticks slower. What else do you want to know? - Wiki quote: <<Spacetime diagram of an accelerating observer in special relativity. The momentarily co-moving inertial frames along the world line of a rapidly accelerating observer (center). The vertical direction indicates time, while the horizontal indicates distance, the dashed line is the spacetime trajectory ("world line") of the observer. The small dots are specific events in spacetime. If one imagines these events to be the flashing of a light, then the events that pass the two diagonal lines in the bottom half of the image (the past light cone of the observer in the origin) are the events visible to the observer. The slope of the world line (deviation from being vertical) gives the relative velocity to the observer. Note how the momentarily co-moving inertial frame changes when the observer accelerates.>> https://en.wikipedia.org/wiki/Minkowski_diagram

Good job, Celeritas. One could draw a kind if diagram (not exactly a Minkowski diagram although it shows the same information) representing the traveler's path by a straight line (see right diagram on sketch). At traveler's proper time 4 he feels decelertion/acceleration, which makes his 3D reference frame (red on the sketch) rotate in 4D spacetime. It's a matter of relativity of simultaneity. Many people battle to understand this, even by visualizing it with spacetime diagrams!

About events, proper length, length contration, Lorentz Transformation, ... ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------

In that case please lock the thread "Relativity and shared realities". I like the "as a particular case"

If you mesure a length of an object, you have to measure between simultaneous extents of that object in the ground frame. If I ask you to measure a moving train you have to mark off the rear and front of the train simultaneously on your ruler on the ground. Ever thought about that? Or doesn't it make sense to you? Similar for measurement of the ground ruler done with a car ruler: the car passenger has to measure between two simultaneous mark-off events of the ground ruler moving by.

Sorry man, I only defend what Einstein says about his own theory! Furthermore; this is a thread about Special Relativity. "For discussion of problems relating to special and general relativity." No some pre-Einstein Lorentz theory that some try to keep alive.

I hope that you are fully aware of the fact that in above exercise the ground observer does not measure the car that's in ground frame of simultaneous events. The 'proper car events' are simultaneous only in the car's frame, hence they are not in the ground frame. (But at some time one event will be part of two frames: the event at the crossing of the two frames). The car in the ground frame is made of different events than 'proper car events'. That's the reason why in the ground frame the car is shorter, and the 1 meter between holes in car will be less than 1 meter on ground ruler. Why worry about the 1.5 meter measurement between events that are not simultaneous in ground frame?

It would be fair to call 'reality' everything that exists 'now' in your surrounding 3D space: all events that simultaneously occur 'now' (f.ex when I say the word "Now"). An event that does NOT occur simultaneously with "Now", is an event of a past 3D world existence or of a future 3D world existence. For me the car hitting the tree tomorrow is not an event of my present "now" 3D reality. Hence there is no damaged tree part of my present 3D space/world existenc. Because different observers consider a different 3D space of simultaneous events, they consider different 3D realities. But they are all only 3D sections through one and the same 4D 'reality: 4D spacetime, aka block universe. A few Einstein quotes: << Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence. >> (Albert Einstein, "Relativity", 1952). << From a "happening" in three-dimensional space, physics becomes, as it were, an "existence" in the four-dimensional "world". >> (Albert Einstein. "Relativity: The Special and the General Theory." 1916. Appendix II Minkowski's Four-Dimensional Space ("World") (supplementary to section 17 - last section of part 1 - Minkowski's Four-Dimensional Space). <<...for us convinced physicists the distinction between past, present, and future is only an illusion, although a persistent one." >> ( Letter to Michele Besso family, March 21, 1955. Einstein Archives 7-245). Karl Popper about his encounter with Einstein: << The main topic of our conversation was indeterminism. I tried to persuade him to give up his determinism, which amounted to the view that the world was a four-dimensional Parmenidean block universe in which change was a human illusion, or very nearly so. He agreed that this had been his view, and while discussing it I called him "Parmenides".... >> (Karl Popper, Unended Quest: An Intellectual Autobiography.Routledge Classics. Routledge. pp.148–150).

The car that is present in the ground frame is NOT the one that's in the car frame. The set of events of the car in the car frame is NOT the set of car events in the the ground frame. Hence the measurement of distance in the ground frame between two car events that are simultaneous in car frame is NOT what is in the ground frame. And you bet, because that length is LONGER than proper car length. The car in the ground frame is SHORTER. The car events that ground frame measures -to find out what car is in the ground frame- are NOT the ones simultaneously in the car frame. The ground that is present in the car frame is NOT the one that's in the ground frame. The set of events of the ground in the ground frame is NOT the set of ground events in the car frame. Hence the measurement of distance in the car frame between two ground events that are simultaneous in ground frame is NOT what is in the car frame. And you bet, because that length is LONGER than proper ground length. And you bet, because that length is LONGER than proper car length. The ground in car frame is SHORTER. The ground events that car frame measures -to find out what ground is in the car frame- are NOT the ones simultaneously in the ground frame. There is no way to understand this without a spacetime diagram. Why don't you try to draw one showing the events, lengths, and different frames? Use my rocket/muon diagram and make the atmosphere length the street ruler, and the rocket the car ruler.

An observer measures with a ruler AT REST relative to him. For the car passenger a ruler at rest in his car is never contracted for the car passenger (in his 3D frame of simultaneous events). And for the street observer a ruler at rest on the floor is never contracted for the street guy (in his 3D frame of simultaneous events).

Let's call these two events A and B. More specific: event A is "front train is hit by a bird", event B is "rear of train is hit by lightning" If you consider a frame F where THOSE TWO events A and B are NOT simultaneous, then that frame measures a LONGER length between THOSE TWO events A and B. But that's NOT what one does measuring a moving train. Measuring a moving train is measuring between two events of the train that are simultaneous in frame F . Events A and B are NOT simultaneous in frame F. The shorter train in frame F 3D space of simultaneous events is NOT a train with simultaneously lightning hitting the rear and front hit by a bird. That's what the different angles of 3D sections thorug 4D spacetime show visually.

Here I put all the information in one diagram. Check how the green simultaneity lines frame (Stan's) and the blue simultaneity lines (Morton's) move through spacetime. They are totally different. The lines representing ray of light represents the path of a photon going from one location to another. A simultaneity line represents all the events that occur simultaneously (and thus exist) for an observer at one specific moment in time out of observer's life This is only if the events are located at the same distance from you. If lights from those events reach you at the same instant of time, then the events occured simultaneously for you (your ref system). when you look at the stars, all that light hit your retina at the same time, but because the distances are different fom your eye, the events when the light left the star didn't occur simultaneously for you. Below I added the paths of light leaving the event 'nose of rocket / muon' hits upper atmosphere of previous discussion. Following the yellow lines you can read where and when the photons are is both frames. You slowly get the picture why I love spacetime diagrams?

Michel, have you made any effort in understanding what's shown in a Loedel or Minkowski diagram? Have you tried Lorentz Transformations?

You don't seem to understand what I write here: http://www.scienceforums.net/topic/98501-lost-in-langevins-language/#entry943663 Why? Is it because you still don't understand relativity of simultaneity? You don't know yet that different frames of reference involve different simultaneity? Why is it you seem to make no progress at all understanding SR?

I'll give it a shot I admit the Langevin text is a bit confusing at first reading. That's correct. Read on... Bear in mind that in the above case the two events to be measured are not simultaneous in the measuring frame. If you measure -with your ruler at rest- the moving train (and mark off the two train events that are simultaneous for the train frame), you will measure a LONGER train. Lenth contraction is a different story. See below: Measuring the moving train that exists in our 3D space of simultaneous events means measuring distance between two events of the train that are not simultaneous for the train frame. That length is shorter than the distance between two simultaneous train events measured from a frame at rest reltaive to train (a train passenger doing the measurement of his train ... obviously at rest relative to him). Train station observer and train passenger measure different 3D sections of simultaneous events through 4D spacetime [edit]removed quote marks

I'll give it a shot I admit the Langevin text is a bit confusing at first reading. That's correct. Read on... Bear in mind that in the above case the two events to be measured are not simultaneous in the measuring frame. If you measure -with your ruler at rest- the moving train (and mark off the two train events that are simultaneous for the train frame), you will measure a LONGER train. Lenth contraction is a different story. See below: Measuring the moving train that exists in our 3D space of simultaneous events means measuring distance between two events of the train that are not simultaneous for the train frame. Train station observer and train passenger measure different 3D sections of simultaneous events through 4D spacetime.

You forgot to tell us what the traveler's simultaneity lines read between the 2010.2 earth year and the 2013.8 earth. (You refer to Langevin p51 but that's only the story of the light beams reaching the observers. That's not the issue. We are interested in what the traveler simultaneity lines read during turnaround.) During outbound journey the traveler's simultaneity lines read a real physical earth clock. That clock has less time than the traveler's clock. Same story for the inbound journey. No discussion about that. (In case you doubt: put a set of cameras along a railway track, synchronise the shutter of all the camera's. The camera takes a picture of the passing moving clock an inch in front of the camera lens: a real physical clock with hands showing less time.) Does -per traveler simultaneity lines- the earth clock time turns fictitious at 2010.3 and return being real at 2013.8? You must be joking. Or do you consider ALL the earth clock readings per simultaneity lines fictitious? If a car drives fast enough I feel -during a split second-the shorther moving car touching between two fingers. Time dilation and length contraction are real. Fly at high speed to the moon and you will be there after 1 minute of your life because the distance to cover contracted. Nothing fictitious. And in earth simultaneity (3D space) the traveler aged only one minut of his life. Nothing fictitious. PS. It's possible the traveler's simultaneity lines during turnaround are not straight lines (if we don't use momentarily co-moving inertial frames during deceleration/acceleration), but that doesn't change the issue; they still read real earth clock times. and have to cover times between 2010.3 and 2013.8

I see you still made no progress... I show you symmetry of length contraction in the other thread.

To visualize the importance of relativity of simultaneity for the symmetry of length contraction, I added a long spaceship, length at rest = length of atmosphere at rest. The muon sits on the nose (or flies alongside rocket's nose). Relative speed of rocket = relative speed muon Mr Green is in rocket, Mr Red at rest in atmosphere. First diagram: In Mr Green"s 3D space of simultaneous events "the collection of simultaneous rocket events" has rest length. The collection of simultaneous atmosphere events in Mr Greens 3D space has shorter length than the collection of simultaneous events of the "at rest" atmosphere in Mr Red's 3D space. Second diagram: In Mr Red's 3D space of simultaneous events the "collection of simultaneous atmosphere events" has rest length. The collection of simultaneous rocket events in Mr Red's 3D space has shorter length than the collection of simultaneous events of the "at rest" rocket in Mr Green's 3D space. The "at rest rocket" for Mr Green and the "at rest atmosphere" for Mr Red have different slope in 4D spacetime. I.o.w. they are not part of the same 3D space of simultaneous events. Event C (for example a baby born on earth surface/lower atmosphere) happens after event B (a bird sits a split second on the roof). For Mr Green the nose of the rocket (= muon) hits the atmosphere (=event A) when the baby is born (event C). No bird on the roof (the bird was on the roof some time ago). For Mr Red the nose of the rocket (= muon) hits the atmosphere (=event A) when the bird sits on the roof (event B). No baby is born (yet).

Indeed. Length contraction (and time dilation) is not about different scaling. It's about relativity of simultaneity. Talking about gamma factor is one thing, but knowing what you measure is more important to understand SR: a guy sitting on the muon measures the length(distance) of a completely different set of simultaneous events of the atmosphere than a guy at rest relative to the atmosphere does. Different 3D sections through 4D spacetime.