# VandD

Senior Members

113

1. ## How would you know? Thought experiment

Who says it 'developed'? You see the universe 'developing' but that's only an illusion of the mind. Einstein: the distinction beween past, present and future is only an stubornly persistent illusion.
2. ## What is Space made of?

The distance between simulaneous events.
3. ## Is Space-Time a Physical Entity or a Mathematical Model?

Clock retardation (time dilation) follows from relativity of simultaneity: different 3D cuts through 4D spacetime. You don't need a kind of ether to produce or explain time dilation. Do you think ether is NEEDED to produce relativity of simultaneity?
4. ## Is Space-Time a Physical Entity or a Mathematical Model?

I.o.w. you don't agree there can be no non-inertial motion without the existence of an ether. I.o.w. non-inertial motion does not need an ether to exist. Thanks.
5. ## Is Space-Time a Physical Entity or a Mathematical Model?

I.o.w. there can be no non-inertial motion without the existence of an ether?
6. ## Models for making sense of relativity - physical space vs physical spacetime

That thread doesn't give an answer what 'physical' or 'real' means. Tim is stuck in the idea that one NEEDS an ether to motivate light propagation. And even if he needs one, that still doesn't give us an answer what 'real' means.
7. ## Models for making sense of relativity - physical space vs physical spacetime

Yes, Einstein and relativity is 'uncommon use of language' for laymen ;-)
8. ## Models for making sense of relativity - physical space vs physical spacetime

In order to do that you have to define what you mean by 'substantial', 'real' etc. Go for it, I'm all ears. In my previous post I already asked you to define what you mean by 'physical'.
9. ## Models for making sense of relativity - physical space vs physical spacetime

Define 'physical'. Light propagation doesn't need a medium/ether to be physical. Movement is reading successive events, all frozen and co-existing in 4D block universe. We don't know yet why we experience ourselves as a 3D being evolving in time, but part of our 4D existence, part of the block universe events. We don't know why we experience evolving time. That's for further generations to sort out. Co-existing events is all you need. 'Real/physical' 3D space/world is section through 4D block universe. Only if you stubbornly believe you NEED an medium/ether for light propagation to exist. Light is isotropic in 4D block universe, each observer measures with real/physical clocks and rulers. They all consider light propagation c physical/real. I think you don't understand how block universe works.
10. ## 3D Space, relativity and presentism

No. For Einstein the universe is a 4D spacetime structure, not 3D. << 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).
11. ## Events and the Space-Time Manifold

Say event A = <<Car hits man crossing the street at traffic light. The clock in the car shows 5:00 o'clock. The clock of the man crossing the street shows 4:35 o'clock.>> That event is absolute: it's one and the same in all reference frames.
12. ## Models for making sense of relativity - physical space vs physical spacetime

Well, it's unclear to Einstein why you need presentism/aether/preferred frame of reference/etc to make more sense of the equations.
13. ## 3D Space, relativity and presentism

My latest post has been deleted without notification or motivation. Why? Is it not allowed to refer to other threads???? In that case what must I do? Copy all the drawings I made for the other thread and drop them in this thread?
14. ## events popping out

============================================================================================= Below I show you how you feel the shorter contracted car between hands of stretched arms: . . . . ============================================================================================ Have you tried to read symmetry of time dilation in the 4D spacetime diagram? (If you want to know when one of the observers SEES the wristwatch.... add the 45 degree light paths! ) Relative v = 0.7c Hence time dilation factor is about 1.4 . . Michel, I can do no better -and I guess you can not find better on the internet- to show you how the relativity of simultaneity of events leads to time dilation and length contraction. Now it's up to you to draw spacetime diagrams...
15. ## events popping out

I don't see what you are getting at. Every split second you see another set of events black dots. Don't you take them into account to make your "measurements" of space and time?
16. ## Models for making sense of relativity - physical space vs physical spacetime

Some 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). ---------------------- Also, 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). ---------------------- Petkov makes an interesting comment about Lorentz failing to understand all times are equally 'real': Quote: <<It was precisely the view, that successful abstractions should not be regarded as representing something real, that prevented Lorentz from discovering special relativity. He believed that the time t of an observer at rest with respect to the aether (which is a genuine example of reifying an unsuccessful abstraction) was the true time, whereas the quantity t' of another observer, moving with respect to the Ørst, was merely an abstraction that did not represent anything real in the world. Lorentz himself admitted the failure of his approach: <<The chief cause of my failure was my clinging to the idea that the variable t only can be considered as the true time and that my local time t' must be regarded as no more than an auxiliary mathematical quantity. In Einstein's theory, on the contrary, t' plays the same part as t; if we want to describe phenomena in terms of x'; y'; z'; t' we must work with these variables exactly as we could do with x; y; z; t.>> >> http://fqxi.org/data/essay-contest-files/Petkov_PetkovFQXi2.pdf I also found this in my files: <If I had to write the last chapter now, I should certainly have given a more prominent place to Einstein’s theory of relativity [...] by which the theory of electromagnetic phenomena in moving systems gains a simplicity that I had not been able to obtain. The chief cause of my failure was my clinging to the idea that the variable t only can be considered as the true time and that my local time t′ must be regarded as no more than an auxiliary mathematical quantity.> (Lorentz 1916, p. 321, note 72*)
17. ## events popping out

I bet you would never see the light without spacetime diagrams. They show more than only pages of 'shut up and calculate'. Correct, but you don't SEE that proper 'at rest' ruler. Because the light from the simultaneous events of the atoms of that ruler do not reach your eyes simultaneously. No, it's rather 22.2 degrees. Only light paths are at 45 degrees. How can a ruler be 'on' the 45 degree line??? Only light is represented by a 45 degre line. correct I don't understand what you mean by that. If you keep the ruler at rest in your hand (you hold the ruler at the left end), then the other end of that ruler at rest is out there. You don't SEE that other 'now' end of the ruler. But you do see a full ruler, but not light coming from the simultaneous ruler events. What you see of the ruler 'at rest' is light coming from successive 'proper ruler' in time. (I'll add that to the diagram) Correct Nothing is directly observable. Even the light of a fly sitting on your eye takes a split second to reach the retina, and from there to your brain... Science doesn't get far without calculating. The input in the Lorentz Transformations are time and spatial coordinates. I don't know what you mean by that. You don't observe a length. You observe events. Light coming from events. And events can be: leaving a mark on a ruler, etc. Here visualizing what an observer SEES.
18. ## events popping out

Be carefull when you write "as SEEN" ... You probably mean how the blue train is shorter in the orange frame? If you want to know what an observer SEES, then you have to add the 45 degrees light paths and see when they hit an observer's eye. I leave that for you to add in following diagram... I give you the set of 'train events' (the big blue dots) that make the 4D blue train. Light blue zones indicates the set of train events in the blue 3D space of simultaneous events.These are the proper train at different blue instant of time. Orange zones indicates the set of train events in the orange 3D space of simultaneous events. These trains are shorter than proper trains in blue spaces. The shorter train in orange 3D space is made of a different set of train events than the (proper) train in blue 3D space. SR is about realtivity of simultaneous events.
19. ## events popping out

Strictly speaking you are correct, BUT... when I show the bike at the flag pole, the event means what it says: the bike at the flag pole. To be fully exact I should have said: event B = (f.ex) "front of bike is at the left surface paint of the pole". Hence both cars are seeing one and the same image: front of bike at pole paint with its clock at 5:05. An event has no 'length'. But on a larger scale, for practical reasons, we say: at event A the bike is at the pole, at event C the bike is at the tower, a.s.o. Length measurement: Length is measured between events: there would be a length between a front event of the bike and a rear event of the bike. For showing length contraction of the bike of the diagram I should zoom in a lot. Therefore, lets consider the length between the dog and the bike (or make it a long train from dog to bike). Orange car measures the length between events D and B. It's the spatial distance an orange ruler at rest (the orange ruler in a frame is also a set of simultaneous events in orange frame) measures between simultaneous events of the dog and the bike (or rear and front of train) in his orange 3D space frame of simultaneous events. How does blue car measure the distance between dog and bike (rear and front of train) present in his 3D space of simultaneous events? Not between events D and B because those events are not simultaneously occurring in his blue 3D space. Watch which events of the dog and bike (rear and front of train) are in his blue 3D space: events F and B ! Blue car driver measures the distance between events F and B. That length is shorter than what orange measures between dog and bike (events D and B) simultaneously present in orange 3D space of simultaneous events. Length "contraction" occurs because different observers measure distance/length between a DIFFERENT set of simultaneous events. This is to be kept in mind especially when proper length changes over time, see sketch 9 here: http://www.scienceforums.net/topic/98501-lost-in-langevins-language/page-3#entry944651 If blue car driver would measure the distance between (non-simultaneous in blue frame) events B and D on his blue ruler (ruler at rest in his blue 3D space of simultaneous events), the driver will measure a LONGER distance/length than proper orange length between D and B. Because on his ruler he will first tick off one event (event D) and then after some time the other event (event B). You are interested in what blue and orange car drivers SEE of all of this? I hope you understand the proper train and the shorter train are two different set of simultaneous events. All you have to do is draw 45 degree lines from the (train) events you are interested in, and see where and when they hit the blue driver's eye and orange driver's eye.
20. ## events popping out

Blue car and orange car calculate what happens 'now', i.o.w. simultaneous with event A. This leads to relativity of simultaneity. What else do you need? Differ? Impossible. The image from event P is what it is. If event P = bike with clock with hands pointing to f.ex. 5:05 o'clock, then both cars at event A see the same image of the bike and the bike's clock showing 5:05. O.K.
21. ## events popping out

Feel free to think that I don't understand SR.
22. ## events popping out

Whatever convention you use, the convention you use has to measure constant speed of light in all inertial frames. And that will tell you there is relativity of simultaneity between the two reference frames. Simultaneity is a basic concept in SR. Relativity of simultaneity even more. You want to refute simultaneity and relativity of simultaneity? Tell us, you still don't understand SR, or ... you do understand SR but you don't want to accept what SR tells you? Indeed. And based on their individual measuring ruler and wristwatch they will sort out which events did occur simultaneously and which not. And it will tell them relativity of simultaneity. Here the light from the events: Really? Below I added a dog chasing the orange car and bicycle. Watch how the light from simultaneous events reach a car driver's eye simultaneously. Verdict: relativity of simultaneity.
23. ## events popping out

It shouldn't if you accept relativity of simultaneity. In my spacetime diagram (see below), blue car and red car move relative to each other. They don't share the same time axis. The blue time and the orange time flow into a different direction in 4D spacetime. Orange 3D simultaneity reference follows the orange car time direction. The 3D blue simultaneity reference follows the blue car time axis. The blue car drives from right to left. The orange car drives from left to right. Three events are hightlighted: Event A = blue car and orange meet (pass each other). Event B = bicycle at flag pole. Event C = bicycle at tower. Where is the bicycle when the two cars cross (event A)? For the blue car, event A and event B are part of one and the same blue 3D space of simultaneous events: event B occurs simultaneously with event A. I.o.w. when the two cars cross, the bicycle is at the flag pole. But for the orange car, event A and event C are part of one and the same orange 3D space of simultaneous events: event C occurs simultaneously with event A. I.o.w, when the two cars cross, the bicycle is already at the tower. (For the orange car, when the two cars cross, event B (bicycle at tree) happened some time ago, part of a previous orange 3D space of simultaneous events). This is the relativity of simultaneity. When both cares meet at event A, events A and B happen 'now' (occur simultaneously) for the blue car, but not for the orange car. For the orange car, when both cares meet at event A, events A and C happen 'now' (occur simultaneously), but not for the blue car. At event A, event C (bicycle at the tower) has not happened yet for the blue car (event C is not part of his 3D space yet), but it does happen 'now' for the orange car. Yes, you might find this 'very troubling', but don't despair, what Copernicus told the world was also 'very troubling' to many people...
24. ## events popping out

God job, guys! I see Michel made a giant leap forward in understanding 4D spacetime! Yes. Because between the event hitting the observer's simultaneity line (not shown) and the event hitting the 45 degree line, the light from the event happening 'now' in observers frame, had time to reach the observer.
25. ## Lost in Langevin's language

I can do no better. If by this time Michel doesn't understand, it gets pretty hopeless... ;-)
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