michel123456

You mean to be tidally locked to the sun. Google it.

So the common interpretation is that I am moving through spacetime, leaving empty the coordinates I left. And also we know that this last coordinate is not directly observable.(i cannot directly observe my own past) IOW in a spacetime diagram, the Observable Universe is entirely collated along the diagonals that pass through the observer. And the entire rest of the diagram is empty of objects.

If i move, if i change coordinate, that means that the coordinate i leave remains empty. Correct?

What does that mean? That objects change coordinates in spacetime ? Because if that's the common interpretation, I agree with that. I thought that common understanding was different. -------------------------------------------------------------- see also after interminable discussion the same question worded differently: http://www.scienceforums.net/topic/64809-what-we-know-about-time/page-13#entry671796 And what I gathered from md65536 I thank him for that.

-------------------- Other question about space: When we measure a distance, isn't that distance embedded in spacetime? Because distance is a measure of reality and reality is embedded in spacetime. And when we draw diagrams with the "space" axis orthogonal to the "time" axis, is distance something that belongs only to "space" (to be drawn along the space axis) or something that belongs to "space&time" (to be drawn on some diagonal)?

The common understanding is a leading question? In a spacetime diagram, an object is represented by its world line. So, if i understand correctly, an object is considered as occupying more than one set of 4D coordinates. Otherwise an object wouldn't be a line but a point. Isn'it the common understanding?

I learned it from this thread where you may find some other interesting info.

There is a little toggle button on the upper left corner to transform in old mode. I don't know why the other buttons become inactive then though.

What is then the common understanding?

So, why are we putting such kind of different properties to space coordinates against time coordinate ? Another example: an objetc makes the following transformation: A. from (0,0,0,0) to (1,0,0,0) This is considered impossible, because the object cannot change spatial coordinates in zero time. we must consider this: B. from (0,0,0,0) to (1,0,0,1) This transformation is what we call: "motion". The object changed coordinates, the object moved. To be compared to the following transformation: C. from (0,0,0,0) to (0,0,0,1) This last transformation is what we call: "standing at rest". The object changed coordinates, but only the temporal one has changed. Here it seems that 'nothing happened" because only "time passes by". It is not impossible (comparing to A). Which is a monumental difference. Further, the common understanding of this change of coordinates is that the object did not "moved in time", but occupies both coordinates. The object was at (0,0,0,0), now it is at (0,0,0,1). As a matter of consequence, one must reconsider transformation B and make a corrective statement: that the object did not "move" from coordinates (0,0,0,0) to (1,0,0,1) but litteraly occupies both coordinates. That is our common understanding. Isn't that complete madness? I wish everybody a Merry Christmas.

Right. Because the object is inside the past light cone.

d=ct is a condition in order to be observable (directly). One can pick Earth's own past that corresponds to an object (an event) that is zero LY away and 3 years in the past. It is (was) something very real that doesn't correspond to d=ct. Earth's own past is not directly observable (I mean you cannot focus on earth's past when looking in your telescope).. The same goes for an object that is 2LY away and 3 years ago. What's the problem?

What we observe today is an object 2LY away and 2 years in the past. are you saying that an object 2 LY away and 3 years in the past does not exist (has never existed) ?

But this object is in space. Out there, around us. Inobservable. Hidden in time..

i am asking where is that object that lies 2 LY from us and 3 years in the past. That is only 1set of coordinates of a single object. If we observed it last year (we took a picture of it) and we cannot observe it today it must have disappeared in the meanwhile, isn't it? Where is it?

I don't understand your argument. Spacetime is full of objects & events. An object situated at 2 LY from us and 3 yearsis an example of a random object in the past that can rxist (or could have existed). We cannot directly observe such an object. So, where is it ? Or, if you prefer, where are all those random objects that do not lie at the exactly corresponding distance and time that make them observable by us,

The labelling "photon" in your first diagram is disturbing. IIRC the standard model does not state that the photon changed dimension over time.

the question remains unanswered: Q1_where is that object that lies at 2 LY distance and 3 years in the past? it is somewhere out there but _We know that it is not (directly) observable today. _It was not (directly) observable last year because objects do not disappear from one year to another. IOW we could not take picture of it. Where is it?

Standed corrected. But in scenario 3 we can conclude that A & B are the same object (as one possibility) while in scenario 2 it is impossible to be the same object.

(bolded mine) Yes, exactly. And how come I can't see him in the present, and why I can't see him as he was 10 years ago, and why I can't see myself as I was yesterday. All that kind of silly questions. Because if I had a friend here next to me but 10 years ago, I wouldn't be capable of observing him. Time acts as a distance. And ONLY when the time-distance (the duration, the delay) corresponds to the spatial distance, then and only then the object is (directly) observable. And yes you are correct, if you had a signal made of massive particles that traveled at 2/3 light speed, it would reach us in three years from a location two light-years away. Do you have any example of such signal in mind ?

The position of an object (or an event) in spacetime can be described with a set of 4 coordinates: 3 spatial coordinates and 1 temporal. (x,y,z,t) Say we have event A with coordinates (0,0,0,0) That represents something that happens at the origin right now. Then some other random event B. Say event B has coordinates (x,y,z,t) . One can choose a set of spatial axes so that x axis is aligned to the direction between the observer and event B, so that we get simplified coordinates (x',0,0,t). Now: 1.If B has coordinates (0,0,0,0) we can safely conclude that A=B (= sign meaning that A & B are 2 events that took place at the same place at the same time). If A & B are objects (and not events), then A & B are the same object. (or 2 objects that have collided). 2. If B has coordinates (1,0,0,0) we can safely conclude that A & b are different events, because they happened at different spatial coordinates at the same time. If A & B are objects then they are not the same object. That's what happens when we change the spatial coordinate. If we change the temporal coordinate, things are different. 3. If B has coordinates (0,0,0,1) we should safely conclude that A & B are different events, because they happened at the same spatial coordinates at a different time. If A & B are objects then we should conclude that they are not the same object (see point 2 above). But that is not what we conclude. What we conclude is that objects A & B are the same object at 2 different time coordinates. And Michel is asking: why? Why do we incorporate different properties to spatial & temporal coordinates? Why aren't they simple numbers ?

We can observe an object A that is 2 LY away. This object is observed today in a situation as it was 2 years ago. Last year, the same object A that was 2 LY away was observed in the situation as it was 2 years before. And so on in the past, and in the future we will see this object A that is 2 LY away. The object A will not dissapear from sight just because time passes by. That happens because time passes by for us and for the object A as well. BUT we cannot TODAY observe an object B that is 2 LY away in the situation as it was 3 years ago. That cannot happen today, and that could not happened yesterday, nor last year, nor in the future. The delay must match the distance in order for an object to be observable.

No. Could we last year observe an object that we cannot observe today? Did it dissapeared from observation just because time passes by ?

Daniel posted this: It is a fact that space is a familiar entity, in contrast with time which is so mysterious. but do we really know what we think we know so well? Here below some considerations and question marks. _it has been established that there is a relation between space and time: any distance corresponds to a certain duration. We cannot get information from any distance in zero time because nothing can go faster than Speed Of Light. IOW in order to observe real simultaneity, information (EM radiation or any kind of other signal) should transfer at infinite velocity and this is not the case. In short, everything that we observe in the space dimensions is in the past. But not ALL the past. An observer can only observe a slice of the past: the farthest the object the more in the past. There is a standard relation beween the distance to the observer and the past. Putting aside for a while the cosmological expansion of space, the relation between distance and time is ruled by SOL. An object at 2 Light-Years distance is also (observed as it was) 2 years in the past. that being established, here is the first question about this well-known space: Q1_where is that object that lies at 2 LY distance and 3 years in the past? How come that we cannot observe such an object ?

Yes that's what I thought. Space looks so familiar.... I will open a new thread on this so that this thread doesn't go out of tracks. (Sorry for the late response.)