michel123456

I suppose that you have the image of a growing fluid that tears apart filaments of matter when expanding. Is that it?

? But the void is full of photons.

1960 It is part of the "who is talking" thing. I agree.

You don't see that if the voids get larger the filaments are stretched? Ultimately, the filaments will get ripped apart. Just as if there was an interaction between the voids and the filaments. This structured model is not what a thinking mind would expect from an expanding universe that follows the rules imposed by the space expanding paradigm. If there were an interaction between the space expanding and the filaments, a regular force, then yes, maybe. But without any interaction, no. In this last case, the voids should remain roughly the same sizes.

Then that means that there exist an interaction between the expanding voids and the filaments. The filaments are thus stretched by some kind of hypothetical force that expands the bubbles of voids. But we know that there is no such kind of interaction: we know that expansion is occurring without a force being required. Which makes me think that this kind of model is not easily compatible with the space expanding theory.

So, in this kind of structure, when the voids expand the filaments must be stretched I suppose?

So you want to keep in C the "memory" of A + B? The best way is not to make the operation at all, simply keep A+B Otherwise the result C can be the result of many other operations as Swansont pointed.

But these are totally different concepts. In the expanding space paradigm, the grid is increasing. It represents a scale factor. It is an increase of the metric. The distance remains the same: for example, if A and B are separated 3 grids square before, they still are separated 3 grid squares after.

That is not my understanding. I thought it was about a scale factor. IOW that the value of what we call 1 meter has changed.

What I cannot understand is at what time the shrinking of the metric changes and becomes a shrinking of the distance. in all the BB representations I have read till now it is as if the universe was smaller in the beginning (denser), which means that the distances between the objects was smaller. OTOH it is also said that what is expanding is the metric. So, at some epoch, the change in metric must have changed into a change in distance, or reversely. I guess.

The very simple and logical solution for the grandfather paradox is the following: if you travel 10 years back in time, you will become 10 years younger. That is most natural. Why would you expect to be extracted in a time bubble and suddenly duplicated, looking at yourself as a baby playing with a ball in the back yard? You will be 10 years younger and having erased from memory 10 years of your life. Of course that would prevent you to travel before your birth (it would be the end of your existence, something like death, surprisingly, before your birth) After thinking about it, it would be exactly as it is now.

Upgrade https://www.youtube.com/watch?v=DCdwfRA1BZw

I think that Mordred took it better. The shrinking is about the metric. The distances as measured by observers in the beginning of the universe has not changed, if I understand clearly what a metric expansion means. Apparent density has not changed. Or has it? Please expand on this. Eventually correcting me.

I have been reprimanded for asking this question again and again so it would be nice if you could give some reference for this. (the bold part) Does that mean that time dilation is an effect where time is always observed as slowing down and never getting faster? To me it is a fundamental question because in many explanations of time dilation (especially on graphics representations but almost everywhere), when an object travels back to Earth, it is observed as if time was speeding up. If it can be proved that this observation is not possible, then many explanations collapse. Thank you.

I understand now. The train is chasing its own image. Does that mean that the photons that carry the train's image are like arrows that the train could eventually get very close too only by increasing its speed? IOW that for outwards photons, the relative velocity is not c? and that simple velocity subtraction is allowed?

Thank you.

Concerning the bold part: how is that possible? You are seeming to say that the train is already on his way back. Thus the train is in-between the tower and the point where he turned back. Thus the train is an obstacle to its own image that comes from behind him... or do I understand badly?

What you say reminds me the block universe, where nothing happens. Time exists as the 4th dimension and everything in the universe is there like frozen in time. The observer is then given the illusion of time elapsing because of the constraints of observation. IOW the observer cannot contemplate the whole block universe but only bit by bit. In this scenario, nothing really happens in the block universe, it is a solid block of marble that contains all the events, past present and future. To speak frankly, it looks wrong to me.

Nobody talked about backwards. Backwards is against causality, it is not under discussion. I was simply asking about time "speeding up". My question arises from a diagram I have seen about the twins but I don't want to post it here and derail further the discussion. Yes, that is my understanding too. I don't think so. Both will see it the same way. In the examples I know, both observers see the other one as time delayed. My question is: are there examples where time speeds up?

Oh. For example the enclosed triangles that are upon the straight vertical line, are they observable? IOW can we directly observe ourselves in our own past when looking into a telescope? The blue ones. Are they observable directly? PS: yes they are causally connected. It is our past after all. But can we observe directly ourselves in the past? Sure, please stay. You asked a very profound question about the metaphysical existence outside the boundary of the spacetime hypersurface, am I right?

lightlike ------------------ Do I have to split blood before getting a straightforward answer? Are all the triangles on the diagram observable by the observer that stands at the summit of the cone?

Sorry for derailing this thread but: I know this great Ned Wright's diagram (from you 2nd link) On this diagram, where are the observable galaxies? From my understanding, the entire observable universe is upon the red pear-shaped curve. The galaxies inside are not (directly) observable, and the galaxies outside are not (directly) observable either. It corresponds to the analogy of the circle on a sheet of paper as described by Studiot previously. IOW from the result of the Big Bang we are observing only a tiny part.

Does time dilation also include the reverse phenomenon, something like time "speeding up"?

That is all ok to me (almost). _The red part I do not understand. I suppose you meant that every observer at any instant has its own lightcone. Because your statement looks like the reverse and must be inaccurate: for any event they are a multitude of lightcones, a multitude of different (possible) observers. _The bold part is what I am going to. Whatever we are observing from the universe is carried by light signals. Whatever we are observing from the universe lies upon the surface of the lightcone.

Yes (the bold part) And no it is not philosophical, it is physics. Would you agree with me if I say that the parts out of the lightcone (inside and outside) are not directly observable?