michel123456

To me, no time cannot be fundamentally different from space. The reason why I think so is that if they mix so well, they must have some "common stuff". "the flip side of spatial distance" yes, some sort. One can make a parallelism between time & distance: both are always positive, both have 1 dimension. It had even come to my mind that distance & time are the same & one thing. But: 1.Time has a negative signature. It is "the flip" of distance but what does that mean anyway?? 2. As Swansont points out, Time is recognized differently from distance. The ticking of the clock remains to be explained.

That's a yes, a bit rough though.

So are you saying that we are not observing galaxies moving at close to C velocity?

Since we are observing galaxies receding at high velocities, doesn't that mean that reversely, observers living on that galaxy are observing us as if we are moving at close to C velocity?

??? (the bold part) My question arises from the fact (fact?) that length contraction is directional while time dilation is not (?). I used to understand it from the point of vue of an observer in FOR1 looking at another observer moving in FOR2, in which case there is no philosophical question: time dilation & lenght contraction mix in a perfect way. My question arises when an observer, in some FOR3, observes a universe contracted in his direction of motion. If time dilation has no directional component it gets awfully weird. I wonder if the "observes a universe contracted in his direction of motion" is a correct statement. If this was correct, all observers could detect such a kind of observational contraction & thus could detect some "direction of motion" through the universe, which sounds weird to me.

Exactly. However length contraction & time dilation go hand by hand, you cannot have the one without the other. So I reiterate my question: what about time dilation when the observer sees a universe that as you states "will appear to be 11 metres long to the pilot in the direction of motion", but "in a direction at right angles to this the universe will appear to extend to its usual limits, whatever they may be" ?

And what about time dilation?

Are you in Belgium? ------------------------------------ If you want to remember the value of Log 2, look at the face of your colleague next to you: the ear (3) the eye (0) the nose (1) the other eye (0) the ear (3) and you get log 2 = 0,30103 (approx.)

How do you describe this geometrically? if I expand, and you expand, at some time we will touch each other. In order to remain in place (relatively to each other) some displacement must take place. Otherwise it will look (from our expanding point of view) as if something was pushing us against each other.

But we are accelerated, through gravitation (see equivalence principle). And we do see a sea of particles around us (the universe). And it is impossible to stop. Our sliding in time cannot stop. Exponentially-contracting universe, yes that crossed my mind but geometrically how do you make this? With one universal center of contraction? Or multiple centers? I was completely blocked by the incapacity of figuring the idea. I have erased the "depending on the direction of acceleration", after much thought it must be wrong.

The Unruh effect: under the prism of what I posted you in PM, it may be (although I thought it would be impossible) that the observer under ridiculously high acceleration is "kicked out" of his own time line in such a way that he can observe particles of his own future (otherwise unobservable) or belonging to his own past, depending on the direction of acceleration.

Interesting, and maybe right. I can live with that.* Yes. One reality. You should have come here much earlier. There are a bunch of people here that argue about multiple reality and that drives me out of mind. And when an observer sees space, another may see time. * Take into consideration that I am erasing continuously my darkest thoughts🙂

Don't you? If there are 3 observers and they observe 3 different ways the "structure" behaves, is it possible for a "structure" to actually behave differently at the same time . IOW that there exist 3 different "realities" at the same time?

Did i say that? What I dislike is the "structure" of spacetime, the "fabric" of spacetime. The fact that some scientists prefer the empty plate of the balance. We know that the way we are observing things through spacetime is observer dependent. One observer will observe the "structure" bending like this, the other observer will see the "structure" crash upon itself, the 3rd observer will notice nothing. Doesn't that mean that there is no underlying "structure"? Isn't that some sort of evidence?

Nothing. Bad moment I suppose.

Sure. But if you had a look in your basement and see that the foundations hover 1 meter over the ground, wouldn't you say "what kind of a weird architect did this"? Or if your house suddenly crumbles for no apparent reason, wouldn't you blame the architect? Or to paraphrase I don't remember who: I cannot pilot a helicopter but when I see one crashing I know that something went wrong.

This makes my head hurt Mine too. To me, the "what's in it is a function of space" is totally impossible. My opinion is that physicists are used to encounter weird things and when they end up with nothingness full like an egg, they say nonchalantly "here another weirdness", with this little smile that you should read as "I don't know what is going on". Physicists have stopped reacting like Erwin Schroedinger with his cat. Schroedinger's cat is an attempt to directly link a macroscopic event (living/dead cat) with a weird event of the microcosm. If scientists could always link to everyday phenomena the weird results they get, well ...the weird results should vanish. IMHO physicists should think more like in 1935 and be astonished more & more. Not asking everyone to accept the incomprehensible because of lack of education.

You know what a function is. x, f (x) where x is the variable. That should ring a bell. In this case, my bad, I should have put nothing. The bell ringing. Moving relatively to what? Don't you need at least a second test mass? The bell ringing again

+1 The goal of emptying the universe is to make it simpler, like one would do examining a difficult function. Insert easy variables like zero, one, and infinite so that you can guess the diagram. But you must be right: Space is a function of what's in it otherwise it can't exist... You could have said Spacetime, anyway. Naively I use the analogy of the balance. On the left side you have nothing, the void, empty space & the concept of time we are trying to tackle. On the right you have "something" lie mass or anything else, it doesn't matter. It may be that the answer is not on the left side (where there is nothing) but in the right side (where there is something). It is just as if you had to find a treasure & choose between 2 rooms. In the left room there is nothing, in the other there is something. Which one do you choose to investigate?

Please explain. You have a 4D spacetime. How do you end with an interaction like gravity while being empty?

Sorry, time recedes in the sense that it goes into the past. But amazingly, the ticking of time does not change (as far as I know) in function of distance. Trying. I didn't expect any of that. Interesting anyway. Concerning space: we know that space is "made up of" (sorry for the informal statement) distance in 3 orthogonal directions. And we know that there is no negative distance, as there is no negative gravity and no negative time. So, what I am trying to say is that time does not go alone. Time & space are intertwined in such a way that the entire body (the manifold) can curve. And the fact that the curve is obtained by mass means (to me) that space, time & gravity are somehow 3 inseparable elements. Now, if temperature plays a role in all this, I don't know.

Right. So if the observer has mass (which he has) he is influencing the manifold he is observing. The manifold is not flat, it is curved. The observer will measure that gravity will recede its influence in function of the square of the distance. Time around the observer will recede in function of the distance (not squared). What else do we know? For example what is the role of space in this game? Is it just a recipient doing nothing?

We have reached a point of not understanding each other. Let me try to explain my thinking: imagine a completely empty universe. Nothing. Now imagine Space (mathematically). Then imagine time (mathematically). Then curve it. Does gravity come out of this model? Or do you have to put something else in this empty model to make time begin to run & gravity arise? What is this thing you have to put in? But if mass is not a function of the manifold then why do we say that gravity is the curve of spacetime?

What else is required? Particles, interactions, physics. Anything else than pure maths.

Yes & no. Yes, time is observer dependent. But this statement puts time as if it was an observer only kind of concept, and I think that time is more than that. As a hint, there is another feature of the universe that is centered on the observer (and on a lot of other things): gravity. I suppose I don't have to explain that there is a geometric link between Spacetime & gravity. If gravity is the curvature of Spacetime, it means that spacetime is not only a 4D mathematical concept. It is not a pure 3D space and one more dimension (called Time). I say that because I doubt that you can infer gravity from the curvature of a pure 4D mathematical concept (or am I wrong here?) It may look funny, but at the design level, the buildings can be flipped with sometimes spectacular results. It is sometimes a good exercise to flip your design (like in a mirror) so that you can have a fresh look & spot errors. Of course, once build, they usually don't flip. So one good exercise would be to model a universe with the other Higgs multiplet decision.