michel123456

I am simply asking how small was the screw. If the image is microscopic, I guess (it's a guess) that the whole picture is less than a millimeter wide. That would made the screw on the picture less than 1/10 mm. Otherwise, the image is not microscopic. It it is approximatively real size on the screen then yes it can be a screw.

But this is a microscopic picture taken by the microscopic imager. So if the explanation is correct, the microscopic imager took a picture of a part of Mars where the Alpha particle X-ray spectrometer was previously posed. How small is this screw?

That is fully correct. Draw an object in a spacetime diagram. Not a point object, but an object that extend in space. How do you draw that? Is it a horizontal straight line segment? Or is it a broken segment that follows the diagonals? I think it cannot be a horizontal segment, because as you said, a horizontal segment (absolute simultaneity in dt=0) is not observable. I think an object in a spacetime diagram is a broken line along the diagonals. Meaning by that that an object extends both in time & in space.

I would say yes. If an object extends through space, it must also extend through time. But the time interval would be restricted by the dimension of the object. I cannot see how an object could extend infinitely through time. The same way an object occupies a specific section of space, the same object must occupy a specific section of time. Not a point. You said it. In fact your diagrams show that time & space are not so different.

I am happy you have understood my point. The question is: if you occupy coordinate T2, do you occupy coordinate T1? My answer is No. For me the 2 coordinates are mutually exclusive. That is because you have translated from T1 to T2. You simply don't continue to "exist" somewhere in the past. And similarly, you don't already "exist" in the future. Upon your time line you occupy only one coordinate. There is only one single JonG translating in time. Your time coordinate is continuously changing, making you feel as if it were a "flow of time". There is a flaw. A time-tagged photo is an object that travels in time like all other objects around us. There is no object that you can retrieve from the past without having traveled in time with us.

Very interesting post. Indeed! The idea is that "motion through time" acts EXACTLY the same way as motion through space. If you are here, you are not there. Motion through space indicates that multiple positions are muyually exclusive. IOW when you move from x1,y,z to x2,y,z, you hace changed coordinates. There is no a first "you" at x1 and a second "you" at x2. There is a single 'you" that has changed position. That's what motion is about. I believe the same happens with time. Positions in time are mutually exclusive. If I am today here, I cannot be "yesterday" here. i have translated from "yesterday" to 'today". IOW there is no other "me" frozen in time "yesterday". I have not duplicated myself through the passage of time. I have only changed coordinates. And if I am right, that I have changed coordinates, it means that my ancient set of coordinates is empty. And as JonG noted, we cannot directly check that.

Right. Do you agree about the concept of "motion" through time? Similar to motion through space. The concept by which there is only one "yourself" changing coordinates in time ? A concept by which when you change coordinate in time, you leave your ancient coordinate "empty", exactly like motion through space? Because when an object moves, it changes coordinates, IOW the same object does not occupy 2 sets of xyz coordinates. An object changes from one set of coordinates to another. I believe the same happens with time. Does that seem sensible?

Yes of course. That is why I added "through c (SOL)". If you tell me the time you will need to hang your painting (the duration), the only thing I can tell you is the maximum distance you may have reached in order to hang your painting. In no case it would be all x,y,z. But the duration can become a maximum distance (because nothing can go faster than c). If you were a photon, then duration would give me exactly the distance. I could then align one axis on your trajectory, and t (duration) would give me x (the distance on the X axis). In this particular case there would be no difference between the information t and the information x.

You said it. Through c (SOL), one space dimension (be it X, y or Z) is also directly dependent of t.

It's a 5th dimension then.

I thought it was commonly accepted that time is a dimension perpendicular to space. If it is correct, then imaginary time IS space. Sorry for that. Skip it.

I don't think it is a good idea to slip into philosophy and questions about human consciousness. We can stick to physics. "Moving" is a concept that we use to describe a change in coordinate in space. Relativity states that motion also need time. Thus, motion is something that takes place both in space & in time: in Spacetime. The problem is that the currently accepted concept of "motion in time" is completely different with the concept of "motion is space". Motion in space is understood as a change of coordinates. "Motion" in time (duration) is commonly understood as an extension of the object. In a spactime diagram an object displacing through time is a line. IOW "motion" in time is understood as an object occupying many temporal coordinates. Which is very different from a displacement in space. That is why I believe it is wrong. i believe that motion in space and "motion" in time are exactly of the same nature: it is in both case a change of coordinates.

Bienvenue.

I wrote it down on several places on this Forum: _you can easily replace the concept of "flowing time" with the reverse concept of "displacement through time". Etienne Klein has asked the question before I did: does time flow or do we travel through time? Simply consider that all objects travel into an immovable substrate called time, not so different than the substrate we call space. Or to put it differently: If one accepts that objects can change position in space, if one should accept that space alone does not stand but that the correct situation is that objects can change position in the Spacetime continuum, then the result is that objects can displace both in space & time. And as a consequence, there is no "flow" of time, there are objects changing coordinates in spacetime.

You should be unsatisfied. As AJB and so many scientists are. Without being cranks, but understanding that some explanations have leaks, and getting work from it. -------------------------- (edit) I am inattentive many times. Maybe i am stupid. For sure I am unsatisfied. That last is the reason of my presence on this Forum.

IMHO it is because physics is not clear, is full of paradoxes, of mutually exclusive theories and of mind blowing "explanations" that leave the attentive reader with more questions than answers. Only the inattentive or the completely stupid can remain satisfied. Whatever is well conceived is clearly said, And the words to say it flow with ease. (Boileau,Canto I, l. 153.) Which is certainly not the case for physics. That is my opinion.

Speculating a bit: It can be seen as follows: In diagram 1, there is the gravity curve, that diminishes as distance increases, and the red cosmological constant wich is separate from gravity and acts linearly. In diagram 2, there is only gravity. The only "thing" is that the axes are tilted. Which means that for extremely large distances gravity becomes negative, and for extremely small distances, gravity never reaches infinity. But I may be wrong of course.

Quoting from the other thread mentioned by elfmotat: (enhancing) That comes out also from the present discussion: if I understand clearly it looks just as if gravity became repulsive as a function of distance. But not as a function of distance squared, because Hubble law is rectilinear.

But if the metric expansion is at work between the earth and the Sun, we should observe it through light. Because light is the "thing" that makes us know about expansion.

1-1 I don't understand your answer. Wiki says: "Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the FLRW metric, and is a generic property of the universe we inhabit. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence of gravitational attraction and such things do not expand at the metric expansion rate as the universe ages. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the Universe given the matter density and average expansion rate." I still interpret from Wiki's statement that space expansion is not making the Earth receding from the Sun because the Earth and the Sun are gravitationaly bound. Where am I wrong?

What else should I understand from Wiki's article?

In my understanding it is meant that for example the space between the Earth and the Sun does not expand, that the space between stars in the Milky Way does not expand, that the space between galaxies in a cluster does not expand, BUT that the space beween galaxy clusters does expand.

From wiki "Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the FLRW metric, and is a generic property of the universe we inhabit. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence of gravitational attraction and such things do not expand at the metric expansion rate as the universe ages. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the Universe given the matter density and average expansion rate."

Well, if gravity IS the curvature of Spacetime, why is it supposed that space (alone) does not expand in proximity of matter. I mean, why is it considered that gravity has no effect on time in the concept of space expansion. My question arises from the fact that we talk about space expansion, and not about Spacetime expansion. If you see the difference.

After this post it was answered that only Space expands. So it is supposed that gravity curves Spacetime and also keeps space from expanding (because in gravitationally bound regions space does not expand) while doing nothing to Time. Looks like a contradiction to me.