B Is Time Just a Perception or a Fundamental Aspect of Spacetime?

[PeroK]

It's a good "first step" in a journey of thousands of miles. This is why it is used so often.

Though it's to often presented as the start and end as a description of gravity...by the time a decent understanding of gravity is accomplished that rubber sheet thing looks silly.

The journey may be 1000 miles, but the rubber sheet is a step in the wrong direction. The obvious flaw is that it relies on the preconceived notion that things move "down". The only reason things move down into the sheet is gravity: take that sheet to the space station and the ball would refuse to follow the sheet. The other flaw is there is no time dimension on the sheet. It's the geometry of spacetime (not space) that defines gravity, so at least one of the dimensions of the sheet would have to be time. The rubber sheet reinforces the classical concept that space and time can be separated.

The first step in learning GR is to learn SR. Part of that journey will be to redefine your understanding of space and time. The rubber sheet analogy is definitely something you have to unlearn.

 

[nitsuj]

The journey may be 1000 miles, but the rubber sheet is a step in the wrong direction. The obvious flaw is that it relies on the preconceived notion that things move "down". The only reason things move down into the sheet is gravity: take that sheet to the space station and the ball would refuse to follow the sheet. The other flaw is there is no time dimension on the sheet. It's the geometry of spacetime (not space) that defines gravity, so at least one of the dimensions of the sheet would have to be time. The rubber sheet reinforces the classical concept that space and time can be separated.

The first step in learning GR is to learn SR. Part of that journey will be to redefine your understanding of space and time. The rubber sheet analogy is definitely something you have to unlearn.

It's arbitrary if one takes a single step in the wrong direction when the whole journey is 1,000 miles...

Perhaps the flaw is bringing in some presumption that things are moving down; to that point the representation is that the sheet itself is the gravity. If that's an issue, what of the friction between the ball and rubber. There is a million issues with it, however I'd say the least of which is that it doesn't work on the space station. :/

With that said, it teaches orbits quite well; of course forgiving many many comparatives to the reality...as is typically true with analogies. Which is all it is, not a 1,000 mile journey. Not a method to explain spacetime, but it's effect under the influance of a mass.

k, can we agree enough with analogies in a science forum ;) The real issue with this damned rubber sheet being referenced here.

SR a single step in the 1,000 mile journey towards understanding GR! I guess GR is far far more complicated than I thought! :D

I watched that vid and I agree completely with you about the presenter focusing on the bending of space. If I understand right the vast majority of what is experienced (the acceleration) is due to time...and that it takes vast amounts of mass to bend space...such as with light bending. However I also noticed this was high school

 

[Max364]

Time may be an emerging quantity?, ie, arises from mental construct? also from another discussion elsewhere i commented:
Taking the speed of light as a unity value of 1

then v + t = C = 1
ie the sum of total speed and time must always be 1.

so for a photon (speed of light) v is at maximum so v=1 must mean that t=0 for photon, it experiences no passage of time.
But for everything else our speed on Earth is very small compared to light so our v is tiny (even at 1000s mph)
so that means we are almost at the maximum rate of passage for time (as v is almost zero) - so for us on Earth v= almost 0 and t= almost 1, we are at maximum rate of passage for time (opposite to that of a photon)!

Also there is no simultaneity of events - (eg, Andromeda Paradox).
There is no universal special moment "Now" (Einsteins famous Train thought experiment with light)

some quotes:
Stannard 1987: In four-dimensional space-time nothing changes, there is no flow of time, everything simply is . . . It is only in consciousness that we come across the particular time known as ‘now’ . . . It is only in the context of mental time that it makes sense to say that all of physical space-time is. One might even go so far as to say that it is unfortunate that such dissimilar entities as physical time and mental time should carry the same name!

 

[Ibix]

Time may be an emerging quantity?, ie, arises from mental construct?

This is incorrect, at least in the framework of relativity. If you are considering another theory, please cite your source.

then v + t = C = 1
ie the sum of total speed and time must always be 1.

This is clearly incorrect. $U_\mu U^\mu=1$ is true for objects on timelike worldlines (the modulus of the four-velocity is one), which may be what you are referring to, but...

so for a photon (speed of light) v is at maximum so v=1 must mean that t=0 for photon, it experiences no passage of time.

...photons do not follow timelike worldlines, so that would seem to be irrelevant.

Also there is no simultaneity of events - (eg, Andromeda Paradox).

There is no global definition of simultaneity, I presume you mean.

 

[Max364]

By emerging , i mean each person/object experiences its own rate of time passage (hence, can not agree on simultaneity of events etc), my time is unique to me even though it is almost exactly the same as yours, as we occupy almost the same space (relatively)
v+t=1, yes for wordlines or even worldtubes (solid 4d object)
Two people passing each other can not agree on what is defined by exact moment "now" - extended to Andromeda galaxy this gives difference of days not just nanoseconds like on Earth etc.

 

[Max364]

I should have added: Andromeda Paradox explained here: https://en.wikipedia.org/wiki/Rietdijk–Putnam_argument

 

[nitsuj]

By emerging , i mean each person/object experiences its own rate of time passage...

Instead of "emerging", which would likely tend to be interpreted as referring to the concept of emergence. You described Proper Time...so just call it that.

 

[Ibix]

By emerging , i mean each person/object experiences its own rate of time passage (hence, can not agree on simultaneity of events etc),

Everyone experiences one second per second. But different frames do not define time the same way, which is what I think you are getting at.

Everyone at rest in the same inertial frame agrees on simultaneity. Frames in relative motion do not agree on simultaneity, though, no.

my time is unique to me even though it is almost exactly the same as yours, as we occupy almost the same space (relatively)

The space we occupy has nothing to do with it. What matters is our relative velocities, or our history of relative velocities, depending on how exactly you mean "time" in this context.

v+t=1, yes for wordlines or even worldtubes (solid 4d object)

I already told you this was wrong. If you believe otherwise please cite a source.

Two people passing each other can not agree on what is defined by exact moment "now" - extended to Andromeda galaxy this gives difference of days not just nanoseconds like on Earth etc.

They can agree. What you might call the "natural" definition of simultaneity is different for the two, this is true. But neither is obliged to adopt that convention. That it is merely a convention is obe of the points of the Andromeda paradox.

 

[PeterDonis]

Andromeda Paradox explained here: https://en.wikipedia.org/wiki/Rietdijk–Putnam_argument

We have an insights article on this (of which I am the author ):

https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

 

[Max364]

Thank you for that Peter and your further explanation of the Block Universe in that article.
Would it be true to say that every point in space exists in a different time? (due to spacetime nature - ie, its own time for every point) - this is because in objects/mass causing warping of space must then also cause warping of time? ...however small...
or do two points separated by space exist in the same time frame? (in the same inertial frame etc)

 

[Ibix]

The block universe models spacetime as a solid block. Splitting it into space and time is equivalent to (imagining) slicing the block into infinitely thin sheets. Each sheet is all of space at one instant.

The point about no global simultaneity is that there is more than one way to slice the block - you can tip it over slightly and make slices at an angle to the first set of slices. That's a different definition of now, and whether or not two events lie in the same slice (are simultaneous) depends which set of slices you choose. The choice of which way to slice is your choice of inertial frame.

But "every point exists in its own time" makes no sense. It's like saying every point in a block exists at its own height. No. A whole plane shares the same height, and similarly the whole of space now (however you chose to define now) shares the same time.

What is true is that, if you trace the paths of two objects through spacetime, they may meet, move apart and meet again. And the paths may have different lengths. Path length through spacetime turns out to be the elapsed time, which is how the twin paradox works.

 

[StandardsGuy]

As @Nugatory says, that picture is very poor. For a start, it implies that the grid is a 2-dimensional model of spacetime, then draws a 3-dimensional Earth sitting outside spacetime.

The picture is very pretty, but it's not even internally consistent, let alone much like the reality.

I don't particularly like the rubber sheet. It is a 3-dimensional model of 4-dimensional spacetime. Perhaps the Earth should be shown below to indicate that the gravity of the Earth is pulling the sheet downward (the third dimension). Anybody using it should point out the inconsistencies and limitations. However it does give a visual understanding to a difficult concept. Let's don't throw out the baby with the bathwater.

 

[Boing3000]

I don't particularly like the rubber sheet. It is a 3-dimensional model of 4-dimensional spacetime.

That's a quite common stance from physicists. From my layman's perspective this 4-dimensional model of a 2-dimensional slice of space-time is a perfectly fine analogy to introduce some key concepts of GR. I don't think I have even heard some of these internet "guy" pretend that the sheet will emulate a inverse square root law and true elliptical "orbits" (do they ?, I mean ignoring friction ?)

Nonetheless it introduce quite naturally what "curvature" is, and how seen from above some invisible local effect are interfering with object movement and their presence.
This made visible sheet "steepness" quite naturally introduce to the audience the urge to understand how to evaluate that change in "curvature" and learn about derivative. How many will then try to follow the rabbit and extend the computation over a 4D manifold using tensor geometry is not the point.

The point is that thanks to that analogy I can explain my grand-ma that gravity is not a force, that all these marble are in free fall and not accelerating, and I can even explain that a black hole is just that same marble but reduce to the smallest dust and projected to infinity, actually puncturing a (black) hole, into this sheet, but then without actually changing anything beyond where the marble "was", and thus also explaining that black hole don't suck anything.

 

[PeterDonis]

Would it be true to say that every point in space exists in a different time?

I'm not sure what this means, but I think the answer is "no".

this is because in objects/mass causing warping of space must then also cause warping of time?

Since the separation of spacetime into "space" and "time" is not invariant--it depends on your choice of coordinates--there is no such thing as "warping of space" by itself or "warping of time" by itself; there is only warping of spacetime.

do two points separated by space exist in the same time frame?

What does "exist in the same time frame" mean?