Proper (and coordinate) times re the Twin paradox

[PeterDonis]

That's what Einstein meant by his 4D existence quotes

You can always put different interpretations on vague ordinary language, even Einstein's. I interpret that quote from Einstein as saying this: relativity tells us that "3D worlds" do not exist; what exists is 4D spacetime. Cutting up that 4D spacetime into slices called "3D worlds" is a human artifact; nothing in the actual 4D spacetime corresponds to it.

If you want to say that your interpretation is more accurate than mine, then you need to find something in the actual 4D spacetime--not just a coordinate choice, but something actual, physical, observable--that picks out a set of "3D worlds" from that 4D spacetime. In flat spacetime, there is a way to do this (by picking global inertial frames); but as I said in my previous post, the special properties that allow you to do that don't exist in curved spacetime.

 

[Ibix]

It's like looking at a cube of metal and arguing about whether it's "really" a set of thin flat plates stacked on top of each other, or stacked on a slope, or corrugated plates or something. It's not "really" anything except a cube. It may be useful to treat it as made up one way or another but, in the absence of annealing marks or whatever, you're imposing a choice.

Dale isn't denying that you can slice the cube. He's just denying that there is One True Way To Slice The Cube.

 

[tophatphysicist]

I knew it would be just a matter of time before someone would bring the discussion into the context of General Relativity. Until we have a theory of unified fields and a theory unifying QM and General Relativity I'm not sure we can make real headway. It is certainly clear that Special Relativity is extremely difficult (if not impossible) to apply in the region of geons and quantum foam. How would you trace out a world line through such chaos? Nevertheless, Special Relativity works quite well in our world. Some would feel that is all we ask of it -- give us a continuous sequence of 3-D cross-sections that extend into the vastness of space that is found to be locally flat, and let it follow the Special Relativity description locally. The far distant black holes do not need to take from us the world we live in.

We could delve more into the details of the bizarre details and implications of high space-time curvature, and even there we might be surprised at the survival of observer 3-D cross-sections. Trouble is that we have no experimental data associated with an actual observer in those extreme circumstances.

Another problem is that physics doesn't seem to have a clear definition of the observer and consciousness, the later seeming to be outside the realm of physics at this point.

 

[Dale]

You talk physics, full of physical devices, but you won't accept a 3D world as it exists now (=collection of simultaneous events) is a 'physical object ? Do I understand you correctly?

Yes. Have I not clearly stated that several times already? The 3D world you mention is not experimentally detectable, so not physical.

But the clock displaying coordinate time is actually also a proper time display. A frame deals with physical clock displays, proper times displayed on the clock.

A frame defines coordinates on a 4D open subset of spacetime. A clock defines proper time only on the 1D worldline of the clock. Even for a clock whose proper time matches the coordinate time, the coordinate time is defined at events where the proper time is not. The two cannot be equated.

That's what Einstein meant by his 4D existence quotes

I am not sure why you quoted that. It seems to support my position.

 

[Dale]

Nevertheless, Special Relativity works quite well in our world.

For many purposes, yes. But there are lots of experiments here in our world that it does not cover.

However, my point is not predicated on either SR or GR. My point is that there is no known method of experimentally detecting the 3D world. To me that makes it non physical.

 

[tophatphysicist]

To me that makes it non physical

The concept of making physical measurements of position and time in a space-time that is nonphysical seems to be in conflict with common sense as well as physics. However, it's best not to pursue this further since it threatens to take us away from discussions of physics that would not be of interest to forum members.

 

[PeterDonis]

The concept of making physical measurements of position and time in a space-time that is nonphysical

Nobody is saying that 4D spacetime is non-physical. We are just saying that 3D worlds are nonphysical; they are artifacts of a coordinate choice. There is nothing physical that picks out a particular set of 3D spacelike hypersurfaces in a general curved 4D spacetime.

 

[PeterDonis]

give us a continuous sequence of 3-D cross-sections that extend into the vastness of space that is found to be locally flat

There are no such things. Once you get beyond the locally flat region and try to extend your cross sections into "the vastness of space", you are just arbitrarily picking out a particular set of cross sections; there is nothing physical that distinguishes one from any other.

 

[Grimble]

Or simply measurements made using different reference frames.

Yes; but I am specifically referring to those two particular frames: one with the clock at rest adjacent to the observer and one with the clock moving relative to the observer.

 

[Dale]

space-time that is nonphysical

Spacetime is physical, the reference frame is not, nor is any 3D object or world.

 

[Ebeb]

Intersting posts, guys.

Spacetime is physical, the reference frame is not, nor is any 3D object or world.

That's quite a statement to think about ;-) I agree a 3D object is a 3D section through physical 4D spacetime. The 'object' is a physical 4D worldtube. So far so good.
But then it seems obvious a 3D section is a physical object. I don't see why it shouldn't be or cannot be.

This whole discussion started trying to explain to Grimble that the time dipslay on a moving clock is a physical thing, not a mathematical issue. You will probably say that it is a mathematical issue because because you choose the way of synchronizing clocks. Indeed, but the time display on the ('moving') clock, any clock, is physical. A time display is what it reads on the clock. Even when it's spatially removed from an observer. That's why I can state that an event is something physical: it is what it is. If a frame selects a different event = a different physical clock display of a clock, to be simultaneous with my wristwatch time display, then that other clock time display is still a physical fact. So far 'events'.
A 3D world is a collection of events. I don't see why a collection physical items (physical time displays on a row of physical clocks) now all of a makes the set of clock non physical.

I think it is a dead end not consdering the different 3D worlds physical objects. Maybe you consider ONLY your present own event as something physical, but not all the other events happening around you? Would that be logical?Is that what you have in mind? Let's hope not.

Last minute, let me try to understand you... You probably will tell me that the clocks are physical, the time dispaly on those clocks too, but not the set, the selection itself of which physical clock with time display of the clock worldline one considers to be simultaneous?
Well, in that case we can get agreement. I recap:

1/ There's a physical 4D reality, 4D spacetime, invariant for all observers,
2/ There is no preferred 3D section (a 3D section =what we would consider our personal 3D reality 'now' (yes it's a mathematical construct of physical items)
3/ All events, 'read' by a frame, are physical building blocks of that 4D physial reality.

I.o.w. I consider myself now, at this present instant in time, a very small 3D part of one big 4D realtity, where -4D speaking- all events co-exist and thus past, present and future are only 'illusion'.
Perfect.

 

[Ebeb]

Yes; but I am specifically referring to those two particular frames: one with the clock at rest adjacent to the observer and one with the clock moving relative to the observer.

Have you understood what my diagrams (post#124 and #127) show?

 

[jbriggs444]

There is no preferred 3D section (a 3D section =what we would consider our personal 3D reality 'now' (yes it's a mathematical construct of physical items)

i.e. a "hypersurface of simultaneity".

Promoting that notion to a "personal 3D reality" seems unnecessary. Calling it "physical" implies that there is something more physically substantial to such a hypersurface than a choice of simultaneity convention.

 

[Dale]

But then it seems obvious a 3D section is a physical object. I don't see why it shouldn't be or cannot be.

If I have a rock, I can use balances and chemicals to determine the physical mass and composition of the rock. I can also imagine slicing the rock into imaginary slices and on a whim I can imagine slicing it multiple different ways without changing anything physically measurable about the rock. The rock is physical with physically measurable properties, the imaginary slices are not.

Similarly with spacetime. I can use clocks and rods to measure spacetime intervals. Using those measures I can determine the physical geometry of spacetime. I can also draw imaginary lines on spacetime and give them labels like t, x, y, z. I can draw those imaginary lines multiple different ways without changing anything physically measurable about spacetime.

This whole discussion started trying to explain to Grimble that the time dipslay on a moving clock is a physical thing, not a mathematical issue. ... the time display on the ('moving') clock, any clock, is physical. A time display is what it reads on the clock. Even when it's spatially removed from an observer. That's why I can state that an event is something physical: it is what it is.

I agree completely with this.

If a frame selects a different event = a different physical clock display of a clock, to be simultaneous with my wristwatch time display, then that other clock time display is still a physical fact.

Yes, but the simultaneity is not. I.e. There are two proper times on two different clocks at two different space like separated events. That is all physical. Saying those two events were simultaneous is not.

A 3D world is a collection of events. I don't see why a collection physical items (physical time displays on a row of physical clocks) now all of a makes the set of clock non physical.

There is nothing physically measurable that singles out that specific collection. I can use a different collection and obtain all of the same physical measurements.

I think it is a dead end not consdering the different 3D worlds physical objects. Maybe you consider ONLY your present own event as something physical, but not all the other events happening around you? Would that be logical?Is that what you have in mind? Let's hope not

I am not sure how you go from my comments that physical objects are 4D and that 4D spacetime is physical to thinking that I would make this claim.

Last minute, let me try to understand you... You probably will tell me that the clocks are physical, the time dispaly on those clocks too, but not the set, the selection itself of which physical clock with time display of the clock worldline one considers to be simultaneous?

Yes, and I also agree with your recap, where the phrase "now, at this present instant in time" is understood to be just a label for a 3D set of events.

 

[tophatphysicist]

If you think about it, our perception delivered by the brain after receiving
information from the optical nerves is an averaged out "image", average over
somewhere around the last 300,000 miles or so of travel along the 4th dimension
(if you set a mode of consciousness watching the continuous sequence of 3-D
cross-sections of the neuron 4-fibre bundle as it advances along the observer
world line at the speed of light -- and assuming some conscious response time).
One way of interpreting that might be to say that consciousness observes a running
average of 4-D sections (3-D cross-sections extending 300,000 miles along the
4th dimension). In a sense you might say that we are actually observing 4-D objects.

I won't get into the details of whether the 3-D cross-sections make up a continuum
or whether they are discrete, perhaps with Planck time sequences. But, we already
know that the optical nerves/brain system has a relatively enormously long time
constant.

 

[tophatphysicist]

Please don't think I don't know the difference between a superposition of 3-D images and a truly 4-D image.

 

[Grimble]

We use coordinates to describe points in space, times, points in spacetime, that is to events. Coordinates refer to a set of axes that form a reference grid. When we calculate the separation between two events are we not measuring that separation, in that virtual world we have created, our thought world?

I can say that in my thought world there exists a real framework - and clocks and rulers - against which I can measure. I am fortunate in my virtual thought world, for my axer, my measuring grid exists and is real but does not interact with any objects in that world. So my grid, i.e. my frame of reference is a part of that world and can be used to measure against.

 

[Dale]

We use coordinates to describe points in space, times, points in spacetime, that is to events. Coordinates refer to a set of axes that form a reference grid. When we calculate the separation between two events are we not measuring that separation, in that virtual world we have created, our thought world?

I can say that in my thought world there exists a real framework - and clocks and rulers - against which I can measure. I am fortunate in my virtual thought world, for my axer, my measuring grid exists and is real but does not interact with any objects in that world. So my grid, i.e. my frame of reference is a part of that world and can be used to measure against.

In a thought experiment you have to be careful to think clearly and write clearly.

Even in a thought experiment a reference frame is 4D and assigns coordinates (including coordinate time) to every event in a 4D region of spacetime. Even in a thought experiment a clock measures proper time only along its own 1D worldline. Even in a thought experiment there is no natural mapping between events on one clock's worldline and events on another clock's worldline (except where the worldlines intersect), so if you wish to do such a mapping you must describe the convention you are using.

You can indeed put as many clocks as you like in as many locations as you choose in a thought experiment, but each clock follows the rules of clocks. The rules of clocks are different from the rules of coordinate systems, even in a thought experiment.

Do you understand this? In particular do you understand that a coordinate system covers a 4D region of spacetime and a clock covers a 1D worldline?

 

[Ebeb]

If I have a rock, I can use balances and chemicals to determine the physical mass and composition of the rock. I can also imagine slicing the rock into imaginary slices and on a whim I can imagine slicing it multiple different ways without changing anything physically measurable about the rock. The rock is physical with physically measurable properties, the imaginary slices are not.

Similarly with spacetime. I can use clocks and rods to measure spacetime intervals. Using those measures I can determine the physical geometry of spacetime. I can also draw imaginary lines on spacetime and give them labels like t, x, y, z. I can draw those imaginary lines multiple different ways without changing anything physically measurable about spacetime.

I'll keep this for last.

Yes, but the simultaneity is not. I.e. There are two proper times on two different clocks at two different space like separated events. That is all physical. Saying those two events were simultaneous is not.

Correct, but you have to take it one step further: If you choose a frame per which they ARE simultaneous, then it would be fair to say that per that frame all the simultaneous events form a physical 3D world of simultaneous events.

There is nothing physically measurable that singles out that specific collection. I can use a different collection and obtain all of the same physical measurements.

I don't understand exactly what you mean here. Because when you deal with a different set of simultaneous events, the 3D section has different physical property than another 3D section, in the sense that f.ex. the event 'tree is black" (because it took fire), is definitely a different physical property than the 3D section where the tree didn't take fire yet...

I am not sure how you go from my comments that physical objects are 4D and that 4D spacetime is physical to thinking that I would make this claim.

Mea culpa. I just wanted to make 100% sure.

Yes, and I also agree with your recap, where the phrase "now, at this present instant in time" is understood to be just a label for a 3D set of events.

Of course. And I would be more specific; a set of simultaneous events. That's the observer's 3D environment 'now'.
And because all the events are physical items, that makes such a 3D set (also) a physical thing...

Let me now deal with slicing the rock.

The rock is physical with physically measurable properties, the imaginary slices are not.

I find it very awkward not being allowed to say that the 3D section of the rock is a physical thing. The slices of rock are made of physical rock atoms. If you cut a loaf of bread, whatever cutting angle you choose, the slice of bread is stil made of physical bread molecules. Why would the slice of bread not be a physical thing/object?
Same for cutting 3D sections though 4D spacetime. We agreed that an object (let's consider a train), is 4D physical thing, 'made' of physical events'. The act of slicing itself can be considered a "mathematical simultaneity convention". I agree... But, that's not the issue. It's about the slice of bread, the short train for that matter, being (also) a physical thing: it's a 3D physical part of 4D physical object. No offence, but I don't understand why you don't seem to understand this.

 

[Ebeb]

If you think about it, our perception delivered by the brain after receiving
information from the optical nerves is an averaged out "image", average over
somewhere around the last 300,000 miles or so of travel along the 4th dimension
(if you set a mode of consciousness watching the continuous sequence of 3-D
cross-sections of the neuron 4-fibre bundle as it advances along the observer
world line at the speed of light -- and assuming some conscious response time).
One way of interpreting that might be to say that consciousness observes a running
average of 4-D sections (3-D cross-sections extending 300,000 miles along the
4th dimension). In a sense you might say that we are actually observing 4-D objects.

I won't get into the details of whether the 3-D cross-sections make up a continuum
or whether they are discrete, perhaps with Planck time sequences. But, we already
know that the optical nerves/brain system has a relatively enormously long time
constant.

Do you mean with 'running of sections' that different observers look/observe/read/measure/touch a 4D physical object? Yes I would agree, but -and this is where I differ from Dalespam, I would consider the result of that measurement (f.ex. a collection of physical events being simultaneous; the shorter train) a "physical 3D object". Wouldn't you?

 

[Dale]

If you choose a frame per which they ARE simultaneous, then it would be fair to say that per that frame all the simultaneous events form a physical 3D world of simultaneous events.

No, it wouldn't. Making an arbitrary non-physical convention cannot magically produce physical results. If you wish you can say "all the simultaneous events form a 3D world", but the adjective "physical" does not belong there.

when you deal with a different set of simultaneous events, the 3D section has different physical property than another 3D section

No, it doesn't. There is no physical measurement which would be affected by such a choice. No matter what measurement you propose and which 3D world you use, I can show how you get exactly the same measurement using a different 3D world. The choice has no measurable consequence, therefore it is not physical.

I find it very awkward not being allowed to say that the 3D section of the rock is a physical thing. The slices of rock are made of physical rock atoms. If you cut a loaf of bread, whatever cutting angle you choose, the slice of bread is stil made of physical bread molecules. Why would the slice of bread not be a physical?

Same for cutting 3D sections though 4D spacetime.

If you take a knife and physically cut a rock or a loaf of bread then you are making a physical change to the rock or loaf. I can experimentally measure this physical change, e.g. by measuring the shear stiffness of the rock. Are you suggesting that the same thing happens in spacetime? If so, what measurement will allow me to detect this physical change in spacetime from your cutting?

A reference frame doesn't physically cut spacetime, it just puts arbitrary labels on it.

No offence, but I don't understand why you don't seem to understand this

I feel the same.

Let me ask you this. What do you personally mean when you use the word "physical"? I personally mean "experimentally measurable".

 

[Ebeb]

No, it wouldn't. Making an arbitrary non-physical convention cannot magically produce physical results. If you wish you can say "all the simultaneous events form a 3D world", but the adjective "physical" does not belong there.

Let me put forward what I consider physical/ physical property. If the 3D train has a yellow paint then that's a physical property of that 3D train at that instant of time." I won't consider that a property of a different section of the 4D train, made before de train was painted yellow.
If the train is painted yellow at one stage of its life, and painted pink during another stage of it's life, then those two 3D sections show/contain two different properties: One section through the 4D train gives you the 3D train with yellow paint property, and another section through the 4D train gives you the 3D train with pink property. I consider this two different physical 3D trains... but part of one and the same physical 4D train that includes the train events yellow paint and events pink paint. The 4D train is a unit made of the full life of 'train', a 3D train only being one split second instant of time of it. Would this help?

No, it doesn't. There is no physical measurement which would be affected by such a choice. No matter what measurement you propose and which 3D world you use, I can show how you get exactly the same measurement using a different 3D world. The choice has no measurable consequence, therefore it is not physical.

See comment above

If you take a knife and physically cut a rock or a loaf of bread then you are making a physical change to the rock or loaf.

No, I won't really physically cut it. I would only select a section of molecules of that rock! Such a section of molecules is made of rock molecules. They are different than another secion of molecules. Now we can argue whether we make a 'physical change' by considering a different section molecules of the rock? The properties of the full rock won't change of course, but the property of a section molecules will be different than another section of change, for example: the color of the molecules might be different for another section molecules... See above for train...
If the train at rest has the property yellow piant, and a split second later the full train has blue paint, then the 3D moving train per my frame has property: parts have yellow paint, other parts have blue paint.

I can experimentally measure this physical change, e.g. by measuring the shear stiffness of the rock. Are you suggesting that the same thing happens in spacetime? If so, what measurement will allow me to detect this physical change in spacetime from your cutting?

See above for rock.

A reference frame doesn't physically cut spacetime, it just puts arbitrary labels on it.

See comment for slicing the rock

I feel the same.

Let me ask you this. What do you personally mean when you use the word "physical"? I personally mean "experimentally measurable".

See above.
I "experimentally measure" the train at rest per my frame to be made of simultaneous events and that I "experimentally measure" the moving train per my frame also to be made of simultaneous events, but both sets of events are different. Both sets of events show me a different 3D train, different 'properties' (see above). Both the train at rest and the shorter 3D train (moving relative to me) are part of one and the same 4D train object. That's what is meant by making 3D sections through a 4D spacetime unit. I really don't know how I can put it simpler.

I forgot to add following sketch in previous post:
.

 

[jbriggs444]

A "physical" human being touching the ends of the car brings his own a simultaneity convention to the table -- one that is merely conventional, not physical.

 

[Grimble]

Have you understood what my diagrams (post#124 and #127) show?

Yes indeed, they are Loedel diagrams?
But I don't see how, when we look at the red clock's time axis the time for each tick on that axis is the same for both observer's (as the proper time read by the red clock observer and for the coordinate time calculated by the green clock observer).
Both the red and the green axes have the same unit size marked on them. OK. But as you say

Per the green frame[...]Red clock "ticks" slower than the green clock, aka red clock "time dilation".

Surely there ought to be two diagrams with different unit size according to observer?

Simplifying the diagram in #124, I have reduced the green clock's time to 1 second.
Measured by the observer holding each clock, that clock's light will have traveled 1 light second and will have reached its mirror.

Relative to the green clock, however, the light in the red clock will have traveled 1.25 light seconds to the mirror. That is 1.25 seconds between the same two events that measured within the red clock takes only 1 second.​

 

[Dale]

Before proceeding, can you please answer the question I asked above: What do you personally mean when you use the word "physical".

Let me put forward what I consider physical/ physical property. If the 3D train has a yellow paint then that's a physical property of that 3D train at that instant of time." I won't consider that a property of a different section of the 4D train, made before de train was painted yellow

I agree, you can measure the wavelength of reflected light at any given event on the train.

But a reference frame is not paint. It doesn't physically change anything about the train. There is no physical difference between the train which is momentarily painted yellow in one frame and a train which has a yellow stripe flashing across the train in another frame.

That is precisely the point. A reference frame doesn't physically slice anything nor does it physically paint anything. It doesn't physically do anything, so it is not physical. Why on Earth do you insist on putting the label "physical" on something that you recognize does not physically do anything.

I "experimentally measure" the train at rest per my frame to be made of simultaneous events

This may be the heart of the problem. There is no possible experimental measurement whose outcome can depend on whether or not two events are simultaneous.

 

[Grimble]

Even in a thought experiment a reference frame is 4D and assigns coordinates (including coordinate time) to every event in a 4D region of spacetime.

But as soon as you use coordinates you are using a set of axes that those coordinates relate to - are based upon...
It seems to me that when you measure distances and times using coordinates you must be making them based on those axes. That is why different frames have different coordinates for the same events.
Is a reference frame not an observer's map of spacetime?

 

[Dale]

It seems to me that when you measure distances and times using coordinates you must be making them based on those axes.

Actually, the measurements are independent of the axes. You can make a measurement with a clock without ever defining an axis, and your coordinate system axes need not be aligned to your measuring device in any way.

Is a reference frame not an observer's map of spacetime?

You are free to use any reference frame as your map. An observer doesn't have to use the one where they are at rest.

 

[Dale]

Some off topic posts have been deleted

 

[Ebeb]

What it means is that there are physical measuring devices and there are mathematical abstractions. A measuring stick belongs in the first category and a reference frame belongs in the second.

Spacetime is physical, the reference frame is not, nor is any 3D object or world.

Seems to be contradictory.
If some device is doing the measurement, I suppose we agree the device is a physical object.
I guess a 3D clock can be considered being physcial 3D object.
Would you consider a 3D measuring stick a physical 3D object?
If you answer 'yes', I suppose you mean a 3D stick of simultaneous events being an object.

 

[jbriggs444]

.
I guess a 3D clock can be considered being physcial 3D object.

A 3D clock has length, width and height. But in order to be useful, it also has duration. It is a 4D object.

I've never seen a 3D measuring stick. All the ones I've ever used have lasted long enough to reach down and pick it up.

 

[Dale]

I suppose we agree the device is a physical object.

Yes. And physical objects are 4D.

a 3D clock

There is no such thing. Clocks are 4D objects.

a 3D measuring stick

There is no such thing. Measuring sticks are 4D objects.

 

[Ebeb]

A 3D clock has length, width and height. But in order to be useful, it also has duration. It is a 4D object.

I've never seen a 3D measuring stick. All the ones I've ever used have lasted long enough to reach down and pick it up.

Never seen a 3D measuring stick? The cup of tea I keep in my hand at present split instant 'now' is a 3D cup of tea object, or not? But it's a section of the 4D cup object, yes I agree.

 

[Ebeb]

Yes. And physical objects are 4D.

There is no such thing. Clocks are 4D objects.

There is no such thing. Measuring sticks are 4D objects.

O.K, Dale.
I think I'll leave it there. I'm afraid pushing this discussion any further to deal with the 3D sections won't help. But I'm happy we at least agree on 4D objects. Thanks for comments.

 

[tophatphysicist]

If some device is doing the measurement, I suppose we agree the device is a physical object.

Never seen a 3D measuring stick? The cup of tea I keep in my hand at present split instant 'now' is a 3D cup of tea object, or not? But it's a section of the 4D cup object

Your comments are so self-evident logical and correct, Ebeb. l don't understand the confusion. They seem to think there's an equivalence between a 3-D object and a 2-D cross-section slice, just because a 3-D object is a 3-D section of the extensive 4-D object. 3-D objects were understood in physics long before Special Relativity came along, and the 3-D object concept has not changed. I once had a conversation with the well know physicist and author, Paul Davies, while visiting my old alma mater, Arizona State University. He explained it simply as "...that's just the world we live in"... that is, the world of 3-D objects.

 

[Dale]

Your comments are so self-evident logical and correct, Ebeb. l don't understand the confusion. They seem to think there's an equivalence between a 3-D object and a 2-D cross-section slice, just because a 3-D object is a 3-D section of the extensive 4-D object. 3-D objects were understood in physics long before Special Relativity came along, and the 3-D object concept has not changed. I once had a conversation with the well know physicist and author, Paul Davies, while visiting my old alma mater, Arizona State University. He explained it simply as "...that's just the world we live in"... that is, the world of 3-D objects.

"I once had a conversation" is not a valid reference. And your post neglects the entire issue.

Are there any physical objects which are 2D?