Is Coordinate-Free Relativity the Key to Understanding General Relativity?

[Ben Niehoff]

Well, regardless of the definitions of manifolds, reference frames, and coordinate system, let me just reiterate my point:

There is no way you can picture anything, or describe shape, location, or size, without an origin (your eyes), and without picturing it at a certain distance away from you.

Your point has nothing to do with coordinate systems.

 

[Dale]

There is no way you can picture anything, or describe shape, location, or size, without an origin (your eyes), and without picturing it at a certain distance away from you.

I agree completely. In fact, that further emphasizes the idea that a lot of geometry can be done without coordinates.

 

[JDoolin]

Ben says "A coordinate system is a map from an open subset of manifold into R^n."
What are examples of open subsets of manifolds? Are the chalkboard or the computer monitor not fair examples of subsets of manifolds?
What are examples of R^n? If you describe the tension in each contractible muscle in your arm, is that not an example of R^n? If you describe the direction of an image in front of your face, wouldn't the natural inclination be to describe this either in terms of left, right, up and down, forward and backward; either in a rectangular or spherical coordinate system?
Is there any way to describe distance without invoking some kind of numerical measure. (I can acknowledge that a dog may or may not invoke numbers in estimating distances, but if not, he also cannot communicate to other dogs where something is. On the other hand, bees are known to communicate quantitatively about distant locations.)

Let me try some questions and see if you can answer them without invoking any kind of coordinate system.

• How big is the screen you're looking at?
• How far away is it?
• Which direction is it from you?
• When you write or draw a picture on a chalkboard, what position do you hold your shoulder, your elbow, your wrist?
• How far away do you stand from the chalkboard?

What is it exactly about this statement

There is no way you can picture anything, or describe shape, location, or size, without an origin (your eyes), and without picturing it at a certain distance away from you.

...that says it does NOT involve a coordinate system?

 

[JDoolin]

Upon mentioning bees in my last post, it occurred to me that another way of describing things is with landmark based geometry. Instead of stating a distance, you just say from this landmark go to that landmark.

You need not mention direction, distance, shape or size.

 

[lurflurf]

JDoolin Coordinates are almost like a religion to you. The point is not "Is it possible to use Coordinates?" but "Is it helpful to use Coordinates?". If we do decide to use Coordinates we must decide which ones to use and how many. A reasonable answer is none to both. Even when possible, using coordinates is not always worth the trouble they cause. Often even a pro coordinate zealot will be say things like

"Imagine that we have some coordinates, but we do not know anything about them, but they are there really they are, they are really messed up, but that is okay, we love them anyway, they are really complicated, we do not know how to get any numbers, if we had numbers it would not help because there would be so many and there would be so much error and the calculations would be so impossible, that is okay though we are not going to use them anyway, also these coordinates require that we embed the object we are working with into a much more complicated object which might be impossible..."

Why would we want to introduce coordinates if (in a particular situation) we are not going to use them and they are not helpful? At best we have wasted time, and introduced needless complications.

 

[Monocles]

Maybe something that would help understanding is to emphasize the point that, in general, if you are given two different sets of coordinates of some objects (which may or may not be the same object), it is extremely difficult to tell if the sets of coordinates are describing the same object. So, we describe objects in a coordinate-free fashion so that we never have to worry about that problem.

Using a programming analogy, this is (I believe) an equivalent problem to the halting problem, since the halting problem is equivalent to the word problem for groups, and group presentations are 'coordinates for groups'.

For examples of how mathematicians think about things that have no concept of size, distance, etc. consider topological spaces. None of these notions exist until you define a metric.

 

[Ben Niehoff]

JDoolin, the concepts of "distance" and "angle" do not require the use of coordinate systems at all.

Think back to the classical Greek geometry you did in high-school. Suppose I have a triangle ABC composed of three lines AB, AC, and BC. There is an unambiguous notion of the angle A that exists independently of any coordinate system. There is an unambiguous notion of the distance AB that exists independently of any coordinate system. In fact, there is no need to use coordinate systems at all; relying on a simple set of axioms, one can derive all geometrical facts using only the pure geometrical concepts of distances and angles.

It was Descartes who invented (or reinvented) the notion of "analytic geometry": that is, marking the points of the triangle ABC by some coordinate system and then using the coordinate system to derive facts about the triangle ABC. This method makes some geometrical proofs more straightforward, but it is certainly not necessary to use a coordinate system, and in many cases it adds unneeded complexity.

For example, try to show that the so-called "conic sections" are actually sections of a cone. Using coordinate systems, this is an algebraic nightmare. Using pure geometry, there is an elegant trick.

 

[Dale]

What are examples of open subsets of manifolds? Are the chalkboard or the computer monitor not fair examples of subsets of manifolds?

An open subset specifically excludes the border, so if you draw a line and say everything inside the line (but not including the line) then that is an open subset of the manifold of the surface of the chalkboard or monitor.

What are examples of R^n?

For a 2D manifold like a chalkboard it would be R^2, i.e. pairs of real numbers (x,y) or (r,theta) or ...

Let me try some questions and see if you can answer them without invoking any kind of coordinate system.

• How big is the screen you're looking at?
• How far away is it?
• Which direction is it from you?
• When you write or draw a picture on a chalkboard, what position do you hold your shoulder, your elbow, your wrist?
• How far away do you stand from the chalkboard?

1) ~23" on the diagonal
2) ~19" from the monitor to the tip of my nose
3) The center of the monitor is straight ahead and at a ~100º angle from vertical
4) I move them all over
5) ~15" away

Note that none of the above required the specification of a coordinate system.

 

[JDoolin]

JDoolin, the concepts of "distance" and "angle" do not require the use of coordinate systems at all.

Think back to the classical Greek geometry you did in high-school. Suppose I have a triangle ABC composed of three lines AB, AC, and BC. There is an unambiguous notion of the angle A that exists independently of any coordinate system.

Go back to my post 31 and see if you can say without question whether the angle between the paths is 90 degrees or 180 degrees.

There is an unambiguous notion of the distance AB that exists independently of any coordinate system.

And what notion of distance would that be? The cartesian distance? The space-time-interval? The arc length of a geodesic? The arc-length of the null path in the reference frame of the nearest gravitational well (which is, of course zero)? Which null path would you choose to use? Which unambiguous notion of distance are you using?

In fact, there is no need to use coordinate systems at all; relying on a simple set of axioms, one can derive all geometrical facts using only the pure geometrical concepts of distances and angles.

It was Descartes who invented (or reinvented) the notion of "analytic geometry": that is, marking the points of the triangle ABC by some coordinate system and then using the coordinate system to derive facts about the triangle ABC. This method makes some geometrical proofs more straightforward, but it is certainly not necessary to use a coordinate system, and in many cases it adds unneeded complexity.

For example, try to show that the so-called "conic sections" are actually sections of a cone. Using coordinate systems, this is an algebraic nightmare. Using pure geometry, there is an elegant trick.

Well, I will acknowledge that there are surely some interesting things you can do with geometry without defining the locations of points. Like sewing instructions... You can take the corners of a rectangle and sew the ends together to make a mobius strip.

But once you have decided that you are using ANGLES and DISTANCES to describe the location of landmarks and features, you have implicitly defined a coordinate system. I don't know what you mean by "pure" geometric structures. But I can say that you need to look a little deeper for the "impurities" and ambiguities that really do exist in these lofty concepts.

 

[Ben Niehoff]

And what notion of distance would that be? The cartesian distance? The space-time-interval? The arc length of a geodesic? The arc-length of the null path in the reference frame of the nearest gravitational well (which is, of course zero)? Which null path would you choose to use? Which unambiguous notion of distance are you using?

We're talking about ordinary, Euclidean plane geometry here, so I don't see why you're going on about spacetime intervals. There is no time dimension involved. There is a triangle ABC formed by three lines AB, AC, and BC. The line AB has a length we can measure by holding a ruler up against it. The angle A can be measured by holding a protractor up against it. We can rotate and translate the paper in any way we like; the length of the line AB and the measure of the angle A are invariant.

Well, I will acknowledge that there are surely some interesting things you can do with geometry without defining the locations of points. Like sewing instructions... You can take the corners of a rectangle and sew the ends together to make a mobius strip.

The "sewing instructions" thing you've described is topology, not geometry. Topology studies how spaces are connected and how different spaces can be mapped into each other.

Geometry studies what happens once you define a notion of "distance" and "angle".

But once you have decided that you are using ANGLES and DISTANCES to describe the location of landmarks and features, you have implicitly defined a coordinate system.

No, I haven't. Why do you think so?

 

[Dale]

But once you have decided that you are using ANGLES and DISTANCES to describe the location of landmarks and features, you have implicitly defined a coordinate system.

This is not correct. See my above reply to your list of questions. I never defined a coordinate system.

 

[JDoolin]

Let me try some questions and see if you can answer them without invoking any kind of coordinate system.
How big is the screen you're looking at?
How far away is it?
Which direction is it from you?
When you write or draw a picture on a chalkboard, what position do you hold your shoulder, your elbow, your wrist?
How far away do you stand from the chalkboard?

1) ~23" on the diagonal
2) ~19" from the monitor to the tip of my nose
3) The center of the monitor is straight ahead and at a ~100º angle from vertical
4) I move them all over
5) ~15" away

Note that none of the above required the specification of a coordinate system.

See my above reply to your list of questions. I never defined a coordinate system.

To the contrary, almost all of your answers specify coordinate systems.

Your first answer maps an open subset of the space in your room to R^1.
Your second answer maps an open subset of the space in your room to R^1.
The third answer is a projection of a vertical plane in your room onto [0,360).
Your fourth answer only succeeds in avoiding a coordinate system by failing to be specific.
Your fifth answer invokes three dimensions, since you are probably describing the average distance between the surface of the chalkboard and the surface of your body.

 

[JDoolin]

We're talking about ordinary, Euclidean plane geometry here, so I don't see why you're going on about spacetime intervals.

If you are using Euclidean plane geometry (i.e. nothing is moving; nothing has any relative velocity), then I would have to agree that angle and distance have unambiguous meanings.

However, I think it is also interesting (if we are talking about relativity) to consider objects that are moving.

There is no time dimension involved. There is a triangle ABC formed by three lines AB, AC, and BC. The line AB has a length we can measure by holding a ruler up against it. The angle A can be measured by holding a protractor up against it. We can rotate and translate the paper in any way we like; the length of the line AB and the measure of the angle A are invariant.

If you are constraining yourself to talking about Euclidean plane geometry, lying on a stationary page, then all you are saying is correct. But, again, the thread is about coordinate free "relativity" so my question in post 44 about post 31 is still valid in the larger context.

 

[Dale]

To the contrary, almost all of your answers specify coordinate systems.

No, not one of them did.

Your first answer maps an open subset of the space in your room to R^1.
Your second answer maps an open subset of the space in your room to R^1.

The space in my room is 3D, so a coordinate system in my room maps open subsets of the space in my room to open subsets of R^3.

However, for the sake of argument, even considering a 1D embedded manifold in my room (so that we can map open subsets of the manifold to open subsets of R^1), a measure of the distance between two points does not establish a coordinate system.

First, the measure of distance is invariant under shifts of the origin. Is the origin on me or is it on the monitor or on some other point elsewhere? Second, the measure of distance is invariant under reversals of the basis vector. Do coordinates increase from me to the monitor or from the monitor to me? Third, the measure of distance in inches does not preclude the use of a coordinate system using other units. Do the coordinates change at a rate of one coordinate per inch or one coordinate per meter? Fourth, the measure of distance does not indicate if the coordinate system is uniform. Do the coordinates change at a linearly decreasing rate as a function of distance?

A measure of distance simply does not establish a mapping to R^n. There are many unspecified details. By telling you the distance from me to the monitor is 19" you cannot tell me unambiguously what is the coordinate position for me nor what is the coordinate for the monitor nor what are the coordinates for each point between us. Nothing less constitutes a coordinate system.

The third answer is a projection of the horizonal plane in your room onto [0,360).
Your fourth answer only succeeds in avoiding a coordinate system by failing to be specific.
Your fifth answer invokes three dimensions, since you are probably describing the average distance between the surface of the chalkboard and the surface of your body.

Similarly with all of these. None specify a coordinate system because all of them leave a huge variety of details unspecified. Do you understand the difference between measuring a distance and specifying a coordinate system?

 

[JDoolin]

No, not one of them did.

The space in my room is 3D, so a coordinate system in my room maps open subsets of the space in my room to open subsets of R^3.

There is a path from on corner of your screen to the other corner of your screen. I suppose it may not actually be an OPEN subset of your room, since it is only one-dimensional. In the other two dimensions, you might call it a closed subset, since a set containing only one point is a closed set.

I believe some use the word "clopen" to describe such subsets as a line or a plane through space.

However, for the sake of argument, even considering a 1D embedded manifold in my room (so that we can map open subsets of the manifold to open subsets of R^1), a measure of the distance between two points does not establish a coordinate system.

First, the measure of distance is invariant under shifts of the origin. Is the origin on me or is it on the monitor or on some other point elsewhere?

If you are finding the distance from point A to point B, then your origin is at point A. But if you use the distance formula $distance = \left |x_b-x_a \right |$, then x_b and x_a must be defined from some other point (the origin).

Even if you use an unnumbered ruler to measure the distance, you still must determine the "from" point, and by doing so, you have defined an origin.

Second, the measure of distance is invariant under reversals of the basis vector. Do coordinates increase from me to the monitor or from the monitor to me?

This would depend on where you place your oigin (where you are measuring from.)

Yes, you can arbitrarily designate your origin, but that does not mean you can make the measurement without choosing an origin at all.

Third, the measure of distance in inches does not preclude the use of a coordinate system using other units. Do the coordinates change at a rate of one coordinate per inch or one coordinate per meter?

That's correct. You could even choose some weird logarithmic scale if you wanted. But again, similarly, you can arbitrarily designate any unit length you wish, but that does not mean you can make the measurement without choosing a unit length at all.

Fourth, the measure of distance does not indicate if the coordinate system is uniform. Do the coordinates change at a linearly decreasing rate as a function of distance?

Again, that all depends on your choice of how to define your unit size.

A measure of distance simply does not establish a mapping to R^n. There are many unspecified details. By telling you the distance from me to the monitor is 19" you cannot tell me unambiguously what is the coordinate position for me nor what is the coordinate for the monitor nor what are the coordinates for each point between us. Nothing less constitutes a coordinate system.

Nothing less? I disagree.

A coordinate system need only have dimension high enough to measure whatever quantities you are interested in. If I ask you for the diagonal length of your computer screen, we only need a one dimensional coordinate system. If I ask you the dimensions of the computer screen, we need a two-dimensional coordinate system. If I ask you where your left eye is in relation to the computer screen, we need a three-dimensional coordinate system. If I allow for the fact that your eyes may be moving at a relative velocity wih the computer screen, and I ask the same question, we need a four-dimensional coordinate system.

Similarly with all of these. None specify a coordinate system because all of them leave a huge variety of details unspecified. Do you understand the difference between measuring a distance and specifying a coordinate system?

I cannot imagine any way to measure a distance without specifying a "from" point and defining some scale with which to mark off the distance.

I think you could define a coordinate system without determining any distances, (so I see there is a difference), but I don't think you can go the other way, and determine a distance without defining, at least, a one-dimensional coordinate system.

 

[Dale]

But if you use the distance formula $distance = \left |x_b-x_a \right |$, then x_b and x_a must be defined from some other point (the origin). ...

Yes, you can arbitrarily designate your origin, but that does not mean you can make the measurement without choosing an origin at all.

Since you seem to be aware that the distance is completely independent of choice of origin then I don't see why you think it must be defined at all. I made the measurement without defining a coordinate system and choosing an origin.

That's correct. You could even choose some weird logarithmic scale if you wanted. But again, similarly, you can arbitrarily designate any unit length you wish, but that does not mean you can make the measurement without choosing a unit length at all.

I never said it did. But the fact is that making a measurement with a chosen unit of length does not constrain nor inform your choice of coordinates in any way.

You seem to be under the misapprehension that a coordinate system is the same as making a measurement that results in a number. That is incorrect, all measurements are invariant under arbitrary coordinate transforms. So the existence of a measurement does not require nor inform you as to any coordinate system. Nature does not come equipped with a required set of coordinates, regardless of what measurements you may make.

You should know the definition of a coordinate system by now. Can you unambiguously define a unique coordinate system from the fact that the distance between A and B is 6"?

A coordinate system need only have dimension high enough to measure whatever quantities you are interested in. If I ask you for the diagonal length of your computer screen, we only need a one dimensional coordinate system. If I ask you the dimensions of the computer screen, we need a two-dimensional coordinate system. If I ask you where your left eye is in relation to the computer screen, we need a three-dimensional coordinate system. If I allow for the fact that your eyes may be moving at a relative velocity wih the computer screen, and I ask the same question, we need a four-dimensional coordinate system.

You don't need a coordinate system for any of those measurements.

I cannot imagine any way to measure a distance without specifying a "from" point and defining some scale with which to mark off the distance.

I (almost*) agree, but that is irrelevant. The "from" point need not be the origin and the scale to mark off the distance need not correspond to any coordinates.

*The almost is that in order to measure the distance between A and B you do not need to identify one as "from" and the other as "to". The measurement of distance is invariant under that choice.

 

[JDoolin]

You can describe the distance between two points without defining a coordinate system.

But in order to measure the distance between two points, you must define an origin and unit length, which is the same as defining a coordinate system.

 

[JDoolin]

That is incorrect, all measurements are invariant under arbitrary coordinate transforms.

The angles between time-like paths are not invariant under velocity transformation.

I went ahead and made an animation so that you can see what I mean.

The angle between the two marked paths is 90 degrees, but from the perspective of the dot in the middle, the angle between the particle paths is 180 degrees.

To claim that "all measurements are invariant under arbitrary coordinate transforms" is one of those true, but misleading statements. The measurement of the angle depends on the velocity of the protractor that measures the angle. Sure, no matter what reference frame you're in that protractor will measure the same angle, but some observers will note that the protractor is distorted, and the measurement is actually incorrect. Just because the measurement is invariant, but the actual observation is very different.


And I want to reiterate what I said in my previous post:

While it is possible to describe things without defining an origin, it is impossible to measure things without explicitly defining an origin and unit length, and it is impossible to visualize anything without implicitly defining an origin.

Even my ability to describe things without defining a coordinate system: ( "My monitor is 20 bloots across" ) actually conveys no useful information, until I define what a bloot is.

 

[Dale]

To claim that "all measurements are invariant under arbitrary coordinate transforms" is one of those true, but misleading statements. The measurement of the angle depends on the velocity of the protractor that measures the angle.

In what way is that at all misleading? I said exactly what I meant (and what I said was true), and you understood exactly what I meant (and acknowledged its truth).

And I want to reiterate what I said in my previous post:

While it is possible to describe things without defining an origin, it is impossible to measure things without explicitly defining an origin and unit length, and it is impossible to visualize anything without implicitly defining an origin.

OK, I have measured the distance between A and B to be 8.5". Where is the origin?

 

[Ben Niehoff]

But in order to measure the distance between two points, you must define an origin and unit length, which is the same as defining a coordinate system.

No it isn't. A coordinate system is "A continuous map from some open subset U of the manifold M into R^n, where n is the dimension of M". Defining "an origin and a unit length" does not give you a continuous map from U to R^n.

 

[JDoolin]

In what way is that at all misleading? I said exactly what I meant (and what I said was true), and you understood exactly what I meant (and acknowledged its truth).

Measurements, as a rule, are observer dependent, but any given measurement is observer independent, because the reference frame of the observation device is already determined.

To say "all measurements are invariant under arbitrary coordinate transforms" is misleading because it does not specify whether you mean "measurements in general" or "any given measurement." It strongly suggests you mean "measurements in general" which would make the statement false.

I only acknowledged the statement's truth based on one possible interpretation.

OK, I have measured the distance between A and B to be 8.5". Where is the origin?

I don't know. All I can guarantee is that when you made the measurement, you referenced your measurement from some origin, and in order to visualize that distance, I must reference it from some origin.

 

[JDoolin]

You can describe the distance between two points without defining a coordinate system.

But in order to measure the distance between two points, you must define an origin and unit length, which is the same as defining a coordinate system.

No it isn't. A coordinate system is "A continuous map from some open subset U of the manifold M into R^n, where n is the dimension of M". Defining "an origin and a unit length" does not give you a continuous map from U to R^n.

But if you are giving a distance, you already have a continuous map between the two points.

Also, the unit length cannot be described at a single point. In a unit displacement vector, there is a continuous mapping of some one-dimensional space from 0 to 1.

 

[Ben Niehoff]

Measurements, as a rule, are observer dependent, but any given measurement is observer independent, because the reference frame of the observation device is already determined.

You seem to be using a strange definition of "measurement" that requires observers to be intentionally naive. Why in the world would anyone try to measure the angle between AB and AC with a moving protractor?

All geometric quantities are invariant under coordinate transformations. In Euclidean space, geometric quantities include angles and distances. An angle is always measured between two lines at the point they intersect. A distance is always measured between two points along the line that connects them. In Minkowski space, geometric quantities include angles, distances, and relative velocities. Relative velocity is really just the "angle" between two worldlines.

I've used the term "relative velocity", but you should note that ALL geometric quantities are already "relative". An angle is always an angle between two lines. One cannot say "The angle of line AB is 30 degrees", that makes no sense. Likewise, a distance is always a distance between two points.

I don't know. All I can guarantee is that when you made the measurement, you referenced your measurement from some origin, and in order to visualize that distance, I must reference it from some origin.

You realize that your inability to answer this question unambiguously proves that Dale is in fact not using a coordinate system?

But if you are giving a distance, you already have a continuous map between the two points.

You'll have to explain. A continuous map from what space into what space?

In a unit displacement vector, there is a continuous mapping of some one-dimensional space from 0 to 1.

This statement makes no sense. You don't seem to be using the words "vector" and "mapping" correctly. Furthermore, I have not once made any mention of vectors, so it's irrelevant anyway.

 

[JDoolin]

You seem to be using a strange definition of "measurement" that requires observers to be intentionally naive. Why in the world would anyone try to measure the angle between AB and AC with a moving protractor?

Are you saying that measuring an angle with a moving protractor is not valid?

In any case, as an observer performing an experiment, you may not have the option of measuring the system in a comoving frame. For instance, if you wish to measure the temperature of air passing by at 100,000 miles per hour, you can't simply place your thermometer in and hope to get the result. In all likelihood, your thermometer will disintegrate.

If you want to measure the shape of a body passing through our solar system at 90% of the speed of light, you don't have the option to run and catch up and place the protractor on the surface.

It's not a matter of naivete. It's a matter of what is convenient and possible.

All geometric quantities are invariant under coordinate transformations. In Euclidean space, geometric quantities include angles and distances. An angle is always measured between two lines at the point they intersect. A distance is always measured between two points along the line that connects them. In Minkowski space, geometric quantities include angles, distances, and relative velocities. Relative velocity is really just the "angle" between two worldlines.

Sure.

I've used the term "relative velocity", but you should note that ALL geometric quantities are already "relative". An angle is always an angle between two lines. One cannot say "The angle of line AB is 30 degrees", that makes no sense. Likewise, a distance is always a distance between two points.

Right, but you still need one vector, and a continuity of positions in between the first vector and the second vector.

You realize that your inability to answer this question unambiguously proves that Dale is in fact not using a coordinate system?

No. Dale used a coordinate system, and an origin. He is hiding information from me, and he is under no obligation to tell me that information, but that does not mean that the information does not exist.

I just measured my own computer screen was 13 and 1/8 inches across. You don't know whether I measured from the left to the right, or whether I was using a yard-stick or a ruler. But you do know something about how a length is measured, and you know that I must have placed an object near the screen, most likely that has a zero-point on it.

And I placed that zero-point somewhere in order to measure the screen.

However, even if I used an un-numbered ruler, I still had to count from one end to another of the screen. I used one end or the other as the origin. Or maybe I played a trick on you and counted both directions from the center then added. But that just makes the origin at the center.

or hey, maybe I played a really crazy trick on you, counting off random little 1/16 inch segments until they were all marked. So now I've converted this vector quantity into a scalar quantity. Have I now succeeded in describing a distance without having an origin?

I don't think so. Because a distance is not made up of discontinuous chunks of ruler. It's made of consecutive chunks of ruler and the continuous space in-between the atoms.

You'll have to explain. A continuous map from what space into what space?

This statement makes no sense. You don't seem to be using the words "vector" and "mapping" correctly. Furthermore, I have not once made any mention of vectors, so it's irrelevant anyway.

Right, you said unit length, I said unit vector. I brought up the term vector to distinguish between "displacement" and other types of vectors, such as force, velocity, acceleration. These quantities are also vectors, but can exist at a single point.

Either way, a unit length, or a unit displacement vector requires space in one dimension to define.

 

[Dale]

I don't know.

Why don't you know? If the mere act of making a measurement explicitly defines a unique origin (as you have claimed) then you should know.

 

[Dale]

No. Dale used a coordinate system, and an origin. He is hiding information from me

I did not use a coordinate system nor an origin, neither explicitly nor implicitly.

I am not hiding anything. You are the one who claimed that the mere fact that a measurement was performed explicitly determines an origin. I thus provided you the information that you claimed was required.

If you wish to revise your claim, then I will be glad to provide as much detail as you claim is required.

Note, however, that there is more to a coordinate system than just an origin, so this is a much weaker claim than the claim that any measurement defines a unique coordinate system. However, since even this very weak claim is false I think it is instructive to pursue it.

 

[Dale]

However, even if I used an un-numbered ruler, I still had to count from one end to another of the screen. I used one end or the other as the origin. Or maybe I played a trick on you and counted both directions from the center then added. But that just makes the origin at the center.

So which is it? The simple fact is that any origin will do, and you will get the same measurement regardless of the origin. The distance measured does not depend in any way on the origin.

This is conceptually similar to Lorentz's aether. You assert that the coordinate system exists and is necessary even though it has no effect on any physical experiment and any choice is consistent with experimental results.

 

[JDoolin]

I am not hiding anything.

First of all, I did not mean to imply any sort of malice by saying you were hiding information. I neither asked nor expected you to provide this information. However, that does not change the fact that the information exists, (or existed, if you've forgotten it).

You are the one who claimed that the mere fact that a measurement was performed explicitly determines an origin. I thus provided you the information that you claimed was required.

If you wish to revise your claim, then I will be glad to provide as much detail as you claim is required.

You've made a strange proposal.

I say you have hidden information from me, but you say if I revise my claim, then you will provide detail to me? If my claim is incorrect, why don't you provide detail to me now, and show me that the claim is wrong?

But I don't really care about the hidden information, as long as a couple of assumptions hold.

(1) the space is not appreciably warped by gravitation where you're taking this measurement, and (2) the origin is stationary with respect to the thing you're measuring

The simple fact is that any origin will do, and you will get the same measurement regardless of the origin. The distance measured does not depend in any way on the origin.

That is only true in regards distances between points in Euclidean space. It is also true with events in Minkowski spacetime, using the space-time interval between events.

However, if you are measuring distance between objects and time between events in Minkowski spacetime, distances DO depend on the origin, because the origin has an intrinsic velocity.

 

[Dale]

You've made a strange proposal.

I say you have hidden information from me, but you say if I revise my claim, then you will provide detail to me? If my claim is incorrect, why don't you provide detail to me now, and show me that the claim is wrong?

The strangeness is inherent in your contradictory claims. First, you claim that the mere fact that a measurement is performed uniquely identifies an origin. Then second, when you have been given the information that a measurement was performed you claim that unspecified additional required information was withheld. The second claim contradicts the first.

However, I can describe in detail the measurement and then you can feel free to tell me what information in addition to the mere fact of the measurement is necessary to specify the origin.

A and B are two marks on a piece of paper lying on my desk. The marks are stationary wrt the paper but not located at any particularly special location or orientation wrt the paper, and the paper is resting on the top of the desk, but not particularly located in any special position or orientation wrt the desk. The desk is stationary wrt the house, etc. The acceleration due to gravity in my house can be taken to be approximately uniform at 9.8 m/s². The measuring device is an unmarked standard rod of 8.5" length composed of a piece of standard "letter paper" constructed according to the usual specifications for letter paper. I carefully placed the two appropriate corners of the rod on the marks and noted that the length matched. Thus, the distance from A to B was measured to be 8.5". The rod was not moving wrt A or B during the measurement.

(1) the space is not appreciably warped by gravitation where you're taking this measurement, and (2) the origin is stationary with respect to the thing you're measuring

1, gravitation is not an appreciable factor in my measurement
2, there is no origin so since it doesn't exist it is not stationary nor is it moving wrt the thing being measured

The simple fact is that any origin will do, and you will get the same measurement regardless of the origin. The distance measured does not depend in any way on the origin.

That is only true in regards distances between points in Euclidean space. It is also true with events in Minkowski spacetime, using the space-time interval between events.

Then you agree it is true with those caveats?

 

[JDoolin]

The strangeness is inherent in your contradictory claims. First, you claim that the mere fact that a measurement is performed uniquely identifies an origin. Then second, when you have been given the information that a measurement was performed you claim that unspecified additional required information was withheld. The second claim contradicts the first.

But there is a difference between performing a measurement and communicating the results of a measurement.

However, I can describe in detail the measurement and then you can feel free to tell me what information in addition to the mere fact of the measurement is necessary to specify the origin.

A and B are two marks on a piece of paper lying on my desk. The marks are stationary wrt the paper but not located at any particularly special location or orientation wrt the paper, and the paper is resting on the top of the desk, but not particularly located in any special position or orientation wrt the desk. The desk is stationary wrt the house, etc. The acceleration due to gravity in my house can be taken to be approximately uniform at 9.8 m/s². The measuring device is an unmarked standard rod of 8.5" length composed of a piece of standard "letter paper" constructed according to the usual specifications for letter paper. I carefully placed the two appropriate corners of the rod on the marks and noted that the length matched. Thus, the distance from A to B was measured to be 8.5". The rod was not moving wrt A or B during the measurement.

Very well done! Because the lengths matched, in fact, you did not have to decide which point you were measuring "from" and which point you were measuring "to". I admit this is one scenario that hadn't occurred to me.

But how would you modify this process if you needed to measure lengths of things that were not exactly 8.5" long?

Dalespam: "The simple fact is that any origin will do, and you will get the same measurement regardless of the origin. The distance measured does not depend in any way on the origin."

JDoolin: "That is only true in regards distances between points in Euclidean space. It is also true with events in Minkowski spacetime, using the space-time interval between events."

Dalespam: "Then you agree it is true with those caveats?"

Yes.

But if you get into general relativity; for instance, the Schwarzschild metric, even your choice of origin will affect measurement of distance, time, and space-time intervals.

 

[Dale]

Very well done! Because the lengths matched, in fact, you did not have to decide which point you were measuring "from" and which point you were measuring "to". I admit this is one scenario that hadn't occurred to me.

But how would you modify this process if you needed to measure lengths of things that were not exactly 8.5" long?

Get another standard rod that is as long as needed, or (more commonly) get a large number of very small standard rods and count how many are used.

Yes.

That is all we are saying. Those caveats are acceptable. The geometry we are interested in spacetime is the spacetime interval.

But if you get into general relativity; for instance, the Schwarzschild metric, even your choice of origin will affect measurement of distance, time, and space-time intervals.

Not really. The origin can be moved in time as desired without even changing the components of the metric tensor. And you can do a diffeomorphism to a coordinate system with any arbitrary origin. Such a transformation will cause the components of the metric to change, but all measurements of spacetime intervals will be unchanged. Since such quantities do not depend on the choice of coordinate system you can express them without reference to any coordinate system if you wish. That is the point of coordinate-free relativity.