# I Proper (and coordinate) times re the Twin paradox

#### Ebeb

The point of that discussion was that if it's physical, it's four dimensional. And if it's three dimensional, it's not physical.

The converse does not apply. Just because it's four dimensional, that does not mean it must be physical. The definition of "physical" that we are using is "measurable". You have to be able to run an experiment. One cannot measure the future (except by waiting -- and then it's not the future any more). That means that the block universe does not qualify as "physical". It is, as @Dale refers to it in #256, an "interpretation".
Two observers at same location but driving different direction measure a different physical clock display event of the 4D clock located at a distance.
That's measuring a 4D object, be it a 4D clock, or the 4D universe/block universe. What else would above measuring be?
Edit: Calling the block universe an "object" is also a bit of a stretch.
What then is an object if it's not 3D, nor 4D?

#### jbriggs444

Homework Helper
Two observers at same location but driving different direction measure a different physical clock display event of the 4D clock located at a distance.
You seem to be putting a lot of weight on word choice and not much effort at clarity of expression.

Two observers at the same location will be momentarily seeing the same thing through their telescopes: an image of the remote clock from some time in the past. The two observers can each adopt a frame of reference in which they are at rest and calculate a "current" clock reading on that remote clock. The two calculations will produce two different results. Those two calculated results will match the physical clock readings on that remote clock at two different events.

• PeterDonis

#### jbriggs444

Homework Helper
you have to consider 'present world' non-existent
One can accept that there are objects in the neighborhood of an arbitrarily chosen surface of simultaneity. One can accept that those objects "physically exist". That does not require one to classify the surface or even its neighborhood as "physically existent". Mathematically existent is good enough.

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#### Mister T

Gold Member
Really unbelievable you ask me this kind of question.
But we wouldn't call it a 2D slice. It's a 2D plane.
A slice has thickness, hence 3D.
That is not my understanding of the standard way the language is used. I could be wrong, but if that's the case it would seem there's a lot of confusion.

I thought I started to understand what you guys have in mind, but I see I'm still nowhere...
Maybe we differ in what 4D means.
Many people have trouble with even fewer dimensions. I know I do.

If you slice a potato you create two pieces. The potato is 3D and the slice is 2D. Thus the number of dimensions is always reduced by $1$ when we slice, as I understand the way the language is usually used to describe the math. Thus a slice of 4D spacetime is a 3D "hypersurface", and I guess a slice of a 2D surface would be a 1D line.

It could be that we don't all share the same common use of the language here, but as far as I know my understanding of it is consistent with the general understanding.

#### Vitro

That is not my understanding of the standard way the language is used. I could be wrong, but if that's the case it would seem there's a lot of confusion.

Many people have trouble with even fewer dimensions. I know I do.

If you slice a potato you create two pieces. The potato is 3D and the slice is 2D. Thus the number of dimensions is always reduced by $1$ when we slice, as I understand the way the language is usually used to describe the math. Thus a slice of 4D spacetime is a 3D "hypersurface", and I guess a slice of a 2D surface would be a 1D line.

It could be that we don't all share the same common use of the language here, but as far as I know my understanding of it is consistent with the general understanding.
It seems some participants here use the meaning of slice as in a slice of bread, where the bread is 3D and the slice is also 3D since it has a thickness.

#### Mister T

Gold Member
It seems some participants here use the meaning of slice as in a slice of bread, where the bread is 3D and the slice is also 3D since it has a thickness.
I was speaking of the general community of professional physicists and mathematicians. I would certainly agree that when I ask for a slice of bread I'm going to get a three-dimensional object. But that is not the way the word is used in the literature.

But arguing about the meaning of a word is not the the issue. The issue is a working grasp of the mathematics and how it applies to the physics. If you want to talk to physicists and mathematicians about this stuff they are going to use the technical meanings of the words, not the everyday meanings. That is a major cause of confusion for learners.

#### Dale

Mentor
Let's please drop the philosophical discussion at this point and stick to experimentally measurable things.

#### PeterDonis

Mentor
• Dale

#### Grimble

@Ebeb, that was a long post. I will try to keep this response brief.

It seems that we have reached agreement about the distinction between a thin section of an object (e.g. a slice of bread) and an infinitesimal dividing plane through an object (e.g. the place where we intend to cut the bread).

Physically, a car hitting a tree is a process with four-dimensional extent. It extends right, left, up, down, forward into the tree and back into the car. It does not just crumple a point on the bumper. It crumples a three dimensional region. It does not just crush the bark on the tree at one point. It makes a three dimensional scar. It does not occur at an instant. It takes place over a [short] time interval.

For most purposes, we do not care about the complete details of the collision. For purposes of our models, it is enough that we know the mass, momentum and energy of the car and of the tree. For the police report, we only need to know that it occurred on Fourth and Elm at 2:00 pm. [we assume that it took place at ground level]. The "event" associated with the collision is the exact four dimensional location which is only approximately where and when the diffuse process took place. The process is physical. The "event" is a labelled feature in our model.

But I agree that one could reasonably say that a collision "event" is physically a four-dimensional process.
I have the feeling that much of this discussion involves the mixing up of an 'event' which has a specific definition in Relativity/spacetime, and the noun 'event' in the English language, which denotes a specific iteration of a process.

#### Grimble

That sounds likely to me also. Or at least a big part of it. The other part is just the math vs physical bit.

So mathematically an event is 0 dimensional. Just a point at an instant. Nothing physical meets that criteria, but we can use the math as a simplification when the actual 4D interaction is very small compared to our scale of interest.
But is that not the point? That a spacetime event is, in essence a way to refer to a 4D location? The word 'event' has connotations of an action or a process but that is really not the right way to envisage what we mean in spacetime, where an event is a point in space at a point in time and as such cannot be 'doing' anything.
It seems to me, in view of the preceding discussions that an event could be likened to a slice through the 4 dimensions which as has been pointed out is not an object of any number of dimensions.
A spacetime event can have no content; it may be described by a verb, for example, when we say the event where a photon of light was emitted by a light source, but that is using the process of light emission to describe/define the point in time (and space) that is the event referred to.

Or am I misunderstanding again?

#### Dale

Mentor
But is that not the point? That a spacetime event is, in essence a way to refer to a 4D location?
Yes. That is correct

#### sweet springs

Hi. Meanings of word event in physics and in daily life are not different, I think. Information of when and where via a certain coordinate is indispensable.
E.g.
Birth of J.C. 4 B.C. @Bethlehem
Rise of French revolution 14 July 1789 @Paris

My birth was an Event no matter how any people say the birthplace is xxx and birthday is yyy differently with their coordinates. Yours also.

#### Grimble

Hi. Meanings of word event in physics and in daily life are not different, I think. Information of when and where via a certain coordinate is indispensable.
E.g.
Birth of J.C. 4 B.C. @Bethlehem
Rise of French revolution 14 July 1789 @Paris

My birth was an Event no matter how any people say the birthplace is xxx and birthday is yyy differently with their coordinates. Yours also.
Yes, thank you, I can see what you mean, it depends on the time scale that is used, in geological terms the last ice age was a specific event that was just a point on an appropriate time scale.
Yet it seems to me that there is an implication in physics that an event happens at an indivisible point in time and that being a point in time it must measure a specific instant in a process, rather than denoting a process...
(Or am I just being picky?)

#### sweet springs

Hi. Both OK. Accumulation of points denote process. My birth and my death are two point events. Between them my life so to say continuous sequence of events are expressed as line, my world line. Best.

#### Dale

Mentor
Yes, thank you, I can see what you mean, it depends on the time scale that is used, in geological terms the last ice age was a specific event that was just a point on an appropriate time scale.
Yet it seems to me that there is an implication in physics that an event happens at an indivisible point in time and that being a point in time it must measure a specific instant in a process, rather than denoting a process...
(Or am I just being picky?)
Mathematically a point is an indivisible 0D geometric primitive. Physically it is usually understood to be an approximation for something that is just far smaller than the scale of interest.

#### Grimble

I have been rereading and working through this thread for some time now and I think I understand what you were trying to instil in me.
Any clock measures proper time. It doesn't matter if they are at rest or moving, if they are inertial or non inertial, in curved spacetime or flat. They always measure proper time along their worldline.
Proper time is what a clock measures and reads/displays.
The important thing about proper time is its invariance. Everyone agrees the length of your path.[worldline?] The important thing about coordinate time is its arbitrariness. We could have chosen a different way to define our zero point, and we could have chosen a different direction to move in.
The rule of thumb is: if you can measure it with a single clock then it's a proper time, if you need two (or more) clocks then it's a coordinate time. Alternatively, if you measure it at the same location it's a proper time, if you measure it at different locations it's a coordinate time.
And if either or both events don't happen at the location of the clock, it is always a coordinate time.
In any reference frame. It is invariant.
The worldline is the geometric figure itself, irrespective of the coordinate system that you might use for describing it. One worldline will have different coordinates in different coordinate systems but it is the same worldline.
If we assign coordinates using a frame in which the object is at rest we'll label the points that the worldline passes through (t,0,0,0) and if we use a frame in which the object is moving these points might be labeled (t',vt',0,0), but they're the same points and the same worldline either way. When we change frames we're changing the axes of the coordinate system we're using to assign coordinates to points, but this doesn't change the points themselves.
So the invariant proper time is measured on the same clock at the same location – in a frame in which the object is at rest, while in a frame where the object is moving we measure (calculate?) coordinate time?
If the two events occur in the same place (in some frame) then the time that elapses in that frame is a proper time. In the rest frames of observers who move relative to those events the time that elapses will not be a proper time, for them those two events occur in different places. They will therefore need two clocks, one located (local!) at each of the two events. And the time that elapses in those frames will always be larger than the proper time.
If we were to mark the point of each completed tick between two events, then regardless of which frame we use, resting of moving, there will still be the same number of ticks between events? That it is the length of the ticks that will vary not the quantity? A longer coordinate time, but the same proper time?
Minkowski - Space and Time (1920) said:
Suppose we have individualized time and space in any manner; then a substantial point as a world-line corresponds to a line parallel to the t-axis; a uniformly moving substantial point corresponds to a world-line inclined to the t-axis;
The above quotes lead me to conclude that proper time is measured vertically parallel to the t-axis.
So Proper Time may be likened to the spacetime Interval between two events on a resting clock's worldline – a straight vertical line parallel to the t-axis, measured at the same location, on the same clock; the spacetime interval being s2=(ct)2-x2 where x=0.
The coordinate time occurs between two events on the t' axis of a moving clock, - being at different locations, measured on different clocks – x≠0. The spacetime interval s2=(ct')2-x2 being invariant, must be the same value.
The proper time between two events is also invariant, it is the distance travelled between the events in the coordinate time, that makes the difference.
Compare these two measurements: while the proper time τ=√(ct)2-x2the spacetime interval s2=(ct)2-x2
They are in effect the same thing; only, because the Spacetime Interval could be space-like,
it is S2 that is used rather than S.

#### Nugatory

Mentor
The above quotes lead me to conclude that proper time is measured vertically parallel to the t-axis.
No, a thousand times no!

The proper time between two events is measured along the wordline of a clock that is present at both events. If the clock does not experience any acceleration this worldline will be a straight line. (If the clock is accelerated its worldline will not be straight, and the proper time along the non-straight worldline will be less than the proper time along the straight worldline of an unaccelerated clock). There is no reason why that straight line has to be vertical and parallel to the t axis.

#### stevendaryl

Staff Emeritus
Proper time is what a clock measures and reads/displays.
Correct.

The above quotes lead me to conclude that proper time is measured vertically parallel to the t-axis.
Not correct. I really do think that it helps to think of the analogy with Euclidean geometry. You are on a one-way road that twists and turns. There is an intrinsic, coordinate-independent way to talk about your location on the road: You say "I am at a point 12.6 kilometers from exit number 12". There is also a coordinate-dependent way to talk about your location: I am at 36.2134 degrees north latitude, 12.0987 degrees west longitude.

To figure out the first, all you need is a car with an odometer. To figure out the second, you need a whole system of conventions for latitude and longitude, and a lot of information, some of which is completely arbitrary, such as the choice of Greenwich, England as the 0 of longitude.

Proper time is like distance along a road. It has nothing to do with coordinates. Although you can measure proper time using coordinates in the same way that you can measure distance along a road using coordinates. If you know that your longitude has changed by $\Delta \phi$ and you know that your latitude has changed by $\Delta \theta$, then your distance along the road has changed by approximately:

$\Delta s = \sqrt{g_{\theta \theta} (\Delta \theta)^2 + g_{\phi \phi} (\Delta \phi)^2}$

where $g_{\theta \theta} = R^2 (\frac{\pi}{180})^2$ and $g_{\phi \phi} = R^2 cos^2(\frac{\pi \theta}{180})$ are the metric components for the coordinate system of latitude and longitude ($R$ is the radius of the Earth).

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• • Dale and Orodruin

#### Grimble

No, a thousand times no!
OK, thank you. What I understand from this:
[...] if you measure it at the same location it's a proper time, if you measure it at different locations it's a coordinate time.
and this:
The proper time between two events is measured along the wordline of a clock that is present at both events.
is that if the two locations are the same and it is measured on the same clock, then this is represented by a vertical line where x is constant...
If the x location changes, whether it does so at a constant speed or accelerating then the line will be angled or curved, but will, necessarily have a varying x coordinate.
If, as in the case of the travelling twin, he moves away and then back to the original location, he will still have changed his x coordinate.
He will effectively have to a different location and then returned - so two coordinate times, not proper times.
I just don't know which of your explanations to believe... #### Grimble

Sorry, the highlighted word was missing in the original.
He will effectively have moved to a different location and then returned - so two coordinate times, not proper times.
Consider a clock moving at v.
After time t it will have moved from event E1 to event E2 a distance x=vt
The time measured by the observer relative to whom the clock is moving from E1 to E2 would be ct - the diagonal (rotated) line.
In the clock's frame E1 and E2 would be measured at the same location by the same clock giving the proper time τ.

OK this is measured in a different frame but it is invariant so would be the same in any frame in which it is moving.

We have then a right angled triangle where the hypotenuse = ct and the catheti =vt and τ. So:
(cτ)2 = (ct)2-(vt)2
cτ = √((ct)2 - (vt)2)
τ = t √((1 - v2)/c2)
τ = t/γ

The proper time is of course the time displayed by the clock at E2, which is less (by the factor γ) than the coordinate time ct.

The invariant proper time is the same in every frame - a measurement between events on a line parallel to the ct axis (usually vertical)

This is what the posts I quoted tell me.

So where have I misunderstood?

#### Grimble

The proper time between two events is measured along the wordline of a clock that is present at both events. If the clock does not experience any acceleration this worldline will be a straight line. (If the clock is accelerated its worldline will not be straight, and the proper time along the non-straight worldline will be less than the proper time along the straight worldline of an unaccelerated clock). There is no reason why that straight line has to be vertical and parallel to the t axis.
But here you say:
If we assign coordinates using a frame in which the object is at rest we'll label the points that the worldline passes through (t,0,0,0) and if we use a frame in which the object is moving these points might be labeled (t',vt',0,0), but they're the same points and the same worldline either way. When we change frames we're changing the axes of the coordinate system we're using to assign coordinates to points, but this doesn't change the points themselves.
In the clock's frame it is at rest and the worldline and the proper time will be on a vertical line parallel to the t axis.
This measurement is invariant.
The proper time is measured between two events measured by the same clock at which the clock is present (t,0,0,0) which are necessarily at the same location.
In a frame in which the clock is moving the coordinates, as you say, are given by (t',vt',0,0). t' is greater than t (t' = γt) and the clock moves further - vt'

So although the clock does not move vertically in a frame in which it is moving, its invariant measurement of proper time is the vertical distance to E2 from the x axis.

#### Dale

Mentor
The proper time is: $$\tau=\int \sqrt{-g_{\mu\nu}dx^{\mu}dx^{\nu}}$$ Where in an inertial frame $g_{\mu\nu}dx^{\mu}dx^{\nu}=-c^2 dt^2+dx^2+dy^2+dz^2$

This quantity is the same in all frames. If there is some frame where $dx=dy=dz=0$ then the worldline “is measured vertically parallel to the t-axis”, but even if the worldline is not parallel to the t axis the proper time is still the same. The whole point is that proper time does not need to be parallel to the t axis.

If you draw a line both you and someone looking at an angle will agree on its length. It is not necessary for that line to be parallel to any coordinate line.

• vanhees71

#### Grimble

The proper time is: $$\tau=\int \sqrt{-g_{\mu\nu}dx^{\mu}dx^{\nu}}$$ Where in an inertial frame $g_{\mu\nu}dx^{\mu}dx^{\nu}=-c^2 dt^2+dx^2+dy^2+dz^2$
I am afraid that is right over my head!
This quantity is the same in all frames. If there is some frame where $dx=dy=dz=0$ then the worldline “is measured vertically parallel to the t-axis”, but even if the worldline is not parallel to the t axis the proper time is still the same. The whole point is that proper time does not need to be parallel to the t axis.

If you draw a line both you and someone looking at an angle will agree on its length. It is not necessary for that line to be parallel to any coordinate line.
Yes, I do understand exactly what you are saying. Indeed you said the same earlier:
In any reference frame. It is invariant.

The worldline is the geometric figure itself, irrespective of the coordinate system that you might use for describing it. One worldline will have different coordinates in different coordinate systems but it is the same worldline.
But can you clear this up for me? In what way does the Worldline have the same length?
At rest its worldline is parallel to the time axis - it has to be because it does not physically move.
In motion it also travels between locations so it is moving in time and space; which seems to me to lead to a longer worldline.

And this post from Mister T seems to say the opposite to your post:
If the two events occur in the same place (in some frame) then the time that elapses in that frame is a proper time.
It is a proper time if the events are at the same location - i.e. parallel to the t axis.
In the rest frames of observers who move relative to those events
that is frames in which the worldline is sloping...
the time that elapses will not be a proper time, for them those two events occur in different places. They will therefore need two clocks, one located (local!) at each of the two events. And the time that elapses in those frames will always be larger than the proper time.

#### jbriggs444

Homework Helper
In motion it also travels between locations so it is moving in time and space; which seems to me to lead to a longer worldline.
This is Minkowski geometry, not Euclidean geometry. Increasing the spatial displacement (the difference between starting and ending x coordinates) while holding the temporal displacement (the difference between starting and ending t coordinates) constant reduces the length of the (timelike) interval between the end points.

#### Dale

Mentor
And this post from Mister T seems to say the opposite to your post
You will have to ask @Mister T to clarify his own post, not me. I would not phrase it that way because proper time is only defined on a worldline and coordinate time is defined throughout spacetime.

I am afraid that is right over my head!
If you want to understand this stuff then you cannot simply give up on this part. You must pursue it until it is not over your head.

Look at the right hand side of the formula in the inertial frame. Do you understand that? Do you see the similarity with the Pythagorean theorem?

But can you clear this up for me? In what way does the Worldline have the same length?
If you take the Pythagorean theorem and calculate the distance between two points and then you rotate your coordinates and recalculate you get the same result. In spacetime the proper time is the equivalent of the Pythagorean theorem and a Lorentz transform (boost) is the equivalent of a rotation. A rotation/boost does not change the length/proper-time. It is not that hard to prove, I would recommend doing it to convince yourself.

At rest its worldline is parallel to the time axis - it has to be because it does not physically move.
In motion it also travels between locations so it is moving in time and space; which seems to me to lead to a longer worldline.
Note that in a rotation both the x and y coordinates are changed in such a way that the distance is preserved. Similarly here both the t and x coordinates are changed in such a way that the proper time is preserved. In your analysis above you neglected the change in t.

"Proper (and coordinate) times re the Twin paradox"

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