On the fragmentalist interpretation of special relativity - by Martin A. Lipman 2018 - Article review

This document contains article review "On the fragmentalist interpretation of special relativity " by Martin A. Lipman written in 2018
To order to read the article select:https://openaccess.leidenuniv.nl/bitstream/handle/1887/78037/Lipman_1_2018.pdf?sequence=1

Contents

Reflection

Introduction

Consider some faraway place in the universe.
It only makes sense to discuss such a place if it is not empty.
There is no fact of the matter about what happens there at the very moment you are reading this sentence.
Correct. This is the case with every position of the whole universe.
It is a mistake to think that there is an objective course of time through which all current things in the universe are progressing.
This sentence is not clear.
It is also not the case that the things you see have intrinsic mass or intrinsic spatial shapes
This requires a definition of the word intrinsic. All the objects I see (now) have mass and a 3D shape.
These are some of the standardly accepted metaphysical consequences of the special theory of relativity, captured by the Minkowskian conception of spacetime.
SR has no physical or metaphysical consequences.
The Minkowskian conception of spacetime accepts the assumption that what varies across perspectives (such as frames of references) must be mere appearance.
The meaning of the word appearance is very important in this context. My interpretation is: not real, pretend, seems.
In SR the concept of "reference frames" is important. IMO whatever type of reference frame is selected: it does not have any physical consequence of what is observed. Of course, it makes a difference about what is observed, if you are inside a merry go round or if you are observing a merry go round.
In particular, Fine argues that a fragmentalist view of time is in a position to
  1. better account for the passage of time,
  2. better account for the relation between tensed talk and temporal reality, and
  3. that it renders tense realism compatible with (a fragmentalist interpretation of) the special theory of relativity.
Sorry, but IMO no body can understand this.

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1. Fragmentalism

Fragmentalism is the view that the world is inherently perspectival.
What means perspectival?
We standardly assume that we only ever have perspectival representations of a non-perspectival world.
Tricky sentence.
Fragmentalism denies this assumption, allowing that the world is itself an inherently perspectival place where facts do not simply obtain or fail to obtain, as we ordinarily assume, but where certain facts can obtain in the context of one set of facts and yet fail to obtain in the context of other sets of facts
Tricky sentence.

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2. Fragmentalist relativity and the manifest image

The Minkowskian conception effectively rejects the reality of anything that differs across the relativistic perspectives and only takes that which is invariant to be real.
invariant is a typical mathematical concept.
The Minkowskian conception is also a mathematical concept and requires observations of objects. These objects are real.
The result is the familiar fusion of space and time into a single spacetime, rejecting the various properties that are based on a neat separation of space and time, such as shape, duration, rest mass, simultaneity, and so on.
Spacetime does not describe a physical concept, because basically it includes imaginary numbers. You can calculate a length of time (s^2 = it^2), but physical it does not exist.
Imagine that you are in a space shuttle hanging still in an empty black space and that, for as long as you remember, you have experienced your space shuttle as being at rest.
The concept 'at rest' makes only sense if you define 'not at rest' and explain what the difference is,
You can define 'at rest' as 'not moving' and 'not at rest' as 'moving', but now you have to explain the difference between these two concepts.
This is a phylosophical issue.
Imagine that I am in an exactly analogous situation, that is to say, I am also in a space shuttle that, for as long as I remember, I have experienced as being at rest.
What both observer A and B in fact assume that they are alone in this world.
One day, we see each other’s space shuttles.
One day, I observe an other space shuttle, approaching my space shuttle.
You experience my space shuttle as drifting by with a constant speed of 5 km/h towards the east, still experiencing yourself at rest.
The only thing you observe is that the distance between us becomes smaller.
I experience you as drifting by with a constant speed of 5 km/h towards the west and still experience myself as being at rest.
The only thing I observe is that the distance between us becomes smaller.
Our observations of each other’s shuttles therefore disagree about who is moving and who is at rest: you observe that I am moving, whereas I observe myself as being at rest.
Neither can either of us observe that he or she is at rest.
What we will both agree that the distance between us becomes smaller.
The question remains how is this possible. There must be a physical explanation.

When you compare the two results it becomes impossible to conclude that we both are at rest.
At the most only one of us is not moving, however this is impossible to decide.
Most probably neither of us is at rest.

You are standing in the exact middle of your space shuttle (which you still observe as being at rest) and you emit some light towards both ends of the space shuttle.
You cannot observe as being at rest . You can only assume that.
You observe that the light arrives simultaneously at the front and end of the shuttle.
That is physical not possible
I expect there is an error in the text. Maybe what is meant: You observe that the light arrives simultaneous at both the front end and the back end of the shuttle.
However that is also not easy. You can only observe this if there are mirrors at both the front end and the back. In that case you observe the reflected signals. If that is the case and you are standing in the middle, you will always observe the reflected signals simultaneous.
However most probably, the two events when the signals are reflected are not simultaneous.
This is as you would expect: as you are at the middle of a space shuttle at rest, the light has the same distance to travel in both directions and should arrive at the same time (*), given that light always has the same speed c regardless of what direction it moves in.
There are three assumptions: The first assumption is that you are at rest, the second assumption is that the speed of light is the same in both directions and the third assumption is that the speed of light is c.
Also this sentence contains an error. At (*) is not indicated where. I expect at the observer in the middle.

In actual fact you need a real experiment to demonstrate that this is true.
An important experiment is that the speed of light is the same in both directions.
If that is the case to define this speed c (and constant) is of much less importance.

I am in my space shuttle, observing your experiment
This is very tricky, because all what I see is something later.
From my perspective, the beams of light also have the same velocity c as in yours.
This is impossible to establish. The speed of light is a local parameter (in space) independent of the speed of any object involved. If this speed is in all directions the same has to be confirmed by experiments. Such experiments are difficult. To declare the speed in all directions the same (locally) is an easy way out.
But since I observe your shuttle as moving, the forward moving light has to travel more distance, and since it still has the same constant speed c, arrives later than the light sent towards the back.
When your shuttle is moving, the two light signals will still arrive simultaneous (in the middle), as observed by you.
As before, our observations conflict. This time, our observations do not disagree about the constant velocity of light, but about the simultaneity of the light arriving at, respectively, the front and back of your space shuttle: you observe them arriving simultaneously, whereas I observe them arriving one after the other.
See: Reflection 1: The observer in the middle of the train

The general lesson is, that if you want to understand an experiment you should consider the whole experiment from one reference frame.
The starting point is to investigate what each observer observers.
The starting point should not be any assumption.

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The crucial overall point however is that there is no way whatsoever of singling out one of the two conflicting sets of observations.
The crucial overall point is that in any experiment we are only discussing one sitution.
The second crucial point is what are we realy observing.
The fact that each of us is at rest is an assumption.
Could one of us be right?
Generally speaking both observers are right insofar they indicate what they see and observe.
As such an observer can not observe that he or she is at rest. That is an assumption.
According to Newton, only one of us is indeed right.
Does Newton really made this claim? IMO in most situations he only used one reference frame.
Newton took space to be a three-dimensional spatial realm that endures through time, so that any spatial location at one time is taken to be identical to a spatial location at a different time, and which location is which is an entirely objective matter
Newton discussed the solar system, with the planets revolving around the Sun.
It should be mentioned that the only thing we can measure is the position of objects at individual moments.
But how can we figure out which spatial location at one time is identical to which spatial location at a different time?
Starting point should be one reference frame and one clock. The problem of course is how do you define such a reference frame and how do you draw the positions of individual objects in that frame.
It seems that we cannot tell empirically who is right.
That is no issue. The whole issue is to document all observations carefully.
The different locations do not come with intrinsic qualitative differences with which we could individuate them, telling us which point is which.
I doubt that that is true. All science starts with observations. Some of these observations should be discarded because they are wrong.
The constant velocities of objects that we observe also cannot help arbitrate our dispute, given that our observations disagree precisely about which objects have which constant velocity.
Generally speaking astronomers don't observe constant velocities. What they observe are a sequence of positions of the same object in time. Out of these observations you can calculate the velocity and acceleartions involved
The whole issue to the problem is to draw the positions of individual objects at specific moments in one 3D reference frame. Using these objects the velocities can be calculated. There is no specific issue related to constant velocities. In general there are no constant velocities.
It should be mentioned that all of this is not simple.
Properties other than the observed constant velocities do not help either.
This sentence does not make much sense.
In short, it does not help to appeal to the contents of our conflicting observations because our observations agree about everything except for the constant velocities attributed to things.
This whole section about Galileo does not make sense considering concepts like Newton's Law and General Relativity.
One thing IMO is for sure, please study the objects involved in one 3D reference frame.
It seems we could only arbitrarily assign one of us to be the privileged observer, without any possible empirical evidence for any particular privileging.
That is a very good starting point. The problem of course is how to integrate the observations of different observers into one frame.
Moving from a Newtonian to a Minkowskian conception of spacetime eradicates the threat of empirical arbitrariness, as the Minkowskian view eliminates precisely that about which the different perspectival observations conflict.
I doubt that.
The first thing to explain is what the specific observations are on which the Minkowskian concept is based. IMO the initial observations on which the Minkoskian concept is based is identical as the Newtonian concept.
An important role here is played by the Lorentz transformations (together with other transformations, such as those governing electric fields) as they capture exactly what varies and what remains invariant across perspectives.
This required a detailed explanation.
Specific this requires a clear explanation what is measured in the x,y,z,t domain versus the x',y',z',t' domain.
Lorentz transformations are coordinate transformations between two inertial coordinate frames that move at a constant velocity relative to each other.
gamma = 1/sqrt(1-v^2/c^2)
How is the parameter v/c calculated?

Lorentz transformations

There is nothing wrong with the lorentz transformations from a mathematical point of view.
The issue is to what extend they can be used to simulate the movement of the planets in our solar system. IMO not.
The problem is if you want to improve such a simulation you should start from a certain 3D reference frame and mark down which is the point of observation within that frame. When you perform your obsevations of the objects studied for one month both your point of observations and the objects started all will move under influence of gravitational attraction as described by Newton's Law or GR. That is why it is so difficult to transform all your observations (which involves the speed of light) within your 3D model (which does not involve the speed of light)
The so-called spacetime interval between spacetime points is the central invariant property that constitutes the intrinsic structure of Minkowski spacetime.
From a mathematical point of view this sentence is 100% correct.
From a physical point of view spacetime does not exist.

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Here Dx, Dy, Dz, and Dt can each be different in different Lorentz-related frames but only in such a way that Ds2 comes out the same in each frame description (where by ‘frame description’ I mean a description of events in terms of the coordinates given by a particular frame of reference).
This raises the question how are these different parameters calculated in different frames. The second question is why these different frames.
According to the Minkowskian interpretation, the spacetime interval Ds2 is the only real spatiotemporal quantity, which can be broken up into different temporal and spatial components within different coordinate representations.
That is correct, but remember this is a pure mathematical operation.
The different temporal and spatial components only feature within the coordinate representations, which are now deemed to be the perspectival representations of the underlying non-perspectival world, itself only characterized by the spacetime interval.
Again this is a mathematical world.
The fragmentalist framework offers an alternative treatment of the shuttle scenario.
Lets wait and see.
Consider in a little more detail the conflicting observations we encounter in the space shuttle scenario.
Again what you need is a more detailed description of what the observations are.
If there are conflicts for example you should place clocks inside the space shuttle to register the time of the events.
It is easy to understand that my observations of a set of events and your observations of the same set of events are different.
What you need is a set of events on which both observers agree. These events should be observer independent. The important issue what are the laws (of nature) that describe these events. Using these law we can predict the future. Finally these predictions should de transformed in what each observer observes.
I have no good independent reason to discredit your observation, and you do not a have good independent reason to discredit my observation.
That is only true if these observations are 'real' observations and not assumptions.
For example: 'at rest' requires a back ground which is considered 'at rest'.
Between us, it seems that we have equally good evidence that I am at rest as well as evidence that I have a constant velocity.
That is not the case.
When I'm alone at see and I observe no other ship I have no evidence if I'm at rest, moving in a straight line, moving in general or in a circle.
In the imagined case, you experience my space shuttle as moving and your space shuttle as being at rest; I experience your space shuttle as moving and my shuttle being at rest.
This depends what is the definition of being at rest.
One object in the universe can never declare itself at rest.
When there are two objects generally speaking they are both moving. etc etc.
Newton would have the same problems.
Generally speaking to 'solve' this issue you need a common reference frame.

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Given these observations, a simple fragmentalist description of the case imagined looks as follows:
(I doubt if it is simple)
R(your shuttle is at rest, my shuttle moves with 5 km/h to the east, the light beams arrive in your shuttle at the front and back simultaneously) & R(your shuttle moves with 5 km/h to the west, my shuttle is at rest, the light arrives in your shuttle at the front only after it arrived at the back)
The text in your shuttle is added for clarity.
Neither of our observations is arbitrarily privileged according to these descriptions.
That is correct. Locally. But this whole exercise makes only sense within a broader context i.e. from one reference frame
The description of the evolution of the physical reality and what observers see has to be handled with care. Generally speaking observers can only observer locally and not globally i.e. at a distance. An observer can not observe reliable if two events at a distance happen simultaneously or not.
We take what we observe, namely the co-instantiation of various variant properties, at face value.
Physics should be based more on measurements, not so much on human observations.
We accept that your shuttle is at rest and that your shuttle is moving with a constant velocity; and we accept that the light arrives at the front and back simultaneously and that it arrives at the front only after it arrived at the back.
My shuttle is based on my observation at rest, but most probably that is physical not correct.
More generally, we both observe genuine but conflicting facts concerning the simultaneity, duration, length and shape.
What I or you observe are not facts. This are only becoming facts if you transform all observations in one reference frame. These facts can never be conflicting, other wise your transformations are wrong.
The transformations mentioned are not Lorentz transformations.
For the fragmentalist, a complete description of some physical system is a description of the system in all the different fragments, of its total perspectival manifestation.
For every physicist a complete description of some physical system is always important inorder to understand what is happening at different levels of detail.
We should sharply distinguish between what is variant across the frame descriptions within a single collection (i.e. across fragments) and what is variant across the total collection of such descriptions (i.e. across total representations of the overall fragmented world).
In order to understand the evolution of the world a one reference frame approach makes the most sense.

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I will continue to speak of frame-invariant facts and variant properties, even within the fragmentalist picture; I hope I have said enough to avoid confusion.
The deeper reasoning of the difference between invariant and variant (facts or properties) from a physical point of view is not clear.

3. The explanatory potency of variant properties

Giving that the standard Minkowskian interpretation of the special theory of relativity is both successful and explains matters in terms of invariant properties only, does the fragmentalist not introduce a layer of superfluous qualitative structure, unneeded for the scientific explanation of phenomena?
What is important is insofar a new 'theory' can make better predictions as a previous 'theory'.
Say we observe things as follows: Rocket I and Rocket II are initially at rest and then they start to move in the x direction at the same time at the same rate, keeping their spatial separation Dx unchanged.
Dx being the distance between the back tail of Rocket I and the front head of Rocket II.
As the rope has gained at this later moment in time a velocity in the x direction, the length of the tightly strung rope must be contracted.
I assume that the physical length of the rope must be contracted. Why?
Of course this is a dangerous experiment, because if the flight path of both rockets is not 100% identical the rope will break.
Now imagine that the rope is contracted beyond its elastic limit and breaks at the moment that the rockets are moving.
Why should the rope being contracted? A much more logical explanation is that the forces in front and in the back are not the same.
We thus have an explanation in terms of length contraction.
No. You must first clearly describe what is observed.
It is easier to assume that there is length expansion involved because the rocket in front will pull the rope until it breaks.

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As Mermin points out, ‘the mechanism that gives the real explanation for a phenomenon in one frame of reference, may be quite different from the mechanism that gives the real explanation in another’
I don't understand what a physical explanation has to do with a reference frame.

4. Hofweber and Lange on the fragmentalist interpretation

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There is no disagreement about the science itself.
It is strange if there are no different opinions about how to perform science.
First of all, the scientific concepts (used) should be clear.
The debate is about the proper metaphysical interpretation of the relevant science: about how best to conceive of the world so as to accommodate the scientific facts.
Difficult sentence
Physicists explain variant matters on the basis of invariant matters, and would it not obstruct science if fragmentalism demands a change in the current scientific practice?
What is the clear distinction between variant matters versus invariant matters. IMO the background of this distinction lies in mathematics. What does this have to do with physics.
Ultimately, current practice ought to conform to whatever turns out to be the most reasonable overall view of the world.
Difficult sentence.
If there turn out to be major advantages to the fragmentalist interpretation over the standard interpretation, then it also would not obstruct science to change current practice, on the contrary.
If this is what is happening, then it makes sense.
What is the chance?
The fragmentalist approach sketched earlier, which maintains explanations in terms of a fundamental Minkowskian spacetime, is less revisionary than the fragmentalist view currently under discussion, which also reverses the order of explanation.
Given the same observations; what does Minkowskian spacetime better explain and predict than the Newtonian space time concept?
And what extend is the fragmentalists view important in this picture?

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They write that the interval must be what explains Lorentz transformations ‘because the spacetime interval, as a frame-invariant fact, is the reality, whereas the facts related by the coordinate transformations are frame-dependent facts and hence are appearances of that reality’

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Reflection 1: The observer in the middle of the train

At page 5 the moving train experiment is discussed.
The problem in the moving train experiment is which object is at rest and which object is moving. Depending on what has been selected the outcome for the different observers can be different.
The setup of the train experiment can be such that there is a platform considered at rest and that there is a moving train. There are two contacts along the track equally spaced as the distance between the wheels of the train. That means when the front-wheel hits one contact also the back-wheel hits the other contact. When there is contact there is also a light flash.
This means:

At page 5 the experiment is discussed slightly different. The origin of the flash comes from an observer in the middle of the shuttle and the events discussed are the arriving times of the flashes at the front end and the back end of the shuttle.
The problem is that both these events cannot be observed. What can be observed, if there are mirrors, the reflections as observed by me and you.

Reflection 2: A system at rest.

Consider a 1000 objects, free floating in space, subject of gravitational forces.
Which one of these objects is physical at rest?
Generally speaking none.
The best point to select as the origin of a frame is the center of gravity of this system.

However this is not all. The first law of SR states: The laws of physics are invariant (i.e. identical) in all inertial frames of reference (i.e. non-accelerating frames of reference).
The question: is what has a reference frame to do with the way physical processes behave.?
Physical processes in general are dependent about many parameters and if the 'same' experiment (process), performed twice, results in different results, than the reason what the physical cause is, should be investigated and the law that describes these procesess should be modified. The same should be said when studying the twin experiment. In the twin experiment two identical clocks are involved: one at rest and one moving. The results of the experiment is that the accumulated clock count for the moving clock is less than the accumulated clock count of the clock at rest. The explanation should start with the different speeds of the moving clock (accelerations). An simple investigation of the behaviour of the moving clock shows that that the light path inside the moving clock is longer than the light path of the clock at rest.

However does that mean that one clock is at rest and the other one moving? No.. In most cases both clocks are moving. At the same time, the chance that both clocks are ticking at the same rate is almost zero.

Reflection 3: The phylosophy of physical science

This document is partly a phylosophical document.
Special Relativity almost starts at the beginning with two postulates. Postulates are not simple assumptions. They are stronger. They are assumptions without proof. They are facts.
For example. Postulate 1: The speed of light c is constant (in all directions). Postulate 2: The Laws of physics are the same in all inertial frames.
Is that allowed?
A different way to do science is to start from experiments and observations. To establish that certain observations are correlated. Secondly to explain these correlation (often by a cause and effect scheme) and the call such an explanation a theory. Thirdly to quantify (model) these correlations and call such a model a law.
In the article under discussion, starts with 3 examples, with IMO is the preferred way.
The first article starts with the observation that I'm alone. The problem is that you can only make that observation when you know that there are more people on this earth. Next there is an assumption: I'm also at rest. Here the problem is that you can only call yourself at rest if you understand the difference between 'at rest' versus 'not at rest' or moving versus not moving. This creates problems if after a certain time, you discover that you are not alone on this earth i.e. there exists also someone else. The first problem is that immediate you should realize that both of us cannot be at rest and most probably neither of us is at rest. The second problem is that initially I considered my self as the center of the universe, linked to a reference frame with myself as the origin. When you discover that there are more people you should modify this one reference frame concept in such away that all observers are treated equally.

See: Reflection 4: Fragmentalism versus Special Relativity

One very important tool to do science is mathematics. One important phylosophycal concept is that physical process are controlled by physical laws. IMO it is completely the other way around. Physical laws (mathematical equations) are the descriptions of some physical processes. The more stable the physical processes the better this fit. The more unstable the physical processes (also called chaotic) the worse this fit. For example: very difficult to predict involves weather prediction and the stock market. Easy to predict is the movement of the planets around the Sun.

Reflection 4: Fragmentalism versus Special Relativity

To compare Fragmentalism versus SR only can be done if there exists a good definition of each. The problem is that is not the case.
Special Relativity should at least be a law (or a set of laws) which describe and explain certain physical processes. IMO this is are only electromagnetic processes. Simple described as processes where light (photons, radiation) is an issue.
If you compare SR with Newton's Law or General Relativity, both these mechanical laws describe the movement of objects in the universe.
In fact there is a clear physical distinction between the two mechanical laws and SR: The first two are based around the concept of gravitons and the second about photons.
It is even stronger the first two laws have nothing to with the speed of light. That does not mean that the speed of light is not involved in any experiment when GR or Newton's law is involved. It is very important first to do the observations inorder to create a physical picture of the objects studied, at different moments. Secondly to verify the results of the predictions.

Special Relativity is very much related to the concept of time. Time is often considered as a fourth dimension, but is very different compared to the parameters x, y and z. A concept like length of time is typical a mathematical concept and not a physical concept. A concept like l = c*t defines the length that a photon has travelled with speed c in time t, but has nothing to do with time.
Time is measured with a clock. Generally speaking there are three types of clock: Mechanical clocks, Atomic clocks and clocks working with time signals. All clocks can be described by means of mathematics and generally speaking the mathematics for all clocks is different.
There are two types of clocks working on lightsignals: Type 1 in which the light signal moves vertical and type 2 in which the light signal moves horizontal. In a type 1 clock the reflecting mirrors are horizontal (one below and one above). In a type 2 clock the reflecting mirrors are vertical.
The important physical issue is that when either clock is moved the length of the lightpath changes and becomes (physical) longer. As a result the clock ticks slower compared with a clock at rest. The important issue is that the mathematics that describe this behaviour for both types of clocks is different. For the type 1 clock this is the Lorentz transformation, for the type 2 clock this is different.

This tells us what the background is of SR. The origin of SR is the behaviour of a moving clock.
However it teaches more. If you want to do science don't use moving clocks or at least use a clock at the centre of our solar system or at the centre of our Galaxy.

What is the overal conclusion:

  1. The most important laws are Newton's Law and General Relativity.
  2. Use only one clock (or reference clock) and one reference frame.
  3. The concept of fragmetalism is limitted, to say the most

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