Comments about the book "Space and time in the modern universe" by P.C.W. Davies

This document contains comments about the book: "Space and time in the modern universe" by P.C.W. Davies. Dover Publications 1950

• The text in italics is copied from that url
  
• Immediate followed by some comments
  

In the last paragraph I explain my own opinion.

Contents

Reflection

• Reflection 1 - Reference frame.
• Reflection 2 - Physical length, space and time.

	1. The many faces of space and time	page 1
	1.1. General concepts	page 1

- Nevertheless, the word space conjures up a picture of emptiness - what is left when all tangible things have been removed.

IMO it is important to make a clear difference between physical space and mathematical space. The Universe in its totality is 3D physical space or a 3D physical area. Physical space contains objects (matter) but is not empty. This pseudo emptiness takes care of energy transport, which allows us to observe the stars in the Universe. Mathematical space does not physical exist and does not recognize the concept time.

Rather than picturing the universe as contained in space, modern cosmology regards both material objects and space as together constituting the universe. The universe is space and matter.

With the addition that this (physical) space is not empty or pseudo empty i.e. filled with energy (transport), elementary particles.
Page 2

In contrast to the picture of space as a physical entity, which can exist in its own right indepentdently of matter, there is a long tradition among certain scientists and philosophers of attempting to reduce all the properties of space to relations between material bodies.

The issue is that the universe can not be studied without matter. When there is matter there is also gravity, which implies that to study SR without GR does not make sense.
What Newton did in fact was too simple. (But very clever when he lived). In order to study celestrial mechanics he assumed that the force of gravity acts instantaneous. In reality this is not the case.

Many of the properties which are attributed to space (or to the relationsip between objects) are well known to most people and usually taken for granted.

The universe is space and matter. The most important are the properties of matter.

The complexity and richness of this structure is soon revealed when the properties of real physical space are compared with mathematical models of spaces in which some of the structure is absent.

The central issue to understand physics is matter.

The human experience of time is fundamentally different from that of space.

From a scientific point of view human experience of time is of almost no physical importance in order to understand the evolution of the Universe. The most important is matter.
Page 3

Unlike space, the structure of which is only appreciated by observation, and abstraction away from the familiar, time is perceived to possess structure at a most fundamental level.

Time is a rather passif factor. Time is important to evaluate the events that happend simultaneous, the sequence of events and the coordinates of the events.

In addition, space and time are linked together by motion, and it arises from the study of motion of material bodies and light signals that space and time are actually two aspects of a single unified structure called space-time.

It should be mentioned that space-time is a mathematical construct.

It is one of the most perplexing puzzles in physics that the elementary conscious experience of time - the flow or motion of the present moment - is absent from the physicist's description of the objective world.

Understanding physics means unraveling the details of the evolution of the physical processes i.e. the physical changes, that take place in the universe. What is important that these changes have nothing to do with conscious experience of time by observers i.e. all humans. As such the concept: "the flow or motion of the present moment" should not be overestimated.

Whether this is due to a deficiency in the framework of physics, which pays scant attention to the role of the conscious mind in the universe, or whether it is because the passage of time is an illusion, is by no means clear.

It should be accepted that time is not an active part of physical processes. Time is a physical fact, linked to the existance of the universe. It should be mentioned that the concept ct i.e. a distance defined as the speed of light c multiplied by the duration t, is a mathematical artificial construct.

1.2 Mathematical models of space

page 4

A theory of space, like all theories in science, requires a model. In common with most good physical models, it should have a mathematical description.

In order to understand the evolution of the Earth a time line is required, dividing the total time of existance (5 billion years) in major time steps and describing the major characteristics of each time step.
In order to understand the evolution of the universe the same approach is followed. The age of the universe is 14 billion years. The description of each time step involves elementary particle physics and chemical reactions (i.e. evolution of stars). The description of the present involves a physical 3D model, which consists of mathematical equations. The most important aspect of mathematics, is a detailed definition of the measurements involved.

Page 6

Dimensionality A well-known specific feature of real space to be imposed on this continuum is the frequently stated fact that it is three-dimensional.

Real space and real objects, like all physical objects, are all 3 dimensional. 2 dimensional surfaces and 1 dimensional points are not physical objects. To say different: Surfaces and points physical don't exist.

The easiest way to understand the meaning of this is to start with a point, which is defined to have dimension zero.

Mathematical points have dimension zero. Physical points or real points are always 3D. See also page 10

Page 7

There is no limit mathematically to the number of dimensions a space may possess

This should be: mathematically space.

Indeed, an important branch of mathematics with applications in physics deals with infinite dimensional spaces.

Physics only deals with 3 dimensions.

It is not known why real space is three-dimensional.

To answer this question you need experiments which challenge the concept that the results can not be explained with 3 space dimensions x,y and z.

It is interesting to consider the properties of (physical) worlds in which (physical) space is say two or six dimensional.

The surface of a water wave is not a 2D physical object. A water wave is part of a 3D structure which has a length, width and height.

Page 8

Sometimes things such as wave propagation or electric phenomena are very difficult in these imaginary worlds.

To explain the working of a radio does not require the concept of imaginary worlds

We do not know if our universe is like the surface of the sphere, or the torus, or some more complicated system. However, in the region of the universe that we actually observe, it is simply connected, like the surface of the sphere.

It is important to define the universe not strictly based on how it is observed. The universe can be physical much larger. It is also wrong to consider the Universe as a balloon.

A space is defined by its properties.

It should be clear if this is a mathematical or physical description.

For example, a two-dimensional 'flatlander' could deduce that he lived on the surface of a torus solely by observation from within that surface (say by testing whether all circles could be shrunk to a point).

Two-dimensional 'flatlanders' don't physical exist. What exists are three-dimensinal 'flatlanders' or humans
If an observer starts from the North pole and walks in a 'straight' line (distance l) to the equator, at that point makes a turn of 90 degrees and walks along the equator again a distance l, at that point makes a turn of 90 degrees towards the North pole, he will reach the North pole after a distance l. That means the observer returns to his initial position after traveling 3 equal distances and making 3 equal turns of 90 degrees. The shape of the path is a triangle. However this triangle is not flat, because this triangle is drawn on the surface of the earth and is curved. To draw the described triangle is impossible on a flat surface.
On a flat space you can only return to the point of origin, if you travel 4 equal distance and you make equal turns of 90 degrees. The shape of the path is a flat square. However such a flat square cannot be drawn on the surface of the earth.

Mathematically, there is no difficulty in extending the discussion of a two-dimensional toroidal surface to a three-dimensional toroidal volume without introducing an embedding 'superspace'.

But physical it does not make sense. From a physical point 2D surfaces don't exist. 3D volumes can exist.

Page 9

Nevertheless,it is helpful sometimes to imagine a higher dimensional embedding space just for intuitional convenience, but one should not expect any discussion of the nature of this embedding space, for it is only an artifice.

This whole discussion makes only sense from a mathematical point of view. From a physical point of view, only 3D volumes should be discussed.
In order to understand space expansion, physical expanded space is transfered or translated as the surface of a ballon. The problem is there no ballon as such a comparison does not make sense.

Orientability With this caution in mind, we shall frequently describe the properties of real three-dimensional space by analogous two-dimensional models embedded in three-dimensional space, for clarity of exposition .

The physical universe and all objects are three dimensional.

One such example is the Möbius strip, which is a two-dimensional space conveniently described by drawing it embedded in threedimensional space as shown in fig 1.4

The Möbius strip is a 3-dimensional physical construct. To build (create) a Möbius strip you need a flat, long strip of paper. Such a string of paper is a 3D construct.
Then there are two options. Lay the strip of paper, lengthwise, in front of you. Take both short ends in your hand, bend them toward each other, and tape them together. A different option is, before taping both ends together, to turn the shortest end together.
This explanation emphesizes the importance of how 3D physical objects are created.

It consists of a strip with a single twist in it, and a moment's thought will show that a left-handed glove turns into a right-handed glove when transported in a closed curve round the strip.

That is physical correct, but the physical importance compared to the evolution of the universe is limited.

Page 10

But even with these restrictions, we may construct mathematical spaces with wildly different properties from real space.

The most important is to study physical objects in physical space

Further important restrictions must be imposed before we have useful models of the real universe.

The most important is to perform (detailed astronomical) observations of the universe.

One of the most obvious practical properties of space in the way in which points can be located by continuous labels or coordinates.

That is correct, but how is that done for individual 'points'?

To take a familiar example, a town may be located by specifying its latitude and longitude, two numbers which label points continuously on the two-dimensional surface of the Earth.

The Earth is a 3D object, including its surface.

Page 11

A space which carries consistent continuous coordinates is called a manifold
In addition to being a manifold, real space has geometrical structure.

This implies that the universe is also a manifold, but what does that really mean?

For example, there exists a shortest path between any two points?

How is that path measured?
Such a shortest path only exists between two points, part of a material object.

Before 1915 it was assumed that the real universe was a metric space which was restricted to obey the rules of Euclidean geometry, named after the Greek geometer Euclid.

First, the combination between 'the real physical universe' and 'a metric mathematical space' is unlucky. Physics and mathematics should be discussed separately.
Secondly, the wording was restricted to obey the rules is wrong. Physical phenomena don't follow any law.
A more correct sentence is the following:
Before 1915 it was assumed that the universe can be described as Euclidean geometry, named after the Greek geometer Euclid.

In this system (i.e. Euclidean geometry) the angles of a (flat) triangle always add to 180 degrees and parallel lines can always be drawn.

This is a mathematical discussion with has 'nothing' to do with physics.

Before the discussion of the mathematical properties of real space is ended, a word must be said about time and space-time. Clearly time may be assumed to share many of the properties of space.

From a physical point of view, space and time are completely different and have nothing to do with each other.
The concept time is linked to physical objects and has 'nothing' to do with (mathematical) space.

It (time) also has a metric structure because we can define the distance between two points in time as the interval between two events.

This distance is defined by multiplying the interval between two events with the speed of light.

For these reasons time can be regarded as a one-dimensional mathematical metric space, which should not be allowed to confuse the reader into thinking that time is really space in disquise or something.

Just a thought. The astronauts in a space-ship and all the humans have a feeling of our existance and of time. This feeling, at the begin of a round trip, is for all the same. During the trip both the astronauts and the humans will age. After the round trip, the feeling of time is for all still the same but this aging for the astronauts and the humans can be differently. The reason for the astronauts can be the influence of gravity and the high speed of the space-ship. In short this influence is medical e.g. physical and has nothing to do with mathematics. That the clock's of the astronauts run behind is mechanical and not mathematical.

Consequently the word 'space' will often be used in this mathematical context to cover aspects of both physical space and time, or space-time.

This is confusing. To understand the universe you should stick as much as possible to physical concepts.

1.3 Newton space and time

page 11

Page 12

Because the path of a moving body is through space in time, this theory necessarily connects space and time together in a set of laws.

The existance of 3D objects links space i.e. the 3D coordinates, with time.

Thus every object has a unique position and orientation in space, and the distance between two events is well defined, even if the events occur at different times.

This is physical a complex situation. My understanding is, that the measurements should reflect the same time.

Newton's concept of time relied heavily on the notion of simultaneity

You can also describe this differently: Newton's concept of simultaneity implies that two events happened at the same time.

Time in this model is universal and absolute.

This sentence has no physical meaning.

A universal time attributes meaning to the notion of events being simultaneous (i.e occuring at the same time) - when these events are at separate points in space.

You can also describe this differently: Two events are simultaneous when they happen at different points in the universe at the same time.

For although, as we shall see, space is taken by Newton to act on matter under som circumstances, matter does not react back on space.

A more realistic observation is: Matter acts on matter.
An even realistic observation is: Matter acts on matter (causes matter to move), but this action (or influence) is not instantaneous.

Page 13

Space as a physical object only makes sense if it can be detected, or if it can exert physical influences.

A better description is: Space as a physical concept only makes sense if space containes objects, which can mutual influence each other.

Moreover, all then geometrical properties of space are only inferred from observations with material objects and light signals; for example, it is easily verified that to a high degree of approximation the angles of a triangle add up to 180 degrees if we equip ourselves with theodolites and poles.

The most difficult part is that the positions of the three angles, in space, have to be measured simultaneous.

In Newton's model of space and time, it is meaningful to discuss the velocity of an object through space.

The most important mechanical question is to measure the position of an object at a certain moment.

	2. The relativity revolution	page 29
	2.1. Space and time crisis	page 29

The position or velocity of a body in Newton's absolute space cannot be revealed by any mechanical experiment.

In order to understand physics the position and time of events are important. In order to understand the movent of objects all objects involved should be included (in the solar system).

However, with the appearance of Maxwell's electromagnetic theory, there arose the possibility of using optics - the motion of light signals - to measure the velocity of bodies through space.

This replaces the problem to: what is and how do we measure the speed of light.

Nevertheless, in the nineteenth century the ether stream was considered very real. The challenge was to measure its rate of flow.

The problem with the ether theory is, that assuming their exists no ether, that space is completely empty.

Page 31

The absence of an ether stream faced physics with a fundamental and disturbing inconsistency.

What physicists should have done to study the experiments that required an ether, from scratch.

Albert Einstein, one of the world's greatest physicists, exploded the whole conceptual framework in which the ether stream experiments had been conducted.

The reference to the ether stream experiments is weak. New research should not be based on failed experiments. At least ?

At the heart of the Special theory of relativity is a denial of the reality of Newtonian space.

To understand this sentence: you need first an explanation "what the reality of Newtonian space" means. If you understand what that means, you cannot deny that.

The ether cannot be detected because it is not there.

But that also means that the physical concept ether cannot be clearly understood.

The whole notion of an absolute frame of rest against which the velocity of an object through empty space could be measured is a fiction.

The concepts absolute frame of rest and empty space require both a clear definition.

Page 32

The new principle of special relativity at first seems innocuous, and then utterly baffling. It states that the speed of light is everywhere constant.

This raises the question how the speed of light is calculated.
What experiments can demonstrate is that if there are two sources which each, both emit a light signal simultaneous at time 1, these lightsignals will arrive at a distant point also simultaneous. This is also valid when the speed of the two sources at time 1 is different.

It states that the speed of light is everywhere constant

To make such a claim about the physical behaviour of a light pulse is strange, because this claim claims that the physical behaviour is always the same, with no restriction.

Page 33

Something very peculiar must occur if a rocket cannot gain one little bit on a light beam, however powerful the engines it possesses.

Consider a light source which emits flashes of light. Consider a rocket which travels away from this light source. My understanding is that these flashes enter the rocket from the back and leave the rocket from the front. This understanding is in agreement with the concept that the speed of light is higher than the speed of the rocket.
The question is what happens when the speed of the rocket is increased. My understanding is that the speed of light is always higher than the speed of the rocket. Nothing fancy is involved.

2.2. The overthrow of Newtonian time

page 33

To obtain a noticeable effect, the train must be travelling very fast indeed - at a sizeable fraction of the speed of light, relative of course, to the railway track?

Assumed to the length of the railway track?
How does one know that the train is traveling at 0.1 times the speed of light? Or for example 0.001 times the speed of light?

Page 34

At some convenient moment when the carriage passes by, the lamp in the centre flashes briefly, sending a short-duration light pulse in both directions down the carriage.

This part of the description of the experiment is clear.

When these pulses reach the carriage ends the event is recorded by both A and B

The event when this pulse reaches the front of the carriage is called tf. It should be mentioned to observe this event you need an observer at the front of the train. The event when this pulse reaches the back of the carriage is called tb. It should be mentioned to observe this event you need an observer at the back of the train.
Both events cannot directly be observed by observer A and B. They require reflection by a mirror.

The outcome of this imaginary experiment is rather odd.

Why call this an imaginary experiment? To understand the physical world, you should not use thought experiments, but perform experiments, which can be repeated. The outcome of any experiment can never be odd.

A, who is travelling in the train, sees both pulses leave the centre and pass down the inside of the carriage, reaching both ends at the same moment.

The last part is an assumption.
A, cannot see the two moments tf (t front) and tb (t back) instantaneous, when they happen. What A can see (in principle) is a flection of each tf and tb. A, can also not see if these two moments happen simultaneous i.e. at the same moment.

This happens because the speed of light is the same for both pulses and the distances over which they travel are equal.

Both reasons are not convinving. As mentioned "A" cannot see the two moments tf and tb simultaneous. What the experiment should indicate that the two moments or events tf and tb are simultaneous.

The experience of B, standing beside the railway track, is very different.

Observer A is part of the moving train, on the railway track i.e. the surface of the earth. Observer B is part of the platform, i.e. the surface of the earth.
Observer B can be considered at rest. The same is true for the railway track.

He also observes the pulses travel in opposite directions at the same speed, but this speed is now reckoned by him in his own reference frame, fixed to the railway track.

B cannot observe that the speed in both directions is the same.

Page 35

What constitutes the same moment at the two carriage ends relative to A is not the same moment as observed by B.

"A l" is the event that left signal hits the left end of the moving train.
"A R" is the event that right signal hits the right end of the moving train.
A observes both these events later. B observes both these events also later, but at different instances.
Experiments should decide which is which. The results have nothing to do with SR, however this description, the reasoning, can be called SR.

Page 36

Consider two inertial observers A and B in relative motion with velocity V in the x direction (see fig 2.4).

The question is how is this velocity v (of B) measured.

At a later time t we might expect x and x' to be related by

x= x' + v*t

(2.1)

because we must add to x' the distance v*t which the frame S' has travelled relative to S in the duration t.

Page 38

A clock carried in a fast rocket would run slow relative to a similar clock left on Earth.

See also: Reflection 1 - Reference frame.

It is important to realise that the effect here is a property of space and time, and has nothing to do with the mechanism of the clock itself.

My understanding is that the mechanical bahaviour of a clock depends about how the clock is build.
The most important issue is, if a clock, which is a mechanical, physical object, truelly shows the physical, universal time

The theory of relativity which predicts the slowing of clock rates is founded upon the relativity of uniform motion, so that no internal observations of clocks or anything else is permitted to indicate whether a system is moving or not - remember that there is no absolute uniform motion.

Page 39

The clock effect, usually referred to as the time dilation effect, is only manifest when observers may inspect other systems relative to which they are in motion.

At this part of the book it becomes very important to explain an experiment how this inspection is actually performed. The practical problem is that it is very difficult to observe other systems at a distance. The most practical experiments are roundstrips.

Thus the observer in the rocket sees the Earthbound clock to be running slow, but not his own, while, because of symmetry inherent in relative motion, an observer on Earth would likewise see the clock in the rocket running slow relative to his own well-behaved clock.

The issue is what happens with the time of a clock in a rocket, which travels to a far away distance, in a round trip, compared to a clock which stays at home, on Earth. The fast moving clock will run behind.
The explanation of this behavior, of a clock, which works using a lightsignal (running vertical when v=0) is that the path of this light signal becomes longer compared with a clock at rest, and as such runs slower.

It will be seen on inspecting equation 2.6 that when v approaches c, gamma becomes indefinitely large, so that the interval of time delta-t appears to the moving observer to be indefinitely extended.

Equation 2.6 is a mathematical description of (the results) a physical experiment, but can not be used as part of an explanation of a physical effect. What counts are the actual results of the measurements of an experiment.

2.3. The twins 'paradox'

page 39

It is not that each observer merely sees the other clock running slow, it actually is running slow - a real physical effect.

The question is if a mechanical clock, which is a moving object, should be running slow.

A dramatic way of illustrating this is to arrange for two identical twins to participate in an experimemt in which one twin leaves Earth in a fast rocket which travels at near the speed of light to the nearest star and back again.

The issue is if the time of a mechanical moving clock should be running slower as the universal time.

Page 45

2.4 Faster than light?

page 45

It might be wondered what would happen to an object which was accelerated to a speed faster than light.

The simple explanation is to perform an experiment. You need at least a rocket, an engine and fuel. After take off you can increase the speed of the rocket but sooner or later you run out of fuel and you have reached the maximum physical speed possible.

The infinite contraction of its length and dilation of time which occur at v=c might be expected to place a limit on the velocity of an object to prevent it from becoming superluminal.

As mentioned what will happen: perform an experiment.

Page 46

Page 49

2.5 The new four-dimensional space-time

page 49

Since then it (SR) has become one of the foundation stones of the edifice of modern physics, with implications which pass far beyond the rather trite considerations of high speed railway trains and rockets. In fact, the time dilation effect has been verified directly, both at subatomic level and macroscopically by clocks being flown around the world.

If physical clocks being flown around the world behave differently, they should not be used for physical research.
Specific physical clocks which use light signals and when the performance (clock rate) is a function of the speed of the clock i.e. the speed of light, then such a clock should not be used.

Page 50

In Newtonian space and time it is assumed that spatial lengths and temporal intervals are independent of the motion of the observer or system. That is, rod lengths and clock rates do not depend on the relative motion of these objects and on who it is that observes them.

That means, general speaking, taht the evolution of any process, does not depend if there are observers. See: Reflection 2 - Physical length, space and time.

In contrast, the theory of special relativity requires that the length of an object contracts along its direction of motion, and its temporal progress expands.

My understanding is that in physics the leading factor to understand the reality is by performing experiments. Ofcourse before performing the experiment we can predict the outcome based on a theory, but that does not mean that the experiment can be skipped. One very important aspect of the experiment is how the length of the moving train is measured. A second aspect is if the length of a train towards an observer versus moving away from an observer are the same or different. My understanding is that the length is the same.

Now we have seen from the train experiment that a moving object which is extended in space is extended in time also - events at the ends of the train regarded as simultaneous to a passenger are spaced a time interval apart to the obsever on the embankment.

Consider that there are two events that happen inside the moving train. What the text implies that it is possible that an observer in side the train observes these events simultaneous, but an observer along the embankment not.
There are two questions: (1) is it pssible that two events are simulataneous and (2) what is the definition of two simultaneous events i.e. events that are simultaneous.
The underlying question is: how are these two events created.

	3. The asymmetry of past and future	page 56
	3.1. The meaning of time asymmetry	page 56

What is the cause of change in the universe?

What is the cause that the universe is dynamic and not static?
That is an impossible the answer question. One thing is for sure (?) that if the universe was static it would not exist.

The quest for understanding why and how some things around us change while others do not is a long, controversial and disorganised chapter of scientific history.

One of the most interesting (human) observations is that the Universe is dynamic, and subject of change, at all levels of detail. The emphasis is here on human observations, with the remark that there are only humans in the final 3 million years. A small number of changes are human influenced, but most are not. All changes that happened before that period are to a large extend inexplicable and there exist no answer on the question what caused these changes. The most impossible question to answer is: How did it all start.

Consequently, in thermodynamics and biology, information theory and mathematics, electrodynamics cosmology and many more this question has been tackled using the mathematical tools and language of the trade.

My understanding is that the least pratical tool to understand the evolution of physical processes around us is mathematics. The most usefull tool IMO are experiments.

Perhaps the greatest misconception of all surrounding the problem of why things change with time is the confusion between time as it enters into the laws of physics and time as it enters into the human mind.

The laws of physics are a scientific description of physical processes, partly involving their evolution in time. Time as it enters into the human mind is a personal description of how we experience time. There exist no confusion between these two.

In chapter 1 a sharp qualitative distinction was drawn between the human perception of space and that of time.

Page 61

In fact, time-asymetric change appears to be the feature of almost every natural phenomenon, Actually, even processes with at first sight seem to be symmetric in time are often, when viewed over a longer time scale, mildly asymmetric.

This leads to the possible conclusion that the evolution of the universe at grand scale is time-asymmetric, with in turn leads to the conclusion that the time-asymmetry has no meaning.

3.2. Irreversibility and the second law of thermodynamics

page 63

	4. Gravitation and the bending of space-time	page 86
	4.1. Physics from the falling body	page 86

	5. Modern Cosmology	page 141
	5.1.The architecture of the Universe	page 141

No analysis of space and time is complete without dicussion of its totality. The totality of space is the Universe and the totality of time is the history of the universe.

The most realistic aspect of understanding the Universe is to try to understand the present and the local Universe. The most complex is the evolution of the Universe in time, in all its details.

Such matters are often considered to the realm of religion or philosophy, and indeed, that is where they did belong throughout most of human history.

The most important issue related to history of the Universe (the evolution in time) are philosophical and physical considerations.

It is no longer necessary for scientists to have beliefs about when the creation of the universe occured or what form it took, it is now a question of using scientific instruments to see what the universe is like and how it has evolved.

The first part of this sentence is almost impossible to answer. Scientific instruments are mostly used to study the present and the vissible Universe.

Nevertheless, it is important to appreciate that we are dealing here with science, and scientific values, so that while personal religious or philosophical preferences may make a great contribution to a particular individual's conception of the universe, the topics to be discussed in this book deal solely with concrete observational data and the controversies which rage around their theoretical interpretation.

Personal preferences and individual conceptions should be minimised. The most important is to describe our knowledge in simple language.

Page 145

It turns out to have the greatest relevance for the time asymmetry of the universe.

The evolution of the universe is a one in a life time event, and has 'nothing' to do with the concept time asymmetry. See also time asymmetry

	6. The beginning and the end	page 172
	6.1. The unstable universe	page 172

In chapter 3 it was

	7. Mankind in the Universe	page 200
	7.1 The impact of space-time concepts on society	page 200

The conceptual foundation for a scientific model of space, time and the universe is necessarily influenced by a pre-existing picture of mankind's place in the cosmos.

The unraveling of the details of the universe and the cosmos, a description of etc, have nothing to do with mankind.
A small part of what is happening 'at present' in the universe, and what we can observe, lies in our neighbourhood. Most we cannot observe. The rest we can observe happened in the past.
Considering the evolution of the Earth only approx. the final 5 million years are influenced by mankind.

Conversely, experimental and theoretical advances in the scientific understanding of space-time physics and cosmology have an impact on society, as do all forms of human intellectual activity.

It must be said that the benefits of science are not distributed equally among all people. Regarding the disadvantages, the picture is the same.

Traditionally, people have appealed to religion to answer questions concerning the structure and evolution of the universe, the creation and destiny of all things in the great scheme.

Today, because not all people are treated equally, the influence of religion on political issues, does not lead to stable societies.

One common criticism of the scientific explanation of these fundamental issues concerns its tentative nature

There is nothing wrong with doubt. Improved equipment can change the opinion of scientists. Such discussions are healty i.e. democratic.

Religion is founded upon faith and dogma, so that the religious explanation is not expected to evolve in the light of experience.

In religional or etnical discussions there is often no place for doubt. That means there exists no healty exchange of opinions.

However, it is not so that the continual re-arrangement of scientific opinions displays a sign of weakness. On the contrary, this is its strength.

For religion there exists no re-arrangement of opinions.

Reflection 1 - Reference frame.

The most obvious way to predict the positions of the stars in the Milky Way is to use a single frame of reference and take the position of the black hole Sagittarius A as the center. Using this reference frame and a clock at the center it should be possible to create a database with observations in the past, which can be used by all astronomers. The clock at the center shows universal time.
The reason is because every observer should be able to add new observations to a data base of observations (position and time) in the past.

Consider the space, part of the Milky Way, filled with clocks, all synchronized by a clock at the center of the Milky Way. All these clocks show the same (universal) time, however observed from this center they all run behind. This delay is a function of distance to the center. This implicates that if this distance is the same, these clocks run synchronious.

Consider all the planets in the solar system. At each planet there is an observation site. At each hour the position of the planet is calculated using the clock nearest to the position of the planet. All these positions show the positions of each planet at the same moment (at intervals of 1 hour) simultaneous. Using these data the position of the planet at the next instant should be calculated.
Consider that each planet also contains a local clock. All these local clock are synchronised at a certain moment, using the clock nearest to the position of the planet. The next hour when the position of the planet is calculated the local time runs behind the universal time.

This method can be critised. A better way is to use a rocket. Comparing the local time with the universal with the nearest clock in side the Milky Way will show that the local clock runs behind.
The most logical conclusion is that local clocks should not be used for any experiment.

Reflection 2 - Physical length, space and time.

Physical length and 3D space assumes that the 3D dimensions of rods are independent of observers who measure these 3D objects. That means if an observer observes or sees an object from different angles or distances the dimensions of the object does not change. The same is true for stars which move in the universe. The exception is if objects 'eject' mass caused by friction or evaporation, bassically as an interaction with larger objects and gravitation.

Time is also a physical parameter which defines that all events happening now in the universe are happening simultaneous. Time should be measured with logical clocks which are considered at rest. That means the distance between these logical clocks does not change.

Reflection 3 - Moving train under a bridge.

This reflection it not so much about a moving train in general, but more what happens with the length of a the train when the observer stands above the train on a bridge and the train is more or less before and after the observer.

O B<----------M---------->A o e b<----------m-----d---->a -------v------> Figure 1

Figure 1 shows the train twice. The length of the train is 2l The top part shows the train at rest. The observer O is at point M above the train, on a bridge. The bottom part shows the moving train at moment t0. The observer o is at point m above the train, also on the bridge At t0 observer o will see the center of the moving train at point m. At that same moment the front is at point a, and the back at point b. But that is not what observer o sees. The problem is that light from the front at t0, still has to travel to point m. The consequence is that o will see the light from an earlier moment i.e. point d. Assuming that the speed of the train is half the speed of light then the front of the train will reach point d at t0 - l/(c/2). This image of the train will reach point m (distance l/2, speed c) at: t0 - l(c/2) + (l/2)/c = t0.

A similar problem exists for the back of the train, which moves towards the right. That means the image from the train travels with a speed c/2 further towards the left from a distance 2l.
Assuming that the speed of the train is half the speed of light then the back of the train will reach point e at t0 - l/(c/2). This image of the train will reach point m (distance 2l, speed c) at: t0 - l(c/2) + 2l/c = t0.

What this means is when an observer is half way between the front and the end of the train

• and he looks towards the front of the train, than what he observes is that the front is closer to him than the actual position. This is explained because light from the actual position still needs time to reach the observer.
• and he looks towards the end of the train, than what he observes is that the end is closer to him than the actual position. This is explained because light from the actual position still needs time to reach the observer.

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