Comments about "Spacetime Physics" by Edwin F. Taylor and John Archibald Wheeler - Second Edition 1992

This document contains comments about the book: "Spacetime Physics" by Edwin F. Taylor and John Archibald Wheeler
To read the first chapter of the book First Edition select: For a review about the First edition select: Book Review Spacetime Physics In the last paragraph I explain my own opinion.



Chapter 1. Spacetime: Overview

1.4 Same Unit for Space and Time: Meter, Second, Minute or Year page 34

Such a device is a "clock" that "ticks" each time the light flash arrives back at a given mirror.
That means the same mirror. If the distance between the mirrors 1 and 2 is 0.5 meter, this means that the flash travels 1 meter from mirror 1 to 2 and back to mirror 1

Chapter 2. Floating Free

2.4 Regions od SpaceTime page 34

However, questions about the entire trajectory cannot be answered using only one free-float frame; for this we require a series of frames.
Only general relativity can describe motion in unlimited regions of space time.
Exactly . This immediate raises a boundary to the usefullness of SR.

Chapter 3. Same laws for all

3.1 The Principle of Relativity

Einstein's Principle of Relativity says that once the laws of physics have been established in one free-float frame, they can be applied without modification in any other free-float frame.

3.2 What is not the same in different frames - page 56

Notice what the Principle of Relativity does not say. It does not say that the time between two events is the same when measured from two different free-float frames.
What PoR should say that the time measured with two identical clock, but moving at a different speed (going from A to B) should be different.
Neither does it say that the space separation between the two events is the same in the two frames.
I think what should be added is the word 'measured'. The issue is always how the distance between two events is measured.
page 57
These events are recorded not only in the laboratory but also by recording devices and clocks in the rocket latticework.

3.4 Relativity of Simultaneity - page 62

Moreover, using the Principle of Relativity, she knows that the speed of light has the same value in her train frame as for the ground observer (section 3.3 and Box 3-2) and is the same for the light traveling in both directions in her frame.
Specific how do you know the last? The problem is that the moving observer can also make such a claim in her frame. The issue is that the moving observer does not see the two flashes simultaneous as drawn in Figure 3-1. However Figure 3-1 does not give a realistic impression: the speed of light drawn is very low. So every thing seems simple, but it is not in reality.

Exercise 3-12 Michelson-Morley experiment - page 84

An airplane moves with air speed c from point A to point B on Earth. A stiff wind of speed v is blowing from B toward A.
The problem is this experiment is compared with the actual Michelson-Morley experiment which raises certain physical problems.
In the airplane experiment both the air speed c and the velocity v are measured in a frame linked to the surface of earth.
In the Michelson-Morley experiment the speed v of experiment is the speed of the surface of earth and c is the speed of light.
From the physical point of view the speed v at a specific point on the surface relatif to the speed of light is unknown and different for (almost) any point on the surface.

Exercise 3-13 the Kennedy - Thorndike experiment - page 86

This result leaves an important question unanswered: Does the round-trip speed of light - which is isotropic in both laboratory and rocket frames - also have the same numerical value in laboratory and rocket frames?

Exercise 3-14 Things that move faster than the speed of light - page 88

This exercise consists of 4 examples
a) The Scissors Paradox. In this experiment you have two rods which are not parallel to each other but make a small angle around point A. What happens when you move one rod down.
The point A is not something physical. It is not part of either rod. When you move the rod down this point moves toward the right, but nothing physical moves toward the right.
In principle this point can move with any speed, even the speed of light. But again nothing physical moves with that speed.
c) Searchlight Messenger The experiment involves a rotating light.
   2              x
   1        5
     2   4
   O   4  5         x
   Figure A
Figure A shows the search light at point O. The search light moves clock wise.
Figure A also shows 5 points nearest to the search light. This are the points 1,2,3,4 and 5.
Point 1 is the nearest point to the search light in vertical direction. (12 o'clock direction) This point moves in the same direction. This are the points 2,3,4 and 5.
Point 2 is the nearest point slightly later. This point moves away in the 1 o'clock direction. These are the points 3,4 and 5. Point 3 is the next nearest point. This point moves away in the 2 o'clock direction.
Point 4 is the next nearest point. This point moves away in the 3 o'clock direction.
The 3 points x are at the same distance from the origin (the same as point 5 in the vertical direction). These points can be at a large distance. What this means is that the search light physical moves at a slow speed and this virtual point x at high speed (as a function of r).
In this example the points in the same direction each show the same photon, moving at the speed of light.
The points at the same distance each show a different photon. The combined speed of these points can be much higher than the speed of light, but that is more a vissible illusion.

Chapter L. Lorentz Transformation - page 95

Exercise L.2 a bad clock

page 112
A pulse of light is reflected back and forth between mirrors A and B separated by 2 meters of distance in the x direction of the Earth frame.
a What is the physical basis for the "bad" behavior of this clock?
In the text they speak about 'physical basis' of the behavior of a clock. That is exactly what the subject is about. The issue is about the physical bahavior of a clock, specific it innerworkings. In this particular case the mirrors are perpendicular to the direction of movement.
You can also have the mirrors parallel to the direction of movement. The ticking rate is different.
b From the spacetime diagrams show quantitatively that your good clock "runs slow" as observed from the rocket frame - as it must, since the clock is in motion with respect to the rocket frame.
The fact the moving clock runs slow has nothing to do with reference frame. It is physical.
c Explain why the clock of Figure 1-3 (at page 12) in the text is a "good" clock
Figure 1-3 is based on the concept that the light flash follows a full cycle between the two mirrors.
The operation of a good clock is based that counts reflect a point at equal distance between the two mirrors. This is half a cycle.

Chapter 4. Trip to Canopus

4.3 Faster than light?

page 122
etc the lookout station clock print out - 10 year later than the Earth date of our departure.
Our rocket clock reads 6 years.
Have we actually covered a distance of 8 light-years from Earth in a time of 6 years.
Yes and No.
The moving clock physical ticks slower as the Earth based clock.
but we get nonsense when we mix together numbers from two distinct reference frames.
Exactly, as such if you want to understand physics (astronomy, mechanics) you should only use one reference frame
To understand the behavior of a clock you should always do a twin type experiment.

4.7 Lorentz contraction

page 126
First of all it is confusing to combine distances measured in one reference frame with time measured in another reference frame.
That is a wrong representation of the facts.
What is measured is the distance between two objects A and B and the time it takes for a clock to travel that distance forward from A to B and back from B to A versus the time one a clock which stayed at A. The result is that the moving clock shows 20*2 years and the stayed at home clock 2*99 years.
There is nothing confusing about this.
page 127
So think of the entire outward trip in terms of rocket measurements.
If you want to do that you play havoc with the reality.
He continues:"Think of a very long stick lying with one end at the Earth, the other at Canopus.
To measure this you can use rods of 1 km. You need many. let us assume 1000000
In case the rods are 1 lightyear you need 99.
"The factor by which the stick appears contracted in the rocket frame is just the same as the ratio of rocket time to Earth time for the outward trip
The word appears is important.
Hence the rocket observer measures the Earth-Canopus distance to be (99 light-years)(20/101) = 19.6 light-years - just a bit less than the 20 light-years as you said.
But this is not a measurement in the same way as the 'very long stick is measured'. It is a mathematical calculation.
Can this distance really be measured?
Can you use the 1km rods on board of rocket? IMO when you do that you stil need the same number 1000000 as before before. Also these 1km rods are 'contracted' (or not).
Anyway what rocket observer 'measures' is of no physical significance. The only physical issue is that the total flight takes 202 Earth Years while the rocket clock measures 40 years. This physical means that the rocket clock runs slower in the Earth frame.

Chapter 5. Trekking through spacetime

5.6 Wristwatch time along a worldline

page 148
You can also call this paragraph: Moving clock along a worldline.
The particle carries a wristwatch and a sparkplug. the sparkplug fires every meter of time (1,2,3,4 ) as read of the particle wristwatch. The laboratory observer notes which of his clocks the travelling particle is near every time the sparkplug fires.
As mentioned when you consider the wirstwatch a clock, then this whole text becomes much simpler. It is the innerworkings of the clock which generates a flash i.e. fires. As such the final result is that the moving clock runs slower as the clock at rest.

Chapter 6. Regions of spacetime

page 176
Only light "photons" neutrino's and gravitons can move directly between two events connected by a lightlike interval.
What is the physical basis behind this claim?
Figure 6.3
Figure 6.3 Two lightlike pairs of Events AE and AG (with event A arbitrarily chosen as reference event) as recorded in spacetime maps of three free float frames.
Figure 6.3 shows 3 frames:
  • A frame at rest. This frame is identified by the two parallel mirrors L1 and L2. The line in between shows the observer at rest.
  • A rocket frame moving towards the right. This frame is identified by the two tilted mirrors M1 and M2.
  • A rocket frame moving towards the left. This frame is identified by the two weakly drawn tilted lines.
What Figure 6.3 shows is that the observer at rest observes the two simultaneous events E and G at spacetime event B.
The Figure also shows that the moving observer does not see the same events simultaneous. She observes the event G at G' (earlier) and the event E at E'(later)
The Figure does not show what the observer moving towards the left observes. In fact it is the reverse as above.
See for comment about the issue of arbitrarily: Reflection - Physics

Reflection - Physics

All the 4 exercises Exercise 3-14 Things that move faster than the speed of light are based around physical phenomena. IMO they have nothing to do with SR, specific with the postulates of SR.
Figure 6.3 See: 6. Regions of spacetime defines an arbitrarily frame inbetween a frame which has a speed v towards the right and a frame v towards the left. This arbitrarily frame is called the laboratory frame. The physical issue is: where is this frame? Is this the frame centered around my laptop or PC? The center of the earth? Some other free floating frame?

The tricky part of the subjects discussed in the book "Spacetime Physics" that when you compare different clocks in the same reference frame the clock at rest ticks the fastest (without any good (?) defintion what a clock at rest is). This implies that there are clocks which run faster and which run slower. That means when you start from the fastest moving clock there are definitily clocks which run slower. etc.


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Created: 22 August 2017

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