Paul Nahin TIME MACHINES Notes

Below are my notes on Paul Nahin's excellent book, TIME MACHINES. The notes read like a stream of consciousness. My note-taking style is to jot down everything that interests me. For me, the one who took the notes, they plug me into the state I would have been in during, or shortly after, reading the book. I have discovered that occasionally reviewing my notes I can retain the essence of a book far longer than if I just think about the same book. I think they will be useful as a searchable index for anyone who has read the book.


Nahin, Paul J. TIME MACHINES, 1999, 2nd ed, 1st printing. [1st ed. 1993]
ISBN 0-387-98571-9
SPRINGER-VERLAG, NEW YORK, BERLIN, HEIDELBERG
SPIN 10682294

iix.1 [Grandfather Paradox] The name originated in a letter from a seventeen year old reader of Astounding Stories Dec 1931. [x.1, xviii.1, xviii.-1], [65.-2]

ix> FORWARD by Kip Thorne
ix.2 If the fundamental laws of physics permit time travel, even just on subatomic scales, then our present understanding of quantum mechanics is flawed in ways that would explain how information gets lost down black holes – and ways that may have profoundly affected the birth of the universe.
ix.-1 Igor Novikov’s Principle of Self Consistency
x.2 I [Kip S. Thorne] fed the concept of a wormhole to Carl Sagan for use in his novel Contact.
x.3 [T.M. ] It is now not only the most comprehensive documentation of time travel in science fiction; it is also the most thorough review of serious scientific literature on the subject …
xi.2 I think we will have discovered that there is a universal “chronology protection” mechanism that forbids time travel on macroscopic, human scales but that time travel is essential to the fabric of spacetime on subatomic, “Planck-length” scales and thereby has a profound influence on the fundamental laws of physics.

xiii> WHAT’S NEW IN THE SECOND EDITION
xv.1 Do I personally believe in time travel to the past? I believe in physics, so my scientific positions are subject to change as we learn more physics. Indeed that’s precisely why time travel is interesting even to physicists who can’t stand the idea (such as Hawking and Visser). Right now known physics doesn’t forbid time travel, and such physicists want to discover the new physics that they believe will finally forbid time machines.
xv.3 The second word in science fiction is important.

xvii> PROLOGUE TO THE FIRST EDITION
xvii.-1 chronomotion chronoviator
xix.2 The two principle players are Kip Thorne (CIT) and Igor Novikov (Moscow).
xix.-1 but I have learned that theology is a necessary dimension to any informed discussion on time travel.
xix.-1 The first time traveler in English literature. 1733 Samuel Madden’s Memoirs of the Twentieth Century. [1st twonky]
xxi.3 [an outline explaining what is contained in each of the book’s 7 sections] Nahin cites chapter 4 (paradoxes) as the most important.
xxi.-1 [an explanation of the system used in the bibliography]
xxxiii.2 Adolph Baker, “Physics in engaged neither in the development of time machines nor in the fabrication of bombs. But it is the business of physicists to make flights of fancy that carry them far beyond the boundaries imposed by current technology.”

xxxv> ACKNOWLEDGEMENTS TO THE FIRST EDITION

CONTENTS

ILLUSTRATIONS
xxxiii Albert Einstein and Kurt Godel
xxxiv H. G. Wells with skeleton

1> AN OVERVIEW OF TIME TRAVEL
1.-2 > The Mystery of Time Travel
2.2 travel through space. The center of a genre of fiction (now called science fiction) called the “imaginary voyage” or “extraordinary voyage.”
3.-2 Paul, “Time travel is the ultimate fantasy, the scientific addition to the human quest for immortality.”
4.-1 gifts from the future [Twonky]
4.-2 Change the past
10.1 Sleeping into the future as a time travel device.
10.2 Washington Irving, “Rip Van Winkle.”
11.2 Larry Niven’s corpsicles in A World Out of Time
12.2 Prof. H. Bruce Franklin (1966) “When one says time travel, what one really means is an extraordinary dislocation of someone’s consciousness in time.
12.2 Ambrose Bierce, “An Occurrence at Owl Creek Bridge” (perception of time passing)
13.3 In An Age (Aldis) we find such drug-induced time travel aptly named mind travel.

13> Machineless Time Travel Without Dreams or Drugs
14.2 [Bible] Matthew 27:52 “When the Graves Were Opened” (Burks)
14.-2 beyond the pale
14.-2 [Bible] What I call the Shirley MacLaine method of time travel – the “channeling” offered by the New Age Time Travel Agency in Gore Vidal’s irreverent (but awfully funny) spoof of the gospel stories, Live from Golgotha.
16.2 Also out for this book are such literary devices as a knock on the head by a crowbar a la Mark Twain’s A Connecticut Yankee in King Arthur’s Court.
17.2 paradoxes changing the past, meeting yourself, and causal loops.
18.2 The supernatural is precisely what this book is not about. This book is about physics.

18> Time Travel by Machine
18.2 In this book we’re interested in physical time travel by machines that manipulate matter and energy in a finite region of space.
18.-2 Einsteinian spacetime
18.-1 From the first it has been known hat Einstein’s special theory allows of time travel into the future. To return, however, to travel into the past, had been thought impossible. Yet since 1949 it has been known that the general theory, which so far has passed every experimental test it has been subjected to, does allow time travel to the past under certain conditions. It is this availability of a theory that separates time travel speculations from the fantasy speculations with which it is often unjustly lumped – speculations that are in the province of quacks, such as ESP, astrology, and mind over matter (spoon bending).
19.1 Einstein’s famous gravitational field differential tensor equations,.. “Curved spacetime tells matter how to move, and matter tells spacetime how to curve.”
19,2 In the large scale, in fact, curved spacetime can lead to violations of causality – that is, to time travel into the past.
19.3 The scientific, rational basis for discussing time travel. In 1949 the mathematician Kurt Godel found one such solution to the field equations
that describes the movement of mass-energy not only through space but backward in time along what are called closed timelike lines or CTLs), a special case of world lines in spacetime.
19.-2 Godel’s time travel requires a machine. Strong time machine (532.-1) creates its own CTLs. Weak time machine (532.-1) merely travels along the CTLs that are already there.
21.-1 “Come back when you know tensor calculus and I’ll explain to you about n-dimensional forces and the warping of world lines.” “The Little Monster” (Anderson)

22> H.G Wells – Why His Time Machine Won’t Work
23.2 All the theoretical models for time travel discussed in this book (Tipler cylinders, black holes, Godel rockets, cosmic strings, and superluminal spacetime warp drives) require spatial displacement.

25> Traveling Into the Future
26.1 Physicists call personal time (local) “proper time.”
27.-2 Time travel for touring purposes.

30> Traveling Into the Past
31.2 Time Travel fiction is, of course, the ultimate in escapist literature!
33.2 Beatles song “Yesterday” (“I need a place to hide, that’s why I believe in yesterday.”) [see Tim Powers The Anubis Gates]

35> Who Else Might Be Interested in Time Travel
35.-1 Larry Niven
35.-1 Time travel as criminal exile
37.2 Simak Mastodonia [Bible][Christians fear that the truth about Jesus will be discovered]
39.1 As we’ll see in Chapter Four, this idea about repeating closed time loops is not correct. [my “Chiliad” may have found a way around that. Virtual looping.]

40> Some Problems
40.2 The seductiveness of the past has never been written about better than by Jack Finney.
41.1 Was home, then, more in time than in space?
41.3 The dead live again
42.2 Even the “mere” communication across time can result in horror, and no writer has better captured the essence of that experience than have Ambrose Bierce and H.P. Lovecraft.

43> Backward in Time – Can it Really Be Done
44.1 Asimov strong time machine
45.2 Philosophy contrary to time travel
45.-2 Niven’s law: If the universe of discourse permits the possibility of time travel, and of changing the past, then no time machine will be invented in that universe.

47> The Problem of Paradoxes
48.1 Kip Thorne, The Mother paradox
49.3 The unchangability of past events.
49.3 Free Will versus fatalism. The Bible offers us no definitive help on this issue. [Nahin is certainly not a Calvinist]
Nahin quotes eleventh century Jewish philosopher Bahya ibn Paquda who lists several Bible passages supporting both predestination and free will:

[Predestination]
1 Except the LORD build the house, they labour in vain that build it: except the LORD keep the city, the watchman waketh but in vain.
2 It is vain for you to rise up early, to sit up late, to eat the bread of sorrows: for so he giveth his beloved sleep.
(Psalms 127:1,2, KJV).

[Free will]
11 For the work of a man shall he render unto him, and cause every man to find according to his ways.
(Job 34:11, KJV).

51.-2 Nahin’s belief: “Christ…had to die for our sins.” [Bible]
52.1 horrible (that Jesus never existed in Gerrold’s novel The Man Who Folded Himself )
53.-1 Change the past until you get it right.

54> The Fictional Origins of “Change the Past”
54.2 Hale wrote the first such story “Hands Off” in which the Biblical account of Joseph is changed, and thus preventing Calvary.
55.1 Wells’ Time Machine did not include a paradoxical element.

57> Ways to Avoid Paradoxes
57.-1 The various forms of changing-the-past paradoxes, and of what seems to be a lack of free will, are of course prominent among the reasons why philosophers and theologians have been so attracted to the question of the possibility of time travel.
58.1 Inadvertent sound recordings made in the past [like “All the Colors of the Masters.”
58.-2 seeing the past
61.2 change the past / free will quandry
62.1 temporal reluctance, or the Law of the Conservation of Reality. (Fritz Leiber)
62.-1 the immutability of the past.
62.-1 fatalism, “if you go back…you will do the same things again and a repetition of all that happened is inevitable.”
64.-1 free will falling to defeat in the face of the overpowering dominance of predestination.
65.4 free will versus fatalism. But that is a different issue from time travel.

66> Where Are All the Time Travelers? [This is the first time Nahin has used a question mark in one of his section headings, even though many have been in the form of questions.]
66.3 Arthur C. Clarke “The most convincing argument against time travel is the remarkable scarcity of time travelers.”
67.1 Fulmer: “There might be some pettifogging physical limitation on time travel: perhaps the energy expenditure varies as the fourth power of the time traversed, making only very short trips feasible, and its discovery lies too far in the future for its effects to have yet been felt.”
67.2 Heinlein The Door Into Summer (photograph the Crucifixion) [Bible]
68.-1 Stephen Hawking (1992) has invoked the “missing time travelers” argument as experimental evidence for his theoretical studies in support of his version of “Niven’s Law,” the so-called Chronology Protection Conjecture.
69.2 Kriele goes on to suggest that Einstein’s general theory of relativity will eventually prove to be the limiting case of an even more general theory of gravity, in which “ ‘free will’ is something like a second-order effect and therefore it is possible that the classical limit space-time of our world contains closed timelike curves [implying the possibility of time travel to the past] though we will enjoy the comfort of free will.”
69.-2 time travelers would drive interest rates to zero.
71.-1 [Bible] Crucifixion
71.-2 private investigator hired to find a time traveler…finds hundreds of them at the annual Oktoberfest in Munich. Time travelers go there, you see, because everybody gets so drunk there is no danger of giving yourself away.

72> Skepticism and Time Travelers
72.-2 The patron saint of skeptics, the Scot David Hume (1711-1776)
72.-1 Hume proclaimed that a miracle by definition violates scientific law and that, because such scientific laws are rooted in “firm and unalterable experience,” any violation of one or more of these scientific laws immediately provides a refutation for the report of a miracle.
72.-2 Sorenson (1987): :The key question will not be ‘Is time travel possible?’ We shall instead ask whether it is possible to justify a belief in a report of time travel.” This gets to the real heart of Clarke’s puzzle.
73.-1 As Robert Sheckley puts it in “Something for Nothing,” “When the miraculous occurs, only dull, workaday mentalities are unable to accept it.”
74.3 C. S. Lewis rejects Humean [as in David Hume] skepticism.
75.1 [Bible} For Hum, second-hand tales of the return of a man from the dead – the claim that literally kept Christianity alive after Christ’s execution – were suspect.
76.2 Arthur Clarke’s third law: “Any sufficiently advanced technology is indistinguishable from magic.”
76.3 Heinlein’s Lazarus Long, “One man’s magic is another man’s engineering.”
76.-2 miracle
77.2 Heath: “even omnipotence [God] is powerless against the extremer forms if skeptical intransigence.” [This powerlessness only holds if God is thought not to want to violate the skeptic’s so-called free will]
77.-1 Modern science fiction writers have often used skepticism as a means of building conflict and tension in their time travel stories.
78.1 photograph of Christ [Bible]

79> Einstein, Godel, and the Past
81.3 Therefore it cannot be excluded a priori, on the ground of the argument given.
82 [Bible] Garden of Eden
84.2 Mach’s Principle, the claim the inertia of any object is determined by the distribution of all the rest of the mass in the universe.
84.-1 De (1969) The conclusion in the last paper was that a general relativistic situation violating causality could be formulated. That is, time travel to the past seems to be no mere anomaly of Godel’s particular solution, but rather is built right into the basic gravitational field equations of general relativity.

85> Quantum Mechanics, Black Holes, Singularities, and Time Travel
85.1 That is, it [Einstein’s Theory of General Relativity] is incompatible with quantum mechanics, which is the theory of the very, very small – the physics of objects on the order of just a single big molecule…Quantum theory, however, seems to work everywhere.
85.2 One of the central concepts in relativity is the world line, which is the complete story of a particle in spacetime…Mixing the two theories – the classically smooth general relativity and the discrete quantum mechanics – to get something called quantum gravity is the Holy Grail of physicists today. (240.-2)
85.-1 quantum time the smallest increment of time that has any meaning – sometimes called the chronon, a term first used in a 1935 non-time-travel story, “The Ideal,” by Stanley G. Weinbaum – may have a non-zero value.
[That is, it may take a certain minimum amount of time for anything to happen]
86.1 As we’ll see later, much of the present controversy over the possibility of time machines hinges on what is called the quantum gravity cut-off. This is a cut-off of destructive spacetime stresses that tend to grow toward infinity whenever a time machine attempts to form. This process goes under the general name back-reaction. This is a cut-off of destructive spacetime stresses that tend to grow toward infinity whenever a time machine attempts to form.
86.2 The so-called cut-off of those stresses, at some finite value, is supposed to occur when the terminal phase of the growth would take place in less than the minimum possible time interval. The cut-off happens because nothing can actually occur in less than the minimum time. The debate over just what the minimum duration is and over whether the cut-off would occur before the stresses could reach finite values high enough to destroy the putative time machine anyway. That is still all very speculative, of course, and the search for a way to connect general relativity and quantum mechanics continues. This is all related to time travel via a fantastic sequence of discoveries made during the last sixty years in relativistic physics.
87.1 Best of all is the anonymous observation that a singularity is “where God is dividing by zero.” [Bible] Historically, however, the occurrence of infinities in physical theories has been the signal that the theories have simply been extended too far.
881. Both kinds of singularities (crushing and incomplete) do signal, of course, that something unusual is afoot, perhaps even the failure of general relativity.
88.-2 Schwarzchild black hole (non-rotating).
88.-1 Kerr-Newman black holes are rotating charged. The Kerr-Newman solution is intriging (sic, intriguing) [typo] because it implies that the interiors of such rotating holes are portals into other spacetime regions that are otherwise inaccessible from our universe. In these holes the central singularity is a ring – a geometrical shape with a hole in it through which matter may be able to pass without destruction. (see Kaufman 1977)
89.-2 black holes still up to debate
90.2 So disturbed was Carter about the possibility of time travel that he felt “the breakdown in general relativity may be [so severe that] the whole theory may have to be abandoned.”
91.2 time machines that require rotation
91.-1 Naked Singularity “this means an observer could receive information from his own future.” (Clarke and de Felice 1982)

92> Tipler’s Time Machine
93.1 an infinitely long very dense cylinder rotating with a surface speed of at least half the speed of light…It is true. On paper.
93.2 Gribbin (1983, Spacewarps) we find the estimate that a 10-to-1 ratio of cylinder length to radius may be enough for Tipler’s cylinder to be “infinite.”
94.1 Showing no lack of imagination, Tipler has himself suggested (1977, “Singularities and Causality Violation.” Annals of Physics 108 (September 22: 1-36) the possibility of speeding up the rotation of an existing star as an alternative approach to that of actually trying to build a cylinder.

370.1 Godel’s time travel idea [that of the rotating universe] still waits for its first fictional use.
370.-1 FTL interactions is a problem with quantum mechanics. Such interactions hold out the tantalizing possibility of being able to transfer information into the past.
373.-1 Hawking, vacuum fluctuation Particle/Anti-particle pairs formed out of nothing. The universe via time travel may have caused itself. Hawking Radiation.
374.1 Hawking showed that the particle entering the black hole could alternatively be thought of as an emitted particle traveling backward in time.
375.1 Nimmersatt (German for “glutton”) giant galaxy black holes.
376.1 to speculate of the cosmological implications of God as a time traveler. [Bible]
376.2 causal loops (Star Trek IV)
376.2 Thorne indicates [in his rebuttal to Hawking that the absence of time travelers proves the impossibility of time travel] that (1) time machines, if possible, must have the property of not being allowed to travel back before their creation (as in the case of a Tipler cylinder), and (2) no time machine has yet been created.
377.1 cosmic censorship [used by Varley in Millenuim?]


97> 2 On the Nature of Time, Spacetime, and the Fourth Dimension

98> What Is Time?
98.1 Christian clerics had identified time as something unusual long before science fiction writers and their time travel stories. [Bible]
100.-2 Einstein: according to his general theory of relativity, time and space would cease to exist if the universe were empty. Spinoza: “there was no Time or Duration before Creation.”

102> Speculations on the Reality of Time
103.-2 watch the crucifixion and then the resurrection of Jesus. [Bible]
104.-1 [Bible] Ecclesiastes 1:9 “there is no new thing under the sun” seems to be circular time.
105.3 Stephen Hawking’s famous book A Brief History of Time. In it he concludes that there is no need for God because in circular time there is no first event and hence no need for a First Cause.
106.-1 The method of the paper is to deny the reality of time via an infinite-regress argument that one philosopher, Mink (1960), has called the pons asinorum of the riddle of time. That is, the argument is so difficult to follow that it is the “bridge of asses” over which most people balk at crossing.
108.2 anachrony (the telling of a story out of normal time sequence, such as occurs in time travel movies. [and Memento]
108.-2 block universe “detensers”

109> Has the Past Been for Ever?
109.3 Early Biblical scholars, of course, believed that the answer was finite (the world came into being because of a First Cause, God’s creation of everything) [I like this, because it places God outside of time, and includes time as a created thing]
109.-2 Ironically, then, although Christian theology may be given credit for introducing linear time, it certainly did not provide very much of it. [only about 4000 years worth]
109.-2 [T-shirt slogan] “What was God doing before he created the world?” “Creating Hell for those who ask that question.” (see Leftow 1991)
109.-1 geologic time or deep time, a subtle play on the metaphor of the “ocean of time.”
110.-2 birth of Christ [Bible]
111.1 St. Thomas Aquinas arguing for the opposite view of an infinite past.
111.2 St Bonaventure argued for a finite past.
111.-1 English political philosopher Thomas Hobbes’ Leviathan (1651), with its arguments against the power of the church and for civil power
111.-1 [typo] Laviathan [should be e]
112.1 Hobbes had long been fascinated by, and was considered an expert on, the ancient problem of “squaring the circle” – a task that has been known to be impossible since 1882
112.-1 [philosopher] Kant’s rejection of an infinite past.
113.2 now famous Big Bang [113.2]
113.3 There is, however, a philosopher for every point of view.
114.4 chronon [the smallest unit of quantum time]
115.1 Creationists also avoid the question of God’s cause, merely saying he needs no cause.
115.2 [grammar] The right question, for Grimbaum, is “Did the universe have a beginning?” and Maddox’s “What caused the beginning?” is a question not for physics but rather for theological metaphysics.
115.2 [Nahin’s belief] Maddox himself had introduced religion into the debate with the claim that creationists love the Big Bang [we do?] because it seems to endorse science by “imagination”; Maddox thereby stained the Big Bang with his unfair (I think) juxtapositioning of it with the pseudo-science of creationism.

115> Time and Clocks
117.2 Progress was steady if sporadic, and by Newton’s time
seagoing chronometers [see Dava’s Longititude] were precise enough to make worldwide navigation possible.
117.3 For our purposes in this book, however, the clock of most interest is one that has never actually been built, the photon clock. To visualize this clock, imagine two parallel mirrors with a “pendulum of light,” – a photon – reflecting back and forth with each pair of bounces being a single tick-tock of the clock…This is important because, as is shown in Tech Note 2, it allows us to derive the central result of special relativity: the conclusion that time is not Newton’s absolute time but in fact is Einstein’s relative time, which depends on the spatial state of the observer relative to the clock. This mixing together of space and time was one of the greatest intellectual insights in all of human history…

117> Hyperspace and Wormholes
118.-1 our universe appears to be a four-dimensional (three spatial and one temporal) world called spacetime.
119.2 In imagery generated by thinking of the sphere as an apple, and of the hyperspace path as a tunnel through the apple, it has become popular to call such shortcuts through any hyperspace of any dimension wormholes…
119.-2 All of these various solutions to the field equations are often called “Einstein-Rosen bridges” in the physics literature…
119.-1 The term wormhole was coined in the 1950s by John Wheeler…(Indeed Wheeler has claimed that the observation of what we call electricity is experimental proof that space is not connected simply.)
123.-1 mirrors have long been used as spatial portals. [see Gene Wolfe’s The Book of the New Sun, Father Inire’s mirrors]

125> Monsters in Hyperspace

130> Space as the Fourth Dimension
135.2 identification of the fourth dimension with the spirit world
136.3 [typo] Raymond Chandle [Chandler]
137.2 Early science fiction was literally overrun with nutty professors whose experiments with the fourth dimension go amiss.

140> Time as the Fourth Dimension

143> H.G. Wells on Space and Time
143.-1 It was H.G. Wells who pioneered time travel as we think of it in this book…
144.2 a recondite matter

148> Spacetime and the Fourth Dimension
148.-1 The modern view of reality, that the past and present and future are joined together into a four-dimensional entity called spacetime can be attributed to the work of Hermann Minkowski (1864-1909), Einstein’s mathematics professor when he was a student in Zurich…”Henceforth space by itself , and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve independence.”
149.2 world-line
150.2 , Block Universe [150.2-167.-2, 387.2]
151.-1 James argued for free will and indeterminism, concepts disallowed in a block universe.
151.-1 [distinction between determinism and fatalism]
154.2 [the block universe] looks like fatalism disguised as physics. It seems to be a mathematician’s proof of a denial of free will dressed up in geometry.
154.2 Geach (1968); On the one hand, he does not like the block universe because of its implication that the future already exists, but on the other hand, if the future is not there, then what sense can be made of being able to change it? How do you change something that does not exist?
156.-1 This idea [that the mind can exist outside of spacetime] has no foundation in physics, however, and it is Minkowski’s block universe idea that will play a central role in this book.

161> Spacetime, Omniscience and Free Will
162.-2 [Bible] 1 Kings 21. [God seems to change his mind about punishing Ahab]. God is aware of everything in this tale, but only as it happens; that is, God’s knowledge is subject to growth. This Hebrew concept of God as a participant in history is at odds with the contemporary Christian concept of divine knowledge of all that has been, all that is, and all that will be.
163.1 [Bible] Bible references in support of God’s eternality: Malachi 3:61, John 8:58, James 1:17
163.2 H.G. Wells’ The Time Machine [missing passage. See if this lost passage is in the Dover special edition.]
164.1 What should we think of a God who follows rules different from those that govern all he is supposed to have made…does he have a frame of reference called “divine immediacy”? Theologians have argued these questions for decades – see Wilcox (1961), Ford (1968), Fitzgerald (1972), Hasker (1985, 1987), and Reichenbach (1987)
164.2 Hasker (who believes free will and divine foreknowledge are not compatible)
166.2 God’s foreknowledge of our destiny can impose the destiny upon us.
166.3 [Bible] Christian theologians are left with the puzzling task of explaining what could possibly be meant by the Biblical exhortation Deut 30:19 I set before you life and death…therefore choose life.. [Nahin hits upon the key without knowing it: God is exhorting the people by His words to do what He knows they will later do.]
167.1 Shakespeare’s Macbeth. (Act 5, Scene 5)(we are merely players following a script)
167.3 Boethius’ influential De consolatione Philosophiae (A.D. 500) …he tried to argue that God’s vision of all temporal reality does not limit the freedom to act.
167.-2 Kurt Vonnegut called this ability (of God to see all things simultaneously) as “chrono-synclastic infundibulated” vision in The Sirens of Titan. In his novel Slaughterhouse Five…”When a Tralfamadorian sees a corpse, all he thinks is that the dead person is in a bad condition at that particular moment, but that same person is just fine in plenty of other moments.” [block universe mentality]
168.2 Chaucer supports predestination, Troilus and Criseyde (Book IV.140)

Some say “If God sees everything before
It happens – and deceived He cannot be –
Then everything must happen, though you swore
The contrary, for He has seen it, He”
And so I say, if from eternity
God has foreknowledge of our thoughts and deed,
We’ve no free choice, whatever books we read.

168.-1 Kubilius “Turn Backward, O Time” (Free will is an illusion; it is synonymous with incomplete perception.)
169.2 Aristotle’s famous ancient dilemma: “Will there be a sea fight tomorrow”?
169.-2 Newcomb’s problem.

170> Does the Future Already Exist? Is the Past Still Around?
173.2 Hypothetical arguments ridiculed. “Twice upon a time in another space no distance in any direction from here…”
174.-3 fatalistic block universe spacetime
174.-2 Nahin’s belief: “One that point, in fact, my personal sympathies lie with Reitdijk. Being unknown is not the same to being undetermined.”
174.-2 obsolete concepts: ether, caloric, philogiston
175.2 Fitzgerald (1969) presented a summary of the “traditionally” competing philosophical views about the ontological status of the future” and then followed that with analyses [nice plural] of how each such view transforms after reformation in relativistic terms.
173.-1 light cone 175.-1
178.-2 Saint Augustine’s Confessions “I am in a sorry state, for I do not even know what I do not know.
380.-1 TCP Theorem. Only in a spacetime with an even number of dimensions does the TCP Theorem hold…There is strong reason to believe the TCP Theorem, because quantum field theory is compatible with special relativity only if the TCP Theorem holds…The TCP Theorem predicts the anti-neutron.
381.1 Chandraskar limit for stars.
382.1 Fifth space dimension
383.-1 Mobius Strip
384.-2 Wells’ use of time as the fourth dimension.
384.-1 Klein bottle
385.2 Wells’ The Chronic Argonauts
385.-1 Time Machine sequels
387.2 A deterministic universe has plenty of room for free will. A fatalistic universe, like the block universe, simply says, “You will do A and B will happen.”
388.4 More recently, it has been shown that time travel does not imply any fatal violation of conservation of energy.


179> 3 The Arrows of Time

180> The Language of Time Travel
180.1 The psychological arrow of time.
180.2 causation

181> Does Time Have a Direction?
181.-1 The central debate here is between what is called objective time (the idea that time really does flow) and mind-dependent time (the belief that time’s flow is simply an illusion, an artifact of our incomplete perception of reality).
182.-2 Donald Williams (1951a) presents a truly staggering collection of ‘time as metaphor’

185> Cause and Effect
185.-2 Cause and effect can be pretty slippery concepts.
186.2 The central puzzle of time travel to the past is its apparent denial of causality – that is, its denial of the belief that we live in a world where every effect has a cause and that the cause always happens first.
186.2 Electronic circuit designers insist that their designs be causal. (i.e. No output before input)
187.-1 literary prescience
187.-1 kaons (K mesons) violate what is called CP-symmetry [one way time direction] [208.3]
185.-1 Electrical engineers…reciprocity theorem [two antennas will pick up the same signal regardless of their design as long as the same signal is sent through both]
189.-2 Who made God?

191> Backward Causation
192.-1 one clearly does not have to discuss time travel to get into serious trouble about cause and effect.
193.2 omniscient God
193.-2 We cannot alter the past. But then we cannot alter the future either, although we can affect it.
193.-1 claimed retroactive effects in quantum mechanics.
194.3 [Dirac 1938] …if a charged particle is subjected to an external disturbance (Dirac considered a passing pulse of electromagnetic radiation), then the charge will start to move before the pulse reaches the electron'’ position. Now that does seem to be a pretty clear example of backward causation.
196.1 Dirac radios in science fiction.
196.2 causal loops and for the potential for breaking such loops, a prime ingredient in many of the very best time travel stories…Modern philosophers call this possibility of breaking a causal loop a bilking paradox. [212.-2][see ch 4]

198> What Does “Now” Mean?
199.3 The doctrine of process theism, the concept that God is a temporal entity that participates in becoming…
200.-2 the idea of a specious present with non-zero duration.
200.-2 the objective theory of time flow.
200.-1 There is no moving now in the block universe except for its subjective presence in our conscious minds.
201.2 The Hopi language had no tenses and therefore no concept of time-as-an-entity…more recent scholarship has conclusively shown that Whorf’s assertion that the Hopi language is tenseless is simply incorrect.
202.3 My only quibble with that is, as I’ve argued before, that the block universe should be associated not with determinism but with fatalism.

205> Irreversibility
205.-1 time as destroyer, runs all through the works of Shakespeare.
206.-2 the phenomena of memory

208> Worlds in Reverse
209.1 [Bible] 2 Kings and Isaiah 38 King Hezekiah – God moved the shadow on a sundial ten degrees backward.

220> The Philosophy and Physics of Reversed Time
225.-1 If the whole of the world is but one of God’s films…
227.1 Earman (1967a): “General relativity seems to offer more hope than special relativity to the proponent of time travel”

227> Entropy as Time’s Arrow
227.-1 The second law of thermodynamics states that a measure of the internal randomness or disorder – what is called the entropy – of any closed system continually evolves toward that of maximum disorder, toward the condition called thermodynamic equilibrium.
232.1 googol (a 1 followed by 100 zeros), googolplex (a 1 followed by a googol of zeros)
232.-2 wonderful idea for science fiction writers (our “normal” time universe and two other universes, one containing tachyons and the other being a time-reversed, anti-matter universe)
235.2 stasis field (zero entropy bubble)

236> Other Arrows of Time
[entropy does not always hold, so the search for the perfect indicator of time’s direction continues]
239.1 cosmological arrow (expanding universe)
240.2 electromagnetic arrow (electromagnetism always propagates into the future)

240> Multidimensional Time
[time not only as the fourth dimension]

245> 4 Time Travel Paradoxes and (Some of) Their Explanations
245.1 quote from Varley’s Millennium

246> Paradoxes
246.-3 Varley’s cosmic disgust theory
248.2 Paradox of God’s omnipotence [free will and the existence of evil]

251> Early Science Fiction Speculations on Time Travel Paradoxes

256> Two Basic Time Travel Paradoxes

259> Can the Present Change the Past? Can the Past be Undone?
260.-3 …the consensus is that the idea of backward time travel is perfectly consistent with the four-dimensional block universe discussed in chapter two.
263.-2 [God] Aristotle quoting Agathorn, “For even God lacks this one thing alone, To make a deed that has been done undone.”
263.-1 Agathron and Aristotle not withstanding, some medieval theologians argued passionately that the past could be changed (but only by God). The eleventh-century Italian cleric Peter Damian is a famous exponent [shouldn’t that be proponent?] of that radical view. See McArthur and Slattery (1974), Remnant (1978), and Gaskin (1977). Writing in his De Omnipotentia Dei (On the Divine Omnipotence in Remaking What Has Been Destroyed and in Undoing What Has Been Done”), Damien made it clear that he believed nothing could withstand the power of God, not even the past.
264.3 time travel is a question for mathematical physics, not theology of grammar.

269> Changing the Past vs. Affecting It
272.-2 The Principle of Self-consistency “The only solutions to the laws of physics that can occur locally in the real Universe are those which are globally self-consistent.”
275.1 [Bible] The Lord’s Prayer Mt 6 and Luke 11 (prayers about the present or future, not the past)
277.2 Dwyer (1975, 1977), 278.1 For the thesis of this book, the importance of Dwyer’s 1977 paper is that it in it he explicitly gives us a rational mechanism: “One can provide an explanation for time travel in terms of the field equations of General Relativity, together with initial conditions on the distribution of mass-energy in a certain region of space-time.”
280.-1 Cumulative audience paradox fills the Holy Land with visitors at the Crucifixion.
185.2 Physicists who worry about cosmic disaster if a time machine were actually to be built, and those who reject physical theories simply because they allow time travel to the past, are worrying about a non-problem. Even if time travel to the past is possible, you cannot go back and kill your grandfather before your father is conceived – or even “better,” you cannot kill yourself in the past. But it is logically possible that you could be the one who introduces your grandfather to your grandmother.

285> Why Can’t a Time Traveler Kill His Grandfather?
288.-2 Either the gun is not behaving as the normal physics object we take it to be, or the notion of voluntary action does not apply in the usual way. [free will]
289.1 [typo] know [knows]
290.-1 Robert Forward: “Once time machines exist, no event is low probability if it is needed to make the past consistent.”
293.1 But such questions (about the unseen hindrance to committing a grandfather paradox) are like asking: “How do the laws of logic prevent the geometer (draftsman) from trisecting the angle or squaring the circle (both proven to be impossible using purely graphical techniques)? …Arntzenius (1990) “Surely it is not an impairment of ‘freedom of action’.”

294> Quantum Mechanics and Time Travel
295.1 The theory of alternate realities with parallel time tracks.
295.2 This apparent fantastic view seems actually to have some scientific plausibility because of the so-called many-worlds interpretation of quantum mechanics, pioneered by Hugh Everett III in his 1957 Princeton doctoral dissertation. For this book, however, the underlying scientific theory of time travel is classical (that is, non-quantum) general relativity, and that theory has nothing to say about alternate time tracks. …As Bell (1987) points out in that paper, in the many-worlds interpretation “there is no association of the particular present with any particular past.”
295.-1 One analyst who believes this [that quantum mechanics itself may make an absolutely crucial contribution to time travel] is David Deutch at the Oxford University Mathematical Institute. Deutch holds (1991) that general relativity is not the proper theory with which to study the physical effects of closed timelike lines.
303.2 Lem’s ergotic theory of history (nearly all possible histories have the same general characteristics, differing only in the details), 300.2,

304> Causal Loops
304.2 [Bible] Photograph of the Crucifixion.
307.3 In Fulmer (1980) a philosopher tells a causal-loop tale that puts the grandfather paradox to shame: “If James cannot decide whether to marry Alice or Jane, he simply travels to the future and learns that he is to choose Alice; he then chooses her for this reason.
310.3 As discussed earlier, the so-called principle of self-consistency has been around in physics for decades, certainly before the modern, serious physics treatments of time travel began in the late 1980s.
311.2 [typo] lops [loops]
311.2 God, uncaused.
311.-2 the question of ultimate origin.
311.-2 sub specie aeternitatis this difference disappears.
312.2 Deutch, however uses the evolutionary principle [knowledge comes into existence only by evolutionary, rational processes] to (for example) reject creationism, the anti-evolutionary theory that “explains away” fossils millions of years old with the assertion that they were created, fully formed by God just a few thousand years ago. [Nahin grossly misunderstands that creationism does not teach that the fossils were created in situ thousands of years ago, but rather that “millions of dead things buried in rock layers, laid down by water all over the world” (to borrow a phrase from Ken Ham) were formed by the Flood of Noah just thousands of years ago. We creationists do not maintain that the fossils are apparently millions of years old, but rather that conventional science has wrongly attributed great age to them despite evidence to the contrary. There is no need to “explain away” fossils that are clearly millions of years old, since all the fossils are merely thousands of years old.]
315.3 smacks of deus ex machina
317.1 To say that causal loops are counter-intuitive is barely to hint at their mystery, but that is not an argument against time travel.

319> Sexual Paradoxes
319.-2 “Harrison’s story [of Jocasta, Dum and Dee] is “ a story so extravagant in its implications that it will be regarded as an effective reductio ad absurdum of the one dubious assumption on which the story rests: the possibility of time travel,.”
320.2 Godfrey-Smith: “The biological problem is the following. Dee is the son of Dum and Jocasta. So Dee obtained half his genes from Dum and half from Jocasta. But Dum is diachronically identical with Dee and is therefore genotypically identical with him (i.e., himself). That is, Dee is both genotypically identical with and distinct from Dum, which is absurd.”

321> Maxwell’s Equations and Advanced Effects
324.-2 Thus the advanced solution to Maxwell’s equations holds out the possibility of sending messages to the past…
324.-2 enfant terrible
326.1 a priori

327> Communication with the Past
331.2 Dirac on pre-acceleration [Dirac radio]
331.-1 Dirac: “it is possible for a signal to be transmitted faster than light through the interior of an electron. The finite size of the electron now reappears in a new sense, the interior of the electron being a region of failure, not of the field equations of electromagnetic theory, but of some of the elementary properties of space-time.” [space-time = general relativity]

332> Wheeler and Feynman and Their Bilking Paradox
332.1 Wheeler and Feynman (1945) [beautiful paper]
332.-2 Wheeler and Feynman proposed that an accelerated charge will not radiate unless there is to be absorption at some other distant place and future time. The future behavior of a distant absorber determines the past event of radiation; there is simply no such thing as just radiation into empty space. The entire universe, spatially and temporally, is a very “connected” place!
335.1 paradox machine – “logically pernicious self-inhibitor.”
335.-1 advanced fields [traveling backward in time]

336> Absorber Theory and Signaling to the Past
338.2 bilking paradox
339.3 microwave background radiation (opposed by Hoyle 1975)
340.1 One surprising result from the Hubble Space Telescope was that the oldest stars appear to be about twice as old as the universe itself…measurement errors are now thought to be the real answer.
340.-2 precognition
342.2 Dirac radio puzzle: If signals arrive at a receiver simultaneously from all future times, how can they be separated?

342> Tachyonic Signals, Spooky Actions, and the Bell Antitelephone
343.3 Special relativity is not violated by FTL motion.
344.2 Reinterpretation Principle
346.1 [typo] decides not to sent [send] the signal
346.1 [M.C.] Escher’s Ascending and Descending and Waterfall.
346.1 bilking paradox
348.-1 Bell’s theorem
350.-1 Bell’s great contribution, then, was to remove the debate about quantum mechanics from metaphysics and to replace it squarely in the realm of experimental physics.
351.3 Because of the work by Bell and Aspect, however, it is now known that quantum mechanics as it stands leads to correct predictions.
397.4 Lewis makes it clear that he believes it is a sin to pray for something known not to have occurred…[a retroactive petitionary]
404.1 Robert Forward thinks it will be possible to build gamma ray transmitters into the past in the twenty-first century. It would seem, then, that what should be done now is to build gamma ray receivers…the technical details will not matter as long as they are put into the public record. That way the future will know about those details merely be reading about them from a musty library book…
405.1 Superluminal speeds – Supernova 1987A in the Large Magelanic Cloud displays apparent expansion speeds of up to twenty times the speed of light.
410.1 so that “well defined” is given a stochastic interpretation. 512.1 Thus the Cauchy problem is stochastically well-defined.


355> EPILOGUE
356.-2 [Nahin laments the obscurity of Godel’s rotating time travel universe)
357.-1 Orson Scott Card Maps in a Mirror: The Short Fiction of OSC. Card refers to time travel as simply a “magic trick” for writers of fiction.
358.-1 Time travel, with its double burden of very difficult mathematics and non-intuitive philosophical implications, is not easy to come to grips with, even for those who are at least open to the possibility of time travel…[Thorne] in a recent visit with Stephen Hawking in Cambridge. They had met to review the current status of their work on closed timelike curves, and their joint conclusion concerning time travel was a sobering one: “We now think that we understand these issues less well than either he or I believed a few weeks ago.”
359.2 Three reasons why Thorne and Hawking felt less sure about the physical possibility:
(1) Space-time may be simply connected and so wormholes may not exist.
(2) Even if wormholes can exist, briefly, quantum mechanics may enforce the average energy condition and thereby forbid wormhole throats to remain open
(3) The Cauchy horizon may be unstable and may collapse into a singularity.
359.-1 On the other hand, Kim and Thorne (1991) disagree with some of the details of how Hawking does his quantum gravity!
360.2 [typo] “The global solutions of the dynamical [Eistein] equations. [should be Einstein]
360.2 Novikov also is not quite ready to put his faith in quantum theory’s ability, as we presently know it, to forbid time travel. As he and a colleague have written – see Lossev and Novikov (1992) – “our understanding of the fundamental structure of the vacuum and the effects of quantum fluctuations is so inadequate that from existing quantum theory only we should definitely exclude the possibility of the very existence of the Universe…but experimentally it exists.”
361.-2 Tipler wrote that he tends to side with Nivokov but also that the present quantum arguments are all “order of magnitude handwaving.”
362.-1 To get right down to it, I don’t know whether it is possible to visit the past, and I fear the odds are that I’ll learn the truth about Heaven (or Hell) in the next world before I learn the truth about time travel in this one.
365.2 The principle of self-consistency around closed timelike curves is going to have to become as much a part of the science fiction writer’s craft (or else she will be a writer of fantasy) as it will have to become part of the fundamental axioms.

415> TECH NOTE 1
What Time is Now?
415.-2 The present for any given observer is that instant in time that seperates her future from her past. To be a little poetic perhaps, it is always the present for us…
417.2 Newton’s absolute time
417.-2 Aristotle “We are not aware of time when we do not distinguish any change.” [So when time is “flying by” a lot of things must be happening. Conversely, when “time drags on” few things must be happening.]
417.-1 God’s timepiece according to Newton.
418.-2 – 419.-2 Einstein’s boxcar analogy for perception of simultaneity.
420.2 Kurt Godel: “The very starting point of relativity theory consists in the discovery of a new and very astonishing property of time, namely the relativity of simultaneity.
421.2 Einstein: “Common sense is that layer of prejudices laid down in the mind prior to the are of eighteen.”

423> TECH NOTE 2
Time Dialation via the Photon Clock
424.-1 The famous Einstein time dilation formula:
t-t’/√(1-(v/c)2

429> TECH NOTE 3
The Lorentz Transformation
431.1 Any frame of reference where Newton’s laws of mechanics hold true is said to be inertial.
435.-3 Einstein derived the Lorentz transformation by first assuming that the speed of light (c) is a constant.
436.-2 Experimental verification of the invariance of the speed of light was finally achieved in 1932. (R.J. Kennedy and E.M. Thorndike, “Experimental Establishment of the Relativity of Time,” Physical Review 42 (November 1, 1932):400-418.

439> TECH NOTE 4
Spacetime Diagrams, Light Cones, Metrics, and Invariant Intervals
442.1 light cones
442.1 timelike – projection on the time axis is greater than their projection on the space axis. (the interior of light cones)
442.2 spacelike – projection on the space axis is greater than their projection on the time axis.

458> TECH NOTE 5
Proper Time, Curved World Lines, and the Twin Paradox

467> TECH NOTE 6
A High-Speed Rocket Is a One-Way Time Machine to the Future
471.3 [typo] both spellings are used: “von Hoerner” and “Von Hoerner,” which is it?
471 table on the experienced acceleration at one gee
[Does this time dilation mean that we need fuel for only 40 years at one gee to reach 29,610 light years distant? Or do we need 59,223 years worth of fuel? It seems that having a lead-foot delivers better fuel mileage at relativistic speeds.]
474.3 Fuel savings at time dilation speeds

475> TECH NOTE 7
Superluminal Speeds, Backward Tie Travel, and Warp Drives, or Faster-Than-Light into the Past
483.3 bilking paradox and tachyons transmission to the past.
485.2 Star Trek Warp Drive
486.-2 A simple presentation on how to turn the Alcubierre warp drive into a time machine can be found in Parsons (1996)
487.2 Krasnikov tube warp…would make a wonderful science fiction gadget.
487.-1 electronic circuit simulates tachyon bilking paradox.

489> TECH NOTE 8
Backward Time Travel According to Godel and Tipler
490.2 tilting light cones – “dragging of inertial frames” effect
490.2 Recent Astronomical evidence…these results are tentative, however, and confirmation will have to wait until the launch, in 2000, of NASA’s Gravity Probe B satellite.
491.2 This kind of round trip, whose trajectory winds back into the past without ever becoming spacelike [FTL], is an example of a closed timelike curve [time travel]…Because these timelike lines are present from the very beginning of the spacetime, Godel’s universe is called a weak time machine.
493.-2 Tipler’s rotating, infinite cylinder is a mechanism for artificially producing the tipped-over light cones effect, thus creating closed timelike curves. For this reason, Tipler’s cylinder is called a strong time machine.

497> TECH NOTE 9
Wormhole Time Machines
498.1 The spacetime wormhole is presently the most promising of the approaches that have been advanced for building a time machine.
498.-1 [typo] the distances through the wormhole itself could [be] very small.
499.-1 Kutner coined the term mugwump for a time traveler using a wormhole.
502.2 antimatter has been observed [!]
503.-1 Experimental detection of the Casimir effect. (negative energy). Robert Forward theorizes that using the Casimir effect one can extract energy from vacuum. [See Robert Forward: “Extracting Energy From the Vaceem by Cohesion of Charged Foliated Conductors,” Physical ReviewB 30 (August 15, 1984): 1700-1702. ]
505.2 Thorne has conceived of a Casimir effect to keep the nouths of a wormhole open.
513.2 arbitrarily advanced civilization
513.3 And Gonzalez-Diaz (1996) suggests that a spacetime with the topology of a torus (think Donuts) [or Baxter’s Ring] could, under certain conditions, form in a natural way from the collapse of rotating matter. Such “ringhole” spacetimes appear to be convertible to time machines with no violations of the AWEC.
513.-1 Big Bang version of the origin of the universe may not be entirely correct [!] for the very earliest moments…a process called inflation..speculated that such an inflation could have enlarged primordial submicroscopic wormholes to macroscopic size..it would allow for such entities to have existed somewhere in the universe since the beginning of time. And so, Tipler’s words…notwithstanding, perhaps the dinosaurs are accessible.
515.-1 dark matter halo around galaxies (search for wormholes underway)
516.-1 automated spacecraft construction plant utilizes even unknown wormholes to time travel.
519.-2 [typo] That is entering A’ and exiting A’ are events [should be: entering A and exiting A’]
522.2 Visser (1997a): “…semi-classical quantum gravity is fundamentally incapable of answering the question of whether or not the universe is chronology protected. To really answer this question we will first need an acceptable theory of quantum gravity.
522.-2 Li-Xin Li…his anti¬-chronology protection conjecture: “there is no law of physics preventing the appearance of closed timelike curves.”
523.1 Li and Got (1998), however, have found a self-consistent, closed timelike solution to the field equations that is not forbidden by quantum effects.

527> TECH NOTE 10
“Solving” the Einstein Gravitational Field Equations, Unphysical Mass-Energy, and the Cosmic String Time Machine
529.-1 those who pale at the very thought of time travel
527.-2 [typo, spelling] Baxter Ring, 535.1 (manuevering [sic] a rocket at 1/2c) [should be maneuvering]

537> TECH NOTE 11
Time and Gravity
Gravitational time dilation is, in fact, the mechanism used by Frolov and Novikov (1990) to achieve a time shift between the two mouths of a wormhole time machine, as opposed to using time dilation bymoving one of the mouths at high speed…

547.-5 Larry Niven “Niven’s Law” of chronology protection.