the physics of time travel
DESCRIPTION
Review of time, with respect to the bending, stretching, folding, & tormenting thereof. Given at Philcon & Balticon (in various versions) in 2003.TRANSCRIPT
![Page 1: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/1.jpg)
• “Really this is what is meant by the Fourth Dimension, though some people who talk about the Fourth Dimension do not know they mean it. It is only another way of looking at Time. There is no difference between time and any of the three dimensions of space except that our consciousness moves along it.” - The Time Traveler
The Physics of Time Travel�
John Ashmead�[email protected]
![Page 2: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/2.jpg)
• What is time? • Is time travel
possible?
![Page 3: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/3.jpg)
Time is Weird (1)
• Pole moving • House at rest • Pole is shortened, so
fits in the house • (per Sartori)
![Page 4: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/4.jpg)
Time is Weird (2)
• Pole at rest • House moving • House is shortened • But “fortunately” both
doors are open at the same time!
![Page 5: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/5.jpg)
Time is Weird (3)
• Pole & house together on one space-time diagram.
![Page 6: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/6.jpg)
Tachyon loops (plain) • In the diagram (drawn to
scale) Alice sends a message at a which gets to Bob at b. Bob processes the message then at c sends Alice a reply which she receives at d, before she sent the first message.
t-axis
x-axis
Primed observer going at .75 c relative to unprimed. Both are ableto emit 3c tachyons (in own rest frame).
Tachyon loop
Alice
Bob (!=
.75)
wab=3
w’ cd=-3,w cd=
1.8
a
b
c
d
![Page 7: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/7.jpg)
Tachyon loops (musical)
• Bob might choose to send Alice the first few notes of Beethoven’s Fifth symphony…
t-axis
x-axis
Primed observer going at .75 c relative to unprimed. Both are ableto emit 3c tachyons (in own rest frame).
tachyon loops 2
Alic
e
Bob (!=
.75)
wab=3b
c
d
w cd=1.8
ditditditdah
a
dit d
it di
t d
ah
![Page 8: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/8.jpg)
Frame-dragging
![Page 9: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/9.jpg)
• One of the CTC inventors
• And also of a way of estimating time ranges from very small samples
![Page 10: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/10.jpg)
Two string time machine�By Gott
![Page 11: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/11.jpg)
• Wormholes, Tachyons, & Time Travel, Oh My
• Excellent introduction to the physics
• And short! • With a first rate
bibliography (he has taken time to write a short one)
![Page 12: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/12.jpg)
Wormholes to go back in time
![Page 13: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/13.jpg)
On Rotating Cylinders & the Possibility of Global Causality Violation
• 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.”
![Page 14: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/14.jpg)
Positive Feedback in Time
![Page 15: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/15.jpg)
• Chronology protection conjecture:
• “The laws of physics do not allow the appearance of closed timelike curves.”
• “Thus the possibility of time travel remains open. But I’m not going to bet on it. My opponent might have the unfair advantage of knowing the future.”
![Page 16: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/16.jpg)
• Novikov has taken a stronger position, in his Novikov consistency conjecture : inconsistent trajectories are forbidden: self-consistency is treated essentially as one of the boundary conditions imposed on any problem.
![Page 17: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/17.jpg)
Visser’s Four Possibilities
• Radical rewrite • Novikov consistency conjecture • Hawking’s Chronology Protection
Conjecture • Boring Physics Conjecture
![Page 18: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/18.jpg)
Three Paradoxes
• Consistency paradoxes – Killed own grandpa?
• Bootstrap paradoxes – Where did the book come from?
• Plausibility paradoxes – No human acts like that!
![Page 19: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/19.jpg)
All You Zombies • A Paradox can be paradoctored! • “I’m My Own Grandpa” • “I know where I come from, but
where do all you zombies come from?”
• (Drawing is from Kaku’s Hyperspace)
![Page 20: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/20.jpg)
• “If you want to see what would happen if you turned a machine gun on Brutus and the conspirators, we can arrange that--but not on this tour! We don’t want to have too much fun, do we?”
![Page 21: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/21.jpg)
The Anthropic Cosmological Principle
• “intelligent life…selects out the actual Universe from among the different imaginable universes: the only “real” universes are those which can contain intelligent life…”
![Page 22: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/22.jpg)
• The large number N: gravity in the cosmos
• The stars, the periodic table, and e.
• The fine-tuned expansion: dark matter and W.
• The number l: is cosmic expansion slowing or speeding?
• Primordial ripples: the number Q
• Three dimensions (& more)
![Page 23: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/23.jpg)
The Life of the Cosmos
![Page 24: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/24.jpg)
A Sound of Thunder • Eckels … fumbled crazily
at the thick slime on his boots. He held up a clod of dirt, trembling, “No, it can’t be. Not a little thing like that. No!”
• Embedded in the mud, glistening green and gold and black, was a butterfly, very beautiful and very dead.”
![Page 25: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/25.jpg)
Science Fiction
• Why pick on grandfather? It seems that the only way to prove that time travel is impossible is to cite a case of killing one's own grandfather. This incessant murdering of harmless ancestors must stop. Let's see some wide-awake fan make up some other method of disproving the theory.” – letter to Astounding Stories
![Page 26: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/26.jpg)
TANSTAAFL
• "The law that entropy always increases holds, I think, the supreme position among the laws of nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations - then so much the worse for Maxwell's equations.…but if your theory is found to be against the second law of thermodynamics, I can give you no hope; there is nothing for it but to collapse in deepest humiliation.” – Eddington
![Page 27: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/27.jpg)
Many arrows ➧ Big Bang - the Mother of all arrows ➧ Radiation ➧ Decay ➧ Evolution ➧ Biology ➧ Economics ➧ Linguistic ➧ Psychology ➧ …
![Page 28: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/28.jpg)
![Page 29: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/29.jpg)
Cognitive psychology
![Page 30: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/30.jpg)
“We need to guard against the double standard fallacy--that of accepting arguments with respect to one temporal direction that we wouldn’t accept with respect to the other.”
![Page 31: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/31.jpg)
Black Holes & Timewarps�- Kip S. Thorne
![Page 32: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/32.jpg)
Complex time
• "In other words, (the pilotʼs) life as observed from other reference frames must be
consistent with his own observations just as spatial observations must be consistent. This
does not necessarily mean (the pilot) can never appear in his own past, but rather that
he cannot change that past after he has experienced it."
– Asaro
![Page 33: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/33.jpg)
Opposite Thermodynamic Arrows of Time
time
time
![Page 34: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/34.jpg)
Opposite Arrows of Time (2)
![Page 35: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/35.jpg)
Paradox Noise
• “In any case where a signal from the future seems liable to tampering, either that signal will come through sufficiently muddled and unclear, or the overall system will otherwise conspire to subvert any schemes for tampering.”� – Peres and Schulman
![Page 36: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/36.jpg)
If wave functions can continue backwards in time (through one of his stargates) then only self-consistent solutions are allowed.
(By analogy with Bohr-Sommerfeld quantization rules for the atom!)
![Page 37: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/37.jpg)
• Kuin sends chronoliths 20 years & 3 months into the past to presage his intended victory…
• Like the three witches telling Macbeth a partial truth to damn him.
![Page 38: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/38.jpg)
Coming to terms with your inner robot
• “Of course I have free will. What choice do I have?” – Irving Berlin
![Page 39: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/39.jpg)
Elbow Room
• “What we want when we want free will is the power to decide our courses of action, and to decide them wisely, in the light of our expectations and desires. We want to be in control of ourselves, and not under the control of others. We want to be agents, capable of initiating, and taking responsibility for, projects and deeds.” – Dennett
![Page 40: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/40.jpg)
Levels of time
• Cause & effect • Entropic • Parametric • Strings • Loop quantum gravity
Presented for your consideration…
![Page 41: The Physics of Time Travel](https://reader033.vdocument.in/reader033/viewer/2022060110/555a2ce4d8b42ab3088b4f41/html5/thumbnails/41.jpg)
Why time travel?
• High fun factor – Fight foo fearlessly
• You never know – “That which is not forbidden is
compulsory!” • Thought experiments
– Witness the references