Transcript The Nature of Time Travel

The Nature of Time
Travel
Xiaolong Shawn Liu
Dinna Ramlan
David Simons
Overview
 Introduction
 Worm Holes
 Paradoxes
 Feasibility
Introduction
 In many science fiction books/movies one can
often find the story of people traveling backward
or forward in time
 Who among us has not seen “Back to the
Future”
 What is really allowed if one were to take into
account the laws of physics? Is it allowed?
How Would We Do It?
 Special Relativity Theories by Einstein
 Time slows down if you go really really really really
really fast (e.g. >95% the speed of light)
 But…mass at these speeds becomes infinite
 This still doesn’t get you backwards in time
 In 1935, Albert Einstein and Nathan Rose
realized that general relativity allowed the
existence of “bridges,” originally called EinsteinRosen bridges but are now called wormholes.
Will this work???
Definition of a Wormhole
 A geometry of 4-dimensional space time in which
two regions of the universe are connected by a
narrow throat
 They are hypothetical connections between a
black hole and a white hole
Wormholes Cont…
 Black Hole:
everything (and I do
mean everything) is
sucked into a black
hole with an
inescapable
gravitational force
 White Hole: this will
take a little more
explaining…
White Holes Cont…
 Theoretical opposite of a black hole
 Not impossible to escape but impossible to reach
 Repels everything (including massive particles
and photons) and nothing can ever enter them
 We have not yet
observed one
Wormholes Cont…
 They can provide relatively easy means of
traveling distant regions of space or even
backwards in time
 By journeying through a wormhole, you could
travel between two regions faster than a normal
beam of light would be able to in normal space
time
Challenges to Transverse
Wormholes
 Worm holes are not static structures
 It’s a shape that expands from zero
throat radius and then back to zero
 Even light won’t
make it through
in time
Challenges Cont…
 Travel through one may be extremely
challenging because any object thrown in would
pull the wormhole together through gravity
 We would need matter with negative energy
density (the magnitude of the tension of the
matter must be greater than the energy density
of the matter itself) to stabilize the opening
 “Exotic matter” is required (we have not yet
found it yet but is not unproven under the laws of
physics)
Implications of Traveling
Through a Wormhole
 One of the wormhole
mouths is placed in Tech
and another mouth is
placed at Norris
 Shawn and David get in
the time machine, travel
away from the earth at
5:00 pm 6/4/03 at nearly
the speed of light from
Tech
 Dinna stays in Norris,
waiting for them to return
Implications Cont…
 Shawn and David return
after 12 hours of their
time but Dinna has gone
through 10 years of her
life (Twin Paradox)
 From the mouth at Tech,
Dinna can see her
younger self at Norris.
 The wormhole forms a
“loop”
Causality
 Law of identity applied to action
 The relationship between causes and
effects
 Events must be in the order of causes
then follow by effects
Paradox
 A statement that is seemingly
contradictory or opposed to common
sense and yet is perhaps true (from
Merriam-Webster Dictionary)
 Violates the law of causality
 Argument that time traveling is impossible
 Can be broken down into: matricide
paradox, Polchinski paradox, and the freelunch paradox
Matricide Paradox
 Also known as grandfather paradox
 A time traveler goes back in time to kill his young
grandfather
 His success means that he could never have
existed in the first place
Polchinski Paradox
 Also known as billiard ball
paradox
 Utilizes time travel to
prevent time travel from
occurring
 Ball is launched from the
right hole and gets out
from the left before it
enters the right hole
 It “collides” with itself to
“deflect” himself so that it
misses the time machine
Free-Lunch Paradox
 Leonardo da Vinci travels
back in time to meet his
younger self and gives him
the portrait of Mona Lisa
 Young Leonardo gets a free
lunch without putting any
effort in drawing the portrait
 Where is the origin of the
portrait?
Resolution to Paradoxes
 Parallel universe or multi-verse
 Chronology protection conjecture
 Entanglement theory
Parallel Universe or Multi-verse
 Theory by Dr. Hugh Everett, III
 Wormholes do not necessarily connect to
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our own universe
Consider the wave function as real object
Specifying the state of one system leads to a unique
specification of the state of the other subsystems
Each system only observing one of the possible results
of a measurement and unaware of the other results
All outcomes exist simultaneously but do not interact
with each other
Each universe is mutually unobservable but are equally
real worlds
Chronology Protection
Conjecture
 Theory by Stephen Hawking
 The law of physics conspire to
prevent time travel by
macroscopic objects
 Universe will react in such a way
as to destroy the time machine
that tries to get into the wormhole
 When someone tries to kill his
young grandfather, he misses his
shot and only hits his
grandfather’s leg
Entanglement Theory
 Your grandfather is represented as a pure
quantum state vector
 All the men at that time are
“indistinguishable”
 Characterized by their “entanglement” with
each other
 You cannot recognize your grandfather to
kill him
Feasibility Overview
 Transversable wormholes permitted 
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wormholes can be “time machines”
Event Horizon forbidden
Wormholes often require violation of
“Weak Energy Condition”
Alternative: Cosmic Strings?
String Theory: Hope for the Future
Requirement: No Event Horizon
 Virtual Particles that enter Wormhole could trigger
infinite energy density  Destruction!
 This is an event horizon, just like a black hole
Terminology
 Weak energy condition: The idea that the
mass (energy) density in any one frame
would always be at least equal to or
greater than zero is called the "weak
energy condition.”
 Averaged Null Energy Conditions (ANEC)
synonym for Weak Energy Condition
(WEC)
Quantum Mechanics
Restrictions
 Quantum Mechanics allows for Negative
Energy Density!
 Quantum Fluctuations make this possible
 Wormholes also require Negative Mass,
similar to Negative
Energy (E = Mc2)
 See right for
wormhole example
Violations of Averaged Null
Energy Conditions
 How much “exotic matter” is required?
 We know Quantum Mechanics allows
some energy condition violations
 2 Possibilities: Do we need
 Infinitesimal ANEC violations?
 Large WEC (weak energy condition)
violations?
Energy Density Theory Results
 Only small amounts of ANEC (Averaged
Null Energy Conditions) needed!
 May 2003 Physical Review Letter: Visser,
Kar, and Dadhich
 Requires proper geometry choice: Static
spherically symmetric space-times
 Special circumstance of theory supports
idea of time travel
Further Research: Cosmic
Strings
 Cosmic Strings are under enormous pressure &
tension, cross section smaller than an atom
 Extremely fast acceleration: accelerate the sun
from 0 to 60 miles per hour in 1/30 of a second
 The disproportional orientation of the universe
makes the existence of these strings highly
possible
Cosmic String Theory Results
 Interesting case: “Closed Time-like Curves”
produced by pairs of moving cosmic strings
 Research: Gott, 1991 Exact Solutions to
Einstein’s Field Equations for cosmic strings
 “Closed Time-like Curves” physics-speak for
time travel, produced with cosmic string speeds
greater than a certain threshold speed
 Research shows in nature these speeds “should
be possible!”
Cosmic String Cont…
 Advanced Civilization could accelerate
separate strings to high speeds by towing
with massive rockets
 Some Closed Time-like Curves (CTC) do
not violate Weak Energy Conditions 
even ‘easier’ for CTC to actually happen!
String Theory
 If successful, will unite General Relativity and
Quantum Theory
 Includes 4 dimensions we’re currently familiar
with, + 6 very small dimensions
 Implications not completely known yet
References
 “Black Holes and Time Warps” by Kip S. Thorne, 1994, W.W. Norton
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& Company.
Time Trips, Black Holes and Relativity website
http://www.geocities.com/CapeCanaveral/Hall/5803/index.html
The “big” time travel
http://www.anu.edu.au/Physics/courses/A07/studentsites/studentsite
s2001/The_Big/
Time travel homepage
http://www.anu.edu.au/Physics/courses/A07/studentsites/studentsite
s2001/Benz_Group/
The Everett Interpretation
http://www.hedweb.com/manworld.htm#believes
Wikipedia, the free encyclopedia http://www.wikipedia.org
References
 “The Universe in a Nutshell” by Stephen Hawking, 2001, Bantam
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Books.
Public Lectures from Website of Stephen Hawking:
http://www.hawking.org.uk/lectures/lindex.html
Definition of Weak Energy Condition:
http://www.aip.org/enews/physnews/1998/split/pnu398-2.htm
Morris, Thorne, and Yurtsever. Wormholes, Time Machines, and the
Weak Energy Condition. Vol 61, Number 13, 26 September 1988
http://prl.aps.org/
Gott, Richard. Closed Timelike Curves Produced by Pairs of Moving
Cosmic Strings: Exact Solutions. Vol 66, Number 9, 4 March 1991.
http://prl.aps.org/
String Theory Overview: http://superstringtheory.com
Visser, Kar, and Dadhich. Traversable Wormholes with Arbitrarily
Small Energy Condition Violations, Volume 90, Number 20, 23 May
2003. http://prl.aps.org/
Thank You!!!