1. The Nature of Time Travel Xiaolong Shawn Liu Dinna Ramlan David Simons

2. Overview  Introduction  Worm Holes  Paradoxes  Feasibility

3. 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?

4. 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 Einstein- Rosen bridges but are now called wormholes. Will this work???

5. 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

6. 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…

7. 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

8. 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

9. 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

10. 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)

11. 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

12. 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”

13. 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

14. 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 free- lunch paradox

15. 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

16. 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

17. 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?

18. Resolution to Paradoxes  Parallel universe or multi-verse  Chronology protection conjecture  Entanglement theory

19. Parallel Universe or Multi-verse  Theory by Dr. Hugh Everett, III  Wormholes do not necessarily connect to 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

20. 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

21. 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

22. Feasibility Overview  Transversable wormholes permitted  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

23. 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

24. 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)

25. 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 = Mc 2 )  See right for wormhole example

26. 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?

27. 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

28. 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

29. 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!”

30. 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!

31. 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

32. References  “Black Holes and Time Warps” by Kip S. Thorne, 1994, W.W. Norton & Company.  Time Trips, Black Holes and Relativity website http://www.geocities.com/CapeCanaveral/Hall/5803/index.html 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/ http://www.anu.edu.au/Physics/courses/A07/studentsites/studentsite s2001/The_Big/ 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/ http://www.anu.edu.au/Physics/courses/A07/studentsites/studentsite s2001/Benz_Group/ http://www.anu.edu.au/Physics/courses/A07/studentsites/studentsite s2001/Benz_Group/  The Everett Interpretation http://www.hedweb.com/manworld.htm#believes http://www.hedweb.com/manworld.htm#believes  Wikipedia, the free encyclopedia http://www.wikipedia.org http://www.wikipedia.org

33. References  “The Universe in a Nutshell” by Stephen Hawking, 2001, Bantam Books.  Public Lectures from Website of Stephen Hawking: http://www.hawking.org.uk/lectures/lindex.html http://www.hawking.org.uk/lectures/lindex.html  Definition of Weak Energy Condition: http://www.aip.org/enews/physnews/1998/split/pnu398-2.htm 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/ 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/ http://prl.aps.org/  String Theory Overview: http://superstringtheory.com 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/ http://prl.aps.org/

34. Thank You!!!