Time Travel I

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Time Travel I

• by
• Robert Nemiroff
• Michigan Tech

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Physics X About This Course

• Officially "Extraordinary Concepts in Physics"
• Being taught for credit at Michigan Tech
• Light on math, heavy on concepts
• Anyone anywhere is welcome
• No textbook required
• Wikipedia, web links, and lectures only
• Find all the lectures with Google at
• "Starship Asterisk" then "Physics X" 
• http//bb.nightskylive.net/asterisk/viewforum.php?
  f39

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Time Travel to the Future

• Time travel to the future
•  
• allowed by Special Relativity (SR)
• basis of the twin paradox
• just go really fast for really long and come back
• or go near a black hole
• can always see what is going on
• can send messages and affect things
• during travel, on the average, Earth's clocks
  will appear to run fast compared to your clock
• expensive and hard to do

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Time Travel to the Past

• Time travel to the past
•  
• has never been done
• generally thought not possible
• unfundable without clear demonstration
• leads to many paradoxes
• might not conserve mass, energy, momentum,
  angular momentum, etc.
• popular plot device in science fiction

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Time Travel Grandfather Paradox

• A person travels back in time and kills her/his
  grandfather.  This happens before (s)he had
  children.  Therefore, there is no way for the
  time traveler to be born.  If the time traveler
  is not born, how can they go back in time to kill
  their grandfather?
• This paradox need not involve a grandfather, but
  could be any direct ancestor including either
  parent.  In perhaps the simplest case, the person
  killed could also be the time traveler before
  they entered the time machine.

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Grandfather Paradox Possible Resolutions

• Parallel Universes Resolution
• One can only time travel to a parallel universe.
• History remains intact at the time of arrival.
• Anything goes after that.
• Restricted Action Resolution
• Actions are restricted -- you cannot kill your
  grandfather (etc.), even if you try
• Related to the Novikov Self Consistency Principle.

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Grandfather Paradox Possible Resolutions

• Destruction Resolution
• Killing your grandfather will cause the
  destruction of the universe (science fiction)
• Predestination Resolution
• Whatever a time traveler does was really supposed
  to happen, even if the time traveler didn't know
  that at the time (science fiction)

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Time Travel Ontological Paradox

• What happens if you get an idea from a
  time-traveler?
•  
• If an idea comes from a time traveler from our
  future, where did the original idea come from?  
• New information must have an origin.

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Searching for Time Tourists

• Hawking Paradox if time travelers existed, they
  would be here now.  Since they are not here, time
  travelers -- and possibly time travel itself --
  does not exist.
• Similar to Fermi Paradox about intelligent alien
  life.
• Sagan Corollary Perhaps they are somehow
  disguised.
• physical laws say they can't be seen
• they might not want to be found
• for personal gain
• to avoid paradoxes in the future

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Searching for Time Tourists

• How would we find time tourists?
•  
• Big lottery winners?
• Images captured in old photographs?
• Google searches for events before they happen?
• (A personal hobby of RJN.)

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Time Travel Chronology Protection Conjecture

• A conjecture by Hawking that the law of physics
  will prevent time travel (closed time-like curves
  in GR) on all but microscopic scales.
• This applies only to "normal matter", matter that
  satisfies the GR Weak Energy Condition.  
• The CPC does not apply to matter with a negative
  energy density (dark energy). 

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Novikov self-consistency principle

• NSCP It is impossible to create time
  paradoxes.   OR
• The probability of something occurring that
  creates a true time paradox is zero.  HOWEVER
• "Changing the past" paradoxes are guaranteed
  self-consistent in that they only create a
  "closed loop" in a self-adjusted, cyclical,
  self-consistent way.

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Novikov self-consistency principle

• Time Loop Logic use the NSCP and a time machine
  to compute things faster than is possible with
  current computers
• Example How to use the future and a time machine
  to compute a prime factor F for a very large
  number N.
• Algorithm Establish N.  
• Receive F from the future.
• If F is a true prime factor of N, send F to the
  past
• If F is NOT a prime factor of N, send F1 to the
  past

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Novikov self-consistency principle

• Time Loop Logic 
•  
• Note that if F is not a prime factor of N, then
  the F sent in step 3 will not be the same as the
  F received in step 1.  This is a time paradox,
  the the F received must the same F that was sent.
  Such time paradoxes are not allowed by the NSCP. 
  Therefore, this cannot happen, and only step 2
  can occur.  So one can determine "very fast" if F
  is a factor.

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RJN Comment Instantaneous Time Travel

• Let's say you go back in time one minute.  When
  you appear, there is twice your mass around as
  was before.  Then, when one copy disappears back
  into the time machine, mass seems to just
  disappear without going anywhere
• Mass and energy conservation appear violated at a
  high level. 
• Even over one minute, the two copies of the time
  machine would develop a relatively high relative
  velocity.  This is due to the Earth rotating, the
  Earth moving about the Sun, etc.  How come you
  don't come flying out of the time machine at a
  very high velocity?