Nature of Time and Block Universe Theory

• Philosophers and physicists debate what time actually is
• Generally accepted view: universe is unchanging block of spacetime
  • Contains three spatial dimensions + one temporal dimension
  • Past, present, future all equally real
  • Dinosaurs exist "out there" in past doing dinosaur stuff
  • All future exists somewhere in spacetime
• Block universe model: other places in time like other places in space
  • Just as Sydney exists while Singapore/London also real but we're not there
  • From time travel perspective: nothing stops swapping current location for somewhere/somewhen else

Christy Miller's Research at University of Sydney Center for Time

• Philosopher studying discrepancy between physics descriptions of time vs lived experience
• Goal: account that makes sense of lived experience given what physicists say time is really like
• Einstein's famous quote: distinction between past, present, future is "stubbornly persistent illusion"
• Still party line in physics today

Time Travel Paradoxes and Constraints

• Grandfather paradox example: backward time travel to murder infant grandfather
  • Creates contradiction: must exist to travel back, but killing grandfather prevents own existence
  • Both exist and don't exist simultaneously - impossible
• Past is fixed/certain way - always going to have been that way
• Time traveler not free to kill infant Hitler or change established past
• Future appears different - like blank book pages to write in
• Block universe model: entire book written from Big Bang to universe's end
  • Not just past is fixed, future also predetermined
  • Difference between past/future is just how much we know about it

Lee Smolin's Alternative View at Perimeter Institute

• Controversial position: time is real, not illusion
• Time is fundamental - most essential part of world experience
• Activity of time: continual construction of future from present
• Experience time as flowing because that's how nature is
• Criticism: lacks mathematical equations, only words so far
• Need mathematics suited to change/growth/time (like Euclidean geometry for static relationships)
• Such mathematics doesn't exist yet but "coming"

Implications for Time Travel Possibilities

• If Smolin correct (only present real, past/future don't exist): time travel probably impossible
  • Past only real as memories/records in present
  • Future doesn't exist yet, no fact of matter about it
  • Nowhere to travel to
• If block universe correct (time is illusion): things look brighter for time travelers
• Quantum physics perspective: nothing is impossible
  • Particles travel through walls via quantum tunneling (classically impossible)
  • Causality (A before B) may not be fundamental quantum mechanically
  • If not fundamental, could exploit/break the rule with right method
  • Very distant possibility but "never say never"

Theoretical Time Travel Methods

• Manipulating entangled particles
• Constructing and controlling wormholes
• Currently tantalizingly out of reach but drawing closer with daily learning