Internet was buzzing on October 28, 2010 withsearchers looking for a glimpse of a possible time traveler after news broke that an Irishfilmmaker spotted someone apparently talking on a cell phone in a 1928 Charlie Chaplinfilm.
Many asks how the time traveler receives cellularservice in 1928 (one might also wonder who she was talking to, decades before othershad cell phones).
Meanwhile, experts suggest an antiquated hearingaid as a plausible explanation.
The serendipitously named Time, on the otherhand, went to physicist Michio Kaku to ask whether time travel was possible.
He claims that time travel is possible, includingthe possibly paradoxical backwards time travel.
He suggests that if someone were to attemptto change their past, they would simply make another possible world a reality and becomesomeone else.
Various Philosophical Objections to Time TravelPhilosophers have discussed a number of objections to the possibility of time travel.
One objection is that if backwards time travelwere possible, then there would be evidence of time travelers around us.
Hearing-aid theory aside, perhaps the CharlieChaplin film provides such evidence.
Another line of reasoning argues that if timetravel were possible, then it would be possible for the same person to have different propertiesat the same time.
For example, you might have red hair in 1985, then you dye your hair black in 2010 and travel back to the time you had red hair.
This would violate Leibniz’s law of theIdentity of Indiscernibles.
Philosopher Paul Horwich argues for a simplesolution by allowing that a ‘proper time indexing’ be a property of individuals.
The Grandfather ParadoxThe most discussed philosophical objection to time travel, however, is the grandfatherparadox.
Although Michio Kaku’s suggestion of creatingan alternate universe would avoid this paradox, it would also raise a further puzzle: undercommon sense conceptions of causation, the person in this newly created reality wouldseem to exist without cause or history.
The grandfather paradox is often framed interms of human agency: what if you tried to go back in time and kill your grandfather? If you were successful, you wouldn’t beborn, which means you wouldn’t be able to go back in time and kill your grandfather.
John Earman provides an alternate formulationwhich sidesteps human agency: “Consider a rocket ship which at some space-timepoint x can fire a probe which will travel into the past lobe of the null cone at x.
Suppose that the rocket is programmed to firethe probe unless a safety switch is turned on and that the safety switch is turned onif and only if the ‘return’ of the probe is detected by a sensing device with whichthe rocket is equipped.
” (as quoted in Ismael, J.
, “Closed CausalLoops and the Bilking Argument”, 136) The paradox can be simply put: the probe isfired if and only if it is not fired.
Since these paradoxes need not be caused byhuman time travelers, philosophers often call such attempts at creating a causal paradox‘bilking’.
Resolving the ParadoxOne argument that bilking does not prove backwards time travel metaphysically impossible beginswith the suggestion that any attempts at bilking could be thwarted.
For example, you go to kill your grandfather, but the gun backfires.
You pick up a knife to complete the job, butyou slip on a banana peel.
An asteroid hits the rocket ship.
It might be countered that thwarting bilkingattempts would require a large number of improbable occurrences.
In an article titled “Bananas Enough forTime Travel?” Nicholas Smith likens the probability of bilking-thwartingevents to the probability of a tomato being squished by a car.
200 years ago, the probability that a tomatoon the road would be squished by a car was 0.
The fact that we don’t see strange coincidencesthwarting events could just mean that time travelers aren’t here now.
Related to Kaku’s suggestion, Smith alsopoints that the bilking argument rests on accepting counter-factual assertions like“if the time traveler killed her grandfather, then she would not exist”.
He says there is no reason to accept sucha counter-factual over the alternative “if the time traveler killed her grandfather, then the time traveler would not be her younger self” (Smith, 372).
Kaku’s suggestion amounts to accepting thelatter counter-factual.
He says, “If the river of time forks andyou get into the hot tub, you’re basically meeting someone else’s teenage mother wholooks like your teenage mother, but it’s not really your own.
” (quoted in Townsend, “The Charlie ChaplinTime Traveler”).
He further describes this as opening up a“parallel quantum reality”.
Paradox Resolution and the Nature of CausalityThis does solve the causal paradox but it raises a further mystery.
If the person you meet in the past is no longeryour mother, then who is your mother? In other words, the parallel quantum realitywould seem to lack a causal history.
This puzzle isn’t paradoxical, and not atall unanswerable, but it does seem to require that we think of causality in a way differentfrom what we are used to in our everyday lives.
A probabilistic view, for example, would providean answer: it’s not true that every person necessarily has a mother, it’s just highlyprobable.