Internet was buzzing on October 28, 2010 with searchers looking for a glimpse of a possible time traveler after news broke that an Irish filmmaker spotted someone apparently talking on a cell phone in a 1928 Charlie Chaplin film.
Many asks how the time traveler receives cellular service in 1928 (one might also wonder who she was talking to, decades before others had cell phones). Meanwhile, experts suggest an antiquated hearing aid as a plausible explanation.
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The serendipitously named Time, on the other hand, went to physicist Michio Kaku to ask whether time travel was possible. He claims that time travel is possible, including the possibly paradoxical backwards time travel.
He suggests that if someone were to attempt to change their past, they would simply make another possible world a reality and become someone else.
Various Philosophical Objections to Time Travel
Philosophers have discussed a number of objections to the possibility of time travel. One objection is that if backwards time travel were possible, then there would be evidence of time travelers around us.
Hearing-aid theory aside, perhaps the Charlie Chaplin film provides such evidence. Another line of reasoning argues that if time travel were possible, then it would be possible for the same person to have different properties at 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 the Identity of Indiscernibles. Philosopher Paul Horwich argues for a simple solution by allowing that a ‘proper time indexing’ be a property of individuals.
The Grandfather Paradox
The most discussed philosophical objection to time travel, however, is the grandfather paradox. Although Michio Kaku’s suggestion of creating an alternate universe would avoid this paradox, it would also raise a further puzzle: under common sense conceptions of causation, the person in this newly created reality would seem to exist without cause or history.
The grandfather paradox is often framed in terms of human agency: what if you tried to go back in time and kill your grandfather? If you were successful, you wouldn’t be born, which means you wouldn’t be able to go back in time and kill your grandfather. John Earman provides an alternate formulation which sidesteps human agency:
“Consider a rocket ship which at some space-time point 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 fire the probe unless a safety switch is turned on and that the safety switch is turned on if and only if the ‘return’ of the probe is detected by a sensing device with which the rocket is equipped.” (as quoted in Ismael, J., “Closed Causal Loops and the Bilking Argument”, 136)
The paradox can be simply put: the probe is fired if and only if it is not fired. Since these paradoxes need not be caused by human time travelers, philosophers often call such attempts at creating a causal paradox ‘bilking’.
Resolving the Paradox
One argument that bilking does not prove backwards time travel metaphysically impossible begins with 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, but you slip on a banana peel. An asteroid hits the rocket ship.
It might be countered that thwarting bilking attempts would require a large number of improbable occurrences. In an article titled “Bananas Enough for Time Travel?” Nicholas Smith likens the probability of bilking-thwarting events to the probability of a tomato being squished by a car.
200 years ago, the probability that a tomato on the road would be squished by a car was 0. The fact that we don’t see strange coincidences thwarting events could just mean that time travelers aren’t here now.
Related to Kaku’s suggestion, Smith also points 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 such a 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 the latter counter-factual. He says, “If the river of time forks and you get into the hot tub, you’re basically meeting someone else’s teenage mother who looks like your teenage mother, but it’s not really your own.” (quoted in Townsend, “The Charlie Chaplin Time Traveler”). He further describes this as opening up a “parallel quantum reality”.
Paradox Resolution and the Nature of Causality
This does solve the causal paradox but it raises a further mystery. If the person you meet in the past is no longer your mother, then who is your mother?
In other words, the parallel quantum reality would seem to lack a causal history. This puzzle isn’t paradoxical, and not at all unanswerable, but it does seem to require that we think of causality in a way different from what we are used to in our everyday lives.
A probabilistic view, for example, would provide an answer: it’s not true that every person necessarily has a mother, it’s just highly probable.