The allure of time travel, particularly journeys into the past, is undeniable. The proliferation of time travel-centric science fiction on television screens is testament to this fascination. These shows often depict scenarios where characters travel back to alter events, a concept philosophers term “inconsistent time travel stories,” or what is also known as the “second time around fallacy”. These narratives explore the tantalizing possibility of reshaping history, often with unforeseen consequences.
Why are we so captivated by these narratives? They invite us to consider the fragility of our present. To what extent is our reality a product of chance, where minor alterations in the past could drastically reshape the present? These questions resonate deeply because we all ponder the influence of our choices on who we are and how resistant our identities are to subtle shifts in our past decisions.
However, this narrative framework presents a philosophical challenge: can you change the past? Philosophical thought typically argues against the possibility of altering what has already occurred. While it may seem possible to travel to a parallel universe that mirrors ours up to a specific point and influence events to create a different future, this does not equate to changing our past. You’re merely transitioning to a universe with an alternative timeline, not rewriting your own history.
Consider a scenario: you travel to a universe resembling ours, eliminate baby “Hitler,” and prevent World War II in that universe. This action doesn’t erase World War II from our history; it simply creates a universe where it never happened. Our universe retains its original timeline.
The core issue lies in the nature of change. For something to change, it must exist in one state and then transition to another. A dog, Annie, weighting 21.2 kgs on Monday at 2pm doesn’t change her mass by altering her mass at that specific time. Instead, she needs to affect her mass at a later time. Applying this to time itself, altering the past would require a past moment to transition from one state to another. But a specific moment in time only exists at that singular moment within a temporal dimension.
Introducing a second temporal dimension, often called “meta-time,” could potentially allow a single moment in one temporal dimension to exist at multiple points in the second. In this framework, changing the past would involve altering a moment from one state at one location in meta-time to another state at a different location in meta-time.
However, most philosophers reject the existence of meta-time. And, even if it did exist, this wouldn’t truly constitute changing the past. The original state of that moment would still exist within meta-time. You’d simply be creating another version of that moment, with different events unfolding. If the goal is to erase past events entirely, this approach falls short.
While philosophers generally agree that changing the past is impossible, they often acknowledge the potential to causally affect the past. What’s the distinction? Changing the past implies altering a moment from one state to another, hence the “second time around fallacy.” Causally affecting the past means influencing a moment to become the way it is (and was). Time travelers, by their very presence, will inevitably causally affect the past, breathing air, interacting with people, and so on. However, their actions don’t change the past; they are integral to shaping that moment in time.
Imagine Tim, who despises his abusive grandfather. He travels back in time to kill him, failing to grasp the impossibility of changing the past (as his grandfather is alive in the present). Tim shoots his grandfather, causing brain damage. Ironically, this brain damage is what causes his grandfather to become abusive, ultimately motivating Tim’s time travel. This is a consistent time travel story. Tim causally affects the past: he shoots someone. But he doesn’t change the past; he is causally responsible for the past being the way it always has been.
Given that time travelers can causally affect, but not change, the past, what actions are justifiable? If Tim’s grandfather is alive, Tim knows he won’t succeed in killing him. This raises an interesting point: can someone deliberate about doing something if they already know the outcome? For instance, if you know you cannot turn blue, you cannot deliberate about whether to turn blue. Similarly, if you know you will time travel to a specific day and wear a duck suit, you cannot deliberate about that action.
However, we rarely possess complete knowledge of past events, allowing room for potential time travelers to deliberate. While some actions might be immediately dismissed, like attempting to kill Hitler, given our certainty of his adulthood, weaker evidence about the past can justify deliberation.
If evidence suggests a famine led to ten thousand deaths, one might travel back to provide agricultural advice. The weak evidence of the famine could be misleading, and the intervention could prevent the tragedy. Perhaps the famine was averted due to the time traveler’s actions.
Intriguingly, some suggest manipulating evidence about the past. If significant historical evidence points to a famine, the traveler knows there’s no point in intervening. But if there’s no famine (perhaps due to prior intervention), the historical records are misleading. Hence, fabricating evidence of a famine could be seen as rational. One might travel to a later time, inform historians of a devastating famine, and then travel further back to prevent it. This creates an explanation for the historical evidence: the future self created it through time travel. This undermines the certainty of the famine and justifies the intervention.
This leads to an odd conclusion: is it rational to manipulate evidence to justify preventing a past event? Ultimately, the rationality lies in influencing events where the traveler’s knowledge of the past is incomplete.
The principles that apply to the past also extend to the future. If one knows what they will do tomorrow, deliberation becomes pointless. However, while we typically know more about our past actions than our future ones, this disparity is a matter of degree.
If overwhelming evidence suggests a future bank robbery, the certainty of that event diminishes the rationale for deliberation. Similar to manipulating historical records, one might fabricate evidence of a bank robbery to explain the predictions. This undermines the certainty of robbing the bank and makes deliberation rational.
Ultimately, the key factor is knowledge. The less one knows about the time to which they travel, the greater the capacity for deliberation. Conversely, extensive knowledge severely undermines deliberation. If one prefers a different outcome, manipulating evidence to justify intervention might be rational.
These scenarios highlight the inherent complexities of time travel and the decision-making processes of time travelers, even without the ability to alter the past. Time travel offers a fascinating lens through which to examine causality, knowledge, and free will.
