Does going back to the past change our future? It’s a question that has fuelled science fiction for decades. Most of us imagine that travelling to the past would create an alternative timeline, influencing a new flow of events that becomes our “new” future. But this leads to a deeper, more frustrating question: How do we travel to our exact past, the one we actually lived, without it turning into a different future?
Before we can solve the mechanics of time travel, we must understand the fundamental laws of the Arrow of Time.
What is the Arrow of Time?
In physics, time is defined as a continuous sequence of events occurring in succession: from the known past, through the present, and into the unknown future. The “Arrow of Time” is the concept that postulates time only moves in one direction.
While many physical processes at a microscopic level are time-symmetric (meaning they look the same moving forward or backward), the macroscopic world is different. The direction of the arrow is dictated by Entropy.
Entropy: The Universe’s One-Way Lock
According to the Second Law of Thermodynamics, the entropy of an isolated system can only increase or remain constant; it can never decrease. Entropy is essentially a measure of disorder.
As time increases, entropy increases, making systems more complex and disordered. Because time and entropy are proportional, the “arrow” cannot be reversed. To go back to your “exact past” would require reversing the entropy of the entire universe. This is physical impossibility under our current understanding of thermodynamics.
Psychology vs. Physics: Memory and Volition
Our mental perception of time is a direct consequence of this thermodynamic arrow. This phenomenon manifests in two ways:
- Memory: We can remember the past because those events have already left an entropic trace in our brains. We cannot remember the future because those states of disorder haven’t happened yet.
- Volition: We feel we can influence the future through our actions, but we are powerless to change the past. Our present choices are the causes of future effects.1
Why Movies Like ‘Avengers’ and ‘Back to the Future’ Get It Wrong
Pop culture has given us a thrilling but scientifically flawed view of time. In movies like Back to the Future or Avengers: Endgame, characters treat time as a highway with off-ramps. While these stories are great for enthusiasm and gaining knowledge about the concept of time, they ignore the Thermodynamic Barrier.
You cannot change your “exact past” because that past is the very foundation upon which your present “self” is built. Reversing the event would be like trying to “un-burn” a piece of wood back into a log; the energy and information have already dispersed.
Conclusion
We cannot change our past. We must deal with our present. However, the future remains under our control. Instead of being sombre about past mistakes, we should focus on making a “better past” for our future selves to look back upon.
The Moving Finger writes; and, having writ, Moves on: nor all thy Piety nor Wit Shall lure it back to cancel half a Line, Nor all thy Tears wash out a Word of it. -Rubaiyat, Omar Khayyam
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