The grandfather paradox is a classic time travel puzzle: If you travel back in time and kill your grandfather before he has children, how can you exist to make the journey in the first place? To understand why this paradox doesn’t actually occur, we need to establish two key principles and then explore three frames of reference.
Key Principles
- The Past Cannot Be Changed: Your past is already set and cannot be altered. Time travel doesn’t change what has already happened.
- Physical Existence Is All That Matters: If you physically exist in a time period, then you are there, and your actions are part of that time’s reality. You cannot simply unexist.
Applying These Principles
Let’s explore these principles using the grandfather paradox by examining three frames of reference: your perspective, the original universe’s perspective, and the destination universe’s perspective.
1. Your Perspective (The Time Traveler)
For you, the time traveler, your journey through time is linear. Here’s how it unfolds:
- You were born.
- You grew up knowing your grandfather.
- You built a time machine.
- You traveled back in time to when your grandfather was young.
- You kill your grandfather before he has children.
From your perspective, you experienced all these events in a straight line. However, despite killing your grandfather, you don’t disappear because your personal timeline is already established. You remember being born, growing up, and building the time machine.
2. The Original Universe’S Perspective
In the universe you left (the original universe), events unfold as follows:
- Your grandfather was born, grew up, and had children, including your parent.
- Your parent was born and eventually had you.
- You were born and grew up.
- You built a time machine and traveled back in time.
- You disappeared from this universe the moment you activated the time machine.
In the original universe, your actions in the past don’t affect the timeline because this universe’s past is already set. When you disappear into the past, the original universe doesn’t track what happens next; it only records that you vanished. Your grandfather remains alive and has children, leading to your birth.
3. The Destination Universe’S Perspective
In the universe you arrive at (the destination universe, e.g., when your grandfather was young), events unfold as follows:
- Your grandfather is alive and young.
- You suddenly appear out of nowhere.
- You kill your grandfather before he has children.
From this universe’s perspective, you appear suddenly. It doesn’t care if you claim to be from the future; it just sees an individual who appears and commits an action (killing your grandfather). There’s no paradox because:
- Everything Occurs in the Present: The universe views all events as happening now. Whether you arrived from the future or not, your actions are just part of the current timeline.
- Physical Existence: Since you physically exist in the past now, you don’t disappear. Your existence as a time traveler is already established when you arrive. If you physically exist in the past, you are already there, and your actions are seen as current events.
Addressing The Paradox
In summary:
– Your Perspective: You live your life linearly. You are born, grow up, build a time machine, and travel back. You kill your grandfather, but you don’t disappear because your personal timeline is already established.
– Original Universe: You vanish the moment you activate the time machine. The universe continues without you, with your grandfather living his life and having children, leading to your birth.
– Destination Universe: You appear suddenly, and the universe treats you as part of the current reality. You kill your grandfather, but there’s no paradox because the universe doesn’t recognize you as a time traveler; it just sees a person committing an action. Physical existence is the only thing that matters in this universe. Since you exist in the past, your actions are part of its present.
What Happens If You Travel To The Future?
You might be wondering: what happens if you travel to the future? Here’s how it works:
- Traveling Forward in the Destination Universe: If you fast forward in the destination universe, the event of your birth might never occur since you killed your grandfather. However, this doesn’t affect your physical existence because you are already there as a time traveler.
- Returning to Your Original Timeline: If you travel back to your own origin timeline just after you left, your life continues where you left off. Your grandfather is still alive because the past in your original universe cannot be changed.
A Warning
Do not come back to a time before you leave. This can create complex and potentially dangerous situations that we will explore in future content.
Conclusion
By understanding these three perspectives and the key principles, we can see that they all work without conflict. Each frame of reference—yours, the original universe’s, and the destination universe’s—can coexist and observe events without causing paradoxes. The grandfather paradox doesn’t occur because the original universe’s past is unchanged, the destination universe integrates your actions into its present, and your personal timeline remains consistent. This allows all three perspectives to record events seamlessly, ensuring that time travel, as described, does not create any contradictions or impossible situations. Happy traveling!
