The Andromeda Paradox Confuses Physicists…and it will Blow Your Mind Too!

Physics has a way of introducing concepts that defy our common sense. The Andromeda Paradox is one such mind-bending thought experiment. It is a direct consequence of Einstein’s special theory of relativity and presents bizarre facts about time and simultaneity over large distances. It may just blow your mind!

Relativity and “Now”

Our intuition tells us that “now” is the same instant everywhere. If something occurs at present on Earth, we would suppose that something else is occurring at the same moment right now in the Andromeda Galaxy, 2.5 million light-years away. Yet Einstein’s special theory of relativity demonstrated that time and space are relative, not absolute. Most importantly, the theory asserts that ‘now’ is relative. Two events occurring at the same moment for one observer may occur at a different time for another observer travelling at a different velocity.

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The Paradox Explained

Suppose Alice and Bob are walking past one another on Earth. Alice is walking towards the Andromeda Galaxy, and Bob is walking away from it. Even this slight difference in speed (walking speed) alters their “plane of simultaneity” under relativity. This plane contains all the events throughout the universe that each individual deems to be occurring “now.”

Since Alice is travelling towards Andromeda, her “now” slice through spacetime is tilted slightly. For far-off Andromeda, her “now” contains events slightly in the future compared to a stationary observer. Bob travels away from Andromeda. His “now” slice tilts in the opposite direction. His “now” in Andromeda contains events slightly in the past compared to a stationary observer.

A Fleet Launching or Not?

Physicist Roger Penrose graphically demonstrated this. He envisioned a choice occurring in Andromeda: whether to send an invasion fleet toward Earth. For Bob (going away), the choice may still be in Andromeda’s future in his “now.” But for Alice (coming towards), the choice may already be made, residing in Andromeda’s past in her “now.” They are standing right next to each other, yet disagree on whether the fleet has been launched right now in Andromeda!

No Causality Violation

This sounds absurd, but it doesn’t violate any physical principles such as causality (cause and effect). Neither Bob nor Alice can know what’s occurring in Andromeda “now.” Light travels 2.5 million years from there to Earth. The paradox merely points out that there is no universal “present moment” throughout the universe. Our sense of simultaneity is entirely a function of our movement. The Andromeda Paradox compels us to embrace the bizarre, interdependent nature of space and time as described by relativity.

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