The Andromeda Paradox Shows That "Now" Is a Local Illusion

The Science of Thought · Explore this series ›

Tormod Guldvog

Editor & Duowriter

May 1, 2025
Updated March 25, 2026

Key Takeaways

• Two pedestrians walking opposite directions disagree about "now" at Andromeda by three days.
• The relativity of simultaneity is confirmed physics, not a perceptual error.
• The block universe interpretation remains philosophically contested among experts.

✓ Fact-checked · See verification details

AI assistant: Research, Drafting, QA

Roger Penrose was working through the implications of special relativity when he noticed something unsettling about the Andromeda Paradox. Two pedestrians on the same street, walking in opposite directions, would disagree about what is happening "now" at the Andromeda galaxy by roughly three days.

Not three milliseconds. Three days.

The Oxford mathematician, who would later win the 2020 Nobel Prize in Physics for his work on black holes, proposed the thought experiment in his 1989 book The Emperor's New Mind. He wanted to disturb a comfortable assumption: that the present moment is real, universal, shared.

It worked.

"Now" at Andromeda Depends on How You Walk

The effect has nothing to do with light travel time. Light from Andromeda takes 2.5 million years to reach Earth, and both pedestrians receive it identically. The paradox concerns something subtler: the relativity of simultaneity, a direct consequence of Einstein's special relativity.

Each observer has a "plane of simultaneity," a slice through spacetime representing all events they consider present. When two people move at different velocities, their planes tilt relative to each other. On Earth, the tilt is negligible. But Andromeda is 2.5 million light-years away. Over that distance, even a tiny angular difference adds up to days.

For Alice, walking toward Andromeda, the present moment there might include a fleet of alien ships preparing to launch. For Bob, walking away, those ships launched three days ago. Same street, same moment, different realities.

The Block Universe Takes the Stage

Penrose built on an argument that Dutch philosopher C.W. Rietdijk had developed in 1966, and that Harvard philosopher Hilary Putnam had reached independently a year later. Their reasoning was strict: if events in the future for one observer are already in the past for another, then those events must exist.

This is the block universe, or eternalism. Past, present, and future coexist as a single four-dimensional structure. Your sense of moving through time is not a journey but a perspective.

Hossenfelder, the physicist and characteristically blunt science communicator, recently observed in a video essay (at the top of this article) that the paradox has "got multiple physicists' brains tied into knots." The knots are genuine. The underlying physics is not in dispute, but its philosophical meaning very much is.

Not Everyone Accepts the Andromeda Paradox's Conclusion

Presentists push back. Their position: only the present moment exists, and the relativity of simultaneity reveals something about the structure of measurement, not the structure of reality.

Two people disagreeing about "now" does not necessarily prove that all times coexist. Two people disagreeing about "here" does not prove all locations are the same place. The analogy is imperfect, but the presentists consider it instructive.

Tim Andersen, a principal research scientist at Georgia Tech, has argued that the paradox, while mathematically sound, does not conclusively establish eternalism. Andersen, who writes with the patient persistence of someone accustomed to swimming against philosophical currents, contends that the block universe interpretation reads more into the physics than the physics demands.

Other YouTubers also criticises StarTalk's explanation, like Lukas Rafaj:

The debate remains live, productive, and genuinely unresolved.

What Your Brain Builds When It Builds "Now"

Strip away the philosophy and a starker observation remains. Your brain constructs "now" from sensory signals that arrive at different speeds, are processed at different rates, and are stitched together after the fact. Visual processing takes roughly 100 milliseconds. Auditory processing is faster. Touch depends on how far the signal travels along your nerves.

The present moment you experience is a reconstruction, a best guess assembled from lagging data. Penrose's pedestrians simply demonstrate that this construction has no cosmic counterpart. There is no universal clock confirming that your "now" matches anyone else's.

More On Spacetime

Black Hole Information Paradox: Could Spacetime Remember?

Physicist Florian Neukart proposes spacetime stores quantum information at the Planck scale, offering a new approach to the black hole information paradox.

→

Whether the block universe is the correct interpretation of this fact remains contested among physicists and philosophers alike. The deeper question of how time actually works connects relativity, quantum mechanics, and thermodynamics in ways that remain far from settled. But the fact itself, that simultaneity is relative, is experimentally confirmed physics. Your experience of an absolute present appears to be a cognitive achievement, not a discovery about the universe.

The question the Andromeda Paradox leaves behind is precise, and several research groups in the philosophy of physics are actively pursuing it. If "now" is something brains build rather than something the cosmos provides, what else about our experience of time is a construction?

The answer may depend on which interpretation of quantum mechanics ultimately prevails, a question that remains as stubbornly open as time itself.

Sources

• Primary Research: The Andromeda Paradox Even Confuses Physicists (Sabine Hossenfelder, 2025)
• Additional Context:
  • The Andromeda Paradox (4rknova.com, Nikolaos Papadopoulos' blog)
  • Rietdijk-Putnam argument (Wikipedia)
  • The Andromeda Paradox is not a good argument for the Block Universe (Tim Andersen)

Tormod Guldvog

Editor & Duowriter

Tormod Guldvog is the editor of Science Reader, covering the space where human and artificial intelligence meet. With a background in technology and science communication, he writes about how AI is changing scientific discovery — and what that means for us. More about Science Reader

Cite this article

Tormod Guldvog. (2025, May 1). The Andromeda Paradox Shows That "Now" Is a Local Illusion. Science Reader. https://sciencereader.com/the-andromeda-paradox-even-confuses-physicists/