Long Summary
The Andromeda paradox, as discussed by Sabine Hossenfelder, is an intriguing consequence of Einstein's theory of relativity that highlights how the concept of 'now' or simultaneity becomes relative depending on the observer's motion. The paradox emerges when two people at the same place and time but moving differently observe events in the distant Andromeda galaxy, situated 2.5 million light years away, and perceive those events as occurring days apart despite observing stars at the same moment. This counterintuitive result comes from special relativity's effects on time perception and simultaneity, not from what the observers actually see since their visual information is always delayed by the travel time of light.
Originating from Roger Penrose's work, the paradox explores the notion that simultaneous events for one observer may not be simultaneous for another in a different inertial frame. Penrose's illustration involves two people passing each other who disagree on whether an Andromedan space fleet has already departed because their “now” slices of spacetime differ. Hossenfelder clarifies this concept by emphasizing that neither observer actually sees the fleet's launch at that moment; instead, the difference lies in their hypothetical slices of simultaneity — how they define what is happening 'right now' at a vast distance.
Using spacetime diagrams to illustrate, Hossenfelder explains that while both observers receive the same light signals from Andromeda and agree on what they see, their planes of simultaneity are tilted differently due to their relative motion. Thus, the disagreement is about when events are considered to happen simultaneously rather than actual observed events. This leads to the profound insight that the notion of 'now' is not absolute but frame-dependent, a foundational aspect of Einstein's relativity and a source of conceptual confusion even among physicists.
More broadly, the Andromeda paradox raises philosophical questions about the nature of time and whether the future is as fixed as the past, supporting the 'block universe' interpretation, where past, present, and future coexist and are equally real. Hossenfelder expresses her preference for this interpretation over alternate views that deny a meaningful concept of simultaneity for distant events. She notes that debates about 'now' in relativity are subtle and that even experts like Neil deGrasse Tyson and Hakeem Oluseyi can struggle to explain it accurately.
Finally, Hossenfelder relates this discussion to the importance of understanding physics deeply, citing her own book "Existential Physics" and recommending learning platforms like Brilliant for engaging with complex topics such as quantum mechanics and differential equations. She encourages exploration of foundational physics, emphasizing how these ideas provide insights into the fundamental structure of reality.
