What's your favourite paradox?

[Justin Swanton]

I think I get it. You have no absolute stationary reference point, so the speed of one object is determined by the rate at which it approaches or recedes from another object, or better, of the rate the two objects are getting closer or further from each other. That being the case, they perceive the flow of time in themselves as different from the flow of time in the other. Objectively however, they exist in the same instant. No object can be in the past relative to another. One will see the other doing things more quickly or slowly than itself, but they both move from present moment to present movement together.

I guessing that if they stop receding and start approaching each other, the perceived flow of time will adjust until when they meet up, they will have both accomplished the same rate of change relative to each other.

 

[OuttaInc]

they all see the timing and position of each event differently, and you can easily create scenarios where they even see them in different orders. So everyone has a different take on where they are at any given time, what the time is from any given person's viewpoint, and what order things are happening in.

I remember reading a very helpful analogy many years ago using a loaf of bread to illustrate this effect, where each slice represented the "now" of multiple observers. The more oblique the slices were cut, the more differences arose in the perceived ordering of events for each observer on the loaf. Can't remember exactly which book it was, but I want to say Brian Greene's The Fabric of the Cosmos.

 

[StilLearning]

I am not entirely sure of my ability to go into further detail without resorting to diagrams and equations, and this is a general paradoxes thread, not a 'John hijacks things to try and express his personal take on special relativity' thread (and I have tea to make). But the path we're heading down leads to a paradox called the twins paradox - you have astronauts who are twins, and you extend one astronaut's view of things to be that of the galaxy generally (so he's stationary relative to the rest of the galaxy), and the other astronaut's view just remains that of his ship (once it has already gotten up to speed). This wikipedia page ( Twin paradox - Wikipedia ) has some good links to further explanations, right down the bottom.

 

[OuttaInc]

I misremembered- it wasn't the book I read, it was the PBS series on Brian Greene's The Fabric of the Cosmos. Happily it's available for free online:
https://www.pbs.org/wgbh/nova/video/the-fabric-of-the-cosmos-the-illusion-of-time/

For a very thought-provoking explanation of why there is no universally agreed-upon "Now" among observers (and why that suggests that past, present, and future all exist simultaneously) fast forward to ~ 18:40.

 

[Omits]

Have we proved time travel is impossible?

 

[Wayne Mack]

But both subjects remain in the same instant of the present. There aren't two presents (Santa is tightfisted).

Normalizing time across the universe to define some point as a common present is certainly mathematically possible. It just as little meaning as all befores and afters diverge. Given the distances, I find even the meaning of 'the same time' to be hard to understand. How would one know that the time at A and B was the same? What happens if one includes D, E, and F?