I Understanding Spacetime simultaneity in twin paradox scenarios

[ESponge2000]

How can there be ambiguity on what choice of simultaneity coordinates to use for 2 objects at rest in the same resting frame? It would be the one for that resting frame not any other one ?

I guess if we factor the distance apart we can disclose that there won’t be simultaneity in other reference frames if they are occupying different points in space

 

[jbriggs444]

How can there be ambiguity on what choice of simultaneity coordinates to use for 2 objects at rest in the same resting frame? It would be the one for that resting frame not any other one ?

Because there is no obligation to judge simultaneity according to the frame in which two objects happen to be at rest.

We are in a thread about [some modification of] the twin paradox. So there will normally be three reasonable choices of simultaneity convention. Infinitely many other choices are also possible.

 

[PeterDonis]

How can there be ambiguity on what choice of simultaneity coordinates to use for 2 objects at rest in the same resting frame?

Because "simultaneity" is a convention. It's not a law of physics. The "obvious" convention you are implicitly assuming is still a convention. There are an infinite number of other possible conventions, which simply don't occur to you, but they're still valid.

The real issue is not "ambiguity"; it is that "simultaneity" is not an invariant; it's not an actual physical thing; it's not an observable. It's an abstract property of your choice of coordinates. So it contains no actual physics. But you are trying to treat it as if it does. That doesn't work.

 

[ESponge2000]

Because there is no obligation to judge simultaneity according to the frame in which two objects happen to be at rest.

That’s a fair point , it is factually correct. We run a trip up with quantum particles entangled when we say
Entangled particle in location B is correlated to particle location A based on what property and angle particle A is measured … We measure particle A from angle X it has a spin of 45mm/s among other unknowns we couldn’t measure
Particle B is entangled to particle A and we now know among other unknowns , a detector of Particle B measuring particle B from angle X will determine it with 100% certainty has a spin of 45mm/s.
They 45 mm/s was not a discovery we learned from something the universe predecided by a code but was not made known to the universe till the measurement and choice of measurement was taken ! Particle B’s rotating speed, however, was determined by the measurement on particle A, “simultaneous” to the measurement on Particle A … well that is simultaneity we still have difficulty defining … superposition to collapse of a photon property we don’t know the way that plays on our concept of time

 

[Nugatory]

Particle B’s rotating speed, however, was determined by the measurement on particle A, “simultaneous” to the measurement on Particle A

We have known for decades now (start with a Google search for “Bell’s theorem experiments”) that that is incorrect.

Aside from the fact that none of these entanglement experiments have anything to do with speed of rotation (it’s the component of the spin in a particular direction that we’re talking about), we cannot conclude from the true statement that “if we measure B we will find….with 100 certainty” that any property of B has in fact been determined.

Even if it were correct, in most entanglement experiments the two measurements are spacelike-separated, meaning that whichever was done first is frame-dependent. Any description that starts with A being measured first is prima facie wrong.

 

[PeterDonis]

After moderator review, this thread is closed. Thanks to all who participated.