Which Watch Will Have Slower Time: Object 1 or Object 2?

[nitsuj]

Edwin Hubble said universe is expanding at the speed faster than light. Galaxy at 14 billion light years are going far away from Earth at speed of light. Some galaxies farther than 14 billion light years cannot be seen by human as light will not reach to earth.

Why it will happen if you all say its only "C" my above question?

The universe is allowed too! That is spacetime "itself" that, cumulatively over vast distances sum to recession velocities greater than c.

The stuff "within" obeys c.

imo it's a fair point the op raised regarding recession velocity, and seems a common consideration when learning about c. The distinction is vast & easily understood and "important".

 

[Ibix]

The practical result is called differential aging.

No - differential aging is what you get in a frame-invariant experiment like the twin paradox, where two clocks meet up, part, and meet up again. Time dilation is an effect between two inertial frames where clocks can only meet at most once.

While you can view it in terms of the same "interval" based analysis as is often used for the twin paradox, you need more than just distance. The two worldlines are at an angle to each other and their own planes of "now" are orthogonal to the lines. Thus they differ over what counts as "the same time" except when they meet and the distance from the meeting point "now" differs.

 

[nitsuj]

No - differential aging is what you get in a frame-invariant experiment like the twin paradox, where two clocks meet up, part, and meet up again. Time dilation is an effect between two inertial frames where clocks can only meet at most once.

Was presuming the OP was meaning the clocks are compared (at rest to each other) after time dilation, worded as "practical result".

Why must the clocks meet up at the start? I appreciate they need to be synchronized, but don't follow why they must "meet up" at the start for it to be called differential aging.

Err..never mind, I see it now. You said it in the first sentence, frame invariant scenario.