Eli5 How it is decided whose clock is running slower, given motion is always relative?

Question

Eli5 How it is decided whose clock is running slower, given motion is always relative?

In: 4

Answer 1

TL;DR – Whoever is the one feeling the seatbelt push against their chest.

Assuming that the velocity between two observers is constant – their speed or direction doesn’t change – then both observers will measure the others clock as ticking slower relative to theirs.

If you’re driving down the road at some arbitrary but constant speed and pass a stationary hitchhiker then from *your* perspective it’s *his* clock that ticks slower, because from your perspective he’s the one moving and not you. From the hitchhikers perspective you’re the one moving, so conversely he sees *your* clock ticking slower. This **symmetry** in time dilation – both of you observing the others clock as ticking slower – is what forms the basis of [the Twin Paradox](https://en.m.wikipedia.org/wiki/Twin_paradox).

So how does this get resolved? It doesn’t *until* the symmetry is broken. As soon as one observers reference frame becomes non-inertial – i.e accelerating – then the symmetry is broken. It’s important to note that the term “acceleration” in this context means changing speed *or* direction which can also mean speeding up or slowing down.

So, using our example, if you’re driving down the road at a constant but arbitrary speed and pass a stationary hitchhiker then both of you will observe each others clock as ticking more slowly relative to your own. Your symmetric.

However, once you slam on the brakes the symmetry is broken because you’re undergoing an acceleration. Your frame of reference is no longer inertial and you know this because you can feel the force of the seatbelt pushing into you as your speed changes – a force the hitchhiker never feels. In other words, something about your frame of reference changed but nothing about his did. Your asymmetric.

This asymmetry is where the difference in measured time becomes realized. You’ll observe the hitchhikers clock ticking faster, he’ll observe yours ticking slower, and when you compare clocks you’ll both agree that you, the driver, aged less than him.