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Comment by anyfoo | Hacker News Reader

It's even more mind-blowing to remember that on that scale, the concept of "things happening at the same time" does not exist, because the reference frames don't allow for it.

That's so bizarre: Stand here, thing A happened before thing B. Stand over there, thing B happened before thing A. Stand at this other place, now they did happen at the same time, but only for you! Nobody's more right than the other. Universally, "same-timeness" (simultaneity) does not exist.

So, by asking for:

> to know things that are happening right now or relatively recently by our standards

You're really only asking for your particular flavor of "right now or relatively recently".

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It's always possible to create shared clock between two points and keep it accurate, then measure time of events A and B according to the clock.

Then you reunite the clocks and discover they show completely different time!

That’s some beautiful hand waving in regards to creating a shared clock.

I’m an idiot and would love to know how this would be possible.

Where they stand does not matter. If those three observers do not move in relation one to another, they will see events in the same order.

This is actually not true. Look at the following constellation:

A--1--B--2--C

Imagine 1 and 2 are satellites and observer B launches a rocket at both of them at the same time. From B's perspective, both satellites explode at the same time.

A, however, will see 1 explode before 2. C, on the other hand, will see the reverse, that 2 explodes first.

There's no right ordering here, just different reference frames.

If Alice, Bob, and Charlie knows distance to satellites, then they can calculate the true timestamps of events. Alice will register explosion of satellite 1 at time Ta1 and distance Da1, satellite 2 at time Ta2 and distance Da2, and so on, then Alice will subtract time of propagation to receive true timestamps and will see that `Ta1 - Da1/c = Ta2 - Da2/c`.

The key idea is that there is no true timestamp. This is really hard to get across in simple examples, because we almost always describe the example from an objective, external viewpoint, and then start discussing how things appear from different perspectives in the example. But the supposedly "objective" viewpoint is just a different viewpoint that we've invented, it is no more accurate than either of the other ones.

So yes, Alice, Bob, and Charlie can all calculate the timestamps relative to the other positions, and they could collectively decide together that one of those options will count as the "true" measuring point. But this is still a measurement decision that they have made.

True, and any other "true" measuring point would be just as right. Hence, no simultaneity in the universe.

You're right. I meant they are each standing in their own reference frame, so effectively they are moving with reference to each other, but I guess that wasn't entirely clear. For example: One observer is on Earth, the other on another planet outside the solar system.

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