# Is simultaneity of events relative or just our observation of it?

I have already read this post about Relativity of Simultaneity but it doesn't seem to answer my question or it confuses me even more.

Please feel free to close this if I am mistaken.

I (think I) understand Einstein's thought experiment about the 2 lightings striking at "the same time" for one observer standing still but not for another one who is moving towards one of the light sources.

But it's only the simultaneity in regards to the observer's perception of the light emitted by the event and not the event itself that is in question.

The event itself: 2 lightings striking the ground, does happen at the same time for both observers, but it's only about their individual perception of both beams of light reaching their eyes that they cannot agree on right?

In other words, observing events and deciding whether they are simultaneous could indeed be relative in certain scenarios but events themselves, by themselves, in themselves, a priori can be simultaneous in an absolute manner no?

I mean if absolute simultaneity of events, independent of human observance doesn't exist then Alpha Centauri doesn't exist and the rest of the universe surrounding us doesn't exist right?

Because for things to exist there needs to be a continuity of existence, and if there is a continuity of existence then it must be simultaneous to/overlapping with the continuity of "our" (e.g. that of the planet earth) existence.

In other words, if the Milky Way is "happening right now" but we're not allowed to say that Alpha Centauri is "happening right now" as well, at the same time, then that would mean that Alpha Centauri doesn't exist or worse the rest of the universe itself.

And if Physics is indeed interested in events in themselves, independent of our observation, why does it care about the relative character of observable simultaneity?

I am a philosopher who is a Physics enthusiast with a high-school level knowledge of Math if that wasn't already obvious in an absolute manner.

I think the post you have linked in your question does answer your question but since you are explicitly aware of the post and don't seem to find an answer there, I will give it a try.

Now, for relativity, you don't need to bring in "observers" for anything. You can replace the word "observer" with "frame" and nobody would bat an eye. So, the claim of relativity regarding simultaneity has nothing to do with whether someone is actually observing the events or not. Rather, it says that the notion of the simultaneity of a pair of (spatially separated) events is a frame-dependent notion. This is to say that it doesn't mean anything to talk of the simultaneity of two events without specifying the frame of reference. Also, one should be absolutely aware of the fact that this is not an ideologically motivated claim that relativity goes around making--in the form of some rhetoric like "everything is frame-dependent". Quite the opposite. All the physically "useful/important/basic" notions and quantities in relativity are actually frame-independent. For example, causality (the notion as to whether a pair of events are causally connected or not) is frame-independent.

Now, Einstein's thought-experiment regarding the simultaneity of events is not about "the observer's perception of the light emitted by the event" or the "perception of both beams of light reaching [an observer's] eyes". It really is about "the event[s] [themselves]". And it turns out that the events themselves occur simultaneously in one frame but not in the other. The soul of the "trick" lies in the existence of a finite-valued invariant speed (let's call it $$c$$). In other words, the non-intuitive result of the frame-dependence of simultaneity comes out of the non-intuitive ingredient of the invariance of $$c$$.

At this point, it should be made clear that the facts that this $$c$$ is the speed at which light travels and that light also happens to be the signals that we use to see things have no importance whatsoever. This invariant speed could have been any finite value and the result that simultaneity is a frame-dependent notion would stay the same.

As the prior post states, observations and underlying reality are different in terms of timing. The two are connected by the fact that all observers agree that to each observer light travels at the same speed. Thus all observers can calculate when something in the past happened based on how far away it was when it happened.

If one observer calculates that two events happened at the same time, then another observer, moving with respect to the first, will calculate that the two events happened at different times.

So, on to your questions:

No, the two lightning strikes do not happen at the same time as calculated by the two observers. Only at most one will agree they happened at the same time.

No, there is no absolute agreement on simultaneity.

Alpha Centauri is about three light years from us. We know that it existed three years ago because we can see it in the sky as it was, for us, three years ago. We really and truly don't know if it exists right now. Check back with me in three years.

You have no idea what is happening right now except right where you are.

Lastly, physicists are not actually all that interested in simultaneity. It is a concept mainly of use in the non-relativistic limit. It is only fun to think about when learning special relativity.

• Thank you for your answer Paul.Regarding Alpha Centauri,I will check back with you in 3 years and if it turns out that it did exist at the time I posted the original question,it means that its existence was happening at the same time I posted the original question right?Also,I don't care about "what" is happening anywhere across the universe right now and whether I can know anything about it. I'm only interested as to whether anything can be happening right now anywhere across the universe and whether this is a valid statement. In any case,I understand there way more reading that I need to do. – Sprout Coder Feb 23 at 20:53
• And one last thing. If we extend your answer about Alpha Centauri to the whole of the universe, then it means that we can't know if the rest of the whole universe exists right now, which to me sounds absurd, because it would mean that it is possible that the universe "ends" where our observations of it "end" and that is is a lot smaller than we thought, but based on other calculations we know (?) that it's not so we know that "more "universe" than the observable one is "happening" right now. :-S (I'll stop here :-D) – Sprout Coder Feb 23 at 20:58
• I wish I could give you good news, but I can't. You are going to get the hang of special relativity very soon when you do that reading. But is is going to get worse for you. Study "wave function collapse" and "bell's theorem". The best minds in the universe are gappling with where "reality" exists and they are not doing very well :-( – Paul Young Feb 23 at 21:03
• And yes, if in three years I confirm for you that Alpha Centauri exists, then YES. It will turn out to have been correct, in hindsight, that Alpha Centauri exists Feb, 23, 2019. – Paul Young Feb 23 at 21:05
• One last bit: there is an old riddle "if a tree falls in the woods and nobody is around to hear it, does it make any noise?" We want to believe that the obvious answer is "yes" but, in fact, sometimes in physics the answer really and truly seems to be "this is not a question that can even have an answer" – Paul Young Feb 23 at 21:10