Simultaneity is at least as well defined as clock time. Both clock time and the velocity of your reference frame can have arbitrary constants added to them to yield equally valid coordinate systems. So "it's not really simultaneous" is analogous to "it's not really 6:30 PM."

Depends what type of physicist you ask.

According to the energy-time uncertainty principle we don't even know when exactly the RF waves that transmitted information hit the receiver on Earth either.

In the absence of a physicist, I suppose that a software engineer in the room might do. After all, the counterintuitive consequences of relativity have their counterparts in counterintuitive effects in distributed systems and concurrent programming. In both cases, the core issue that misleads our intuition is the lack of a shared global clock that would impose a total ordering [1] on events/reads/writes/etc. Instead, events in both situations are only ordered partially [2]. In relativity the ordering is determined by the speed of light, in distributed systems the ordering is determined by what messages have been exchanged by two nodes and in concurrent programming reads and writes are ordered by synchronization actions such as lock acquisition and release, memory barriers etc (c.f. the happens-before relationship in JMM [3] and other memory models).

See for example [4] and [5].

[1]: https://en.wikipedia.org/wiki/Total_order

[2]: https://en.wikipedia.org/wiki/Partially_ordered_set

[3]: https://en.wikipedia.org/wiki/Java_memory_model

[4]: https://www.youtube.com/watch?v=UYZIHP120go

[5]: https://www.microsoft.com/en-us/research/publication/time-cl...

S/he already did. We just don't know yet.

> simultaneity

not in the true accurate to the picosecond sense of the word, no, but the exact word simultaneity is used when discussing number and density of satellites about a given latitude/longitude in the starlink beta program. Since they're LEO and orbiting at only 550 km, the satellites above a given spot on the ground vary greatly in the not-yet-complete sparse network.

Usually related to discussions of whether a beta test customer terminal will briefly hiccup and lose connection to its default gateway, or if somebody is at a sufficiently high latitude that they can have full coverage for all 86400 seconds in a day.

https://satellitemap.space/ has a good animated visualization of this.

Came here to suggest The Order of Time by C Rovelli, which explains this in such a captivating way.

Oh but there is. Just not in the same frame of reference.