Simultaneity of Astronauts

Question

Let us image a spacecraft in fast orbit. Since it took off its clock has lost several minutes relative to Earth time. From the ground we can speak to the astronauts, although there is 1 second radio delay. Aside from the signal delay we can say we are coexistent, talking, albeit in different frames and quite different clock time. So is this coexistence an existential, experiential construct disconnected from the mathematics of physics? i.e. Physical processes on the ground are happening at a different rate, so in what way are we coexistent with the astronauts?

Restatement

In physics the tangible reality is the radio signal, so ground crew and astronauts need coinciding future causal/light cones, and the signals take time to travel. If the delays are known signals could be sent to multiple spacecraft to arrive 'simultaneously' - We can know they are receiving the signal 'right now' - in the present. Also, there is a horizon of the present beyond the spacecraft, but physics is only concerned with what can be causally connected. So physics ignores the 'now', but it metaphysically exists, doesn't it? And is it not really the only thing that truly exists, (unlike memories of the past)?

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Addendum (edited)

The 'coexistence' referred to in my question is simply 'absolute simultaneity' as described here: Einstein, Relativity and Absolute Simultaneity. I quote from a review below.

The astronomer Van Flandern contributes a highly readable essay on "Global Positioning System and the twins' paradox", arguing that GPS is "a practical realization of Lorentz's 'universal time', wherein all clocks remain synchronized despite being in many different frames with high relative speeds". In essence, the GPS system considers one frame, the local gravitational field (read, Lorentzian ether), to be privileged: here alone clocks tick at universal time, lending a new, technology-infused meaning to Lorentzian relativity.

Answer 1

Your example contains an internal fallacy, as follows:

For the spacecraft to have experienced a time dilatation onboard of several minutes due to special relativity, it cannot be only one light-minute from earth.

For a complete explanation of why, I suggest you post this to the physics stack exchange.

Answer 2

It's like information in physics, where it having been defined there as the inverse of entropy, can lead to confusion.

In relativity time hasn't been discovered to be absolute or not, it's been defined, as related to the speed light moves. That's also, the limit at which signal information can be transmitted between locations.

Physicists had to think harder and more precisely about information, in order to account for Maxwell's Demon. Relativity forced similar more precise thinking about time. So I ask you, what do you think time is?

"What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know." - St Augustine

More on time in philosophy here: Can time be understood conceptually without experiencing it?

Answer 3

The crux of this question is that it is asking whether "really the only thing that truly exists" is what exists 'now', as, for instance, experienced in common by astronauts referencing a privileged frame.

To begin with, the 'now' that is experienced should be separated from the scientific 'now' of clock-time, such as now 3 o'clock. E.g. Heidegger's Being & Time, page 387-388

That Present which is held in [Dasein's] authentic temporality and which thus is authentic itself, we call the "moment of vision". This term must be understood in the active sense as an ecstasis. ... The moment of vision is a phenomenon which in principle can not be clarified in terms of the "now" [dem Jetzt]. The "now" is a temporal phenomenon which belongs to time as within-time-ness: the "now" 'in which' something arises, passes away, or is present-at-hand. ... [As] an authentic Present or waiting-towards, the moment of vision permits us to encounter for the first time what can be 'in a time' as ready-to-hand or present-at-hand."

That should clarify the separation of the moment of experience from the scientific 'now' of synchronised GPS satellites, for instance.

In defining the scientific 'now' we need only to deal with clock-time and physical information. However, this runs into problems with universality.

Re: "In essence, the GPS system considers one frame, the local gravitational field (read, Lorentzian ether), to be privileged: here alone clocks tick at universal time"

This specific conceptualisation of synchronised absolute time using calculated adjustment to clock-time depends on clock-time, which fails to be universal in a gravitational singularity, e.g. from Wikipedia

A gravitational singularity ... is a condition in which gravity is so intense that spacetime itself breaks down catastrophically. As such, a singularity is by definition no longer part of the regular spacetime and cannot be determined by "where" or "when".

And from within a singularity, according to Stefan Hensel's account

time doesn't stop at the singularity. Au contraire, it's accelerated to the max. What does stop is space.

So it is not possible to define a universal, synchronised manifold of simultaneity: it is stymied by the asymptotes of singularities.