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?


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.

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    We are "coexistent" with everything in our causal cone, as we can send and receive signals from there. What does it have to do with rates? However, this "coexistence" is entirely inferential, with mediation from physics and mathematics, it is not a "disconnected experiential construct", I think (what does that mean exactly?).
    – Conifold
    Feb 7, 2022 at 22:38
  • Disconnected meaning the ground crew and the astronauts are experiencing different physical environments; but they continue to coexist even if the astronauts orbit so fast they become frozen in time. Or are we not coexisting when time dilates? Does it start to concern what we mean by existing? The experiential construct. Feb 7, 2022 at 23:27
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    I think it concerns what we mean by co-existing in the spirit of eternalism/presentism debate. In relativity we are coexisting with the entire spacetime, the global simultaneity intuitions, that probably motivate your "coexistence" (existence "at the same time"?), are invalid there. What remains of them is just the possibility of causal interaction, which would restrict "coexistence" from the entire spacetime to our causal light cone. Arguably, we do not "coexist" with the part of it that we can never interact with.
    – Conifold
    Feb 8, 2022 at 11:45
  • @Conifold That is getting close to the heart of the matter. Does anything contradict the 'horizon of the present' being unobservable, but actually real. Physics says we can't see it so forget about it. i.e. from your interesting link, Relativistic Physics "Positing otherwise unnecessary unobservable structure—absolute simultaneity—does violence to Ockham’s razor." Feb 8, 2022 at 12:46
  • So I'm asking, isn't the matter present in the "absolute simultaneity" in the "unobservable structure" actually the only physical reality? Feb 8, 2022 at 13:08

3 Answers 3


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.

  • I think the physics perspective is that the ground crew and the astronauts are in different frames and their only point of knowable reality contact is the delayed radio signal (light cone). I am trying to get beyond that to the awareness of the simultaneity/coexistence, for instance "I know they will be receiving my signal right now." However, can 'simultaneous' be said when they are in different frames with different time flow? Feb 8, 2022 at 7:46
  • Lol Physics Stack Exchange refers it back to Philosophy (or at least not Physics) : How do we mathematically know for sure that absolute time is abandoned in relativity? Feb 8, 2022 at 13:58

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?

  • Actually I think time is a separate issue from 'the moment of existence', which is what Einstein declared as unobservable. There is an amusing PDF about that here: Science and the Unobservable (Nature, 1937). However, absolute simultaneity is used for GPS; it can be calculated given knowledge of the topology. Physically, it is what exists, regardless of how fast or slow time is flowing at a specific location. Mar 7, 2022 at 13:24
  • So the 'moment' is the instantaneous configuration of everything. Mar 7, 2022 at 13:38
  • @ChrisDegnen: Instanteneity is relative. We are finding rather than everywhere existing all-at-once, it's more like waves of types of change bouncing around, and we see intersections as causes, or objects. Have a look at Is the idea of a causal chain physical (or even scientific)? You might find the Loop Quantum Gravity picture of time as emergent interesting also.
    – CriglCragl
    Mar 7, 2022 at 17:45
  • @ChrisDegnen: "Absolute" simultaneity is not really absolute, it's a convention (albeit a fantastically convenient and useful convention).
    – Kevin
    Mar 7, 2022 at 18:54
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    @CriglCragl Hi. I see "Presentism and Quantum Gravity" philsci-archive.pitt.edu/2308 concludes: "Given that physics is currently moving in the direction of M theory and loop quantum gravity, presentism’s future prospects do not look good, at least from the standpoint of scientific realists who take current developments in quantum gravity as getting us close to a true account of reality. Nevertheless, based on the existence of potentially viable theories of fixed foliation quantum gravity, I conclude that presentism is compatible with our most fundamental physics – for now." Mar 13, 2022 at 16:56

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.

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