Article Spacetime: is it real and physical, or just a calculational tool?

[C C]

https://bigthink.com/starts-with-a-bang/...ulational/

EXCERPT: . . . In plain English, the contents of your Universe determine how spacetime is curved. You can then take the spacetime curvature and use it to predict how every quanta of matter and energy will move through and evolve in your Universe...

[...] What we don’t measure, though, is spacetime itself. We can measure distances and we can measure time intervals, but those are only indirect probes of the underlying spacetime. We can measure anything that interacts with us — our bodies, our instruments, our detectors, etc. — but an interaction only occurs when two quanta occupy the same point in spacetime: when they meet at an “event.”

We can measure every one of the effects that curved spacetime has on the matter and energy in the Universe, including:

• the redshifting of radiation due to the Universe’s expansion,
  
• the bending of light due to the presence of foreground masses,
  
• the effects of frame-dragging on a rotating body,
  
• the additional precession of orbits due to gravitational effects that go beyond what Newton predicted,
  
• how light gains energy when it falls deeper into a gravitational field and loses energy when it climbs out of it,
  

and many, many others. But the fact that we can only measure the effects of spacetime on the matter and energy in the Universe, and not the spacetime itself, tells us that spacetime behaves indistinguishably from a purely calculational tool.

But that doesn’t mean that spacetime itself isn’t a physically real entity. If you have actors acting out a play, you’d justifiably call the location where the play took place “their stage,” even if it was simply a field, a platform, bare ground, etc. Even if the play took place in the weightlessness of space, you’d simply note that they were using their freely-falling reference frame as a stage.

In the physical Universe, at least as we understand it, you cannot have quanta or interactions between them without the spacetime for them to exist in. Wherever spacetime exists, so do the laws of physics, and so do the fundamental quantum fields that underpin all of nature. In a sense, “nothingness” is the vacuum of empty spacetime, and talking about what occurs in the absence of spacetime is as nonsensical — at least from a physics perspective — as talking about a “where” that’s outside of the boundaries of space or a “when” that’s outside of the boundaries of time. Such a thing may exist, but we have no physical conception of it... (MORE - missing details)

[Magical Realist]

"But despite all the things that spacetime enables us to predict and know, it isn’t real in the same way that an atom is real. There’s nothing you can do to “detect” spacetime directly; you can only detect the individual quanta of matter and energy that exist within your spacetime. We’ve found a description of spacetime in the form of Einstein’s General Relativity that can successfully predict and explain every physical phenomenon we’ve ever observed or measured, but as far as exactly what it is — and whether it’s “real” or not — that’s not a question that science has yet discovered the answer to."

Space seems to be an a priori abstraction of our bodily experience. We do not perceive it directly. It extends indefinitely beyond us but is never real in itself. It takes matter and events happening within it to evoke it into being. We do not originally posit space in the sense of being a universal continuum containing all things. We encounter space thru our everyday living in terms of places that are mapped out in terms of matter and motion. We know place before we know space. It is the primitive non-abstract experience of places and of traveling that shows us that space is not an in-itself thing like matter is. It is sheer possibility of location and movement as dynamically perceived in the primordial awareness of self-embodiment. Space is not real in-itself, yet it is the language thru which all that is emerges as objectively here or there. It is not irreducible. It is an infinitely extending corollary of our own presence in a here and now.

[confused2]

For this experiment you need a dog and a ball. You throw the ball up away from the Earth. The ball is in free fall and the Earth is accelerating towards the ball. Any reasonably smart dog knows this and can catch the ball without going into any sort of existential crisis mode.