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There's a compelling theory put forth (or at least reiterated) I believe by Feynman(?) (Edit: Sorry it was Penrose) , that, assuming all subatomic particles eventually decay, the resulting empty universe becomes a singularity and a big bang happens again, birthing a new universe.

Surely spacetime isn't changing size in such a case - lest the planck length change with it.

It seems, then, that in the absence of anything to measure it, spacetime just doesn't really exist and is just a mathematical construct simplifying the description of how forces and matter interact based on whatever laws govern physics. This might seem like semantics, but when it comes to discussing space being shaped to connect two different "regions" as with wormholes the distinction seems to matter.

So is spacetime a real tangible fabric of the universe or is it more of a description of a range of interactions and behaviors?

Edit: I'm not dissenting spacetime's success at describing reality. But rather, it seems to me that in reality, the way particles interact is based on individual and entangled wave functions and the way those particles interact with all of the fundamental forces. So then isn't spacetime just a simplified construct? Einstein's equations predict wormholes but I'm curious if they just show up as a mathematical quirk because of the equations treat spacetime as a 3+1D fabric, when it seems like perhaps it's more complicated than that.

Or maybe it's not. Maybe spacetime really is a fabric filled with dark energy that exists even in a true vacuum. Seems like a valid question to me, despite the comments it's getting.

john doe
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  • Can you provide a source claiming that Feynman said that? – user1379857 Apr 24 '21 at 16:05
  • This seems essentially a duplicate of the many versions of "what is spacetime ?" we've had on this site and I don't see any difference here apart from the very wild speculation about wormholes. – StephenG - Help Ukraine Apr 24 '21 at 16:47
  • In the actual universe we inhabit there is always "something" present, and that will be true even if all matter evaporates into radiation (there would still be the radiation present). As for whether space and time are mathematical constructs... look around you. Do you experience space? Do you experience time? What more evidence would you need to consider these "real"? – Eric Smith Apr 24 '21 at 19:48
  • @StephenG Yes, there are variations on this question, each with answers. None of them are satisfying, Although the answer by Niels Nielsen below gets closer than most. – garyp Apr 24 '21 at 20:58
  • "spacetime just doesn't really exist" What do you mean? I walk my dog through spacetime once every day. – my2cts Apr 24 '21 at 23:50
  • The theory you mention on the first paragraph sounds like Roger Penrose's Conformal cyclic cosmology - https://en.m.wikipedia.org/wiki/Conformal_cyclic_cosmology – Lucas Baldo Apr 25 '21 at 03:33
  • @LucasBaldo is correct it was Penrose. As for the rest of you commenting along the lines of "I walk my dog through spacetime. You're asking what spacetime is?" etc: You're all ignoring the distinction between shortcuts and reality. Virtual particles depicted in Feynman disgrams are aptly named virtual, since they describe a drastic simplification of a massive range of the real interactions that occur. Those shortcuts are helpful, but if I were to say "ok because of the math behind these virtual particles, we can do XYZ." If spacetime is a shortcut, perhaps wormholes rely on flawed shutcut math – john doe Apr 25 '21 at 06:44

2 Answers2

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I'll address the question in the title.

All of physics consists of mathematical descriptions, written down by humans, of natural phenomena that we observe and study in the interests of understanding those phenomena well enough to enable predictions to be made. So our description of spacetime is a mathematical construct created by humans to formalize and codify the behavior of the real world, and enable accurate predictions to be made about the real world via calculation.

But at the deepest levels, the real world behaves exactly as if spacetime were an integral part of that reality; furthermore, we also know that if it wasn't, there would be easily-observed consequences which have been assiduously sought after for over a hundred years (for which the finder's reward would be a Nobel Prize)- and never seen.

This suggests quite strongly that the functioning of the real world with respect to for example things like 4-dimensional spacetime really is governed by things which admit precise mathematical description i.e., the story of the universe is actually written in the language of mathematics- whether or not there happen to be any sufficiently smart & clever humans hanging around to figure out and write down the equations.

niels nielsen
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    I think the definition Sabine Hossenfelder mentions here in the 10th paragraph: http://backreaction.blogspot.com/2021/01/is-time-real-what-does-this-even-mean.html is relevant here: Something is considered real if it is necessary in our model to explain things. For example, are numbers real? Or time? Or ideas? You can't see them, touch them, or even point to them. In some cases you can't even directly perceive them, let alone measure them. It is easy to forget that we are so accustomed to our senses that we do not treat things we can directly sense with the same amount of rigour. – DKNguyen Apr 24 '21 at 18:54
  • @DKNguyen, I tend to steer clear of philosophical treatments of physics. Philosophy is a journey of many paths, all leading from nothing to nowhere. – niels nielsen Apr 24 '21 at 19:03
  • I understand. I am not trying to treat it philosophically so much as point out how familiarity or acclimation with something affects our thinking. To me, this question comes off the same as if someone asks whether electric fields or lines of flux real. Presumably the OP is already past this point and no longer questions whether those are real because they have acclimated but is now wondering the exact same thing about space-time. – DKNguyen Apr 24 '21 at 19:04
  • @nielsnielsen I think this answer is pretty philosophical. – Deschele Schilder Apr 25 '21 at 03:19
  • @DescheleSchilder, I've spent enough time on philosophy SE to know that my answer isn't. You want philosophy? Hoo-boy. I go over there from time to time to keep them honest with respect to physics and haven't made much of a positive impression... – niels nielsen Apr 25 '21 at 04:41
  • @nielsnielsen Sabine Hossenfelder's statement is very relevant and i do not think it is philosophical. What she is saying in those lines is very much grounded in reality. – silverrahul Apr 25 '21 at 06:25
  • I want to point to my comment I edited into the question to highlight that Im not dissenting the real behavior of the universe that spacetime describes: just that math extrapolated from equations that are themselves simplifications could possibly be flawed. The distinction seems to matter. Wormholes don't seem to make sense outside of just looking at math, so I'm trying to figure out if it's just due to a mathematical extrapolation from a shortcut. Spacetimes describes relative interactional positioning between particles / waves based on the fundamental forces, speed, momentum. (1/2) – john doe Apr 25 '21 at 06:55
  • (2/2) But our equations fail to correctly describe 3 body systems. In reality the equations are usually simplifying the more complex reality, including quantum entanglement and wave functions of trillions of particles. My curiosity is not whether spacetime describes reality, but whether it's really the fabric the equations make it look like, causing Eistein's equations to predict wormholes. – john doe Apr 25 '21 at 07:00
  • @nielsnielsen The answer sounds pretty philosophical though, without addressing the Nature of spacetime What else is physics about than examining the Nature of things? Now you can argue that we'll never know that (and that's probably so), but what I mean to say is how does spacetime look like at the smallest level (like is done in loop quantum gravity)? Your gravitons will do a good job in explaining that further though. Which is not to say that they exist. Spacetime can be and behave without gravitons. How is a fluctuating topology, with disconnected piece emerging described by gravitons? – Deschele Schilder Apr 25 '21 at 09:13
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Einstein told us that an empty globally developing spacetime can't exist. For a globally developing spacetime to exist there has to be matter inside it. That is not to say that an empty spacetime that only locally develops (while the global structure stays the same) can't exist. You can read this here. It could be the Feynman universe you talk about, before a new big bang. But you can't touch it... You can't measure the time there neither can you make distance measurements, because of the simple fact that there are no clocks and distance measurement devices. Notice that for time to exist in such a structure there is no need for the irreversible processes we encounter in thermodynamics. That is, there isn't a global direction of time, only a local one. To deny that spacetime exists, independently of human beings, would be to deny that we exist, though there are people who see the material universe as a mere chimera.
To say what spacetime (or the matter inside it) is an Sich, who knows? We can't know. It's something very mysterious though. Luckily! It makes us feel, love, fight and think.

Urb
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Deschele Schilder
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