Is Spacetime Real?
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This is a false dichotomy. Atoms too are "just" a mathematical construct used to describe how certain observable, physical systems behave. It's even called the atomic _theory_, and like all other good theories it knows its limits. At low enough energies, it's too fine and at high enough energies it's too coarse a model. Spacetime is exactly as real as any other current model of observable physical phenomena that physics can offer.
The same applies to atoms, we all agree on what they are, and we can show pictures of them etc.
The question here is whether spacetime is something like that, or if it is a proxy concept with no grounding whatsoever in the universe, but with good predictive powers?
For example, for a long time we believed that Aether filled space, and although we now know this is not the case, the mathematical model provided by the Aether theory was pretty useful. You could very well imagine a world where we know Aether is not real, but continue using it for lack of a better theory as it is a useful framework.
> It's even called the atomic _theory_
Theory, in scientific theory, means the opposite of theory in our everyday language though. Theory does not mean a guess, it means a body of work accepted as valid and robustly tested.
This is where you bury the lede. It's clear if you read many of the comments here that there is no such agreement. Some people _think_ of an atom as something you can reach out and touch, or grok in some deep sense distinct from performing specific experiments. Others take an approach that's more in line with theory in the sense I use it (https://en.wikipedia.org/wiki/Scientific_theory). Your definition of theory is too loose: it's not just a body of work. That sounds like a pile of experimental evidence to me. A theory is a robust model or family of related models that match the experimental evidence. Atoms exist in that sense: they're a well tested model of experimental phenomena.
I'm certainly not claiming that atomic theory is a wild speculative guess which you seem to imply I've done.
I guess I should have said "all experts agree" instead? Lack of knowledge about something by some people doesn't change the definition of that thing. I would add however, that you can reach out and touch an atom with a certain definition of touch (as touch is mostly a macroscopic concept).
> I'm certainly not claiming that atomic theory is a wild speculative guess which you seem to imply I've done.
You will have to explain this one to me then. You said "it's *even* called a theory" to reinforce your point that it was just a mathematical tool (and hence not real following your definition). This point is valid only if theory means guess, which it does not.
And unfortunately for your argument, the bit you elided here is the critical bit. If we understand "touch" meaning to interact with something "physically" rather than "chemically" or indirectly (light, sound), then "touch" refers essentially to electromagnetic force and the Pauli exclusion principle. Since the latter can only be defined in terms of yet more "matter", in fact we have no way to observe matter in any way but electromagnetism in its various forms, viz. "light" in a wide sense, chemical interaction, and via magnetic repulsion on macroscopic or microscopic scales. Given this, I would say it's largely correct to understand atomic theory as useful abstraction; indeed, an abstraction which dates back hundreds of years which has been refined and found still useful. That said, something being an abstraction doesn't make it not real! Indeed I'm not entirely sure I know what else "real" is supposed to be.
Don't be so sure... atoms and even relatively large molecules definitely experimentally exhibit [1] wave particle duality in double slit experiments demonstrating they move as waves. Given that absolutely nothing is stationary at any temperature above absolute zero, what are you touching and with what?
[1] see end of article https://arstechnica.com/science/2021/01/the-curious-observer...
Scientists, like everyone, have a realist perspective. They consider reality to correspond with appearences. But this is a convention or a metaphysical belief or even a religous belief, it's not scientific.
The fully formalized ("axiomatic") mathematical descriptions oftentimes lag behind the theoretical models ("postulated") in use that are used to compare with real data for instance.
That said, I would say Atoms are as real as it can get. There are so many experiments about them and applications as well, for instant nuclear fission. How can they not be real? :-) Speaking about Spacetime, of course it is real. There is such a crazy amount of predictions based on Special Relativity.
Well put, and also not any specific degree of real, if you accept that the simulation hypothesis can't be disproven. The specific 'real' discovered properties of atoms or spacetime are actually artifacts of simulation.
I'm not sure how to postulate this idea further, but I've had it for years.
Maybe when you close your eyes you fade to black, and existing is a constant choice. Maybe situation and circumstance are handshake deals made in the collective unconscious, an onion growing layer by layer from the kernel.
Just something to think about :)
Right, with an atomic force microscope.
...but it almost sounds like you're suggesting that atoms and molecules are NOT something you could individually pick up with an AFM?
Consider that "human", "chair", and "atom" are also just concepts are minds apply to certain patters of energy. We all "know what that is" because it is advantageous for our minds to place things in these categories for the purpose of modeling. You may know what a "chair" is, but there are an infinity of gradations between "chair" and any two other objects, like the beanbag chair sitting between "chair" and "pillow".
The concept of a chair has no grounding in the universe, it's all in the mind. There are no physics that only apply to chairs.
> For example, for a long time we believed that Aether filled space, and although we now know this is not the case, the mathematical model provided by the Aether theory was pretty useful. You could very well imagine a world where we know Aether is not real, but continue using it for lack of a better theory as it is a useful framework.
Yes, that's how it works. When we have a better model we use that unless a simpler one will do. Newtonian gravitation isn't "real" by your definition, but we still use it in a lot of circumstances even though we have more accurate models.
So the question is whether spacetime is one of those patterns we can point to, like a "human" a "chair" or an "atom", which is what we call "real" in our everyday language, or whether it is only a useful modelization tool, like the Aether. There are no pattern anywhere in the universe matching the Aether.
It is a valid question.
If we discover a new model that makes better prediction than our current spacetime based model, just like relativity was to Newton's theory, will spacetime stop being real?
The article was clearly using "real" with the definition we attach to it in our everyday language, not in deep philosphical sense.
Yeah, pretty much.
> If we discover a new model that makes better prediction than our current spacetime based model, just like relativity was to Newton's theory, will spacetime stop being real?
In so far as Newtonian gravitation is also not-real. We still use that model for a lot of things of course. I don't know much about Aether theory or its predictions, so instead I'll use another long gone theory for my example: People used to model the motion of "heavenly bodies" using epicycles, complicated circles-within-circles patterns that assumed the Earth was the center of the universe and everything was taking a very complicated path around it. These epicycles were actually quite good at predicting the motion of planets and stars, so as far as anyone was concerned they were real. Some clever people eventually came around to the idea that all of that could be much more easily and more accurately modeled if we put the sun at the center. Of course we know today that that isn't quite right either.
We don't, and possibly cannot, ever know if the models we use to describe the world around us are the most accurate or not. They are real when they work, because we have no better metric to determine their reality.
The way you use "real" is the way I would use "valid": A theory is valid until proven otherwise.
And the way I use "real" (and I think it is the same way the article used it), is whether something has any grounding in our universe (some pattern as you said before).
For example, wormholes are possible following the theory of general relativity, but we don't know if they are "real", e.g. can there actually be a wormhole in our universe. If we do end up finding one, then they are real. If we however find conclusive evidence that they cannot exist in our universe, then they are not real, but only an artifact of our (incomplete) theory of general relativity.
Many of the things you described as "real" don't have grounding in our universe, i.e. things like "chair". It's possible I misunderstand your usage of the term "grounding".
> For example, wormholes are possible following the theory of general relativity, but we don't know if they are "real", e.g. can there actually be a wormhole in our universe. If we do end up finding one, then they are real. If we however find conclusive evidence that they cannot exist in our universe, then they are not real, but only an artifact of our (incomplete) theory of general relativity.
Sure, but how does that apply to spacetime? We know, so far as anything can be known, that space and time are two different perspectives of the same thing. Relativity described it, and we have measured and the effects it predicts with startling accuracy.
Here's a question: It matters if wormholes can or cannot exist regardless of if the current model permits them or not. Does it matter if spacetime fits your definition of "real" or not?
Worm holes aren't real until we can't describe phenomenon without them.
Same with the aether. Black holes and chairs are real because we can't make a theory that doesn't include them while still making accurate prediy
Hm? Can you elaborate on this?
The photo of the building is an accurate diagram of the model of that one individual building. It describes the building, it is not actually the building. However, your model (roof height, walls, location, etc) is the building. If that building is destroyed in a fire and remade to those exact specs, do we not say it is still a building?
You said "a model which makes accurate predictions is as real as real gets". That the picture is the building is your claim here, that there's nothing more real than an accurate model, which I'm countering by absurdum. Wind can turn a turbine, a weather predicting model of wind cannot. You say "the differentiation [between a model and reality] is meaningless", I say a model is a leaky abstraction - when you send a single atom through a Young's double slit experiment and it interferes with itself and generates a wave interference pattern, that's leaking a reality outside the billiard-ball model of atoms, that's one time when the distinction between model and non-model is meaningful.
> "If that building is destroyed in a fire and remade to those exact specs, do we not say it is still a building?"
If the building is destroyed in the fire, and your model of it isn't, how can you say there is no difference between the building and the model?
My claim is that the model describes a real thing, not that it itself is a real thing, and the extent to which that thing being described is real is a matter of the accuracy of the model. You can model the motion of planets with geocentric epicycles, but that is less real than a model that uses heliocentric ellipses.
Let me turn this around for a second:
> when you send a single atom through a Young's double slit experiment and it interferes with itself and generates a wave interference pattern, that's leaking a reality outside the billiard-ball model of atoms, that's one time when the distinction between model and non-model is meaningful.
Now ask yourself: if we have a 100% accurate model of physics that employs a concept of spacetime such that there are no leaks, no unaccounted for observations whatsoever, then would you say that it matters if spacetime is just some mathematical concept and the real mechanism works differently but produces exactly the same results?
I submit that it doesn't. A model that produces perfectly accurate descriptions describes reality in so far as any description of reality can be said to. We don't have perfect models of reality, that is known, however lacking a more accurate model it is meaningless to say that our current ones (which are astonishingly accurate despite being imperfect) don't describe reality.
This example actually weakens your argument. Heliocentric ellipses are chosen because they explain our model well and importantly are simpler than epicycles; but we can make epicycles arbitrarily accurate by piling on as many of them as you like.
So which is real? That Earth and the other planets revolve around the sun or that the sun and other planets revolve around the Earth?
> if we have a 100% accurate model of physics that employs a concept of spacetime such that there are no leaks, no unaccounted for observations whatsoever,
Let's say I have a box with a green LED, a red LED, and a button. One of the LEDs is always lit. Whenever I press the button, that LED becomes unlit, and the other becomes lit. If I try to open the box, it explodes. My model of how this box works is very simple: A demon is sitting inside the box and touches the LED that becomes lit. When I press the button he gets poked with a tack and switches to touching the other LED. If I try to open the box, he gets angry and blows up.
This model is 100% accurate. Every one of these boxes I have seen always behaves this way. Is there a demon sitting in the box?
Epicycles allowed us to map the motions very well but they were incredibly complicated and didn't provide for explanations of how they came into being. Simpler models that provide the same results are considered to be better. Given two models that make the same predictions, it makes more sense to use the simpler one than the more complicated one (Occam's razor). The heliocentric model no doubt had its detractors until it was refined enough to make the same or better predictions, but its simplicity made it much more attractive as an explanation.
> This model is 100% accurate. Every one of these boxes I have seen always behaves this way. Is there a demon sitting in the box?
Typically we apply Occam's razor in this. If we know how we could construct such a device without a demon, a being for which we have no other evidence of its existence, then our simpler and more readily explainable solution is what we generally go with. However, if this box is indeed unopenable, and the light always changes, then it doesn't really matter does it? Neither the demon nor a more conventional explanation makes any difference to how we interact with the box, nor could it ever.
The "reality" of what happens inside the box is not relevant and some would argue not even something that exists in the realm of scientific thought. It's like, say you live in Conway's Game of Life, and ask "what are cells made of?", it is completely unknowable and even if you somehow had the answer it would be meaningless since cells always behave in exactly the same way that is 100% explained by a simple model.
> ... the extent to which that thing being described is real is a matter of the accuracy of the model. You can model the motion of planets with geocentric epicycles, but that is less real than a model that uses heliocentric ellipses.
This statement doesn't suggest simplicity as a measure of realness and it is unclear why reality would necessarily prefer the simpler thing. We prefer it, of course. However, the two models on offer are mathematically equivalent and differ only in: (1) simplicity, and (2) explanation. So I'm not sure if you're modifying your stated position or not when you apply Occam's razor here.
As for whether it matters, I think that's rather the point: We don't know if it matters until we find out it does. I don't think we can know if matters until the moment it definitely does. A 100% accurate full-knowledge model is, by our own models, an impossibility. And even if we had such a model, how would we ever know it?
Because it doesn't matter if the results are the same, and simpler things are inherently preferred. Assume that both of these models are 100% correct in their predictions: there is no reason whatsoever to believe that the more complicated one is a "truer" model of reality, nor is there a reason to believe the simpler one is, so we proceed on the notion that the simpler one is because that is more to our liking and the end results are the same.
I guess one of the fundamental impasses here is that I am not convinced, on a philosophical level, that there is any deeper "realness" than can be modeled. If we can't model it, or rather if modelling it serves no purpose, then it may as well not exist.
> As for whether it matters, I think that's rather the point: We don't know if it matters until we find out it does. I don't think we can know if matters until the moment it definitely does.
Right. We must assume reality is what it appears to be until we make observations and tests that prove otherwise, then we construct a new model. We must assume that new model represents reality because we simply have no alternative until we build a better model.
> A 100% accurate full-knowledge model is, by our own models, an impossibility. And even if we had such a model, how would we ever know it?
To a certain extent we can't. No matter how many correct predictions we make there is always the possibility that we will eventually make an observation that defies our model. That's sorta my point. What we call reality is, by necessity, a product of observation, deduction, testing, and modelling. We have to construct our reality from that, and talking about whether or not an accurate model represents some deeper "reality" or not is pointless without an alternative.
If right now we question the "reality" of spacetime, what changes? Can we even build an accurate model of reality as we currently know it without spacetime? Can that model be made simpler than the ones with spacetime? If so, no one has done it yet. Therefore we have no choice but to believe that the universe isn't deceiving us and spacetime actually does exist, because believing the opposite is completely pointless.
I'd add: is it something one can develop an intuition for?
I don't think there is anyone who says gluons or general relativity are intuitive. You have to do the calculations (or memorize heuristics) to get something useful. Atoms, on the other hand, or electromagnetic radiation as another example, can be visualized (and mentally simulated) more or less correctly. (Note: atoms. Not nuclear mechanics.)
The question the author is talking about isn't whether a particular theory is correct, or even whether a theory contains explanatory power. I believe the author is assuming general relativity for the sake of argument, and discussing the concept of existence within the general relativity theory's framework.
For example, take the three-body problem [1]: does the _theory_ of general relativity require us to add initial conditions to the three body problem? In other words, are there more fundamental initial conditions not derivable that must be inserted as inputs into the equations when we use general relativity to solve the problem instead of say Newtonian mechanics? The answer is No. While the equations will change, the initial conditions (the degrees of freedom) do not.
What does that mean? It demonstrates indivisible properties of the universe given a set of theories. With general relativity now 'energy' and 'mass' are interchangeable, but the existence of mass/energy isn't a fictitious force, it can't be explained away by coordinate transformations. Gravity can't either, but a gravity well points to an underlying reality of mass/energy, not the other way around. If it were the other way around, we would say gravity is the 'real' thing, and mass/energy are resulting effects.
If we adopt that same definition for spacetime, we also have photographs of it, e.g. https://en.wikipedia.org/wiki/Solar_eclipse_of_May_29,_1919#... and https://en.wikipedia.org/wiki/Gravitational_lens#Gallery.
What does this even mean? Unless you're talking about some supernatural "atomic sense", there's obviously no way to "sense them directly" - we sense them through their interaction with other things combined with our model of those things and the interaction.
I think atomic phenomena may feel somehow more familiar because it takes far less energy to make them dynamic than it does for gravitational phenomena. If we were somehow huge beings made of binary stars or something, gravity would probably be the very familiar force and atomic phenomena almost inconceivably small to you.
Basically you start with some base information(axioms and postulates), then you gather some experimental evidence for those axioms and postulates. You establish some rules on which your experimental and observational universe works. Then what follows is volumes and volumes of Math.
If Im not wrong relativity itself was developed over a decade. And I'd assume it took tons and tons iterations, retries and tweaking to make it work.
Where common people like us get lost is looking at Symbol manipulation directly. At that stage Math scares the day light out of us.
The right approach is to start with your own axioms, postulates, truths and symbols and start working from there.
This is really like some newbie seeing a super massive code base and thinking it could be outside their reach to ever work with such a thing. But if you look at it carefully. All there is to it is loops, condition statements, operations, variables and basic abstractions.
> You sense spacetime directly too. It's what holds you do this planet we inhabit.
Nope.
At the same time not every real image is real. We could sometimes see two Suns in the sky, or we can see a rainbow, but it doesn't mean that the second Sun and the rainbow are real.
Yes every theory is just a model to fit observations etc etc. But Atoms are conceivable - they work as a model, but we can also imagine how to fit into the reality that we experience. We can 'see' them to an extent with special instruments. And their abilities/behaviours are evident in all the physical world of compounds and elements around us. You can disolve salt in water, you can light up a neon sign.
Quantum Mechanics is a model that fits experiment results very well, but is very hard to conceive of. Wave function collapse has no obvious analogue in the world as we experience it. Thats the point of schrodingers cat. You either have to go for the many worlds interpretation, or get into spooky stuff about observers.
Spacetime/relativity is similar, we can do stuff like fly a clock around the world in an airplane, or observe mercury transitioning past the sun, but its much less conceivable based on our everyday world. So spacetime might just be a constuct, like wave function collapse, that doesnt map easily onto our experience of the world. Certainly its a question worth asking.
Another historical example would be electric and magnetic field lines. My impression is that even Faraday originally had doubts about their reality; others certainly did. They could have just been forever considered a useful mathematical construct. At some point it became clear that thinking of them as really existing, permeating space, and having physical properties akin to those of accepted real stuff (momentum etc) was more useful. Many years after GR was formulated there were arguments about similar mathematical objects (dynamical components of a tensor, or "gravitational waves") should be considered real or not. The story is that Feynman convinced many with a simple thought experiment about how they transmit energy.
"Quantum Mechanics is a model that fits experiment results very well, but is very hard to conceive of."
These two statements stand in contradiction. Atoms only work as a model if quantum mechanics holds. In a non-quantum world, electrons collapse into a nucleus, shedding electromagnetic radiation and lowering their energy as they do so. That's also necessary to understand why neon signs have their colors.
More broadly, if you want to compare one scientific theory to another and contrast their "realness" you have to take quantum mechanics along with your atomic theory.
If you're saying I 'have' to take QM along with my atoms, then you also have to take Relativity along with your QM, and at the moment they are irreconcilable. Until we have a 'theory of everything' its ok to use simpler models within boundaries
I think a more interesting question then is whether spacetime arises as a result of the presence mass & energy?
Spacetime is 4D array of data: [x,y,z;t]. Can you translate your question to plain English, please?
It still doesn't make sense to me how gravity is put in the same bucket as the other three "fundamental" forces.
Since the universe is expanding, are we making more space (or spacetime)? If so, how does that work?
I'm not even sure if the universe is finite or not. I mean the observable universe is but how that relate to the actual universe? This seems like it would be somewhat unknowable.
Honestly, I'm still fuzzy on what the Higgs boson is and does and how it seems to be this redheaded stepchild in the Particle Zoo that doesn't really fit it anywhere else.
Speaking of the particle zoo, why are there (generally) three generations of particles/
The article touched on this but is space and/or time discrete or continuous? I'd learned that there was a concept of Planck distance, being the limit of how small a distance you could have. But is this the quanta of distance or just a limit on how low you can measure?
Is it just me or are we in a drought of theoretical physics breakthroughs? When I was younger, I guess I imagine we'd not be in the same position we were 20+ years ago. I mean there have been advances of course but we have things like the LHC pretty much confirming everything we already knew. Even the Higgs boson breakthrough was confirmation of a ~50 year old theory. A bunch of contender theories have been disproven, which of course has value.
But it seems like we're really no closer to reconciling quantum mechanics and gravity (other than eliminating candidates).
In some frame of reference, a photon leaving the Sun and hitting your retina, is a 0-distance, instantaneous connection between the Sun and your retina. And in another frame of reference, the photon "should" be stretched a hundred million miles long - or, not be a discrete corpuscular thing with a front and a back which travels through the intervening space.
It would be simpler to imagine if there was a nice Aether already connecting the Sun and your eye, for the photon to be a peturbation of, and then the emission of a photon wouldn't be a generation of a new thing de novo, but an electron dropping down an orbital level and its electric charge yanking on the electromagnetic field Aether like shoving a compression wave down a slinky. :/
For some reason the idea of photons stretching a hundred million miles long reminded me of the Asimov story 'The Dead Past' showing how a 'time viewer' would in reality be a Total Informational Awareness device.
Higgs field? https://simple.wikipedia.org/wiki/Higgs_field
Quantum vacuum energy is just noise.
Superposition is for waves only.
Dark matter can be explained by "SISO" principle.
Wave-paticle duality is boring simple: https://dotwave.org/single-particle-creates-interference-aft...
My discussion with my friend went back and forth, and we eventually decided that most things don't inherently exist, including people, chairs, and atoms. We ended up on the fence as to whether the "wave function of the universe" inherently exists. Anyway, I captured the essence of our conversation in a dialogue that may be of interest to other HNers:
As for the actual question of spacetime, I like the author’s conclusion for the time being:
“ 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.”
For the future, I think ...“it’d be really cool if”.... holography stitched spacetime from entanglement/entropy such that we could measure directly... something quantum... and believe it to be a real measurement of spacetime itself, right here in good ol’ de Sitter (or wherever we are, should experiment find things afresh;). Is there any hope there I wonder.
1. made poorly by design
2. approved by someone that have no clue why it was designed this way
So yes, my comment may seem hyperbolic but I still think that mixing with such people is a bad idea as long as they show no remorse knowingly doing damaging deeds.
That said, this is clearly off-topic and I regret starting this discussion here.
But then, someone once asked me 'what is energy?' So I rattled through the whole orthodox list, mechanical, kinetic, potential, electrical, heat, audio - and how each form may be transformed into another form. But then, the question was repeated 'So ... what is energy?'
'Ummm, matter moving in space, relative to some other matter.' Doesn't apply to potential, that's an object in a field. That snow and those boulders up the hill aren't moving relative to me ... but could be, any minute now.
In the end, there's matter moving (or not, relative to me) in space. The rest of what's 'real' is in our minds - each of which is different.
To put it another way, if we are not the only scientifically capable species, would a different species discover different physics? How different? Would there be at least some ideas that overlap?
These topics get explored in science fiction. Carl Sagan and James Burke also wrote quite a bit about how humans discover new ideas and suddenly the universe changes.
But if you think these ideas are so abstract as to be meaningless, then there is no question to pose, and nothing to discuss.
- Morpheus
https://en.wikipedia.org/wiki/Spacetime
But you need to warp space as in curve it. If you would fly close to a black hole space might be warped.
https://www.space.com/1976-black-hole-puts-dent-space-time.h...
You probably need to think of space in higher dimensions than three for example four dimensions.
But practically, we behave as though reality, consciousness, and freewill have substantial basis.
Entanglement yields locality. Space emerges. I'm not convinced time is emergent, though (I give the idea low credence).
I'm not a scientist, I just listen to Sean Carroll's podcasts.
Could you please expand on that?
Better explanation here: http://www.preposterousuniverse.com/blog/2016/07/18/space-em...
> whether it’s “real” or not — that’s not a question that science has yet discovered the answer to.