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“What exactly is spacetime? Is it a real thing like an atom, or just a mathematical construct that is used to describe how mass 'generates' gravity?” This is a

“What exactly is spacetime? Is it a real thing like an atom, or just a mathematical construct that is used to describe how mass 'generates' gravity?” 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.

ncann 6y ago
While not a perfect definition of "real", don't we already have images of atoms?
- thisiscorrect 6y ago
  We do. Thinking a bit about what that means, we have photographs showing how the physical entity we model as atoms interact with light, and can confirm that the theory lines up with experiment. 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#Observations https://en.wikipedia.org/wiki/Solar_eclipse_of_May_29,_1919#... and https://en.wikipedia.org/wiki/Gravitational_lens#Gallery https://en.wikipedia.org/wiki/Gravitational_lens#Gallery.
  - jakelazaroff 6y ago
    Atoms aren’t just something we’ve modeled — they’re a physical thing, i.e. matter. We can sense them directly, not just infer their existence by measuring related phenomena. I realize when you get into theoretical physics the “thingness” of concepts becomes more and more abstract, but atoms don’t fall into that bucket.
    - thisiscorrect 6y ago
      You sense spacetime directly too. It's what holds you do this planet we inhabit. 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.
      - jodrellblank 6y ago
        Atomic phenomena are almost inconcievably small to me. There's around a hundred trillion atoms in a human body cell, and around a hundred trillion cells in a human body, plus or minus an order of magnitude or two. It's astonishing to me that people in the early 1900s could measure and calculate the charge on an electron, or that Marie Curie had to process tons of pitchblende to purify micrograms of Radium - and could, and did. Or that Rutherford could say where most of the mass inside an atom is, and split the nucleus of Nitrogen atoms - 1/1000th of the size of the whole atom - and tell what was happening down at that scale - using the technology of 1917.
        kamaal 6y ago
        As a kid I read the whole Michelson–Morley experiment in a public library. It was a quite a peek into how the science of that era was done. 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.
      - 6y ago
    - enkid 6y ago
      I don't see the distinction you're making between "sensing" and "measuring related phenomena." What's different from using a specialized microscope to take a picture of an atom and using a telescope to take a picture of black hole?
      - Balgair 6y ago
        The amount of grant funding! ;P
    - bobcostas55 6y ago
      >We can sense them directly 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.
      - jakelazaroff 6y ago
        Okay, that's fair. But taken to its extreme, we sense everything through its interaction with other things. But obviously, things exist; not everything can be just a mathematical model.
        headsupernova 6y ago
        Why not?
        drran 6y ago
        Mathematics is too powerful. It can express anything, both real and unreal things. Physical world has limitations.
        bobcostas55 6y ago
        Check out Max Tegmark's writings on the mathematical universe, some very interesting stuff in that direction.
        jakelazaroff 6y ago
        Will do, thanks for the rec!
- ordu 6y ago
  Those images, as I understand, constructed by heavy calculations based on a theory of atoms. Isn't it? So it is arguable that images are not "real", they are visualizations of a data. 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.
- mhh__ 6y ago
  We have images of black holes merging too.
- yes_man 6y ago
  We have devices that register electromagnetic radiation spectrum emitted (by ”atoms”) which can be visualized as an image. But this is at the heart of the parent comment: does the atom exist, or is it just a concept that describes the end result when strong interaction, electromagnetic force and gravity make particles stick together in a (usually) consistent way? If we call the atom real, we can call other aggregations like the economy or game theory real as well. But they are just concepts
  - AnIdiotOnTheNet 6y ago
    It isn't much of an argument. Everything we think of as "real" in the sense that you describe it is actually just a conceptual model we have for how certain patterns of energy in the universe behave. The universe has no special rules for apples, floors, wifi access points, cars, or basketballs.
- umvi 6y ago
  Yeah but what are we actually seeing? An atom is made of protons, neutrons, and electrons, and 99.9% empty space. But you can't really look at an image of an atom and say "there's a proton there" so what is the image of exactly? Rinse and repeat with quarks.
codeulike 6y ago
I dont agree its a false dichotomy. 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.
- enkid 6y ago
  Our ability to conceive of something should not be in the definition of reality. Quantum mechanics and relativity have just as much, if not more experimental evidence to support them as atomic theory, and make even better predictions. This has implications in the "real world." If we base our definition on what is "intuitive" instead of what is provable, we end up with things like humour theory in medicine.
  - codeulike 6y ago
    I'm not saying we should reject anything if it isn't intuitive. I'm just saying the statement Is Spacetime a real thing like an atom, or just a mathematical construct that is used to describe how mass 'generates' gravity? is not a false dichotomy
- thisiscorrect 6y ago
  "But Atoms are conceivable - they work as a model, but we can also imagine how to fit into the reality that we experience." "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.
Voloskaya 6y ago
I disagree with you. Your argument could be made about anything, there is no such thing as a "human" either, it's just a big bunch of wave-particles that does not have some clear limits. Yet when we talk about a human, we all know what that is, and when a human is seating on a chair we don't think the chair is part of it. 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.
- AnIdiotOnTheNet 6y ago
  > 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? 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.
drewcoo 6y ago
A scientific theory is a body of knowledge. The special use of the term seems like a false monocotomy.
f154hfds 6y ago
I think you and the author are talking about different concepts. To your credit, for the author's 'real thing like an atom' here, the term real is ambiguous. 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. [1]: https://en.wikipedia.org/wiki/Three-body_problem https://en.wikipedia.org/wiki/Three-body_problem

ncann 6y ago

While not a perfect definition of "real", don't we already have images of atoms?

thisiscorrect 6y ago
We do. Thinking a bit about what that means, we have photographs showing how the physical entity we model as atoms interact with light, and can confirm that the theory lines up with experiment. 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#Observations https://en.wikipedia.org/wiki/Solar_eclipse_of_May_29,_1919#... and https://en.wikipedia.org/wiki/Gravitational_lens#Gallery https://en.wikipedia.org/wiki/Gravitational_lens#Gallery.
- jakelazaroff 6y ago
  Atoms aren’t just something we’ve modeled — they’re a physical thing, i.e. matter. We can sense them directly, not just infer their existence by measuring related phenomena. I realize when you get into theoretical physics the “thingness” of concepts becomes more and more abstract, but atoms don’t fall into that bucket.
  - thisiscorrect 6y ago
    You sense spacetime directly too. It's what holds you do this planet we inhabit. 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.
    - jodrellblank 6y ago
      Atomic phenomena are almost inconcievably small to me. There's around a hundred trillion atoms in a human body cell, and around a hundred trillion cells in a human body, plus or minus an order of magnitude or two. It's astonishing to me that people in the early 1900s could measure and calculate the charge on an electron, or that Marie Curie had to process tons of pitchblende to purify micrograms of Radium - and could, and did. Or that Rutherford could say where most of the mass inside an atom is, and split the nucleus of Nitrogen atoms - 1/1000th of the size of the whole atom - and tell what was happening down at that scale - using the technology of 1917.
      - kamaal 6y ago
        As a kid I read the whole Michelson–Morley experiment in a public library. It was a quite a peek into how the science of that era was done. 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.
    - 6y ago
  - enkid 6y ago
    I don't see the distinction you're making between "sensing" and "measuring related phenomena." What's different from using a specialized microscope to take a picture of an atom and using a telescope to take a picture of black hole?
    - Balgair 6y ago
      The amount of grant funding! ;P
  - bobcostas55 6y ago
    >We can sense them directly 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.
    - jakelazaroff 6y ago
      Okay, that's fair. But taken to its extreme, we sense everything through its interaction with other things. But obviously, things exist; not everything can be just a mathematical model.
      - headsupernova 6y ago
        Why not?
        drran 6y ago
        Mathematics is too powerful. It can express anything, both real and unreal things. Physical world has limitations.
      - bobcostas55 6y ago
        Check out Max Tegmark's writings on the mathematical universe, some very interesting stuff in that direction.
        jakelazaroff 6y ago
        Will do, thanks for the rec!
ordu 6y ago
Those images, as I understand, constructed by heavy calculations based on a theory of atoms. Isn't it? So it is arguable that images are not "real", they are visualizations of a data. 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.
mhh__ 6y ago
We have images of black holes merging too.
yes_man 6y ago
We have devices that register electromagnetic radiation spectrum emitted (by ”atoms”) which can be visualized as an image. But this is at the heart of the parent comment: does the atom exist, or is it just a concept that describes the end result when strong interaction, electromagnetic force and gravity make particles stick together in a (usually) consistent way? If we call the atom real, we can call other aggregations like the economy or game theory real as well. But they are just concepts
- AnIdiotOnTheNet 6y ago
  It isn't much of an argument. Everything we think of as "real" in the sense that you describe it is actually just a conceptual model we have for how certain patterns of energy in the universe behave. The universe has no special rules for apples, floors, wifi access points, cars, or basketballs.
umvi 6y ago
Yeah but what are we actually seeing? An atom is made of protons, neutrons, and electrons, and 99.9% empty space. But you can't really look at an image of an atom and say "there's a proton there" so what is the image of exactly? Rinse and repeat with quarks.

codeulike 6y ago

I dont agree its a false dichotomy. 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.

enkid 6y ago
Our ability to conceive of something should not be in the definition of reality. Quantum mechanics and relativity have just as much, if not more experimental evidence to support them as atomic theory, and make even better predictions. This has implications in the "real world." If we base our definition on what is "intuitive" instead of what is provable, we end up with things like humour theory in medicine.
- codeulike 6y ago
  I'm not saying we should reject anything if it isn't intuitive. I'm just saying the statement Is Spacetime a real thing like an atom, or just a mathematical construct that is used to describe how mass 'generates' gravity? is not a false dichotomy
thisiscorrect 6y ago
"But Atoms are conceivable - they work as a model, but we can also imagine how to fit into the reality that we experience." "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.

Voloskaya 6y ago

I disagree with you. Your argument could be made about anything, there is no such thing as a "human" either, it's just a big bunch of wave-particles that does not have some clear limits. Yet when we talk about a human, we all know what that is, and when a human is seating on a chair we don't think the chair is part of it. 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.

AnIdiotOnTheNet 6y ago
> 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? 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.

drewcoo 6y ago

A scientific theory is a body of knowledge. The special use of the term seems like a false monocotomy.

f154hfds 6y ago

I think you and the author are talking about different concepts. To your credit, for the author's 'real thing like an atom' here, the term real is ambiguous. 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. [1]: https://en.wikipedia.org/wiki/Three-body_problem https://en.wikipedia.org/wiki/Three-body_problem