• MindForged
    703
    Right, spacetime a real concept, just like unicorn is. The fact that it's extremely useful separates it from the concept of a unicorn, which is not so useful. However, this just places it more like the concept of Santa Clause, or the perfect circle, a very useful concept.Metaphysician Undercover

    False, spacetime is real as in it's part of the model of physical reality as understood by both QM and Relativity. It's not merely extremely useful, the fact that it can be distorted by matter is a fundamental finding in modern physics. Ideologically-motivated rejection of this, a century later, is ridiculous.

    No it doesn't it just gives us the means for modelling the effects of gravity. General relativity gives us no understanding of gravity itself, none at all. If it did, it could point us to the graviton.Metaphysician Undercover

    Whether or not a graviton exists isn't even understood. There are numerous problems in trying to even add the graviton to the Standard Model of physics (namely the current inability to renormalize certain results at certain levels of energy). Quantum field theory can explain gravity in terms of particle exchange but this doesn't work out at Planck scales so it's obviously not workable currently. GR's understanding of gravity is that it's not a force in the usual sense (just a feature of space in the presence of matter), so no gravitons are needed to explain what we observe at large scales. Is there more to learn? Sure, but that doesn't make the current models inaccurate, just incomplete. That's a non sequitur.

    I've talked to many physicists, and your claims, that space-time is more than just a conceptual tool, is just not consistent with what these physicists tell me. You're just taking an extremely speculative metaphysical proposition, and claiming that physicists believe this proposition. Maybe some do.Metaphysician Undercover

    "I've talked to many" (likely just a few) is the textbook example of an anecdote. What I'm taking is the standard model and understanding of gravity and spacetime.
  • TheMadFool
    2.7k
    In the Wheeler-DeWitt model (for example) time is absent, because the universe as a whole is at rest. This is because the universal wavefunction is in an eigenstate of its Hamiltonian. This in turn is necessitated because otherwise physical quantities would depend on the unphysical c-number parameter t.Inis

    Like I suspected but could be mistaken, change is essential for the concept of time. So, the universe at rest isn't changing and ergo, no time.

    What of this wavefunction? I guess it's a theoretical wavefunction and having no physical counterpart, fails to provide the tick-tock of a clock.

    How does one define a wavefunction of such kind? A wave must have, if I remember correctly, at least 2 dimensions. What are they? Thanks.
  • Inis
    170
    Like I suspected but could be mistaken, change is essential for the concept of time. So, the universe at rest isn't changing and ergo, no time.TheMadFool

    The universe is completely stationary, just like the block-universe of general relativity, except this one (probably best to give it a more appropriate name - the multiverse) is much bigger.

    What of this wavefunction? I guess it's a theoretical wavefunction and having no physical counterpart, fails to provide the tick-tock of a clock.TheMadFool

    Maybe I should have just used the word "state" or "quantum state", but in realist theories, the wavefunction is a mathematical object that is in one-to-one correspondence with reality, and evolves in accordance with reality. Fortunately in this particular case it doesn't evolve.
  • Metaphysician Undercover
    5k
    Infinite divisibility is a red herring. Continuous motion through space-time is the fundamental reality.aletheist

    What does "continuous motion" mean other than that the intervals of time and distances are infinitely divisible?

    False, spacetime is real as in it's part of the model of physical reality as understood by both QM and Relativity.MindForged

    Yeah sure, it's part of the model, we've been through this already. The model is conceptual, so it's real in the same sense that models are real, and concepts are real. Are you familiar with the analogy of the map and the terrain? Spacetime is part of the map.

    Whether or not a graviton exists isn't even understood.MindForged

    Right, see why I say that your claim that general relativity provides us with an understanding of gravity is false? If general relativity provided us with an understanding of gravity we wouldn't have to question whether or not a graviton exists.

    (just a feature of space in the presence of matter)MindForged

    Right, so space-time is different where there is matter from where there is no matter. That's what I meant, instead of the concept accounting for the existence of gravity, the concept changes to account for gravity. That's like saying that I have a concept of the boiling of water, "water boils at 100 degrees Celsius". But for some reason water on top of a mountain boils at a different temperature. Instead of producing a new concept of the boiling of water, employing pressure as well temperature,, I just adjust the concept, to say that water boils at 100 degrees Celsius, with some exceptions for elevation. There are exceptions to the concept of space-time to allow for differences in the presence of matter, but space-time, as a concept, does not incorporate within itself, an understanding of gravity.
  • aletheist
    984
    What does "continuous motion" mean other than that the intervals of time and distances are infinitely divisible?Metaphysician Undercover
    Infinite divisibility is an insufficient criterion for continuity. After all, the rational numbers are infinitely divisible--thus serving as the basis for Zeno's paradoxes--but no one takes them to be truly continuous. I now find magnification to be a more perspicuous illustration--no matter how much we were to "zoom in" on continuous space-time, we would only ever "see" continuous space-time--never discrete point-instants.

    For various purposes, we arbitrarily mark points and instants, and then measure the distances and intervals of time between them by comparing them with arbitrarily established standard units. There is no unit of distance or time embedded in the universe itself, only velocity--the speed of light--consistent with my contention that continuous motion is the more fundamental reality.

    Are you familiar with the analogy of the map and the terrain? Spacetime is part of the map.Metaphysician Undercover
    No, space-time itself is the terrain, and mathematical models of it are the map. The latter can be incorrect precisely because they purport to represent something real--that which is as it is regardless of what any individual mind or finite group of minds thinks about it.
  • Metaphysician Undercover
    5k
    Infinite divisibility is an insufficient criterion for continuity. After all, the rational numbers are infinitely divisible--thus serving as the basis for Zeno's paradoxes--but no one takes them to be truly continuous. I now find magnification to be a more perspicuous illustration--no matter how much we were to "zoom in" on continuous space-time, we would only ever "see" continuous space-time--never discrete point-instants.aletheist

    Isn't zooming in basically the same thing as dividing? Anyway, space-time is completely conceptual, there is no such thing as zooming in on it. If you were zooming in on something, it would be an object, like a molecule or an atom or something like that. You couldn't even produce an absolute vacuum, and try to zoom in on it, whatever that would mean, because the absolute vacuum is purely conceptual. The idea of zooming in on an absolute vacuum is nonsensical, and the idea of zooming in on space-time is even more nonsensical (if that makes any sense) than the idea of zooming in on an absolute vacuum.

    No, space-time itself is the terrain, and mathematical models of it are the map.aletheist

    Space-time is a mathematical model, just like a triangle is a mathematical model. I don't see where you think you might find this thing called space-time other than in the minds of physicists.
  • aletheist
    984

    Conflating reality and existence, as usual. Cheers.
  • Pattern-chaser
    720
    The real is that which is as it is regardless of what any individual mind or finite group of minds thinks about it. The actual (or existent) is that which reacts with other like things in the environment. Hence reality and actuality are not coextensive--besides real actualities (e.g., individual events), there are also real possibilities (e.g., qualities) and real conditional necessities (e.g., laws of nature) that cannot be reduced to collections of their actual instantiations.aletheist

    I have long argued that Harry Potter and Sherlock Holmes are as real as Donald Trump and Barack Obama, but not in the same sense/way, of course. I think you're saying the same here? :chin:
  • aletheist
    984

    What J. K. Rowling and Arthur Conan Doyle have written about Harry Potter and Sherlock Holmes, respectively, is real--their books (and all derivative literature, movies, etc.) are as they are regardless of what anyone thinks about them. However, Harry Potter and Sherlock Holmes are not themselves real, they are fictional--they only are as they are because, and to the extent that, their creators (and others) have thought about them.
  • Pattern-chaser
    720
    :up: There is also a sense in which they are more real than simply being the written thoughts of Rowling and Conan-Doyle, though. Harry's and Sherlock's words and actions - though they may be fictional - have influenced many people just as real humans - like Obama or Trump - might do. This influence they share with real people, even though they are not real. For some purposes - not rigid or demanding ones like philosophy or science :wink: - they can be considered the same as real people.... :smile:
  • aletheist
    984

    As I see it, Potter and Holmes have not actually influenced anyone as real agents; those effects are more properly attributed to Rowling and Doyle, along with the creators of subsequent derivative works.
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