4 ms·

Isn't a "medium" necessary to establish some kind of causality? Medium just means the means by which objects interact with each other.

BobbyJo 3y ago
Not to mention that there is vacuum energy, which to me (total layman) would indicate that not only is there a medium, but it also has its own influences on the objects within it (like generating Hawking radiation from black hole horizons).
tux3 3y ago
There's different ways to interpret what people expect there to be by an extra medium on top of the vacuum. We can observe and describe stuff interacting. The stuff is positioned in space. We can see that some stuff takes up space, and it can move to take up some other space. What we describe interacting is the stuff itself, the space around it is where it is. Throw an apple through the air, and you can watch it sail through that medium. Remove the air, put it in vacuum, the apple is still able to move around despite the lack of anything meta or extra besides the apple and where the apple is. The story is similar with light, gravity, other phenomenon. Things have some particular location in space, and that location can evolve with time. You can still try to define the word, pick something within that description and call it a medium if you like, but it's not fundamentally necessary. And it won't be something "present in a vacuum that allows things like gravity to propagate". Things propagates on their own, their location within space simply evolves.
- cheschire 3y ago
  Would spacetime be the medium? Or is spacetime just the language to describe the medium? Again, presupposing there must be a medium.
- lacy_tinpot 3y ago
  I think the confusion here is "in space", maybe you can expand on that. Is space itself not the medium? Localization is precisely what prevents backwards causality and is at the basis of relativity, where spacetime is invariant. And from what I understand that's also the source of conflict in the quantum world where that's not necessarily the case, retrocausality is entirely possible, spooky interactions at a distance are entirely possible. Like the problem of gravity is described by the curvature of spacetime. Does spacetime "exist"? If not it's just a tool, and not an actual observation that maps onto reality. In other words we don't know how gravity actually "works" and interacts or what "spacetime" is, if it is a thing, regardless of how powerful the description is. The description is just a mathematical tool that is very useful, but that's just about it. "You can still try to define the word, pick something within that description and call it a medium if you like" Yes... that's how language works. "Spacetime" is mapped onto something. Language is a medium of communication. Spacetime is a medium of interaction/causality. Whether such things "exist" is an epistemic discussion or a discussion for experimentalists. But any medium is just the means by which something occurs or happens. So what is the means by which gravity "happens", or exerts influence on other bodies if not the curvature of "spacetime", which itself hasn't really been established. Does gravity just magically happen?
  - tux3 3y ago
    >I think the confusion here is "in space", maybe you can expand on that. Is space itself not the medium? I think using that definition is fine! I tried to talk about things positioned in space directly instead of using a notion of medium because that word comes with a lot of imagery, and it can make discussion difficult, it's easy to talk past each other when we start using words that don't have a very well settled definition I was originally responding to a formulation where the medium is defined as something "present in a vacuum" that allows propagation of things. This is where imprecise language creates trouble, and why I don't like that word. You can say space is the medium, but you can't say space is a thing present in the vacuum that allows propagation. Vacuum is located in space, there's not a very meaningful sense in which space is a medium found within vacuum that helps things propagate, or at least that definition of the word is not very helpful to my understanding of the world For gravity, I'm okay with calling spacetime the medium, as long as you're careful to not imagine that there's some more ethereal thing within the vacuum that physics doesn't say anything about >But any medium is just the means by which something occurs or happens That's fine. That's not an invalid definition. I just think it's important to separate the concept of a physical medium from more philosophical notions that everything happens in a medium, even if that sometimes means defining the word medium as an immaterial concept whose existence makes people question the definition of the word "exist". In response to the original post, that definition of a medium inside the vacuum is something that doesn't exist. With your definition of space as the medium for gravity, you can make that work, as long as you can communicate what you really mean to other people when using that particular word!
    - denton-scratch 3y ago
      > For gravity, I'm okay with calling spacetime the medium I thought spacetime was supposed to be identical with the gravitational field. It can't be gravity's "medium" if that medium is itself gravity. I find this extremely hard to think about; gravity both generates spacetime, and warps it. It's really hard to escape from the earthman's experience of space as a sort of instantaneous volume; we don't really experience time at all, except in the sense that we have a sense of certain events as being not the present (i.e. the past). We know that time exists in some sense; but all we experience of spacetime is the space bit.
- Chris2048 3y ago
  > Things have some particular location in space but then, according to the Uncertainty principle, it has a less particular momentum..
godojo 3y ago
String Theory tries to say that.
roywiggins 3y ago
In quantum field theory, space is just full of various fields that mediate interactions.
joquarky 3y ago
Sabine Hossenfelder has a video which may be helpful [0]. She discusses nine levels of what we could call "nothing" as in "there is nothing present in this container". It may be useful since she discusses the various substrates that are still present even in what we might normally think of as a "vacuum" [0]: What is "Nothing"? https://youtu.be/PhfqdBk8qxk https://youtu.be/PhfqdBk8qxk
layer8 3y ago
There is no causality in the fundamental ontology of physics. There are only physical laws (which incidentally are time-reversible), and what they imply logically. Causality is an emergent phenomenon, or an interpretation we apply at a higher level.

BobbyJo 3y ago

Not to mention that there is vacuum energy, which to me (total layman) would indicate that not only is there a medium, but it also has its own influences on the objects within it (like generating Hawking radiation from black hole horizons).

tux3 3y ago

There's different ways to interpret what people expect there to be by an extra medium on top of the vacuum. We can observe and describe stuff interacting. The stuff is positioned in space. We can see that some stuff takes up space, and it can move to take up some other space. What we describe interacting is the stuff itself, the space around it is where it is. Throw an apple through the air, and you can watch it sail through that medium. Remove the air, put it in vacuum, the apple is still able to move around despite the lack of anything meta or extra besides the apple and where the apple is. The story is similar with light, gravity, other phenomenon. Things have some particular location in space, and that location can evolve with time. You can still try to define the word, pick something within that description and call it a medium if you like, but it's not fundamentally necessary. And it won't be something "present in a vacuum that allows things like gravity to propagate". Things propagates on their own, their location within space simply evolves.

cheschire 3y ago
Would spacetime be the medium? Or is spacetime just the language to describe the medium? Again, presupposing there must be a medium.
lacy_tinpot 3y ago
I think the confusion here is "in space", maybe you can expand on that. Is space itself not the medium? Localization is precisely what prevents backwards causality and is at the basis of relativity, where spacetime is invariant. And from what I understand that's also the source of conflict in the quantum world where that's not necessarily the case, retrocausality is entirely possible, spooky interactions at a distance are entirely possible. Like the problem of gravity is described by the curvature of spacetime. Does spacetime "exist"? If not it's just a tool, and not an actual observation that maps onto reality. In other words we don't know how gravity actually "works" and interacts or what "spacetime" is, if it is a thing, regardless of how powerful the description is. The description is just a mathematical tool that is very useful, but that's just about it. "You can still try to define the word, pick something within that description and call it a medium if you like" Yes... that's how language works. "Spacetime" is mapped onto something. Language is a medium of communication. Spacetime is a medium of interaction/causality. Whether such things "exist" is an epistemic discussion or a discussion for experimentalists. But any medium is just the means by which something occurs or happens. So what is the means by which gravity "happens", or exerts influence on other bodies if not the curvature of "spacetime", which itself hasn't really been established. Does gravity just magically happen?
- tux3 3y ago
  >I think the confusion here is "in space", maybe you can expand on that. Is space itself not the medium? I think using that definition is fine! I tried to talk about things positioned in space directly instead of using a notion of medium because that word comes with a lot of imagery, and it can make discussion difficult, it's easy to talk past each other when we start using words that don't have a very well settled definition I was originally responding to a formulation where the medium is defined as something "present in a vacuum" that allows propagation of things. This is where imprecise language creates trouble, and why I don't like that word. You can say space is the medium, but you can't say space is a thing present in the vacuum that allows propagation. Vacuum is located in space, there's not a very meaningful sense in which space is a medium found within vacuum that helps things propagate, or at least that definition of the word is not very helpful to my understanding of the world For gravity, I'm okay with calling spacetime the medium, as long as you're careful to not imagine that there's some more ethereal thing within the vacuum that physics doesn't say anything about >But any medium is just the means by which something occurs or happens That's fine. That's not an invalid definition. I just think it's important to separate the concept of a physical medium from more philosophical notions that everything happens in a medium, even if that sometimes means defining the word medium as an immaterial concept whose existence makes people question the definition of the word "exist". In response to the original post, that definition of a medium inside the vacuum is something that doesn't exist. With your definition of space as the medium for gravity, you can make that work, as long as you can communicate what you really mean to other people when using that particular word!
  - denton-scratch 3y ago
    > For gravity, I'm okay with calling spacetime the medium I thought spacetime was supposed to be identical with the gravitational field. It can't be gravity's "medium" if that medium is itself gravity. I find this extremely hard to think about; gravity both generates spacetime, and warps it. It's really hard to escape from the earthman's experience of space as a sort of instantaneous volume; we don't really experience time at all, except in the sense that we have a sense of certain events as being not the present (i.e. the past). We know that time exists in some sense; but all we experience of spacetime is the space bit.
Chris2048 3y ago
> Things have some particular location in space but then, according to the Uncertainty principle, it has a less particular momentum..

godojo 3y ago

String Theory tries to say that.

roywiggins 3y ago

In quantum field theory, space is just full of various fields that mediate interactions.

joquarky 3y ago

Sabine Hossenfelder has a video which may be helpful [0]. She discusses nine levels of what we could call "nothing" as in "there is nothing present in this container". It may be useful since she discusses the various substrates that are still present even in what we might normally think of as a "vacuum" [0]: What is "Nothing"? https://youtu.be/PhfqdBk8qxk https://youtu.be/PhfqdBk8qxk

layer8 3y ago

There is no causality in the fundamental ontology of physics. There are only physical laws (which incidentally are time-reversible), and what they imply logically. Causality is an emergent phenomenon, or an interpretation we apply at a higher level.