It depends what answer you're looking for! We can tell you how photons behave, how to recognize them, and how to predict what they do. For hard philosophy questions, that's more under sections 100-119 of the Dewey Decimal than something physics aims to answer
>what medium is present in a vacuum that allows things like gravity to propagate
That's a little like asking who is in charge of pushing the sun around the Earth. There's a presupposition in that question, you should first ask why you expect there to be a medium, besides nondescript 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.
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?
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!
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.
but then, according to the Uncertainty principle, it has a less particular momentum..
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
Hope I cleared that all up. Don't ask any follow up questions.
IMO, this desire to "simplify" something that cannot really be simplified is why so much pop sci around quantum stuff is just nonsense. Even normal words mean entirely different things in the realm of quantum physics. The layman doesn't understand what quantum computing does because the layman doesn't even have a knowledge of calculus, and all the intuition about the behavior of mathematical systems that comes with it, let alone the kind of linear algebra, discrete math, and higher level math like fields.
People need to stop pretending that the most advanced mathematical constructs humankind has conceived, standing on ten thousand years of the smartest minds we have ever birthed, is something that John Q Public can expect to understand.
It's no different than me being unable to discuss influences on Picaso's work because I have never taken an art history class. Sure, I could parrot some dumbed down thought someone else feeds me, but I emphatically do not have the context and surrounding knowledge to extract any meaning from that.
Photons are merely quantized ripples in the electromagnetic field (according to quantum mechanics, the momentum and energy in these ripples come in discrete chunks).
It's instructive to wonder what could possibly be a simpler ontology. Perhaps one imagines a "medium" like that of sound, consisting of little atoms of some sort spread throughout spacetime. But then immediately this would beg way more questions: what are the atoms themselves made out of? When neighboring "atoms" are displaced why and how do they affect a given "atom" in order to propagate disturbances? What other properties might these atoms have and what would such a word as "properties" even mean since they are supposed to be the elemental constituents of light/gravity itself?
According to modern physics, none of this: just 4 + 10 = 14 numbers, no substructure whatsoever.
(Of course, there are other fields too, like matter fields and gluon fields and one a necessary part of the whole story is a declaration of what these numbers look like according to different observers, but these features are just elaborations on the basic picture of a collection of numbers attached to events.)
Also quantum field theory seems be just plain bad at explaining classical phenomena, which dominate general relativity. To me at least it seems quite difficult to describe a basic electromagnetic from the QFT, other than pointing at the Lagrangian in the path integral and saying something about how contributions outside its extremum will cancel out. Though perhaps I'm simply missing some trick here.
The way to deal with that is to use mathematics. Once you yield to its power and give up trying to "understand" particle physics, things are a lot simpler (for certain values of "simple" ofc!)
They may turn out to be right, but you have to admit they sound arbitrarily made-up for lack of a better understanding.
Well, yeah. Dark matter and dark energy are the bug ticket names, so to speak. Thousands of physicists are coming up with ways to "look harder."
Dark matter and dark energy are just placeholder names for something we don't understand and don't know what it is. We didn't make them up. We just observe the effects and conclude that there must be something there that is making the universe behave the way it does.
Mostly, but they have a lot of behaviors attached to them at this point due to various measurements and accompanying assumptions. Not to mention that labeling the sets of interactions each exhibit with a single name creates the appearance to the public that they are created by a single source. Dark Matter could be several different things interacting, likewise for Dark Energy.
But you have to name them something to talk about them. There probably would have been better names, but physicists are no better at naming than programmers