Spacetime, quantum field, and aether

[Mohd Abdullah]

Hey guys,

What is the difference between spacetime, quantum field and aether? For a beginner, maybe these terms seems similar because these terms indicating "something" that which has no shape. But what are the differences exactly between these?

 

[hilbert2]

Spacetime is the set of all points that have t, x, y and z coordinates. A field is a function that assigns some (scalar, tensor, spinor..) value for every spacetime point. Aether is a hypothetical material medium that fills the whole universe, but almost no one believes in it anymore because we now know that light doesn't need any substance to propagate in.

 

[Vanadium 50]

You can't start physics in the middle. If you want to understand advanced topics, you need to understand things like force, power and how they differ first.

 

[robining]

Conceptually, if spacetime distorts or curves, demonstrating the property of being something "substantial" (ie. able to have shape that can be distorted) doesn't that make it a substance? How can "nothing" have a shape that can be distorted? In other words, perhaps the early theorists trying to understand the substance of the universe got it wrong, but doesn't spacetime's curvature confirm that there's something (some thing that can be distorted--or that acts like a substance) comprising the "medium that fills the whole universe"?

 

[Nugatory]

Conceptually, if spacetime distorts [doesn't that imply that there is something there to distort]

You'll hear this "distorts" description a lot, even from competent physicists, because there's no other way of describing spacetime curvature with the limitations of the English language. But the truth is in the math not the words, and the conclusion that you're trying to draw here doesn't show up in the math.

We cannot say "distorted" without implicitly assuming that we're dealing with something that has a shape to distort - there's a natural undistorted shape that's been acted on to distort it. But that's not how spacetime works - it just is. When we say that spacetime is "curved" or "distorted", we're really just saying that the distance between nearby points is calculated using rules more complicated than those of Euclidean geometry (where the Pythagorean theorem is all you need). That doesn't mean that spacetime is a distortion of Euclidean geometry, just that it's different.

 

[robining]

You'll hear this "distorts" description a lot, even from competent physicists, because there's no other way of describing spacetime curvature with the limitations of the English language. But the truth is in the math not the words, and the conclusion that you're trying to draw here doesn't show up in the math.

We cannot say "distorted" without implicitly assuming that we're dealing with something that has a shape to distort - there's a natural undistorted shape that's been acted on to distort it. But that's not how spacetime works - it just is. When we say that spacetime is "curved" or "distorted", we're really just saying that the distance between nearby points is calculated using rules more complicated than those of Euclidean geometry (where the Pythagorean theorem is all you need). That doesn't mean that spacetime is a distortion of Euclidean geometry, just that it's different.

I'm not sure that the word "distorts" is central to the conceptual understanding I'm trying to discuss. Yes, we are limited by conventional English as our only means of communicating--and, as with any language, it reflects a certain worldview, a specific understanding of how the world works. That's inescapable. What we can do is use the words we have as best we can to understand what actually is as accurately as possible.

What I'm trying to say is that "substance" is also a word/category. Substance implies "stuff" is there to be shaped. If mass changes the shape of something then, by implication, there's something there to be changed. Certainly, in the image of a ball rolling on a sheet--the interaction between the ball and the "sheet" is what guides the ball in a curving path. If that's in anyway a depiction of what mass does to space, what is the "sheet" (or trampoline or "space") that's holding the ball on it's path? If mass changes something, what is it changing?

I'm saying that Einstein didn't change the idea that there's some "stuff" being affected by mass, he just changed the word from "(a)ether" to "spacetime". Conceptually, there's still "something" capable of having the property of "shape" which, in generally, we would normally categorize as a "substance".

 

[Delta2]

There are some theories that say that spacetime consists of some sort of particles, but we are not allowed to discuss them here according to the rules of this site that allows only discussion on mainstream science.

 

[PeterDonis]

I'm saying that Einstein didn't change the idea that there's some "stuff" being affected by mass, he just changed the word from "(a)ether" to "spacetime". Conceptually, there's still "something" capable of having the property of "shape" which, in generally, we would normally categorize as a "substance".

Even if we accept all of this as correct for the sake of argument, what does it have to do with the question you asked in the OP of this thread? "Spacetime" and "quantum field" could both be "substances" by your definition, but that doesn't make them the same. Nor does it make the term "aether" any more meaningful in terms of modern physics.

The answer @hilbert2 gave you in post #2 is basically correct. Do you have a question about that answer?

 

[Nugatory]

Certainly, in the image of a ball rolling on a sheet--the interaction between the ball and the "sheet" is what guides the ball in a curving path. If that's in anyway a depiction of what mass does to space

It's not. That rubber sheet analogy is terribly misleading, in part because it doesn't represent time at all so is an unworkable as a representation of spacetime, and in part because it suggests that there is sheet to distort. A much more accurate depiction is this video by our member @A.T.:
which shows the apple following its natural path through spacetime except when it is being tugged off course by its stem.

what is the "sheet" (or trampoline or "space") that's holding the ball on it's path? If mass changes something, what is it changing?

There is no sheet, just an apple and the surface of the Earth on a collision course. Nothing is changing (except at the moment when the stem breaks, and then it's the stem that is changing).

 

[robining]

Much appreciated! I'm going to have to watch that video several more times!