B How do space and time fuse together to form “spacetime?”

[Wiredcerebellum]

TL;DR

If neither space nor time have any physical properties of matter underlying them, how can space and time merge to form spacetime?

Here is the definition of spacetime?

“In physics, spacetime is any mathematical model that fuses the three dimensions of space and the one dimension of time into a single four-dimensional continuum.”

But if space is literally the absence of matter or physical properties, and time has no physical properties, how do the two merge to form the 4-dimensional structure called spacetime?

Spacetime can bend and ripple like a physical object so it seems like spacetime should have physical properties of matter underlying it that allow for the bending and rippling, but it doesn’t. So how do space and time “fuse” to form spacetime if neither have physical properties?

 

[Dale]

TL;DR Summary: If neither space nor time have any physical properties of matter underlying them, how can space and time merge to form spacetime?

So how do space and time “fuse” to form spacetime if neither have physical properties?

Who says they don’t have physical properties? You can measure them quite accurately.

 

[Wiredcerebellum]

Who says they don’t have physical properties? You can measure them quite accurately.

What are the physical properties of matter? That’s where I’m lost. Thanks for your help in advance.

 

[DaveC426913]

What are the physical properties of matter? That’s where I’m lost. Thanks for your help in advance.

Well, notably, length, width and height.
In the absence of matter we can measure distances using 3 coordinates, such as x, y and z.

 

[Dale]

What are the physical properties of matter? That’s where I’m lost.

Spscetime has geometrical properties. Lengths, widths, heights, durations, angles, etc.

Matter also has charge, spin, etc.

 

[Wiredcerebellum]

Well, notably, length, width and height.

But what is the length, width, and height measuring? What are the properties of matter underlying it that allow spacetime to bend and ripple? That’s where I’m lost.

 

[ProfuselyQuarky]

What are the physical properties of matter? That’s where I’m lost. Thanks for your help in advance.

What do you think matter even is to ask that question?

 

[Wiredcerebellum]

Well, notably, length, width and height.
In the absence of matter we can measure distances using 3 coordinates, such as x, y and z.

What do you think matter even is to ask that question?

I’m wondering what is bending and rippling when we say spacetime can bend and ripple? Is there matter underlying spacetime that allows it to bend and ripple? When I say matter I’m referring to atoms.

 

[DaveC426913]

What are the properties of matter underlying it that allow spacetime to bend and ripple?

To be clear, spacetime is not really a 'thing', in the sense that it does not actually bend and ripple.

 

[Wiredcerebellum]

To be clear, spacetime is not really a 'thing', in the sense that it does not actually bend and ripple.

Then how are gravitational waves formed if spacetime cannot literally bend and ripple?

 

[Ibix]

So how do space and time “fuse” to form spacetime if neither have physical properties?

How do up/down, left/right and forward/backward "fuse together" to make the 3d space you see around you? I don't think that question really makes sense, and it doesn't get clearer if you add time as a fourth dimension to the mix.

Note that the paragraph you quote is specifically talking about the mathematical model. Historically we have tended to view space as "where stuff happens" and time as some separate thing that kerps everything from happening at once. But relativity turns out to be easier to understand if you think of time as just another dimension, albeit with some slightly different properties. We aren't "fusing" anything in reality - just fitting two parts of our models together more closely than we used to.

You are reading too much into popsci descriptions of physics, in other words.

Spacetime can bend and ripple like a physical object

No it can't, although you do see that kind of thing written. The curvature of spacetime is about how the rules of geometry vary through space and time. It's not really anything like a distorted rubber sheet.

Unfortunately it's really hard to talk about this stuff precisely without post-grad maths. So some physicists try to talk in more casual allegorical terms to try to get their ideas across. But that often just ends up confusing people in new ways...

 

[DaveC426913]

Then how are gravitational waves formed if spacetime cannot literally bend and ripple?

That's why we're looking for the existence of gravitons.

 

[Wiredcerebellum]

That's why wer'e looking for the existence of gravitons.

Ok I understand. So currently there is no matter underlying the rippling and bending of spacetime that we know of? It’s all theoretical?

 

[DaveC426913]

Ok I understand. So currently there is no matter underlying the rippling and bending of spacetime that we know of? It’s all theoretical?

Not 'theoretical'; it's an analogy. It is really the behavior of the geometry of things that move through spacetime. Their paths bend due to applied forces (such as encountering the Earth's surface)

 

[Wiredcerebellum]

Not 'theoretical'; it's an analogy. It is really the behavior of the geometry of things that move through spacetime. Their paths bend.

Ok then…

 

[Ibix]

Then how are gravitational waves formed if spacetime cannot literally bend and ripple?

Time is a part of spacetime, so talking about spacetime changing is already a mistake. All that changes is which "slice" of spacetime you currently think of as "now" - the gravitational waves are already baked in.

 

[DaveC426913]

As Ibix said:

Unfortunately it's really hard to talk about this stuff precisely without post-grad maths. So some physicists try to talk in more casual allegorical terms to try to get their ideas across. But that often just ends up confusing people in new ways...

The nature of things IS the math that describes them. To try to frame fundamental things in terms of what we intuitively know means inevitably that we use simplified, flawed analogies.

 

[Wiredcerebellum]

Time is a part of spacetime, so talking about spacetime changing is already a mistake. All that changes is which "slice" of spacetime you currently think of as "now" - the gravitational waves are already baked in.

Well alright then.

 

[Wiredcerebellum]

Law o

As Ibix said:The nature of things IS the math that describes them. To try to frame fundamental things in terms of what we intuitively know means inevitably that we use simplified, flawed analogies.

What about the law of parsimony?

 

[DaveC426913]

What about it?

 

[Wiredcerebellum]

Time is a part of spacetime, so talking about spacetime changing is already a mistake. All that changes is which "slice" of spacetime you currently think of as "now" - the gravitational waves are already baked in.

Alright.

 

[Wiredcerebellum]

What about it?

You shouldn’t need advanced math to explain how spacetime works.

 

[Ibix]

What about the law of parsimony?

GR is pretty parsimonious. At its core is literally one equation plus a couple of "energy conditions" that help to define what we call reasonable solutions.

Unfortunately it doesn't map very well into everyday language, and you can write very long textbooks on the topic.

 

[Ibix]

You shouldn’t need advanced math to explain how spacetime works.

The entire history of modern physics was kicked off by Newton developing calculus. I'm afraid that the maths has done nothing but get harder to learn since then. We can do analogies and hand-waving all day long, but the only working models of any physical system are mathematical ones.

Spacetime is the background on which physics happens. General relativity makes the geometry of that background depend on what's in it, whereas most other theories just assume there's a background there and don't interact with it. That's really all there is to know at the high level. It isn't really anything except whatever aspect of reality that furnishes notions of dustance, angle,and speed.

 

[Dale]

You shouldn’t need advanced math to explain how spacetime works.

Why not?

 

[Nugatory]

I’m wondering what is bending and rippling when we say spacetime can bend and ripple? I

Nothing. That "bending and rippling" description isn't right, it's just the best that we can do to describe the theory without using the math needed for a complete and accurate description.

 

[Vanadium 50]

You shouldn’t need advanced math to explain how spacetime works.

Advanced math? Like algebra?

 

[PeterDonis]

if space is literally the absence of matter or physical properties

It isn't.

and time has no physical properties

It does.

Reasoning from false assumptions never works out well.

 

[PeterDonis]

What are the physical properties of matter?

Why is that relevant? You asked about spacetime, not matter. In John Wheeler's famous statement, spacetime tells matter how to move. That is the key physical property that spacetime has.

 

[Dale]

@Wiredcerebellum A table top is physically flat, and the legs are physically perpendicular to the surface. Those words “flat” and “perpendicular” are geometrical words, but they describe a physical object. So geometry is part of physics.

The geometry that most people are used to is called Euclidean geometry. It is captured in this equation, which comes from the Pythagorean theorem: $$ds^2=dx^2+dy^2+dz^2$$ Everything from Euclidean geometry comes from this equation, including things like the interior angles of a triangle sum to 180 degrees. This is the geometry we use to say that the table top is flat and that the legs are perpendicular.

When we say that spacetime is space and time together, what we mean is simply that the geometry of physics is given by $$ds^2=-dt^2+dx^2+dy^2+dz^2$$ You cannot accurately describe the geometry of physics without including time as well as space.

This in no way implies that spacetime is matter. Nor does it imply that spacetime has no properties. That is a false dichotomy.