Questions about Spacetime

[Assassin in Burgundy]

Hey, is anyone out there an astronomer or something like it? Because I've been looking up what spacetime is, but haven't gotten a clear answer. Here's what I have so far:

Imagine the universe as flat. You lie all the planets, stars, and particles onto the surface. Depending on the gravity strength of the object, it will create ripples. Black holes will cause tears. Right now, I'm wondering if this is correct, what that fabric is, and what a flat universe would look like. If you know anything about this, please respond because I haven't gotten anywhere on other sites. 

[soyperson]

Ohh dear. Gettin' into the confusing (or as my biology teacher puts it, cornfusing) stuff. 

Well, people, let's learn some stuff. 

[Glamdring804]

Wow. This is a big scale question. I'm by no means an expert on relativity, but I do have a working knowledge.

The two dimensional model of space-time is a bit of an over-simplification. The 'fabric' is not exactly a fabric, but a four dimensional continuum. And even that's not quite right. As for black holes, well, they don't exactly "tear" space-time, rather, they create an infinitely deep pit. And as for a flat universe, well, as far as we can tell, we are living in a flat universe. We've found no evidence of curvature on a large scale, and the distortions caused by stars and planets are rather small and don't really contribute on the larger scale of things.

[Tsidqiyah]

@Assassin in Burgundy I will need to reach out to my brothers for the details (one got his PhD in  Math for Celestial-Mechanics and ADVANCED number theory and the other is working toward a PhD in physics.)  

But the very basic idea is that spacetime is a fourth-dimensional model that combines height, width, length and time. this model has simplified a lot, if not everything, about astrophysics since it was proposed. In general, think back to graphing equations: One dimensional is a number-line; Two is a X,Y coordinate grid; Three would be an X,Y,Z coordinate box, while Four (spacetime) would be X,Y,Z,T system where every "event" (i.e. planet, star etc) has it's own unique position and the system is the concatenate of ALL possible events..

The idea that gravity creates "wells" within the "fabric" of spacetime comes from a lot lot of recent math development starting with the Theory of Relativity. GPS uses this math BTW. The idea that black-holes tear holes in the "fabric" is a Hollywoodism from the fact that EVERYTHING we understand about physics becomes meaningless past the event horizon. Infinite deep pit is a good description. In general space time is modeled to be smooth and continuous. The last model of the universe I saw has it as a torus, or donut shaped,  but this is due to mathematical extrapolations  not physical observation. 

[Yezrien]

I think the simplest answer, in terms of general relativity, is that space and time are two sides of the same coin. When you distort one, you also distort the other. A large mass (like a planet, star, black hole, etc.) warps both space and time around itself. The space warp is basically gravity, and the time warp is time dilation -- time slows down as you approach the center of gravity. Time is moving slightly slower here on Earth than it is in the vacuum of space.

A black hole has way more mass than Earth does, so its effects on both space and time are more pronounced. If you get close enough, the gravity is so intense that it's physically impossible to escape (even for a beam of light). And time practically slows to a complete stop.    

Edited January 27, 2017 by Belzedar

[Erunion]

Just to add to what others are saying. 

Time is just another dimension. Up/down, forward/back, left/right. Those are the three dimensions you can manipulate pretty easily. The fourth dimension, time, you travel through at a near-constant rate of 1 second per second. 

When trying to understand space/time, you just have to plot the fourth dimension, time. We can't really visualize that though, so we make a simplifying assumption that space is 'flat', with time being the z-axis (so where the flat surface changes Z-height then time changes). 

We can then use that simplification for calculations and modelling. 

 

(AFAIK, space isn't actually flat - it's 3D, and roughly spherical. However, for the sake of simplification, this model assumes the 3rd physical dimension to be irrelevant and plots time on that axis). 

 

This is all very rough. Someone who knows more, feel free to correct me. 

 

Fun Fact: The Christian God (and likely many other religions' gods') is considered to exist outside of space-time, and is supposed to be a being who can 'see' and interact with time the same way we can see or interact with distance; hence His omniscience/omnipresence. (This is a fairly old conception too, I know Lewis held it, and I believed he was referencing Augustine, but I could be wrong there). 

Why is this fact fun? 

Because as this is a Sanderson forum, it really makes you wonder what happens when you interact with the Spiritual Realm. 

[Assassin in Burgundy]

@Glamdring804 @Tsidqiyah @Erunion Thanks for clarifying. For Glamdring, how would space be flat? I'm not contradicting, but trying to figure out how. Like, what happens if you go too far off the charts. Would you, flat earth style, fall into an abyss?

[Erunion]

28 minutes ago, Assassin in Burgundy said:

@Glamdring804 @Tsidqiyah @Erunion Thanks for clarifying. For Glamdring, how would space be flat? I'm not contradicting, but trying to figure out how. Like, what happens if you go too far off the charts. Would you, flat earth style, fall into an abyss?

AFAIK it's more of a simplifying concept. So Space isn't really flat. But you can compress 3D space into a 2D model in order to use Time as the 3rd dimension, so you can visualize what things are like. 

[Glamdring804]

2 hours ago, Assassin in Burgundy said:

@Glamdring804 @Tsidqiyah @Erunion Thanks for clarifying. For Glamdring, how would space be flat? I'm not contradicting, but trying to figure out how. Like, what happens if you go too far off the charts. Would you, flat earth style, fall into an abyss?

Well that is actually a very good question that cosmologists would love to know the answer to. Before I continue, let me clarify something. When I say "space is flat," What I am referring to is a large scale curving of all three dimensions of space. I don't mean that space is actually a two dimensional sheet, I'm discussing weather the universe as a whole has a larger shape and structure to it. I am also ignoring distortions made by massive objects, i.e. gravity wells, because they are just blips in the large scheme of things.

We don't really know what the universe is like beyond the region within which light has had enough time to reach us, since the beginning of the universe 14 billion years ago. We do not know if the universe is finite or infinite. Does it go on forever, or does if have discernible edges somewhere out there, that we could fall off of, as you put it? Or maybe spacetime is finite in volume, and instead of having edges, it loops back on itself, like the edges of a Pacman map. These are questions we don't have the tools to answer, and probably won't for thousands of years, if ever. It's still fun to speculate, though.

4 hours ago, Erunion said:

The fourth dimension, time, you travel through at a near-constant rate of 1 second per second.

To a point. It's actually somewhat more complicated than that. The working mechanics of relativity say that we are actually moving through the four dimensions of spacetime at a constant rate: the speed of light. If we are sitting still, away from anything with strong gravity, then 100 percent of our movement is in the direction of time. We experience time at one second per second. However, if we begin moving at hi speeds, we direct more and more of our speed towards spatial movement, and our progress through time begins to decrease. As we approach the speed of light, nearly all of our motion is through the spatial dimensions, and we have practically stopped moving through time. From an outside perspective, time slows nearly to a crawl for us.

[Erunion]

@Glamdring804 - great explanation, and especially the clarification on my point! 

 

While I knew higher velocity = slower time, is it then because we are moving constantly at the speed of light? So any physical movement will slow down our movement through time (as our actual velocity in space time would be constant, c?). 

That explanation would make a lot of sense. 

And has this corollary: were I to leave planet earth and the solar system, and somehow make my spatial velocity == 0 (with an arbitrarily defined centre of the universe as my absolute reference point), then I would experience time at its highest possible velocity? 

Obviously, this wouldn't significantly change my 'time velocity' as our planet/solar systems velocity is insignificant w.r.t c, but it would make the wait for Doors of Stone & Winds of Winter feel a little faster. 

[Assassin in Burgundy]

So @Glamdring804, the universe extends in the first three dimensions, up/down, left/right, and forward/backward. So it'll be a roughly spherical shape? Sorry if I'm misunderstanding. 

[Briar King]

Yes if the Big Bang is fact it's going outward in all directions 

[Elenion]

2 hours ago, Erunion said:

While I knew higher velocity = slower time, is it then because we are moving constantly at the speed of light? So any physical movement will slow down our movement through time (as our actual velocity in space time would be constant, c?). 

That explanation would make a lot of sense. 

And has this corollary: were I to leave planet earth and the solar system, and somehow make my spatial velocity == 0 (with an arbitrarily defined centre of the universe as my absolute reference point), then I would experience time at its highest possible velocity? 

Indeed. To visualize this, I think of the relationship between space and time is like a circle in the coordinate plane centered on the origin with a radius of c, where the x-axis is velocity and the y-axis is time. When you have absolutely 0 velocity through space (x), you would have the greatest speed through time (y) at exactly c. When you add a little velocity through space, you shift your point to the right along the circle, but in doing so you move slightly away from maximum speed through time. The more towards velocity through space you go, the more you affect your speed through time, until when your velocity through space becomes infinitely close to c, your speed through time becomes infinitely close to 0.

Hope that helps, but it might just come out mumbo-jumbo-y. It's hard to explain, but it makes sense to me.

[Erunion]

1 hour ago, Elenion said:

Indeed. To visualize this, I think of the relationship between space and time is like a circle in the coordinate plane centered on the origin with a radius of c, where the x-axis is velocity and the y-axis is time. When you have absolutely 0 velocity through space (x), you would have the greatest speed through time (y) at exactly c. When you add a little velocity through space, you shift your point to the right along the circle, but in doing so you move slightly away from maximum speed through time. The more towards velocity through space you go, the more you affect your speed through time, until when your velocity through space becomes infinitely close to c, your speed through time becomes infinitely close to 0.

Hope that helps, but it might just come out mumbo-jumbo-y. It's hard to explain, but it makes sense to me.

No, that's exactly how I thought of it. Well explained. 

 

 

Plus, by now all those who aren't hardcore nerds have already fled. 

[Glamdring804]

13 hours ago, Assassin in Burgundy said:

So @Glamdring804, the universe extends in the first three dimensions, up/down, left/right, and forward/backward. So it'll be a roughly spherical shape? Sorry if I'm misunderstanding. 

We don't know. The visible universe, the region of space and time that we can see, is indeed spherical. Light has had 14 billion years to reach us, so the very farthest edges of the region were 14 billion light-years away when they emitted the light we see today. Since then, the expansion of the universe has pushed those regions to a distance of about 46 billion light-years. So the current visible universe is a sphere 93 billion light-years across, with Earth at the center. We don't know if this is the actual center of the universe (It's probably not) and we don't know what, if anything exists beyond this limit. Maybe it's a sphere, maybe it's a tetrahedron, or maybe its a doughnut. Each of those scenarios works with the math. We don't know what the actual shape is simply because the universe isn't old enough for the information to reach us, and it probably never will.

9 hours ago, Erunion said:

Plus, by now all those who aren't hardcore nerds have already fled. 

This is a badge I wear with pride.

[Assassin in Burgundy]

1 hour ago, Glamdring804 said:

We don't know. The visible universe, the region of space and time that we can see, is indeed spherical. Light has had 14 billion years to reach us, so the very farthest edges of the region were 14 billion light-years away when they emitted the light we see today. Since then, the expansion of the universe has pushed those regions to a distance of about 46 billion light-years. So the current visible universe is a sphere 93 billion light-years across, with Earth at the center. We don't know if this is the actual center of the universe (It's probably not) and we don't know what, if anything exists beyond this limit. Maybe it's a sphere, maybe it's a tetrahedron, or maybe its a doughnut. Each of those scenarios works with the math. We don't know what the actual shape is simply because the universe isn't old enough for the information to reach us, and it probably never will.

This is a badge I wear with pride.

Ok, thanks. When you said "flat universe," I was thinking of laying all planets and stars and everything on a flat surface or something. 

[Briar King]

I certainly hope my fellow geeks here watch How The Universe Works narrated by Mike Row on Sci chan! I think it's on S7 or 8 currently. Always good to watch even though they include some of the crazy doomsday stuff in a few epi. You can learn a lot just watching it.

[kenod]

Here's a good playlist on the subject of general relativity and curved space-time:

The channel also has a lot of other videos on the subject of science, like quantum mechanics.

[Assassin in Burgundy]

12 hours ago, kenod said:

Here's a good playlist on the subject of general relativity and curved space-time:

The channel also has a lot of other videos on the subject of science, like quantum mechanics.

I really wished I hadn't watched that. I stayed up way to late watching all his videos and this entire day has just been exhausting. Thank you. 

[Briar King]

Tonight's ep of How Uni Works will be about Dark Matter. Should be an interesting watch. It's suspected to be time shape ing force for our Uni.Dark Matter/Energy is a bit weird...