The outer limits of spacetime

[QuantumT]

It is something I have always wondered about, since I was a child. What is there beyond spacetime? Back then I imagined a wall of rock, but I was only a kid, inspired by 70's sci-fi, so there you go...

But what happens if a hypothetical spacecraft goes to the outer limits of space? Does it just stop or bounce back, because it is a part of spacetime and can't go beyond? Or does it continue into an endless empty void?

Obviously nobody knows. But what does the theories predict?

[beecee]

6 minutes ago, QuantumT said:

It is something I have always wondered about, since I was a child. What is there beyond spacetime? Back then I imagined a wall of rock, but I was only a kid, inspired by 70's sci-fi, so there you go...

But what happens if a hypothetical spacecraft goes to the outer limits of space? Does it just stop or bounce back, because it is a part of spacetime and can't go beyond? Or does it continue into an endless empty void?

Obviously nobody knows. But what does the theories predict?

We don't know if the universe/space/time is either infinite or finite. If it was finite, the topology would be closed, and we would simply arrive back to the point we started from. 

https://www2.lbl.gov/Science-Articles/Archive/SNAP-3.html

[Airbrush]

On 3/21/2019 at 4:18 PM, QuantumT said:

It is something I have always wondered about, since I was a child. What is there beyond spacetime? Back then I imagined a wall of rock, but I was only a kid, inspired by 70's sci-fi, so there you go...

But what happens if a hypothetical spacecraft goes to the outer limits of space? Does it just stop or bounce back, because it is a part of spacetime and can't go beyond? Or does it continue into an endless empty void?

Obviously nobody knows. But what does the theories predict?

I don't know what the theories predict, but if I was the author of a scifi novel about a spaceship speeding outward, in my story I would have the ship going out into "an endless void" because given what I have learned about astronomy and cosmology, that would seem most realistic.  This stuff about "arrive back to the point we started" seems unrealistic.

[Strange]

On 3/21/2019 at 11:18 PM, QuantumT said:

It is something I have always wondered about, since I was a child. What is there beyond spacetime?

There. is no "beyond". 

On 3/21/2019 at 11:18 PM, QuantumT said:

But what happens if a hypothetical spacecraft goes to the outer limits of space? Does it just stop or bounce back, because it is a part of spacetime and can't go beyond? Or does it continue into an endless empty void?

There is (according to current models) no limit or boundary of space. Space is either infinite (so you would just keep going forever) or it is finite but unbounded (so would end up back where you started).

[beecee]

10 minutes ago, Airbrush said:

I don't know what the theories predict, but if I was the author of a scifi novel about a spaceship speeding outward, in my story I would have the ship going out into "an endless void" because given what I have learned about astronomy and cosmology, that would seem most realistic.  This stuff about "arrive back to the point we started" seems unrealistic.

Any supposed void you talk about is actually just space and part of our universe.

The stuff about arriving back to the point we started from, depends on the topology of the universe, which evidence so far tells us is flat. The three possible popular topologies are flat, open and closed, but also other more exotic shapes have been discussed, such as torus and doughnut shaped. 

The following describes some of these possibilities better then I....https://phys.org/news/2015-05-universe.html

[geordief]

31 minutes ago, Strange said:

 

There is (according to current models) no limit or boundary of space. Space is either infinite (so you would just keep going forever) or it is finite but unbounded (so would end up back where you started).

Can space be both finite and continuously expanding?

[swansont]

1 hour ago, geordief said:

Can space be both finite and continuously expanding?

A sphere has a finite surface area, and could be continuously expanding, so yes.

[geordief]

1 hour ago, swansont said:

A sphere has a finite surface area, and could be continuously expanding, so yes.

Is a sphere a mathematical rather than a physical object? (an idealization)

The universe (which is what I was thinking of by "space") cannot be spherical  even allowing for it being unobservable  in places.

So can this universe be both finite and expanding?

Would the expansion itself have to be bounded for this to be possible?

[QuantumT]

In this computer simulated model of our universe, the shape seems almost cubic

 

[Airbrush]

3 hours ago, beecee said:

Any supposed void you talk about is actually just space and part of our universe.

Speeding out in any direction in a hypothetical superfast spacecraft, you will soon find yourself entering a great void, because voids are most common.  Even if you pass thru a galaxy or 2, you will soon enter a great void.  When you leave any super cluster you will enter a void that is so vast and expanding so fast that you will never emerge from it (unless you can travel faster than light).  Right?  The destiny of any object moving fast enough, such as a hypervelocity star or black hole, is to end up alone, in a great void...forever.

Edited April 8, 2019 by Airbrush

[geordief]

20 minutes ago, Airbrush said:

Speeding out in any direction in a hypothetical superfast spacecraft, you will soon find yourself entering a great void, because voids are most common.  Even if you pass thru a galaxy or 2, you will soon enter a great void.  When you leave any super cluster you will enter a void that is so vast and expanding so fast that you will never emerge from it (unless you can travel faster than light).  Right?  The destiny of any object moving fast enough, such as a hypervelocity star or black hole, is to end up alone, in a great void...forever.

The void itself is expanding...? Not the distance between objects in the void?

[beecee]

1 hour ago, Airbrush said:

Speeding out in any direction in a hypothetical superfast spacecraft, you will soon find yourself entering a great void, because voids are most common.  Even if you pass thru a galaxy or 2, you will soon enter a great void.  When you leave any super cluster you will enter a void that is so vast and expanding so fast that you will never emerge from it (unless you can travel faster than light).  Right?  The destiny of any object moving fast enough, such as a hypervelocity star or black hole, is to end up alone, in a great void...forever.

Yep, no problems with that really, It's just I thought you seem to be speaking of voids beyond our spacetime/universe: Point I'm making being that we can only ever be aware of our own universe/space/time, I'm pretty sure.

1 hour ago, geordief said:

The void itself is expanding...? Not the distance between objects in the void?

Bingo! Or more simply it's the voids/space expanding over large scales [cosmological redshift] between galactic clusters and walls, not the galactic clusters and walls moving away.[Doppler redshift]

[MigL]

In Swansont's example, the universe is represented ( in fewer dimensions ) by the surface of the sphere, not the interior.
As a matter of fact, any topology you can 'picture' is unrealistic, as they would involve 'embedding' in a higher dimension ( extrinsic ).
For example, the non-embedded, finite but unbounded torus ( topological donut ) is a flat torus ( intrinsic curvature ). And this has the qualities of the old 'Asteroids' game; when you travel off one end of the screen, you re-appear on the opposing side.

[LaurieAG]

1 hour ago, MigL said:

In Swansont's example, the universe is represented ( in fewer dimensions ) by the surface of the sphere, not the interior.
As a matter of fact, any topology you can 'picture' is unrealistic, as they would involve 'embedding' in a higher dimension ( extrinsic ).
For example, the non-embedded, finite but unbounded torus ( topological donut ) is a flat torus ( intrinsic curvature ). And this has the qualities of the old 'Asteroids' game; when you travel off one end of the screen, you re-appear on the opposing side.

The Cycloid Arc has some inherent geometric and realistic physical properties.

https://en.wikipedia.org/wiki/Cycloid

(1) the area of the plot, of one complete revolution of a Cycloid Arc on a 2D plane, equals the surface area of a sphere of the same radius as the Cycloid.

(2) the area under the line of the Cycloid Arc on the 2D plot exactly equals the area of a circle with the same radius. A circle with the same radius and the same surface area cannot be duplicated on the surface of a sphere due to the curvature of the sphere.

(3) the area above the line of the Cycloid Arc on the 2D plot exactly equals 3 times the area of a circle with the same radius.

(4) the Tautochrone curve is half a Cycloid Arc that, in its physical form, has an interesting property.

Quote

The curve is a cycloid and the time is equal to π times the square root of the radius (of the circle which generates the cycloid) over the acceleration of gravity.

https://en.wikipedia.org/wiki/Tautochrone_curve

Do we theoretically mistake these intrinsic properties for something other than what they really are?

Edited April 8, 2019 by LaurieAG
tidy up quotes etc

[Strange]

6 hours ago, geordief said:

Is a sphere a mathematical rather than a physical object? (an idealization)

The universe (which is what I was thinking of by "space") cannot be spherical  even allowing for it being unobservable  in places.

So can this universe be both finite and expanding?

Would the expansion itself have to be bounded for this to be possible?

That was a 2D analogy for 3D space. The surface of a balloon has a finite area and can expand without limit (bearing in mind this is an analogy so it isn’t going to burst!)