expansion of space

[michel123456]

 

Of space, spacetime is normally considered a static thing.

Well then, if Spacetime is static and if space expands, it must mean that time contracts.

[Strange]

No, space "expands" (at this point, it becomes clear this is a poor analogy) as you move through the time dimension.

 

Imagine a conical shape where the circular cross section represents two of the spatial dimensions, and the length of the cone is the time dimension. At different points along the time axis, the spatial dimensions will be different.

[Endy0816]

 

I like to think of it as a conical seashell personally. Easiest to visualize in terms of area increasing from the tip outwards.

[Strange]

 

I like to think of it as a conical seashell personally. Easiest to visualize in terms of area increasing from the tip outwards.

 

Nice. And if you measure the rate at which the distance between a pair of spots changes as you go up the shell, it should be proportional to how far apart the spots are.

[michel123456]

You have taken as granted that time is not expanding nor contracting. How do you know that for sure? How is it possible that C remains constant when space expands ? by which mechanism do the photons "know" that they have to "jump" more and more in order to keep their velocity constant?

[Strange]

You have taken as granted that time is not expanding nor contracting.

 

it is just a convenrtion. I think it is quite possible to choose alternative coordinates where, for example, the universe does not expand but the speed of light (and other constants) changes over time. The convention chosen (expanding space) is simple and intuitive.

 

 

by which mechanism do the photons "know" that they have to "jump" more and more in order to keep their velocity constant?

 

They don't. They always move at the same speed. For objects far enough away from us, space between us and them is expanding fast enough that light from there will never reach us.

Edited August 8, 2014 by Strange

[michel123456]

 

it is just a convenrtion. I think it is quite possible to choose alternative coordinates where, for example, the universe does not expand but the speed of light (and other constants) changes over time. The convention chosen (expanding space) is simple and intuitive.

 

 

They don't. They always move at the same speed. For objects far enough away from us, space between us and them is expanding fast enough that light from there will never reach us.

That makes little sense to me. In which coordinates do one measure that "they move at the same speed"?

[Strange]

That makes little sense to me. In which coordinates do one measure that "they move at the same speed"?

 

Distance divided by time.

 

I'm not sure what you are asking. The speed of light is defined to be a constant: the metre is defined in terms of the speed of light.

[`hýsøŕ]

the shell analogy i have one clarification question for, namely does the idea that those spots getting further away from each other modelling the galaxies in our universe mean that our universe is expanding when observed from a 4+ dimensional space? (since the spots on the shell are being viewed from a 3 dimensional space and they're effectively on a growing 2 dimensional ring)

 

ty for dis analogy it does help a bit i guess, a little clearer than the balloon analogy which has caused me much confusion over the years

[Strange]

the shell analogy i have one clarification question for, namely does the idea that those spots getting further away from each other modelling the galaxies in our universe mean that our universe is expanding when observed from a 4+ dimensional space? (since the spots on the shell are being viewed from a 3 dimensional space and they're effectively on a growing 2 dimensional ring)

 

You can think of it that way but it is important to note that the description of the curvature of 4D spacetime in GR does not require a higher dimension for the curvature to "happen in". It is "intrinsic curvature", which can be a tricky concept to get your head round....

[michel123456]

 

Distance divided by time.

 

I'm not sure what you are asking. The speed of light is defined to be a constant: the metre is defined in terms of the speed of light.

OK the speed of light is defined to be a constant.

So i understand that when space is expanding, SOL remains a constant.

But when space is expanding, what expands is the metric.

So, in order for SOL to remain constant in the new metric, the metric of time must expand too (and not contract as a firstly assumed). Otherwise, SOL would not be a constant under space expansion.

 

The other way round would be to consider that "our" metric is universally correct and unchanging, that photons always move at velocity c as measured by "our" metric, and that photons are not affected by the expansion of space. But is that acceptable?

[Strange]

The speed of light being constant is a "local" thing. You will always measure the speed of light locally to be c. Zarg the Magnificent in another galaxy will also measure the speed of light locally to be c. But you are right, you may not agree on the distances or times involved and therefore will each may say the other's speed of light is different. (I think. This is a complex area and attempting to apply "common sense logic" will almost certainly get you the wrong result.)

[`hýsøŕ]

 

You can think of it that way but it is important to note that the description of the curvature of 4D spacetime in GR does not require a higher dimension for the curvature to "happen in". It is "intrinsic curvature", which can be a tricky concept to get your head round....

 

are you saying like... take a square, 2d, you could curve its surface by warping and bending it in the plane it lies in, without having to warp it 'up' into a third dimension (say, perpendicular to the plane the square lies in before its warped)? and that this is whats happening with our 4D spacetime? (i hope you guys get that this isn't easy to imagine lol)

[Strange]

 

are you saying like... take a square, 2d, you could curve its surface by warping and bending it in the plane it lies in, without having to warp it 'up' into a third dimension (say, perpendicular to the plane the square lies in before its warped)? and that this is whats happening with our 4D spacetime? (i hope you guys get that this isn't easy to imagine lol)

 

Basically, yes. But I think intrinsic curvature is only possible in 3 dimensions and above.

 

A 2D analogy (very crude) is the 3D graphics technique of bump mapping. This takes a 2D surface and distorts the "surface normals" (the vectors that point directly away from the surface). This affects things like lighting and makes the surface appear curved, even though it is still flat.

[MigL]

If I may help clarify...

Space expands but light does not slow down or speed up. It cannot.

However it can lose energy and change frequency, and this change is inversely proportional to the expansion.

This is where the red shift=>expansion relation comes from.

 

So I would say that michel123456 is right about expansion affecting time ( as in the timebase of the EM wave ) but Strange is right about the invariance of c .