# Why do Gravity waves move at c?

sonhouse
Science 12 Jan '16 12:57
1. sonhouse
Fast and Curious
12 Jan '16 12:571 edit
I was thinking, gravity waves are ripples in spacetime. At the BB the universe expanded about 22 orders of magnitude greater than c (speed of light).

So, if space itself can exceed c than why are gravity waves which are ripples in spacetime, why is the propagation of gravity waves limited to c?
2. 12 Jan '16 13:327 edits
Originally posted by sonhouse
...if space itself can exceed c ...
space itself doesn't move at any definable speed else in what sense would space be moving with a 'speed' if not relative to something else? Or what would space be moving relative to or through? You cannot say space moves through space, right?
3. 12 Jan '16 14:54
Originally posted by humy
space itself doesn't move at any definable speed else in what sense would space be moving with a 'speed' if not relative to something else? Or what would space be moving relative to or through? You cannot say space moves through space, right?
Or how can space time move at all? What is there to move?

I know, I know, stop asking questions no one can explain to me.
4. 12 Jan '16 14:54
Originally posted by humy
space itself doesn't move at any definable speed else in what sense would space be moving with a 'speed' if not relative to something else? Or what would space be moving relative to or through? You cannot say space moves through space, right?
Or how can space time move at all? What is there to move?

I know, I know, stop asking questions no one can explain to me.
5. 12 Jan '16 15:141 edit
Originally posted by humy
space itself doesn't move at any definable speed else in what sense would space be moving with a 'speed' if not relative to something else? Or what would space be moving relative to or through? You cannot say space moves through space, right?
One patch of space can move away from another patch of space faster than c if the space in between is expanding. It is a little difficult to pin down what is space or where it is. Maybe we should refer to the sum of everything within a particular region rather than 'space', or even use the word space to refer to those things.
6. 12 Jan '16 15:17
Originally posted by sonhouse
I was thinking, gravity waves are ripples in spacetime. At the BB the universe expanded about 22 orders of magnitude greater than c (speed of light).

So, if space itself can exceed c than why are gravity waves which are ripples in spacetime, why is the propagation of gravity waves limited to c?
Gravity waves could travel faster than the speed of light just as certain types of waves in light waves can. What cannot happen is send information using them faster than the speed of light. Of course here we are talking about the speed of light relative to the local environment. Relative to something far away, there is nothing stopping anything from travelling faster than the speed of light. In fact everything is travelling faster than the speed of light relative to some other point in the universe.
7. sonhouse
Fast and Curious
12 Jan '16 15:31
Gravity waves could travel faster than the speed of light just as certain types of waves in light waves can. What cannot happen is send information using them faster than the speed of light. Of course here we are talking about the speed of light relative to the local environment. Relative to something far away, there is nothing stopping anything from trav ...[text shortened]... thing is travelling faster than the speed of light relative to some other point in the universe.
That faster than light separation of stuff though is only for that stuff separated by what, a billion light years or so? The word I heard is although space was expanding at some 22 orders of magnitude faster than c at the start of the BB era, now the expansion has slowed to something more like 3 or 4 times c, the recently found speed up of the expansion notwithstanding. Not sure how much that has changed things.

But for sure, a star say on one edge of the Milky way is not flying away from another star on the other edge of our galaxy at 150,000 light years or so.

Even the Andromeda galaxy 3 million odd light years away is actually flying TOWARDS the milky way as we are in the middle of an impending collision of the two in a few billion years because the sun total of the bending of space is leading to a net attractive force between those two galaxies.

I don't think there is even much of a velocity built up between our galaxy and one say 50 million light years away. Too close for much stretching at that kind of local distance, cosmologically speaking.
8. 12 Jan '16 17:086 edits
One patch of space can move away from another patch of space faster than c if the space in between is expanding....
I could be wrong and I am really not sure about this but my intuition shouts out at me saying there is something wrong with that concept. I will try my best to pin down what is bothering me:

Because none of us can really accurately mentally visualize (as opposed to merely conceptualize) any goings on beyond in the limitations of 3D, is impossible for any of us to mentally visualize something expanding without two points either side of it moving relative to each other by moving further apart. But that is visualizing something expanding through space. So when we try and visualize space itself expanding, we (including myself) are visualizing space expanding through space, which is circular and makes no sense. Thus I cannot help wonder if that mental visualization is insidiously misleading in the narrow sense it may be wrong to conclude from it that two points either side of an arbitrary region of 'space' move relative to each other by moving further apart just because it is impossible for us to visualize those two points not moving apart when visualizing that space expanding!

Perhaps when we refer to the so called 'expansion' of space, we should say what we are referring to is merely analogous to something 'expanding' but simply not the same thing as what we call 'expansion' in everyday life so we shouldn't take what we see in our resulting mental visualization of that 'expansion' of space too literally and emphasize some of the things that visualization shows will be completely false?
9. sonhouse
Fast and Curious
12 Jan '16 17:59
Originally posted by humy
I could be wrong and I am really not sure about this but my intuition shouts out at me saying there is something wrong with that concept. I will try my best to pin down what is bothering me:

Because none of us can really accurately mentally visualize (as opposed to merely conceptualize) any goings on beyond in the limitations of 3D, is impossible for ...[text shortened]... oo literally and emphasize some of the things that visualization shows will be completely false?
I think we have to think of space as connecting with or part of a higher dimension and it would be analogous to a thin puddle spreading out on a road Vs an ocean say in outer space, a sphere of water with a big pipe going to the center and pumping new water in.

The thin puddle would be us in our dimension, the sphere of water would be space expanding into a yet higher dimension where there would be plenty of room for a universe to fill in to.

From our POV, stuff would just be receding but of course that would look the same pointing to stuff up, down, left, right, forwards and back, from any angle you look, stuff is receding.

I am thinking it would have to be like that but of course there are the cadre of upper dimension doubters but how else would you go about explaining something like that?

Especially since we feel the universe was smaller than a proton at the start of the BB and then filled in the space we see now somehow pumping in space between each and every atom perhaps.

But local collections of atoms don't experience a separation, gravity sees to that.

Space is wavy.
10. 12 Jan '16 20:54
Originally posted by sonhouse
The word I heard is although space was expanding at some 22 orders of magnitude faster than c at the start of the BB era,
Well whoever you heard it from is either outright wrong, or didn't tell you the whole story.

Space expansion cannot be measured in units of speed. Space expansion must be measured in units of proportion. So for example space might grow 1% per year. The only way you can bring speed into the measurement is if you say space is finite, or you are talking about a specific region of space (say the visible universe for example). The problem with the visible universe of course is that it is expanding all the time as light comes towards us. Nevertheless we could estimate the relative velocity between us and a point on the edge of the current visible universe.
11. 12 Jan '16 21:00
Originally posted by sonhouse
Especially since we feel the universe was smaller than a proton at the start of the BB
I don't know how many times I have to say this: it is not known that the universe is finite. If you disagree and feel that there is some evidence that the universe is finite then please provide references. Until then stating that 'we feel the universe was smaller than a proton' is simply incorrect.
12. sonhouse
Fast and Curious
12 Jan '16 21:371 edit
Well whoever you heard it from is either outright wrong, or didn't tell you the whole story.

Space expansion cannot be measured in units of speed. Space expansion must be measured in units of proportion. So for example space might grow 1% per year. The only way you can bring speed into the measurement is if you say space is finite, or you are talking a ...[text shortened]... timate the relative velocity between us and a point on the edge of the current visible universe.
I mentioned in a post being at a talk at Bell Labs given by Alan Guth. He said the BB started at basically zero size, I imagine not exactly zero but close enough for government workðŸ™‚ and expanded to the size of a football in something like 10^-23 second, something like that anyway.

I used those figures and came up with that number, 22 orders of magnitude faster than c 'growth' for that time period only.

I was able to mention that during a question and answer session, I was almost unable to speak, I was so nervous, and he said and I quote "Well, you did your arithmetic right'
I asked him that question about the speed of light limit and he answered that does not apply to space, only to matter and electromagnetic radiation and such, but space itself has no 'speed' limit. And just measuring the 'speed' by measuring such effects as doppler shift and the like.

So he was saying at least in the first tiny moments of the BB it was about that order of magnitude but exponentially slowed the 'growth' rate which has slowed down to this day, the speed up at 5 billion years ago notwithstanding.

Now they think it is still cranking out about 3 c or so. Meaning the universe is several times larger than telescopes can see, which we see out to about 14 billion light years but light from a much larger volume has not and will not reach us and so we will never actually see that stuff with telescopes.

Anyway I got that more or less from the horses mouth. Guth was a biggie in BB stuff and inflation in particular.
13. 12 Jan '16 22:202 edits
... stating that 'we feel the universe was smaller than a proton' is simply incorrect.
Didn't you know that many of us had earlier felt the whole universe around with our fingers and felt from our tactile sense its tiny size when it was small and judged purely from our tactile sense at our finger tips that it must have been roughly smaller than a proton when we touched it?
14. 13 Jan '16 08:48
Originally posted by sonhouse
I used those figures and came up with that number, 22 orders of magnitude faster than c 'growth' for that time period only.
Assuming a finite universe and your figures are correct, what you measured there is the relative speed of two points on opposite sides of the universe relative to each other, and by 'speed' here we mean how much space is between them over time. Neither the particles nor the space is really moving. Further, depending on the geometry of the universe, two points on opposite sides might actually be in the same place - but of course at 22 orders of magnitude that doesn't really matter.

Now they think it is still cranking out about 3 c or so.
Again we should presumably assume you are referring to the relative velocities of points on opposite sides of the universe. This is not some fundamental speed that the universe is 'cranking out' but merely a factor of the size of the universe in relation to the amount it is expanding which can only really properly be measured in terms of proportions. And given the size of the universe that would work out to a minuscule amount in terms of proportions (which is why we cannot detect it locally).

Meaning the universe is several times larger than telescopes can see, which we see out to about 14 billion light years but light from a much larger volume has not and will not reach us and so we will never actually see that stuff with telescopes.
To actually calculate that would be extremely complicated.

It remains the case however that it is not known that the universe is finite.
15. sonhouse
Fast and Curious
13 Jan '16 15:251 edit