22 May 2011 9:13:24 UTC

Topic 195797

(moderation:

Can gravitational waves from pulsars and ect. travel from our universe to the outside void or not? Can they travel outside of our universe (in theory)? Since they have never been detected, than maybe we do not know.

Just asking.

Tom

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## Can gravitational waves travel outside of the universe?

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Where does our Universe end and the 'void' begin? If we 'know' about the 'void' doesn't that then mean it is 'in' our Universe too? I thought our Universe extended to everything we knew about.

## I think Tom is referring to

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I think Tom is referring to the work of Lisa Randall and others, the hypothesis being the universe has many more dimensions than we immediately know of. The relative weakness of gravity compared to other forces ( as we experience them ) is attributed to it's 'dilution off the brane into the bulk'. The brane being the lower dimensional space we inhabit that borders upon the bulk, a higher dimensional component. Somewhat like the walls of a cardboard box lining the surrounds of the volume it contains. The various force strengths are then explained as due to their 'coupling' to our brane. Hence gravity couples weakly but the EM and nuclear forces much more so, thus we experience the latter more formidably while gravity has a role to play deeper into the bulk. Probably keeps everything together I expect.

However this line of argument ultimately spirals into the squirrel's nest of string theory or somesuch variant, as in order to avoid too much handwaving one needs a detailed mechanism to predict observational results with. Is 'the string' attached to the brane at one end and is flapping about in the bulk? Perhaps Ms Randall's book 'Warped Passages' would help you here.

But I guess if gravity is acting on us creatures lying within the brane then I guess it would act into the bulk, waves included. Whether this is just a cute effective theory ( as by definition we won't be sticking measuring equipment out into the bulk, with some data cord trailing back to us on the brane ) or can actually make a prediction testable on the brane which distinguishes itself from other competing/established explanations, remains to be seen.

Cheers, Mike.

( edit ) 'brane' is a contraction of 'membrane' : I tend to think of it like some higher dimensional GladWrap or wallpaper that lines some hyper-volume. Our universe as traditionally defined by EM reception becomes the totality of the wallpaper. Interestingly this might also explain the observed correlation of features beyond strict light-speed causality links ( shortcut through the bulk rather than ripple round the edge ). I think if no gravitational waves were found by us, then one explanatory route ( which keeps GR as an effective theory on the brane, but needs the bulk to explain wave-less-ness ) becomes said radiation of waves into the bulk. Yes, we are just a fly on the wall ... :-)

( edit ) "squirrel's nest" ??? What are they Mike, birds or something ... ? :-)

( edit ) Oh yeah, they do have nests after all. Don't mind me, just thinking our loud.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

## I guess the related question

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I guess the related question is why 4-D spacetime for GR? Naturally having just melded 3-D's of space with 1-D of time then Einstein wasn't going to over-reach his new theory. Anyway one line of argument is the inverse square law ie. double the distance and quarter the strength. If there was significant projection of the force to some other dimension(s) then ought that not diminish more savagely?

[ Inverse square laws are a reflection of measured surface area at a given fixed radius then scaling strictly. This leads to the relative area enclosing a volume going like inverse radius. That is : r^2 divided by r^3 as seen some distance away. This is certainly not true around really large & dense bodies, a good example being the Shapiro delay in binary systems. Also the geodetic effect illustrated by Gravity Probe B - the 'missing inch' in the orbital circumference - implies a lower surface area of the spherical surface for such an orbital radius than a pure Euclidean case. Allowing PI to vary doesn't help here as it would appear in numerator and denominator ..... ].

Trouble is that 'inverse square' is only grossly true in the limit of lower mass and separated bodies ( hence GR becomes the correction to Newton with higher densities and we really don't know what happens at QM scales ). Things like solar systems aren't stable in the long term for other force vs distance dependencies - hence we slide into 'anthropic' arguments that state we couldn't be here to describe anything other than an inverse square law because circumstances wouldn't then be constant enough for us to crop up and be describing it! However exhausting such logic might be, it is still true that solar systems are stable but may yet still be within some slight variation from inverse square. So Mercury's perihelion anomaly ( attributable to GR corrections ) won't bring Jupiter down on our heads or fling Neptune away. That doesn't narrow down specific alternate candidates though.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

## RE: I think Tom is

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I saw a tv show about the 'big bang' and 'branes' the other day and was thinking about ESP in relation...I was wondering if people who 'see' the future are actually just more sensitive to when the 'branes' are closer together than at other times. And they aren't 'seeing' our future necessarily, just sensing another 'branes' reality. Probably no way to prove it one way or the other, but thought provoking.

## Here's an interesting and

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Here's an interesting and relevant quote on this topic ( my highlight in red ) from the ET design document page 12 ( 14 of 455 ) :

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal