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Comment by kristianp | Hacker News Reader

So, is that a strike against string theory's extra dimensions, or is it another type of extra dimension being ruled out?

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It is a strike against the idea that over long distances, the spherical shell of "expanding and thinning" gravity can go down faster than 1/r^2 (which would come about from A=pi*r^2, the area of a sphere in three dimensions), achieving that goal by slipping in to dimensions higher than three. However now we have found that gravitational waves are about as strong as we expect it looks like gravity isn't leaving the world of 3+1D.

Sphere's have surface area and volume. Neither of those quantities is given by the formula for the area of a circle. I’m not a physicist and can’t speak to the merits of the rest of your post but this error of yours makes me doubt your post's veracity. Did you make a mistake in your parenthetical remark? Maybe it edit if you have.

I assumed it was a reference to the surface area of a sphere - which is 4pir^2. The point would still hold.

Makes sense. That’s what I would have thought if not for the area of a sphere in three dimensions remark.

Area = surface area not volume.

Physicists tend to drop constant factors when they do calculations, I’ve noticed.

A sphere has surface area and volume. I didn’t claim or imply that I thought area meant volume. It does not have area and the formula for the surface area of a sphere is not pi r^2. I mentioned this in my original post. Then there is the remark that whatshisface made: A = pi r^2, the area of a sphere in three dimensions. That is wrong and most likely a mistake.

When talking about dropping a constant factor are you referring to the 1/r^2 part or dropping a 4 in the statement A=pi r^2. Because if the latter then why keep pi? That’s a constant.

The constants perfectly divide out in this case, adding the pi was an afterthought so that people would recognize it as an area. Yes... It should have had a 4 as well. Too late to edit now.

[Complete layman here.]

Could it also not mean that gravity is "just" a 3D (or 3+1D) phenomenon?

For example, what does gravity mean in 2D?

Can we rule out that our 3D† universe is not a rarity in a larger, 4D universe, much like how true-2D planes are a rarity in our 3D universe?

† D as in spatial dimension, excluding time.

If there are extra dimensions that gravity doesn't exit into, this experiment won't detect them. It is not curtians for string theory.

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