
Gravitational waves provide dose of reality about extra dimensions - dnetesn
https://phys.org/news/2018-09-gravitational-dose-reality-extra-dimensions.html
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Animats
Nice. A related result from the neutron star collision is that the speed of
light and the speed of gravity are the same, to within something like 1 part
in 10^15. Both light and gravity waves from an event 130 million light years
away came in less than 2 seconds apart.

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BurnGpuBurn
I don't get this. Shouldn't the speed of gravity be instant? If it has the
same speed as light, the earth would gravitate towards the location of the sun
roughly eight minutes ago (like we see the sun basically where it was eight
minutes ago) in stead of where it actually is, and thus not hold its orbit.

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kirykl
I think this helps answer your question
[https://physics.stackexchange.com/questions/252015/gravity-v...](https://physics.stackexchange.com/questions/252015/gravity-
vs-gravitational-waves)

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BurnGpuBurn
Thanks! That's a long read. I've started at the original question, but it'll
take a while to process.

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mirimir
> The gravitational waves from the collision reverberated in LIGO the morning
> of Aug. 17, 2017, followed by detections of gamma-rays, X-rays, radio waves,
> and optical and infrared light. If gravity were leaking into other
> dimensions along the way, then the signal they measured in the gravitational
> wave detectors would have been weaker than expected. But it wasn't.

Do they really mean to say "followed by detections"? Don't gravitational waves
propagate at c? Also, I'm puzzled by "weaker than expected". One issue is
calculating expected intensities for gravitational waves vs electromagnetic
radiation. The other is calibration of LIGO signals. It's not obvious that
resulting uncertainties are small enough for drawing reliable conclusions
about gravity leakage.

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JeromeBonnet
My understanding is that the detection order follows the underlying phenomena:
first the neutron stars fell towards each other, producing gravitational
waves, and only then did the stars collide, producing electromagnetic waves.

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candiodari
As I understand it gravitational waves aren't slowed down by anything whereas
electromagnetic waves interact with matter along the way, which takes some
amount of time.

There isn't much matter to interact with, but not quite zero either. Something
like 1e-23 of the distance had some amount of matter in it (mostly very
diffuse gas clouds, nothing more).

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rotorblade
I'm confused over the size of these extra dimensions... They say that they do
not rule out small compact extra dimensions, but only large ones, the number
they give is ~100 km. But this must surely be ruled out by ordinary
gravitational experiments that establish the 1/r^2 law for gravity, since I
would assume that that law would be seriously broken for extra dimensions of
those sizes for even everyday physics.

The number that is quoted in some textbooks on String theory (but I do not
have the refs.) is that gravity is 1/r^2 down to sizes of ~1 cm for extra
dimensions.

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dogma1138
Gravitational waves are the best litmus test we have for gravity on a large
scale which is why they put to rest most of the modified gravity theories.

Newtonian gravity on large scales wasn’t “proven” explicitly in fact we have
had to introduce dark matter to make it work to match observations, there is
also the issue of empty space having weight which is attributed to various
favtors depending on the theory in question.

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k__
How come there are people saying there are higher dimensions and people saying
it's all holographic and the truth is 2D

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konschubert
The discussion about how many dimensions "really" exist is pointless because
there is no other way to define "real" than to define it as the reality that I
experience. And I experience 3 dimensions.

Physics is a collection of models expressed as formulas, plus recipes of how
to translate these formulas to the reality that we experience.

If I have a formula with 5 dimensions in it that tells me how to build a
teleporter then that's great but it doesn't change the fact that the world
that I experience is 3 dimensional in space.

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kristianp
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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whatshisface
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.

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skh
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.

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

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

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empath75
Area = surface area not volume.

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

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skh
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.

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whatshisface
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.

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crazygringo
Are there particular theories of quantum mechanics or string theory that are
now any less likely?

Or is this just confirming what everyone expected all along?

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T-A
It's yet another limit on the general idea of large extra dimensions:

[https://en.wikipedia.org/wiki/Large_extra_dimension](https://en.wikipedia.org/wiki/Large_extra_dimension)

It doesn't say anything about compact extra dimensions (the tiny curled up
ones you usually hear about in string theory).

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crazygringo
Thanks! That's _exactly_ the context that was totally missing from the
article.

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blind
'Scientists still confusing themselves by taking a 200 year old interpretation
of an experiment as scripture.'

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make3
the title of the article implies that there is new evidence for the existence
of multiple dimensions, while the content of the article is about the opposite
being the case

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Arnavion
Hard to see how. "Dose of reality" usually means evidence _to the contrary_.
Even if there was some ambiguity, the first line of the article as well as the
caption of the opening image make it clear.

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mullikine
It's probably more cryptic than ambiguous, due to the decorative language. A
lengthy terms and conditions is not ambiguous, yet it's very vexatious. This
is neither ambiguous nor vexatious. Perhaps though, it's a little bit cryptic.
A simple man must try a little harder. Spock would probably say the statement
is intrinsically ambiguous due to the metaphor, however.

