
After 100 years, scientists are finally closing in on Einstein’s ripples - pavornyoh
http://arstechnica.com/science/2016/02/after-100-years-scientists-are-finally-closing-in-on-einsteins-ripples/
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delinka
I wonder of someone can help me with a misunderstanding. If "spacetime" is the
medium which conducts the matter and energy we (and our stars and planets) are
made of, how can we ever know that it's expanding or contracting? If we look
at it like Flatland, how could a Flatlander know that the sheet he lives on is
actually rippled? He simply traverses his "space" unaware that he's bobbing
about in a third dimension. Also in this case, you'd never be aware of being
spaghettified[1] by the changing of space.

So the laser light has to travel via space and if space contracts, it still
has to cover the same amount of space (although, from an external extra-
dimensional perspective, it might 'slow down' in a contracted space), and
would thus not produce these interference patterns.

If, however, our energy and matter are not 'conducted' by space, then I
suppose I can begin to comprehend why such an experiment might work.

1 -
[https://en.wikipedia.org/wiki/Spaghettification](https://en.wikipedia.org/wiki/Spaghettification)

~~~
T-A
> If we look at it like Flatland, how could a Flatlander know that the sheet
> he lives on is actually rippled?

He builds a device which emits flat-balls (all right, circles) traveling at a
known constant speed. Then he measures the time it takes a flat-ball to reach
a target and bounce back to him (they are made of flat-rubber). And he
discovers that there are fluctuations in the travel time, well above what he'd
expect from known error sources.

Being a flat-Einstein, he then deduces that Flatland is rippled in a third
dimension, and the flat-balls are taking longer to traverse a rippled surface
than a flat surface, because /\/\/\/\/\/\is longer than ------------ when you
have to move along the surface of the ripples rather than go through them.

~~~
delinka
I feel you've done little but directly translate the LIGO experiment into
Flatlandian physics. That doesn't answer the question.

Use a sheet of paper as your Flatland. Draw two points, measure the distance.
Crinkle up the paper. There's still the same 'distance' of paper between the
two points. Should the points be able to travel, they have to traverse the
same number of paper molecules whether the paper is crinkled or not. No one
living in this Flatland can measure the crinkles in this space because they
live inside this space. They would need to step outside their own space with
some ability to measure variations in a third dimension.

If the physics of a universe necessitates movement from one point to the next
(let's suppose these points are Planck length apart, call them 'Planck-
points'), if particles must be translated from Planck-point to Planck-point
and can't skip anything in between, how are we to ever measure that Planck
length expanded or contracted? Surely if it "takes longer" to traverse an
expanded Planck length, then it hasn't expanded, but there now exists more
space - more Planck-points.

If gravity wells stretch and contract _matter_ (rather than space), then this
fits my mental model. If they actually stretch _space_, within which matter
exist and along which matter traverses, I struggle to see how we could ever
detect space deformations without stepping outside our own space.

~~~
T-A
> Use a sheet of paper as your Flatland. Draw two points, measure the
> distance. Crinkle up the paper. There's still the same 'distance' of paper
> between the two points.

Because the paper is not elastic. Spacetime is; it stretches. So a better
analogy would be the rubber sheet with masses rolling around on it, and
deforming it, which is often invoked in popular introductions to spacetime
curvature. Like all analogies, it has its limitations, but it does capture the
essence of the classic Wheeler quote: "Mass tells space-time how to curve, and
space-time tells mass how to move."

> If gravity wells stretch and contract _matter_ (rather than space), then
> this fits my mental model.

Gravity stretches and contracts space AND matter, the latter to the extent
that it can overcome the chemical bonds (and other forces) trying to keep
matter together (gravity is weak, so that takes a lot of mass). We see the
stretching of matter all the time as tides. Moons torn apart by it when they
get too close to their planets (the Roche limit) is a more dramatic example.
Spagettification of matter falling into a black hole an even more dramatic
one. You can think of gravitational waves as an oscillating, self-sustaining
tide traveling at the speed of light.

By the way, there is a thread of thought which takes the analogy of spacetime
with an elastic solid quite seriously: [http://arxiv.org/abs/gr-
qc/0408051](http://arxiv.org/abs/gr-qc/0408051)

