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Re: [Phys-L] gravitational waves

On 04/12/2016 07:37 PM, Ludwik Kowalski wrote:

According to:

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

That's an article by Steve Carlip, hosted on John Baez's excellent
site: http://math.ucr.edu/home/baez/

The physics FAQ is at: http://math.ucr.edu/home/baez/physics/

" the speed of gravity has not been measured directly in the
laboratory—the gravitational interaction is too weak, and such an
experiment is beyond present technological capabilities. The "speed
of gravity" must therefore be deduced from astronomical
observations, and the answer depends on what model of gravity one
uses to describe those observations.

In the simple newtonian model, gravity propagates instantaneously:
the force exerted by a massive object points directly toward that
object's present position. For example, even though the Sun is 500
light seconds from the Earth, newtonian gravity describes a force on
Earth directed towards the Sun's position "now," not its position
500 seconds ago. Putting a "light travel delay" (technically called
"retardation") into newtonian gravity would make orbits unstable,
leading to predictions that clearly contradict Solar System
observations."

I would encourage people to read all of Catlip's FAQ article,
not just the first two paragraphs.

In particular, the third paragraph points out that general
relativity doesn't suffer from the aforementioned problems.
The way to fix the problems is to upgrade from Newtonian
gravitation to GR.

What is the officially "accepted value" of the speed of
gravitational waves? My intuitive assumption is that is that the bars
of error are larger than 30%. But this is only a guess.

It is not necessary to guess. In the 9th paragraph of Catlip's
article, it explains that pulsar data (as of several years ago)
provides 1% error bars. A more recent bound is 0.2%. The
observations must be interpreted in the light of theory, but
almost any imaginable theory gives the same bounds, or better.
GR itself predicts that in the linear regime the waves travel
at the speed c. If we account for nonlinearity, that introduces
error bars on the order of 1 part in 10^20 or better, for typical
gravitational waves.

1) Cavendish, who died 200 years ago, would certainly measure the
speed of a gravitational disturbance (with his simple laboratory
model), if he had an ultra-fast clock, similar to clocks available to
modern scientists.

2) Did LIGO scientists try to do something like this?

3)They probably did this;

No, they didn't. Instead they used GR to predict that the wave
from any terrestrial source would be too small to observe. It's
an easy calculation. Suppose the receiver is sensitive to h on
the order of 10^-20. If the transmitter would put out h on the
order of 10^-40 or less, it's not worth expending much effort
on it. Small changes in the transmitter don't help.

This is yet another item on the long list of reasons why the
disk model is worse than nothing. It predicts a lot of things
that don't exist.
-- It's wrong about the magnitude of the waves.
-- It's wrong about the direction of propagation.
-- It's wrong about the polarization.
-- It's wrong about the x-dependence.
-- It's wrong about the t-dependence.
-- It's wrong about conservation of energy.

You can maybe get something to happen in the near field, but it
isn't a proper wave. In the far field, in the radiation zone,
it is zero.

Yesterday I compared the disk model to a flat-earth model, where
the sun was carried west-to-east each night in a golden cup.
However, that was much too generous, insofar as a flat-earth model
makes "some" correct predictions. The surface of the earth is
/locally/ flat to a useful approximation; I can lay out square
tiles on my kitchen floor using Euclidean geometry, without
worrying about the curvature of the earth.

I reckon the disk model is more like saying storks bring babies.
AFAICT it doesn't make any correct predictions, not even to a
first approximation. It doesn't tell you want to do if you want
babies, and it doesn't tell you want to do if you don't want babies.
Also it makes crazy predictions about the size of babies, and the
seasonal and geographic distribution.

Just because you can construct the model doesn't mean it's
faithful to reality.

http://physics.stackexchange.com/questions/235450/do-gravitational-waves-travel-faster-than-light

" [with the discovery of gravitational waves, we will be able to]
Track Supernovas hours before they're visible to any telescope
because the waves arrive Earth long before any light does, giving
astronomers time to point telescopes like Hubble in that direction "

On stackexchange, it pays to read the answer ... as opposed to
relying on random hypotheses that are mentioned in the questions.

In this case, the third and fourth sentences of the answer explain
the time difference. To wit:

In the case of a supernova, it's actually a dynamic process
instead of a flip of a switch, and so the change in the magnitude
of light emission can indeed lag behind by several hours from the
start of collapse of the star's core - the detection of
gravitational waves could allow us to "buy back" that several hour
window by detecting the gravitational waves produced by core
collapse instead of having to wait for the light magnitude
increase. There's no disconnect here, just sloppy reporting.

Just for fun, we might consider the hypothetical effect of the index
of refraction of the interstellar medium. Hypothetically, it delays
the arrival of a light pulse. Alas, as it turns out, this is too
small to be observable. Something like 1 part in 10^29 for visible
light.

=============

The speed c is 299 792 458 m/s exactly, by definition, with no
error bars whatsoever.

In spacetime, c is to the xt plane what 1 radian is to the xy
plane. It plays a central role in all of relativity, including
general relativity. Calling it the speed of light is mildly
misleading, because c is super-important even in the absence
of light.