# The Naked Scientists Forum

### Author Topic: Defining the action of gravity  (Read 6311 times)

#### Scilleterate

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##### Defining the action of gravity
« on: 22/11/2007 22:27:45 »
Millions of pounds of taxpayers money has been & is being spent on gravitational wave detection. I know they are predicted by Relativity, but a tuppence ha'penny experiment would prove it once & for all:

During a total eclipse of the Sun, surely we have the technology to measure to the millisecond, the instant of maximum gravitational attraction, here on Earth? We have mirrors on the Moon, reflecting laser light; we have satellites measuring surface heights of oceans & continents. Tell me it can be done?

If it can, we get the instant of the alignment of the masses of the Sun & Moon, as this point of maximum attraction occurs. This is then compared with the instant of optical alignment (Don't ask me! you're the scientists).

If the two events are simultaneous, gravitational waves travel at the speed of light. If the alignment of masses is 1¼ seconds ahead of optical alignment, gravity is action at a distance & we can turn off the funding tap to gravitational wave experimentation.

For what it's worth, my layman's money is on action at a distance.

#### syhprum

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##### Defining the action of gravity
« Reply #1 on: 23/11/2007 11:36:31 »
The experiment has already been done with radio waves from a quasar passing close to Jupiter and confirming the propagation of gravity waves is very close to c.
I will hunt for the original paper and publish the URL

#### Soul Surfer

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##### Defining the action of gravity
« Reply #2 on: 23/11/2007 11:50:24 »
Aggre with syphrum I've read the paper some years ago.  That is not the reason that scientists are trying to detect gravitiational waves.  Their detection and location will give insight into parts of the universe that can not be detected and measured by other means notably merging black holes, closely orbiting neutron stars and other cataclysmic processes very close to their origins.

#### lyner

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##### Defining the action of gravity
« Reply #3 on: 23/11/2007 22:53:24 »
Quote
During a total eclipse of the Sun, surely we have the technology to measure to the millisecond, the instant of maximum gravitational attraction, here on Earth?
This a severely non-trivial measurement. You are talking of measuring the  phase of  the (broad) peak of a very noisy signal with an accuracy of  better than 1ms in  several hours. The event doesn't repeat itself very often, either.

#### lyner

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##### Defining the action of gravity
« Reply #4 on: 23/11/2007 22:55:37 »
Quote
The experiment has already been done with radio waves from a quasar passing close to Jupiter
Do you mean a partial occlusion? - I hope you do!

#### Scilleterate

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##### Defining the action of gravity
« Reply #5 on: 25/11/2007 10:21:27 »
The experiment has already been done with radio waves from a quasar passing close to Jupiter and confirming the propagation of gravity waves is very close to c.

I think you will find that the eminent Professor Will is very sceptical about the conclusions of this experiment.

#### Scilleterate

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• Posts: 13
##### Defining the action of gravity
« Reply #6 on: 25/11/2007 10:28:20 »
This a severely nontrivial measurement. You are talking of measuring the  phase of  the (broad) peak of a very noisy signal with an accuracy of  better than 1ms in  several hours. The event doesn't repeat itself very often, either.

This is a knee-jerk, negative reaction. You know the 'total' eclipse is a matter of minutes. Some of the accuracies of measurements, publicised by the scientific community, are almost beyond belief. In positive mode, can't you think of a way it might be done?

#### Scilleterate

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##### Defining the action of gravity
« Reply #7 on: 25/11/2007 10:51:50 »
That is not the reason that scientists are trying to detect gravitational waves.

What if there aren't any gravitational waves? What if every piece of matter in the Universe is linked to every other piece instantaneously? Action at a distance, in fact. The explosion of a super nova would affect the gravitational 'environment' of every molecule of matter in the Universe instantaneously. My body is not too bothered about a radiation that turns my skin brown, but I think all the cells would like to know about a gravitational change at the same time.

Gravity is so different to every other form of electromagnetic radiation. If William Hill has odds on this, my layman's gut feeling would lead me to risk at least ten bob on it.

#### lyner

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##### Defining the action of gravity
« Reply #8 on: 25/11/2007 16:39:16 »
Quote
In positive mode, can't you think of a way it might be done?
I would, first, like to improve my signal to noise ratio considerably.
Going into space would remove all the  slamming door and tube train disturbances.
The problem is that you are trying to identify a single peak - or, worse, still, a single dip, the height of which you can't be sure of.
BUT, the LIGO equipment uses masses, separated by several km (?) and tries to detect their relative movement, using laser interferometry. This gives you positional accuracy to within the wavelength of light.

I can't help being a bit negative, because of years of engineering reality.
One question I would have is about the probable wavelength of the gravitational waves. It would seem that they would be very long. This would mean that diffraction effects would be very noticeable. So, as the moon goes across the sun, although the light is cut off in a very well defined manner, the diffraction of 'gravity waves' would produce a very broad dip in gravitational field and a very poorly defined minimum - possibly over an hour..

It would be very, very easy to compare the speeds of red and blue light by simply looking at the leading and trailing edges of the eclipse to see if their colours were different but this gravitational thing would be a much harder problem.
Let's face it, if the only problem was that it is very costly to do, someone would have done it by now - it is such a hot subject. Funding would have been there, for sure. You could have my tuppence ha'penny, for a start.
« Last Edit: 25/11/2007 16:42:57 by sophiecentaur »

#### lyner

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##### Defining the action of gravity
« Reply #9 on: 25/11/2007 16:40:48 »
Quote
my layman's gut feeling would lead me to risk at least ten bob on it.
or even half a crown?

#### syhprum

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##### Defining the action of gravity
« Reply #10 on: 25/11/2007 23:02:57 »
The experiment has already been done with radio waves from a quasar passing close to Jupiter and confirming the propagation of gravity waves is very close to c.
I will hunt for the original paper and publish the URL
Here is a good account of the Jupiter experiment, you must judge for yourselves the validity of the various objections to the conclusions.

http://www.csa.com/discoveryguides/gravity/overview.php

#### Scilleterate

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##### Defining the action of gravity
« Reply #11 on: 26/11/2007 22:03:05 »
One question I would have is about the probable wavelength of the gravitational waves. It would seem that they would be very long. This would mean that diffraction effects would be very noticeable. So, as the moon goes across the sun, although the light is cut off in a very well defined manner, the diffraction of 'gravity waves' would produce a very broad dip in gravitational field and a very poorly defined minimum - possibly over an hour..

Thanks for being positive. I'm on the brink of losing the thread of your explanations of the difficulties. I'm under the impression that the only detectable gravitational waves would be from massive events: super nova explosions, black hole collapse, etc.. Does the mix of 'minuscule' events from the front to the back of the Sun give a 'white noise' gravitational 'beam', of immeasurably small amplitude? Is this the beam you believe will be diffracted by the Moon?

My simple, mechanical engineer's, idea was to measure the instant at which the Earth's crust reaches its maximum height, which then pinpoints the time of the coincidence of alignment of the centres of masses of the Sun & Moon. This is the moment of maximum gravitational attraction & I was hoping that by, say, bouncing a laser beam off this mirror on the Moon, the distance between the mirror & the receiver would be measurable to a few wavelengths of light & the timing accurate to a few milliseconds. No?

#### Scilleterate

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##### Defining the action of gravity
« Reply #12 on: 26/11/2007 22:12:40 »
even half a crown?

Sophiecentaur; a double first in maths, but just too young to know ten bob = 4 half a crowns.

Let me know anytime you need help with farthings & thrupenny bits.

#### lyner

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##### Defining the action of gravity
« Reply #13 on: 26/11/2007 23:59:36 »
And tanners???

#### lyner

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##### Defining the action of gravity
« Reply #14 on: 27/11/2007 00:04:23 »
As for the experimental design parameters, could you do any sums to predict just how much relative movement we're looking for? That might encourage me to be positive - or boringly realistic.
Btw, would we be looking for a screening effect or a lensing effect?

#### Scilleterate

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##### Defining the action of gravity
« Reply #15 on: 27/11/2007 23:32:52 »
As for the experimental design parameters, could you do any sums to predict just how much relative movement we're looking for? That might encourage me to be positive - or boringly realistic.

I've read that the Earth's crust rises & falls a measurable amount,with the passing of the Moon, just like the oceans. I imagine the point of maximum height of the Earth's crust is always likely to be 1¼ seconds ahead of the line from the optical centre of the Moon, to the centre of the Earth? Whether this has been proved experimentally is irrelevant, since it will not tell us anything about the speed of propagation of gravitational waves, assuming they exist.

However, since we know that the neap tides are produced by the coincidence of the alignment of the masses of the Sun & Moon, it seems reasonable to assume there is a measurable neap tide on the Earth's crust. Somebody out there knows what instruments are measuring the height of the Earth's crust & whether they can be used in the experiment I am proposing.

Trouble is, Sophiecentaur, there's only thee & me talking about it. A science forum has lower useful productivity than a Cabinet Meeting; peeing & wind come to mind.

#### Soul Surfer

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##### Defining the action of gravity
« Reply #16 on: 28/11/2007 00:27:55 »
I do not think that the experiment is either useful or feasible.  As I have already pointed out the efforts to detect gravititional waves are not aimed at proving the velocity of these waves which is already known.

#### lyner

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##### Defining the action of gravity
« Reply #17 on: 28/11/2007 13:02:51 »

I have only four words to say to you about the subject of your proposed measurement--

"Signal to noise ratio."

But I can't resist a few more - there would be a phase lag in any tidal effects. There are a lot of perturbing factors to vary the phase and amplitude; seismic movements (random) other planets (cyclic), the seasons (axial tilt) , ground temperature, water table, even atmospheric pressure.
Am I a complete misery?

#### syhprum

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##### Defining the action of gravity
« Reply #18 on: 28/11/2007 21:10:22 »
I think that this problem will only be finally resolved when a distant gravity wave generating event is observed simultaneously by gravity and EM radiation.

#### Scilleterate

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##### Defining the action of gravity
« Reply #19 on: 01/12/2007 00:44:44 »
Am I a complete misery?

Completely. "But I like you"

#### Scilleterate

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##### Defining the action of gravity
« Reply #20 on: 01/12/2007 00:49:13 »
the velocity of these waves which is already known.

I think not. See what Professor Will has to say about that experiment.

#### Scilleterate

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##### Defining the action of gravity
« Reply #21 on: 01/12/2007 00:51:19 »
I do not think that the experiment is either useful or feasible.

Why not? And why not?

#### Scilleterate

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##### Defining the action of gravity
« Reply #22 on: 01/12/2007 00:57:11 »
I think that this problem will only be finally resolved when a distant gravity wave generating event is observed simultaneously by gravity and EM radiation.

Maybe there's no such thing as a gravitational wave. I wonder what the cost per wave detected is? What does £a.lot/0 equal?

#### The Naked Scientists Forum

##### Defining the action of gravity
« Reply #22 on: 01/12/2007 00:57:11 »