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Post by carnyx on Jan 18, 2011 16:01:04 GMT 1
Gravitational waves may exist, and are actively being sought. Some predict that they will propagate at light-speed.
I should like to suggest a simple experiment that would confirm this view.
We know that the Sun exerts a gravitational attraction to objects on the earth. At the equator we see this effect as a diurnal tide, as each molecule of seawater is attracted upwards in response to the sun's gravitational pull.
Now, if we measure the phase angle between the elevation of the sun's image, and the height of the sun-tide, we could then tell if gravitational waves travel faster, or slower, than the light coming from the sun.
Does anybody know if this has been done?
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Post by speakertoanimals on Jan 18, 2011 16:19:59 GMT 1
This effect ISN'T a gravitational wave, but just the normal gravitational pull of the sun.
Checking the ANGLE of the suns gravitational pull relative to the viewed position of the sun is actually something called gravitational aberration.
The effect can be explained like this. Relativity says information can only be transmitted at lightspeed. Hence the gravitational pull of the sun can be considered as a signal telling you where the sun is or where the sun was, yes? Hence if travels at lightpseed, direction of pull should tell you where the sun was eight minutes ago (time it takes light to reach earth). And ditto, earths pull on sun tells us where the sun where we were eight minutes ago.
except that simple argument would give a problem -- with delayed pulls like that, the net effect on the earth-sun system would not ne what Newton demands -- for every action there is an equal an opposite reaction. It would mean there was a net torque due to mutual gravitational effect, which would mess up earths orbit, lead to non-conservation of angular momentum etc etc.
Actual solution in relativity: the mistaken assumption is that in relativity (ie treating speed of light as finite), the correct expression for gravity is just a force between centres that depends on position. The correct expression is actually force depending on position eight minutes ago (delayed knowledge of position bit), PLUS a term which depends on the speed as well. This knowledge of past position and past speed enable a prediction of CURRENT position, hence why net gravitational force points at where the sun IS, not where it was, just as me knowing that you were on this mark a second ago travelling at ten metres per second allows me to predict that you will be ten metres past that mark now (give or take a bit allowing for acceleration).
So what are gravitational waves? They are the predicted oscillatory part of a gravitational field, which is different to the oscillatory nature of the force we experience from the sun, because we happen to be moving in an oscillatory manner relative to the centre of the sun, hence moving through different parts of the suns gravitational field.
So, if we had a black hole in a fixed position, we would experience a gravitational pull. If we also vibrated the black hole, there would be an additional effect, the gravitational wave, on top of the constant pull.
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Post by carnyx on Jan 18, 2011 19:10:31 GMT 1
STA ,
Your explanation is unconvincing because it assumes an a-priori knowledge of future position.
And so on the face of it, as the phase angle indicates an 8-minute lag for light, then changes in the gravitational field appear to be communicated instantaneousy.
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Post by abacus9900 on Jan 18, 2011 20:00:17 GMT 1
Gravitational waves may exist, and are actively being sought. Some predict that they will propagate at light-speed. I should like to suggest a simple experiment that would confirm this view. We know that the Sun exerts a gravitational attraction to objects on the earth. At the equator we see this effect as a diurnal tide, as each molecule of seawater is attracted upwards in response to the sun's gravitational pull. Now, if we measure the phase angle between the elevation of the sun's image, and the height of the sun-tide, we could then tell if gravitational waves travel faster, or slower, than the light coming from the sun. Does anybody know if this has been done? Gravity wave experiments are in place at the moment but to the best of my knowledge have yet to be detected.
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Post by speakertoanimals on Jan 18, 2011 22:17:56 GMT 1
STA , Your explanation is unconvincing because it assumes an a-priori knowledge of future position. And so on the face of it, as the phase angle indicates an 8-minute lag for light, then changes in the gravitational field appear to be communicated instantaneousy. Wrong. Not just because if what you said was true, we would have disproved Einstein....................... What I said was if I know the position of the sun IN THE PAST and the speed of the sun IN THE PAST, then I can make a decent stab at predicting the position a little into the future. To the same degree of accuracy, the leading terms in the expression for the gravitational force at the earth due to the sun are those based on the position in the past (that's the bit like Newtonian gravity, but with a time-delay), AND a second term which depends on speed and position in the past. The combination of the two gives a result which LOOKS like it is based on instaneous position, but actually is just the rough prediction of current position I mentioned at first. Google on Carlip and gravitational aberration if you really want the full papers on this topic. It DOESN'T assume a priori knowledge of future position (not that I think you know what a priori means either......................) Gravitational waves haven't been directly detected, by the measured time-decay of binary pulsars due to energy loss through gravitational radiation fit almost perfectly (and earned some geezers the Nobel prize).
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Post by buckleymanor1 on Jan 19, 2011 0:10:39 GMT 1
They had better hand it back then
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Post by speakertoanimals on Jan 19, 2011 19:46:49 GMT 1
WHy? That 'by' should have been a 'but' BTW..................
A remark that doesn't actually add anything to the debate, but that's par for the course in this poor excuse for a science board.
Seems the time-wasters always win, especially when the moderation is a total joke.................
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Post by carnyx on Jan 21, 2011 17:24:17 GMT 1
Can anybody critique the idea that if Gravitational waves existed, then they ought to show up as jitters in the red-shift ( e.g. fraunhofer line positions) of light from distant objects.
And if there was a difference in the effect on the speed of light and on mass, then we might see these waves as 'jitters' in the position of stellar objects.
Then there is the redshift phenomenon itself. If a lamp was to be in orbit around a black hole, it's light as seen by us would be redshifted .. even though it was not actually speeding away, or at an incredible distance.
Could it be that some of the stuff claimed to be at the edge of the Universe because of red-shift, is in fact close-by but are in areas of very strong gravity? Or even that the light has passed through such areas?
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Post by abacus9900 on Jan 21, 2011 17:49:35 GMT 1
Can anybody critique the idea that if Gravitational waves existed, then they ought to show up as jitters in the red-shift ( e.g. fraunhofer line positions) of light from distant objects. And if there was a difference in the effect on the speed of light and on mass, then we might see these waves as 'jitters' in the position of stellar objects. Then there is the redshift phenomenon itself. If a lamp was to be in orbit around a black hole, it's light as seen by us would be redshifted .. even though it was not actually speeding away, or at an incredible distance. Could it be that some of the stuff claimed to be at the edge of the Universe because of red-shift, is in fact close-by but are in areas of very strong gravity? Or even that the light has passed through such areas? This short video might be of interest to you: www.5min.com/Video/What-are-gravitational-waves-7858
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Post by Progenitor A on Jan 21, 2011 17:51:29 GMT 1
Can anybody critique the idea that if Gravitational waves existed, then they ought to show up as jitters in the red-shift ( e.g. fraunhofer line positions) of light from distant objects. Well, as light waves are red or blue shifted by gravity, then any variation in the gravity field , which is what gravity waves would be, if they exist, should indeed cause changes in the spectral lines of light. Whether the increasing/decreasing acceleration of gravity waves will be enough to make a measurable shift I do not know, but certainly when stars collide then the change in gravity should, I think be enough to do that And if there was a difference in the effect on the speed of light and on mass, then we might see these waves as 'jitters' in the position of stellar objects. Better to stick to blue/red shift rather than the speed of light I think (unless you specifically want to make that point). Otherwise what you say sounds very reasonable to me Then there is the redshift phenomenon itself. If a lamp was to be in orbit around a black hole, it's light as seen by us would be redshifted .. even though it was not actually speeding away, or at an incredible distance. Yup, They say that bothe velocty of separation causes a redshidt as does gravity (simultaneous red and blue shift depending upon the position of the observer.) Could it be that some of the stuff claimed to be at the edge of the Universe because of red-shift, is in fact close-by but are in areas of very strong gravity? Or even that the light has passed through such areas? ;DI suppose that the redshift due to a lamp orbiting a black hole can be discriminated from other red shifts because it would disappear as the lamp went behind the black hole. Now light pasing through such heavy gravitational fields may be blue shifted as well as red-shifted, but no matter, your question still holds; how do we discriminate between universe-expansion red shift and gravitational red-shift as distant light passes through strong gravity fields?
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Post by speakertoanimals on Jan 26, 2011 17:07:56 GMT 1
The essential point about gravity waves is that they are waves consisting of stretching and contraction perpendicular to the direction of travel (see animations on en.wikipedia.org/wiki/Gravitational_wave). Hence as regards cosmological red-shift etc, the stretching and compression will cancel out, to give no net effect. That would be the rather ODD assumption that we are at the top compared to the rest of the universe, and that all the rest of the universe is downhill compared to us. Which seems rather unreasonable. ANy light passing through/past a strong gravitational field of very massive object -- the net effect is ZERO, since even though the light gets blueshifted as it falls into the gravity well, it gets redshifted by the same amount as it comes out again. Some creationsist have however used such daft ideas to try and explain why six days in heaven looks like several billion years here on earth, but none of them can be considered as anyway reputable or sensible. Might as well twist geology to try and 'explain' the existence of a fiery hell under our feet, and why all the demons can fit in without the cavity collapsing....................
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Post by robinpike on Jan 26, 2011 19:31:37 GMT 1
STA, or anyone, i don't understand the explanations for why the speed of gravity can be finite, and yet the earth still feels the effect of the sun's gravity as if it was instantaneous. Is it possible to explain without using maths?
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Post by speakertoanimals on Jan 26, 2011 20:40:04 GMT 1
STA, or anyone, i don't understand the explanations for why the speed of gravity can be finite, and yet the earth still feels the effect of the sun's gravity as if it was instantaneous. Is it possible to explain without using maths? Rather than the sun, let's take the position of a car. We will assume that the cars acceleration is limited on the timescales we are talking about. Now how can I predict the future position of the car? I take its position NOW and its speed NOW, and that gives me a good prediction not too far into the future. Now replace the car by the sun, Replace known position and speed with position and speed a little bit in the past. And same argument shows we can make a decent prediction of its position NOW, based on past measurement, provided time inetrval not too big. And that is what gravity DOES, to still obey relativity -- the EXACT gravitational force NOW depends (to lowest order, hence time not too big!) on position and speed of the sun in the past. That is why gravity as computed from relativity has the EXTRA velocity-dependant terms, so the guess current position trick can apply.
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Post by buckleymanor1 on Jan 27, 2011 1:23:26 GMT 1
What if there was two strong gravitational fields(two massive objects close together) and the light passed through the middle on it's way to us. Don't you get the Shapiro effect. www.wordiq.com/definition/Shapiro_effect
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Post by speakertoanimals on Jan 27, 2011 2:00:26 GMT 1
Except the Shapiro effect is a time-delay effect, caused by the route near a gravitating body being that bit longer than when the body is not there. But we aren't measuring the flight time of light from distant sources -- we don't know precisely when it started out, after all. To measure it, you'd need something like a pulsar, which emits regular signals -- then you can measure a delay, relative to the last pulse, if you like. This is what was done for pulsars: physicsworld.com/cws/article/news/2644But for signals without this regularity, and where these isn't a variation as to what the light passes through (as there is in binary pulsar systems), you wouldn't know it was happening.
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