23 Jul 2008 14:57:50 UTC

Topic 193777

(moderation:

This is from a thread that I hijacked over on s@h. However, this is likely a more appropriate forum. Any takers?

Quote:

Quote:... It is not an electromagnetic force. Therefore, I go back to my question of why and how do we know (believe) that its effect would travel at the speed of light.See:

First speed of gravity measurement revealed

Also:

The Speed of Gravity What the Experiments Say

That also describes very nicely some of the sort of orbital simulations I tried playing with for myself where you must 'rediscover' that gravity is 'already there' for us to stay in orbit. It is the propagation of changes of the gravitational field that is at the 'speed of light'.

Further thoughts...

(Sorry, this does get rather heavyweight but should still be interesting. It keeps us in orbit!)

[edit] Whichever way we look at this, the 'speed of gravity' question appears to question the fundamental principles of science of "causality" and "conservation of energy/matter". There's something not understood or more likely I've missed something... [/edit]

There looks to be quite a 'discussion' between S Carlip (SC) and T v Flandern (TvF)...

The main parts of the argument appear to be (to my understanding):

TvF notes (amongst other examples) that the position of our sun as seen from Earth is at an angle away (and behind, described as "retarded") from the line of acceleration the Earth experiences towards the sun. That is, we see the sun as it was and where it was about 500 seconds ago, yet we experience acceleration due to gravity for the sun's present (instantaneous) position.

Indeed, you must assume an instantaneous position for calculating gravitational attraction or your orbital mechanics simply do not work.

At first glance, this appears to invalidate the fundamental principle of physics of "causality in forward time". So do we really have "Magic" and "Spooky (infinitely fast) action at a distance"?

TvF proposes that one possible answer is that gravitational fields (and also electrostatic fields) act (update) at a propagation speed of many times greater than the speed of light. (This should not be confused with the very different "gravitational waves" which can be viewed as ripples in the field whereby their propagation speed at the speed of light is uncontested.)

TvF also notes that Lorentzian Relativity allows for faster than light speed and has never been experimentally invalidated.

SC counter argues that a gravitational field updates its new position at light speed propagation from any variation due to acceleration of the source mass. With this is the implication that the gravitational field for a source mass follows exactly the position for the linear velocity of that source in forward time.

What that means for the Sun - Earth example is that for experiencing the gravity of the sun at any instant, we feel the gravity from the linear extrapolated position of the sun that is 500 seconds along the tangent from a point on its orbit from 500 seconds ago.

Hence, gravitationally we 'feel' the sun for a sun orbit that is very slightly further away and very slightly retarded transversely. Similarly, so too for the sun experiencing the gravity from the earth.

This situation whereby the gravitational pull is at a slight angle such that the earth is pulled forwards slightly (and similarly the earth pulls the sun forwards slightly in the sun's orbit) should cause the earth to gain angular momentum and be flung away. (And similarly so for the sun. ... But energy cannot be created, merely converted...) This happens for our own moon for example for an analogous 'angle of gravity' effect. The earth rotates more quickly than the moon orbits, the earth's tidal bulge is dragged ahead of the moon's orbital position, and so the skewed position of the centre of gravity from the earth as experienced by the moon pulls the moon ever faster ahead in its orbit. Hence, the moon is gaining angular momentum (from the earth) and so moves to an ever higher orbit to ultimately be flung away from the earth. (Actually, I'd expect the moon to eventually slow the earth down to a point where earth's tides and moon maintain lockstep and the moon gains no further angular momentum.)

So...

Assuming that a field of force can follow the linear velocity of the field source, and that it is only the perturbation of position due to any acceleration that then takes light speed to propagate outwards:

Does the small weakening of the gravitational effect due to the greater distance of apparent effect exactly cancel out the gain you get for the forward component felt for the small retarded position distance offset (difference between linear tangent vs following the circumference)?

My view is that the sun and earth due to gravitational light speed propagation of the new position of each other due to acceleration "appear" to take orbits that are slightly larger and slightly retarded with respect to each other.

On a related thought: Does the sun have a (multiple?) tidal bulge(s) that influences the orbits of the inner planets?...

Regards,

Martin

A few links:

Does Gravity Travel at the Speed of Light? (Steve Carlip, Matthew Wiener and Geoffrey Landis)

See also:

Sunlight hides a thousand-year journey that actually began in the core

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## Gravity Waves...

)

Hi Martin, thanks for porting that over, a lot to digest in the thread and all the links. Don't know if this is helpful, but they've run simulations that start with an even distribution of elements formed from the big bang (mostly hydrogen & helium) to see which theories, models, and equations of gravity will produce the structures and formations that are observed today, some 14 billion years after the initial conditions. Don't have time to look for the links at the moment, (they're posted on one of the threads here too) but the point is that the equations of general relativity work. In the meantime, see this hyperphysics page on the Friedmann Equation. I'll try to post the links later...

Not quite sure what you're asking here, but events that appear simultaneous in one inertial frame of reference may appear to happen at different times in other inertial frames. It sounds like a question of what's the proper order for performing the calculation at two different times for two different inertial frames of reference? If not done correctly, the predicted orbit doesn't equal the observations...

## RE: ... and all the

)

An interesting aspect is that I've not found anything to say that T v Flandern's thesis for this has been proven or disproven by any experimental results.

I think there is no dispute for the very good agreement with observation for the GR formula. The question is more in the interpretation of the formula and whether/how an infinitely fast action of gravitation is assumed with the use of instantaneous positions rather than retarded positions for gravitational sources.

Did those simulations disprove LR?

Aside: How are Cosmology at home and Milkyway at home different to what has (presumably) already been simulated?

Thanks, but I'm not considering expansion. Just the one case for the earth and sun orbiting about each other and the propagation time for gravity. Expansion and the rest might come later!

That is a good mind-twister for whether you assume to be observing all this from the earth or from the sun or from a point inbetween.

However, my question is actually with regard to what I think S Carlip describes and that T v Flandern dismisses due to the assumptions made in the maths...

If you assume a 'static' gravitational field at any instant, then you (should) see the retarded gravitational position as described by T v F and as is seen for light. However, orbits don't work for such a case.

If you take S Carlip's description and assume that a gravitational field exactly follows a source for a linear vector, then with the displacement from the linear vector for the acceleration for an orbit, you then gravitationally see a very much smaller error between the instantaneous position and the retarded position. Such a position, having followed a tangent from a past point on the orbit, is slightly retarded and also slightly outside the line of the orbit. Hence, you still have a retarded angle that will cause a gain in angular momentum for an orbit, but you also have a slightly weaker force of attraction acting due to the slightly greater distance of the retarded position being a little outside the actual orbit. Does the slightly greater distance (slightly weaker attraction) of the retarded position conspire to cancel the orbital angular moment gain effect of the retarded position being transversely behind the body's instantaneous position?

(The length of the tangent is the orbital velocity times the time taken for light speed propagation sun to earth, or vice versa.)

Mmmm... A diagram may be needed to avoid the use of a thousand words?

Another form of the question is: Must we assume a light-speed defying instantaneous position of everything when considering a gravitational field?

Is this an uncommon question or something that has already been definitively thrashed out?

Regards,

Martin

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## RE: RE: ... and all the

)

It's my understanding (& I'm no expert) that the time/distance between the two positions can be as close to zero as you like, but it means performing many more calculations to get this more precise answer.

I don't know, they were mentioned in an online lecture given by Frank Wilczek, I think this one: The Origin of Mass and the Feebleness of Gravity

Wow, first I've heard of these projects, thanks Martin! I'll have to check them out.

In that case I'll only mention Gravity Probe B in pointing out there's much to consider â€“ many parameters :)

Maybe, but does this Wiki page help to answer the question? Orbit

Quoting the specific parts, there is this in the history section: â€œ Albert Einstein was able to show that gravity was due to curvature of space-time and was able to remove the assumption of Newton that changes propagate instantaneously. In relativity theory orbits follow geodesic trajectories which approximate very well to the Newtonian predictions. However there are differences and these can be used to determine which theory relativity agrees with. Essentially all experimental evidence agrees with relativity theory to within experimental measuremental accuracy. â€?

And there is this in the Analysis of Orbital Motion section: â€œ Please note that the following is a classical (Newtonian) analysis of orbital mechanics, which assumes the more subtle effects of general relativity (like frame dragging and gravitational time dilation) are negligible. General relativity does, however, need to be considered for some applications such as analysis of extremely massive heavenly bodies, precise prediction of a system's state after a long period of time, and in the case of interplanetary travel, where fuel economy, and thus precision, is paramount. â€?

I'd say definitively thrashed out, in many different experiments testing relativity...

## It can be hard to gather the

)

It can be hard to gather the threads on these issues sometimes, but I seem to read that the basic problem here is definition of measurement. That is when a data point is created, typically recording a co-incidence of events, and then what is meant for something to be 'true'.

Einstein sorted this all out with the Special and General theories based on the realisation of finite delay of electromagnetic influences. So differently placed points of view in spacetime, differently moving and accelerating, would disagree about event ordering, passage of time etc. Hypothecated 'observers' could thus relate their measurements ie. transform a measurement made in one frame to another. [ There does not need to conscious entities involved, simply material particles which have a response in the circumstances described. ] It all comes out brilliantly if, and only if, the speed of light ( in vacuo ) is set as a constant.

This was more than just a change in worldview but also came with a mathematical setting to give hard and testable figures. In all cases where experiment has been performed well enough to make such distinctions b/w SR/GR vs other theories, then Einstein is right. That's not to say that it explains all, certainly not, but there has yet to be a better rival theory in the relevant domains of applicability. In particular the micro realm, where QM rules, gravitational theory has yet to be experimentally testable at that scale. Nor does GR theory give either non-singular solutions or even those that 'abut' sensibly to QM.

The other aspect is that of the 'effective' scale of theory, in that at human scale our senses are electromagnetic only, thus our experiments are based on chains or cascades of meanings from other scales back to human ones. Hence the description of any other force - gravity, strong and weak nuclear forces - are inevitably going to be phrased in EM terms ie. speed of light constancy. Had we or the universe begun differently we might be saying how light travels at the speed of gravity - because we'd be using gravitons to measure up photons, not vice versa! :-)

Cheers, Mike.

( edit ) and Newtonian dynamics and Newtonian gravity are respectively the limited domain approximations ( low speed, low acceleration ) of Special and General Relativity.

( edit ) What Einstein's work also really highlighted was that you can really only construct workable theories based upon measurable quantities in given reference frames, and the nuances of that. This was considerably expanded upon with QM. While formulations are often done based upon some diagram-like god's-eye view in our minds, realistically such views are 'outside' of spacetime. The centres of black holes is such a case. With a theory constructed outside of black holes, ie by us humans, all that one can really say is that there is a boundary ( the event horizon ) within which we can make no testable statements. We can hypothecate some internal black hole behaviours by extrapolating from the outside and going inwards - but it doesn't matter as from the outside we will never obtain data to know for sure. The only persistent external artifacts of black hole production are overall mass, angular momentum and charge. That's it. Nothing else. Absolutely no detail whatsoever of internal structure and machinations can be either deduced or tested.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

## I think that the big problem

)

I think that the big problem is that of reconciling general relativity and quantum mechanics. It may be interesting to read what an American physicist, Lisa Randall of Harvard University, thinks about it as reported in the July/August issue of CERN Courier:

Lisa Randall

Tullio

## RE: I think that the big

)

Terrific Tullio! Plus the links at the bottom of the article to some of her lectures. Her Warped Passages is great, I learn a little bit more with each re-reading ..... :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal