- #36
Chronos
Science Advisor
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You can lead a horse to accolades . . . A physicist is a mathematician with a hobby horse.
pervect said:The main problem with regarding gravity as "just a force", is that it cannot possibly explain gravitational time dilation.
To be honest I did not know all the reference you was mention, I am not highly educated and can not relate to the same background materiel, - sorry.You addressed me, but your comments suggest that you did not bother to read my long and thoughtful post #26-27!
yogi said:consider the gravitational force exerted upon an object of mass M as a deformation of an accelerating spacetime surface in proportion to its inertial reactance M. In other words, there is no gravitational force per se, but only a reactionary force that deforms the acceleration of the spacetime surface - ergo, both the space are affected in proportion to the accelertion-mass product
Bjarne said:can not relate to the same background materiel
Bjarne said:Space seems to play a more important and central role that we usually think... We also off course accept that space somehow seems to be deeply involved in gravity phenomena’s... I think that space related to gravity questions doesn’t get the central attention it deserves. Think for instance (a while) of the expression “Space curves”... I mean space seems clearly to be deeply involved in the cause of gravity, its not only a question of curved space.
Bjarne said:When we think about space, - we accept it curves and also that it can expend... since we know that space can expand, - it is obvious to ask: can space also have the opposite option: Can space contract, - and could the presence of matter be responsible for the force it requires?
Bjarne said:If space is nothing, how can nothing curve?
Bjarne said:It’s a quite mathematically / geometrically (cold) language, not minded for anything else’s like calculations. This would be the end of the road, that means that we on that basis even should not attemp to understand what gravity is.
Chris Hillman said:yogi, you proposed:
Unless I missed something, you didn't define the terms "deformation", "accelerating spacetime surface", "inertial reactance" (of a body? a surface?), and I see no indication that you took the point that the acceleration of a curve (e.g. the world line of an object) is a vector field defined along the curve, with units of reciprocal length, while the Gaussian curvature of a two-dimensional surface (and the sectional curvatures of higher dimensional submanifolds) is a scalar field defined in the surface, with units of reciprocal area. Without meaningful mathematical definitions of your terms, IMO your proposal is too vague to evaluate/discuss.
We can't keep repeating this indefinitely, but as I and others have already noted, part of your confusion may be caused by your insistence on thinking of "space" as something like a material which can "expand" or "contract", perhaps somewhat like a heated/cooled metal bar.
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I.
Pervect had commented to the effect that if gravity is simply a force, there could be no time dilation
yogi said:I was merely suggesting an alternative to Einstein's static origin of curvature (that is, to suggest that curvature may be the result of some dynamic) where the deformation is created by the interaction of accelerating spacetime (a momentum rate of flow which is equivalent to stress). As we are told, Einstein was happy with Riemannian curvature as a description of spacetime - but at the time gtr was developed he had no knowledge of global expansion, and therefor no reason to suspect that curvature could be consequent to motion rather than static mass
yogi said:Perhaps he had grave doubts when he referred to the left side of the equation as made of fine marble, and the right side as a house of straw.
yogi said:So my point is, if you consider expanding space as having an intrinsic acceleration, the deformation of such can be equated to stress.
yogi said:Pervect had commented to the effect that if gravity is simply a force, there could be no time dilation. But if the force is consequent to a dynamic, you can arrive at the left side of the Einsteins equation by a different route.
yogi said:Not everyone has the same view of space or spacetime.
yogi said:While it may not be in vogue to think in terms of a spatial medium that can contract and shrink, most persons do not have the ability to deal with these interesting questions from an abstract mathematical perspective
Like Bjarne (whom I've put in my "ignore" list)
What contradiction between the stone's fall and gravitational tides? Who ignores such phenomena? They are basic to understanding the theory of gravitation.Bjarne said:The problem for Chris and a whole world of scientist is that they are dealing with fragmented knowledge, and also puts contradiction gravity phenomena on the “ignore list”, - (a stone fall in one direction,- the tide the other in the opposite).– I can not understand why physicist at least nor tries to understand such huge mysterious phenomena as a whole. There a huge number of broad hints. Unfortunately are all written on the ignore list
What contradiction between the stone's fall and gravitational tides?
If a person is going to criticize a theory, rather than just ask questions about it for enlightenment, then they need to first fully understand that theory; otherwise they demonstrate only their own ignorance.Bjarne said:A stone fall in one direction, - the tide in the exact opposite.Garth said:What contradiction between the stone's fall and gravitational tides?
These phenomena’s is off course not a contradiction, - only our attemp to solve the cause of these often seems to be.
We have no general accepted gravity theory able to comprehend the cause of both these phenomena’s.
yogi said:I consider Einstein's words regarding the conditioning of space by matter as definitive as to how we should interpret space in the gtr...Einstein viewed space as something. That may not be the view held by many on these boards - but it was Einstein's view.
yogi said:in de Sitters spherical universe where both space and time dilate equally.
yogi said:If the de sitter sphere is uniformly expanding (uniform radial dilation c) the spacetime surface(s) [3 space and one time] of the deSitter universe will have normal components of acceleration.
Chris Hillman said:Hi, yogi, I must demur on at least one point:
- Einstein's views changed so radically and so often that IMO statements of the form "X was Einstein's view" have little meaning.
- Einstein died in 1955, well before the "Golden Age of Relativity" (c. 1960-1975). You might be able to argue that some statement on topic T by Einstein represents his definitive statement of his own most mature view on topic T, but IMO it would be ludicrous to insinuate that physics/mathematics stopped when Einstein died.
True - his views did change - what I detect was a shift in the direction of attempting to explain physics in terms of space as a substantive
No, "the moons circulation causes the acceleration of the Earth to change a bit and this is the cause of the tide" this is totally wrong, did you not understand the careful illustration I gave you?Bjarne said:Garth
OK the moons circulation causes the acceleration of the Earth to change a bit and this is the cause of the tide. Well probable nothing wrong with that. I am focused of the variation of force that space passes on between the bodies, and the cause of that. It’s off course the same force that pulls down a stone. We can explain both phenomena’s based on the acceleration of gravity (the expression of a such force) but this is of course only a superficial solution, - and well you are right nothing wrong with that.
Thank you for sharing your knowledge.
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Bjarne
Chris Hillman said:This sounds like the same fallacy I seem to have been debunking with unusual frequency in recent days here at PF. If so: "length" does not "shorten" and "time" does not "slow down".
Regardless, given that the term "dilation" is seriously misleading but well entrenched, I suggest that everyone here should always take the time to write out more carefully what one really means. Never forget that so-called "time dilation" effects always involves a comparison between two clocks in different states of motion. Thus, it never makes sense to say "time dilates" without the context of what clocks are being compared and how. Similar but even more nuanced remarks hold for "space dilates".
Chris Hillman said:However, I have been mulling the prospect of trying to write some brief expositions of frame fields and "beacon" null geodesic congruences in some simple and often encountered cosmological models including
So we should postpone further discussion until this material appears.
- Milne frame in Minkowski vacuum (as a foil for genuine cosmological models),
- expanding inertial frame in deSitter lambdavacuum,
- frame of dust in FRW dust models, plus FRW lambdadust models, plus frame of inertial particles not comoving with dust, plus contracting FRW matched to Schwarzschild as per Oppenheimer-Snyder collapsing dust ball model,
- frame of dust for LTB dusts (spherically collapsing dust clouds),
- ditto, for McVittie dust ("interpolates" between Schwarzschild and FRW),
- for Szekeres dusts (a simple solution with no Killing vector fields at all),
- for planar symmetric Kasner dust (a homogeneous but nonisotropic example),
- for Kantowski-Sach dust (plus matching to Frolov observers in Schwarzschild vacuum future interior)
- for Mixmaster dust (or perhaps its NIL analog, which is simpler)
- for Van Stockum dust (a swirling dust cloud),
- for Godel lambdadust (another swirling dust cloud)
We certainly can and do use Newtonian theory in regimes where it is accurate enough, such as when NASA sling shot a spacecraft around the solar system with incredible accuracy.Bjarne said:Garth
Right, - Space-time is a better expression, even though it belongs to a higher dimension.
I mean space can certainly deform whereby time also changes.
But when we want to understand why a horizontal moving cannonballs lose its potential energy (100 MPH) before a football, - we still depend on a Newtonian way of calculation / theory, - right?
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Bjarne
yogi said:what I detect was a shift in the direction of attempting to explain physics in terms of space as a substantive
yogi said:in the spherical space-time universe of de Sitter, the two sphere surface grid is composed of time and space.
yogi said:As the spherical universe expands, both the time dimension and the space dimension increase in relation to their previous lengths ...its the inflating balloon model except that the surface is composed of one time dimension and one space dimension rather than two space dimensions - so the reference for change is the previous surface itself
yogi said:If you like - with intense effort I will probably to be able to understand about every third word
I don't understand your question of "a horizontal moving cannonballs lose its potential energy (100 MPH) before a football".
With the correction that we are talking about curved 'space-time' and not just 'space', yes space-time curvature as defined in GR can accurately determine free falling trajectories, that is the point.My point is that it can’t only be ‘curved space’ that determinate how fare an objects travel
because observations have been made, such as the light deflection by the Sun or time delays of spacecraft whose radio pulses pass close to the Sun en route to Earth, that are accurately predicted by GR but not Newton. It is called doing science, theories stand or fall on empirical testing and falsification.To my opinion why should it not be possible that both Newton and Einstein both was right?
is gravity stronger than we think possibly due to Planck sized dimensionsmarcus said:the most accurate theory of gravity, currently, represents it as the way matter affects geometry. I think this remains mysterious. How can matter affect geometry?
and there is the puzzle about inertia. why should stuff follow geodesics? and why should a thing's inertia ("inertial mass") be the same as the ("gravitational mass") strength with which it bends geometry? this does seem elusive, to use your word.
I've just been reading a 2001 book by Smolin called *Three Roads to Quantum Gravity* and I'm amazed at how good it is. Didn't expect such clarity and depth in a popular-written book. The last chapter has a prospective on how these very same problems might eventually (over next 10 years say) be addressed and solved. nice thing is that he doesn't just trivialize the problems---he takes a serious look into them. Great book.
gravity can be repulsive ?
andrewj said:gravity can be repulsive ?