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Thread: How gravity works

  1. #1

    How gravity works

    Introduction

    See this thread What do you think is mainstream in cosmology? I think it was an interesting conversation. One thing that cropped up was why does the horizontal light beam curve downwards, and why does your pencil fall down? There was a suggestion that it isn't mainstream to explain this, and that we don't use physics to make claims about why the beam curves. I disagree with that. My sentiment is that we do physics to understand the world, not to shut up and calculate. So I'm going to tell you why I think the light beam curves downwards, and why your pencil falls down. I think most of what I'm going to tell you is mainstream, or was, or should be, or will be. See what you think.

    An optical clock isn’t some cosmic gas meter with time flowing through it

    If you had a clockwork clock that was running slow, and I told you it was because time was running slow inside the clock, you’d laugh in my face. You’d know that it was due to some issue with the mechanism. Perhaps the oil was drying out and gumming up the works. Whatever the reason, you’d know that your clock was going slower because the clockwork was going slower. So, what if I showed you an optical clock going slower? An optical clock is typically an atomic clock which employs ytterbium atoms instead of caesium atoms, and visible light instead of microwaves. An optical clock isn’t some cosmic gas meter with time flowing through it. Nor is a clockwork clock. When a clockwork clock goes slower it’s because the clockwork goes slower. And when an optical clock goes slower, it’s because the optics goes slower. And when does an optical clock go slower? See the interview with David Wineland of NIST: "if one clock in one lab is 30cm higher than the clock in the other lab, we can see the difference in the rates they run at". An optical clock goes slower when it’s lower. This is said to be the hard scientific evidence for gravitational time dilation. But it’s really the hard scientific evidence for light goes slower when it’s lower.

    Einstein said light curves where the speed of light is spatially variable

    Einstein came up with special relativity in 1905 with his two postulates, one of which was the speed of light is constant. But only two years later in 1907 he was saying light curves in a gravitational field because the speed of light isn’t constant. He said it again in 1911, 1912, 1913, 1914, 1915, and 1916. You don’t find Einstein saying light curves because spacetime is curved. Instead you find him saying "the curvature of light rays occurs only in spaces where the speed of light is spatially variable”, and talking about the energy-density of space varying. You can liken this to the density of air varying. Sound waves in air tend to curve downwards at night because the cooler air near the ground is denser:


    Image from Rod Nave’s hyperphysics, see refraction of sound

    It's similar for sonar waves in the sea, see figure 10 of the US Navy ES310 web page. It's also similar for light. A light wave “veers” downwards rather like a car veers left when it encounters mud at the side of the road. We can do something similar with light with something as simple as a piece of glass. That’s why we have lenses in spectacles. But we can also do it with an energy-density gradient in space. That’s why we have gravitational lensing. See Professor Ned Wright’s Deflection and Delay of Light article for more. He doesn’t say the light is deflected because spacetime is curved. Instead he says this: "In a very real sense, the delay experienced by light passing a massive object is responsible for the deflection of the light. The figure below shows a bundle of rays passing the Sun at various distances":


    Gif from Ned Wright’s Deflection and Delay of Light

    The delay is there because the speed of light is lower closer to the sun. This is why Irwin Shapiro said "according to the general theory, the speed of a light wave depends on the strength of the gravitational potential along its path". Also see Is The Speed of Light Everywhere the Same? It’s a PhysicsFAQ article by Don Koks. He says the answer is no, and that "Einstein talked about the speed of light changing in his new theory". Also see the Wikipedia article on the variable speed of light. Pay attention to the section on Einstien's updated proposals. It says "there is no other way to interpret the velocity of light in this usage except as a variable scalar speed". I'd say https://arxiv.org/abs/0705.4507 is worth a read too.

    The rubber sheet analogy

    Once you know all this, you can make sense of the rubber-sheet analogy. It’s sometimes said to be a tautology, in that gravity is used to try to explain gravity. However it can explain gravity rather well provided you know what it’s really depicting. The starting position is that the curvature you can see in the images relates to spacetime curvature. That’s associated with a “curved metric", and a metric is associated with measurement. As for what you’re measuring, imagine you could place a 15 x 15 array of optical clocks throughout a horizontal slice of space around the Earth. Then you plot all the clock rates, such that the lower slower clock rates generate data points lower down in a 3D image, and the higher faster clock rates generate data points higher up in the 3D image. When you join the dots, your plot looks like this:


    CCASA image by Johnstone, see Wikipedia

    That's an image from the Wikipedia Riemann curvature tensor page. It's effectively the rubber-sheet depiction of curved spacetime. And because it's derived from optical clock rates, it's also a plot of the speed of light. Some might say that it’s just a plot of the “coordinate” speed of light, but it’s more than that. There is no time flowing inside an optical clock, so the height at some location on the plot depicts the real speed of light at that location. It also depicts the gravitational potential at that location. Meanwhile the slope at some location depicts the first derivative of gravitational potential, and therefore the force of gravity at that location. The curvature at some location depicts the second derivative of gravitational potential, and therefore the tidal force at that location. That’s where the force of gravity changes most. That’s spacetime curvature. If you don’t have any spacetime curvature, your plot can’t get off the flat and level, which is why spacetime curvature is said to be the defining feature of a gravitational field. But note that a marble rolls down where the sheet is sloping rather than curved, and that your plot is what’s curved, not space. Your plot of measurements is curved so your metric is curved, so spacetime is curved, but space is not.

    Light does not follow the curvature of spacetime

    This is why light does not follow the curvature of spacetime. You can appreciate this if you zoom in on a section of the plot. If we represent a light beam with a yellow line, it curves wherever there’s a gradient in gravitational potential. That’s where the plot has a slope, where the grid lines are tilted as opposed to curved.



    The force of gravity and so the curvature of light is greatest where the tilt is greatest. The tilted light-cones in the 2009 Stanford singularities and black holes article by Erik Curiel and Peter Bokulich depict this. Alternatively you can emulate the tilt with a piece of stiff board. Lift one side up, and roll a marble across it. It follows a curved path because the board is tilted, not because the board is curved. It’s similar for the room you’re in. The force of gravity is 9.8 m/s² at the floor and at the ceiling, so there’s no detectable tidal force, and so no detectable spacetime curvature. But your pencil still falls down. That’s detectable. The point to note is that gravity isn’t there because spacetime is curved, it’s there because there’s a gradient in potential, and a gradient in the speed of light.

    Gravitational time dilation does not occur because spacetime is curved

    Another point to note is that gravitational time dilation does not occur because spacetime is curved. That’s confusing cause and effect. Curved spacetime corresponds to your curved plot of optical clock rates, and optical clocks don’t go slower when they’re lower because your plot of clock rates is curved. They go slower when they’re lower because light goes slower when it’s lower, along with all other electromagnetic phenomena. And light goes slower when it’s lower because space near the Earth is different to the space further away. Because a concentration of energy in the guise of a massive planet “conditions” the surrounding space, this effect diminishing with distance in a non-linear fashion.

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  2. #2
    Space is a polarizable medium

    Einstein talked about it in his 1920 Leyden Address. He said this: "According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that 'empty space' in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty". Einstein didn’t talk about spacetime curvature. Instead he talked about space that was neither homogeneous nor isotropic. He also spoke of "the refraction of light rays by the gravitational field". So did Newton, see Opticks query 20: "Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies in empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines?” All this talk of aether might sound archaic, but it isn’t. Julian Schwinger wrote a paper quantum electrodynamics II : vacuum polarization and self-energy. Space is a polarizable medium. You can also find modern authors saying much the same thing. See for example the 2008 paper Inhomogeneous vacuum: an alternative interpretation of curved spacetime.

    Space is not some ideal Newtonian emptiness

    Also see the Wikipedia aether theories article and note the quote by Nobel laureate Robert B Laughlin: "it is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed". Laughlin also said space is more like a piece of window glass than ideal Newtonian emptiness. He finished up saying this: "the modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo". Einstein is supposed to have done away with the aether in 1905 but in the end, he didn’t. He thought of space as a something rather than a nothing. In his 1929 essay on the history of field theory, Einstein described a field as a state of space. He was talking about gravitational fields and electromagnetic fields, and he said this: "it can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds". The point to appreciate is that according to Einstein a field isn’t something that exists in space, it’s a state of space. As to what sort of state, I’d say a gravitational field is a place where space is inhomogeneous because it has an energy-density gradient. Or perhaps it’s better to say it’s a pressure-gradient, and that space has an "elastic" quality.

    Space is modelled as an elastic solid

    This is why if you google on Einstein elastic space, there’s plenty of hits. This is also why I think general relativity is related to continuum mechanics. This is why we have the stress-energy-momentum tensor, which "describes the density and flux of energy and momentum in spacetime":


    Public domain image by Maschen, based on an image by Bamse see Wikipedia

    The shear stress term on the right tells you we’re dealing with something that could be modelled like some kind of elastic solid. The energy-pressure diagonal tells you it's an elastic solid subject to pressure. For an analogy, imagine you have a block of gin-clear ghostly elastic jelly, with grid lines in it so you can see what's going on. You slide a hypodermic needle into the centre of the block, and inject more jelly. This represents a concentration of energy bound up as the matter of a massive star. It creates a pressure gradient in the surrounding jelly. Stress is directional pressure, the pressure is outwards, and Einstein’s equation Gμν = 8πTμν is modelling the way gin-clear ghostly elastic space is conditioned by the energy you added.

    Curved spacetime is not curved space and time

    Yes, some people talk about elastic spacetime as opposed to space, wherein gravitational waves are elastic ripples that propagate through spacetime. I think that’s the wrong approach myself, because spacetime models space at all times, so there is no motion in spacetime. So it’s space that’s elastic, not spacetime. People also talk about curved spacetime as if it’s curved space and curved time, but I think that’s the wrong approach too. See John Baez and Emory Bunn’s preliminaries article dating from 2006: “Similarly, in general relativity gravity is not really a ‘force’, but just a manifestation of the curvature of spacetime. Note: not the curvature of space, but of spacetime. The distinction is crucial”. Space isn’t curved where a gravitational field is. Instead as Einstein said, space is “neither homogeneous nor isotropic” where a gravitational field is. Because a concentration of energy conditions the surrounding space. Because of this the speed of light is spatially variable, and because of that light curves. When you plot the spatial inhomogeneity using optical clocks at different elevations, your plot is curved because the inhomogeneity decreases with distance. That’s why the force of gravity diminishes with distance in a non-linear fashion, in line with the inverse square rule. Eventually when you’re a long long way from Earth your optical clock readings no longer exhibit any variation with elevation. At such a location space is homogeneous, spacetime is flat, light goes straight, and your pencil doesn't fall down.

    A better analogy

    It’s important to appreciate that the curvature in the rubber sheet picture is not the curvature of space in some higher dimension. We have no scientific evidence for any such higher dimension. However we do have evidence that space is three dimensional. That’s why the three-dimensional gin-clear ghostly elastic jelly is a better analogy than the rubber sheet. You can draw it by imagining you’re looking at the Earth in the rubber-sheet lattice from underneath. Think in 3D, and you can get a feel for the way the surrounding jelly is pressed outwards rather than pulled down or inwards. Like this:

    [image removed]
    Image credit: NASA (I added the lattice lines)

    However this depiction can still cause issues because the Earth is spherical. As a result the grid lines are curved, sending the wrong message about curved spacetime. To clear the air on that we need to zoom in to get rid of the curvature of the Earth. Like this:

    [image removed]
    Image credit: NASA (I removed the moon and added the lattice lines and the light beam)

    The height of each rectangle in the lattice relates to the optical clock rate at that location. Because they’re optical clocks, the height of each rectangle also relates to the speed of light at that location. And because the speed of light is spatially variable, a beam of light going across the picture will curve downwards like the sonar wave. Hopefully you can see how this relates to Ricci curvature. Whilst the image above shows rectangles instead of geodesic balls, you can easily imagine that the more flattened the rectangle, the lower the gravitational potential. And usually when gravitational potential is lower, the gradient in gravitational potential is steeper, so light curves downwards all the more.

    Why matter falls down

    Anyway, once you understand why light curves, it’s easy to understand why matter falls down. All you need to know about is the wave nature of matter, as demonstrated by the Davisson-Germer experiment. And about the Einstein-de Haas effect which "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". And about pair production, wherein we can make an electron and a positron out of light. It's worth reading See Hans Ohanian’s 1984 paper what is spin? Then like Feynman said, if you look at the energy flow, you find that it "just circulates around and around". So just think of an electron as light going around and around. Then simplify it to light going around a square path. Like this:

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    Now imagine it’s in a gravitational field. The vertical parts of the path stay vertical, but the horizontal parts bend down a little. So the electron falls down:

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    In essence the reducing speed of light is transformed into the downward motion of the electron. Internal kinetic energy is converted into external kinetic energy. But only the horizontal component bends down, so the Newtonian deflection of matter is only half the deflection of light. Since you can diffract protons and neutrons and other things too, the same principle applies to matter in general. To things like your pencil. Like Newton said in his 1692 letter to Richard Bentley, there is no magical mysterious action at a distance. There are no gravitons flying back and forth either.
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  3. #3
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    fools and angels. I am not qualified really to challenge you but it seems to me you are talking about both words and interpretations using words. Just as I complained that relative to me the sun is going round me, the interpretation that light slows down in a gravity field like it does in glass as opposed to space time is curved in a gravity field is just words. We now use lightspeed as a fundamental constant from which length and mass can be derived but that is another use of words to be convenient for our model and for what we use our model.
    You, I guess, link time dilation to the speed of light. One defines the other and vice versa. Is that a criticism of the "mainstream" or an interpretation issue where we choose our words to suit the problem? OK another way to say that is your interpretation of light slowing down in an energy field is another set of words from curved space time. Both "energy field" and "curved space time" are shorthand for maths that explains what we see, or think we see when a distant star is late arriving in our telescope while "transiting" a planet. Better models make better predictions of those kind of observations. In order to check that light slows down we have to fix both distance and time (or spacetime for short) , as well as visiting distant space, and we are in a circular argument to explain what we observe (from here).
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  4. #4
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    Yes, there is the constant confusion between what is a testable physics prediction, and what is human everyday language used to bring some understanding to the physics. Until that distinction is made, there's the potential of spending a lot of time with little return, because there is confusion about the different standards of truth applied to each of those. Physics is tested, words are judged. I'm not saying it isn't interesting to try to use words to describe what is happening, I'm saying it isn't interesting to confuse that with testable physics.

    I'm not trying to be too negative about what was obviously a great deal of effort to think about gravity, I'm saying that the aim is off target. For example, in the above we have "when an optical clock goes slower, it’s because the optics goes slower." The statement is almost completely without value, because it does not give empirical meaning to its terms. What does it mean for a clock to "go slow"? Here we have the confusion between measurable time, and coordinate time. Let me explain each.

    Measurable time: This is the time measured, locally, by a good clock. How do we know, empirically, when a clock is "good"? Simple-- it is good when it is consistent with all the other available clocks in the same region. It is consistency that supports the concept of time-- if every clock in the universe runs slow, then none of them do. If two good clocks measure a different time between two events, then the two clocks took different paths between the events, and neither is "running slow" or "running fast" (how could you even say which?). Rather, a different amount of time elapsed, an amount of time that both clocks measured perfectly correctly. That latter statement is the only one that can be verified empirically, because it is easy to verify empirically that both clocks worked fine-- by virtue of their consistency with other clocks following the same path. This is why it is not possible for an optical clock to have its "optics run slow"-- that optical clock will agree quite well with all other local clocks, including those that don't use any optics at all. Measurable time is local, this is the key notion behind "proper time."

    Coordinate time: Coordinates are something quite different from empirical measurables, they are mathematical abstractions that we use to arrive at a solution. As such, they are really absolutely anything we want them to be, and indeed Einstein went to great pains to make sure that the laws of physics work for any coordinates. However, to call something a coordinate "time", instead of a coordinate blarglesnoot, we merely need that coordinate time to correspond to measurable time somewhere-- the somewhere of the observer in question. But when you extend the coordinates nonlocally, as one invariably does (otherwise you'd just use measurable time instead), that's when they can be anything. It's all ruled entirely by convenience, not physical truth. And those who confuse convenience with physical truth were basically the ones throughout history who have allowed dogma to slow the advance of science. Examples include the flat Earth (it is convenient to coordinatize the Earth as flat, we do it all the time with north-south-east-west etc.), and the idea that the Earth is the stationary center of the universe (it is convenient to talk about the Sun rising and setting, and the stars moving in diurnal circles, etc.).

    So what then did Shapiro mean when he said the speed of light changes with gravitational potential? He meant the coordinate speed of light. He would certainly realize that he has chosen a particular coordinate system from which to make his remarks, and he could easily explain what those coordinates are and what conveniences they offer him. He could also explain other coordinates that get a different result.
    Last edited by Ken G; 2019-Jan-27 at 01:44 AM.

  5. #5
    Quote Originally Posted by profloater View Post
    fools and angels. I am not qualified really to challenge you but it seems to me you are talking about both words and interpretations using words. Just as I complained that relative to me the sun is going round me, the interpretation that light slows down in a gravity field like it does in glass as opposed to space time is curved in a gravity field is just words.
    It isn't just words. Optical clocks go slower when they're lower. They don't go slower because some unseen thing called "time" goes slower. They do it because light goes slower. Einstein said the speed of light is spatially variable, so did Shapiro, so did Don Koks. And others. See what John Moffat and Joćo Magueijo said in Comments on “Note on varying speed of light theories". They said this: "Can c vary? Could such a variation be measured? As correctly pointed out by Ellis, within the current protocol for measuring time and space the answer is no. The unit of time is defined by an oscillating system or the frequency of an atomic transition, and the unit of space is defined in terms of the distance travelled by light in the unit of time. We therefore have a situation akin to saying that the speed of light is “one light-year per year”, i.e. its constancy has become a tautology or a definition”. What they were saying is people use the local motion of light to define the second and the metre, which they then use to measure the local motion of light. So they always say light moves at 299,792,458 m/s. So they will tell you that both light pulses in the (idealized) gif below are travelling at the same speed.

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    Quote Originally Posted by profloater View Post
    We now use lightspeed as a fundamental constant from which length and mass can be derived but that is another use of words to be convenient for our model and for what we use our model.
    Unfortunately, it isn't constant. And do you remember me talking about the mass deficit? A 1kg brick at rest at an elevation of 1000 metres has more mass than the same brick at rest at sea level.

    Quote Originally Posted by profloater View Post
    You, I guess, link time dilation to the speed of light. One defines the other and vice versa. Is that a criticism of the "mainstream" or an interpretation issue where we choose our words to suit the problem?
    I think it's a criticism of the mainstream. I think the mainstream claims the speed of light to be absolutely constant, and says the spatially variable speed of light is merely the "coordinate" speed of light, which doesn't matter.

    Quote Originally Posted by profloater View Post
    OK another way to say that is your interpretation of light slowing down in an energy field is another set of words from curved space time. Both "energy field" and "curved space time" are shorthand for maths that explains what we see, or think we see when a distant star is late arriving in our telescope while "transiting" a planet. Better models make better predictions of those kind of observations. In order to check that light slows down we have to fix both distance and time (or spacetime for short) , as well as visiting distant space, and we are in a circular argument to explain what we observe (from here).
    I don't think it's a circular argument. Einstein said what he said, as did Shapiro etc, and the spatially variable speed of light is demonstrated by the NIST optical clocks. People who dispute that are dismissing the evidence they can see, and believing in something they can't see. They believe that time goes slower, but light doesn't.

  6. #6
    Quote Originally Posted by Ken G View Post
    Yes, there is the constant confusion between what is a testable physics prediction, and what is human everyday language used to bring some understanding to the physics.
    Like I said, we do physics to understand the world, not to shut up and calculate.

    Quote Originally Posted by Ken G View Post
    Until that distinction is made, there's the potential of spending a lot of time with little return, because there is confusion about the different standards of truth applied to each of those. Physics is tested, words are judged. I'm not saying it isn't interesting to try to use words to describe what is happening, I'm saying it isn't interesting to confuse that with testable physics.
    Trust me, when you know how gravity works, things get very interesting indeed. Things like black holes.

    Quote Originally Posted by Ken G View Post
    I'm not trying to be too negative about what was obviously a great deal of effort to think about gravity, I'm saying that the aim is off target. For example, in the above we have "when an optical clock goes slower, it’s because the optics goes slower." The statement is almost completely without value, because it does not give empirical meaning to its terms. What does it mean for a clock to "go slow"? Here we have the confusion between measurable time, and coordinate time. Let me explain each.
    There's no confusion on my part. A clock goes slow when its reading lags that of another clock 30cm higher up. Note what David Wineland of NIST said: "if one clock in one lab is 30cm higher than the clock in the other lab, we can see the difference in the rates they run at". And note that there is no actual time flowing through an optical clock.

    Quote Originally Posted by Ken G View Post
    Measurable time: This is the time measured, locally, by a good clock. How do we know, empirically, when a clock is "good"? Simple-- it is good when it is consistent with all the other available clocks in the same region. It is consistency that supports the concept of time-- if every clock in the universe runs slow, then none of them do. If two good clocks measure a different time between two events, then the two clocks took different paths between the events, and neither is "running slow" or "running fast" (how could you even say which?).
    Because both clocks are sitting in the same lab, and one is 30cm above the other one. Neither is taking a different path between events. Spacetime models space at all times, there is no motion in it or through it.

    Quote Originally Posted by Ken G View Post
    Rather, a different amount of time elapsed, an amount of time that both clocks measured perfectly correctly. That latter statement is the only one that can be verified empirically, because it is easy to verify empirically that both clocks worked fine-- by virtue of their consistency with other clocks following the same path. This is why it is not possible for an optical clock to have its "optics run slow"
    It is possible, it happens. You have two optical clocks, one's a foot higher than the other, and the lower clock runs slower. There is no sign of anything called "time" elapsing. Neither clock is actually measuring something called time flowing or passing or elapsing. Like I said, a clock is not some cosmic gas meter with time flowing through it.

    Quote Originally Posted by Ken G View Post
    -- that optical clock will agree quite well with all other local clocks, including those that don't use any optics at all. Measurable time is local, this is the key notion behind "proper time".
    Proper time is merely some cumulative measure of local motion. You aren't really measuring some unseen thing called time.

    Quote Originally Posted by Ken G View Post
    Coordinate time: Coordinates are something quite different from empirical measurables, they are mathematical abstractions that we use to arrive at a solution.
    Those two optical clocks are only 30cm apart, and the lower clock is running slower than the upper clock. That's empirically measurable.

    Quote Originally Posted by Ken G View Post
    As such, they are really absolutely anything we want them to be, and indeed Einstein went to great pains to make sure that the laws of physics work for any coordinates. However, to call something a coordinate "time", instead of a coordinate blarglesnoot, we merely need that coordinate time to correspond to measurable time somewhere-- the somewhere of the observer in question. But when you extend the coordinates nonlocally, as one invariably does (otherwise you'd just use measurable time instead), that's when they can be anything.
    In the example I gave, something only 30cm away was non-local. I believe that distance has now been reduced by the latest clocks at NIST. You will be aware that the parallel-mirror light-clock is used as a gedanken clock in relativity. Let me show you the situation when I replace the NIST clocks with parallel-mirror light-clocks:

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    Those two two light pulses are not moving at the same speed. See what I said to profloater about the tautology.

    Quote Originally Posted by Ken G View Post
    It's all ruled entirely by convenience, not physical truth. And those who confuse convenience with physical truth were basically the ones throughout history who have allowed dogma to slow the advance of science...
    The physical truth is that the lower of the two optical clocks goes slower, and there isn't any time flowing through either of them. An optical clock goes slower when it's lower because light goes slower when it's lower.

    Quote Originally Posted by Ken G View Post
    So what then did Shapiro mean when he said the speed of light changes with gravitational potential? He meant the coordinate speed of light. He would certainly realize that he has chosen a particular coordinate system from which to make his remarks, and he could easily explain what those coordinates are and what conveniences they offer him. He could also explain other coordinates that get a different result.
    No he didn't. Irwin Shapiro meant what he said. Einstein meant what he said too, and he said it time and time again:

    1912: “On the other hand I am of the view that the principle of the constancy of the velocity of light can be maintained only insofar as one restricts oneself to spatio-temporal regions of constant gravitational potential”.

    1913: “I arrived at the result that the velocity of light is not to be regarded as independent of the gravitational potential. Thus the principle of the constancy of the velocity of light is incompatible with the equivalence hypothesis”.

    1914: “In the case where we drop the postulate of the constancy of the velocity of light, there exists, a priori, no privileged coordinate systems.”

    1915: “the writer of these lines is of the opinion that the theory of relativity is still in need of generalization, in the sense that the principle of the constancy of the velocity of light is to be abandoned”.

    1916: “In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity”.

    1920: “Second, this consequence shows that the law of the constancy of the speed of light no longer holds, according to the general theory of relativity, in spaces that have gravitational fields. As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable”.
    Last edited by The Physics Detective; 2019-Jan-27 at 03:09 PM.

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    And here's another example of the confusion between physics and sociology that is rife in that post: "Another point to note is that gravitational time dilation does not occur because spacetime is curved. That’s confusing cause and effect." The first thing we must recognize is that there is no equation in physics that asserts cause and effect, cause and effect is a concept that we overlay on top of physics because of the way we are conditioned to think about nature. So it is a product of our intelligence and our language, but it oddly missing in the equations of physics. Take any equation you like from physics, including every equation used in GR, and you will see this. The only physics equation that even gives a glimmer of cause and effect is the second law of thermodynamics, but this only asserts an arrow of time, that all complex processes move in the same direction of time in the sense that they all show entropy increasing in the same direction. But that's just an arrow to time-- it's not a cause and effect in the way The_Physics_Detective appears to mean.

    So of course some will misconstrue these words and think I'm saying that cause and effect is not an important concept, or that I'm saying we never use that concept in how we describe a physics example. No, I have not said that, because cause and effect is a very important concept, and we use it all the time in life, and sometimes even in physics. What I did say, however, is that it is a notion we overlay on top of physics, that is not actually in physics. This is quite easy to show, just give me the place in physics where cause and effect appears, and when you fail, my point is made. So what does that mean, abandon cause and effect? No, it means do with it the same thing you do with all the sociological constructs that we overlay over physics: recognize they come from us, are subjective, perspective-dependent, interpretation-dependent, and nonunique. In short, don't confuse them with statements of physics, recognize they are like coordinate systems-- convenient modes of understanding something that can also be understood in other ways. This is a pretty subtle point, and I don't know if it will be taken in, but even if it isn't, I have a simple challenge: if you claim there is a cause and effect in gravity, demonstrate how it is needed! In other words, demonstrate why I can't get all the same answers without it. When you fail, my point is taken, if if you don't take it.

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    Quote Originally Posted by The Physics Detective View Post
    Like I said, we do physics to understand the world, not to shut up and calculate.
    Please listen: Of course we do physics to understand the world, that doesn't release us from the need to differentiate when we are doing physics from when we are using physics to understand the world. The former is a quite different exercise, because it is objective, well-tested, observer-independent, and unique within a given choice of theory. The latter is none of those things. I don't think you will understand anything I'm saying until you understand that difference, and not understanding that difference is the primary flaw in everything you are saying about gravity.

    A classic example of this flaw in your logic is your tendency to use quotes from authorities without recognizing that the quotes fall into the latter category. The former category contains the equations of physics, the mathematical rules and symmetries they obey (like being coordinate independent, in the case of GR-- importantly), and the body of observations that demonstrate the domains of validity of those laws. The latter category are human language, not physics, used to help us understand the laws from some particular, non-unique, coordinate-dependent, sociological perspective. You need to see that difference, or you will continue to paint non-objective claims as if you thought they were statements of objective truth-- as you have done so many times in the above.

    I will give you a classic example: the concept of curvature. It seems from what you write that you think there is a difference between a curved object, like a sphere, that is a 3D-embedded 2D manifold and supports a concept of tangential Euclidean planes on small scales, and a 2D flat object, like a filled circle, which has a closed non-Euclidean metric on it that causes parallel lines to intersect and alters the meaning of distances within that circle. But there is not a difference, those are simply two sociological pictures (sociological in the sense that they connect with our perceptions of daily life) that are completely mathematically equivalent, and you can use either picture you like to describe closed curvature! So when you talk about spacetime curvature, and try to distinguish a curved spacetime with locally tangent Minkowski spaces as if that was something different from a flat spacetime with a non-Minkowski metric inserted right into it that causes light rays to curve, all you are doing is failing to notice what is a sociologically generated picture you are forcing everyone to adopt, versus what is the mathematical meaning of curvature. It's not that your picture is necessarily wrong (it's hard to tell if it's necessarily wrong as you don't actually use it to do anything, such as make any calculations), it's that it isn't necessarily right either. You are selling something as unique, but I'm not buying it that way-- I know it is not a unique description.

    That's why you use quote mining so much. The quotes come from people who are using a particular perspective, a particular coordinate system. If someone was at a bank when a robber entered, they might later recount that the robber entered from the left and crossed to the right. Now ask yourself this: did that witness "mean what they said"? Obviously they do. But all the same, no one would later use that testimony to claim that the witness was stating that the robber really entered "from the left" and crossed "to the right" in some absolutely correct sense, we would all automatically know their statement was coming from a particular human perspective based on where they were in the room. We would know that automatically because it is such a simple point, but physics and GR are anything but simple. Thus, we miss this point when it happens in GR. That's what you are doing when you mine those quotes. When the person said those things, just like our robbery witness, they either assumed their perspective would be taken into account, or they simply weren't focusing on the need to assert that perspective because they didn't realize someone would later take it as a statement of truth independent of all such considerations.
    Last edited by Ken G; 2019-Jan-27 at 03:41 PM.

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    Quote Originally Posted by The Physics Detective View Post
    It isn't just words. Optical clocks go slower when they're lower. They don't go slower because some unseen thing called "time" goes slower. They do it because light goes slower. Einstein said the speed of light is spatially variable, so did Shapiro, so did Don Koks. And others. See what John Moffat and Joćo Magueijo said in Comments on “Note on varying speed of light theories". They said this: "Can c vary? Could such a variation be measured? As correctly pointed out by Ellis, within the current protocol for measuring time and space the answer is no. The unit of time is defined by an oscillating system or the frequency of an atomic transition, and the unit of space is defined in terms of the distance travelled by light in the unit of time. We therefore have a situation akin to saying that the speed of light is “one light-year per year”, i.e. its constancy has become a tautology or a definition”. What they were saying is people use the local motion of light to define the second and the metre, which they then use to measure the local motion of light. So they always say light moves at 299,792,458 m/s. So they will tell you that both light pulses in the (idealized) gif below are travelling at the same speed.

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    Unfortunately, it isn't constant. And do you remember me talking about the mass deficit? A 1kg brick at rest at an elevation of 1000 metres has more mass than the same brick at rest at sea level.

    I think it's a criticism of the mainstream. I think the mainstream claims the speed of light to be absolutely constant, and says the spatially variable speed of light is merely the "coordinate" speed of light, which doesn't matter.

    I don't think it's a circular argument. Einstein said what he said, as did Shapiro etc, and the spatially variable speed of light is demonstrated by the NIST optical clocks. People who dispute that are dismissing the evidence they can see, and believing in something they can't see. They believe that time goes slower, but light doesn't.
    I am feeling the same here as when you insisted the earth goes round the sun, but the sun does not go round the Earth. A simpler example. Now you ask me to consider the clock going slower as light slowing down, not time, and not presumably distance. I am aware of the evidence and I have read Carlo Ravelli too. He, like KenG in my opinion, is clear about the story we are telling to explain what we observe.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    It is very difficult to disprove your ATM since it is totally devoid of any mathematical formalism.
    A few points:

    1. Light clocks tick slower due to the fact that "coordinate" light speed is variable (proper light speed is constant). KenG already pointed out the difference between the two, I could easily post the mathematical formalism that shows why light clocks' periods are dependent on gravitational potential. As an aside, all clocks' periods are dependent of the gravitational potential as explained by GR and tested by many experiments. GOS is living proof of this fact.

    2. You completely misunderstand two GR effects:

    -light bending by the gravitational bodies

    -Shapiro delay

    Both these effects affect light VELOCITY, not light speed. Light speed is constant.
    Light bending by gravitational bodies, contrary to your ATM is not a refraction effect. If it were, the different wavelength components of white light would be bent by different angles, which they aren't.

    Now an exercise for you:

    Q1. Using your ATM provide the mathematical prediction of the outcome of the Shapiro experiment.
    Q2. Using your ATM provide the mathematical prediction of the advancement of the Mercury perihelion

    Please answer Q1, Q2

  11. #11
    Quote Originally Posted by Ken G View Post
    And here's another example of the confusion between physics and sociology that is rife in that post: "Another point to note is that gravitational time dilation does not occur because spacetime is curved. That’s confusing cause and effect." The first thing we must recognize is that there is no equation in physics that asserts cause and effect, cause and effect is a concept that we overlay on top of physics because of the way we are conditioned to think about nature. So it is a product of our intelligence and our language, but it oddly missing in the equations of physics. Take any equation you like from physics, including every equation used in GR, and you will see this. The only physics equation that even gives a glimmer of cause and effect is the second law of thermodynamics, but this only asserts an arrow of time, that all complex processes move in the same direction of time in the sense that they all show entropy increasing in the same direction. But that's just an arrow to time-- it's not a cause and effect in the way The_Physics_Detective appears to mean.
    The point is that people who consider themselves tp be mainstream will tell you that gravitational time dilation occurs because of spacetime curvature. See this example. But it seems you agree with me on this point, so I won't dwell on it.

    Quote Originally Posted by Ken G View Post
    So of course some will misconstrue these words and think I'm saying that cause and effect is not an important concept, or that I'm saying we never use that concept in how we describe a physics example. No, I have not said that, because cause and effect is a very important concept, and we use it all the time in life, and sometimes even in physics. What I did say, however, is that it is a notion we overlay on top of physics, that is not actually in physics. This is quite easy to show, just give me the place in physics where cause and effect appears, and when you fail, my point is made.
    I gave you one/ Light goes slower when it’s lower because space near the Earth is different to the space further away. Because a concentration of energy in the guise of a massive planet “conditions” the surrounding space, this effect diminishing with distance in a non-linear fashion. Then because light goes slower when it's lower, the horizontal light beam curves downwards, and your pencil falls down.

    Quote Originally Posted by Ken G View Post
    So what does that mean, abandon cause and effect? No, it means do with it the same thing you do with all the sociological constructs that we overlay over physics: recognize they come from us, are subjective, perspective-dependent, interpretation-dependent, and nonunique. In short, don't confuse them with statements of physics, recognize they are like coordinate systems-- convenient modes of understanding something that can also be understood in other ways. This is a pretty subtle point, and I don't know if it will be taken in, but even if it isn't, I have a simple challenge: if you claim there is a cause and effect in gravity, demonstrate how it is needed! In other words, demonstrate why I can't get all the same answers without it. When you fail, my point is taken, if if you don't take it.
    I reject your assertion that cause and effect are not part of physics. Cause and effect are crucial to understanding, and crucial to physics.

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    Quote Originally Posted by The Physics Detective View Post
    The
    .....
    I reject your assertion that cause and effect are not part of physics. Cause and effect are crucial to understanding, and crucial to physics.
    Aha, now I really disagree. Cause and effect is an assumption in our model of reality, possibly untestable, and certainly questionable in the quantum model. It is the model we use in everydaylife of course, but to base physics on it without a caveat is a closed path of reasoning. Maybe it is the basis and maybe it isn’t. It is not the chicken and egg issue that we started from in who orbits whom, but more fundamental. Gravity is all around us but it might be the integrated effect of entanglement or it might be gravitons, the mainstream does not have a mechanism for gravity. So i will not be able to provide one here. But with no mechanism the observation of exotic clocks remains open to interpretation,. I hate dogma, prefer discussion of the evidence.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Quote Originally Posted by The Physics Detective View Post
    The point is that people who consider themselves tp be mainstream will tell you that gravitational time dilation occurs because of spacetime curvature. See this example. But it seems you agree with me on this point, so I won't dwell on it.
    That, of course, comes from exactly the problem Ken highlights - people taking a particular stand on cause and effect, when the physics offers none.
    We plot gravitational time dilation against distance from mass, and we get a curved line - time curvature. Does the underlying maths cause time dilation? Of course not. But if we reify the derived coordinate system, we find ourselves claiming that there is some real stuff, space-time, which causes thing to happen. But at that point we've stepped away from the physics and started editorializing about the physics. It's extremely common, and it's fairly harmless, so long as everyone keeps in mind, at least from time to time, that they're actually talking about mathematical descriptions.

    Grant Hutchison

  14. #14
    Quote Originally Posted by Ken G View Post
    Please listen: Of course we do physics to understand the world, that doesn't release us from the need to differentiate when we are doing physics from when we are using physics to understand the world. The former is a quite different exercise, because it is objective, well-tested, observer-independent, and unique within a given choice of theory. The latter is none of those things. I don't think you will understand anything I'm saying until you understand that difference, and not understanding that difference is the primary flaw in everything you are saying about gravity.
    What flaws? You haven't pointed out something that's wrong with my explanation of how gravity works. You aren't addressing the physics. You're claiming I don't understand anything you're saying, but there's no substance to your critique.

    Quote Originally Posted by Ken G View Post
    A classic example of this flaw in your logic is your tendency to use quotes from authorities without recognizing that the quotes fall into the latter category.
    Einstein made it crystal clear: "the curvature of light rays occurs only in spaces where the speed of light is spatially variable". We can see that the speed of light is spatially variable from those NIST optical clocks. We can see that light curves. We can see that pencils fall down. We can then see the cause and effect, which is that light waves curve downwards just like sonar waves curve downward, because there's a gradient in wave propagation speed:

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    What we can't see is some magical mysterious thing called time that you claim those clocks are measuring. Don't dismiss the hard scientific evidence you can see because you believe in something you can't.

    Quote Originally Posted by Ken G View Post
    The former category contains the equations of physics, the mathematical rules and symmetries they obey (like being coordinate independent, in the case of GR-- importantly), and the body of observations that demonstrate the domains of validity of those laws. The latter category are human language, not physics, used to help us understand the laws from some particular, non-unique, coordinate-dependent, sociological perspective. You need to see that difference, or you will continue to paint non-objective claims as if you thought they were statements of objective truth-- as you have done so many times in the above.
    I reject your assertion that cause and effect is not part of physics.

    Quote Originally Posted by Ken G View Post
    I will give you a classic example: the concept of curvature. It seems from what you write that you think there is a difference between a curved object, like a sphere, that is a 3D-embedded 2D manifold and supports a concept of tangential Euclidean planes on small scales, and a 2D flat object, like a filled circle, which has a closed non-Euclidean metric on it that causes parallel lines to intersect and alters the meaning of distances within that circle. But there is not a difference, those are simply two sociological pictures (sociological in the sense that they connect with our perceptions of daily life) that are completely mathematically equivalent, and you can use either picture you like to describe closed curvature!
    There is a difference between a sphere and a circle. Please address the physics of my thread.

    Quote Originally Posted by Ken G View Post
    So when you talk about spacetime curvature, and try to distinguish a curved spacetime with locally tangent Minkowski spaces as if that was something different from a flat spacetime with a non-Minkowski metric inserted right into it that causes light rays to curve
    A metric is an abstract thing, Ken. It's related to measurement. It doesn't cause light rays to curve. The spatially-variable speed of light causes light rays to curve, just as a depth-variable speed of sound makes sonar waves curve.

    Quote Originally Posted by Ken G View Post
    all you are doing is failing to notice what is a sociologically generated picture you are forcing everyone to adopt, versus what is the mathematical meaning of curvature.
    You're forcing everybody to adopt a sociologically generated picture. I'm telling you how gravity works. And I will reiterate: light doesn't curve because of spacetime curvature. Your measurement of the force of gravity g is the same at the ceiling as at the floor. So there's no measurable spacetime curvature in the room you're in. But your pencil still falls down. Light doesn't curve because of some abstract thing called a metric either.

    Quote Originally Posted by Ken G View Post
    It's not that your picture is necessarily wrong (it's hard to tell if it's necessarily wrong as you don't actually use it to do anything, such as make any calculations), it's that it isn't necessarily right either. You are selling something as unique, but I'm not buying it that way-- I know it is not a unique description.
    I'm selling what Einstein said, and what the evidence says.

    Quote Originally Posted by Ken G View Post
    That's why you use quote mining so much. The quotes come from people who are using a particular perspective, a particular coordinate system. If someone was at a bank when a robber entered, they might later recount that the robber entered from the left and crossed to the right. Now ask yourself this: did that witness "mean what they said"? Obviously they do. But all the same, no one would later use that testimony to claim that the witness was stating that the robber really entered "from the left" and crossed "to the right" in some absolutely correct sense, we would all automatically know their statement was coming from a particular human perspective based on where they were in the room.
    /quote]You're trying to find a reason to dismiss Einstein, Newton, Shapiro, Koks, Wright, and Magueijo and Moffat. But your reason just isn't good enough, and your example is weak. We'd know what happened from the security cameras in the bank.

    Quote Originally Posted by Ken G View Post
    We would know that automatically because it is such a simple point, but physics and GR are anything but simple. Thus, we miss this point when it happens in GR. That's what you are doing when you mine those quotes. When the person said those things, just like our robbery witness, they either assumed their perspective would be taken into account, or they simply weren't focusing on the need to assert that perspective because they didn't realize someone would later take it as a statement of truth independent of all such considerations.
    The truth is that optical clocks go slower when they're lower, and all observers agree with that. And all observers will also agree that they can't see any time flowing through those optical clocks.
    Last edited by The Physics Detective; 2019-Jan-27 at 05:21 PM.

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    Quote Originally Posted by The Physics Detective View Post
    The point is that people who consider themselves tp be mainstream will tell you that gravitational time dilation occurs because of spacetime curvature. See this example. But it seems you agree with me on this point, so I won't dwell on it.
    All that is happening is you are misconstruing the meaning of "cause." We don't even need GR to see this, Newtonian gravity works fine. One of the many essentially equivalent ways of asserting Newtonian gravity in one dimension (for simplicity) is that the second derivative of the potential is proportional to the negative of the mass density. Consider that equation for a moment. Most people attempting to attribute some sociological "cause and effect" to the acceleration of gravity would say that it is "caused" by the presence of mass, and the presence of mass is "caused" by the presence of mass density, ergo, the acceleration of gravity is caused by mass density. Is that not a logical conclusion?

    But look again at that equation-- it doesn't say the acceleration of gravity is proportional to the mass density, it says the gradient in the acceleration is proportional to mass density. So by the exact same logic you used above, we should all conclude that mass density does not cause gravity, but that masses cause gradients in gravity! You see the point? This is just what you are saying about spacetime curvature! We could argue until we are blue in the face if masses cause gravity, or if masses only cause changes in gravity, or if masses cause time dilation, or if masses cause curvature, and what would be the point? We'd only be arguing our own sociological constructs about the meaning of a cause. Now to be clear, there are times when identifying causes is useful because it can help guide a correct solution, and avoid specific errors that can stem from mistaken ideas about causes. This, unfortunately, is not one of those cases, for one very simple reason: your challenge is to identify a single situation, any one at all, where your perspective leads to a correct solution, and the "mainstream" perspective leads to an incorrect solution. When you fail to do that, the point is made.
    I gave you one/ Light goes slower when it’s lower because space near the Earth is different to the space further away.
    No, that is not an example of where cause and effect appears, that is merely an example of you placing the word "because" into your language. Show me an equation of physics where cause and effect appears in the equation. Show me a situation where I need your concept of cause and effect or I get the answer wrong. If you cannot do that, then you have failed to show that your notion of cause and effect is part of physics. Someone else may use a different cause and effect, or no concept of cause at all, and also get the correct result. What does that tell you?

    Whenever I try to argue cause and effect in physics, and I do, I always do one very important thing: I show the testable outcome the other person is getting wrong. That is essential, it always has to be there or the argument is merely personal opinion.
    I reject your assertion that cause and effect are not part of physics. Cause and effect are crucial to understanding, and crucial to physics.
    But you have offered zero evidence. Look at your argument right now-- your logic rests on conflating "crucial to understanding" with "part of physics." But that's wrong, what's part of physics are its laws, mathematical symmetries and logic, and body of experimental evidence that support them. You can tell this is true by picking up any physics book, and notice that this is all you will find in it. When you understand this, you can then ask, then why do we use cause and effect language in physics? The answer is, because it can help us understand. But it's often not necessary to understand it the same way we do! To show necessity, you always have to show where the alternative leads to a mistake. This is what I have asked you to do, and this is what I know you will not be able to do, in regard to what you imagine gravity is.

    I can give you a hundred examples of what I'm talking about. Indeed, I already gave one: the laws of Newtonian mechanics. One can easily use cause and effect language in Newtonian mechanics, and it might look like this. We might say that when you exert a force on a rope, it pulls on a mass, and that causes the mass to accelerate. Inspired by that language, you might assert a= F/m, and indeed I often do-- especially when I am using that picture for the cause and effect there. However, more often we see the equation F = ma, which has a rather different flavor, and a wholly different accounting of cause and effect. In that form, it sounds like we are saying that we are not causing a force on the rope, we are causing mass m to have acceleration a, and because we are causing that, we say by definition that we have exerted a force F. Some books will define force that way for one very simple reason: they realize force is not a "thing" independent from that equation. I don't particularly like that, I like to think of force as an empirical entity that we can measure by using it to stretch a spring, and then once we have empirically defined forces, we can say they "cause" acceleration. Nevertheless, I know that anyone using the equation F = ma, or a = F/m, is going to get the right answer. If I cannot think of a way they get anything wrong by saying that accelerations are what is caused and forces are just a language for bean counting whatever accelerations are happening, and not the causes of anything, then I recognize it is a perfectly valid alternative picture of the cause and effect of Newtonian physics.

    Now, if on the surface you think it sounds absurd to say that accelerations cause forces, you should recognize there is a deeper form of Newtonian mechanics that uses something called a Lagrangian instead of a = F/m. In Lagrangian mechanics, we hardly ever talk about forces at all! Instead, the cause and effect sounds like the scalar action being "caused" to be minimized by requirements of nature, and once the action is caused to be minimum, we get acceleration. Then when we are all done, we can go back and attribute forces if we like, or not-- they never appear anywhere in the attribution of cause and effect.
    Last edited by Ken G; 2019-Jan-27 at 05:39 PM.

  16. #16
    Quote Originally Posted by profloater View Post
    Aha, now I really disagree. Cause and effect is an assumption in our model of reality, possibly untestable, and certainly questionable in the quantum model. It is the model we use in everydaylife of course, but to base physics on it without a caveat is a closed path of reasoning.
    I will never give up on cause and effect. If somebody says why does the apple fall down? I will never shrug my shoulders and say that's not part of physics.

    Quote Originally Posted by profloater View Post
    Maybe it is the basis and maybe it isn’t. It is not the chicken and egg issue that we started from in who orbits whom, but more fundamental. Gravity is all around us but it might be the integrated effect of entanglement or it might be gravitons, the mainstream does not have a mechanism for gravity.
    I'm telling you the mechanism for gravity. Einstein described it, but the mainstream threw it away, even though general relativity is the best-tested theory we've got. See https://arxiv.org/abs/1403.7377. Gravity is nothing to do with entanglement or gravitons.

    Quote Originally Posted by profloater View Post
    So i will not be able to provide one here. But with no mechanism the observation of exotic clocks remains open to interpretation,. I hate dogma, prefer discussion of the evidence.
    The evidence is that optical clocks go slower when they're lower. You can see that. But you can't see any time in those optical clocks.

  17. #17
    Quote Originally Posted by grant hutchison View Post
    That, of course, comes from exactly the problem Ken highlights - people taking a particular stand on cause and effect, when the physics offers none.
    I reject that assertion. Light curves downwards for a reason. Einstein said it's because the speed of light is spatially variable, which matches the evidence of optical clocks, and matches our experience of sonar waves. So I think saying the physics doesn't offer a reason is giving up on physics.

    Quote Originally Posted by grant hutchison View Post
    We plot gravitational time dilation against distance from mass, and we get a curved line - time curvature. Does the underlying maths cause time dilation? Of course not. But if we reify the derived coordinate system, we find ourselves claiming that there is some real stuff, space-time, which causes thing to happen. But at that point we've stepped away from the physics and started editorializing about the physics. It's extremely common, and it's fairly harmless, so long as everyone keeps in mind, at least from time to time, that they're actually talking about mathematical descriptions.
    What we're talking about is understanding gravity. Einstein told us that the massive body alters space, so light goes slower when it's lower, so light curves. What's not to like? Especially when it's just like the curvature of other types of waves.

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    Quote Originally Posted by The Physics Detective View Post
    The truth is that optical clocks go slower when they're lower, and all observers agree with that. And all observers will also agree that they can't see any time flowing through those optical clocks.
    I can't see space flowing through a metre rule either - does that mean rulers don't measure distance, they measure wood?

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    Quote Originally Posted by The Physics Detective View Post
    I reject that assertion.
    I know. That's not a coherent argument, though. Against that, we have the undoubted fact that multiple coherent "causal" narratives can be constructed using the same underlying mathematics - lots of different reasons for why the rope breaks in Bell's "Paradox", for instance. They're all just ways of telling the same mathematical story.

    Quote Originally Posted by The Physics Detective View Post
    What we're talking about is understanding gravity. Einstein told us that the massive body alters space, so light goes slower when it's lower, so light curves. What's not to like? Especially when it's just like the curvature of other types of waves.
    Einstein invoked a particular set of coordinates. Lots of people invoke that coordinate set, in similar circumstances, because it makes some things easier to visualize. But it makes other things harder (see pretty much any layman's question about black holes on this forum, for instance). We choose other coordinates to do other jobs, and the attached "causal" stories come out with different plots.
    The secret is to keep going back to the maths, to check that you're working things out correctly, and not to to try to reify one set of coordinates as if they tell the whole story.

    Grant Hutchison

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    Quote Originally Posted by The Physics Detective View Post
    There is a difference between a sphere and a circle. Please address the physics of my thread.
    Go back and read what I said. If you won't read what I say, there's definitely no point in continuing.

  21. #21
    Quote Originally Posted by Ken G View Post
    All that is happening is you are misconstruing the meaning of "cause." We don't even need GR to see this, Newtonian gravity works fine.
    I'm not misconstruing it. As for Newtonian gravity, don't forget that Newton talked about space growing denser and refracting light rays by "bending them gradually in curve lines". It's essentially the same as what Einstein said. And yet you reject how gravity works by claiming cause and effect are not part of physics? Really Ken, that argument is no argument at all.

    Quote Originally Posted by Ken G View Post
    One of the many essentially equivalent ways of asserting Newtonian gravity in one dimension (for simplicity) is that the second derivative of the potential is proportional to the negative of the mass density. Consider that equation for a moment. Most people attempting to attribute some sociological "cause and effect" to the acceleration of gravity would say that it is "caused" by the presence of mass, and the presence of mass is "caused" by the presence of mass density, ergo, the acceleration of gravity is caused by mass density. Is that not a logical conclusion?
    No, because force of gravity relates to the first derivative of potential, while the tidal force (or the change in the force of gravity) relates to the second derivative of potential. Also no because most relativists would say that the mass of a body is a measure of its energy content. They'd say gravity was caused by a concentration of energy, typically in the guise of a massive planet. They'd be right, but they wouldn't have given enough details.

    Quote Originally Posted by Ken G View Post
    But look again at that equation-- it doesn't say the acceleration of gravity is proportional to the mass density, it says the gradient in the acceleration is proportional to mass density. So by the exact same logic you used above, we should all conclude that mass density does not cause gravity, but that masses cause gradients in gravity!
    Sorry Ken, but you seem to have confused yourself here. Let me paraphrase what Einstein said. The concentration of energy, the "mass", alters the surrounding space. But this alteration diminishes with distance. So space is inhomogeneous. It is not uniform. If it was, light wouldn't curve and your pencil wouldn't fall down. On top of that when you plot the alteration using that array of optical clocks, you see a curvature on your plot, because the inhomogeneity reduces as you move away from the planet.

    Quote Originally Posted by Ken G View Post
    You see the point? This is just what you are saying about spacetime curvature! We could argue until we are blue in the face if masses cause gravity, or if masses only cause changes in gravity, or if masses cause time dilation, or if masses cause curvature, and what would be the point?
    No I don't see your point. The science is clear. A massive body causes gravity. Take away the massive body, and you take away the gravity.

    Quote Originally Posted by Ken G View Post
    We'd only be arguing our own sociological constructs about the meaning of a cause.
    No Ken, understanding how gravity works is not just some "sociological construct". It's physics.

    Quote Originally Posted by Ken G View Post
    Now to be clear, there are times when identifying causes is useful because it can help guide a correct solution, and avoid specific errors that can stem from mistaken ideas about causes. This, unfortunately, is not one of those cases, for one very simple reason: your challenge is to identify a single situation, any one at all, where your perspective leads to a correct solution, and the "mainstream" perspective leads to an incorrect solution. When you fail to do that, the point is made.
    I hinted at that earlier. The mainstream leads to an incorrect solution when it comes to black holes. It can't explain gamma ray bursts, but I can. With a little help from Friedwardt Winterberg.

    Quote Originally Posted by Ken G View Post
    No, that is not an example of where cause and effect appears, that is merely an example of you placing the word "because" into your language. Show me an equation of physics where cause and effect appears in the equation. Show me a situation where I need your concept of cause and effect or I get the answer wrong. If you cannot do that, then you have failed to show that your notion of cause and effect is part of physics. Someone else may use a different cause and effect, or no concept of cause at all, and also get the correct result. What does that tell you?
    It tells me you're claiming that cause and effect is not physics because you can't explain how gravity works. And that you're beating about the bush instead of addressing the physics.

    Quote Originally Posted by Ken G View Post
    Whenever I try to argue cause and effect in physics, and I do, I always do one very important thing: I show the testable outcome the other person is getting wrong. That is essential, it always has to be there or the argument is merely personal opinion. But you have offered zero evidence.
    I've offered ample evidence. Those optical clocks go slower when they're lower, remember?

    Quote Originally Posted by Ken G View Post
    Look at your argument right now-- your logic rests on conflating "crucial to understanding" with "part of physics." But that's wrong, what's part of physics are its laws, mathematical symmetries and logic, and body of experimental evidence that support them. You can tell this is true by picking up any physics book, and notice that this is all you will find in it. When you understand this, you can then ask, then why do we use cause and effect language in physics? The answer is, because it can help us understand. But it's often not necessary to understand it the same way we do! To show necessity, you always have to show where the alternative leads to a mistake. This is what I have asked you to do, and this is what I know you will not be able to do, in regard to what you imagine gravity is.
    Again, you're beating about the bush instead of addressing the physics.

    Quote Originally Posted by Ken G View Post
    I can give you a hundred examples of what I'm talking about. Indeed, I already gave one: the laws of Newtonian mechanics. One can easily use cause and effect language in Newtonian mechanics, and it might look like this. We might say that when you exert a force on a rope, it pulls on a mass, and that causes the mass to accelerate. Inspired by that language, you might assert a= F/m, and indeed I often do-- especially when I am using that picture for the cause and effect there. However, more often we see the equation F = ma, which has a rather different flavor, and a wholly different accounting of cause and effect. In that form, it sounds like we are saying that we are not causing a force on the rope, we are causing mass m to have acceleration a, and because we are causing that, we say by definition that we have exerted a force F. Some books will define force that way for one very simple reason: they realize force is not a "thing" independent from that equation. I don't particularly like that, I like to think of force as an empirical entity that we can measure by using it to stretch a spring, and then once we have empirically defined forces, we can say they "cause" acceleration. Nevertheless, I know that anyone using the equation F = ma, or a = F/m, is going to get the right answer. If I cannot think of a way they get anything wrong by saying that accelerations are what is caused and forces are just a language for bean counting whatever accelerations are happening, and not the causes of anything, then I recognize it is a perfectly valid alternative picture of the cause and effect of Newtonian physics.
    Please stay on topic. This just isn't relevant to how gravity works. When I pull on the rope, I cause the mass to move.

    Quote Originally Posted by Ken G View Post
    Now, if on the surface you think it sounds absurd to say that accelerations cause forces, you should recognize there is a deeper form of Newtonian mechanics that uses something called a Lagrangian instead of a = F/m. In Lagrangian mechanics, we hardly ever talk about forces at all! Instead, the cause and effect sounds like the scalar action being "caused" to be minimized by requirements of nature, and once the action is caused to be minimum, we get acceleration. Then when we are all done, we can go back and attribute forces if we like, or not-- they never appear anywhere in the attribution of cause and effect.
    As above. Please address the OP. Go through it step by step and point out anything that's wrong. You will not be able to.
    Last edited by The Physics Detective; 2019-Jan-27 at 06:22 PM.

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    Quote Originally Posted by The Physics Detective View Post
    I will never give up on cause and effect. If somebody says why does the apple fall down? I will never shrug my shoulders and say that's not part of physics.

    I'm telling you the mechanism for gravity. Einstein described it, but the mainstream threw it away, even though general relativity is the best-tested theory we've got. See https://arxiv.org/abs/1403.7377. Gravity is nothing to do with entanglement or gravitons.

    The evidence is that optical clocks go slower when they're lower. You can see that. But you can't see any time in those optical clocks.
    I accept all the evidence, I accept also the maths, but maths is not any measure of cause and effect, it’s a model. I do not know if the whole explanation is that light slows down, but that is not cause and effect any more than saying gravity is why the apple falls. Gravity is a work in progress for physics. By illustrating that you are not challenging the mainstream. If i ignore the cosmos i can work with gravity using Newton, but he fails to give a cause for the effect, only rather easy to use Maths.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  23. #23
    Quote Originally Posted by Shaula View Post
    I can't see space flowing through a metre rule either - does that mean rulers don't measure distance, they measure wood?
    No, because you can hold your hands up a metre apart, and you can see the space between them. You can see the distance between them, and you can see that it's the same distance between the two ends of the metre rule. You can also waggle your hands and see them moving. So space and motion are empirical. But you can't see time flowing through those optical clocks. Or any clock. Because it isn't. Open up a mechanical clock and you see cogs and gears and things, moving. The clock is clocking up some kind of regular cyclical motion, and giving you a cumulative display that you call "the time". So when that clock goes slow you know it's because the cogs and gears are going slow, not because time is going slow. It's the same with the NIST optical clock at teh lower elevation. It goes slower because light goes slower at the lower elevation.

  24. #24
    Quote Originally Posted by grant hutchison View Post
    I know. That's not a coherent argument, though. Against that, we have the undoubted fact that multiple coherent "causal" narratives can be constructed using the same underlying mathematics - lots of different reasons for why the rope breaks in Bell's "Paradox", for instance. They're all just ways of telling the same mathematical story.
    I'm sorry Grant, but that's back to front. What happens happens. The mathematics is just some abstract symbolism that you're using for calculation and prediction. The reality underlies the mathematics, not the other way around.

    Quote Originally Posted by grant hutchison View Post
    Einstein invoked a particular set of coordinates. Lots of people invoke that coordinate set, in similar circumstances, because it makes some things easier to visualize. But it makes other things harder (see pretty much any layman's question about black holes on this forum, for instance). We choose other coordinates to do other jobs, and the attached "causal" stories come out with different plots. The secret is to keep going back to the maths, to check that you're working things out correctly, and not to to try to reify one set of coordinates as if they tell the whole story.
    Now I'm really sorry Grant. Because when you understand how gravity works, you will appreciate that just about everything you thought you knew about black holes is wrong.

    All: It's been good talking to you all, but I'm afraid I have to go now. As I said to Ken, please address the OP. Please go through it step by step and try to point out anything that's incorrect.

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    Quote Originally Posted by The Physics Detective View Post
    I'm sorry Grant, but that's back to front. What happens happens. The mathematics is just some abstract symbolism that you're using for calculation and prediction. The reality underlies the mathematics, not the other way around.
    Yes, that's my point.
    Because the story you are telling us is all about the mathematics. Light goes slower in a gravity field only in a particular set of coordinates. In another set of coordinates, it's travelling at c. You're making a category error by claiming you're talking about what really happens.

    Grant Hutchison

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    Quote Originally Posted by The Physics Detective View Post
    No, because you can hold your hands up a metre apart, and you can see the space between them. You can see the distance between them, and you can see that it's the same distance between the two ends of the metre rule. You can also waggle your hands and see them moving. So space and motion are empirical. But you can't see time flowing through those optical clocks. Or any clock. Because it isn't. Open up a mechanical clock and you see cogs and gears and things, moving. The clock is clocking up some kind of regular cyclical motion, and giving you a cumulative display that you call "the time". So when that clock goes slow you know it's because the cogs and gears are going slow, not because time is going slow. It's the same with the NIST optical clock at teh lower elevation. It goes slower because light goes slower at the lower elevation.
    But two observers in motion won't agree on the distance between my hands, even if they have their own rulers to compare with. My main point is that the statement that you can't see time flowing in a clock there fore you can't say a clock measures time effectively rules out most methods of measurement. An optical clock compares a calibrated system to another to give a measure of time. So does a ruler.

    Your argument seems to boil down to you claiming one degree of freedom is special, without any strong justification.

    But anyway, the key question is - does your interpretation lead to different predictions to the more popular one? If the answer is yes then there is a chance it is interesting and new. If the answer is no then it doesn't matter. Pick whatever interpretation makes you happy. Science is about the model, the predictions, the tests. If your interpretation does lead to new predictions can you please present them here?

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    Quote Originally Posted by The Physics Detective View Post
    As I said to Ken, please address the OP. Please go through it step by step and try to point out anything that's incorrect.
    Well, there's this:
    Quote Originally Posted by The Physics Detective View Post
    Why matter falls down

    Anyway, once you understand why light curves, it’s easy to understand why matter falls down. All you need to know about is the wave nature of matter, as demonstrated by the Davisson-Germer experiment. And about the Einstein-de Haas effect which "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". And about pair production, wherein we can make an electron and a positron out of light. It's worth reading See Hans Ohanian’s 1984 paper what is spin? Then like Feynman said, if you look at the energy flow, you find that it "just circulates around and around". So just think of an electron as light going around and around.
    I see a sequence of non sequiturs.

    Please explain why you think an electron is "light going around and around".

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Please explain why you think an electron is "light going around and around".
    On the face of it, it seems this idea that matter is confined light should make predictions at variance with GR.
    In GR, slow-moving objects sample primarily the time curvature to determine their acceleration under gravity, whereas relativistic objects increasingly sample space curvature, too, producing a velocity-dependent additional acceleration, with the observed deflection of light (2x Newtonian) as a limit. If objects fall simply because matter is "light going around and around", then all objects are maximally relativistic all the time, aren't they? So why doesn't the Newtonian approximation correctly predict the Einsteinian gravitational deflection of light?

    Grant Hutchison

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    Quote Originally Posted by giorgio View Post
    It is very difficult to disprove your ATM since it is totally devoid of any mathematical formalism.
    A few points:

    1. Light clocks tick slower due to the fact that "coordinate" light speed is variable (proper light speed is constant). KenG already pointed out the difference between the two, I could easily post the mathematical formalism that shows why light clocks' periods are dependent on gravitational potential. As an aside, all clocks' periods are dependent of the gravitational potential as explained by GR and tested by many experiments. GOS is living proof of this fact.

    2. You completely misunderstand two GR effects:

    -light bending by the gravitational bodies

    -Shapiro delay

    Both these effects affect light VELOCITY, not light speed. Light speed is constant.
    Light bending by gravitational bodies, contrary to your ATM is not a refraction effect. If it were, the different wavelength components of white light would be bent by different angles, which they aren't.

    Now an exercise for you:

    Q1. Using your ATM provide the mathematical prediction of the outcome of the Shapiro experiment.
    Q2. Using your ATM provide the mathematical prediction of the advancement of the Mercury perihelion

    Please answer Q1, Q2
    Sorry your post was in the mod queue a bit long. So here's a push to make sure OP sees it.
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    Quote Originally Posted by The Physics Detective View Post
    As I said to Ken, please address the OP. Please go through it step by step and try to point out anything that's incorrect.
    That actually isn't how things work here, we are free to pick whatever part of the OP we wish and simply ask about that element. As for my part, I've already done that-- I'm convinced you will never answer any questions I pose to you that require you to demonstrate with any concrete examples that anyone needs to take your perspective or else they will get some testable answer wrong involving gravity. Now that I am satisfied that you cannot give any examples of any predictions that anyone would get wrong if they don't share your opinions, I have satisfied myself that you are not doing physics. As such, there is no more to say. Not everything you are saying is wrong, but everything is just your opinion, and you tend to quote mine without understanding the context of the quotes you are using. In particular, the difference between a perspective someone is taking to help them understand GR, versus a truth that is required to take or else some answer comes out wrong. Until you can see that difference, I see no point in the discussion. You say physics isn't just shutting up and calculating, but you never actually calculate anything, so that's rather a wide gulf, wouldn't you say? For example, even after a long exchange, I still can't tell whether or not you now recognize that the equivalence principle can in fact be used, and quite easily so, to get the bending of a horizontal laser beam across a room. If your picture of gravity leads you to imagine that the equivalence princple gets that answer wrong, then it is I who am giving you an example of a testable conclusion that you are getting wrong, rather than the other way around.
    Last edited by Ken G; 2019-Jan-27 at 09:12 PM.

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