Tag Archives: General Relativity

Light Cone Confusion In The Here And Now

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The light cone graphic below is taken from a Wiki article. The discussion therein gets the basics right, at least with regards to where the concept of a light cone comes from and the dimensional issues with the illustration.

… a light cone (or “null cone”) is the path that a flash of light, emanating from a single event (localized to a single point in space and a single moment in time) and traveling in all directions, would take through spacetime. (…)

In reality, there are three space dimensions, so the light would actually form an expanding or contracting sphere in three-dimensional (3D) space rather than a circle in 2D, and the light cone would actually be a four-dimensional version of a cone whose cross-sections form 3D spheres (analogous to a normal three-dimensional cone whose cross-sections form 2D circles)

https://en.wikipedia.org/wiki/Light_cone

That’s fine as far as it goes – with two caveats. First of all, the spacetime term should be understood as referring to a relational concept of space and time, not to Wheeler’s causally interacting spacetime. Secondly, contracting spheres of light do not exist in physical reality. Much of the rest of the article is gibberish well encapsulated by the labeling of the illustration which basically renders the image incoherent.

Observer should be labeled Galaxy. A galaxy (TSV) on cosmological scales emits ESWs of light and absorbs ElectroMagnetic Radiation from all remote sources that can reach it.

Future LC = Diverging Light Cone -essentially a projection of an ESW emitted by a galaxy.

Past LC= Converging Light Cone – the aggregate of all the incoming EMR that can reach a galaxy from remote sources.

Hypersphere Of The Present is an imaginary mathematical construct that does not exist in physical reality.

For starters, the image has an observer at the shared apex of the two cones but an observer is not mentioned in the text of the Wiki article. In terms of physical reality an observer is at the apex of a “past light cone” – the observer observes light emitted from distant sources, usually omnidirectional emitters like stars and galaxies.

The “past light cone” is the aggregate of all the inbound radiation from those distant sources onto the observer. Rather than calling it a “past light cone” it would be more accurate to label it a Converging Light Cone, with the understanding that the light cone is a relative, point-of-view phenomenon that has no physical relevance except with respect to the observer.

The “future light cone” does not have an observer at its apex, it has an omnidirectional emitter such as a star or galaxy there. The “future light cone” is an aggregate of the successive expanding spherical wavefronts of electromagnetic radiation emitted by the emitter. The “future light cone” should be more accurately labeled a Diverging Light Cone. The DLC is a physical entity, consisting of sequentially emitted expanding spherical wavefronts of electromagnetic radiation. That understanding flows from Maxwell and Einstein – it is standard physics

Borrowing from radio terminology the emitter/observer can be thought of as a transmitter/receiver or transceiver (TSV). The term transceiver will also be used for an observer-only by considering a non-transmitting observer (such as a human) to be a subcomponent of a transceiver such as a star or galaxy system. With respect to the space and time (relational) labels of the illustration, the apex can be labeled “Here and Now”. So the apex represents the HAN of a TSV.

The rest of the labeling is adequate with the relational nature of space and time caveat being understood. What the illustration then presents us with is a stark refutation of the modern conception of the Cosmos as a simultaneously existing Universe. The TSV (galaxy) is always and only at some unique spatio-temporal location.

The TSV is at the center of the omnidirectionally expanding spherical wavefronts of electromagnetic radiation that it emits – the Diverging Light Cone. A TSV is also at the center of all the electromagnetic radiation that is arriving at its particular place and time from all directions – the Converging Light Cone.

The following statement applies to every possible TSV – everywhere and everywhen. Every TSV is at the center of its own unique “universe” which is just its own unique view of a Cosmos that cannot be simultaneously accessed from any three dimensional HAN.

No TSV can detect the state of a remote TSV that is simultaneous with its own HAN. The finite speed of light prohibits any and all such knowledge. The nearest galaxy to our own, Andromeda, is 2.5 million lightyears distant. We see it in our frame as it existed 2.5 million years ago. We do not have and cannot have any knowledge of its “current” state. Andromeda’s “current” state is not part of the Cosmos we have access to. Andromeda’s HAN does not exist in our unique cosmological frame – Andromeda is always There and Then (TAT) in our cosmological frame.

The two dimensional projection labeled the Hypersurface Of The Present illustrates this clearly. The HAN of any TSV is always and only a local state. All other spatio-temporal locations lie outward – TAT- along the surface of the Converging Light Cone. No TSV has access to the HOTP and in fact the HOTP is only a mathematical/metaphysical construct that has no physical correlate. The HOTP does not exist in physical reality because it represents a universal simultaneity which cannot exist because lightspeed has a finite maximum. There is no physical meaning to the concept of a “universal now” – that is the reason there is no universal frame or “now” in General Relativity.

The apex point represents the only HAN available to any TSV. All remote objects exist only in the transceiver’s past – on the TAT of the Converging Light Cone.

Unfortunately, modern cosmologists are of the opinion that they do have knowledge of this simultaneous something (the HOTP) that does not have any existence in physical reality. That is what the term Universe refers to as employed by cosmologists. They believe themselves to be in possession of knowledge of this imaginary, simultaneously existing Universe that, by the known laws of physics, cannot exist. That 13.8 billion year old entity does not exist by normal scientific standards – it is not an observable.

What modern cosmologists have, of course, is just a mathematical model based on some simplifying assumptions adopted @ 100 years ago at a time when the known Cosmos barely extended beyond our own galaxy. One of the model’s assumptions is that the Cosmos has a “universal” spacetime frame (the FLRW metric) even though, in the context of General Relativity, no universal frame exists. A universal spacetime metric inherently includes a universal time with a universal now. Despite the incongruency, the FLRW metric was applied to the GR field equations. The result of this misbegotten effort speaks for itself:

The Standard Model of Cosmology is a miserable failure; it describes a Universe that looks nothing like the Cosmos we observe. To the extent that it can be said to agree with actual observations, it only arrives at such agreements by insisting that physical reality contains entities and events that physical reality, by all direct empirical evidence, does not appear to contain.

The SMC is junk science or perhaps more accurately, it is a mathematicist confabulation presented as science by people who don’t understand basic physics – that the speed of light in the Cosmos has a finite maximum of @3×108 meters/second. It’s not that they don’t know that fact, they do, but rather they don’t understand what it means in the context of the vast Cosmos we observe. They only know what the SMC tells them and that model, they believe, can’t be wrong because if it were smart people like them wouldn’t believe in it.

In fact though, we have no scientific reason to think that the limited view of the Cosmos we have provides us with knowledge of an unobservable, simultaneously-existing, and expanding Universe. The consensus belief of cosmologists that they have such knowledge can be attributed to the fever dream of mathematicism that deeply infects the theoretical physics community. Modern cosmology is a mess.

Science is not perfect. Mistakes are to be expected in science. The Standard Model of Cosmology is a mistake. The model’s foundational assumption of an “expanding universe” is a mistake. It is a mistake in the same way that geocentrism was a mistake. It is fundamentally wrong about the nature of the Cosmos, It is time to move on from the expanding universe model. I’ll give the last word to the astrophysicist Pavel Kroupa:

Thus, rather than discarding the standard cosmological model, our scientific establishment is digging itself ever deeper into the speculative fantasy realm, losing sight of and also grasp of reality in what appears to be a maelstrom of insanity.

https://iai.tv/articles/our-model-of-the-universe-has-been-falsified-auid-2393

The Radiation Density Gradient II

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I want to correct and extend a point raised in the thought experiment of the previous post. Under consideration, a blackbody the size and density of the sun placed in the InterGalactic Medium. Such a body should come to equilibrium with the Ambient Cosmic Electromagnetic Radiation. The ACER has never been properly accounted for in its entirety, though a preliminary effort has been made,

The blackbody, being in equilibrium, is emitting as much energy as it is receiving and it therefore has an energy density gradient surrounding it consisting of the outbound radiation which drops off in density as 1/r2, the radial distance from the center of the blackbody. The underlying ACER density (presumed approximately constant) does not change with distance and may well be considered an inertial field.

Now we flip the fusion switch and make the blackbody a more realistic astronomical object, a star. Compared to the blackbody, this star has a relatively enormous radiation density gradient consisting of all the omnidirectionally emitted radiation produced by the star. The density of that radiation will again drop off as 1/r2.

When a remotely sourced electromagnetic wave passes close by a star the wave is observed to curve as if it were traversing a medium with a density gradient. This is commonly attributed to a gravitational field though such a field has never been observed.

What is observed are a curvature of light and a radiation density gradient. It strains credulity to believe that those two physical facts are uncorrelated. This in turn suggests that observed gravitational effects, commonly attributed to an unobserved gravitational field, are in fact a consequence of matter-electromagnetic energy interactions.

The Radiation Density Gradient

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A thought experiment: Imagine a spherical blackbody the size and density of the sun. Now place that body deep in the InterGalactic Medium. In such a location a blackbody will be absorbing all of the Ambient Cosmic Electromagnetic Radiation that flows continuously through any location in the IGM. Eventually we should expect the sphere to settle into a state of equilibrium with the ACER, continuously absorbing and reemitting all the energy it receives.

This means that there must exist a Radiation Density Gradient surrounding the blackbody consisting of all the inbound and outbound radiation. By normal geometric considerations we should expect the gradient to drop off as 1/R2, R being the radius.

The question is, just how much energy is that? The answer depends on the surface area of the sphere, which is easy to calculate, and the energy density of the ACER which doesn’t seem to be cleanly known, Various frequency bands have been observed to varying degrees. This recent survey of the Cosmic spectrum as a whole suggests that the aggregate nature, behavior and effect of the ACER has not been properly and fully assessed.

Given some reasonable value for the ACER density it would be possible to estimate the RDG of the sphere. With a reasonable value for the RDG a calculation could then be made, using optical considerations, of the curvature to be expected for the path of a light ray passing through the RDG, feeding that curved light back into the RDG calculation. The result could then be compared to the standard curvature predicted by General Relativity but first the additional energy directly radiated by the sun would have to be factored in. Something to think about. Somebody else will have to do the math though. That’s not my gig.

The Mathematicist’s Tale

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19Jun22 The following was posted as a comment to this Medium post by Eric Siegel. It has been slightly edited.

This is a good overview of the illogical mathematicism at the root of the inane Big Bang model. The Friedmann equation is indeed the headwaters of all the nonsense that currently engulfs and overwhelms any possibility of a meaningful scientific account of the Cosmos.

The Friedmann equation rests not only on the two simplifying assumptions of mathematical convenience, isotropy and homogeneity, the author cites. There was also an unstated but implicit assumption that the Cosmos could be treated as a unified, coherent, simultaneous entity, a Universe.

Further it was assumed that the field equations of General Relativity, derived in the context of the solar system, could be stretched to encompass this imaginary Universe. The results speak for themselves.

The standard model of cosmology is an incoherent, unscientific mess. It claims that once upon a time 13.8 billion years ago the entirety of the Cosmos was compressed into a volume smaller than a gnat’s ass that began expanding for an inexplicable reason, then accelerated greatly due to the fortuitous intervention of an invisible inflaton field, which set invisible spacetime expanding at just the right rate to arrive at the current state of the Universe, which we are told is 95% composed of some invisible matter and energy that has no other purpose than to make the model agree with observations; the remaining 5% of this Universe is the stuff we actually observe.

To be clear, all of the entities and events the standard model describes are not part of the Cosmos we actually observe except for that rather trivial 5% of the BB Universe. That 5% of the model is the Cosmos we observe.

Science is the study of those things that can be observed and measured. At some point in the mid 20th century theoretical physicists adopted the conceit that science was the study of mathematical models. This categorical error was compounded by treating scientific hypotheses such as “universal expansion” as axioms (true by definition). In science, all truths are provisional; nothing can be held true by definition.

Axioms belong to the mathematical domain. Math is not science and the mathematicist belief that math underlies and determines the nature of physical reality has no scientific basis. That the Cosmos is a unified, coherent and simultaneously existing entity can only be true if the Cosmos had a miraculous simultaneous origin – the Big Bang,

The problem with miracles is that they are not scientific in nature; they cannot be studied only believed in. Believing in the Big Bang and weaving a fantastical account of an imaginary Universe is a mathematical/metaphysical endeavor that has nothing to do with science or physical reality.

Imaginary Universe? That’s right, the Universe of the Big Bang has no basis in any known physics. In the physics we know, there is a finite, maximum limit to the speed of electromagnetic radiation – 3×10^8 meters per second (186,000 miles per hour). It is via electromagnetic radiation that we observe the Cosmos.

The galaxy nearest to our own is Andromeda; it is detectable by the unaided eye as a fuzzy patch in the night sky. Andromeda is @ 2.5 million light years from Earth. Everything we know about Andromeda is 2.5 million years out of date, of its current state we do not have and cannot have any direct knowledge.

At the far extent of our observational range we observe galaxies that are @10 billion light years away. We do not have and cannot have any knowledge of the current state of any of those galaxies. The same argument holds for all the other galaxies we observe.

Galaxies are the primary constituents of the Cosmos we observe. It follows, therefore, that we do not and cannot have knowledge of the Cosmos’ current state. The very idea of the Cosmos having a current state is scientifically meaningless. Mathematicists believe otherwise:

“If you want to understand the Universe, cosmologically, you just can’t do it without the Friedmann equation. With it, the cosmos is yours.“


See that’s all you need, a simple mathematical model derived 100 years ago by “solving” the field equations of General Relativity (which does not have a universal frame) for a simplistic toy model of the Cosmos (that does have a universal frame). Suddenly you can know everything about everything, even things you can’t possibly observe or measure. That’s mathematicism hard at work creating an imaginary Universe for mathematicists to believe in. The standard model of cosmology has no scientific basis; it is a nerd fantasy and it is scientifically useless.

Two Big Lies

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This fundamental idea — that matter and energy tells spacetime how to curve, and that curved spacetime, in turn, tells matter and energy how to move — represented a revolutionary new view of the universe. Put forth in 1915 by Einstein and validated four years later during a total solar eclipse — when the bending of starlight coming from light sources behind the sun agreed with Einstein’s predictions and not Newton’s — general relativity has passed every observational and experimental test we have ever concocted.

How to understand Einstein’s equation for General Relativity

Sooner or later it seems Ethan Siegel will trot out every disingenuous argument employed by the Big Bang cult in support of its peculiarly unscientific belief system. Two big lies about General Relativity popular among the faithful are succinctly presented in the above quote. The first is that Einstein “put forth” the idea of that GR reduced gravity to the geometrization of a substantival spacetime.

Einstein opposed that view throughout the years subsequent to GR’s introduction, whenever it was proposed. That is a matter of historical record. The formulation that Siegel presents here is a paraphrasing of John A. Wheeler’s well known assertion. That assertion directly contradicts Einstein’s clearly and repeatedly stated position on the matter.

The second big lie is that GR “has passed every observational and experimental test…“. That is true only of tests performed on the scale of the solar system. GR does not pass tests on galactic and cosmological scales without the ad hoc addition of dark matter and dark energy. The existence of neither of those hypothetical entities is supported by direct empirical evidence; the only support they can realistically be said to have is that they reconcile GR with observations. Siegel knows this, he just chooses not to mention it. That is what is known as a lie of omission:

Lying by omission, also known as a continuing misrepresentation or quote mining, occurs when an important fact is left out in order to foster a misconception… An omission is when a person tells most of the truth, but leaves out a few key facts that therefore, completely obscures the truth.

https://en.wikipedia.org/wiki/Lie#Types_and_associated_terms

Adventures in Theoretical Physics II – Fun with General Relativity

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Well here’s a cute little video that manages to do a good job of conveying just how daft and detached from reality theoretical physics has gotten over the last century:

The first 11 minutes or so are effectively a sales pitch for one of the structural elements of the Big Bang Model – Spacetime. The deal is, you’re supposed to believe that the force of gravity is not really there – nothing is holding you to the surface of the earth, rather the earth is accelerating upward and pushing against you.

And the reason this is happening is that you are not following a curved path in –Spacetime, because according to the video you are being knocked off of that curved path by the earth that is accelerating upwards and you are in the way and that’s gravity, tada! How do we know this? Well that’s obvious, it’s in the math and the math tells reality what’s going on and if reality doesn’t like it, that’s too bad. So don’t go trusting your lying eyes, alright.

In addition to Spacetime, this fairy tale is predicated on a ridiculous over-extension of the Principle of Equivalence that Einstein used in developing Special Relativity. Einstein was very clear that the POE applied only under the severely constrained circumstances of a thought experiment. His main purpose seems to have been to provide a physical interpretation for the observed equivalency between gravitational and inertial masses. Einstein presented the POE as informing his ideas about gravity.

The video ignores Einstein’s constraints and pretends the POE is fundamental to General Relativity, so it winds up insisting that things that are obviously not true in physical reality, are, nonetheless, true simply because the math can be framed that way – your lying eyes be damned.

We are told that a man falling off a roof is in the exact same situation as an observer in a non-accelerating rocket ship far from any gravitating body. This claim is made even though it is obviously not true; the falling man will be injured, if not killed, when he hits the ground, whereas no such fate will befall the observer in the rocket ship.

So the idea is, until the falling man meets his unfortunate fate, the situation is the same and therefore both situations are the same, the different outcomes not withstanding – because the math is the same. Observers free falling in orbit won’t be able to tell they’re not in an inertial frame – unless they look out the window, so that’s just like being in an inertial frame too. Right, of course.

In a similar vein, the video insists that an observer in a rocket accelerating at 9.8 m/s^2 will not be able to tell the difference between that situation and standing on the surface of the earth. The presenter fails to mention however, that only holds true as long as the observer doesn’t observe out the window, which will alert the observer that the rocket and therefore the observer are not at rest on the surface of a large gravitating body and therefore the situation is not comparable to standing at rest on the surface of the earth. Also, if any observer steps off the rocket, they will be left behind as the rocket accelerates away. But nevertheless, it’s all the same – as long as no one looks out the window, and maybe you remember that the earth is actually accelerating upwards under your feet, like the floor of the rocket. Sure, of course.

For the sake of introducing some sanity in this matter, here is Einstein on the POE. Note that the second paragraph completely contradicts the claims made in the video implying the equivalence of all inertial and non-inertial frames.

We must note carefully that the possibility of this mode of interpretation rests on the
fundamental property of the gravitational field of giving all bodies the same acceleration, or, what comes to the same thing, on the law of the equality of inertial and gravitational mass…

Now we might easily suppose that the existence of a gravitational field is always only an apparent one. We might also think that, regardless of the kind of gravitational field which may be present, we could always choose another reference-body such that no gravitational field exists with reference to it. This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the earth (in its entirety) vanishes.

RELATIVITY THE SPECIAL AND GENERAL THEORIES, ALBERT EINSTEIN, authorized translation by Robert W. Lawson, original version 1916, translated 1920, appendices 3 and 4 added 1920, appendix 5 added to English translation 1954

It is clear from this statement that the POE of Einstein’s thought experiment is the Galilean version, commonly referred to nowadays as the “Weak” POE. The so-called “Einsteinian” and “Strong” POEs of modern cosmology are post-Einstein formulations attributed initially to Robert Dicke, though there were doubtless others who perpetrated and embellished this nonsense. Neither extension of the POE has anything to do with the foundations of Einstein’s Relativity Theory. It is those mid-20th century extensions that are misleadingly presented in the video as fundamental features of General Relativity.

The POE, in its current, extended usage, is mostly just a conjecture of mathematical convenience, allowing theorists to use Special Relativity math instead of the more difficult General Relativity formulations. It also results in a theoretical claim that the speed of light in a vacuum is a universal constant. That claim contradicts both GR which predicts that the speed of light varies with position in a gravitational field and observations which confirm that prediction.

This unwarranted belief that the speed of light is a universal constant has also produced a cottage industry of theorists expounding a theory of undetected structures called Black Holes with the physically absurd properties of an event horizon and a singularity. No such structures exist. The relativistic slowing of light in a gravitational field precludes their existence. It does not preclude the existence of massive high-density objects.

Ok, let’s grant that this video presentation is of dubious scientific quality and does not, perhaps, represent the consensus view of the scientific community, particularly with regard to the so-called Principle of Equivalence, although if not the consensus, the Strong POE certainly commands significant support by a majority of theoretical cosmologists . The usual suspects will whine, of course, that pop-science presentations like this video cannot be trusted.

That complaint is also lodged against anything written for a general audience, even when the author is a fully accredited scientist with a relevant FAS (full alphabet soup) after their name. If it’s written so non-experts can understand it, then it is, on some level, wrong.

The reason for this situation is straightforward: much of what theoretical physicists believe cannot be translated into clear, logical, statements of scientific fact. What you get instead is confident handwaving consisting of metaphysical assertions that have no factual basis in empirical reality and a lot of math. According to theorists this is because theoretical physics can only be properly understood by those steeped in years of study of the underlying mathematical esoterica that informs only the truly knowledgeable. To which the only proper retort is: math is not physics and if your math cannot be translated into empirically verifiable physical terms – then your math is inadequate to the task of being a proper scientific model of physical reality.

The modern POE is just a conjecture of mathematical convenience, nothing more. Nonetheless, this modern POE permeates and perverts the scientific literature. Here is an Encyclopedia of Britannica entry for the POE:

In the Newtonian form it asserts, in effect, that, within a windowless laboratory freely falling in a uniform gravitational field, experimenters would be unaware that the laboratory is in a state of nonuniform motion. All dynamical experiments yield the same results as obtained in an inertial state of uniform motion unaffected by gravity. This was confirmed to a high degree of precision by an experiment conducted by the Hungarian physicist Roland Eötvös. In Einstein’s version, the principle asserts that in free-fall the effect of gravity is totally abolished in all possible experiments and general relativity reduces to special relativity, as in the inertial state.

Britannica, The Editors of Encyclopaedia. “Equivalence principle”. Encyclopedia Britannica, 31 Mar. 2019, https://www.britannica.com/science/equivalence-principle. Accessed 6 June 2021.

It should be noted that, according to the encyclopedia’s referenced article on Roland Eötvös, his experiment “… resulted in proof that inertial mass and gravitational mass are equivalent…“, which is to say, that it demonstrated the Weak POE only. It is also clear, that the authors of this entry are confused about the distinctions between the three POEs. But what of that; it’s only an encyclopedia trying to make sense of the nonsensical world of the modern theoretical physicist and modern theoretical physics is an unscientific mess.

Nobel Prize in Physics Awarded to 3 Scientists for Work on Fairy Tale

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It’s sadly, depressingly true. Some mathematicist nonsense that attends modern cosmological theory has been awarded science’s grandest prize:


This award has been given to mathematical work on a mathematical theory that has nothing to do with actual physics. Black holes, the ostensible subject matter of the work are fictitious entities that only arise in a mathematical models (such as Schwarzschild solution to the field equations of General Relativity) that ignore one of the fundamental consequences of General Relativity – the speed of light varies with position in a gravitational well.

Specifically, the steeper the gravitational well, the slower the speed of light. It follows directly from this, an observed fact, that gravitational collapse is a self-limiting event. In the Schwarzschild and similar solutions, the speed of light is held constant and results in a theoretical collapsed state (a black hole) that is physically nonsensical, containing as it does a singularity (a mass of zero-volume and therefore infinite density).

Modern theoretical cosmology is a mess, in which Einstein’s General Relativity has been dumbed-down in service of mathematical convenience. The result has been dumb-downed, mathematical models (the Big Bang, black holes), that present physically nonsensical accounts of observed reality. These models do not remotely resemble the physical reality we can actually observe and measure. They are mathematical fairy tales.

The Nobel Prize in Physics was just awarded for work on a dumbed-down mathematical model that has nothing to do with real physics.

The Speed Of Light Is Not A Universal Constant

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One of the great unforced errors of late 20th century theoretical physics was to declare the speed of light in a vacuum, a universal constant. This was done despite the fact that, according to General Relativity, the speed of light varies with position in a gravitational field. This variation in light speed has been observed.

The idea that the speed of light varies with position in a gravitational field is not new. It comes on good authority:

…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. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity: its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light).

Albert Einstein, Relativity The Special And The General Theory, 15th edition

The claim of those who wish to maintain the universal constancy of light speed (c) is that it is justified by the Equivalence Principle. The rather lengthy linked discussion goes into some detail but no mention is made of the constancy of c being consequent on the EP. None of the tests of the principle cited in the article involve measuring the value of c directly.

Einstein invoked the EP in the derivation of General Relativity to provide an interpretation of the observed fact that inertial mass and gravitational mass are equivalent. Given the above quote, he obviously did not find justification therein for asserting the universal constancy of c.

The only reasonable conclusion is that the EP does not justify the claim of a universally constant light speed. The claim appears to be only a lazy mathematicist assumption of mathematical convenience. Theoretical physics is a remarkably irrational, unscientific mess because of this sort of nonsense.