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.