Markus Hanke

+1 You are completely right, this is actually an important distinction - thanks for correcting me on this 👍

It should be noted that a simple curve (1D manifold) has no intrinsic curvature - the Riemann tensor vanishes identically in 1D. But it can of course have extrinsic curvature when embedded in a higher dimensional space.

Great insight! Never thought about it from this particular angle, though in retrospect it seems obvious +1

That seems more reasonable to me - not that I’m an expert, this is quite a subtle question. My own guess - the field equations for torsion in ECT contain no derivatives, and at the same time torsion is completely determined by local matter fields. This implies that torsion vanishes in regions where T=0, and no wave-type equation exists for torsion to “radiate” through vacuum. So it can’t have any propagating degrees of freedom - it’s purely a local phenomenon subject to the local presence of matter.

To be honest, I’m not so sure about this. The energy-momentum that forms the source term in the Einstein equations comes from the Noether current associated with spacetime translations, whereas spin comes from Lorentz invariance. These are different things. It is not in fact possible, AFAIK, to define a unique 4-momentum vector for intrinsic spin, so I don’t see how it could - if taken in isolation - act as a source of gravity. Or am I missing something?

I was under the impression that a recent experiment has cast serious doubts on the viability of Bohmian mechanics: https://www.nature.com/articles/s41586-025-09099-4 This is essentially a direct conflict between what BM predicts in that situation, and the observed outcome. What it means is that, if I understand the implications correctly (and I’m not sure that I do), the concept of “particle” that BM is constructed on does not correspond to particles in the real universe.

I think it goes even deeper - it’s about the wish to reduce an increasingly complex world that inherently operates in shades of grey, to a simple and easy to understand world view that has just black and white. We are more comfortable with what we can emotionally understand. This is why all conspiracies, without exception, are based on “us the good guys” vs “THEM”.

I don’t think this is correct. k=0 just means that the universe is spatially flat, but that doesn’t imply that a(t) must necessarily be unity. You can have a spatially flat, metrically expanding/contracting universe. But in either case, the Riemann tensor does not vanish, irrespective of the value of k.

But metric expansion is a gravitational effect…? That’s purely spatial curvature though. The Riemann tensor as a whole does not vanish in FLRW spacetime, even for the case k=0.

Lol, nice one ☝️

You need to remember though that just because you don’t understand it, doesn’t mean it’s not useful or doesn’t work. It evidently does, because we are using those findings in practical applications. I myself do not understand in detail how a mobile phone is constructed, since electrical engineering is not my area of expertise. But it still works. The average person in any math or physics department at a university isn’t a genius, with very few exceptions - they’ve just decided to put in the work necessary to learn the concepts. In-depth mastery of any subject requires time and effort, that’s just how it is.

You know, that’s a pretty useful general guideline to have, so far as personal speculations in physics go 👍 The reality is that issues such as dark matter/energy etc have been deeply thought about for a long time by a large number of very brilliant minds. You can’t just dismiss and disregard that. It is therefore exceedingly unlikely that the next major breakthrough is going to happen on some social media forum. That being said, I think that most in the physics community agree that our current models are provisional, and that our understanding is evidently off somewhere. The problem is being looked at from all angles - not a day goes by where not a new paper appears on arXiv about proposed new particles, modifications of gravity, discretization of spacetime etc etc. It’s an area of very active research.

I rather suspect that the opposite might happen - he’ll become a bit of a legend in…well…let’s call it “certain circles”. Somewhat similar to what happened to Tesla.

No. There are time-dependent processes that do not involve motion, such as the decay of elementary particles for instance. Locally at any given location, all clocks always tick at exactly “one second per second” - so there is no meaningful way to say that it is different in different locations. The only thing that changes is the relationship of clocks in spacetime, but that’s not the same thing. Again, clocks don’t have different “speeds” - it’s only that clocks at different events are related in non-trivial ways. This may at first glance appear to say the same thing, but it doesn’t. I have personally done it twice - once in high school physics class with an apparatus basically consisting of an assembly of rotating mirrors, and once for fun using the classic setup involving marshmallows in a microwave. And there are many other ways to do it at home, it’s not really that difficult. Note though that the level of precision within such DIY tabletop experiments is naturally limited, so don’t expect too much in terms of accuracy of the final numerical value.

I absolutely agree with you, which is why, in my post, I added the caveat that it wasn’t entirely rigorous. I chose to use it anyway as I thought it might be the best fit to what I perceived (perhaps incorrectly?) to be the level of background knowledge the OP possesses. It’s not always an easy task to balance technical rigour with the needs of the audience. But +1 from me for the excellent explanation for what really happens 👍

Photons do not experience deceleration or acceleration. What happens in a medium other than vacuum is basically that they start interacting with electrons present there; you could perhaps say (not entirely rigorously) that they get absorbed and re-emitted, the process of which leads to a tiny but measurable delay. So the overall measured speed appears to be lower, even though the photons themselves always locally propagate at exactly c. But there is never any deceleration involved, since massless particles cannot travel at anything other than exactly c.

Yes, as I said a few posts ago, LIGO would in principle see both types of waves. However, you need to remember that gravitational waves often aren’t the full story - some events such as BH/neutron-star mergers, or mergers of SMBH with accretion discs, will also produce an EM signature. If gravitational waves were advanced, we would see them passing LIGO long before the actual source event happens (as seen in other channels). Needless to say, so far at least nothing of the sort has ever been observed.

You’re just repeating the same meaningless stuff over and over again.

This doesn’t make any sense at all. If only the speed of light changes, no matter in what way exactly, then this will be detectable, because it will have an effect on certain other fundamental constants. For example, the fine structure constant as locally measured by us would have been different in the past compared to what is now, which would be obvious in a number of ways, most notably in natural fission reactors such as Oklo. And “rate at which we change in the universe” is not a well-defined term. What is it that changes exactly, and with respect to what does it change? You have to be a lot more precise.

So if there aren’t any electrons, for example if I have a sample consisting only of protons and neutrons (ie ionized hydrogen) in a stationary state, then this sample will exhibit no gravity? Also, you are forgetting that gravity is generated by things other than massive particles - for example electromagnetic fields have a gravitational influence, as do pressure, stress, strain etc. Don’t be ridiculous - if you propose an alternative theory of gravity, you need to be able to quantify its predictions. We are doing physics here. Verbal claims aren’t enough, and as an engineer you should understand this. You can’t even know yourself if your proposal is consistent with reality, if you can’t quantify things. For example, if I’d ask you to tell me the trajectory of some test particle (eg a probe) given initial and boundary conditions, how would you do it? You have no mathematical framework, nothing at all, other than verbal claims. It’s useless.

It is a constant only within the same medium, but it is always invariant. I’ve already provided a link to show that the experimental and observational data available to us is inconsistent with the notion of a spatially varying speed of light. You can’t just ignore that and claim the opposite. Let’s think about this for a moment (it’s not like no one has ever considered this before). Ordinary cosmological redshift is due to the expansion of spacetime between the emitter and the observer; it is not a local effect. Therefore, all spectral emission lines of distant sources are shifted by the exact same amount, preserving the overall spectral pattern. This is what we observe. The same is not true for a c that varies with location, because also the fine structure constant directly depends on c, so it would change as well - and as it so happens, the relative energy level splittings in atoms scale with \(\alpha^2\). This means that a varying speed of light would lead to different spectral lines getting shifted by different amounts, so the overall pattern is not preserved - in direct contradiction to what we actually observe. So no, these two effects do not produce the same results, and observation tells us clearly which one actually applies.

These are scenarios where quantum effects become important, so just GR alone isn’t going to be enough here. Also, the Big Crunch is hypothetical.

I think these would look just like ordinary retarded waves to LIGO - what is measured there are essentially just tidal effects. But crucially, causality would be violated - we’d see the wave front arriving here before the source event actually happens in its own rest frame (ie all other signals would arrive much later). Needless to say this has not been observed.

Evidence? This claim is not consistent with any of the data available to us, which shows that speed of light in vacuum is an invariant. See here for example. The speed of light isn’t constant, it’s invariant. That’s not the same thing.

I’m sorry, but this makes even less sense. We evidently are a part of this universe, so there’s no “our” vs “the universe’s” speed of light. There’s only one c, which is the same everywhere (AFAWCT).