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 👍