andrew s

I would speculate that it is in the A/D and gain circuits. Providing high stability in a consumer chip is perhaps too much to hope for. If you stress the circuit with multiple reads with no time to stabilise I would not be suprised noise were introduced. Regards Andrew

But, you need to make both GR and SR corrections to make GPS work as the clocks the satellites carry need correcting both for both their velocity and hight in the earths gravitational well. Regards Andrew

It amuses me (easily done) that it is always mentioned that we don't have a quantum theory of gravity but never that we don't have a quantum theory in curved spacetime. QFT is based on the flat spacetime of SR. Regards Andrew

I obviously agree but it is not mass that is the cause of the internal forces but the other forces principally electromagnetic (at normal scales) but also the strong and weak forces at nuclear scale. Simply gravity is just geometry and so impacts even zero rest mass particles. Regards Andrew

I find that a very sad view. We know what gravity is as well as we know anything. It allowed the prediction of the bending of the path of light by a massive bodies and the gravitational waves that allowed us to observe the merging of neutron stars and black holes. Yes it may not be the ultimate answer but what more could you reasonably expect? Regards Andrew

Exactly, analogous in fact. Regards Andrew

Yes, a google of "Gravity Inside a hollow sphere " will quickly answer the question in the affirmative. In fact Newton proved the theorem himself. Regards Andrew

@Tiki all I can do is quote one of the mentors PeterDonis from Physicsforums In response to Martin Scholtz who said: two observers, although both following geodesics, have non-zero mutual acceleration is coordinate-independent thing and it happens only in the presence of curvature. PeterDonis replyed: This is tidal acceleration, and since both observers are in free fall, there is no force involved in the GR sense. It's simply spacetime geometry--as you say, tidal acceleration is a sign that spacetime is curved. One could talk of "tidal force", but once again, the force involved is not gravity, but internal forces inside objects that cause stresses due to parts of the object not moving on the geodesic paths that they would move on if the object had no internal forces and every part of it could move independently. Source https://www.physicsforums.com/threads/is-gravity-a-force.975552/#post-6214766 Regards Andrew

No in my example it is electromagnetic (em) just as it is the force that stops you free falling through the earth. When you jump you free fall back to earth until em stops you. Maybe, we will have to agree to disagree 😀. Regards Andrew

Indeed, but science is about the interplay of observation/experiment and theory. There are no theory free observations anymore than there are observation free theories (or at least there shouldn't be). Regards Andrew

If you have two non interacting test particles side by side free falling in a non uniform curved spacetime then they will drift apart. However, if they are bound, say by covalent bonds, then they will resist this and it is this resistance that is felt as the tidal force. Regards Andrew

Of course we might be wrong but the models we have today are the best fit we have to the data we have today. Strangely, we need dark matter and dark energy to make our current best cosmological model "LCDM" work so I struggle to see how they are "so whacky that they might never fit in our current models". They already do. Regards Andrew

Correct on tidal forces as different parts of a finite object want to free fall on different trajectories. In GR gravity is the curvature of space time not just time and it is not a force. Free fall is the trajectory taken when no forces are acting on the object. The GR equivalent of Newtons first law. However, here the straight lines are geodesics. Regards Andrew

You need to look at the tidal effects. Maybe two large masses head and foot. Dark matter so you can't see them. Regards Andrew

If I accept this then I think what you can tell is the you are not in an inertial frame. I am less clear that you could tell you were spinning as opposed to experiencing a radial force. Only our previous referenced experience would allow a rotating interpretation. Another might be being stretched like a rubber band. Regards Andrew

I think the scenario you propose is impossible to realise. If you were alone you could not set your self spinning. If one one were to set you spinning they would spin in an equal and opposite way to you. Your spin would be relative to theirs. You can't just spirit them away. Regards Andrew

Unfortunately, the mathematical underpinnings of GR and QFT are totally incomparable. This is why reconciling the two in a so called "quantum gravity" is proving, how should I put it, difficult. Regards Andrew

As I am sure you all know F=ma only holds in an inertial frame. In a rotating frame as in @vlaiv thought experiment you have to add the pseudo forces to make it work, hence, the centripetal force. Similar forces are needed to explain Foucault pendulum as viewed in the rotating frame of the earths surface. Regards Andrew

Now there is a good research topic. Regards Andrew

I have looked at most of the current theories but until we have observations that can differentiate them my preferred choice is a residual cosmological constant. Regards Andrew

Thanks I have found the paper and will look into it more carefully. Just reading the abstract it seems to rely on rotation of the Universe which is currently ruled out by observation. Regards Andrew Just checked it took 7 yrs and 4 versions to get on arXiv and has not been referenced according to Google scholar.

@ollypenrice would they not crash at the equator on the opposite side to where they started? 😱 Regards Andrew

Can you provide a reference to that view? It is more common, but not universal, view that dark energy is just the cosmological constant of GR I.e. a net residual curvature of spacetime. Regards Andrew

In GR gravity is caused by the curvature of spacetime. It predicts gravitational waves. Gravitons are an unobserved prediction from a yet to be completed quantum theory of gravity. The force you feel as the pull of gravity is in fact the push of the electromagnetic force between the material in your feet and the floor as it prevents your free fall along a spacetime geodesic. (Yes in Newtons theory it is a force but this theory is less accurate than GR) Regards Andrew

@SgtBilko sorry I misunderstood your original post. Regards Andrew