Genady

Is there a stable circular orbit for a massless test particle around a Schwarzschild black hole? Photon sphere is unstable, AFAIK.

You are free to guess.

45 years ago President Carter has helped me to escape from the USSR.

Moreover, it is not necessary for c to be a speed of anything. It is a coefficient in the spacetime metric.

I am sorry, I didn't pay attention and did not notice that the question was directed to you. Deleted.

Another way to look at it is that mass of an affected body was introduced into the equation to make it fit into the force-based model. Then, it disappears when the model is not force-based anymore.

deleted

In QFT, photon does not have dimensions.

A gravitational acceleration being independent of a body's mass also follows from the Kepler's third law.

Sure, we need a standard of force, but I don't see how it makes F and m interdependent. I see that F and m are defined separately while F=ma sets their interdependence.

I am not sure it is necessary. We only need to define that in any given conditions (i.e. under given albeit undefined forces) mass is inversely proportional to acceleration. We take a spring, again, stretch it by 10 cm, attach a "unit" of mass, release and measure the acceleration. Then we attach another body to the same spring stretched by the same amount and measure its acceleration. We get the mass of the second body. Etc. Now we have independent definitions of mass and of force and can put them together without any circularity. We could've discovered that \(F \propto m^2a^2\) or \(F \propto e^{ma}\), ... PS. To establish that "mass is inversely proportional to acceleration under given conditions" we can do, e.g., the following. First, the same spring, body, stretch, release, measure acceleration procedure. Then, halve the body and define that its half has half the mass. Repeat the procedure and find that the acceleration doubles. ...

Are you sure? I seem to remember (I think, from Shankar's lectures, but would need to look back to make sure), that force is defined first statically, like the following. Take a spring. Some force needs to be applied to stretch it say by 10 cm. Take two identical springs like that attached parallelly. A force needed to stretch both of them together by 10 cm is defined as double the first force. Etc. After force is defined in such a way, its use in dynamics is a law / equation.

Gravity is not a force nor acceleration. Gravity, or gravitation, is a set of phenomena when bodies affect motion of other bodies without touching or having electromagnetic or other specific interactions. One can talk about 'force of gravity', 'gravitational acceleration' and other characteristics of these phenomena.

Right. It is not there to start with. 😉

Evidently, "Details matter" in the other thread... Let's fix it:

Force and acceleration are never the same. They are measured in different units.

The gravitational acceleration on the Earth surface is g=9.8 m/s2. A body moving with the speed v=28,437 km/h and accelerating toward a center with the g would rotate at the distance R=v2/g=63,670 km from the center. This distance, 63,700 km, is about the Earth radius. Thus, this body would rotate around the Earth center without pushing on the ground, "levitating".

Yes, it is good now, when you write ma=GMm/R2 But be careful: the second time you've written ma=Gmm/R2 instead of GMm/R2 Details matter.

Okay, let's look at the first "equation", F = ma= (G*M1*M2)/R2 It is not an equation, but rather a shortcut of two equations: F=ma ma=\(\frac {GM_1M_2}{R^2}\) The first equation is the Newton second law. It is fine. However, the second equation is meaningless, unless you have a reasonable interpretation for it. PS. You are only a bit older than me.

In Newtonian physics, none.

No. No.

My mentor's advice was that if a journal asks to recommend possible reviewers, recommend authors from your references - it will improve chances to get positive reviews. I did so once and received a very good review indeed. (This does not apply to the OP, evidently.)

If it were true, it'd be possible to attach a quantity to it: for example, how much new information is being created when an electron goes from one orbital to another in an atom?

Fushta!

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