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Comment by madaxe_again | Hacker News Reader

As a physicist, I find the treatment of gravity as a "force" rather bizarre.

It's not a force per se, rather a perceived force as a result of the curvature of spacetime - it's no more a force than centripetal acceleration, which is a perceived force brought about by a rotating frame of reference.

Gravity similarly is a perceived force as a result of a moving frame of reference in a dimension we aren't built to directly perceive, other than as an acceleration towards mass.

But that's physics for you - the particle physicists want particles everywhere. The quantum physicists want collapsing waveforms everywhere. The cosmologists just see warped space.

If anything, the thing preventing resolution of any variety of GUT is culture.

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  as a result of the curvature of spacetime

So... why does this curvature, or slope have any affect on systems of bodies at all?

An abstract gradation of some property alone doesn't necessarily provoke further side effects, so why would gravity (this curvature of spacetime) set objects in motion?

If you have an undisturbed system of cold stones suspended in a void, that are otherwise motionless over time, and then place a strong, cold, static gravity well in proximity, what impels the stones into the curvature? Why does an otherwise motionless stone move into the curvature of said given gravity well at all?

>"otherwise motionless"

There's the trick to it - in any context where you are talking about space's curvature, those "motionless" rocks are going to be moving through time. The geodesics that they will follow are parametric curves of the form f:(s)->(x,y,z,t).

While there are many possible choices of parameter s (as with any parametric curve), one that physicists particularly like is proper time: the time as shown on the watch of the walking man.

So, the only "truly" motionless object would be one that for every second of time it experienced, it would find itself zero seconds in the future. Quite impossible - which is why you can't "pause" yourself along gravitational geodesics and float.

Ah, see, but this is where the communication breakdown emerges.

Most people do not intuitively consider a body suspended in space as a constituency of atoms, with subatomic particles operating in static, yet relativistic motion about a nucleus of protons and the like, subject to such curvatures.

This is where the fundamental shift in teaching the conceptual principles of gravitation and motion need to occur.

Whether it's intuitive or not, one needs to lift away from the concept of the stone moving from point A to point B, and instead, think in terms of a collective glob of photons and other particles being subjected to the same warping of paths that affects the paths of photons, when they transit too close to the sun's gravity well.

I thought the problem was that you can't teach both of those things at the same time (masses in spacetime vs. matter as clouds of bosons and fermions statistically obeying laws of quantum behaviour) . Hence the interest in finding a theory that accounts for both of them.

I don't agree about that being a problem.

The general relativity-based description of gravity you've provided is no more in conflict with a quantum (i.e. particle) theory of gravity than Newton's Laws are in conflict with the Schrodinger equation.

They're just different tools that are applicable to different domains, and if we had one without the other, or if the two were inconsistent, we would definitely have an incomplete model of physical law.

This isn't just some sociological quirk: The three other forces of nature are well described by quantum field theories---with plenty of experimental data to back those theories up. So why would the fourth be any different? It's just that gravity is the weakest force by far, so its QFT machinery is the hardest to observe.

> If anything, the thing preventing resolution of any variety of GUT is culture.

Doesn't your mindframe lead inexorably to string theories? Those haven been empirically successful. So, no, I'd say the thing preventing the GUT is reality.

Popper would respond that the thing preventing their resolution is their unfalsifiability.

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