6 Comments
User's avatar
Steven Verrall's avatar

Great post! I have read many Substacks, but this is the first that has impressed me enough to subscribe to. Keep it up!

Expand full comment
Fabien's avatar

Hard to believe this is still controversial a century after Einstein theory of general relativity and given how many times he was proven right.

Expand full comment
Dan James's avatar

It should be pointed out that you are talking about a local statement? At a point, particles of different properties follow the same geodesic. But in a global spacetime structure, different initial positions lead to different geodesics — and their separation evolves according to curvature. So different regimes: local, equivalence principle, nonlocal, geodesic deviation

Expand full comment
Raymund Schippers's avatar

What if all my cells were magnetic and I orbit a huge iron mass in flat space. Do I feel a force keeping me in orbit? Am I in free fall, weightless?

Expand full comment
Steven Verrall's avatar

Magnetic forces cannot cause stable orbits. See Bertrand's theorem. https://en.m.wikipedia.org/wiki/Bertrand's_theorem

You could make the analogy to electric forces, as was done with early atomic models. However, accelerating charges emit EM radiation at a far greater rate than accelerating masses emit gravitational waves.

Therefore, purely classical electric force orbits will quickly destabilize. Objects governed by significant electromagnetic forces can't be in freefall and therefore can't experience weightlessness.

It is worth remembering that all charged fundamental particles are massive, but some massive particles are uncharged. Therefore, gravity acts in the complete absence of EM forces, but EM forces are always accompanied by gravity.

Expand full comment
Grey Hodge's avatar

This can't be true, as we can experimentally see clumps of dark matter. For dark matter to be the universe itself then that would mean that there's more "universe" in some parts of the universe than others. This is a reply to @mpc755

Expand full comment