A question about why gravity is only attractive

[Narandi]

TL;DR Summary

Relationship between elecricity and gravity

It is know that Coulomb"s law is similar to Newton"s law of universal gravitational and that gravity is always attraction, and electricity is both attraction, and repulsion. Why is gravity only attraction? I have such an idea that electric forces are gradually inferior in strength to gravitational ones, starting with the Plank mass. And in the world of stars and galaxies only attraction. Here the rotation of their central more massive bodies already plays a role. I would like to know the opinion of physicists.

 

[Dale]

Summary:: Relationship between electricity and gravity

Why is gravity only attraction?

Isn’t the cosmological constant repulsive?

 

[PeterDonis]

I have such an idea that electric forces are gradually inferior in strength to gravitational ones, starting with the Plank mass. And in the world of stars and galaxies only attraction. Here the rotation of their central more massive bodies already plays a role. I would like to know the opinion of physicists.

We can't discuss personal theories here, so this part of your post is out of bounds. Please bear that in mind for future posts.

Why is gravity only attraction?

As @Dale has pointed out, this isn't quite true: there are spacetime geometries in General Relativity, which is our best current theory of gravity, that can be described as having repulsive rather than attractive "gravity". Gravity is attractive for ordinary matter and radiation, however, so it is a valid question why that is the case.

The reason is twofold: first, electric charges of ordinary objects can be positive or negative; whereas all ordinary matter and radiation has a positive gravitational "charge" (which is usually described as "energy" but that isn't quite correct; the full technical details are beyond the scope of a "B" level thread), and second, for gravity, because it is a spin-2 interaction rather than a spin-1 interaction like electromagnetism, like charges attract instead of repel (as like charges do in EM).

 

[Drakkith]

Why is gravity only attraction?

We don't know. We observe that something keeps large objects bound together and something alters the path of moving objects, such as planets, moons, asteroids, etc. Our first description of this was in the form of Newton's Law of Universal Gravitation, followed by Einstein's General Theory of Relativity. GR potentially answers your question on why gravity is attractive, but that just implies another question. Why is spacetime curvature the way it is? The answer to which is, "We don't know. That's just what we observe."

 

[sysprog]

And let's please not forget that the electromagnetic force is much stronger than gravity $-$ on the order of $10^{36}$ times as strong. http://solar-center.stanford.edu/activities/jeff/Gravity.html

 

[jbriggs444]

I absolutely agree with you, so I'm looking for reasons

You understand that eventually the answer to "why" questions is always "because", "I don't know" or "it's turtles all the way down".

 

[sysprog]

A description of the experiment to test the gravity of the smallest particles was published in the journal Nature on March 10, 2021.

I think that this is a mischaracterization of the experiment. Presumably you were referring to:

Westphal, T., Hepach, H., Pfaff, J. et al. Measurement of gravitational coupling between millimetre-sized masses. Nature 591, 225–228 (2021). https://doi.org/10.1038/s41586-021-03250-7​

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Abstract​

Gravity is the weakest of all known fundamental forces and poses some of the most important open questions to modern physics: it remains resistant to unification within the standard model of physics and its underlying concepts appear to be fundamentally disconnected from quantum theory1,2,3,4. Testing gravity at all scales is therefore an important experimental endeavour5,6,7. So far, these tests have mainly involved macroscopic masses at the kilogram scale and beyond8. Here we show gravitational coupling between two gold spheres of 1 millimetre radius, thereby entering the regime of sub-100-milligram sources of gravity. Periodic modulation of the position of the source mass allows us to perform a spatial mapping of the gravitational force. Both linear and quadratic coupling are observed as a consequence of the nonlinearity of the gravitational potential. Our results extend the parameter space of gravity measurements to small, single source masses and low gravitational field strengths. Further improvements to our methodology will enable the isolation of gravity as a coupling force for objects below the Planck mass. This work opens the way to the unexplored frontier of microscopic source masses, which will enable studies of fundamental interactions9,10,11 and provide a path towards exploring the quantum nature of gravity12,13,14,15.​

The experimenters do not claim to have examined the gravitation of "the smallest particles"; in fact, they claim "sub-100-milligram", and explicitly distinguish "objects below the Plank mass" as something for "future improvements to our methodology".

90 mg exceeds 4 thousand times the Planck mass, it's like a flea more than an imperceptible speck of dust, this is our world and an excess of 4 thousand is not so scary. Will overcome. But this is the first experience on such a scale and I'm interested in how the universal gravitation will behave

At least to me, this seems overly dismissive of the difference in the scale of current experimentation compared to what is anticipated for possible future experiments.

 

[PeterDonis]

Physicists tested the law gravity on the smallest bodies weighing 90 milligrams which is not so far from the Plank mass.

You seem to have a misconception that the Planck mass is the smallest possible mass something can have. It's not even close. The electron mass is about 25 orders of magnitude smaller than the Planck mass, and neutrino masses are much smaller still.