Why gravity is not like the other forces

[C C]

https://www.quantamagazine.org/why-gravi...-20200615/

INTRO: Physicists have traced three of the four forces of nature — the electromagnetic force and the strong and weak nuclear forces — to their origins in quantum particles. But the fourth fundamental force, gravity, is different. Our current framework for understanding gravity, devised a century ago by Albert Einstein, tells us that apples fall from trees and planets orbit stars because they move along curves in the space-time continuum. These curves are gravity. According to Einstein, gravity is a feature of the space-time medium; the other forces of nature play out on that stage.

But near the center of a black hole or in the first moments of the universe, Einstein’s equations break. Physicists need a truer picture of gravity to accurately describe these extremes. This truer theory must make the same predictions Einstein’s equations make everywhere else. Physicists think that in this truer theory, gravity must have a quantum form, like the other forces of nature. Researchers have sought the quantum theory of gravity since the 1930s. They’ve found candidate ideas — notably string theory, which says gravity and all other phenomena arise from minuscule vibrating strings — but so far these possibilities remain conjectural and incompletely understood. A working quantum theory of gravity is perhaps the loftiest goal in physics today.

What is it that makes gravity unique? What’s different about the fourth force that prevents researchers from finding its underlying quantum description? We asked four different quantum gravity researchers. We got four different answers... (MORE)

COVERED: Gravity Breeds Singularities ... Gravity Leads to Black Holes ... Gravity Creates Something From Nothing ... Gravity Can’t Be Calculated

[Zinjanthropos]

Golfing the other day and my buddy pulls out the latest gadget from his bag. Called a golf ball balancer. A machine that spins your ball until centrifugal forces position the ball in its most balanced position and by inserting a pen/ marker into a small opening one is able to draw an equator on the ball’s surface. You then try and putt the ball using this most balanced area to your advantage. I Assume the equator here is where the gravitational force is (most)equal all around. I tried to tell him that once he whacks it with a club the equator will change and he’d need a new equator for putting every time. There’d be so many balanced equators on it that one might not recognize it as a golf ball.

I assume the equator in this case indicates a line on the surface where the centre of gravity force is equal at any point on the line. Does the centre of gravity exert (wrong word?) the gravitational force or the most gravitational force?

[C C]

Thanks for the article. Very interesting and useful.

Welcome to the forum, Ricardos.

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[...] Does the centre of gravity exert (wrong word?) the gravitational force or the most gravitational force?

Switching from golf ball to the Earth, the latter's center of mass is just where gravity points or wants to pull you. You have roughly equal amounts of material tugging you from both halves of the Earth's sphere. Kind of like two beefy midgets grabbing your arms on each side and each struggling to tug you exclusively toward him. Those balance out or cancel so that the net result is a "straight" downward pull towards the center of the Earth. (Going with only "two slices" of the Earth for the sake of simplicity.)

However, if you could travel through the Earth and survive, you would feel weightless at the center due to the Earth's matter surrounding you and pulling you from all directions (counteracting again). On the Earth's surface, there's no matter above you like that detracting from the attraction of the matter below. (Gravitational influences from Moon, Sun, etc, though.)

Although it's not like hitting the planet with a golf club, setting aside variations in density and elevated terrain one could say the attraction towards the center is the same everywhere on the Earth's surface (not just the equator). But the planet isn't a perfect sphere -- it's slightly flatter at the poles -- so your weight as measured on scales would be slightly more at the poles than at the equator because the former are closer to the center of the Earth's mass.

[Secular Sanity]

Sorry, I couldn’t find a Muppets video but Professor Julius Sumner Miller had a children’s show. 

[Catastrophe]

From Post #4 yesterday:

" On the Earth's surface, there's no matter above you like that detracting from the attraction of the matter below. "

That is directly analogous to surface tension whereby molecules at the surface (of, say, water) are attracted downwards and sideways by other water molecules,  but obviously only to a negligible extent upwards by air molecules. (from a surfactant chemist    )

I should add that the forces involved in the surface tension of water are primarily hydrogen bonding. In other liquids it depends on their polarity. For non-polar molecules (liquids) it is down to van der Waals forces. This is why water has such a high surface tension.