Comparing Gravity to Other Forces in Physics

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The discussion highlights the puzzling relative weakness of gravity compared to other fundamental forces, which physicists consider a major unsolved problem. Participants question the arbitrary nature of units like charge (Coulomb) and mass (kilogram) when making these comparisons. They emphasize that while one might not expect a relationship between the forces, the vast difference—40 orders of magnitude—poses challenges for theoretical frameworks. The conversation suggests that comparing the coupling constants of forces, rather than their units, provides a clearer understanding. Ultimately, using natural units allows for a more consistent comparison of the four fundamental forces in physics.
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I've often heard physicists say that the relative weakness of gravity is 'one of the greatest unsolved problems in physics'. When physicists talk about how puzzling it is that gravity is so much weaker than the other forces, how are they comparing the two? Isn't the unit of charge (Coulomb) and the unit of mass (kilogram), fairly arbitrary?
And if we Compare the two based on the smallest possible unit (charge of electron, mass of a quark or whatever), why should we EXPECT them to be related in the first place?
 
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waveicle said:
Isn't the unit of charge (Coulomb) and the unit of mass (kilogram), fairly arbitrary?
Yes

And if we Compare the two based on the smallest possible unit (charge of electron, mass of a quark or whatever), why should we EXPECT them to be related in the first place?
You wouldn't necessarily expect them to be similar - but 40 orders of magnitude is difficult to fit into a theory!
 
When we speak of comparing forces, we mean to compare the (force-mediated) *coupling* between two objects- that is, not 'm' or 'q', but 'G' and 'e_0'.

More precisely, using natural units for the four forces:

http://en.wikipedia.org/wiki/Fundamental_force

allows one to compare them in an 'apples to apples' manner.
 

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