5 ms·

Please correct me if I'm wrong, but the second sentence of the first paragraph already seems weird: > The gravitational force is the weakest of these four. Is

by k_sze 6y ago

Please correct me if I'm wrong, but the second sentence of the first paragraph already seems weird: > The gravitational force is the weakest of these four. Isn't that distance dependent? At macroscopic distances, the weak and strong interactions are even weaker, no?

irjustin 6y ago
Technically yes, but usually this statement is assumed at the quantum level/distances.
_Nat_ 6y ago
Yeah, there're qualifications behind it. [This PDF](http://web.mit.edu/sahughes/www/8.022/lec01.pdf http://web.mit.edu/sahughes/www/8.022/lec01.pdf) starts with some potentially helpful context. Usually this claim is discussed in connection to the so-called ["hierarchy problem"](https://en.wikipedia.org/wiki/Hierarchy_problem https://en.wikipedia.org/wiki/Hierarchy_problem).
cbkeller 6y ago
Not exactly: if you had a kg of free protons and a kg of free electrons, the electromagnetic forces between them would dwarf the gravitational forces between them even at macroscopic distances. By a lot, as Feynman liked to note in his Lectures [0,1]. It’s just hard to get a kg of free protons exactly because the forces involved are so large. So charges tend to pair up and screen [2], which just camouflages the true strength of the force. For the strong force the situation is the same in a sense, if you had a kg each of quarks of different colors — except you literally can’t have that, because the energy involved in separating a pair of quarks is so large that it actually causes two new quarks to snap into existence and pair off with the two you were trying to separate before you can separate them on anything approaching macroscopic scales [3]. So the strong force is obligatorily screened (camouflaged) at long distances, but only because it is so strong. The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t. [0] https://www.feynmanlectures.caltech.edu/II_01.html https://www.feynmanlectures.caltech.edu/II_01.html [1] https://sieste.wordpress.com/2012/04/22/feynmans-electric-force-analogy/ https://sieste.wordpress.com/2012/04/22/feynmans-electric-fo... [2] https://en.wikipedia.org/wiki/Electric-field_screening https://en.wikipedia.org/wiki/Electric-field_screening [3] https://en.wikipedia.org/wiki/Color_confinement https://en.wikipedia.org/wiki/Color_confinement
- povik 6y ago
  To nitpick here: You need less than kilogram of electrons to screen a kilogram of protons (roughly 2 grams it seems)
  - cbkeller 6y ago
    Quite true! I thought about saying “kg of positrons and kg of electrons” or such so that the mass/charge ratio would be more equivalent.
    - jschwartzi 6y ago
      You're actually looking for a mol of electrons and a mol of protons.
- benlivengood 6y ago
  > The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t. What if you want to build a fusion reactor? For now neutrons have to be along for the ride but that's only because we can't squish and heat plain protons hard enough.
  - cbkeller 6y ago
    In that case, go for it ;)
l33tman 6y ago
The weak and strong interactions have other properties that screen their influence at longer ranges (just like with the EM force due to objects being mostly neutral in case of the strong interaction for example), but the strong force is indeed much stronger. The EM force between a proton here and at the edge of the visible universe is the same as the gravitational force between two protons at 1000 meters distance. The scale difference in the "hierarchy problem" is truly not graspable by human intuition.
deleted 6y ago
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by k_sze 6y ago

irjustin 6y ago

Technically yes, but usually this statement is assumed at the quantum level/distances.

_Nat_ 6y ago

Yeah, there're qualifications behind it. [This PDF](http://web.mit.edu/sahughes/www/8.022/lec01.pdf http://web.mit.edu/sahughes/www/8.022/lec01.pdf) starts with some potentially helpful context. Usually this claim is discussed in connection to the so-called ["hierarchy problem"](https://en.wikipedia.org/wiki/Hierarchy_problem https://en.wikipedia.org/wiki/Hierarchy_problem).

cbkeller 6y ago

Not exactly: if you had a kg of free protons and a kg of free electrons, the electromagnetic forces between them would dwarf the gravitational forces between them even at macroscopic distances. By a lot, as Feynman liked to note in his Lectures [0,1]. It’s just hard to get a kg of free protons exactly because the forces involved are so large. So charges tend to pair up and screen [2], which just camouflages the true strength of the force. For the strong force the situation is the same in a sense, if you had a kg each of quarks of different colors — except you literally can’t have that, because the energy involved in separating a pair of quarks is so large that it actually causes two new quarks to snap into existence and pair off with the two you were trying to separate before you can separate them on anything approaching macroscopic scales [3]. So the strong force is obligatorily screened (camouflaged) at long distances, but only because it is so strong. The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t. [0] https://www.feynmanlectures.caltech.edu/II_01.html https://www.feynmanlectures.caltech.edu/II_01.html [1] https://sieste.wordpress.com/2012/04/22/feynmans-electric-force-analogy/ https://sieste.wordpress.com/2012/04/22/feynmans-electric-fo... [2] https://en.wikipedia.org/wiki/Electric-field_screening https://en.wikipedia.org/wiki/Electric-field_screening [3] https://en.wikipedia.org/wiki/Color_confinement https://en.wikipedia.org/wiki/Color_confinement

povik 6y ago
To nitpick here: You need less than kilogram of electrons to screen a kilogram of protons (roughly 2 grams it seems)
- cbkeller 6y ago
  Quite true! I thought about saying “kg of positrons and kg of electrons” or such so that the mass/charge ratio would be more equivalent.
  - jschwartzi 6y ago
    You're actually looking for a mol of electrons and a mol of protons.
benlivengood 6y ago
> The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t. What if you want to build a fusion reactor? For now neutrons have to be along for the ride but that's only because we can't squish and heat plain protons hard enough.
- cbkeller 6y ago
  In that case, go for it ;)

l33tman 6y ago

The weak and strong interactions have other properties that screen their influence at longer ranges (just like with the EM force due to objects being mostly neutral in case of the strong interaction for example), but the strong force is indeed much stronger. The EM force between a proton here and at the edge of the visible universe is the same as the gravitational force between two protons at 1000 meters distance. The scale difference in the "hierarchy problem" is truly not graspable by human intuition.

deleted 6y ago

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