# Gravity vs Strong Force

1. Jul 25, 2006

### wsdisman

I have heard a number of times that the force of gravity is fairly insignificant compared to such forces as the strong force. Why is this, and how can one mathematically compute the differences in force?

2. Jul 25, 2006

### Office_Shredder

Staff Emeritus
Let's compare gravity to the electric force, because it's simpler (strong force is weaker than gravity at far distances). Gravitational force is proportional to the masses, the square of the distance between objects, and a constant G. Electric force is proportional to the charges, the square of the distance, and a constant k. Now, k is about on the order of 1020 times larger than G is. So unless an object has a much larger mass than charge (a proton has about 108 times as much charge as mass), the gravitational force will be weaker.

You can compute each force individually using the equations F=GmM/r2 and F=kqQ/r2, then subtracting

3. Jul 25, 2006

### Pythagorean

One thing to keep in mind is that you only notice the gravity of Earth because Earth is HUGE. You wouldn't notice the pull of gravity from something the size of a ball bearing, but you would notice the electromagnetic force from something the same size.

electrons are tiny, yet they can keep gravity from pulling you through the sidewalk (the electro force is actually what keep you from just falling throught the side walk, because your electrons repel the sidewalk's electrons)

And then we know the strong force is stronger than the em force because in order for two protons to bond, they have to be able to overcome the force that makes them want to repel (which is the strong force).

Gravity only seems so powerful because we only notice it in HUGE objects. It's relatively weak (and immeasurable still, I think) for individual particles of matter.

4. Jul 25, 2006

### Mickey

We would notice the gravity from a ball bearing made with the same material and density of a neutron star.

I guess the weirdest thing about ordinary matter is that it's such a space hog. Squash the same electrons and atomic nuclei under the pressure of a star and you get matter the size of a birthday cake whose gravity would crush you just for sitting in the same room with it.

Last edited: Jul 25, 2006
5. Jul 25, 2006

### Pythagorean

yes, forgive my ignorance of density, I meant something the mass of a ball baring.

6. Jul 25, 2006

### rbj

who says?! on what non-anthropocentric basis can you make such claims? that's like saying that i weigh about 3 times as much as i am old. (50 years, 150 lbs.)

i like how Frank Wilczek sez it:

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

for fundamental particles, gravity is a helluva lot weaker than E&M because the elementary charge is approximately equal to (order of magnitude) the Planck charge and the masses of these particles are far, far smaller than the Planck mass.

Last edited: Jul 26, 2006
7. Jul 25, 2006

### Mickey

It's confusing because at the planck scale, gravity can be stronger than the other forces in the vicinity of a black hole or supermassive condensate. That's where laws of physics break down, and many new ones are created, it seems. We don't understand planck scale gravity just yet.

Claims that suggest that gravity is a "weak" force only apply from certain perspectives, like particle physics. But particle physics can't explain gravity yet, so that's kind of silly. We really don't even know for certain that gravity is weak. Crazy, huh?

8. Jul 26, 2006

### rbj

my point is the same as in the wikipedia article:

we don't know that gravity is weak (but it is oft said, and the only way to say that is in the context of fundamental particles). we cannot really know that because we can't really compare these apples to oranges.

Last edited: Jul 26, 2006
9. Jul 28, 2006

### Pythagorean

I thought the whole reason we couldn't detect gravity was because it's so weak. And I'm talking the general case here, not in blackholes.

But then, how is strength measured? Force across a distance? Force that acts within a volume? I've never heard a scientific, unitary definition of the 'strength of a force'.

Last edited: Jul 28, 2006
10. Jul 28, 2006

### rbj

we can detect gravity. this chair is pushing up on my butt. i'm detecting it.

but this is illustrative. it depends on the mass of objects how much force of gravity will act. it depends on the electrical charge of objects how much force of E&M will act.

because electric charge has both positive and negative values, it is possible to make sheilding of E&M forces from unwanted sources. so we can block out local light we don't like so we can see stars at a large distance.

but since there is no negative charge mass lying around, we cannot build sheilding from any gravitational forces. the only thing we can do to attenuate unwanted gravitational disturbances (from siesmic and tidal forces) that interfere with detection of gravity waves from distant objects is to put distance in between and let the inverse-square nature of gravity do its thing. i think that means we'll need another "Hubble" for detection of distant sources of gravitational radiation.

anyway, we should someday be able to detect the motion of some pair of black holes orbiting each other if something like that is not too far away.

Last edited: Jul 28, 2006
11. Jul 28, 2006

### Office_Shredder

Staff Emeritus
Umm...... imagine you have two positively charged particles, with mass of equal magnitude to their charge. Do they attract or repel, and why?

I'll give you a hint: it's not because the proton's charge is larger than its mass

12. Jul 28, 2006

### rbj

tell me, what does it mean for their "mass [to be] of equal magnitude to their charge"?

how about this for a hint: is your computer heavier than it is long?

Last edited: Jul 28, 2006
13. Jul 28, 2006

### Pythagorean

That's not quite my point. I was under the impression that we didn't have experimental data to show a quantized unit of gravity. A force carrier, the graviton, what ever the experimental equivalent of the theoretical prediction.

My point was that we know very little about gravity, compared to the other forces, I guess. You can only detect the chair pushing up on your butt because the earth is so massive. Can we detect the pull of gravity between two atoms? Do they have a gravitatinal force? Isn't the EM force between two particles much stronger than the force of gravity between them?

I didn't think we answered these questions in the lab, yet, but I'm also way too premature in my education to study relativity or any modern models of gravity (if they exist).

14. Jul 29, 2006

### rbj

actually, i thought with GR, there is a lot of human knowledge regarding gravity, but they do not know how to tie it to the Standard Model which, if i am not mistaken, ties the other 3 fundamental forces togehter. there is no experimental evidence of gravitons, yet. perhaps String Theory is something that ties them all together, but from what i read, there is little hope of experimental evidence that will support or refute String Theory (if that continues to be the case for long enough time, string theory will move from physics to philosophy in science, i believe).

"so massive" compared to what?? it's not particularly massive compared to a star. or more so a SMBH.

yes they do, but it is very small compared to the force between chair and butt.

depends on the particles. the EM force between two electrically neutral particles is zero. if those particles have mass the EM force is much less than the force of gravity.

true, the repulsive EM force between two protons (alone in free space) is far, far greater than the gravitational attractive force betweent the same two protons. but, until you find a way to compare the amount of charge on those two protons to the amount of mass (two incompatible quantities, like comparing my age to my weight - am i older than i am heavy or the other way around?), you cannot assign that great disparity of force to a notion that the EM force is, in general, so much stronger than the force of gravity. it depends on how much charge is on the objects and how much mass, two independent quantities.

please take a look at the Planck units article in Wikipedia. this is precisely the kind of "issue" that Planck units are meant to address.

this issue is not a matter of modern models of gravity or necessarily of relativity, it's an issue of quantitatively comparing different dimensions of physical stuff. it's an issue of comparing apples to oranges. how do we compare apples and oranges?

Last edited: Jul 29, 2006
15. Jul 29, 2006

### GOD__AM

I think it has more to do with the binding force holding the molecules together. The sidewalk doesnt pass through our bodies because of the electrons repulsion, but if the sidewalk was made of water this repulsion would not stop a person from being pulled through the same way the air doesn't prevent it.

16. Jul 29, 2006

### Pythagorean

But with water, your molecules don't pass through the water molecules, the water rushes around you and makes room for you (again, because of the EM force). But you're right, the molecular bonds between two concrete atoms is apparently much stronger than the molecular bonds between water atoms (which, I believe, is where surface tensoin comes from).

This relates to a remark I made in my third post in this thread:

It is professional physicist (usually explaining to laymen) where I hear the four forces mentioned, and there order in strength, gravity being the weakest, strong being the strongest, but I've never seen any mathematical deffinition of the 'strength of a force' so I don't know how you would compare them, really. So how is it that gravity is considered the weaker force by the professonals? Or is this a misconception?

17. Jul 30, 2006

### rbj

it's not a misconception but they are usually leaving out the context. when we talk of the other three forces together, it is in the context of sub-atomic particles. in that context, the E&M force (assuming particles are charged), weak nuclear, and strong nuclear forces are somewhere in the same league with each other but the gravitational force is not even close to that league. (but it's because the masses of the particles are so much smaller than the natural unit of mass.) you might also want to check out "similitude" and "nondimensionalization". you can compare quantities if they are in the same dimension of physical stuff. one thing we can do is express physical equations in such a way that all physical quantities are normalized against some natural unit. then all of the numbers are dimensionless and non-anthropocentric (or natural) and then it might make sense to compare how heavy something is to how long it is or how old it is.

18. Jul 30, 2006

### Office_Shredder

Staff Emeritus
You're completely ignoring the original context of my point, and drawing me into a semantics game here. My point was to compare electric vs. gravitation force. In standard metric units, mass is measured via kilograms, and charge is measured via culoumbs. To demonstrate WHY, numerically, the gravitational force is so weak, I discussed how, for a standard electrically charged unit (proton), the charge is much larger, and the constant is much larger, on a numerical scale. I agree, we can scale the units so the constant is no longer larger, OR the charge is no longer larger. But, keeping in mind context, that adds nothing to the conversation at hand, and surely detracts from it (especially considering I had already given a proportion between the magnitude of the proton's charge and mass, which would be skewed if the units were changed). Additionally, by scaling one, you make the other part larger, so the net force continues to be much greater electrically rather than gravitationally.

Yes, the weight does have a greater magnitude than the length. But nice try.

19. Jul 30, 2006

### rbj

i was only asking you a question. you're saying that one thing is equal magnitude to another thing. but the things are not commensurate. they're not the same species of animal. you need to be more specific on how you're comparing these two different quantities of completely different dimension (which means you cannot measure them with the same unit).

my original question stands. on what non-anthropocentric or non-anthropometric basis are you comparing or equating the magnitude of mass and charge of some thing?

in general?? the electric force action operates on charge and is independent of mass. the gravitational force action operates on mass is doesn't look at charge at all. like the peak of the Dow-Jones Average and the height of Mt. Everest, what do the two have to do with each other? how can they be directly (or indirectly) compared?

physical reality doesn't give a rat's ass what units are "standard" to us protoplasms we call human. you can't (successfully) base your argument of why something is true in physical reality on what humans choose or decide to do. independent of what humans do, the laws of physical reality remain.

the example you chose is an example. it's been cited before. did you read what (Nobel laurate) Frank Wilzcek said about that example? it's in the Planck units article at Wikipedia with citation. you should check it out.

from the P.O.V. of physical reality, those dimensionful physical constants are crap. they are what they are only because we humans have chosen to measure length in meters, mass in kilograms, and time in seconds, all units that have been arrived at partly because they are meaningful to the human experience (the aliens on the planet Zog will have a completely different experience) and partly because of historical accident. physical reality doesn't give a spit.

i think you need to do some reading.

are you kidding?? (because if you're serious, then this is evidence you don't get it yet. it is not simply an issue of semantics but what is real and what is a human construct.)

Last edited: Jul 30, 2006
20. Jul 31, 2006

### Office_Shredder

Staff Emeritus
I'm comparing the force they cause, as per the original post.

How much force it causes

They don't, yet electric and gravitational force both have a lot to do with..... force.

I agree, the laws of physical reality remain. And that's why electric force generally is greater than gravitational force

Do you really think I made it up on my own? Of course I know it's been cited before, yet it's an excellent example to demonstrate why, numerically, the electric force is considered stronger than the gravitational force

Aliens on the planet zog will still notice that electrical force pulls stronger than gravitational force... like I said before, if you scale the units, the constants scale in exactly the opposite direction. This isn't just true for electrons and protons, ions have the exact same result (as I'm sure you already know).

Ask a stupid question, get a stupid answer.