On the contrary, gravity is actually the weakest force. This is true when it comes to the fundamental forces anyway. If you order the four fundamental forces from weakest to strongest, it would look like this: gravity, weak nuclear force, electromagnetic force, and the strong nuclear force. If we are to zoom in on two protons that are extremely close together, there are a few forces at play.
These are forces that they are exerting on each other. Since they both have mass, the gravitational force is exerted on both of them by each other, causing gravitational attraction on each other. Both protons will have a positive electric charge, as protons do, so just like gravity, they exert electromagnetic repulsion on the other.
The strong nuclear charge comes from the fact that they have internal ‘color’ charges, this causes attraction. This color charge is something explored in particle physics and is not necessarily important when answering this question.
Because the strong nuclear force is the strongest when the protons are in close range, it overtakes the other forces and dominates the situation. The two protons bind and form a helium nucleus.
Gravity is the weakest force on this scale of atoms. In fact, it is so weak when we zoom in this far, that a lot of scientists choose to ignore gravity readings when they are working in terms of particle physics. They are unlike to encounter any calculations problems when they ignore it because gravity is so weak at this level of scale.
But when we are talking about larger scales, like astronomical ones, gravity is dominant. The main reasons for this are that gravity has a longer range than the other forces (this shows that distance is important in both cases), and the second reason is that there is nothing in existence with negative mass.
Mass is obviously the cause of gravity and the thing that it acts upon. As the two objects that are experiencing the forces become separated, some of the forces die off. Since the nuclear forces are short range (hence nuclear), they drop to zero rapidly as the two objects move away from one another.
The reason for the miniscule size of a nucleus in an atom is directly thanks to the short range that the nuclear forces have. If a nucleus was too big, the nuclear forces would not have as great an effect. Therefore, the protons in the nucleus would not be drawn together.
The nuclear forces don’t even have effect when two particles are merely nanometers apart, so on an astronomical scale, this would be even less effective as a force. This means that the earth and the sun are absolutely too far apart for any sort of nuclear force to act on each other.
They are billions of meters away from one another, so the nuclear forces, both weak and strong, have no effect on the two astronomical giants. But aside from these nuclear forces, the electromagnetic forces and gravity both come into effect in space and have an effectively infinite range in space. They die off in strength as 1/r2.
If we quickly look at what this actually means: when we say that gravity has an effectively infinite range, we are actually using the Newtonian formulation of gravity. In general, gravity is better described when using the formulation of general relativity, or E = mc2 for special relativity, how you might know it. This is where E is the equivalent of energy, m is calculated as the mass of the object and c2 is the speed of light squared.
But the theory of general relativity tells us that gravity is actually not a real force and is in fact a warping or alteration of spacetime. In other words, if you imagine space time as having a 3-dimensional grid pattern through the whole of space, something with a large mass could alter the positioning of these lines, and we call that gravity or gravitational pull.
Since we have two different ideas when it comes to gravity: the Newtonian version is better to use when talking about smaller scales, e.g. not galaxies and black holes. But on a larger scale, we are more likely to use general relativity since this explains everything and all of the possible effects of gravity.
If we consider gravity on a larger scale, bigger than even galaxy groups, we can see that it does not have an infinite range and in fact dips on a scale. But if we were in the scale of galaxy groups, it would appear ‘unlimited’. But gravity still has 1/r2 behavior.
But if gravity still has effectively unlimited range, then why is the universe expanding on not coming together? This is because of the operative word ‘effectively’.
On different scales, the force of gravity can appear to have an infinite reach, and on larger ones we can see that it in fact does not. Our universe is expanding as a whole, meaning that the general relativity explanation for gravity is more fitting, since this behavior is predicted.
On a large scale (like a universe) gravity acts in a strange way that alters spacetime in such a way that causes the universe to expand instead of everything being drawn to each other. But on a smaller scale, gravity works in the usual attractive Newtonian way that we are used to here on earth.
After that brief intermission to explain the differences between Newtonian gravity and the theory of general relativity, we can now get back to the question at hand. Why is gravity the strongest force?
If we, as dwellers in a galaxy, take gravity to still have an ‘infinite’ range (since we are on a small scale compared to the universe at large), then so does electromagnetism.
So, a new question arises: why is the earth orbiting the sun and held in orbit thanks to gravity and not electromagnetic forces? The reason being that although there is no such thing as a negative mass, there is such thing as a negative charge. This means that gravity is always working, but electromagnetic forces can sometimes be a little trickier to decipher.
If there are two objects next to each other, one with a single positive charge and one with a single negative charge, the negative is likely to ‘cancel out’ the positive.
This is called a dipole. Or a pair of equally and oppositely charged poles that are separated by a distance of some sort. Interestingly, the electromagnetic force that is caused by one of these dipoles will die off at 1/r3 instead of 1/r2. You do not necessarily need to know the specifics of this apart from the fact that gravity will die off at 1/r2.
If we take this even further and double the number of charged objects we have: so, two each of a positive charge and a negative charge. We would be left with a quadrupole instead of a dipole. The electromagnetic force dies off at 1/r4, which is even more rapid than a dipole. This is because the negative charges do an excellent job of cancelling out the positive ones.
When you add more and more equal numbers of positive to negative charges, the range of electromagnetic force gets shorted because it dies off faster and faster, as we have seen with the dipole versus the quadrupole.
This is interesting because most of the atoms in our known universe have an equal number of positive charges to negative charges. This is because most things are made up of atoms, and these atoms contain protons and electrons that make up the charges.
So, interestingly, the effective range of electromagnetic force is shorter than the range of a single positive or negative charge, because they work so hard to cancel each other out.
Neutral atoms, that is atoms that are equal in electromagnetic force, only have an effective electromagnetic range that can be measured in nanometers. This is absolutely tiny. The reason for this is that the positive and negative charges are cancelling each other out in a way that is shortening the electromagnetic force reach.
Making the range of the electromagnetic force reach much shorter for objects in space like star and planets. This means that these space giants can exert no real electromagnetic force on each other.
So, this means that gravity is the only force left when it comes to astronomical scales, or things like stars, planets and moons. Which is interesting given that on smaller scales, gravity is the weakest force. Gravity would suffer the exact same way as electromagnetic force is there was such a thing as negative mass.
Since mass and negative mass could act in a way that cancel each other out. But since there is no such thing as negative mass, gravity reigns supreme in outer space at a larger scale, as its reach is far greater than any of the other three fundamental forces. Which is good, because there would be no significant force at an astronomical scale, meaning asteroids would not be drawn together to create planets.
Since there is no such thing as negative mass, objects in space are drawn together in only an attractive and additive way. And gravity is what we have to thanks for things like planets forming, the orbit that earth holds around the sun in its closed trajectory, and the fact that we remain on earth’s surface instead of floating away.
We owe a lot to gravity, since it has kept us in that sweet spot of orbiting the sun in just the right area to sustain light and benefit from the sun’s heat.
To summarise: although gravity is the weakest of the four fundamental forces in general, and especially when we zoom into the nucleus of an atom where the nuclear and electromagnetic forces take control, it is the strongest in space.
When we say it is the strongest in space, wheat we mean is that it takes control of all things astronomical. It draws asteroids together and has a much further reach than the other forces do in space. Because of this, gravity is the strongest force in some respects, as it is what we have to thank for most of the movements in space, as well as the collisions that create planets etc.
So, to answer the question ‘why is gravity the strongest force?’ we can say that it is the strongest force in space on an astronomical level simply by default. But when it comes to zooming in on the atom and the protons in the nucleus, it is outweighed by the other three fundamental forces which are strong nuclear force, weak nuclear force, and electromagnetic force.
