# Gravity stronger than electromagnetic force in a black hole?

Well, the question has somewhat been answered before, but there's one part missing, which - I'd think - is in conflict with the physical laws.

The earlier reply says that the gravitational pull even at the event horizon is so big, that not even the other forces can overcome this. So far, so good...this part I can accept.

However, what makes it possible for the black hole to appear in the first place? In this situation, the gravity between every single atom would have to fight against the electromagnetic force in order to compress the atoms sufficiently to create a singularity. Once the singularity is there, the gravitational pull becomes infinite - but how the he** does the electromagnetic force allow the gravity to create the singularity in the first place? I'd say that since the strength of gravity is directly related to the amount of mass - the more mass, the stronger the gravitational pull...but the same amount of mass generates millions of times stronger electromagnetic resistance between the atoms...or, did I misunderstand something completely here?

• Not an expert on black holes, but if you have, for example, a neutron star, then you don't have to worry about the electromagnetic force. The catch is that gravity is always attractive, whereas electromagnetic forces can be attractive, repulsive, or neutral. Nov 2 '12 at 2:14
• possible duplicate of physics.stackexchange.com/q/38550 Nov 2 '12 at 6:53
• Nov 2 '12 at 10:53

This has little to do with relativity per se and much more to do with the actual effects involved in black hole formation.

All the outer layers of a star weigh down on the interior, and it is this progressive added weight that eventually overcomes all other forms of pressure--electrostatic repulsion, for instance, but in discussions of black hole formation, we don't usually talk about that. Quantum effects start getting very significant as the particles are forced progressively closer together, and electron degeneracy is usually the first roadblock to black hole formation. This is just an effect of fermions (of which electrons are just one kind) being unable to occupy the same state as one another.

Still, there are only so many electrons, and if the object is still too massive, it may collapse into a neutron star, which is held up by the same principles, but by this point all the electrons have merged with protons to form neutrons. Even that may not be enough to stave off collapse into a black hole, for stars that are massive enough.

• Neutrons are fermions as well. Why couldn't an event horizon still form before the singularity and have the core of the black hole remain "non-singular"?
– DWin
Jan 9 at 18:16

In the low mass limit, you're absolutely correct. Electromagnetism absolutely overpowers gravity, to the point where you can completely ignore the latter.

But something funny happens when you get a lot of mass concentrated into one place. In this case, the mass curves the underlying spacetime. This fundamentally alters everything that happens with the physics. In particular, two things happen:

1) There will be a maximum mass beyond which things are no longer physically stable--they must either explode and shed some of the mass, or collapse down to a black hole. In this case, a charged object will either be forced to lose its surplus charge, or it will be forced to simply collapse into a charged black hole

2) Distances between objects will change due to the curvature, which can mean that two charged objects feel forces on each other in a different way than you would naïvely expect just from looking at their coordinate separation from each other and just simply applying $F=k\frac{q_{1}q_{2}}{r^{2}}$

All energy gravitates, not just that associated with mass, which includes the energy of an electromagnetic field and, in a sense, the gravitational field as well--though it would be more proper to say gravity is nonlinear. In your thought experiment, the electromagnetic field trying to counter-act gravity actually makes gravity stronger, and for extreme enough circumstances it loses.

More rigorously, the Einstein field equation describes the behavior of gravity in terms of a stress-energy tensor, with includes (a) energy density, which includes mass-energy, (b) momentum density and energy flux, and (c) mechanical stress. In turns out that in a local inertial frame, the effect of gravity is proportional to the sum of energy density and the principal stresses (the sum of those last terms is proportional to the mean pressure).

To simplify into more intuitive terms, both energy density and pressure gravitate. Normally, energy density is completely dominated by mass ($E \approx mc^2$ unless momentum is high), and ordinary pressures are very low compared to energy density of massive bodies. Thus, the most important term for gravity is mass--but only under ordinary conditions.

Imagine a spherical body that's that is extremely density, with gravity very high. It is supported against gravity by internal electromagnetic forces, or any other means. Therefore, stresses within it will be extremely high--and this contributes to gravity. In other words, at the very act of trying to keep gravity at bay makes it stronger, and eventually it will win against even electromagnetism.

hahaha

Actually, EMF is always applicable, even in a blackhole.

All stars have perfect symmetry, to a degree, gravity is always trying to crush the star and thru fusion SNF / the EMF electrons repel this force outward causing this balance.

Now you can have GRB's and the formation of blackholes if the star is large enough or if not Neutron stars or even smaller dwarf stars (white dwarf ect)

NS's have a very strong magnetic field and are immensely dense, so the subatomic structure basically pushes the electrons and protons together so only neutrons remain. Or rather the electron shells cannot repeal the up or down symmetry and are forced unilaterally to be the same thus annihilating each other, this breaks down the atomic structure and allows for the collapse to further propagate into the process for collapsing mass into near infinite density.

Now a blackhole is just a point where mass bends space time to a point, a singularity, which is more less a super compressed neutron star without even neutrons. However, there still is an EMF present, and it is responsible for allowing a singularity to exist. IE: Some force is keeping gravity from collapsing further. This is not infinite density, but rather near infinite, subatomic particles are literally at Planck length. However, blackholes have a measurable mass, and if we could ever get a neutron star to circle the accretion disk and get close to the event horizon, I think we would see something very cool, with the EMF as the point where gravity is strongest. What these forces are is still uncertain, we dont have the quantum data, to get that, we'd need the left and right limits of mass required to create a NS and BH and the Quantum data from inside a balchole, both of which we will never get unfortunately.

Its not like EMF has become weaker than gravity. Its always stronger than gravity in our Universe. Comparative strength of all the forces may vary in another Universe but not in ours.

What happens actually is, in objects like Black Hole, total mass becomes insanely large. In fact, this is the strength of EMF which restricts the requirement of large amount of mass to form a black hole otherwise it would be quit possible to have black hole of just 1kg. But too much mass is necessary to overcome the effect of EMF.

Now there is a catch in your question,

gravity between every single atom

Its not like force on an atom by every other atom. More precisely we should say, force on an atom by all other atoms. Effect of Gravity becomes more stronger just because overall attraction on an atom increases by increasing the number of atoms in the system. This is also true only for Gravity, not for EMF.

Let me explain.

Nature of force of gravity is attractive only. It doesn't repels. And as we know, it decreases by distance (1/r2). Theoretically, it lasts for infinite distances. Now if we increase the number of atoms then it only contributes in attraction.

But in the case of EMF things are different. Nature of EMF is both attractive and repulsive. and it also not depend on just (1/r2). If we have a system with 3 charge particles (+-+) then the force depend upon (1/r3). If 4 particles (+-+-) then dependency on distance will be (1/r4). And we also know charge always exists in dipole. We can't have a system with a single charge particle. So theoretically, effect of EMF will last for infinite distances but if we increase charge particles, it decreases insanely.

So when we increase atoms in a system, effect of gravitational force increases with compare to EMF. So in systems like black holes, Gravity overcomes the effect of EMF very easily.