Laws of Attraction in Science

The nature of attraction in the physical world obeys various laws of physics, from electromagnetic and gravitational laws to even the theory of relativity. Gravity, electrostatics and magnetic forces have been familiar since ancient times. The scientific revolution, then modern physics, added much refinement to these ancient understandings, including the discovery of additional forces of attraction.

Gravity vs. Electric Force

The electric (electromagnetic) force is a billion-billion-billion-billion (10^36) times stronger than gravity. This is surprising, since we don't feel electric forces in our daily experience; we feel gravity. The explanation of this paradox is that the electric force has two kinds of charges. Gravity doesn't come in two kinds that cancel each other--it builds up cumulatively. So we feel the Earth pull on us, but we don't feel smaller objects, like the pen on the table, pulling on us. But if your mass was made up of entirely positive charges, and a friend's mass a few feet away was made up of entirely negative charges, the attraction would be with a force great enough to lift a third person that was dense enough to have the mass of the Earth.

Why Protons Don't Fly Apart

If the electrostatic force is so strong, then why don't protons in the atom nucleus fly apart from each other? Because a force called the strong force binds nucleons (neutrons and protons) together. Actually, nucleons are composed of quarks, which the strong force binds together within the nucleon. That neutrons and protons attract is just a residual effect of a greater attraction between intra-nucleon quarks.

Attracting Parallel Wires

Magnetism is familiar to us from playing with permanent magnets. But magnetism can also exhibit force better described as oblique than attractive. A proton passing perpendicular to a magnetic field's direction will be deflected in a direction perpendicular to both the proton's direction of motion and the magnetic field's direction. What does this have to do with attraction? Imagine two parallel wires, both carrying equal current in the same direction. They each create a circular magnetic field around each other. As the electrons of the opposite wire travel down their wire, they are traveling perpendicular to the direction of the magnetic field set up by the first wire. They are therefore deflected by the magnetic field in a third direction, perpendicular to both the electron motion and the magnetic field. In other words, the electrons are deflected toward the other wire. So parallel wires carrying current in the same direction attract each other. (If the currents were in the opposite direction, the wires would repel.)

Theory of Relativity

Einstein predicted that gravitational attraction would extend to massless particles. Two lines of reasoning arrive at this conclusion. One is that, observationally, photons were found to exert inertia on incident objects, i.e., they could push on things. Since inertial mass and gravitational mass are believed to be equivalent to a high degree, if not exactly, this indicates that photons should have gravitational mass and be deflected in a gravitational field.

Principle of Equivalence

The other line of reasoning is based on the principle of equivalence, which states that an observer in a closed laboratory cannot distinguish between the effects produced by a gravitational field and those produced by an acceleration of the laboratory. A light beam accelerated transverse to its path should deflect. By the principle of equivalence, a gravitational field should therefore deflect it as well.

Parallel Wires Reconsidered

The theory of relativity can actually be used to explain away the magnetism in the parallel wire demonstration. By the special theory of relativity, objects experience length contraction in the direction of their motion--the closer to the speed of light, the greater the contraction. Electrons don't go very fast in wires. But there are so many of them, that the cumulative effect makes the electrons in one wire, from the point of view of a stationary proton in the other wire, seem to bunch up lengthwise. Meanwhile, the proton density, being motionless, appears the same. So the net perception to a proton is of the opposite wire taking on a negative charge. This perception causes the proton to be attracted to the opposite wire. Note that the argument doesn't work in terms of electrons seeing other electrons moving, since from the electrons' viewpoint, the electrons in the other wire are standing still. No length contraction is perceived between electrons of opposite wires. Remember, the currents were in the same direction, and of same magnitude, so the stationary-perception simplification would hold.