Four Forces of the Universe

(This piece is a bit of a digression from the recent focus on evolution and its discontents, and moves back closer to my own comfort zone in physics. It is also at a high school physics level – the water gets much deeper than I’ve presented here very quickly, but this is a useful place to start.)

In discussions of the universe at the larger scale of cosmology and astrophysics – the formation and motion of stars and galaxies – I’ve recently begun encountering people who claim that ‘it’s all magnetism’, that every force in the end can be reduced to magnetism.

No doubt these voices have been around forever, and it’s just that I’ve started hearing them, but it’s a fascinating phenomenon. These people tend to be skeptics about General Relativity, and sometimes even about whether gravity exists at all. Certainly dark matter and dark energy are rejected passionately – it’s all magnetism!

I’m not sure what motivates it. Perhaps magnetism is the force that seems realest and most tangible. Most of us have played with magnets, and felt that real force, of both attraction and repulsion. It feels quite like the force of gravity holding us down – at least the attractive element does. And imagining a repulsive gravitational force is exciting! No heavy rockets needed to get to space if you’ve got anti-gravity!

As a bit of an antidote, I thought I might talk for a moment about the four fundamental forces that act in our universe. I might include a couple of equations for comparative purposes, but it should be quite easy to understand this post even if you ignore them.

The four forces are gravity, electromagnetism, the strong nuclear force and the weak nuclear force. (Newer physics links electromagnetism and the weak nuclear force together as the ‘electroweak interaction’, but that’s past where we want to go for the moment.)

The nuclear forces govern what happens inside the nucleus: the strong force is what holds the nucleus together in spite of the electrostatic repulsive forces between the protons, and the weak force governs radioactive decay. We won’t say too much more about them, except to note that the balance between the strong nuclear force and the electromagnetic force is what allows stable nuclei – and hence us – to form. Slightly different values of either would not allow matter to form. This has implications for whether divine fiddling with the speed of light or the rate of radioactive decay could happen, but that’s another story for another day.

Electromagnetism can be thought about as two things, although they are tightly tied together – electricity and magnetism.

We’ll take magnetism first. It’s fascinating, because it acts only on a moving charge. If a charged particle remains perfectly still in a magnetic field, no force acts on it. The formula is F = qvBsin(theta), where F is the force (newton), B is the magnetic field strength, q is the charge (coloumb), v is the velocity (metre per second) and theta is the angle (degrees or radians). As you can see, if v is 0, the force will be 0. The calculation is actually a ‘vector cross product’, and that tells us the direction the force will act in, but that’s also probably further than we need to go.

OK, we have enough already to reject the idea that gravity can be reduced to magnetism. We saw that if v is 0, F is 0, but also, if q – the net electrical charge on an object – is 0, the force will be 0. I don’t have a net electric charge on my body, neither do you, and neither does Earth. That means that the gravitational force holding me down on my chair as I type this is not a magnetic force.

(It’s possible the objection will be raised that the protons and electrons in my body have charge and are moving relative to those in the Earth, but again, there is not a net overall charge on an atom, and the directions of motion would all cancel one another out. And, of course, we now tend not to think of the motion of electrons in terms of the ‘orbit’ metaphor anyway…)

The other manifestation of the electromagnetic force is electrostatic attraction and repulsion, and this gets interesting. The formula is F=(kq1q2)/r2 where F is the force, k is a constant, 9 x 109 , q1 and q2 are the charges on the two objects and r is the distance between them.

The reason I said it gets interesting is that this has a direct mirror in the equation for gravitational force, F=(Gm1m2)/r2 where G is a different constant, 6.67 x 10-11 and m1 and m2 are the masses of two objects.

While the similarities are striking, there are also two important differences:

  1. There are both attractive and repulsive electrostatic forces. Professor Paula Abdul had it right: opposites attract! And same charges repel. On the other hand, there is only an attractive force of gravity, not a repulsive one. Objects with mass always pull one another closer, never push one another away.
  2. The relative strength of the forces. I haven’t included the units, but in terms of the normal SI unit conventions the constant for the gravitation force is about 1/1020 or 0.00000000000000000001 times as large as that for the electrostatic force. Inducing a very small charge in a party balloon, for example, will allow it to stick to a wall in defiance of gravity.

So there you have it: a brief rundown of the four fundamental forces, and – in simple terms at least – some discussion of why we still need four, and can’t boil them all down to one.

For ease of navigation I will include links to each of the other posts in this series at the bottom of each post.

Why I think it’s important to understand evolution
Cosmogenesis, abiogenesis and evolution
Evolution and entropy
Facts, Theories and Laws
Radiocarbon dating
Radiometric dating and deep time
Probability and evolution
Species and ‘baramin’, macro- and micro-evolution
Mitochondrial Eve and Y-chromosomal Adam
Transitional fossils
Complexity – irreducible and otherwise