The Four Forces

Hi Guys. I am writing an article and was planning to state that all "change" all "movement" can be attributed (at a fundamental level) to the fundamental forces (electro-magnetic, strong and weak nuclear) plus gravity. Essentailly I am looking for confirmation or refutation before I write it. Is this true? From a physics point of view, is anything (from a sub-atomic particle to a blackhole) known to be able to "move" or "change" without being explained in terms of the three fundamental forces or gravity? Thanks, Brian
- Brian (age 29)
Australia

Yup*, the strong, weak, electromagnetic and gravitational forces are the big four forces we all know and love. Actually, since the 1960’s we’ve known that the electromagnetic and the weak nuclear force are really different forms of the same force, called the "electroweak" force.

But I must complain a small bit about your choice of words. "Change" and "movement" do not need forces. Newton’s first law says that in an inertial reference frame, if an object has no forces acting on it, it will move with constant velocity (that is, with constant speed and direction). You can find a frame of reference moving with an object so that it won’t appear to be moving in that frame, but in other frames of reference the object will be moving. Motion really is the rate of change of position, so if you have motion without force, you have change without force.

Forces govern interactions. They cause accelerations, classically, or they define the allowed quantum states quantum-mechanically, or govern scattering. The quantum picture of the three forces (not including gravity) is one of exchange of force-carrying particles. The strong force is mediated by the exchange of gluons, the electroweak force is mediated by the exchange of photons and the weak force carriers W+, W- and Z^0. We’re still at a bit of a loss as to how to get gravity to work quantum mechanically.

Tom

Not only can you have change (say relative positions changing) without any forces, you can also have forces without change. For example, the lowest-energy state of an atom is held together by electrical forces (and nuclear ones in the nucleus) but it is unchanging in time. So there’s no necessary or sufficient connection between forces and change.

Mike W.

* We now have to add the Higgs interaction to gravity, electroweak, and chromodynamic on our list. /mw

(published on 10/22/2007)

With the confirmation of the Higgs Boson (and thereby it's field), do we therefore now teach the four forces of physics, or the five forces of physics? Thanks - will
- Will (age 70)
Corvallis, OR - USA

I just checked with an expert colleague, Jon Thaler, who says that the Higgs interaction is not part of the electroweak interaction. So it would indeed be a fourth fundamental force. (Fifth if you want to count weak and electromagnetic separately.) Thanks to your question I've gone back and made small changes to several old answers.

Mike W.

(published on 12/06/2013)

Thanks for answering my initial question. Wouldn't it be five forces even if you don't count electromagnetic as two: Gravity, Electromagnetic, Weak, Strong, and "The Higgs"
- Will (age 70)
Corvallis, OR - USA

The theorists now have a unified theory including both electromagnetism and the weak interactions, creatively called the elctroweak interaction. Since the unified theory predicts (observed) effects that are in not in the sum of the two separate theories, it should be considered a single type of interaction. Think of an analogy: the unification of electricity and magnetism predicts electromagnetic waves, not present in the separate theories. Now we see electricity and magnetism as two sides of the same effect, with the two types of fields mixing as you go from one reference frame to another.

It's possible that unified theories will be found including chromodynamics etc., so the number of distinct forces could go down again. That's because whether forces are "distinct" depends not only on the forces but on our ability to see the connections. Of course, as more effects are found the number could also go up again, as it did with confirmation of the Higgs field.

Mike W.

(published on 12/10/2013)