Gauge this!

This one is just about as fundamental as it gets. All of physics comes together because of "gauge fields".

The first to be discovered was gravity, (we studied the effects but didn't "know" what it was) and then electromagnetism. There are two more. But what are gauge fields?

To quote Sir Patrick Moore, "Well, we just don't know". We know what they DO, just not what they ARE.

Taking the first two forces to be discovered, gravity and electromagnetism, this is what happens.

Place something where there is a gravity gauge field (ie "gravity"), and it moves towards another thing according to rules we fully understand. The "field" is like a slope running downhill from the smaller to the bigger.

Place something in an electromagnetic gauge field, and nothing happens. That is, UNLESS that thing has something on it we call "electric charge". We also do not know what this is either, just that things either have it or not, and, what is more, it comes in two types. We call them "positive" and "negative". (We tried "Laurel" and "Hardy" but it didn't sound quite right). All we knew was that two of the same type were unfriendly and pushed each other away, and two which were different, became lovers and wanted to be together all the time. The "field" was like a slope running downhill from one type to another.

Once more, we do not know what these things are, just what they do.

We knew that these "charged particles" exchanged another, the photon, in order to be friendly or hostile. We called this an "exchange" particle.

Then we formulated a theory that to hold an atomic nucleus together, there had to be another gauge field that was stronger than the electromagnetic field, but only at very short distances. If you think abut it, the atomic nucleus has lots of particles in, some with no "charge", others with the SAME charge. Yes, I know I said stuff of the SAME type are hostile, that is why we figured out there must be a stronger "attractive" force only available inside this nucleus thing, otherwise it would fly apart (if you get a big enough nucleus, it actually does "fly" apart because it is simply too big, and we call this "radioactivity". Some guys (embarrassingly calling themselves "nuclear scientists" - my own field) even used this to make a very big bomb called "The Atom Bomb". Because of this force which acts at extremely short range, unlike the electromagnetic field (which has infinite range and therefore whose exchange particle, the photon, has zero mass), the exchange particle had to have real mass. (the mass of the exchange particle is inversely proportional to the range of whichever force we study)

Then, just before the atom smashing started, Hideki Yukawa predicted from analogy with the electromagnetic field in 1935, that the exchange particle for this "strong" force, would be 260-270 times heavier than the electron, and it would show up in our atom smashers.

What we found was startling. We found a new particle only 207 times the electron mass.

What was going on? It was actually the wrong particle, and we now know it as the mu-meson, or muon.

The one we were looking for duly turned up and we called it the pi-meson, or the pion. ("mu" and "pi" are Greek letters, "pi" being better known in mathematical circles)

The upshot was this:

All gauge fields "exchange" particles, rather like they play "catch", in order to do stuff.

Now, we had four forces, electromagnetism; the "strong" force (the one that prevents the nucleus from flying apart); the "weak" force (which doesn't do much except help change particles to others with help from the conversation of energy); and er, gravity (that thing that helps apples to fall from trees)

What are their exchange particles?

electromagnetism: the photon (has been known for years, it is "light")

strong: initially, pions, that stuck nuclear stuff together, but as wisdom blossomed, gluons, that stick the quarks together (theoretically, cannot be detected, only indirectly observable in "three-jet events* (see my post "Three jets are better than two")) ("quarks" are still more bits inside the bits inside the nuclei - a bit like those Russian Dolls - and yes, the buck stops there)

weak: Intermediate vector bosons, the W+ , W- and the Z-0. (predicted in the 1970's, observed in the 1980's)

gravity: ?

Gravity is, and always will seem to be, a complete headache. It's exchange particle is speculated to be the "graviton", but has never been seen, directly or indirectly.

WARNING - SUBJECTIVE BIT!

It is the opinion of this author that Einstein failed in his attempt to unify gravity with the other three forces because gravity is NOT a gauge field, but a geometric effect. I submit that there are only three forces, plus gravity, which Einstein himself proved to be geometric.

I find it hard to believe that if gravity IS a gauge field, spacetime would be, by definition, quantized, that is, there is a smallest bit of space beyond which we cannot measure because nothing smaller exists.

END OF SUBJECTIVE BIT