You might say I was strongly attracted to magnets as a kid. Such a puzzle .. they were attracted to the refrigerator, but not to a piece of paper. I could put several sheets of paper between one and the refrigerator and it was still attracted. Two magnets, depending on their orientation, would either attract or repel each others. What bothered me most was how did action at a distance work - how did the magnet know that there was a refrigerator nearby? Were they talking somehow?
My old friend Ge Molin would say that you had to be very ambitious when trying to understand a problem. For him 野心 meant ambitious and translates as 'wild heart'. You had to be ambitious to make any progress. Molin would say you are driven by a wild heat. How wonderfully poetic even if it isn't the most accurate translation.1
Newton's law of gravitation was one of the most dramatic leaps in science . Two bodies attract each other with a force proportional to the product of their masses divided by the square of their separation. Double the mass of one object and the force doubles. Double the distance and the force drops to a forth of what it was. This, combined with a little more simple physics allows you to work out how most object move in the world we know. It is even good enough to go to the moon.
Newton had a simple and beautiful theory that was more accurate than scientists of the day - or for a few centuries to come - could measure and it was predictive. For some it seemed to be the description of Nature at a deep level, but Newton was troubled. His wild heart thinking made him consider its implications. The sort of thinking that drives science and progress
While it was easy to calculate the forces between two bodies, the mechanism wasn't clear. How did an apple know how to follow the most direct path to the Earth? How did the moon know to follow the path it followed - a path that was predicted by mathematics? The bodies had to sense each others position and mass instantly. Not only that, they needed to know the position and mass of every other object in the Universe instantly if you really wanted to get things right. Newton left the problem for those with more insight, but that is a problem with some bits of insight. His theory allowed people to make predications good enough to change the world, didn't shed light on the inner workings. There were parts of Newton's puzzlement were similar to what had bothered me as a kid...
You're probably wondering why I'm starting with something as well known as Newton's Law of Gravity...
Earlier this week two regular readers wrote asking for comments on the announcement of a new experiment that would "create" matter from pure light. The description given in news reports wasn't very clear and it brings up a deeper question: what are particles made of? Thinking about this I couldn't think of a clear way to describe what was going one using the New York Times level physics that most people have. A practicing physicist uses a different model to think about the process. Electromagnetism and gravity, although very different, turn out give some insight. Strap in and I'll try to sketch out some "modern" 20th century physics using a bit of the history of science rather than math.2
But back to gravity...
In 1783 Pierre-Simon Laplace solved Newton's puzzle. Rather than try to change the inverse square law he rewrote it to describe a field that filled space with the force of gravity just being the slope of the field. Fine - what's a field? Think about temperature. Every point in space has a temperature associated with it. The temperature in your room is a field - every point in space has a value associated with it.3
Imagine a mass like the Earth. Plot the distance from the Earth on the horizontal axis and the gravitational potential vertically. The closer you get to the Earth, the more the slope increases and the force due to gravity is higher.
This was a wickedly beautiful solution. Now you only needed to know the field immediately around the area you were calculating - no longer did you need to consider the mass and position of every object in the Universe. The spooky action at a distance went away - all of the action is embedded in the field. The field is a thing - it is a fundamental element of Nature.
The 19th century saw the brilliant unification of electricity and magnetism by Maxwell. This is another field theory - space is filled with an electro-magnetic field and using Maxwell's equations it is possible to work out a good deal of what our modern world is based on. The view that emerged was one where matter was made of tiny particles and forces are carried by the gravitational and electromagnetic fields that fill space. Conceptually a lot of beginning physics and much of engineering is taught that way to this day.
By the early 1930s quantum mechanics had emerged. It basically states what you observe differs from reality. In classical physics a particle has a position and a velocity. Conceptually you could measure these together to whatever accuracy your apparatus allowed as they are real. You can't measure the position and velocity of a particle to arbitrary accuracy - in fact it is worse than that - there is no such thing as a position and velocity for the particle. What exists is a wave function - a wave spread through space that gives us the answer to what is the probability that you can measure the position of a particle if that's what you want to measure. At tiny scales - the size of an atom and below - this is important, but it is more difficult to see as objects increase in size. What we perceive with our senses is so enormous compared to the quantum world that we don't notice it. But it is real and is highly predicative and has astonishing accuracy.
A more accurate description of Nature is that particles are really just fields. The combination of quantum mechanics and field theory gives quantum field theory. It is one of the core tools - probably the core tool - of physics. Perhaps because it isn't terribly intuitive and requires some mathematical sophistication it usually isn't taught until the junior or senior year of college and rarely outside of physics departments.
QFT is very elegant - now you can describe the interactions between particles and particles or particles and fields as fields interacting with fields. It is a bit of handwaving, but the energy of a particle is related to the frequency of its wave function. Massive particles have very short wavelengths and are mostly "localized". At our scale it is easy to mistake it for a very small mass and one that interacts with the gravitational force as well as one of the other three forces in physics. When you sit on a chair your atoms don't really physically contact those in the chair - the electric repulsion of the electrons that come in close proximity becomes so great that it gives a nice solid surface - a surface that, at a tiny scale, is really the result of the interaction of fields.
That's it - at Nature's core (at least to our current understanding) everything is just a field (although there are a variety of types of fields). Mostly we don't have to think in those terms and we use good enough approximations to throw a ball, watch a sunset or listen to some music. But if you want to get poetic you are an enormous collection of tiny fields - all vibrating away and interacting with each other and the fields of the Universe all with their own music of sorts.
Most of the time is nothing wrong with thinking about the world as made up of atoms and recognizing a gravitational and electromagnetic force, but you can move to a different level when you need to think more deeply about what Nature is doing.
Now the original question that was posed can be answered. If a particle and its antiparticle meet they can convert into a couple of photons. Matter becomes light. About 80 years ago it was predicted that it should be possible to do the reverse - namely to collide two photons and produce two elementary particles that have rest masses.4 This is easy to do in the presence of something like an atom, but the probability of the easiest reaction - two photons turning into an electron and a positron - is difficult. Now an experiment has been proposed that looks very promising - there should be a result within a year.
The Science Times level description has two photons colliding and just vanishing in a poof of energy which turns into an electron and positron pair. The more useful description has two very energetic waves in the electromagnetic field interact with each other with enough energy (waviness or shaking) that the violently shaking shakes another field - this time the field associated with the electron particle - and an electron and positron are created. If you could bang another particle field hard enough, you would get out some other combination, but even this very low mass pair is going to be very impressive.
A final note. Sometimes a new scientific result causes an old one to be overthrown, but there are times when additional insight is added. The old theory is not overthrown, but enhanced. Sometimes the new theories are easier use on questions where the old theories worked well, but sometimes they are much more difficult to use. You pick what is easy. For all intents and purposes most of the world is still accurately described by classical physics.
1 Mike notes that 'wild heart' has a slightly negative connotation. The definition that Molin Ge used applies more to ambitious work - Newton's Law of Gravity is one of the most ambitious pieces of physics.
2 I'm not a history of science person and only give a rough outline. Additionally the natural language of science in general and physics in particular is math and words are inadequate. So this is necessarily very very rough. Those of you with physics backgrounds may wish to leave ...
3 This is only for thinking about a field in terms of something familiar. Temperature is not something that is fundamental, so it isn't a fundamental field. Also note that fields can vary with time.
4 A photon is technically a particle - a particle without a rest mass. The notion of matter is loosely defined .. most people think about it as something that is a particle or collection of particles and particles are usually seen has having mass. To stay away from confusion a photon is usually described as a zero rest-mass particle.