Frank Witte

Perfidiously Pervious Particles

Figure 1: Niels Bohr sought to pin down the discussion on the "true nature" of particles by describing them as being "both waves as well as particles".

 

Things usually are not what they seem. As a scientist this doesn't necessarily cause you a headache. Rather it's your core business to deal with such uncertainties and extract sensible knowledge from them. But sometimes the fundamentals become discomfortingly alien...

'Old particles'

The idea that the world is made up of 'stuff' in the form of lots

of small particles is old. Ancient greek philosophers, notably Democritus

who first conceived of the idea of 'atoms', devoted considerable

energy contemplating this question. During the rise of early-modern

science, under the influence of Descartes and Newton in the sixteenth

century, particle theories of natural phenomena became all the more

en vogue. Even light, according to Newton, should be interpreted

as a particle phenomenon, but, as is often the case, the question

of the nature of these particles went unanswered.

Continuous Nature

A totally different concept, namely that of continuous media carrying

waves, also owes its scientific origins to the same era. Huygens,

in whose honour the principle of wave-propagation is named, proposed

that light was to be thought of as something continuously flowing

and altering in form in a very specific way. A wave-front could

be considered as a result of each point in space within the wave

being a source of waves itself. In his day optic devices were far

too inaccurate to discriminate between these ideas decisively, but,

two centuries later, the 'light as a wave phenomenon' emerged as

the favoured view. In fact, the whole view of nature became a lot

more 'continuous' and even a concept like "heat" was phrased

in terms of a continuous caloric fluid. The extent to which this

view permeated scientific thought is exemplified by the resistance

amongst the nineteenth century physics community to embrace the

concept of molecules.

'Modern particles'

The view most non-physicists have of the nature of particles dates

from the late nineteenth and early twentieth centuries. The Austrian

physicist Ludwig Boltzmann showed that the old "continuous"

view of nature could be very well reproduced by working out the

laws of classical mechanics for very large numbers of very small

particles, dubbed molecules. The emerging science of chemistry showed

that molecules could be thought of as comprising the right pairings

of more fundamental units called atoms. The great triumph came a

short time later when it was revealed that most of the properties

of atoms could be understood in terms of three simple constituent

particles: protons and neutrons, which reside in the atomic nucleus,

and electrons. What a nice place the world was - three little billiard

balls making up this whole astonishing variety of stuff that we

see around us.

'Post-Modern Particles'

The Post-Modern world started before the modern world had ended,

at least in physics. It turned out that the three fundamental building

blocks of stuff were not at all like billiard balls. Quantum mechanics

is the desperate attempt of mankind to come to terms with this.

In the first half of the twentieth century wave ideas came crashing

back into physics to explain the oddities of how these particles

behave. Niels Bohr (figure 1) subsequently sought to break

down the discussion on the "true nature" of particles

by describing them as being "both waves as well as particles".

Figure 2: Quarks in Protons, Neutrons, and other combinations.

Of course, if you call this the "complementarity-principe"

it sounds more credible, and if you add a little Yin-Yang all is

well that ends well. I believe we could just as well say that they

are neither … neither particles nor waves. So what is the nature

of these elusive things that we call particles?

Let us ignore for the moment that the original simplicity of the

electronic-protonic-neutronic trinity has given way to a little

zoo of 6 quarks, 3 types of electrons, and three types of neutrinos.

Still not bad though for an explanation of all of stuff. No, the

real enigma is their nature, not their numbers. The discovery of

their wave-properties was not the last word, for these waves turned

out to be wholly different than any waves we ever had encountered

before. Indeed, quantum waves are not at all like the waves you

see at the beach. They are, at best, waves of probability. These

waves of probability come in package deals that we can either take

or leave. It is those packages that we refer to as particles - hardly

a game of billiards now is it ?

Figure 3: When waves from multiple sources mingle interference occurs.

But those probability waves aren't really probability waves at

all! They are only like that if the probable velocities of the particles

are low. But if they are high, i.e. approaching the speed of light,

these particles reveal their true (?) nature as excitations of an

underlying field! Now where did that come from? Modern particle

physics uses the language of quantum fields; continuous agents (fields)

that somehow manage to make a very discrete impression (particles).

These are the sort of things a royal household dearly needs, but

physics? More and more aspects that we once considered to be sturdily

embedded in the nature of material particles are evaporating before

our eyes. Even the strings in string theory are not strings.

At the End of All Things...

Stuff is not made from little billiard balls. That is, unless you

want to extend the definitions of balls to include things that have

no size, no exact location, no exact velocity, and no precise number.

Things that can change their nature, their charges, that have distinct

left and right hands (weak interactions). Things that appear "hot"

to an accelerated observer in the vacuum (Unruh Effect). A family

of Things of which some are confined to invisibility for most of

their lives (strong interactions). I guess physics, which was long

thought to be a materialistic science dealing only with the dead

and inanimate subjected to soulless forces and interactions has

come to the end of all things. At its deepest level known to man,

nature seems infinitely more subtle and alien than ever before.

The great miracle is that we were actually able to understand it

this far at all.

- February 2006

About the Author

Dr Frank Witte is a university lecturer in the theoretical physics department at Utrecht University, the Netherlands. He appears on regional radio every now and then answering listeners' questions on relativity, which is one of his favorite subjects.