Before we deal with that thing that shook the world of physics, we need to say a few words about what quantum physics is.

Quantum physics deals with the motion and properties of very small things.  When I say small things, I mean REALLY small - too small to see with the naked eye, or even with the most powerful optical microscopes.  I'm talking about atoms and the electrons, protons and neutrons that go to make up those atoms.  (See the Atoms section on the Radiation pages for more about atoms).  No, to "see" atoms and electrons and protons and neutrons you need special microscopes that probe those atoms with something other than visible light.  Electron microscopes are what you need.  Electron microscopes fire electrons at atoms, rather than light, and by looking at how the electrons are scattered through interactions with those atoms, you can build up a picture of what an atom looks like.

This state of affairs, that we cannot easily see what goes in atoms, gives us a first hint that life within an atom is different from what we are used to in our every day world.  In fact, life in the "quantum world" is very different indeed.   "Counter-intuitive" would be a good way to put it.

However, if we go back just over 100 years in time, nobody knew what atoms were.  At that time, the universe was thought to be well understood.  People knew about forces and the laws of motion, electricity and electromagnetism were understood, and the principles of thermodynamics had been developed.  Yes, life was very comfortable and it seemed that the "classical physics" of that time was quite sufficient to understand how the universe worked.

A simple example will illustrate the success of classical physics.  Consider a person standing on top of a building of height h and suppose that the person drops a bowling ball of mass M from the top of the building.  Assume that wind resistance is negligible.  We can ask ourselves some questions about the motion of the bowling ball.  For example, after some time T, with what speed v downwards is the bowling ball travelling and how far down (distance S) has it fallen?  What is its momentum p and kinetic energy E?  These are not difficult questions to answer.  It is straightforward to show that these answers are as follows:

where g is the acceleration due to gravity (equal to roughly 10 m/sec²).  These equations can be verified to be correct to a very high degree of accuracy, and we can make useful predictions from them.  For example, we can work out that if h = 400 m then the time taken to reach the ground is about 9 seconds, and the speed on hitting the ground is about 90 metres per second (which is equal to around 200 mph).

A slight digression is in order here.  The "momentum" of a classical object such as a bowling ball is the product of the ball's mass and its velocity.  Momentum is an important quantity as the laws of mechanics show that the momentum of an object remains constant if there are no external forces acting on that object.  It turns out that it is easier to formulate the theory of mechanics (of which the above calculation is an example) in terms of the momenta of the particles and objects under consideration.  Momentum is also a useful concept in the quantum world.  Just to make life interesting,  quantum objects can have momentum too, even if they have zero mass!

Returning to our bowling ball falling from a building, there is another point that is worth mentioning about the simple calculation above.  If we so wanted, we could measure the position and speed (and hence momentum) of the bowling ball as a function of time, and we could measure both quantities simultaneously at any given time.  This is a general property of "classical" mechanical phenomena, namely that there is no fundamental reason why such simultaneous measurements cannot be made.  In some circumstances and situations this might require sophisticated equipment, but in principle it can always be done, provided the equipment to do so is available.  This might seem a rather trivial and unimportant point, but we will return to this later when we come to discuss the Heisenberg uncertainty principle.

Now let us turn our attention to another interesting problem, which at first sight should also seem to pose no great difficulties.  Everyone knows that if you take an iron bar and heat it in a fire, then after a while the bar will start to glow red - it becomes "red hot".  Now, if you keep the bar in the fire, it will start to glow orange, then yellow, and eventually through to white hot, as the temperature of the bar increases.  Now, if you use special equipment you can analyse the radiation emitted by the bar, and you will find that it consists of radiation at various wavelengths and intensities.  If you plot the intensity of a given wavelength component against the wavelength itself, you will find a curve that has a shape similar to that shown in the figure below.

Around the turn of the century, two scientists, Rayleigh and Jeans, attempted to develop a theory that explained the shape of the curve shown in the figure above.  They used the techniques of classical physics and completely failed in their attempts to explain why the curve takes the form that it does.  Their formula came close to explaining the curve at high wavelengths, but at lower wavelengths (i.e. to the left of the above graph), their formula predicted that the intensity should get higher as wavelength decreases, whereas the curve above shows that intensity DECREASES as wavelength decreases.  This came to be known by the evocative name "ultra-violet catastrophe", because ultra-violet light has a range of wavelengths in the region where the failure of their theory is most catastrophic.

The problem was resolved in 1900 by a physicist called Max Planck.  His inspired solution was, in simple terms, to assume that the atoms in iron bar behave like small oscillators, and that if the frequency of vibration of these oscillators as they heat up is f, then the ONLY allowed energies of the oscillators are

E = hf, 2hf, 3hf, 4hf, ...

where h is a constant.  This is a drastic departure from classical physics, where there is no reason whatsoever why an oscillator should not acquire any value for energy.  The simplest way to accomplish this, for "classical" oscillators, is to increase their amplitudes of vibration.  Imagine a swinging pendulum, which is a simple oscillator.  You give the pendulum more energy by increasing the width of the pendulum swing.

With this assumption, Planck was able to deduce a new formula to describe the shape of the intensity / wavelength curve shown above, and it was found to give excellent agreement when a suitable value was chosen for the constant h.  This constant h came to be known as Planck's constant, and thus came the birth of quantum physics.

Before ending this section it is worth noting that Planck did not come up with the idea of energy quantisation because he felt that there was a justifiable reason for doing so.  In fact, he described the idea as an "act of desperation", something that he tried in order to try and account for the experimental result for intensity vs. wavelength.  Max Planck was awarded a Nobel Prize in 1918.

On to Wave Particle Duality