Light: Wave, Particle, or Both?

From the previous post we know that energy is not continuous, but is quantized, coming in discrete amounts. If energy were continuous then no atom could exist because the electron would spiral into the nucleus according to classical physics. Now let’s take a look into a more bizarre case.

There is in my opinion no better example than the often-discussed wave-particle duality in quantum physics. In essence it defines the quantum element (atoms, electrons, quarks, photons, etc.) as either a wave or a particle depending on if the element has been observed or not. As is always the case, the element will manifest as a particle when observed, but will remain a probability wave when not observed. The fundamental fact behind this is that the particle does not exist until it is observed - it is not a real object that exists independent of observation.

A stream of subatomic particles such as electrons or even light itself is beamed toward a screen. Between the screen and the light/particle source is a barrier that contains two parallel slits. If the slits are open, the light is projected through and out to the screen. The slits can be opened or closed independently at will.The particle stream intensity is reduced until only one particle at a time passes through either of the slits. Meaning a single particle is emitted, it then comes to the barrier where it passes through the slits or hits the barrier and is absorbed. We cannot predict where a particle will land even if the beam is well aimed. If a particle reaches the highly sensitive registering screen it will leave a small spot. Typical everyday thinking would suggest at this point that each particle must pass through one slit or the other to reach the screen.

Unfortunately this is not a time for normal common-sense thinking. Strangely enough, if you have one of the slits closed, more particles will reach certain areas of the screen than if you leave both slits open. There is absolutely no way to make sense of this fact if one thinks of the beam as composed of individual particles shot one at a time toward the screen. One would think that with two slits open, twice as many particles would reach the screen.

It would seem that when both slits are open, the single particle fired from the source, is actually taking both paths (slits) at once and somehow interfering with itself before reaching the screen. It is as if the particle has turned into a wave - extending itself over the entire path then dividing into sub-waves after passing through both slits. If the sub-waves converge in-phase with each other, a spot is made on the screen. Should the waves converge out-of-phase, nothing hits the screen, the waves are canceled out. In fact the pattern made on the screen after this has been going on for sometime looks like a wave interference pattern, much like ripples in a lake.

Here is something to think about, if the electron is watched while traveling on it's path, it will only be found at one slit or the other, never both. If the electron is not observed while on its path, it will apparently take all possible paths. If you look at the screen while observing the electron, you will see it register as a tiny point - as you would expect from a particle. But go away for a time and return - you will find an ever extending concentric ring pattern much like that made when you drop a pebble in water - indicating wave interference. A random stream of separated particles could never produce such a pattern - but these are separate particles and the pattern of interference is real. So how can an interference pattern be created when only one particle at a time is being fired through the slits?