A planet to discover ball and wave

Published on May 14th, 2013 | by Stephanie


Discovering the world – my way

Our planet is organised by humans; humans who wish to discover it.  And I am one of those. I decided to discover this world a little differently to many of my homosapien counterparts. Rather than getting on a plane and discovering the macroscopic world with my senses,  I took a trip to the semi-tangible world to engage my brain.

Throughout the remainder of this year, I will endeavour to blog at least once a month about my trip thus far.   So, before I bequeath to you a rather intriguing adventure, let me give you a sense of my “through line”.  I enrolled in a Bachelor of Arts / Bachelor of Science way back in 2007 (this was my equivalent to a round-the-world ticket) and as you read I slave over the final chapters of my theoretical physics honours thesis.

The first rendition on my – and now your – discovery of the planet will concern the wave-particle duality of light.

This means that light displays both wave like and particle like properties concurrently.

Light behaves like a ball:


Light behaves like a wave:


What are the consequences of duality?

Firstly, it is pretty weird.  We will need to re-examine a few of our common conceptions.

I am assuming that we can all picture light in its particle form minuscule globules of energy whipping through the atmosphere.  Photons — the name physicists bestow upon light particles — can be thought of as marbles: spherical blobs of tangible stuff existing in a physical location in real space.

marble-72152_640Light as a wave, on the other hand, is a tad more difficult to get your head around.  I would put this down to the fact that waves spread out over time.  Imagine that you have a marble and you throw it into a still lake.  The marble strikes the surface, causes a disturbance and then sinks to the bottom.  The marble stays put at the bottom of the lake, but the wave it created continues to spread out.  If you look at one point of the wave and take note of its spatial location, a few seconds later this same point no longer has the same position.  So the way that particles and waves travel through space is quite different.

wave-motion-64169_640Now think about the difference in the interactions that these two states can have.  Colliding particles “bounce” off each other, whereas waves travel “through” each other, and emerge unchanged.  Upon travelling through each other, waves can interfere. If the crest (maximum) of one wave interferes with the trough (minimum) of another, the wave can disappear.

Back in 1801 Thomas Young demonstrated this phenomenon with his double slit experiment.

Imagine two slits in a screen.  If you throw your marbles at it — one at a time — they will either ricochet off the screen or — if you’ve got good aim — go through one of the slits.  And depending on which slit you managed to get your marble through, it will emerge on the other side travelling in one of the two directions.

A spread out wave travelling towards the screen will pass through both the slits simultaneously. The slits split up the incident wave and two separate waves emerge.  These new waves continue to propagate forward and interfere with each other.  In some places a peak and a peak interfere, doubling the strength of the wave.  In other places a peak and a trough interfere, and annihilate the wave.  [When the light hits the screen we say that it diffracts, and by measuring the diffraction pattern a distance away from the screen we can find out all kinds of things — these will be discussed in future travel reports.]

To see the effect that the two slits have on the emerging diffraction pattern a detector can be placed a little down stream of the screen.  The marbles will hit the screen in one of two spots, depending on which slit they went through.  What you see on the detector resembles this


Where as the diffraction pattern caused by the interaction of the waves, will look like this


because as the waves propagate toward the detector they interact with one another and spread out — like in the image above of the two waves interacting.

The hardest thing to imagine is that photons — electrons, atoms, other fundamental particles and just about everything else actually — possess both these wavelike and particle like properties at the SAME TIME.  Light IS a wave AND a particle.

Many mysterious questions arise from this rather disturbing phenomenon: What happens to the energy in the photon when it collides with something? Is the energy spread out over the entire wave, which is distributed throughout space?  Or is the energy in a little packet?

Stay tuned.  I took a fairly long cruise down this line, and will tell you all about it in my next instalment.  If I have time, I might even let you in on how I manipulate this to image single cells and electronic structures in my research.


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