Quantum Mechanics- Wave particle duality

For the last three hundred years there has been debate whether light is made up of particles or waves.   Many scientists have researched this area such as Sir Isaac Newton in the 17th century and Thomas Young in the 19th century, who developed the basic double slit experiment. We now know that not only light acts like a wave but electrons can also act like waves, creating interference patterns.  This area of physics remains an area of intrigue because we still don't know why the act of observing an experiment changes the result.  Now physics is opened up to realities; actual and potential. 

Where did the double slit experiment come from?

The double slit experiment came about when Thomas Young questioned Newton's theory that light was composed of a stream of corpuscles.   Newton's theory stated that :

1. Every source of light emitted tiny particles known as corpuscles

2. These corpuscles are perfectly elastic, rigid, and weightless

Thomas Young noticed some problems with Corpuscle theory such as; why different coloured objects are refracted at different angles and explaining interference.  He came up with the double slit concept when thinking over some of Newton's experiments as he thought light may act in a similar way to sound. In 1800 sound was known then as a compressed wave in air and when two waves of sound crossed they interfered.  The idea that light could show an interference pattern, if proven, would show solid proof that light is a wave.  The experiment did indeed show that light produced an interference pattern and Young went on to measure many of the wavelengths of different colours.

What is light?     We can see that light isn't just what Newton said it was i.e. wasn't just tiny particles called corpuscles. We can show this from two lasers and chalk dust. If you make the laser beams intersect you would expect there to be a collision and that light particles would end up going in random directions.  We can see that this isn't the case.  We see that the light continues without any collisions.  But light isn't just a wave.  In the 20th century it was shown that light when shone on metal transfers it's energy to the atoms in the metal, a particle property.  This explains that light is made of photons, bundles of energy that act as particles and waves. This is unlike anything seen before.  This is the beginning of the revolutionary topic Quantum mechanics.

The double slit experiment

When particles are shot through a single slit it creates a single band. When particles are shot through two slits it creates two bands.

When we look at waves we see that they act differently. When they go through a single slit we see that they act similarly to ripples in water. But when there are two slits we see an interference pattern. There are constructive boundaries where the amplitude increases and destructive boundaries where the amplitude decreases.

If we shoot electrons through a single slit we see that they act just like the particles, creating a single band. But when shot through the double slit we see they form an interference pattern just like the waves.

What is going on ?  The electron acted like the particle when shot through a single slit, but when shot through the double slit it acted like a wave.  Scientist's first idea was that when they shot the electrons, the electrons must have been hitting off each over causing the interference pattern. To test this they made the electron gun shoot individual electrons to see if the interference pattern occurred when electrons couldn't hit each other.  They found that the interference pattern appeared again, built up slowly over time.  Their new hypothesis was that the electrons must be splitting, going through both slits and then interfering with each other before rejoining, just before hitting the observation screen. 

What is interfering with what ? To find out scientists used observation equipment to see if the electrons were splitting, going through both slits and then interfering with itself.  It was shown that the electron just went through one slit and didn't create an interference pattern.  The observation screen showed that two bands were formed, just like the particles. The process of observing the experiment changed the result.

Why does the observation collapse the wave function? The act of observing the electron changes the position and energy of the electron.  As the University of Oregon states, '' If the physicist looks for a particle (uses particle detectors), then a particle is found. If the physicist looks for a wave (uses a wave detector), then a wave pattern is found.

The Double slit experiment can be done with any microscopic particle.  It shows us that where they hit and don't hit the observation screen is down to their wavelike behaviour.  There's more to the experiment than just this.  It shows us that the individual waves interfere with each other.  This ability of individual states of superposition interfering with each other is key to the microscopic world.

So what is the significance of this?  It has led to the invention of quantum computers for example.  They can carry many calculations simultaneously because they are in a superposition of states.  A ten element quantum computer can carry out 1024 calculations at the same time because its in 1024 different states at the same time.  All strands of the parallel calculation are useless if they haven't be combined.  Interference combines these calculations.  Interference enables a single answer to emerge from the 1024 different calculations.  Again it is curious to note that the superpostions can never be observed as mentioned above.  What we can see is just the consequences of their existence, the results of interference.  As with the double slit experiment it is impossible to observe an electron going through both slits at once. 

The double slit experiment has been done many different ways.  In 1961 Claus Jonsson did the experiment with electrons.  His results mirrored Young's, creating interference patterns.  In 1974 it was possible to do the experiment with a single electron for the first time.  Again the interference pattern occurred.  They also added a detector and found that the interference pattern dissapeared.

Science has proven that light is not just a wave or made of particles, but both.  It has a dual potential nature. This holds true at the quantum level.  What's curious is that once a quantum entity is observed, its actual  nature is one or the other.  Unlike Newtonian physics where things are or are not, it is now shown that at the quantum level two kinds of reality exist; actual and potential. More recently, the experiment has been performed with photons, electrons and atoms and each time the same result occurs.  This is one of the big areas of intrigue in quantum mechanics.