The Yin and Yang of Quantum Physics

Light has a split personality. It acts like tiny waves. It acts like tiny particles. Waves and particles behave very differently in certain conditions. Light is made of photons which can act either as particles or as waves, depending on the circumstances. Although we can do the math, nobody really understands just how this happens.

Quantum Physics says beer only comes in discrete quanta, such as 12 oz, 16 oz, 32 oz, six packs and assorted kegs. The store dont sell 3 oz beers or 19 .747545322 oz beers. You can get an available quantum or be thirsty.

Actually it says that about electromagnetic energy, not beer. This is the key to the physics of the Two-Slit Experiment.

Einstein won the Nobel Prize not for E=mc2 or Relativity, but for his explanation of the photoelectric effect. When light shines on a metal in a vacuum, it can knock loose electrons which are detected as a current flowing in the vacuum to another electrode. The light is not always strong enough to knock loose any electrons, depending on the metal and on the voltage between the electrodes. However, physicists saw a strange thing which they couldnt explain. Making the light BRIGHTER made no difference at all, photoelectric emission depended only on the COLOR of the light. But physics had convincingly explained light as a wave. Einstein explained that light was made of photons, each one is a discrete quantum of energy proportional to their frequency (or wavelength, or color). An electron in the metal had to receive a photon with enough energy to promote it from the bound state to the free state. Brighter light has more photons, but the energy of each photon depends only on the color. Using photons with a higher frequency ( shorter wavelength, or more blue ) gave enough energy to free the electrons.

Einsteins strange new paradigm explaining the photoelectric effect as particles of light opened the door to Quantum Physics whose strange mathematics lead to solar cells, the laser, and computer chips. However, the Two-Slit Experiment shows that light also acts as a wave. Despite the enormous success of quantum theory nobody really understands how a photon can act as a particle and also act as a wave.

The Two-Slit Experiment shows that although a photon acts as a particle, it appears to be in many places at once. Particles we see, like billiard balls and bullets, have a definite position. Photons have a quantum probability density that spreads out rather like ripples on water. Imagine hitting a billiard ball and seeing it disperse as waves across the table. These waves of probability engulf the other balls until suddenly at one point of the wave the original ball snaps back into existence and the rest of the wave vanishes. Nobody really knows how this happens. Although the equations of quantum mechanics are very specific in what shape these waves take, they cannot say what point of the wave will snap back into a particle. The amplitude of the quantum wave at a point is only the probability that the ball will materialize at that point.

The Two-Slit Experiment is simply light shining through two small slits in a screen. The light falls on a second screen where it makes an interference pattern. The pattern for particles is very different than the pattern for waves. Even though the light is made of tiny individual photons, the pattern shows it passed simultaneously through both slits in the first screen, like a wave. Even when the light is so dim that only one photon at a time fals on the slits the interference pattern is still produced.

Whats more, things we used to feel certain were just particles, such as electrons and even atomic nuclei of metals, can act this way. Stranger yet, we now have the technology to track individual particles and watch what path they take when they encounter the two slits. Amazingly, they now act as ordinary particles, and the interferenca pattern vanishes. Quantum wave interference only happens when "nobody is watching" them pass thru the slits. As soon as we stop tracking the path of the particles, the quantum wave interference reappears. How do the particles "know" when its okay to turn into waves?

You can produce quantum wave interference at home. Heres how.

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Ingredients:

• Laser pointer or small visible laser
• Microscope slide
• Candle
• Two X-acto blades
• Straight edge
• White paper on wall in a Dark room

ALSO:

• Something to hold the parts steady: stands, tripod, tables etc.

Procedure:

Make the Two-Slit Screen:

Pass the microscope slide thru a candle flame until it is evenly coated with a thin layer of black soot. This soot is an amorphous form of carbon powder called "lampblack". The soot should be just thick enough to block most of the light, but not so thick that it flakes off when scratched.

Using care, hold the slide firmly on the table sooty side up. Carefully position a straight edge to support it over the slide, not touching it. Pinch two X-acto blades together and align their tips using a hard surface. Now draw the tips of the blades smoothly across the sooty surface of the slide to produce a pair of straight, parallel scratches.

Dont worry about messing up, theres room for several attempts on the slide, and you can wipe off the soot with a paper towel and start over. Keep practicing until you get two very thin, smooth, straight, parallel scratches in a fairly dark layer of soot.

Set up according to diagram:

You get the idea. Your dimensions can vary according to your laser and slit spacing, etc. Fiddle around until the light forms a beaded or fringed pattern on the paper. With a bit of adjustment you may be able to see more than 50 fringes. The more the better. Try to keep the laser beam square-on to the two screens.

The alternating light and dark bands are due to "Interference" between light from one slit and light from the other. At some points light from each slit adds together making a bright spot. At other points, light from one slit "cancels out" the light from the other slit, making a dark spot. Only waves can do this, particles like bullets and billiard balls dont "cancel out" each other. An interference pattern like this is a solution of the Wave Equation and we know that even though the light is made of individual photons, each one is somehow passing through both slits at the same time like a wave.

Results :

In this case the solution of the Wave Equation gives some simple solutions that are easy to verify.

Mark on the paper each and every bright fringe as far as you can in each direction. Turn on the light, count the marks and measure the total distance from the first to the last. Divide the total distance by the number of marks less one, which gives an accurate measure of the distance between each fringe.

Also measure the exact distance between the two slit screen and the viewing screen.

Every point on the interference pattern forms a very thin triangle with the two slits. Light spots are when the light from each slit reinforces. When one side of the triangle gets longer than the other by 1/2 of a wavelength of light, the two sides cancel. Another 1/2 of a wavelength and they add up again. Even though the base of the triangle is only the tiny distance between slits, the fringes are very close together because the wavelength of light is very, very much smaller than the spacing between the slits.

Wave Behavior

We will need to use the relationship l = ws/D

l = ws
      D

where

• l     is the wavelength
• w    is the fringe spacing
• s     is the slit spacing
• D    is the distance from the slits to the screen

Click HERE if you want to see how the equation is derived.

 

Worked example: A Young's Double Slit experiment uses a HeNe laser of wavelength 638 nm. The double slit is placed at a distance of 1.5 m from the viewing screen.   65 fringes are counted, spanning a total of 288 mm, what is the spacing between the slits?

First, we need the fringe spacing. Since there are 65 fringes marked, the pattern covers 65 - 1 = 64 fringe spaces. So 288 mm / 64 spaces is 4.5mm per fringe.

Young's original formula               l = ws/D

Rearranging to find s:              s = Dl/w

l = 638 ´ 10-9 m;       s = 4.5 ´ 10-3 m;       D = 1.5 m             Þ s     =     2.50 m ´ 638 ´10-9m       =       0.000212 m       =       0.21 mm                                      4.50 ´10-3 m

To check our experiment, we use a micrometer to measure the thickness of the blade used to scribe the slits. (the slit spacing is half of two blade thicknesses; it is one thickness). Blade thickness measures about 0.0081 inches. Convert our experimental result to english units: 0.000212 m * 39.4 in/m = 0.0083 inches. The micrometer confirms our experimental result within better than 3%.

Avoid this bear trap: Remember always to convert nanometres to metres. 1 nm = 1 × 10-9m.

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