|
Young's Double Slit Experiment!
(Part 1) By popular demand, I bring you the wonders of one of the most startling and counterintuitive experimental results ever fallen into the hands of scientists. But first, a little background on the subject to put everything in context and to see what exactly this test settled.
For hundreds of years, great figures in science have been debating between the true nature of one of the most stunning physical attributes of the universe: light. What is light? What is it made of? How and why does it behave the way it does? Those were the central questions. Light is such a grand, powerful phenomenon it even got an entire field of physics devoted it – optics.
It was first the German physicist Max Planck (the father of quantum mechanics) who shed light on this issue. He proposed that electromagnetic waves (light) consist of “lumps” or “packets” of energy. According to him, the minimum amount of energy each wave can have is proportional to its frequency. The higher the frequency, the more energetic, and vice versa. That is, X-rays, say, would have more energy than microwaves because of X-ray's higher frequency. Perhaps Plank's greatest discovery was that the energy of electromagnetic waves are quantized. In other words, energy could come in some denomination, and any multiple of it, yielding no partial or fractional energy states. This discovery lead him to receive a Nobel Prize in 1918. He also predicted a proportionality constant between the frequency of a wave and the minimum lump of energy it can have. This is known today as h-bar (1.05 x 10^-34 J*s).
All this Plankian talk brings us to wonder what these ‘lumps' or ‘packets' of energy are. What are they made of?” This is where the famous debate started between weather or not light is a wave or particle. More than 300 years ago Isaac Newton speculated that light was simply a stream of particles. Other physicists disagreed with him throughout the years, until a man by the name of Thomas Young devised and carried out the double slit experiment proving Newton wrong. And this leads us finally to the experiment itself…
The double slit experiment: on determining the nature of light (wave or particle):
Materials:
• Variable light source – one where we could vary the rate of photon emission
• A thin, opaque sheet with two, vertically parallel slits cut into it
• A detection sheet of some sort…could be a piece of paper or a computerized photon detection plate
Procedure:
The light source is placed such that the light emitted will shine directly through the two vertical slits of the opaque sheet. On the other side of the sheet with the slits is the detection sheet. Let me note that this test must be conducted in a room or enclosure with complete darkness to make sure absolutely no outside light interferes. First, let's imagine we cover up the left slit and just allow a beam of light to pass through the right one. What would we expect to see on the detection sheet? We see what we would expect: a vertical line illuminated corresponding to the photons passing through the one slit. We would get the same result if we covered up the right slit and just allowed photons through the left slit only. Now, lets see what happens if we open up both slits, allowing light to pass through…what sort of pattern would we expect to see on the detection plate? If the light passing through just the right slit produced an illuminated column, and light passing through just the left column produced the same thing, only moved over to the left a little more, then wouldn't opening both slits just produce the two illuminated strips simultaneously? This is what common sense and intuition tells us, and it's also what the particle theory of light predicts. Here, our common sense is dead wrong (as it is in the vast majority of quantum mechanics). We do not see two illuminated regions. Instead, what we get is an interference pattern. We get some vertical illuminated strips, right next to lines of darkness, next to more, but fainter illuminated strips, and so on. The light is going through both slits, and as they do, from that point onward we can effectively view the light coming out of each slit as two sources of light. These two sources of light spread out, and interfere constructively and destructively with each other. That is, at some points the peaks of the waves coincide and combine to form a wave with higher amplitude (these spots represent the illuminated vertical strips on the detection plate) and at other points the peaks coincide and combine to cancel each other out (the dark spots). This behavior belongs to wave phenomena!
Here is a visual of this set up:
Shock # 1:
Here's where it gets more interesting. What if we slowed down the rate of photon emission all the way down to 1 photon firing per second…each individual photon would have to go through one of the two slits (it couldn't go through both at the same time, right? Or could it?). Now what sort of pattern do we see? As it turns out, if we wait long enough for the photons to build up on the detection plate enough to recognize a pattern, we would see that the exact same wave-like interference pattern builds up!
How could this be possible? If only one photon is traversing this contraption one at a time, what in the world could it possibly be interfering with? And even more startling, even neutrons and heavy atoms can be fired in these same ways, and the same interference pattern builds up. Really? How do neutrons or protons interfere? Conducting this test even with heavy elements produces this same result! How in the world could they have a wave nature to them?
Shock # 2:
So we know that when we slow down the photon (or neutron, proton, etc) emission so much that only 1 photon passes through the apparatus at once, the interference pattern still shows up. Well, we thought we could be clever and set up a device to detect which slit the photon actually goes through. Smart, right? Nice try, man. The instant we set up a way to measure which slit the photon goes through................the interference pattern disappears. What? How could the photon possibly be aware of our monitoring of it?
Who can shed some light on this issue? Questions for the audience:
• Explain Shock # 1
• Explain Shock # 2
• What does this test prove?
• More importantly, what are some of the implications of this test?
Discuss the answers
here!
|
