Double Slit Experiment
The Observer Changes Quantum Reality
The double slit experiment is probably the most well known experiment in quantum theory and quantum mechanics. The reason why the double slit experiment is so well known is because it easily demonstrates the central quantum paradox. Is light a particle or a wave? The behavior of quantum particles and waves (now called wavicles), changes when measured or observed.
The essence of the experiment is as follows
A beam of electrons is directed through a screen with two slits in it. Since electrons act like waves they split when they go through the two slits in the screen. They arrive at a florescent screen on the other side of the slits. The image on the florescent screen shows up as an interference pattern. The spacing of the light and dark patterns on the florescent screen allows us to measure the length of the waves. Simple, right?
Electron waves are probability waves. The electrons arrive at the screen in a very particle-like way. They appear as dots in various concentrations. Where they land can only be determined by probability. All of the dots together make an image that resembles a wave interference pattern. Now it begins to get strange.
Suppose that we make the electron beam so weak that only one single electron passes through the 2 slits. Do we still get an interference pattern? Yes we do.
Can a single electron split, pass through both slits and interfere with itself? Yes it can.
The Observer Changes Quantum Reality
The double slit experiment is probably the most well known experiment in quantum theory and quantum mechanics. The reason why the double slit experiment is so well known is because it easily demonstrates the central quantum paradox. Is light a particle or a wave? The behavior of quantum particles and waves (now called wavicles), changes when measured or observed.
The essence of the experiment is as follows
A beam of electrons is directed through a screen with two slits in it. Since electrons act like waves they split when they go through the two slits in the screen. They arrive at a florescent screen on the other side of the slits. The image on the florescent screen shows up as an interference pattern. The spacing of the light and dark patterns on the florescent screen allows us to measure the length of the waves. Simple, right?
Electron waves are probability waves. The electrons arrive at the screen in a very particle-like way. They appear as dots in various concentrations. Where they land can only be determined by probability. All of the dots together make an image that resembles a wave interference pattern. Now it begins to get strange.
Suppose that we make the electron beam so weak that only one single electron passes through the 2 slits. Do we still get an interference pattern? Yes we do.
Can a single electron split, pass through both slits and interfere with itself? Yes it can.
"This preposterous proposition is mathematical, but this one proposition is responsible for all the miraculous magic that quantum systems are capable of and that has been verified by myriad experiments and technologies. It is easy to get exasperated and to disbelieve this strange consequence of quantum mathematics." (Goswami, pg. 69)
So what do we do? Naturally we check up on the electron to see which slit it is going to pass through. We turn the light on and watch as the electron passes through the slit. What do we see? The interference pattern has disappeared! As soon as we look at the electron and locate it we lose all the information about its momentum and we lose its wave nature.
Uncertainty principle: - As soon as we determine which slit the electron is passing through, the process of looking destroys the interference pattern.
Complementarity principle - The measurements on the electron's position and momentum are complementary - mutually exclusive processes. We can concentrate on the momentum and measure the wavelength (and thus the momentum) of the electron from the interference pattern, but then we cannot tell which slit the electron goes through. Or we can concentrate on the position and lose the interference pattern and thus the information about the wavelength and momentum. (Goswami pg. 70) Depending on what apparatus we use to do the experiment, we can see one attribute or the other but not both.
Notes: Amit Goswami, The Self Aware Universe, N.Y., Tarcher/Putnam 1995. Dr. Goswami's excellent explanation (pages 66-70) of the double slit experiment was used to help with this page.
So what do we do? Naturally we check up on the electron to see which slit it is going to pass through. We turn the light on and watch as the electron passes through the slit. What do we see? The interference pattern has disappeared! As soon as we look at the electron and locate it we lose all the information about its momentum and we lose its wave nature.
Uncertainty principle: - As soon as we determine which slit the electron is passing through, the process of looking destroys the interference pattern.
Complementarity principle - The measurements on the electron's position and momentum are complementary - mutually exclusive processes. We can concentrate on the momentum and measure the wavelength (and thus the momentum) of the electron from the interference pattern, but then we cannot tell which slit the electron goes through. Or we can concentrate on the position and lose the interference pattern and thus the information about the wavelength and momentum. (Goswami pg. 70) Depending on what apparatus we use to do the experiment, we can see one attribute or the other but not both.
Notes: Amit Goswami, The Self Aware Universe, N.Y., Tarcher/Putnam 1995. Dr. Goswami's excellent explanation (pages 66-70) of the double slit experiment was used to help with this page.