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Monday, 16 November 2009 07:46 |
- the two slit experiment is key to understand the microscopic world
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The wave-like properties of light were demonstrated by the famous experiment first performed by Thomas Young in the early nineteenth century. In original experiment, a point source of light illuminates two narrow adjacent slits in a screen, and the image of the light that passes through the slits is observed on a second screen.
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click here to interference movie click here to see a wave experiment
- waves can interfere, for light this will make a series of light and dark bands
- matter particles, such as electrons, also produce interference patterns due to their wave-like nature
- so with a high flux of either
 photons or electrons, the characteristic interference pattern is visible
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The dark and light regions are called interference fringes, the constructive and destructive interference of light waves. So the question is will matter also produce interference patterns. The answer is yes, tested by firing a stream of electrons.
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- if we lower the intensity of light, or the flux of electrons (the electric current), we should be able to see each photon strike the screen
- each photon makes a dot on the screen, but where is the interference pattern?
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However, notice that electrons do act as particles, as do photons. For example, they make a single strike on a cathode ray tube screen. So if we lower the number of electrons in the beam to, say, one per second. Does the interference pattern disappear?
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- the interference pattern is still there, it simply takes some time for enough photons, or electrons, to strike the screen to build up a recognizable pattern
- interference, or a wave phenomenon, is still occurring even if we only let the photons, or electrons, through one at a time
- so what are the individual particles interfering with? apparently, themselves
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The answer is no, we do see the individual electrons (and photons) strike the screen, and with time the interference pattern builds up. Notice that with such a slow rate, each photon (or electron) is not interacting with other photons to produce the interference pattern. In fact, the photons are interacting with themselves, within their own wave packets to produce interference.
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- in order for a particle to interfere with itself, it must pass through both slits
- this forces us to give up the common sense notion of location
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But wait, what if we do this so slow that only one electron or one photon passes through the slits at a time, then what is interfering with what? i.e. there are not two waves to destructively and constructively interfere. It appears, in some strange way, that each photon or electron is interfering with itself. That its wave nature is interfering with its own wave (!).
click here to see a particle experiment The formation of the interference pattern requires the existence of two slits, but how can a single photon passing through one slit `know' about the existence of the other slit? We are stuck going back to thinking of each photon as a wave that hits both slits. Or we have to think of the photon as splitting and going through each slit separately (but how does the photon know a pair of slits is coming?). The only solution is to give up the idea of a photon or an electron having location. The location of a subatomic particle is not defined until it is observed (such as striking a screen). |
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