For Neil

On Particle Physics:

Neil, 'my other half' knows that I absolutely love particle physics and reading up on it is sort of a hobby (weird, I know!). So he kept the link for the following video and got me to watch it. This sent my brain into a science frenzy, because I believe that everything is simple and logical and nothing is 'hocous pocous'! (I admit that this is my opinion and that I am one person and everyone is wrong and right at the same time.) After watching it, I think he was a bit disappointed because I said I couldn't believe it until I understood about the measuring device. What exactly was this 'measuring device', and where exactly was it placed in the experiment. Well I have done a bit of research on what others have to say and I have come to my own conclusion.

Thank you, Neil ... you just opened my mind a little wider. 

 

When you observe it by using a measuring device it acts like a particle?

When you observe it without a measuring device it acts like a wave?

By observing an electron you change it's behaviour, because in order to observe it you have to use light (measuring device?) and this disturbs its behaviour ...

Aside: I like that probability comes into this and this sends us on a new journey. Probability's ability to define am electron currently as a wave and a particle. 

[Perhaps anothe discussion for a later time?]

So then, what is the measuring device? 

Opinion 1:

Source: Richard Feynman, The Feynman Lectures on Physics, vol 3, Addison Wesley Longman, 1970 

"They use some sort of a photo detector followed by a photo multipier to detect which slit the electron went through. This detector can be a light source which is emitting photons. When a charged particle such as an electron passes by, the light from the detector is scattered and we can observe a flash on one side or the other depending on which slit the electron passed through. Of course, when the detection is made, the interferance pattern vanishes. Just like little schoolchildren, you could say. The electrons are misbehaving, doing their strange dance and interfering with each other when you are not looking, and as soon as you turn around and look, they start behaving well. One may argue that the act of watching the electrons disturbed them, and that’s why the interference went away. When the electron, on its way to the screen, interacted with the photons of the light from the detector, a jolt was given to the electron which caused the electrons not to go in some convoluted way and destroyed the interference. Fair enough. Let’s reduce the jolt given to the electron so that it is not disturbed. To do so we reduce the momentum, p, of the photon from the detector by reducing its frequency (i.e., increasing the wavelength). Accordingly, we gradually increase the wavelength of the detector’s photons. Initially nothing happens; i.e. the interference bands continue to be not visible because the electrons have been disturbed due to the high energy of the photons. Then suddenly, when the wavelength is comparable to the spacing between the slits, the momentum seems to have been reduced considerably and the interference bands reappear on the screen. With great enthusiasm and expectation, we look at the detector to see which slits the electrons are coming from. What do we see? The detector is not functioning! The scattered light is no longer precise but is smeared out across the two slits and we can no longer tell which slit the electrons are passing through! The disappearance and the reappearance of the interference bands can be explained using the wave nature of the detector’s photon. The precision with which the scattered flash (produced by the interaction of the electron with the photon) can be pinned down is inversely proportional to the wavelength (compared to the distance between the slits) of the photon from the detector. When the wavelength of the photon from the detector is small, many wavelengths fit between the two slits and we can tell precisely which slit the electron went through (but remember the interference bands vanish). When the wavelength is increased and becomes comparable to the distance between the slits, only one or two wavelengths fit between the slits and therefore the flash cannot be pinned down accurately and, just then, the interference bands reappears. Precisely when the interference bands reappears, we lose our ability to make a meaningful measurement about the electron’s path, and this is again nature’s ploy at work; the photon from the detector (or some such elementary particle) is the only tool available to track the electron’s path and it does not cooperate. It is this disappearence and reappearence of the interference fringes, and the inherent inability to precisely say (with the available tools of nature/technology) through which slit the electron went through, that makes quantum physics so strange. "

Opinion 2:

"The question of observation is a good one. The electron is described by what is called a wavefunction. The square of the wavefunction's amplitude at a point describes the probability of an electron being at that point. Hence, when you pass electrons through a double slit, the wavefunction(which is spread out in space) will hit both slits and interfere with itself afterwards, and you get destructive interference at certain spots, which means there will be zero probability of the electron being found there. Thus, you get a fringe pattern. Now, enter a measurement device. Like you said, let's measure the position of an electron before it goes through the double slit. This measurement forces the electron to take a definite position, effectively collapsing the wavefunction and causing the wavefunction to be zero everywhere except where the electron is found. Now, since the wavefunction only exists at one point, it can't go through both slits and interfere with itself, since it won't spread back out after it has gone through them. Thus, the wavefunction wont be able to interfere with itself, and the interference pattern disappears, leaving the particle-like pattern. "