Edi said:
In the double slit experiment - what exactly is the measuring device?
.. the have to measure witch slit the photon went trough - how do they do it?
- measuring anything means interacting with it. Changing it, really. So, isn't it obvious that interacting with the photon in any way will change its .. trajectory?
The photon doesn't hit all the probable lines in the screen at the same time. All it does is hits the screen one by one and making the interference pattern over time. - witch means that whatever is interacting with the photon creates the probability of the photon hitting one of the lines on the wall, with the highest chance at the middle one and decreasing with the number on the left or right.
For me - this doesn't mean that the photon is interacting with itself or something super weird. All the experiment proves is that interacting with the photon will.. well.. you know.. interact. Interacting in a specific way will produce a specific result. Dohh.
Soooo.. still - I am interested in what way do they actually measure witch slit it goes trough. (?)
I am not an expert in physics, but have read about double slit experiments from a semi-layman's viewpoint.
From what I have read, at least in case of the experiment conducted with electrons, I think 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.
Source:
Richard Feynman, The Feynman Lectures on Physics, vol 3, Addison Wesley Longman, 1970
