aommaster Posted October 30, 2005 Posted October 30, 2005 Hi guys! The diffraction pattern of a double slit when a laser is passed is easily explained my superposition (interference). How do you explain the very distinct sections of light when a laser is passed through a single slit? Surely it can't interfere with itself? Also, if it is a possibility, scientists fired single electrons through a slit, one after the other, with a time gap in between, removing any chance that they may interfere with themselves, but still, the same pattern was produced. Why does that happen? I think this has to do with quantum physics, and at the moment, I am fairly new to it, so, please try and use simple language, for my brain cannot handle extreme physics Thanks alot guys!
Klaynos Posted October 30, 2005 Posted October 30, 2005 Hi guys! The diffraction pattern of a double slit when a laser is passed is easily explained my superposition (interference). How do you explain the very distinct sections of light when a laser is passed through a single slit? Surely it can't interfere with itself? Also' date=' if it is a possibility, scientists fired single electrons through a slit, one after the other, with a time gap in between, removing any chance that they may interfere with themselves, but still, the same pattern was produced. Why does that happen? I think this has to do with quantum physics, and at the moment, I am fairly new to it, so, please try and use simple language, for my brain cannot handle extreme physics Thanks alot guys![/quote'] The first bit I can't remember :| The second question: If you fire single electrons at a double slit you WILL get an interfearence pattern appear on your detector after sending many of these electrons through. This is because the electron (the largest particle this has been demonstrated with is C60) has wave like characterists, and is therefore appearing to travel through BOTH slits, and tehrefore does infact interfear with intself. This is due to the wave-particle duality principle...
5614 Posted October 30, 2005 Posted October 30, 2005 Klaynos answered your first question! The wave interferes with itself.
aommaster Posted October 30, 2005 Author Posted October 30, 2005 But how can it? If you fire a single particle through two slits, I was told that the particle goes through BOTH slits. How come? Is this where the idea of extra dimensions come from? And is this the wave-particle duality principle that Klaynos is talking about, or am i completely off track?
Klaynos Posted October 30, 2005 Posted October 30, 2005 Yes it is the wave particle duality. The particle is acting as a wave and a wave can pass through both holes so it does so it is acting as a wave. The detector that is normally used detects individule particles so when they're detected they displaying particle characteristics. If we use some form of detection to calculate through which slit the electron passes then the interfearence pattern disapears, because you cannot detect a particle without altering it's movement in some way... It is a very odd concept and completely abstract because when you kick a football at a wall with 2 holes in it it doesn't pass through both of them...
Cap'n Refsmmat Posted October 30, 2005 Posted October 30, 2005 The Fabric of the Cosmos by Brian Greene would be an excellent book to read more about this. If we use some form of detection to calculate through which slit the electron passes then the interfearence pattern disapears, because you cannot detect a particle without altering it's movement in some way... Not exactly. It does disappear, but only because the particle now has a definite position, directly in front of one of the slits--if it was not observed, it would not have a definite position and it could pass through both slits and interfere with itself.
Klaynos Posted October 30, 2005 Posted October 30, 2005 The Fabric of the Cosmos by Brian Greene would be an excellent book to read more about this. Not exactly. It does disappear' date=' but only because the particle now has a definite position, directly in front of one of the slits--if it was not observed, it would not have a definite position and it could pass through both slits and interfere with itself.[/quote'] As I have been taught it the reason that we cannot infact see the interfearence pattern is because we alter it's motion. I'd be interested to see what swansont or Severian say about it...
5614 Posted October 30, 2005 Posted October 30, 2005 In Search of Schrödinger's Cat by John Gribbin is also an excellent book to read about this and much of QM. What we have is to do with wave/particle duality, and also to do with uncertainty & the Copenhagen interpretation. On top of what Klaynos said about when you observe which hole the particle goes through you will not get the standard "dual slit" results, as the particle goes through a specific hole and thus cannot interact with itself (if it was not observed it would go through both holes, only by measuring it are we forcing it to make a definite choice). err, yeah, back to what I was going to say when I said "on top of"... if you try firing a particle and then while it's moving then shutting one of the 2 holes the particle will know that there is only 1 hole and will make the interference pattern as if there is 1 hole. The particle knows how many holes are open AND if the holes are being watched or not. The particle cannot be tricked in any way and will always produce the 'right' interference pattern the way the slits are set up at the point when it goes through the slits, ie. whatever setup it is when the particle goes through the slit, that's the interference pattern you will observe. This is an example of when the observer effects the experiment. This is a thing which seperates QM from classical physics, in QM (such as this) by observing or not we are affecting the results.
5614 Posted October 30, 2005 Posted October 30, 2005 As I have been taught it the reason that we cannot infact see the interfearence pattern is because we alter it's motion. I'd be interested to see what swansont or Severian say about it... That's a true point. By observing we will alter the experiment, but how? Is it, like what Cap'n & myself have always though, that by observing we are forcing the particle through a specific hole... or Is it that by observing we would alter it's motion... I assume the reason for this would be observation via photons??? Well we could detect a particle using other methods which wouldn't involve photons, like an electron has a magnetic field which we could observe, this wouldn't change the motion of the electron, I don't think. I'd still rather go for the first one and agree with Cap'n and myself, it makes more sense. Rather than having a superposition of the electron going through both holes and interacting with itself we are forcing an electron through one specific hole (by watching it doing it) and then the electron must act as though it's gone through that hole and so it cannot interact with itself. Like you said, interesting to see what others say.
swansont Posted October 30, 2005 Posted October 30, 2005 Any point on the aperture can be thought of as a point source of light, and the path lengths differ, so you get interference. single-slit diffraction Note that you get the single-slit "envelope" superimposed on double-slit diffraction, due to the finite size of the slits.
aommaster Posted October 31, 2005 Author Posted October 31, 2005 Swansont: Yeah, I realised that when we did the laser diffraction. So let me see if I got this right: When single slit diffraction occurs, the wave passes through the slit, and the slit behaves as it is the light source. And this, in turn, causes the diffraction pattern seen as there is a difference in distance travelled As for the double slit diffraction, the particle goes through BOTH of the gaps (by some means we are not sure of) and causes the normal double slit diffraction. Is the part that I have emphasised above correct? Are we not sure why particles travel through both slits?
swansont Posted October 31, 2005 Posted October 31, 2005 Swansont: Yeah' date=' I realised that when we did the laser diffraction. So let me see if I got this right: When single slit diffraction occurs, the wave passes through the slit, and the slit behaves as it is the light source. And this, in turn, causes the diffraction pattern seen as there is a difference in distance travelled As for the double slit diffraction, the particle goes through BOTH of the gaps ([u']by some means we are not sure of[/u]) and causes the normal double slit diffraction. Is the part that I have emphasised above correct? Are we not sure why particles travel through both slits? It goes through both slits because it can - it's a wave.
aommaster Posted October 31, 2005 Author Posted October 31, 2005 Sorry if this may sound annoying, but why can a single wave pass through both slits? How can one wave split into two? And if it does split, does it loose its energy in the intermediate stage? Thanks Swansont!
Klaynos Posted October 31, 2005 Posted October 31, 2005 Sorry if this may sound annoying' date=' but why can a single wave pass through both slits? How can one wave split into two? And if it does split, does it loose its energy in the intermediate stage? Thanks Swansont![/quote'] Waves are not bound to a single point in space, if you get a fish tank and put a screen in it with 2 gaps in it and you make a little wave with your hand or droping something into it you'll notice the wave spreads out and passes through both holes and then spreads out from those holes...
swansont Posted October 31, 2005 Posted October 31, 2005 Sorry if this may sound annoying' date=' but why can a single wave pass through both slits? How can one wave split into two? And if it does split, does it loose its energy in the intermediate stage? Thanks Swansont![/quote'] It may be a problem to think of the wave as a water wave, made up of a bunch of molecules. It doesn't split, per se. The wave is the description of the photon at any given time. According to the Feynman path integrals, light takes all possible paths available to it. So in that view, it has to go through both slits unless you do something to show that it hasn't gone through just one.
BhavinB Posted October 31, 2005 Posted October 31, 2005 It may be a problem to think of the wave as a water wave' date=' made up of a bunch of molecules. It doesn't split, per se. The wave is the description of the photon at any given time. According to the Feynman path integrals, light takes all possible paths available to it. So in that view, it has to go through both slits unless you do something to show that it hasn't gone through just one.[/quote'] All possible SHORTEST paths. Either way...perhaps its easier to understand this concept through the idea of infinite secondary sources. Consider the wave arriving at both slits. Assuming the intensity at both slits is comparable, you can say they now act as secondary wave sources. The two sources interfere and give you the diffraction pattern (in the far field).
swansont Posted October 31, 2005 Posted October 31, 2005 All possible SHORTEST paths. AFAIK the integral is done over all possible paths.
aommaster Posted October 31, 2005 Author Posted October 31, 2005 AFAIK the integral is done over all possible paths. This is the idea that I find really hard to grasp. HOW does it go through all possible paths? Does the photon split up? And if it does, does it loose energy during this intermediate stage? If not, does it go through a dimension that we can't sense?
5614 Posted October 31, 2005 Posted October 31, 2005 To answer how, well, the best advice I could give you is stop thinking classical. Everything you imagine is classical physics and you imagine it classically... even when people think of wave/particle duality as a 'wavicle' they are using classical physics to imagine QM and this does NOT work! A photon will take all possible paths, because, well, it can so it will... unless you are watching it, if you are watching it then it must take a path for you to watch.
aommaster Posted October 31, 2005 Author Posted October 31, 2005 Hehe... that ois probably why my physics teacher hates quantum mechanics. He cannot understand it. I think I just have to accept that if a photon has been given many paths to go through, it will go through all these paths, no energy is lost. It just clones itself! Is that a correct way of thinking about it?
5614 Posted October 31, 2005 Posted October 31, 2005 Again this is a classical way of viewing things and like I just said in another thread viewing a QM problem with classical mechanics will involve some "technically incorrect" points. For you and this example the photon does not clone itself at all in any way. Can't you just imagine it like this: http://img.photobucket.com/albums/v601/5614/waves.jpg but with two holes, so it'd end up like this: or this (basically same thing): http://img.photobucket.com/albums/v601/5614/doubleslit1.jpg A wave can go through 2 holes at the same time, it's as simple and as classical as those pictures above. A wave going through 2 holes is a classical idea and holds true in this scenario (of light going through 2 holes).
aommaster Posted October 31, 2005 Author Posted October 31, 2005 Ok. Thanks alot for that! Sorry for bothering you!
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