robinpike Posted August 5, 2014 Share Posted August 5, 2014 Unable? No. While water, specifically may not have been observed, atoms and molecules have been. My graduate work was in this field. I work next door to a guy who observed sodium atoms and dimers interfere. It's been observed in C-60 (buckyballs) and that was 15 years ago! http://physicsworld.com/cws/article/news/1999/oct/15/wave-particle-duality-seen-in-carbon-60-molecules To paraphrase Josh Billings, there is a problem here with the things you know that just ain't so. It's the wave explanation. The wave goes through both slits. Yep okay, fair point about the atoms and molecules. What I want to understand is how the wave explanation (for example for the electron) describes the path of the wave as it travels between the two slits. For example, is there any time when the wave is inside the small part of the barrier that divides the two slits? If the wave does spend some time inside the barrier that divides the two slits, I don't understand why the wave doesn't collapse? Or if it doesn't collapse how the wave is able to progress while inside the barrier? If the wave does not spend any time inside the barrier - then I don't understand what is preventing the wave from taking that path? Link to comment Share on other sites More sharing options...
imatfaal Posted August 5, 2014 Share Posted August 5, 2014 Robin - does a water wave ever occupy part of the barrier? Simplistically it is the fact that there are two paths and they are of different length that creates interference. Waves do not pass through the barrier - they go through the slits/openings; just take a look at a photo of a water wave experiment showing interference. When you talk of a quantum mechanical object any classical approximation is going to be just that - an approximation; and any claim that the qm explanation is flawed because it fails to comply with classical understanding is missing the point. You ask where the wave is - take a look at a harbour opening; where is the wave? It covers open water in troughs and peaks - it is not localised. The qm object behaves as wave in that it interferes with itself. If you don't like the water-wave analogy then don't bother with it. As per his usual standard, Richard Feynman's explanation in the University of Auckland / Douglas Robb Lectures is masterly http://www.vega.org.uk/video/subseries/8 Link to comment Share on other sites More sharing options...
robinpike Posted August 5, 2014 Share Posted August 5, 2014 Imatfaal, I don't mind referring to the analogy of the water wave to help the discussion, but my questions always refer to the behaviour of the electron / photon. You mention that the water wave does not go through the barrier - I accept that. But my question is why the electron / photon wave does not go into the barrier? What is stopping the 'quantum wave' from passing through the inside of the barrier as it is passes down each slit? Link to comment Share on other sites More sharing options...
JoeLight Posted August 5, 2014 Share Posted August 5, 2014 What properties of photons are particle like. What makes us think the photon is ever a particle at all and not exclusively a wave. Link to comment Share on other sites More sharing options...
imatfaal Posted August 5, 2014 Share Posted August 5, 2014 What properties of photons are particle like. What makes us think the photon is ever a particle at all and not exclusively a wave. That's a bit of a side question so if you want to discuss at length please ask the question in a new thread. Basically a lot of physics can only be explained if light is quantised into discrete quantum mechanical objects (initially the ultraviolet catastrophe and the photoelectric effect) Imatfaal, I don't mind referring to the analogy of the water wave to help the discussion, but my questions always refer to the behaviour of the electron / photon. You mention that the water wave does not go through the barrier - I accept that. But my question is why the electron / photon wave does not go into the barrier? What is stopping the 'quantum wave' from passing through the inside of the barrier as it is passes down each slit? There is stuff in the way. Link to comment Share on other sites More sharing options...
Strange Posted August 5, 2014 Share Posted August 5, 2014 But my question is why the electron / photon wave does not go into the barrier? What is stopping the 'quantum wave' from passing through the inside of the barrier as it is passes down each slit? The electron (or photon) interacts with the electrons in the material of the barrier. This is what makes the barrier opaque to electrons/light. I second the recommendation for Feynman's lectures. He explains some of the subtleties very clearly. Link to comment Share on other sites More sharing options...
robinpike Posted August 5, 2014 Share Posted August 5, 2014 The electron (or photon) interacts with the electrons in the material of the barrier. This is what makes the barrier opaque to electrons/light. I second the recommendation for Feynman's lectures. He explains some of the subtleties very clearly. When the experiment is performed with NO slits in the barrier, the electrons / photons never get through to the other side, the barrier is opaque to the electrons / light. So, when the two slits are present in the barrier, and the wave meets that small part of the barrier that divides the two slits, how does the wave manage to progress? Why isn't the wave absorbed by that part of the barrier? Link to comment Share on other sites More sharing options...
Delta1212 Posted August 5, 2014 Share Posted August 5, 2014 When the experiment is performed with NO slits in the barrier, the electrons / photons never get through to the other side, the barrier is opaque to the electrons / light. So, when the two slits are present in the barrier, and the wave meets that small part of the barrier that divides the two slits, how does the wave manage to progress? Why isn't the wave absorbed by that part of the barrier? There is some probability that the wave will be absorbed at that point, but there is also a probability it won't interact (i.e. That the photon "passed through one of the slits"). The wave doesn't automatically collapse the moment it reaches something that it could potentially interact with if there is some probability that it could travel down a path that doesn't run through the barrier. Link to comment Share on other sites More sharing options...
Strange Posted August 5, 2014 Share Posted August 5, 2014 When the experiment is performed with NO slits in the barrier, the electrons / photons never get through to the other side, the barrier is opaque to the electrons / light. So, when the two slits are present in the barrier, and the wave meets that small part of the barrier that divides the two slits, how does the wave manage to progress? Why isn't the wave absorbed by that part of the barrier? Some of it is (thinking classically). Some photons are. The probability of an electron/photon being absorbed or passing through a slit is proportional to their relative area - just as in the classical case. As a simpler example, consider the case of a sheet of glass. We can see through it but also see a reflection in it. That is easy in the classical view: the wave is split, some passes through and a proportion of the energy is reflected. In the quantum view, each photon either passes through or is reflected. How does each photon know what to do? It is just probability; each photon has a probability of passing through or being reflected, which corresponds to the amount of energy transmitted/reflected in the classical case. How do those probabilities come about? It is the sum of the probabilities of interaction of the photon with all the atoms in the glass, and the surrounding air, and ... well everything else. Link to comment Share on other sites More sharing options...
swansont Posted August 5, 2014 Share Posted August 5, 2014 Yep okay, fair point about the atoms and molecules. What I want to understand is how the wave explanation (for example for the electron) describes the path of the wave as it travels between the two slits. For example, is there any time when the wave is inside the small part of the barrier that divides the two slits? If the wave does spend some time inside the barrier that divides the two slits, I don't understand why the wave doesn't collapse? Or if it doesn't collapse how the wave is able to progress while inside the barrier? If the wave does not spend any time inside the barrier - then I don't understand what is preventing the wave from taking that path? It might spend some time in the barrier and be reflected, absorbed or possibly (rarely) transmitted. Not all electrons or photons make it through the slits. The interference is from the ones that do. Some of it is (thinking classically). Some photons are. The probability of an electron/photon being absorbed or passing through a slit is proportional to their relative area - just as in the classical case. As a simpler example, consider the case of a sheet of glass. We can see through it but also see a reflection in it. That is easy in the classical view: the wave is split, some passes through and a proportion of the energy is reflected. In the quantum view, each photon either passes through or is reflected. How does each photon know what to do? It is just probability; each photon has a probability of passing through or being reflected, which corresponds to the amount of energy transmitted/reflected in the classical case. How do those probabilities come about? It is the sum of the probabilities of interaction of the photon with all the atoms in the glass, and the surrounding air, and ... well everything else. Exactly. Before it happens it's a probability, after it happens, it's the statistics. And the numbers match up. Link to comment Share on other sites More sharing options...
JoeLight Posted August 6, 2014 Share Posted August 6, 2014 Thanks for all the help guys I have a few more questions Do electrons when behaving like a wave use the electro magnetic field as a medium? If so, does this play any role in inductance? Is there a name to describe particles such as photons and electrons that can have wave like properties. For instance particles that have both particle and wave like characteristics are called _______? I know photons are sometimes call Quanta. Lastly if photons are massless how does gravity have an effect on light? For instance black holes. Link to comment Share on other sites More sharing options...
Strange Posted August 6, 2014 Share Posted August 6, 2014 Do electrons when behaving like a wave use the electro magnetic field as a medium? Firstly, they always behave like waves (as much as they behave like particles). They are quantizations of the electron field, not the electromagnetic field (otherwise they would be photons). But moving electrons will create electromagnetic radiation. Is there a name to describe particles such as photons and electrons that can have wave like properties. I think they are just called particles. I don't know if there is another term. (Note that all particles have a wave nature. Even macroscopic objects do, but it becomes irrelevant at that scale.) Lastly if photons are massless how does gravity have an effect on light? For instance black holes. In general relativity, gravity is not a force acting on mass, it is a curvature of space time. Light, just like anything else, follows the curvature of space time and so is affected by gravity. But actually, even classical (Newtonian) gravity can be shown to affect light. For example, we know that the rate at which an object falls is independent of its mass. You can take the limit as the object's mass approaches zero and work out the effect of gravity on light. As Newton developed both the theory of gravity and the method of limits, he was able to do this. His prediction of the amount of deflection of light by a massive body is different from that of general relativity so this became one of the first tests of GR. Link to comment Share on other sites More sharing options...
swansont Posted August 6, 2014 Share Posted August 6, 2014 I think they are just called particles. I don't know if there is another term. (Note that all particles have a wave nature. Even macroscopic objects do, but it becomes irrelevant at that scale.) To add to this,in the context of QM, it is typically understood that a generic mention of "particle" is not referring to something that behaves like a marble, i.e. a classical particle. In context, like "wave-particle duality", you can infer a classical meaning. Link to comment Share on other sites More sharing options...
Delbert Posted August 6, 2014 Share Posted August 6, 2014 As a simpler example, consider the case of a sheet of glass. We can see through it but also see a reflection in it. That is easy in the classical view: the wave is split, some passes through and a proportion of the energy is reflected. In the quantum view, each photon either passes through or is reflected. How does each photon know what to do? It is just probability; each photon has a probability of passing through or being reflected, which corresponds to the amount of energy transmitted/reflected in the classical case. And I understand the thickness of the glass is a factor as well. Which posses the question that does the photon know about the other surface? Well clearly it doesn't. Please! After all it's called an elementary particle. It seems to me that Richard Feynman's Sum Over Histories is saying that there's an event at the source followed by an event at the destination. What happens in between is a maelstrom of interactions and exchanges of who knows what. In other words it isn't a particle travelling from A to B, which is why we get stuck on the particle/wave business. It isn't a particle and the wave is a probability pattern of where an interaction might occur. Link to comment Share on other sites More sharing options...
JoeLight Posted August 6, 2014 Share Posted August 6, 2014 It seems to me that Richard Feynman's Sum Over Histories is saying that there's an event at the source followed by an event at the destination. What happens in between is a maelstrom of interactions and exchanges of who knows what. In other words it isn't a particle travelling from A to B, which is why we get stuck on the particle/wave business. It isn't a particle and the wave is a probability pattern of where an interaction might occur. Is there any studies going on to discover what is actually occurring ,if anything, between A and B or since the math is there and if we know A we can accurately predict B, is it something that were not looking into at the moment? I know it has been in question for a long time. Link to comment Share on other sites More sharing options...
Strange Posted August 6, 2014 Share Posted August 6, 2014 (edited) There are experiments that use "weak measurements" to determine the paths of photons. Not surprisingly, these show that the most common paths are the most probable ... http://scienceblogs.com/principles/2011/06/03/watching-photons-interfere-obs/ Edited August 6, 2014 by Strange Link to comment Share on other sites More sharing options...
Delta1212 Posted August 6, 2014 Share Posted August 6, 2014 Of course, at some point it may just be that what happens between points A and B just isn't knowable with absolute confidence. We have some pretty good ideas, and people have made some solid attempts at coming at it from an angle, but fundamentally what we're talking about is finding a way to test what a particle is doing when no one is testing it to see what it's doing. Link to comment Share on other sites More sharing options...
swansont Posted August 7, 2014 Share Posted August 7, 2014 And I understand the thickness of the glass is a factor as well. Which posses the question that does the photon know about the other surface? The reflection probability does not depend on the thickness. Just the index of each medium. It's a surface phenomenon. R = ((n2-n1)/(n1+n2))2 Link to comment Share on other sites More sharing options...
Strange Posted August 7, 2014 Share Posted August 7, 2014 (edited) It can do, if there is interference between the light reflected from the front and the back surface. (And another plug for the Feynman lectures. Or book, if you prefer that medium.) Edited August 7, 2014 by Strange Link to comment Share on other sites More sharing options...
swansont Posted August 7, 2014 Share Posted August 7, 2014 It can do, if there is interference between the light reflected from the front and the back surface. (And another plug for the Feynman lectures. Or book, if you prefer that medium.) Yes, but as you say that's interference and depends on some of the wave traveling through the material and reflecting. Link to comment Share on other sites More sharing options...
Strange Posted August 7, 2014 Share Posted August 7, 2014 (edited) Yes, but as you say that's interference and depends on some of the wave traveling through the material and reflecting. But if you consider an individual photon, then its probability of being reflected from the top surface or not varies with the thickness of the glass. So it seems that somehow it "knows" how thick the glass is even though it only encounters the top surface ... Edited August 7, 2014 by Strange Link to comment Share on other sites More sharing options...
swansont Posted August 7, 2014 Share Posted August 7, 2014 But if you consider an individual photon, then its probability of being reflected from the top surface or not varies with the thickness of the glass. So it seems that somehow it "knows" how thick the glass is even though it only encounters the top surface ... I agree. I'm just saying this is an interference effect and not a reflection effect. You have to account for all possible paths of the photon. Link to comment Share on other sites More sharing options...
robinpike Posted August 7, 2014 Share Posted August 7, 2014 Yes, but as you say that's interference and depends on some of the wave traveling through the material and reflecting. But that explanation based on interference fails to work when the experiment is performed with individual photons. When the experiment is performed with individual photons, and the amount of light returned from the front of the glass is counted over time, it is found that the amount of light depends on the thickness of the glass. An individual photon cannot interfere with itself through reflection from the front and back surfaces of the glass - as by the time the 'reflection' from the back surface reaches the front surface, the photon is no longer there. Link to comment Share on other sites More sharing options...
Strange Posted August 7, 2014 Share Posted August 7, 2014 But that explanation based on interference fails to work when the experiment is performed with individual photons. Have you watched the Feynman lectures on QED yet? He covers this in great detail. Link to comment Share on other sites More sharing options...
robinpike Posted August 7, 2014 Share Posted August 7, 2014 (edited) Have you watched the Feynman lectures on QED yet? He covers this in great detail. Yes, the QED calculations are covered in detail, and I have no problem with their accuracy and ability to agree with what is observed. I'm just saying that QED doesn't appear to work as an explanation. If someone could explain how it works for a single photon (in terms of this discussion), I would much appreciate it ! If the response is going to be: it is not possible to explain - QED works as a calculation, the calculation is all that is required of Physics? That is okay, but it should be clearly stated that QED is just a means to calculate, and QED is invalid as an explanation... Edited August 7, 2014 by robinpike Link to comment Share on other sites More sharing options...
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