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Natural Science Forum / Physics / Particle Physics / September 2004Newbie questions
Hi there, Please forgive my layman's lingo and clumsy descriptions, but I'm wondering if somebody could point me in the right direction (books etc) to learn about the issues related to the following thoughts. . . 1. If a sun emits light as a finite scattering of individual photons with linear paths, viewing the star from many light-years away, wouldn't it be staggeringly unlikely for enough photons to have been emitted at the incredibly tight angle to actually hit our eye (or sensor) all that distance away, and still be perceived as a constant steady point of light? 2. Is the "brightness" of light considered to be a "number of photon hits over time" or can a single photon have a brightness as well as a frequency? Also is light frequency the photon actually changing position in space like a sign wave shape, or is the frequency just a "pulsation". 3. Polarizing light using a grating seems to infer that the frequency wave of light has a rotation axis, and only the photons with an axis matching the grate are able to pass through. Does this mean that the path of a photon is a flat strip with a measurable width ? Or is it a single infinitely thin line and I'm missing something silly? 4.Could the dual slot experiment actually be dealing with waves, but the sensors pick up the peak "fuzz" of a weak wave, like a bell buoy on the sea. The wave is always there, but the bell (or photon detector) only "rings" if the wave disturbs it enough? 5. Is the wave particle duality just incompatible ways of visualizing the same thing? 6. If light is a wave, doesn't it require a medium to travel through? Something to "remember" it's peaks and troughs over time? Thanks to any helpful persons out there ! Richard > Please forgive my layman's lingo and clumsy descriptions, but I'm wondering > if somebody could point me in the right direction (books etc) to learn about > the issues related to the following thoughts. . . Your questions are about astronomy, classical electrodynamics and quantum mechanics. It depends on your education what kind of book are suitable for you. I think as a layman popular science books are the best choice. Unfortunately I don't know such books, but I will try to answer your questions. > 1. If a sun emits light as a finite scattering of individual photons with > linear paths, viewing the star from many light-years away, wouldn't it be > staggeringly unlikely for enough photons to have been emitted at the > incredibly tight angle to actually hit our eye (or sensor) all that distance > away, and still be perceived as a constant steady point of light? Obviously the star light which we can see has got enough intensity (or enough photons). Otherwise we wouldn't see it. Of course there are also stars that are too far away or too weak, so that we cant't see them with our eyes. > 2. Is the "brightness" of light considered to be a "number of photon hits > over time" or can a single photon have a brightness as well as a frequency? Yes, brightness can be considered as photon flux density. A single photon has no brightness. It has a particular energy, which is directly related to its frequency and its colour. > Also is light frequency the photon actually changing position in space like > a sign wave shape, or is the frequency just a "pulsation". No, light is an electromagnetic wave (in the classical theory), i.e. the electric and magnetic field strength is pulsating. In quantum theory the amplitude of the wave gives the probability to measure a photon, which is the smallest possible energy packet that can be emitted or absorbed. The photon was proposed by Einstein in 1905 to explain the photoelectric effect. > 3. Polarizing light using a grating seems to infer that the frequency wave > of light has a rotation axis, and only the photons with an axis matching the > grate are able to pass through. Does this mean that the path of a photon is > a flat strip with a measurable width ? Or is it a single infinitely thin > line and I'm missing something silly? When we study the propagation of light we must use the wave picture. It's the oscillation plane of the electromagnetic field that has to match the grating in order to pass through. > 4.Could the dual slot experiment actually be dealing with waves, but the > sensors pick up the peak "fuzz" of a weak wave, like a bell buoy on the sea. > The wave is always there, but the bell (or photon detector) only "rings" if > the wave disturbs it enough? I think that's not a good picture. When we condider a single photon it behaves like a wave when passing the slot (it makes no sense to ask which slot the photon has passed through). When the detector absorbs the photon then the wave diappears completely. > 5. Is the wave particle duality just incompatible ways of visualizing the > same thing? That's almost right. I wouldn't say the two ways are "incompatible", they are different sides of the same coin. When we want to describe propagation of "particles" we need the wave picture. When we want to describe the detection or interaction with other particles we need the particle picture. > 6. If light is a wave, doesn't it require a medium to travel through? > Something to "remember" it's peaks and troughs over time? That was the big question in the 19th century. Maxwell's theory of classical electrodynamic predicted electrodynamic waves that where found by Hertz. It also turned out that light is such a wave. But the theory said nothing about a medium for these waves. This hypothetical medium was called "ether" and there was a famous experiment of Michelson and Morley, who tried to measure the motion of the earth through this ether. But the ether couldn't be found. This led to the theory of Special Relativity (again by Einstein and again in 1905). Thanks for your help! Richard <snip> > 1. If a sun emits light as a finite scattering of individual photons with > linear paths, viewing the star from many light-years away, wouldn't it be > staggeringly unlikely for enough photons to have been emitted at the > incredibly tight angle to actually hit our eye (or sensor) all that distance > away, and still be perceived as a constant steady point of light? If all the photons between the sun and our eye were scattered, then you're right, it would not be perceived as the localized source as it is. However, the fact is that the majority of the photons are not scattered at all (or appreciably) between the surface of the sun and our eye. The teeny fraction that have been, are responsible for the blue sky, among other things. > 2. Is the "brightness" of light considered to be a "number of photon hits > over time" or can a single photon have a brightness as well as a frequency? The brightness has to do with the amount of energy deposited (multiplied of course by our sensor's sensitivity at that frequency). The amount of energy deposited depends both on the number of photons and the frequency of the photon. > Also is light frequency the photon actually changing position in space like > a sign wave shape, or is the frequency just a "pulsation". Depends on what you mean by motion of the photon. The photon is not a BB that wiggles back and forth through space, if that's what you mean. The electric field (and magnetic field) that are responsible for carrying the energy DO wiggle in the sense that they point one direction and then the other. This does not imply actual movement back and forth, though. If it helps, go to a doorway and stand in the open doorway and put your hands flat on either side of the wall next to the doorway (one on the inside wall and one on the outside wall). Now push first with your right hand and then with your left, then with your right and then with your left. Though the wall doesn't move, your "push" is wiggling back and forth. You could certainly arrange the push's magnitude to vary sinusoidally. The electric field is like that, expressing the capacity to push back and forth even though nothing actually moves unless you put something there (like an electron) that CAN wiggle back and forth through space. > 3. Polarizing light using a grating seems to infer that the frequency wave > of light has a rotation axis, and only the photons with an axis matching the > grate are able to pass through. Does this mean that the path of a photon is > a flat strip with a measurable width ? Or is it a single infinitely thin > line and I'm missing something silly? See the previous. A polarizing filter is something that can be wiggled by an electric field in one direction (say, left-right) and not in the other (say, up-down). In the direction that it can be wiggled, it absorbs the energy of the push (the field) and none of the push survives past the filter. In the other direction though, none of the energy of the push is absorbed and so it carries on past the filter. Ordinary light contains a mix of pushes in both directions, and so some, but not all, is absorbed by the filter. > 4.Could the dual slot experiment actually be dealing with waves, but the > sensors pick up the peak "fuzz" of a weak wave, like a bell buoy on the sea. > The wave is always there, but the bell (or photon detector) only "rings" if > the wave disturbs it enough? Experiments with the photoelectric effect proved that light will not behave this way in this experiment. You can't make the wave of a given frequency arbitrarily weak. It has a lower bound, the energy of one photon of that frequency. If you could, then photoelectric detectors would behave completely differently than the way they do. > 5. Is the wave particle duality just incompatible ways of visualizing the > same thing? In a way. If I asked you to describe a US penny, you might say that on one side it has a president and one the other it has a pillared building. It doesn't mean that presidents have pillars or that buildings hold government office. Nor does it mean that, sure a penny has a pillared building, but FUNDAMENTALLY it's more president-like. That's all nonsense. Wave-particle duality means that we have inadequate language to describe what a photon is, and we end up describing it by describing "both sides" of it. > 6. If light is a wave, doesn't it require a medium to travel through? > Something to "remember" it's peaks and troughs over time? No, it does not. Many other waves rely on a distinction between the inertial objects -- chunks of a medium (loops of a Slinky, air molecules) -- and the coupling between them that are responsible for passing "the message" from one inertial object and the next. In light, the coupling and the inertial objects (the electric and magnetic fields) are one and the same. Ask yourself what a medium is, exactly, and you'll see what I mean. It's not enough to say, "Well, water is the medium for surfing waves, and air is the medium for sound waves, and so on." That's trying to define something by examples. What is the definition of a medium, such that water and air satisfy that definition? When you can answer that, then you'll have a better understanding of why a medium is sometimes present, sometimes not. PD Very helpful, thanks for taking the time to clue me in. Richard |
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