Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: lightarrow on 08/08/2006 16:29:57
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Hello to everyone,
I hope this subject hasn't already been covered; if so, I ask you to forgive me.
A Laser emits a weak beam of low frequency (I will explain later why low frequency) light, so that a revelator "ticks" once every, let's say, minute. Most physicists explain: "one single photon is emitted from the Laser every minute, travels to the revelator and it's revealed".
Question: How can we say that, in this case, a spatially localized particle travels from the source to the revelator? A way to prove it would be to put...another revelator in the middle of the route, so destroing the photon (here is why low frequency and hence low energy); so we are left with the same question: What is the photon from the source to this new revelator?
From my point of view, and I've thought a lot about it, the "photon" doesn't really exist from the source to the revelator; the "photon" actually is "The Tick of The Revelator".
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Classical light consists of a continuous form of electromagnetic wave energy. Quantum theory divides this into packets of defined energy which as you say can be detected individually. but it is all part of "quantum wierdness" that this wave energy has passed through all of space between the transmitter and receiver and not just the single track of a particle. This can be proved by setting up a classic two slit intereference pattern which causes the light to form interference fringes where parts are brighter than the other. These intereference fringes can be shown to exist even if ONLY ONE photon is inside the apparatus at any one time so there is no doubt that both the particle and the wave actually travel between the source and the detector (revelator).
Learn, create, test and tell
evolution rules in all things
God says so!
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Originally posted by lightarrow
A Laser emits a weak beam of low frequency (I will explain later why low frequency) light, so that a revelator "ticks" once every, let's say, minute. Most physicists explain: "one single photon is emitted from the Laser every minute, travels to the revelator and it's revealed".
I am curious as to how a laser can emit a single photon.
Is not the acronym LASER implicitly telling you that the light is amplified, and thus must be greater than the minimum intensity, otherwise if it is merely at its minimum intensity, what is it amplified from?
George
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Quote, from another someone:
"I am curious as to how a laser can emit a single photon.
Is not the acronym LASER implicitly telling you that the light is amplified, and thus must be greater than the minimum intensity, otherwise if it is merely at its minimum intensity, what is it amplified from?"
It's an intelligent observation another someone!
However, other important features of a Laser beam are coherence, monocromaticity and parallelism: the first is a precise phase correlation among the waves in the beam; the second is a single, precise wavelenght of the beam; the third...you understand perfectly (important for writing-reading devices, as in computer drives). Coherence, for example, is necessary to make holograms
You can make a Laser emit one single photon at a time reducing enough its intensity, for example putting a filter in front of it.
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Originally posted by Soul Surfer
... This can be proved by setting up a classic two slit intereference pattern which causes the light to form interference fringes where parts are brighter than the other. These intereference fringes can be shown to exist even if ONLY ONE photon is inside the apparatus at any one time so there is no doubt that both the particle and the wave actually travel between the source and the detector (revelator).
We have an electromagnetic wave that hits the shield with two (or more) slits, then it hits the revelating shield. Without the concept of a flying photon, I can easily imagine a low intensity wave that hits this last shield, making it "clicks" only in some precise point of it and only once every minute: the low intensity wave means low probability of interaction. The fact that THE INTERACTION is particle-like, doesn't mean that a particle travels from source to revelator.
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OK so you are quite happy with the wave version of the photon. the proof of the particle aspects of the photon comes from tha fact that it is always emitted and detected in discrete quantities of energy one of the best experiments that prove this is the photoelectric effect (the experiment that einstein got his nobel prize for) if you bombard a surface with light of a particular frequency it will emit electrons because the photons have dislodged the electrons from the atoms but if the wavelength of the light is just too long it will not dislodge electons no matter how intense the light is the individual photons are just not energetic enough to do the job.
Learn, create, test and tell
evolution rules in all things
God says so!
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First of all, I wish to thank you to have answered to my topic. I appreciate in particular your answers, soul surfer, because they are not trivial. I'm happy to have found someone with wich to discuss these subjects in a deeper way.
Of course I know photoelectric effect, as well as Compton effect, blackbody spectrum, and other things. My problem is: in my personal opinion, all these facts only prove that THE INTERACTION is quantized, not that a particle flies from source to revelator.
Why it is quantized, and why the energy acquired from the electron is proportional to frequency...this is something else I would like to know. Quantum mechanics doesn't prove it, it takes it as a matter of fact. Where does h (Planck's constant) come from? The speed of light, for example, another very important constant in physics, is explained from classical electromagnetism: c^2=1/mu(0)*epsilon(0), but h is not.
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Originally posted by lightarrow
We have an electromagnetic wave that hits the shield with two (or more) slits, then it hits the revelating shield. Without the concept of a flying photon, I can easily imagine a low intensity wave that hits this last shield, making it "clicks" only in some precise point of it and only once every minute: the low intensity wave means low probability of interaction. The fact that THE INTERACTION is particle-like, doesn't mean that a particle travels from source to revelator.
I suppose the underlying question is – what is a particle? How would you define something as being, or not being, a particle?
George
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The present state of physics does not describe what a photon IS, we have only a description of how it is behaving and interacting. Such a state of affairs is very common in physics. We can always try to get to a deeper level (better description) but we never get at the bottom.
Personally (without reference) I think of a photon (or other particles) as a disturbance of the quantum vacuum (an update of the "old ether"). Then, one could understand better why one can associate a wavelength (De Broglie) to each moving particle and why exact localisation is not possible.
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Originally posted by Nieuwenhove
Personally (without reference) I think of a photon (or other particles) as a disturbance of the quantum vacuum (an update of the "old ether"). Then, one could understand better why one can associate a wavelength (De Broglie) to each moving particle and why exact localisation is not possible.
That would seem very much in tune with lightarrow's argument.
George
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Originally posted by another_someone
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Originally posted by lightarrow
We have an electromagnetic wave that hits the shield with two (or more) slits, then it hits the revelating shield. Without the concept of a flying photon, I can easily imagine a low intensity wave that hits this last shield, making it "clicks" only in some precise point of it and only once every minute: the low intensity wave means low probability of interaction. The fact that THE INTERACTION is particle-like, doesn't mean that a particle travels from source to revelator.
I suppose the underlying question is – what is a particle? How would you define something as being, or not being, a particle?
As something with limited extension, that we can see without destroying it. An atom, an electron, an electromagnetic train of waves or anything else with enough energy to be little affected in its properties after have been revelated.
In this case, from my point of view, the concept of particle is assigned to that region of space we are measuring; when, on the contrary, that region of space has so little energy that it is destroyed after revelation, the particle is, instead, in the revelator (its "click"). All the other cases among these two extremes: the particle is in both, and IS both.
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Of course I know photoelectric effect, as well as Compton effect, blackbody spectrum, and other things. My problem is: in my personal opinion, all these facts only prove that THE INTERACTION is quantized, not that a particle flies from source to revelator.
Why it is quantized, and why the energy acquired from the electron is proportional to frequency...this is something else I would like to know. Quantum mechanics doesn't prove it, it takes it as a matter of fact. Where does h (Planck's constant) come from? The speed of light, for example, another very important constant in physics, is explained from classical electromagnetism: c^2=1/mu(0)*epsilon(0), but h is not.
The subject you have raised is indeed a perplexing one, which has generated great debate. One thing we need to keep in mind is that although the wave particle behavior is something difficult for the average person to make any sense out of, the mathematics describing it are very well developed. If we really want to have any hope of understanding this subject, we need to look closely at the math. In particular, a study of the Schroedinger "wave" equation is in order. When we look at that, we see that although it is a "wave" equation, it is an equation markedly different than any of the other wave equations we traditionally run into in physics (sound, water, classical electromagnetism). It differs markedly especially in the fact that the number of independent variables (coordinates or dimensions) over which the wave is formed, is not 1, 2, or 3 as in classical equations, but typically much larger. That means that when we calculate a "wave" from it, it is not a wave in the classical sense. That means we can't use it as the basis of conventional wave thinking untill we have somehow extracted from it an expression which is over 1, 2, or 3 conventional dimensions that has some resemblance to a classical wave.
If this sounds like I am saying that what is propagated in the double slit experment may not actually be the wave we think it is, that is what I am saying.
Another view of this problem can be had by examining how quantum electrodynamics is mathematically constructed. The quantization of the electromagnetic field, (to boil it down to its simplified essence) originates in the results which quantum mechanics gives us for the behavior of a particle confined within a potential well, in this case a parabolic well. A parabolic well corresponds to the situation in which the particle is confined to the vicinity of a point by a force which is proportional to the distance -- the simple harmonic oscillator problem. Quantum mechanics replaces the "particle" with a wave equation within the well, such that the wave's rate of propagation varies with the "speed" of the particle at any point, such that the wavelength is shorter in regions of high speed (near the origin) and longer near regions of low speed, and becomes imaginary beyond a certain distance away. This creates a standing wave within the well, the width of which depends upon what energy is assumed for the particle. Owing to the need for discrete solutions of this problem, only certain solutions are possible, meaning that only certain energy levels are possible. In turns out that in this sitiation, the energy levels are spaced equally apart in energy. As to the position of the particle: the absolute square of the wave function at any position and moment, is proportional to the probability that an operation to detect a particle would be successful at yielding the registration of a particle, if carried out at that point. Other than that, we know nothing about the position of the particle. As to the energy of the particle: We have said that the quantum hypothesis and the assumption that there must exist a Schroedinger wave, requires that onlyh certain energies are possible. More exactly, that means that if an operation to measure the energy is carried out, it can yield only one of the discrete permissible values. However, the wave principle also tells us that any linear combination of wave states that are valid, is also a valid solution, so that the particle is capable of existing in several energy states at once, so long as no operation to measure the energy exactly, is carried out. So in other words, we have a situation in which the classical quanties of position and energy are, in general, apparently indefinite, and become definite only when an operation to measure one or the other is carried out, and (because a wave state corresponding to a definite location cannot be a wave state correspoinding to a definite energy and vice versa) the two quantites cannot both be simultaneously definite.
What quantum electrodynamics does is say that this law (behavior of a particle in a situation where the energy varies as the square of the amplitude, as with a particle held by a linear spring) applies to any variable the square of which is proportional to a classical energy. In particular, it applies to the electromagnetic field strength. Solving the equations in an analogous manner (although the situation is somewhat more complex here because of the possibilities of various field polarizations) yields analogous, and profoundly signficant, results: The electromagnetic field strength, like with the position of a particle in a parabolic potential, is, in general, of indefinite value, and for any particular spatial wavelength is capable of existing only in discrete energy levels, and zero is not a permissible observable and fixed value of the field. For an electromagnetic field which has a particular direction of propagation, polarization, and spatial wavelength, the phase of oscillation is indeterminant, and the "value" of the field remains time-invariant but multivalued, assuming a whole range of values between some effective maximum and the negative of the same. This is a very important point to note, because this description of the electromagnetic field is quite unlike the classical view, in which it has, at any one point in space and time, definite strengths and directions (electrostatic and magnetic), which oscillates according to a well defined function through all phases, in definite relation to time. Another key point to note is that the "width" of the parabolic curve (energy as a function of field strength) varies with spatial wavelength in such a way that the spacing between energy levels is simply h times the frequency -- which of course is nothing other than the photon energy. The conclusion is inescapable therefore that the "photon" is nothing more or less than the difference in energy between one permissible eigenstate of the field at that wavelength, and the next adjacent one (up or down).
So when you talk about the photon being, I understand, some artifact of the process of detection, that is only part of the story; you have to get into an understanding of what a quantized radiation field looks like, and that shows us that the wave propagating from transmitter to detector is in fact not structured the way a classical wave is structured, but has features unimaginable to classical physics.
Of course, this new picture of the electromagnetic field raises many questions, which, however, I will not attempt to deal with here.
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why the energy acquired from the electron is proportional to frequency...this is something else I would like to know.
The answer appears to lie in Special Relativity. The wavelength and frequency of an electromagnetic wave depend entirely upon the reference frame in which it is viewed. What will look like a gamma ray to one observer will look like light to another and like a microwave to a 3rd. Each observer will assign a different value to the energy of a photon. I believe that when you run the quantization equations for the radiation field through in different reference frames, you end up with this conclusion.
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Why it is quantized
The best answer to that seems to be, why is the classical world not quantized? If the Schroedinger equation and the things related to it define the microscopic world, what that means is that the shape of nature is simply that way, and the mystery is not that it is that way, but that the larger world is not also that way. And the answer to that question is that the quantum physics, when aggregated into classical magnitudes, simplifies down into classical behavior, but that the universe at its most fundametal level is not classical.
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From my point of view, and I've thought a lot about it, the "photon" doesn't really exist from the source to the revelator; the "photon" actually is "The Tick of The Revelator".
As noted above, the "photon" can't even be said to be a particle at all; it is a difference of energy states in the electromagnetic field.
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Originally posted by Atomic-S
As noted above, the "photon" can't even be said to be a particle at all; it is a difference of energy states in the electromagnetic field.
But can the photon be said to be any less of a particle than any other fundamental particle, or is it the case that no fundamental particle can truly be said to be a particle?
George
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Originally posted by Atomic-S
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why the energy acquired from the electron is proportional to frequency...this is something else I would like to know.
The answer appears to lie in Special Relativity. The wavelength and frequency of an electromagnetic wave depend entirely upon the reference frame in which it is viewed. What will look like a gamma ray to one observer will look like light to another and like a microwave to a 3rd. Each observer will assign a different value to the energy of a photon. I believe that when you run the quantization equations for the radiation field through in different reference frames, you end up with this conclusion.
Of course frequency of light depens upon the Reference System, but an electron's speed too. If in a RS, an almost still electron (not exactly still, because in that case its wavelenght would be infinite and the electron couldn't have neither defined position nor little dimension) is hit from a beam of light with frequency f, in another RS moving at speed v (parallel to the beam of light) with respect the first, the beam has an higher frequency f', for example, but the electron is not still anylonger, it travels in the same direction and versus of the beam. After the electron has interacted with this beam of f' frequency, we have to subtract the initial kinetic energy of it, to compute the energy of interaction.
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Originally posted by Atomic-S
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Why it is quantized
The best answer to that seems to be, why is the classical world not quantized? If the Schroedinger equation and the things related to it define the microscopic world, what that means is that the shape of nature is simply that way, and the mystery is not that it is that way, but that the larger world is not also that way. And the answer to that question is that the quantum physics, when aggregated into classical magnitudes, simplifies down into classical behavior, but that the universe at its most fundametal level is not classical.
When a pure sound of specific frequency f hits a diapason with own frequency f, it resonates. Let's suppose we still hadn't found an explanation of it, we could say: "That's the way the world is!".
I know that quantization could never been explained, but it would seem very strange to me.
You can say: "world doesn't have to behave in a classical way!" Ok, but i still have to see in a book of physics a better classical study of the interaction between a light beam and an electron.
I say this because you cannot say: "an electromagnetic wave interacting classically with a point of charge, or a little sphere of charge, doesn't provide the right solution of what happens". Thanks! The electron is not a point or a sphere or anything like this! NOW, we know it! So we are not forced to think with quantum mechanics theory (from which, for example, h is a postulate, it's not explained), to study classically the light-electron interaction.
For example, if the electron were a wave of charge, (it's just an example, I don't have believes on it), how would an electr.magn. wave interact, classically, with it? I mean, classicaly in the sense without quantum mechanics, but with special relativity of course.
I'm aware it wouldn't be a simple problem, every charged region of that space would move according to sp.relat. and would produce others electr.magn. waves ecc. Are we sure in this case the interaction would be the same as it is in the normal classical way?
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Originally posted by another_someone
But can the photon be said to be any less of a particle than any other fundamental particle, or is it the case that no fundamental particle can truly be said to be a particle?
George
That's an interesting question. Maybe I should have titled my initial question: "What really is a particle?"
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I have been away on holiday for a week and have just come back to the board and am working my way from where it was when I left it.
"Lightarrow" you have moved your question into a much more philosophical area and I will do my best to explain things a I see them.
I have always beleived ever since I was a child and started to think about these things that we would eventually find out that the "particles" in the universe wre really persistent elements built by scrumpling and distorting the fabric of space time in various ways. Such that although they are dynamic and always changing they possess precise propeties that can be observed.
There have been several attempts to propose a theory of this nature that I have seen over the years. the most recent (and best I think) is covered in the 12 August issue of the magazine "New Scientist".
Another way of lioking at the same thing is that particles are similar to the stable structures or "gliders" that you get in the computer based simulation "life" This was well illustrated by Stephen Wofram in his book "a new kind of science" but he only illustrated it using stable complexity generated in two dimensions whereas it is really in four dimensions (the classical einsteinian space-time)
There are several reasons why I strongly expect that this will be the solution to a theory of everything. The most notable one is that in particle interactions just about any sort of particle can be turned into any other provided of course that you observe the rules by putting the right sort of things into your reaction and use the correct energy level. so this implies that right at the most fundamental level they are really all made out of the same stuff.
Another important fact is the laws of the conservation of energy and angular momentum which are essential for a universe to be stable, understandable and last for a significant length of time. Plancks constant is a vital part of this because it represents a tiny piece of angular momentum and defines the smallest possible angular momentum element.
The most fundamental property of quantum mechanics is its connectedness which by the possibility of quantum entanglement means that every particle in the universe is somehow "aware" of every other particle in the universe and has a common set of physical laws and that in some way in at least one "dimension" the universe is extremely small
Another important book to read is by Martin Rees "just six numbers" he clearly demonstrates that the whole structure of our universe are defined by just six critical values one of which is the number of extended dimensions in space time. However he takes Planck's constant as given. one important property is that the values of the numbers if selected at rendum would appear to be very improbable but they may not appear so improbable if initially an evolutionary process had been at work that tried to make our universe last as long as it possibly could.
To conclude I would like to come bact to consider the nature if a photon the wave/particle of electromagnetic energy. When for some reason an electrical field changes it creates a magnetic field when a magnetic field changes it crerates an electrical field so a photon is a confined area (moving at the speed of light through the classical vacuum)in which the energy is being moved continuously between electrical and magnetic fields.
Gravity is associated with mass and the graviton is a quantised tensor wave in space time that communicates gravitational fields in just the same way that photons communicate electromagnetic fields. I vsiualise normal (non composite) fundamental particles (like electrons and quarks as being long lived combinations of thest two sorts of elementry structures. their precise shapes at any instant are continually changing but they continually recycle themselves.
Learn, create, test and tell
evolution rules in all things
God says so!
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But can the photon be said to be any less of a particle than any other fundamental particle, or is it the case that no fundamental particle can truly be said to be a particle?
No fundamental particle can be said to be any more or less a particle than any other. As to why some might appear to be such, that has to do with certain of their quantum properties which differ.
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Of course frequency of light depens upon the Reference System, but an electron's speed too. If in a RS, an almost still electron (not exactly still, because in that case its wavelenght would be infinite and the electron couldn't have neither defined position nor little dimension) is hit from a beam of light with frequency f, in another RS moving at speed v (parallel to the beam of light) with respect the first, the beam has an higher frequency f', for example, but the electron is not still anylonger, it travels in the same direction and versus of the beam. After the electron has interacted with this beam of f' frequency, we have to subtract the initial kinetic energy of it, to compute the energy of interaction.
Correct. However, you must remember that in the photoelectric effect, the electrons are not free-flying but are confined in a target.
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For example, if the electron were a wave of charge, (it's just an example, I don't have believes on it), how would an electr.magn. wave interact, classically, with it? I mean, classicaly in the sense without quantum mechanics, but with special relativity of course.
I don't think we can begin to speak of how an electromagnetic wave acts classically with a wave of charge, until we have first constructed a description of the wave of charge. What is the description of the wave of charge? What is its equation? What does it look like? How do its dynamics work? Until we know that, we can't really say much of anything about how it would classically interact with a classical electromagnetic wave.
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That's an interesting question. Maybe I should have titled my initial question: "What really is a particle?"
The comments by Soul Surfer appear to at least partly answer this question. The following might also be added, in regard to the nature of an electron. I mentioned before, how by treating the electromagnetic field as an assembly of harmonic oscillators (one for each state of wavelength, propagation direction, and polarization), using the quantum equations for the harmonic oscillator, we end up with a field composed of "photons" (discrete steps in energy). It turns out that a similar treatment seems possible for the eletron (and also for other particles). Going back to the photon case, one of the interesting results of this is that because photons are not "objects" like little balls, but rather steps in energy, they are not individually identifiable. I.e., they are indistinguishable "particles". That results in the principle that any equation that describes an assembly of photons in terms of their "particle coordinates" must be symmetric with respect to the interchange within the equation of any 2 sets of coordinates. That has statistical implications, that cause an assembly of photons to behave differently, as in thermodynamics, than they would if they were little planets that we could individually name. Now with the electron, that is also true, but the electron differs from the photon in one important respect: its spin. You may remember from the equations of orbits in an atom, that the requirement of periodic continuity requires that the angular momentum about any one direction can assume only integer values. Something like that governs individual particle spins also, the phenomenon of particle spin being (in the case of these "particles" which are not really particles at all in the classical sense, but differences of state) related to the directional characteristics of the wave function at each point. (Thus, a particle having a scalar wave function, describable at each point by a single (complex)number, has no directional properties and therefore no spin. A value describle by three (complex) vector components at each point is a vector, and corresponds to a particle spin of 1 and to equations of the first degree. A value describable by nine (complex and possibly in part redundant) components at each point is a tensor and corresponds to spin 2, and to equations of the 2nd degree (That would be your gravity.) But in this view, only integer values of spin are possible. But it turns out that Dirac, by mixing in Einsteinian relativity and the element of time, discovered that half-integer spin states are also possible, provided that the quantity which represents the directional property of the wave function is not a scalar, not a vector, not a tensor, but is something else. This led to the matrix solution of the electron, which turns out to have a spin 1/2 and also directional properties which are, however, not like those of the photon. Subject to that, we can proceed to derive the existence of the electron from an assumption that it is a quantized energy step in some kind of an energy field, as is the photon. However, this solution came with a price: The principles of particle indistinguishability which lead to the requirement, for photons, of a symmetric overall state function for an assembly of them, lead in the case of half-integer particles to a requirement that the state function be antisymmetric. That is, on exchanging the coordinate sets of any 2 of the particles in a coordinate based state function, the value of the function must everywhere reverse. Obviously, if the function is such that 2 particles occupy the same individual state, the only way this is possible is if the function is everwhere zero, i.e., that state of the system cannot exist. This is known as the Pauli exclusion principle, and says that no 2 electrons can occupy the same state. The result of this is that when building up the quantized electron field in, say, an atom to higher and higher levels, it cannot simply assume greater "amplitude states" as would photons, but each additional piece of self-energy added to do this must take the form of a distinct wave state. Thus, electron shells are formed, each of which can be occupied by only 2 electrons (of opposite spins). These blips thus appear to us to be very definable and particulate in their nature, even though at the most fundamental they are simply increases in state. One other thing leads the electron to have a "particulate" appearance more than the photon, and that is the fact of its nonzero rest mass. Where there is a nonzero rest mass, the Schroedinger equation takes a form which is somewhat more complex than the electromagnetic case, and is somewhat similar to the situation in which a classical string is connected to the tabletop by an assembly of light springs. There will be a minimum possible frequency of propagation, at which the entire string will be in phase. That would correspond to an electron at classicdal rest, and the minimum frequency is simply its rest energy divided by h . (That is, f = mc^2/h , m being the rest mass.) That is, it is possible for an electron to assume a state of classical rest, something which is a particle-like behavior rather than a wave-like behavior, but you will note that this behavior is nonetheless fully derivable from wave considerations, by simply changing the form of the wave equation, in the same way as loading a classical string with springs would. So again, we see that a seemingly particulate behavior of an elementary particle actually does not prove that it is fundamentally different from a more "wavelike" particle, but only that the form of its equation is different.
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Thank you all for your explanations.
I wish I have a better knowledge of quantum mechanics. I would like to make some other questions, however, but I don't know if you all have enough of this thread; however, here are the questions:
1) As you say, Atomic-S, there is a minimum frequency for electron's wave: mc^2/h ~ 1.4*10^20 Hz. But wich is the wavelenght?
2) Is a particle's spin an intrinsec property of that particle? We know that "hidden variables" doesn't actually exist.
3) One of quantum mechanics postulates and the very essence of it, is Schrodinger equation, where there appears the constant h.
We are worried about where the mass of every particles comes from, (we have to build more powerful accelerators to find Higgs's boson) but we are not worried at all about where h comes from. This is another mistery for me. How could we believe to find a unification between gravity and quantum mechanics, for example, if we don't go a little deeper in our understanding of physics?