Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: waytogo on 21/03/2013 22:28:30
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As the title says. Tnx.
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Most materials emit photons from their surface in a random direction and phase. If the energy source is heat (like an incandescent light), the frequency is also randomly drawn from a probability distribution which depends on the material's temperature.
One exception is the laser - the initial photons are emitted in a random direction, but only the small percentage which are aligned with the mirrors see a long path through the lasing medium and are amplified. The rest are lost.
When the laser material is "pumped" to a semi-stable high energy state, it will emit a photon in the same direction and phase as the incoming photon, amplifying the light bouncing between the mirrors. One of the mirrors is partially reflective, allowing a narrow beam of light to be emitted in a single direction with a common phase and frequency.
Other light sources can also have their randomly emitted light redirected into a single direction by using:
- lenses (eg lighthouses and LEDs)
- mirrors (eg car headlights and battery-powered torches)
- refraction (eg optical fibers)
- absorbent materials (eg X-Ray machines)
- At lower frequencies, microwaves can be directed using waveguides
- and radio waves can be directed by using an array of antennas which add the signal together in the wanted direction, and cancel it out in unwanted directions.
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Most materials emit photons from their surface in a random direction and phase.
Hi there, first of all I would ask you, where did you find that info?
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Most visible light emission and absorption occurs at the level of individual atoms, with each atom independently absorbing energy (which pushes an electron into a higher orbit), and radiating energy (as an electron falls into a lower orbit).
For incandescent lamps or the Sun, this produces photons with a random direction, frequency and phase. See http://en.wikipedia.org/wiki/Black-body_radiation#Explanation
Another class of light emitters produces light in a narrow range of frequencies, because the emitting material is kept under conditions where most of the electrons will jump between specific energy levels, producing light of one or a few specific frequencies. Examples are LEDs and sodium vapour lamps. However, each atom is still acting quite independently, so the direction and phase of the photons is still random. See http://en.wikipedia.org/wiki/Emission_spectrum#Origins
The difference with a laser is that the light between the mirrors forms a "standing wave" in which all of the atoms are immersed, so the atoms and their electrons no longer act independently, but act as a group, producing light of a common direction, frequency and phase. This group behaviour is assisted by starting with a material which produces a line spectrum. See http://en.wikipedia.org/wiki/Laser#Design
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Let's make a simple example: a single atom in an excited state returns to its fundamental level and at the same time it emits a photon.
Which direction does the photon go?
Simpler/simplistic answer: in every directions at the same time, as a spherical wavefront of the electromagnetic wave.
More complex answer: that is true until you measure the photon momentum. But to measure the photon momentum is not necessary to make the light interact with some device: you could measure the atom's recoil (if it's large enough) because the photon momentum is exactly the opposite, for the law of conservation of momentum. The photon and the atom states are "entangled", so if you measure the momentum of one of the two, then you already know that the other momentum will be opposite when you will measure it. So you can measure the atom's momentum and let the electromagnetic wave extend to infinite: you already know that the photon, one day, will be detected in the approximate direction opposite to the atom's recoil. It is approximate because the uncertainty principle.
But before you measure one of the two momentums, you can't say in any way which value/direction it has!
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A good explanation light arrow but it does not explain why individual photons come off in particular directions "when no one is looking" Most people tend to think that when you say measure you mean that some person has to observe it. It is I think important to say that the term measure does not require any human input in any way. as yo say because action and reaction are always equal and opposite the photon momentum ust be in a specific direction but the reaction of the emitting atom is most likely to be "measured" by its interaction with surrounding atoms
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As the title says. Tnx.
For an isolated atom which transitions to a lower energy state a photon is emitted. The direction is random. It can go in any direction with equal probability.
Since there is only one photon per transition there really is no spherical wave front of em radiation.
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A good explanation light arrow but it does not explain why individual photons come off in particular directions "when no one is looking" Most people tend to think that when you say measure you mean that some person has to observe it. It is I think important to say that the term measure does not require any human input in any way. as yo say because action and reaction are always equal and opposite the photon momentum ust be in a specific direction but the reaction of the emitting atom is most likely to be "measured" by its interaction with surrounding atoms
Certainly.
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The direction is random. It can go in any direction with equal probability.
Since there is only one photon per transition there really is no spherical wave front of em radiation.
But before measuring the photon's momentum, you treat the emitted EM field as a spherical wavefront, for example to compute its intensity in every point of a distant screen or to compute the probability of finding the photon after a diffraction grating. This is equivalent to your statement: "The direction is random. It can go in any direction with equal probability".
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Nicely put Pete.
What about a atom getting hit by a photon annihilating in the collision, emitting a new one. Will that one also have a unspecified direction until measured? (Btw, not directed to Pete but to you all)
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You might want to talk about it in terms of 'absorption' and 'dropping off' then, as it seems to me :)
Weird stuff.
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What about a atom getting hit by a photon annihilating in the collision, emitting a new one. Will that one also have a unspecified direction until measured?
Yes, where is the difference with the example I made? There isn't.
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Don't get me wrong Lightarrow. I'm actually trying to see it, how would you define a refractive index from that notion?
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Don't get me wrong Lightarrow. I'm actually trying to see it, how would you define a refractive index from that notion?
Refractive index is a quantity that gives an average value for the result of many photons interacting with many atoms. You can start from the above ideas and make a lot of simplifying assumptions to compute average values and get to refractive index. Trying to get there by literally computing every atom-photon interaction would be impossible due to the number of calculations required.
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What I was thinking of JP was in terms of having a number of atoms interacting with the 'force/energy/momentum' represented by the light, forcing a directionality upon the new 'photon'. But I can't see how it should be done, although that would be the most plausible explanation for me. But then you have one 'photon' interacting with one atom too?
As if the amount created a directionality but when having a one to one representation, it disappear.
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What do you mean by a single photon? The usual Fock state definition is a particle with a precisely defined energy and momentum. Therefore that photon has a perfectly defined direction, but is spread out over all space somehow.
What Lightarrow's referring to is a spherical wave, which could be formed by a quantum state of photons traveling in all directions at once. If there is only a single photon in that state, then you can write it as a quantum sum of individual Fock states over all possible directions. If you were to measure it and find the photon, then you'd know after the fact which direction it took. But like the particles in the two slit experiment, until you measure it, it's apparently moving in all directions at once.
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Synchrotron radiation is a different source of light, which has been used as a tunable X-Ray source for research into materials science, chemistry, biology and even paleontology. Synchrotron radiation is emitted at a tangent to the electron storage ring.
Since the electrons are traveling at almost the speed of light, the photons are emitted within a very narrow angle around the direction that the electrons are traveling, producing a very directional beam from a source with a very small diameter.
http://en.wikipedia.org/wiki/Synchrotron_light_source#Beamlines
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Actually, according to the Neil's Bohr interpretation of quantum theory, which is the prevailing interpretation, the photon has no direction and does not propagate through space. At "birth" there is only a quantum probability field which is only a mathematical abstraction without what we would think of as a physical presence. The means of restricting the possible direction, which where mentioned in the previous post, are only parameters that restrict the probabilities.
When there is a point of possible interaction, such as the presence of an electron at one of the highest probable positions for photon occurrence, then the photon actualizes. It is the actual testing for a photon that can determine which of several paths the photon would be associated with even though it never propagated along that path. If the photon continues it is again reduced to a new quantum probability field with different possibilities. This is the basis for the uncertainty principle.
If this concept troubles you, you are not alone as many Nobel prize holders in physics, starting with Albert Einstein up through present time with Frank Wilczek, have not accepted this concept. Never the less the concepts of non-locality and detection induced quantum field collapse are generally regarded as the most accepted mainstream interpretation.
I have an hour long lecture and demonstration on this subject. If its not breaking any rules I will try and post a link in the new theories section, although no new theories are introduced.
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I have an hour long lecture and demonstration on this subject. If its not breaking any rules I will try and post a link in the new theories section, although no new theories are introduced.
I don't know if it breaks the forum rules, in case, would you send me a message with the link?
Thank you.
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lightarrow
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Can you see how it is done JP?
I would love too.
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Can you see how it is done JP?
How what is done?
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This.
"The usual Fock state definition is a particle with a precisely defined energy and momentum. Therefore that photon has a perfectly defined direction, but is spread out over all space somehow."
How do I make that intuitive, without using a wave picture?
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You don't. If this were intuitive, it wouldn't be quantum mechanics. :p
You can make a pretty intuitive analogy to classical waves though. In classical waves, you have something called a plane wave that is a sinusoid over all space and time, but which is traveling in one particular direction. You can see a nice picture of such a wave here:
http://en.wikipedia.org/wiki/Plane_wave#Arbitrary_direction
A plane wave of light is defined in terms of two physical properties: its direction and its wavelength.
Quantum mechanically, a Fock state is defined as the smallest packet of energy that makes up that plane wave. Since that plane wave has a well-defined direction, so does the small packet making it up. Since the plane wave has a well-defined wavelength, so does the photon (which is why it has a precise energy). The difference, of course, is that a classical wave isn't destroyed by a measurement (you could stick multiple probes in the plane wave to detect it at different points). With a Fock state, if you detect the photon at a point, you collapse the state so it can't be measured elsewhere.
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I missed one point above. An electromagnetic plane wave also has polarization, which in a Fock state translates to photon spin.
Another important point is that a Fock state isn't the only single-photon state, but it is the simplest I've seen since it only has three quantum numbers (direction, wavelength/energy and polarization). You can always have a photon that's in a superposition of Fock states (another quantum oddity). So you might have a 50% change of detecting a particle with one direction and a 50% chance of another. In lightarrow's example of photons emitted in all directions, you have a single photon that's in a superposition of Fock states traveling in all directions at once.
While I appreciate that diving right into "what is a photon" is intriguing, I unfortunately haven't found a good way of explaining it that doesn't involve an understanding of both classical plane waves and quantum superpositions. On the other hand, if you build a solid understanding of those two ideas, a photon isn't that hard to grasp.
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Excellent Description JP!
However, it may leave some with the impression that you are describing a real wave like entity. I am including some quotes from http://www.plato.stamford.edu/entries/qm-copenhagen/#5 .
" Bohr spoke of the mathematical formalism of quantum mechanics, including the state vector or the wave function, as a symbolic representation.....The reason is, according to Bohr, that a quantum system has no definite sinematic or dynamic state prior to any interpretation. "
Anyone looking for an intuitive explanation is not going to find it. Our intuitive concepts of reality are based on the way our brains evolved to react to our senses' interpretation of our surroundings. For example our concept of hard physical presence comes from the inability of the very vacuous fields that we are made of to penetrate those of other objects.
I think theoretical physics has moved away from ping pong ball particles and waves in honey to a reality that is only a mathematical abstraction, or more correctly is unknown and can only be described by mathematical abstractions. Some of you are aware that I am critical of the Bohr interpretation of light. However, I am not critical of his basic concept of reality. I only have a different opinion as to when and where the photon actualizes.
There are of course a lot of hybrid interpretations that try to combine intuitive with the non-intuitive which may be beneficial to some.
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I don't believe in none-locality. I never will. It's turning mathematics into reality, but mathematics isn't reality. The problem of none-locality isn't in the Universe it's in the mathematics. Don't solve the Universe in this case, find the problem in the maths. An experiment that shows none-locality exhibits missing physics. Join the dots together with... something. I'm not just saying that with a human origin in my head, I'm saying that with a theory that I am not posting.
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Glad to hear it.
I also do not accept non-locality and quantum filed collapse. Some time ago when I posted experiments that challenged non-locality they were sent to the new theory dungeon. The video I just listed in new theory contains some of these that anybody can perform.
I think the dots are connected but not by anything that is in our conceptual experience. My thought is that the same phenomena is responsible for the illusions that we perceive as matter, energy, space, time, etc.. Presently math is the only language with which we describe this process.
I feel obligated to correct people when they think that Bohr was describing real waves. Otherwise they will not understand why some very prominent physicist refer to it as voodoo physics.
I am getting off the original topic. If anyone is willing to accept the orthodox, or Bohr, view of light they cannot have photons with directional propagation through space.
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To me it goes back to a duality. And I think it is a 'real' duality. I do not expect it to be waves giving us excitations, although you gave me a very competent description. Neither do I expect it to be ping pong balls, although I still would like that concept better :) And I agree JP, it's not intuitive. But it should be, if we find the right description.
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My point, if I now have one :) is that if one accept a duality then it's high time to find out from what that would come. If one stops at one of the descriptions, then?
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One reason why Lightarrows description was interesting to me is just that, it's a duality. But a photon do not have a momentum in his description, although you can refer to it as a idealized fock state in where a presumption is just that directionality.
But if you allow a 'photon' to exist, as it comes to be measured, then you don't need directions. Instead you need relations, and that's also why I wondered about if the amount might be what gives a direction. The other case is our 'ping pong ball' bouncing, which can't be correct if the direction is unspecified experimentally.
So, is it?
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I would go with the Spin Spin...
http://www.sciencemag.org/content/339/6122/928.abstract (http://www.sciencemag.org/content/339/6122/928.abstract)
Using spacetime as a spin locator, and the photon with it own spin sitting in a spin locator, so that's just like pointing an invisible gun of Dark Matter. Spin Spin. It would use the area of least resistance per C cycle. Sort of like Dandelion seeds. I always go with the simplest idea, because the Universe is always supposed to go with the simplest idea.
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Nicely put Pete.
What about a atom getting hit by a photon annihilating in the collision, emitting a new one. Will that one also have a unspecified direction until measured? (Btw, not directed to Pete but to you all)
No. The photon must be in the same direction and have the same energy, and hence the same momentum, in order to conserve momentum. That's how laser works.
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No. The photon must be in the same direction and have the same energy, and hence the same momentum, in order to conserve momentum.
Don't understand why: after having absorbed the photon, the atom recoils, taking the photon's momentum and this conserves it. Then the atom can emit a subsequent photon in any other direction or in all of them simultaneously (as a spherical wave) and as a consequence it will recoil again accordingly.
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Nicely put Pete.
What about a atom getting hit by a photon annihilating in the collision, emitting a new one. Will that one also have a unspecified direction until measured? (Btw, not directed to Pete but to you all)
No. The photon must be in the same direction and have the same energy, and hence the same momentum, in order to conserve momentum. That's how laser works.
No, Pmb, a laser works because of one particular type of emission of photons called stimulated emission. This requires that a background EM field is present when the atom emits a photon, and that background field causes the atom to emit photons identical to those making up the background field.
More commonly, atoms emit through spontaneous emission, which is random in direction, and is what Lightarrow described.
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No, Pmb, a laser works because of one particular type of emission of photons called stimulated emission. This requires that a background EM field is present when the atom emits a photon, and that background field causes the atom to emit photons identical to those making up the background field.
What you and Lightarrow spoke of is spontaneous emission where the incident photon does not match one of the excited states of the atom. I was referring to stimulated emission. The background EM field you speak of is in the form of photons. I just looked it up to be sure. From Modern Physics - Fourth Edition by Paul A. Tipler and Ralph A. Llewellyn, page 419
Called stimulated emission, its probability of stimulated emission per atom pr unit time (transition rate) can be written B21u(f), where B21 is called Einstein's coefficient of emission. In this process the electric field of an incident photon with energy hf equal to the energy difference E2 - E1 ... stimulates the atom or molecule in state 2 to emit a photon with energy E2 - E1 = hf which is propagated in the same drection and with the same phase as the incident photon. Such photons (or radiation) is called coherent.
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No. The photon must be in the same direction and have the same energy, and hence the same momentum, in order to conserve momentum.
Don't understand why: after having absorbed the photon, the atom recoils, taking the photon's momentum and this conserves it. Then the atom can emit a subsequent photon in any other direction or in all of them simultaneously (as a spherical wave) and as a consequence it will recoil again accordingly.
What I said was incomplete. See above.
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So, can you have a setting in where something gets its direction by the amount around it? Not referring to photons, just trying to see in what way such a thing could be possible? On the other tentacle, in my mind it seems to either have to with waves interacting, quenching and reinforcing, or with momentum(s), if we now assume that a wave have a momentum?
Because there is a momentum, as shown by Pete's idea about a laser?
And yes, I'm slightly bicycling.
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Both photons and waves have momentum. A monochromatic plane wave has a precise momentum and is made up of photons which each have precise momenta.
In other words, a classical plane wave and the photons making it up have fixed frequencies and are moving in a precise direction, giving them a precise momentum.
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Yes, JP the spontaneous emission of a photon follows the background EM direction. Agreed; but what if that EM direction is created in a star where the magnetization is backwards because the stars environment have positron shells instead of the normal negative electron shells?
So I think that a stars photons look identical BUT are exact opposites magnetically and electrically thus can annihilate each other. If we fire a earth formed photon into the sun it will annihilate the stars photon and only the momentum energy will remain.
Happy Easter
CliveS
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Yes, JP the spontaneous emission of a photon follows the background EM direction. Agreed; but what if that EM direction is created in a star where the magnetization is backwards because the stars environment have positron shells instead of the normal negative electron shells?
So I think that a stars photons look identical BUT are exact opposites magnetically and electrically thus can annihilate each other. If we fire a earth formed photon into the sun it will annihilate the stars photon and only the momentum energy will remain.
Happy Easter
CliveS
You need to picture what you are calling a shell. The sun has photons that hit us all of the time. I do actually feel that you have thought it through, and got confused by the electron mainly.
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Clive - please keep on topic and leave the speculations to the New Theories board
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But then we have this.
"it's interference between the different atoms that a photon is absorbed by then re-emitted that makes photons keep going the same direction in glass. It's not that an individual atom absorbs a photon and them re-emits it in the same direction; you can see this because only the interference between different atoms would lead to Snell's law of refraction. I believe Feynman has an excellent explanation of this in his book QED, which I highly recommend as an elementary presentation of quantum mechanics. – Peter Shor "
Which is exactly what I was thinking of.
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although, rethinking it :) It's a wave picture he presents here, calling it photons. But if it is possible to define it such from waves, then there should be some equivalent explanation treating it as 'particles', or maybe not? It is after all a 'duality'.
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stars [with] positron shells instead of the normal negative electron shells?
Recent experiments with antiHydrogen suggest that it produces photons with the same spectrum as normal hydrogen.
If the Sun were made of antiHydrogen = positron circling an antiproton, the solar wind would be made of antiHydrogen, and Earth's Aurora would be far more spectacular, as positrons and antiprotons annihilate in the upper atmosphere.
The same would occur for other stars whose antimatter solar wind was interacting with the matter-based galactic medium, bathing the galaxy in a glow of gamma rays - which is not seen.
It is a long-term mystery why the universe we live in seems to be made mostly of matter, but astronomers are convinced that antimatter is not common in our galaxy.
So I think that a stars photons look identical BUT are exact opposites magnetically and electrically thus can annihilate each other.
The photon is its own antiparticle, so photons produced by normal matter can annihilate each other if they have sufficient energy. They don't need to originate in atoms and anti-atoms.