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Author Topic: How The Photon Direction Is Determined When Its Born?  (Read 10014 times)

Offline waytogo

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As the title says. Tnx.


 

Offline evan_au

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #1 on: 22/03/2013 08:44:37 »
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.
 

Offline waytogo

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #2 on: 22/03/2013 09:44:49 »
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?
 

Offline evan_au

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #3 on: 22/03/2013 20:11:37 »
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
 

Offline lightarrow

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #4 on: 23/03/2013 19:35:08 »
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!
« Last Edit: 23/03/2013 19:43:19 by lightarrow »
 

Offline Soul Surfer

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #5 on: 23/03/2013 23:17:09 »
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
 

Offline Pmb

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #6 on: 24/03/2013 00:34:14 »
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.
 

Offline lightarrow

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #7 on: 24/03/2013 11:25:36 »
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.
 

Offline lightarrow

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #8 on: 24/03/2013 11:28:33 »
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".
 

Offline yor_on

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #9 on: 24/03/2013 13:02:51 »
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)
=

You might want to talk about it in terms of 'absorption' and 'dropping off' then, as it seems to me :)
Weird stuff.
« Last Edit: 24/03/2013 13:08:16 by yor_on »
 

Offline lightarrow

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #10 on: 24/03/2013 17:15:21 »
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.
 

Offline yor_on

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #11 on: 24/03/2013 19:09:19 »
Don't get me wrong Lightarrow. I'm actually trying to see it, how would you define a refractive index from that notion?
 

Offline JP

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #12 on: 25/03/2013 13:25:55 »
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.
 

Offline yor_on

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #13 on: 25/03/2013 14:39:33 »
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.
 

Offline JP

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #14 on: 25/03/2013 15:16:37 »
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.
 

Offline evan_au

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #15 on: 25/03/2013 16:40:56 »
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
 

Offline sciconoclast

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #16 on: 25/03/2013 17:24:00 »
       
       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.
 

Offline lightarrow

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #17 on: 25/03/2013 18:06:22 »
       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.

--
lightarrow
 

Offline yor_on

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #18 on: 25/03/2013 22:19:50 »
Can you see how it is done JP?

I would love too.
 

Offline JP

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #19 on: 26/03/2013 00:54:36 »
Can you see how it is done JP?

How what is done?
 

Offline yor_on

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #20 on: 26/03/2013 01:09:55 »
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?
 

Offline JP

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #21 on: 26/03/2013 01:31:29 »
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.
 

Offline JP

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #22 on: 26/03/2013 13:29:50 »
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.

 

Offline sciconoclast

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #23 on: 26/03/2013 16:15:31 »
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.

 

Offline Pincho

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Re: How The Photon Direction Is Determined When Its Born?
« Reply #24 on: 26/03/2013 18:13:28 »
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.
« Last Edit: 26/03/2013 18:15:26 by Pincho »
 

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Re: How The Photon Direction Is Determined When Its Born?
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