Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 14/05/2019 18:36:21
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It might be useful to try to answer this question. Does a photon self propagate due to the interactions of the electric and magnetic fields? Taking into consideration the wavelike nature of the photon.
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Some sources say that the photon is both wave like and particle like, depending on the method of observation.
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Some sources say that the photon is both wave like and particle like, depending on the method of observation.
All reliable sources say this.
How does this relate to the topic?
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It might be useful to try to answer this question. Does a photon self propagate due to the interactions of the electric and magnetic fields? Taking into consideration the wavelike nature of the photon.
Some sources say that the photon is both wave like and particle like, depending on the method of observation.
All reliable sources say this.
How does this relate to the topic?
I was responding to jeffreyH’s comment, quoted above, about the propagation of a photon, i.e., in the form of the particle and the wave…
Is the photon particle itself “light”, or does the photon, which is made up of multiple quanta of energy contained in the photon particle, emit electromagnetic wave energy, which is “light”?
It looked like the thread was continuing into that discussion.
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I’ve split this off from the relativity thread as it’s now moving away from that topic.
Is the photon particle itself “light”,
If you fire a single photon at a photographic film then, as long as the film is sensitive to that frequency, it will register a spot. Same for any detector sensitive to that frequency including the eye. So, we assume the photon is light.
or does the photon, which is made up of multiple quanta of energy contained in the photon particle, emit electromagnetic wave energy, which is “light”?
The photon is a single quantum it does not contain multiple quanta.
It does not emit anything, it is an oscillating em field disturbance moving through space.
The photon displays wave properties eg diffraction/interference and particle properties eg transfer of momentum.
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If we fire a photon of a particular frequency at a sensor in the vicinity of the Sun (say,exactly 1000 light seconds-so about twice the distance) can we say that both the magnetic field and the electric field would "wobble" there in a way that is a function of the photon's associated wave at that point as the photon passed by?
And could the size of this "wobble" also be increased if several photons of the same wavelength were travelling together ? (ie as a laser,I think)
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To add to what Colin2B has already said. A photon, being a quantum of electromagnetic radiation, is the smallest divisible unit of radiation energy you can have for EMR of a given wavelength.
And, as pointed out, if a single photon strikes photographic paper, it produces a single localized spot. ( like a particle would).
However, if you send a series of single photons, one at a time, through a double slit interferometer, While each individual photon will produce a localized dot on the paper, The pattern formed after many photons have passed through will be an interference pattern like you would expect from a wave.
A classical particle would not create the interference pattern, while with a classical wave, you'd get the interference pattern, but it wouldn't be formed from individual dots.( the whole pattern would form at once and just get more distinct as more light passed through the slits.
While you will hear it said that exhibits both the properties of particles and waves, it is probably more accurate to say that the classical distinction between wave and particle just doesn't hold, and that this becomes more apparent at the quantum level.
This characteristic is not limited to light alone. Electrons( original considered classical particles) show the same behavior. You can pass them through a diffraction grating and produce an interference pattern composed of individual dots also.
It is something built into the universe at that scale and not something exclusive to this or that "particle".
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In what way would it self propagate Jeffrey? Don't you then need to add something else to it?
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Does a photon self propagate due to the interactions of the electric and magnetic fields? Taking into consideration the wavelike nature of the photon.
If you fire a single photon at a photographic film then, as long as the film is sensitive to that frequency, it will register a spot. Same for any detector sensitive to that frequency including the eye. So, we assume the photon is light.
This raises a few questions:
1. Does the photon propagate, or does the EM energy propagate only as a wave?
2. In the same way that sound waves are vibrations, that become sound only when interpreted by the appropriate auditory organs; could the same thing be true of light?
I.e. the radiation travels as “waves” of energy. Only when “observed” can it be considered as light. It then appears as a spot.
No “ears”, no sound, no “observer”, no light. (?)
3. What is the nature of the “interactions of the electric and magnetic fields”?
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Electromagnetic radiation propagates according to Maxwell's equations,which are based on the observation that a moving charge creates a magnetic field (see any electromagnet) and a varying magnetic field induces charge (see any dynamo).
The electromagnetic spectrum is continuous but some sources of em radiation emit discrete quanta with a specific frequency.
The "observer" is irrelevant since the photon, phonon or whatever has no concept of observation and the source, which may be a few billion light years away, really doesn't care what happens to the energy it radiates.
Sound is longitudinal pressure waves of any frequency in any medium. The effect of a sound wave has nothing to do with the auditory system. You can play your violin in front of an audience of bats, humans and neurologically deaf humans - the effect on their tympani and auditory ossicles is exactly the same, and the same on a microphone diaphragm that converts it to an electrical signal. The violin and the connecting air have no interest in the nature or presence of a receiver.
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Thanks Alan. There are a few points I will return to, but it may be a day or so; in which case, I wouldn't want to appear unappreciative.
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In what way would it self propagate Jeffrey? Don't you then need to add something else to it?
Well why is it moving at all in that case? The speed of the photon in vacuum could have been variable but it isn't. There is an upper limit. It has to be because of the properties of the electromagnetic field itself. So we are back to the interactions of electric and magnetic fields. Please refer to Alan's post above.
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The "observer" is irrelevant since the photon, phonon or whatever has no concept of observation….
As we are dealing with quantum scale objects (photons); this does seem a restricted interpretation of “observer”.
Do quantum particles have a “concept of observation” ?
Sound is longitudinal pressure waves of any frequency in any medium.
Looked at another way, we could say that “longitudinal pressure waves of any frequency in any medium” are defined as sound, because the most readily observable examples are converted to sound by auditory systems.
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1. Does the photon propagate, or does the EM energy propagate only as a wave?
Don’t understand the question. The photon is the smallest energy unit (quantum) of the wave. As @Janus said, it has wave properties.
2. In the same way that sound waves are vibrations, that become sound only when interpreted by the appropriate auditory organs; could the same thing be true of light?
I.e. the radiation travels as “waves” of energy. Only when “observed” can it be considered as light. It then appears as a spot.
No “ears”, no sound, no “observer”, no light. (?)
I’m with Alan on this one. Philosophical questions often come down to poor definition of terms or misunderstanding physics and this is a classic example.
As we are dealing with quantum scale objects (photons); this does seem a restricted interpretation of “observer”.
Do quantum particles have a “concept of observation” ?
No, of course not, how could it. In QM observation is synonymous with taking a measurement. How you take that measurement determines whether you see wave or particle behaviour.
Your question is similar to asking whether ocean waves have a concept of breaking on the shore, it makes no sense.
.....because the most readily observable examples are converted to sound by auditory systems.
Infrasound? Ultrasound?
Even bats can’t hear the ultrasound used in maternity scanners or therapeutic ultrasound.
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Does a photon self propagate due to the interactions of the electric and magnetic fields?
Yes.
a moving charge creates a magnetic field (see any electromagnet) and a varying magnetic field induces charge (see any dynamo).
Well why is it moving at all in that case?
To see why it is moving in a particular direction, you can look at the source:
When an excited electron drops to a lower energy level of an atom, it emits a photon of a characteristic energy/frequency/wavelength, but in a random direction.
- The quantum transition creates the conditions of a changing electric field (the electron changing orbital) which induces a changing magnetic field, which in turn induces a changing electric field.
- In the "near field" (close to the atom), the electric field of the electron remains bound to the atom.
- But part of those changing electric and magnetic fields detach from the atom, creating "far field" conditions. This energy propagates away from the atom "to infinity" (or until it runs into something).
- Conservation of energy ensures that the photon has the characteristic energy
- But quantum uncertainty in the position of the electron means that the direction must be very uncertain
- Once an electromagnetic wave starts propagating in a particular direction, the conditions of the electric and magnetic fields in front, behind and in the middle interact to ensure that it keeps moving in that direction.
- The speed that it moves depends on the permeability and permittivity of whatever it is propagating through (including a vacuum). This can be considered as the capacitance and inductance of the medium (including a vacuum).
- These parameters affect the extent to which a changing electric field produces a magnetic field, and the extent to which a changing magnetic field produces an electric field.
There are some exceptions where it changes direction:
- If the light hits a mirror, the current induced in the metal surface cancels the incoming electric field, and produces an outgoing photon which follows the laws of reflection.
- If you change into a different medium (eg air to glass), the permittivity of the medium changes (glass has a higher permittivity, and the speed of light is lower). This changes the direction of the light, according to the laws of refraction.
- If the "wavicle" strikes a dual slit, it interferes with itself, and it's final direction will be more likely in some directions than others, creating an interference pattern.
- If you change into a stronger gravitational field (eg near a black hole), time dilation causes the direction to bend.
Different sources of light produce different kinds of light with different characteristics:
- Movement of electrons driven at different rates by a crystal oscillator (eg in a WiFi router) produces light/radio waves of different frequencies.
- Random movements of atoms above absolute zero produce black body radiation, which is emitted in all directions with a characteristic spectrum
- In a laser, quantum effects produce light which is almost monochromatic, and interference produces light which has low divergence (diffraction-limited)
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As we are dealing with quantum scale objects (photons); this does seem a restricted interpretation of “observer”. Do quantum particles have a “concept of observation” ?
The only things in the universe that have a concept of observation are the humans who write excessively anthropic physics texts!
The fundamental principle of physics is that stuff happens consistently throughout the universe. This is an implicit belief of everyone except corrupt Health and Safety Executive inspectors and devotees of religion. Humans have not been around very long in cosmic terms, but the quantum phenomena we observe to have happened in other galaxies, millions of years before life evolved on Earth, are entirely consistent with those we observe to be happening here and now, so the observer has no role in it.
That does not preclude the role of an "observation" in modifying what is observed, and this is where you need to be very careful in the use of language. Heisenberg's thought experiments concern locating an electron in space and time. The only way to do this is to ping a photon off it, but in doing so you will transfer energy and momentum to the electron and thus alter the numbers you wanted to determine. From which he produced the principle of indeterminacy, which is probably the most important and most misinterpreted insight ever made!
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- If the "wavicle" strikes a dual slit, it interferes with itself, and it's final direction will be more likely in some directions than others, creating an interference pattern.
People should read this point over a few times. Then we might get less nonsense spouted about the double slit experiment. Thanks for posting this Evan.
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Don’t understand the question. The photon is the smallest energy unit (quantum) of the wave. As @Janus said, it has wave properties.
I’m not arguing with that. Sometimes one sees the assertion that light travels as a wave, but is “detected” as a photon. I was looking for opinions on that idea.
No “ears”, no sound, no “observer”, no light. (?)
I’m with Alan on this one. Philosophical questions often come down to poor definition of terms or misunderstanding physics and this is a classic example.
I’ve long suspected that semantics could be blamed for much misunderstanding. Perhaps this comes down to the point at which we define longitudinal pressure waves as sound, or EMR as light.
.....because the most readily observable examples are converted to sound by auditory systems.
Infrasound? Ultrasound?
Even bats can’t hear the ultrasound used in maternity scanners or therapeutic ultrasound.
All I meant was that because people were hearing sounds, long before anyone knew about sound waves, the popular definition of sound might be that which is audible to the human ear. This might be extended to those wavelengths that can be heard by various animals, etc. but is there a point beyond which these waves might not be defined as sound?
A similar question could be asked about light; especially in view of definitions such as “Light is a transverse, electromagnetic wave that can be seen by the typical human.” https://physics.info/light/
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No Jeffrey
Ask how light can 'propagate' instead.
Don't look at the equations
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The "observer" is irrelevant since the photon, phonon or whatever has no concept of observation and the source, which may be a few billion light years away, really doesn't care what happens to the energy it radiates.
Could greater clarity be achieved if “observation” were used instead of “observer” when talking about quantum reactions?
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Even then, you need to remember that "observation" is introduced as an auxiliary and explanatory concept in physics, rather like "cross section" in engineering. Nobody is going to cut a ship in half to make it work, but drawing what it would look like if cut in half, is very helpful in explaining how it works.
The natural world does what it does, and we petty beings try to understand and model it as though we were observers to every action.
To go back a couple of paragraphs, the properties of longitudinal compression waves are pretty much independent of frequency, so they are all "sound" whether we, bats or transducers can sense them, but the interaction of electromagnetic radiation with matter is strongly dependent on its frequency so we assign specific names to parts of the spectrum, and the tiny bit that excites transient chemical responses in the eye is called light.
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Thanks Alan. I like the "cross section" analogy; it's worth keeping in mind when thinking about QM.
As far as terminology is concerned, I guess we are back to Humpty Dumpty!
That should work, as long as each of us is aware of how the other is using a term.
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Sometimes one sees the assertion that light travels as a wave, but is “detected” as a photon. I was looking for opinions on that idea.
Simple view:
At the source an atom cannot emit photons with any old energy. When an electron moves between higher to lower energy levels in the atom an em wave is emitted with the specific energy for that particular transition equal to the difference in energy between those levels. The energy of this wave is =hv and is called a photon. Clearly this is the wave that travels to the detector. This process has a symmetry, just as the movement of a source electron creates an oscillating em field (wave) then at the detector end the oscillating field can move an electron.
One of the key points in understanding photons is to understand the quantum nature of the detector. When an em wave over a certain energy hits a detector the photoelectric effect releases an electron which triggers a photomultiplier and a counter goes click. If the wave does not have that energy then the counter doesn't go click.
When Einstein wrote his paper on the photoelectric effect they didn’t know about the quantum nature of the electrons in the photoelectric metal so the view was that the photons were particles that transferred their momentum to the electron kicking it free, just like billiard ball interactions, and this led Einstein to conclude that light is a stream of particles, photons.
However, we now understand much more about the quantum nature of the detector where each electron is in a quantum potential well - defined by the binding energy. If the incoming energy is large enough to expel the electron from the well it leaves, if not it dissipates its energy in collisions with nearby electrons and atoms and remains trapped in the potential well, ie not emitted. So what you are measuring is not the quantum nature of light, but the quantum nature of the detector. Crucially, in QED you don’t have to assume that the incoming light is quantised for the photoelectric effect - the quantisation can be derived separately. Even Einstein stated in his paper on photoelectric effect that his working was heuristic.
So in QED and QFT the energy is transferred by a disturbance of the em field (as described by Maxwell’s equations) but detected as a photon. I think it was Pete who said it isn't a photon until it's detected.
A wave by any other name ........ ;)
By the way you may have missed this:
You're confusing the term observer with person. The observer could be a system of clocks and rods and detectors at the location at the time of the event. It may be a person but that's not necessary.