Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jsaldea12 on 30/03/2019 09:54:22
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Light is found out to be the same: forward, back, sideward. Michelson-morley with use of interferometer prove it. Thus .ight was found independent from source. Is that correct? jsa 3.30.19.
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If you mean the speed of light, yes.
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Wherever the source of light, say from super-flying saucer,, is moving, 30% the speed of light,, moving forward, back, side ward, light is the same 186,000 miles per second.. Why? jsa 3.30.19
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Because the speed of light depends on the properties of the material it is traveling through.
In a vacuum it's c (in water it's about 3/4 c).
We know the relevant properties of the vacuum
https://en.wikipedia.org/wiki/Vacuum_permittivity
and
https://en.wikipedia.org/wiki/Vacuum_permeability
and we can calculate the speed of light
https://en.wikipedia.org/wiki/Electromagnetic_wave_equation
None of that depends on anything to do with the source of the light.
So the speed of light does not depend on the source.
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Because the speed of light depends on the properties of the material it is traveling through.
In a vacuum it's c (in water it's about 3/4 c).
We know the relevant properties of the vacuum
https://en.wikipedia.org/wiki/Vacuum_permittivity
and
https://en.wikipedia.org/wiki/Vacuum_permeability
and we can calculate the speed of light
https://en.wikipedia.org/wiki/Electromagnetic_wave_equation
None of that depends on anything to do with the source of the light.
So the speed of light does not depend on the source.
Could one not say the same about the speed of sound in a medium..?
.and yet in that case the speed of the sound wave does depend on the state of relative motion of the source....
Unless I am ,as usual mistaken..
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Unless I am ,as usual mistaken..
Good guess.
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Wherever the source of light, say from super-flying saucer,, is moving, 30% the speed of light,, moving forward, back, side ward, light is the same 186,000 miles per second.. Why? jsa 3.30.19
Because light in a vacuum travels at c, and in our universe c is a finite invariant speed. ( space and time are intertwined and the value c plays an important role in their relationship.) Its not just light, but anything that travels at c (gravitational waves, for example) also shares this trait.
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Wherever the source of light, say from super-flying saucer,, is moving, 30% the speed of light,, moving forward, back, side ward, light is the same 186,000 miles per second.. Why? jsa 3.30.19
Because light in a vacuum travels at c, and in our universe c is a finite invariant speed. ( space and time are intertwined and the value c plays an important role in their relationship.) Its not just light, but anything that travels at c (gravitational waves, for example) also shares this trait.
Would you agree with the idea that absolutely everything travels at c even when not moving wrt another frame of reference?
I have heard it often stated that ,by sitting in my seat I am thus traveling at c.
Do we somehow need to extricate ourselves from the idea of relative motion having a "more substantial" meaning to see this more clearly perhaps?
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Light independent from the source since it's massless there's no usual interaction between the two.
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Because the speed of light depends on the properties of the material it is traveling through.
Could one not say the same about the speed of sound in a medium..?
.and yet in that case the speed of the sound wave does depend on the state of relative motion of the source....
Unless I am ,as usual mistaken..
As @Bored chemist says you are mistaken, but it is a common mistake.
A sound wave propagates at speed of sound in air independently of the speed of the source. What does vary is the speed of sound with the motion of the observer. If the observer is moving through the air they will measure a different speed to an observer who is stationary. The same is true if the air is moving, for example an observer in a balloon travelling with the wind vs an observer on the ground with the air moving relative to them.
This doesn’t happen with light where the speed measured by the observer is independent of the observer’s motion.
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Because the speed of light depends on the properties of the material it is traveling through.
Could one not say the same about the speed of sound in a medium..?
.and yet in that case the speed of the sound wave does depend on the state of relative motion of the source....
Unless I am ,as usual mistaken..
As @Bored chemist says you are mistaken, but it is a common mistake.
A sound wave propagates at speed of sound in air independently of the speed of the source. What does vary is the speed of sound with the motion of the observer. If the observer is moving through the air they will measure a different speed to an observer who is stationary. The same is true if the air is moving, for example an observer in a balloon travelling with the wind vs an observer on the ground with the air moving relative to them.
This doesn’t happen with light where the speed measured by the observer is independent of the observer’s motion.
The frequency does change though ,doesn't it? (the Doppler effect)
Is it possible (or useful) to say that the rate of propagation of em energy does vary with the relative motion of the observer?
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The frequency does change though ,doesn't it? (the Doppler effect)
Yes, but the speed of light doesn’t
Is it possible (or useful) to say that the rate of propagation of em energy does vary with the relative motion of the observer?
Not only useful but essential. All energy measurement depends on motion and/or position of the observer.
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Not only useful but essential. All energy measurement depends on motion and/or position of the observer.
So does that kind of override our (my) intuitive difficulty in accepting the experimentally verified fact that the speed of em propagation does not depend on the motion of the observer?
Can we treat this "energy propagation" as more fundamental ?
It seems to me that the two processes are linked...
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Is it possible to create a region of space where there is no em field?
Is it possible to create some kind of an enclosure where existing em fields do not enter?
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Yes - sort of. There is no electromagnetic field inside a mumetal box, apart from the black-body photons emitted by the box itself. In principle, if you cool it to 0 K, even that will cease.
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Yes - sort of. There is no electromagnetic field inside a mumetal box, apart from the black-body photons emitted by the box itself. In principle, if you cool it to 0 K, even that will cease.
Would such a space be of any particular interest ?
Would a beam of light propagate through it in exactly the same way as through any other space that had an em field?
Also,if we shine a beam of light in the vicinity of an em source (let's say the Sun) would that environment (in terms of em fields and photons) be any different from any region of space in the universe where em radiation has propagated since the earliest times?
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Yes. We use mumetal and even ordinary sheet steel to keep radiofrequency interference out of such things as MRI machines. We use brick walls to contain or reflect infrared radiation. We use curtains to keep visible photons in or out of places where we don't want them to enter or escape from. And we use chunks of lead to absorb x-rays.
There seems to be some confusion in this thread. A photon is an oscillating electromagnetic field, which as Maxwell pointed out, is why light can propagate through a vacuum.
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There seems to be some confusion in this thread. A photon is an oscillating electromagnetic field, which as Maxwell pointed out, is why light can propagate through a vacuum.
How was Maxwell able to show this?
Was it an inescapable logic or rather a Eureka moment?
Also is this description the same as the "particle is an excitation in a field" idea ? ( maybe that is a description /definition that just applies in Quantum theory ?)
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Yes - sort of. There is no electromagnetic field inside a mumetal box, apart from the black-body photons emitted by the box itself. In principle, if you cool it to 0 K, even that will cease.
What? no passing gammas?
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How was Maxwell able to show this?
We know from experiment that electrical and magnetic fields are related. Maxwell combined the known mathematical relationships to show that an electromagnetic wave can selfpropagate, and in the case of propagation in vacuo the speed of propagation can be predicted from simple static measurements. The derivations are well described in Wikipedia and far too involved to replicate here, but be assured they are based on entirely conventional calculus applied to very simple experimental results.
To be consistently pedantic, I should retract the phrase "a photon is...." and replace it with "a photon can be modelled as....", otherwise people will start spouting nonsense about duality and the collapse of wave functions.
And yes, BC gets the class pedantry prize for today - if you wait long enough, some cosmic photon will probably pass through your mumetal box, along with a zillion neutrinos, and you will never know about it.
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Not only useful but essential. All energy measurement depends on motion and/or position of the observer.
So does that kind of override our (my) intuitive difficulty in accepting the experimentally verified fact that the speed of em propagation does not depend on the motion of the observer?
It doesn’t override, it’s just not related to speed of propagation at all.
It seems to me that the two processes are linked...
No, it is just part of the way we measure relative energies.
Let’s say you are driving along and a car comes up behind and hits you, travelling at twice your speed. Compare the relative kinetic energy of impact to the case where the other car is travelling only slightly faster than you - nowhere near as much kinetic energy transferred nor damage done.
Kinetic energy is relative even at non relativistic speeds. Light is no different except that because it’s speed relative to us is always c and it’s rest mass is zero, we need to use its measured momentum which it was discovered is related to its frequency.
I notice you are trying to make sense of some of the more esoteric terminology bandied around, but reread and take to heart @alan comment about “a photon being modelled as ...” because it is easy to be led down rabbit holes.
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Not only useful but essential. All energy measurement depends on motion and/or position of the observer.
So does that kind of override our (my) intuitive difficulty in accepting the experimentally verified fact that the speed of em propagation does not depend on the motion of the observer?
It doesn’t override, it’s just not related to speed of propagation at all.
It seems to me that the two processes are linked...
No, it is just part of the way we measure relative energies.
Let’s say you are driving along and a car comes up behind and hits you, travelling at twice your speed. Compare the relative kinetic energy of impact to the case where the other car is travelling only slightly faster than you - nowhere near as much kinetic energy transferred nor damage done.
Kinetic energy is relative even at non relativistic speeds. Light is no different except that because it’s speed relative to us is always c and it’s rest mass is zero, we need to use its measured momentum which it was discovered is related to its frequency.
I notice you are trying to make sense of some of the more esoteric terminology bandied around, but reread and take to heart @alan comment about “a photon being modelled as ...” because it is easy to be led down rabbit holes.
Well,I am aware of the warning(prohibition?) against taking "energy" to be a "thing" rather than a property of a system and am now reminded of the importance of keeping in mind that all we have is models.
Can I ask ,then if we can model the propagation of light in terms of the propagation of energy through a system?
Say the Sun explodes,could the energy distribution** in the Solar system be modeled as propagating from one region to other regions ?
The speed of this propagation would be limited at c in a vacuum ,wouldn't it?
** does that term ""energy distribution" have a meaning or is it word salad (googling it ,all I find is Energy Distribution Companies and perhaps a few uses in physics that I don't think I had in mind)
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Yes - sort of. There is no electromagnetic field inside a mumetal box, apart from the black-body photons emitted by the box itself. In principle, if you cool it to 0 K, even that will cease.
I do not think this is strictly correct, zero point energy exists all through out space and is electromagnetic in nature. It prevents absolute zero from being reached. There is no such thing as empty space.
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There seems to be some confusion in this thread. A photon is an oscillating electromagnetic field, which as Maxwell pointed out, is why light can propagate through a vacuum.
Again this may not be strictly correct viewed from QED stand point which regards a photon as a particle with a field, or a localized field that behaves like a particle.
ie Photons are known to be able to travel billions of light years and can then still be detected by relatively small photon detectors. Waves are not capable of performing that trick. Wave packages disperse when they move. Single waves spread over space and their amplitude diminishes with increasing distance from the source.
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There seems to be some confusion in this thread. A photon is an oscillating electromagnetic field, which as Maxwell pointed out, is why light can propagate through a vacuum.
Again this may not be strictly correct viewed from QED stand point which regards a photon as a particle with a field, or a localized field that behaves like a particle.
Are you saying that the field doesn’t oscillate? Or that it isn’t electromagnetic?
If you agree with both of those I don’t see why you consider @alancalverd statement to be incorrect?
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There seems to be some confusion in this thread. A photon is an oscillating electromagnetic field, which as Maxwell pointed out, is why light can propagate through a vacuum.
Again this may not be strictly correct viewed from QED stand point which regards a photon as a particle with a field, or a localized field that behaves like a particle.
Are you saying that the field doesn’t oscillate? Or that it isn’t electromagnetic?
If you agree with both of those I don’t see why you consider @alancalverd statement to be incorrect?
I did not say incorrect, I said not strictly correct :) .
An oscillation is something that repeats, a fluctuation in a field need only increase decrease and disappear. I do not think that can be regarded as an oscillation. Clearly a photon has some properties that can be defined by electromagnetic theory, which is not compatible with both Quantum mechanics and relativity. QED is compatible with both, and explains the photon as a field, which is more akin to a particle. A electromagnetic wave spreads and disperses as it passes through space. A photon field does not.
A short discussion is given here on this link whereby the contradictions I point out above are loosely discussed. https://www.researchgate.net/post/Is_Einsteins_photon_really_the_same_as_the_QED_photon
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An oscillation is something that repeats
I would suggest that if something has a frequency and wavelength we would say it is oscillating.
a fluctuation in a field need only increase decrease and disappear. I do not think that can be regarded as an oscillation.
That sounds more like a virtual photon.
However, it is not unusual to refer to a single cycle as a single oscillation and a regular fluctuation is an oscillation.
Imagine a single wave passing over a flat ocean. It appears to be a single, moving fluctuation, in reality it is oscillating up and down in a very regular pattern about a moving point (because it is a travelling wave). Think also pulse travelling down a rope.
A electromagnetic wave spreads and disperses as it passes through space. A photon field does not.
Muddled thinking in that link.
This is only true for point source or spherical waves. A plane monochromatic wave will propagate forever through space with no loss. Take a perfectly parallel beam of light, it will not disperse as it passes through space.
A short discussion is given here on this link whereby the contradictions I point out above are loosely discussed. https://www.researchgate.net/post/Is_Einsteins_photon_really_the_same_as_the_QED_photon
A collection of posts where some haven’t really thought before putting fingers to keyboard and don’t even know the history of photon thinking.
Also you need to remember that intensity is not a single photon property, it is a mass photon property.
It’s worth going back to the origins of Planck’s light quanta, Bohr’s model of energy levels, and Einstein’s photoelectric effect and then thinking what you mean by a photon being either a wave or a particle and why @PmbPhy says “It’s not possible to state whether a photon is a particle or a wave unless you state how its observed.“
Also, what do you mean by a particle? Do you mean a piece of matter surrounded by a field? Or do you mean a wave variation of the em field that in some circumstances has the properties of a particle?
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Think also pulse travelling down a rope.
There is a kind of single wave that propagates without changing shape, a soliton.
See: https://en.wikipedia.org/wiki/Soliton
Take a perfectly parallel beam of light, it will not disperse as it passes through space.
Unfortunately, there is no such thing as a perfectly parallel beam of light.
Lasers can be focused to a very parallel beam, but the degree of focus is limited by diffraction and the size of the source.
For sufficiently large distances, it starts to follow an inverse square law.
See: https://en.wikipedia.org/wiki/Diffraction-limited_system
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Quote from: flummoxed on Yesterday at 14:38:40a fluctuation in a field need only increase decrease and disappear. I do not think that can be regarded as an oscillation. That sounds more like a virtual photon. However, it is not unusual to refer to a single cycle as a single oscillation and a regular fluctuation is an oscillation.Imagine a single wave passing over a flat ocean. It appears to be a single, moving fluctuation, in reality it is oscillating up and down in a very regular pattern about a moving point (because it is a travelling wave). Think also pulse travelling down a rope.
Er damn you are right (how can I mean what I say when I don't say what I mean) The description is more like a virtual particle, which is not what I meant.
I should have wrote the photon field increases as it approaches and decreases as it passes a region in space and then that same photon is not repeated in the same region in space ever again because it is travelling at c :) which is not an oscillation unless it hits a mirror :( an is repeated.
Quote from: flummoxed on Yesterday at 14:38:40A electromagnetic wave spreads and disperses as it passes through space. A photon field does not. Muddled thinking in that link.This is only true for point source or spherical waves. A plane monochromatic wave will propagate forever through space with no loss. Take a perfectly parallel beam of light, it will not disperse as it passes through space.
It is true for a single photon, as opposed to an electromagnetic wave like a radio wave. and yes once you start reading all the other posts there is some muddled thinking. I do have better links but the 2nd post in that link possibly nails the differences, between a single photon field and an electromagnetic field. Also you need to remember that intensity is not a single photon property, it is a mass photon property.It’s worth going back to the origins of Planck’s light quanta, Bohr’s model of energy levels, and Einstein’s photoelectric effect and then thinking what you mean by a photon being either a wave or a particle and why @PmbPhy says “It’s not possible to state whether a photon is a particle or a wave unless you state how its observed.“Also, what do you mean by a particle? Do you mean a piece of matter surrounded by a field? Or do you mean a wave variation of the em field that in some circumstances has the properties of a particle?
The photon field has no mass it only has inertia. E=pc
By the Bohr model I assume you are referring to electrons (fields/clouds/particles) changing energy levels around a nucleus and giving of photons or absorbing photons of set energy levels which can be argued are fields/particles which interact with things in their path, giving the appearance of a wave particle like property.
Does QED get around the problems of how the photon is observed via interacting with things in its path ? Am I correct in thinking QED describes a photon better than wave particle duality? Is wave particle duality and QED like newtons theories versus Einsteins ? :)
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There is a kind of single wave that propagates without changing shape, a soliton.
I was thinking of the soliton when I described the water wave, didn’t want to mention it because it relies on dispersive effects which a photon doesn’t. But I think the travelling wave pulse is a good picture.
Unfortunately, there is no such thing as a perfectly parallel beam of light.
I was keeping quite about that :). After all we are talking theories/visualisations.
However, in qft the photon often looks like a plane wave, the perfect light beam?
Lasers can be focused to a very parallel beam, but the degree of focus is limited by diffraction and the size of the source.
For sufficiently large distances, it starts to follow an inverse square law.
Problem with laser following inverse sq law is that it is usual to invent a false origin to make the numbers work. However, they are always dispersive (at the moment).
I should have wrote the photon field increases as it approaches and decreases as it passes a region in space and then that same photon is not repeated in the same region in space ever again because it is travelling at c :) which is not an oscillation unless it hits a mirror :( an is repeated.
Yes the disturbance of the photon field does increase as it passes by and then decreases, but the field view also considers the that field is the em field and it is oscillating.
the 2nd post in that link possibly nails the differences, between a single photon field and an electromagnetic field.
The 2nd post says “The EM field relies on the nearby existence of electric charges.”. This is not the view of field theory, which considers that the field is always there even though it may have a value of zero. Also, it views the em field and photon field to be the same. Quote Matt Strassler “.....if two electrons pass near each other they will, because of their electric charge, disturb the electromagnetic field, sometimes called the photon field because its ripples are photons. “ - for ripples read oscillations.
The link below gives more detail on the derivation of the photon field and you can see it is all based on em field and oscillations (frequencies) of it. Note the comment “We need to quantize the EM field into photons satisfying Plank's original hypothesis” in other words a photon is a quantisation of the em field. https://quantummechanics.ucsd.edu/ph130a/130_notes/node418.html
You might also want to read the section “Remember that one exponential corresponds to the emission of a photon and the other corresponds to the the absorption of a photon. We view Α as an operator which either creates or absorbs a photon, raising or lowering the harmonic oscillator in the vacuum.” Which I was going to raise with you in a different thread.
The photon field has no mass it only has inertia.
Sorry, I didn’t mean that sort of mass. Intensity is an en-mass property of photons, as is inverse sq law. Imagine a point source light bulb, billions of photons are leaving it in all directions, intensity is proportional to the number of photons/s passing a unit area. Obviously the number of photons decreases as the radius increases following the inverse sq law. Similarly the mass of photons carry the wave properties that we see in the classical wave - frequency, polarisation, etc.
Does QED get around the problems of how the photon is observed via interacting with things in its path ?
Wave theory for photons en-mass, QED for individual photons and atoms.
Am I correct in thinking QED describes a photon better than wave particle duality?
As the previous comment, you wouldn’t want to do an optics problem using QED!
Is wave particle duality and QED like newtons theories versus Einsteins ? :)
There are similarities. Newton is a good approximation at low speeds - you wouldn’t want to work out a billiard ball trajectory/collision using relativity, but you would for particle accelerator collisions.
However, to me QED vs wave is more like atoms and lattice structure vs mechanical properties of materials eg young’s modulus. It’s detail vs gross (en-mass) properties - sometimes the detail gives an understanding of what’s going on, but other times I just want to get on and compare the gross strength of 2 materials!
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Sorry, I didn’t mean that sort of mass. Intensity is an en-mass property of photons, as is inverse sq law. Imagine a point source light bulb, billions of photons are leaving it in all directions, intensity is proportional to the number of photons/s passing a unit area. Obviously the number of photons decreases as the radius increases following the inverse sq law. Similarly the mass of photons carry the wave properties that we see in the classical wave - frequency, polarisation, etc.
the en-mass through me of :) I even had to google it in case it was some new unit I had never heard of. en-masse I think might be the correct spelling.
There are similarities. Newton is a good approximation at low speeds - you wouldn’t want to work out a billiard ball trajectory/collision using relativity, but you would for particle accelerator collisions.However, to me QED vs wave is more like atoms and lattice structure vs mechanical properties of materials eg young’s modulus. It’s detail vs gross (en-mass) properties - sometimes the detail gives an understanding of what’s going on, but other times I just want to get on and compare the gross strength of 2 materials!
Understood
The link below gives more detail on the derivation of the photon field and you can see it is all based on em field and oscillations (frequencies) of it. Note the comment “We need to quantize the EM field into photons satisfying Plank's original hypothesis” in other words a photon is a quantisation of the em field. https://quantummechanics.ucsd.edu/ph130a/130_notes/node418.htmlYou might also want to read the section “Remember that one exponential corresponds to the emission of a photon and the other corresponds to the the absorption of a photon. We view Α as an operator which either creates or absorbs a photon, raising or lowering the harmonic oscillator in the vacuum.” Which I was going to raise with you in a different thread.
Yes photons and EM fields travel at c. BUT Does an EM field which disperses really represent a photon field which does not. ie It is a mathematical model, which might not truly represent reality. Are there other/better quantum interpretations?
I was thinking of the soliton when I described the water wave, didn’t want to mention it because it relies on dispersive effects which a photon doesn’t. But I think the travelling wave pulse is a good picture.
I am thinking pilot waves and De-Broglie Bohm Bohemian mechanics give a better picture :)
Just how many interpretations of quantum mechanics are there?
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Does an EM field which disperses really represent a photon field which does not.
I think you must have missed a bit in my reply.
The em field does not disperse, it is the same as the photon field. Photons are quantisations of the em field.
It’s the photons which (en-mass) disperse to give inverse sq law effect.