[litespeed]

Geezer,

I always assumed light travels through space/time almost as if it were the Either Michaelson and Morely were looking for. The only thing is M&M did not understand that matter traveling through space/time interact with it.

And it is true light travels at different speeds when you put certain transparent mass in front of it. At least I think that is true. If so, matter can interact with light without absorbing it. It sort of makes sense to me since mass interacts with space/time by bending it.

[yor_on]

Geezer I find it questionable too
Not your statement, but what a vacuum really is. My thoughts on it is that if distance and motion is something defined to SpaceTime under our arrow of time (-past--'now'-> future-->)and elastic, then at and possibly 'under' Planck size and without any 'arrow of time' there should be other definitions and what we call distance here might not exist at all, as it craves a times arrow to exist. So I sort of agree with that you said

[LeeE]

The issue with waves in a vacuum is that classical waves are not a fluctuation within the medium, but a fluctuation of the medium itself, so if there's no medium there's nothing to be 'waved'; you can't wave a flag if there's no flag to be shaken about.

If space-time is a medium, then it needs to consist of something.  That though, is rather like saying the a distance or a length of one foot/metre/whatever needs to be made of something and is not just the distance between two points; it's like the difference between the distances marked off along a rule or tape measure, and the material that makes up the matter of the rule or tape measure itself.

As it is though, it seems to be that the distance between two points exists regardless of whether the rule or tape measure is there to measure it (which is not to say that the distance remains the same regardless of whether the rule or tape measure is there and when it's not, and depending upon where you're watching it all from).

[Geezer (OP)]

The more closely we examine matter, the less tangible it appears. I seems to me that all particles are simply different manifestations of energy in "space". Of course, this requires that "space" actually is something. But there are indications that it actually is - gravity seems to suggest that it is, as do the wave characteristics of photons and all particles.

The fact that we don't observe space as "something" is hardly surprising if everything is actually made from space itself.

If you assume for a moment that this model is valid, many things do seem to fall into place. Of course the big question then becomes "what the heck is space?"

[Soul Surfer]

Light arrow the "size" of a photon is approximately the wavelength multiplied by the reciprocal of the fractional  bandwidth of the frequency of the photon over which the observation is made.  Say for example I was observing radio signals at 100Mhz,  this is around the frequency of FM radio,  the wavelength is around 3 metres and, if I observed the signal with a receiver with a bandwidth of 10MHz, that is one tenth of the frequency.  The receiver therefore needs about ten waves to respond.  So the "size" of the photons being observed is about ten wavelengths, that is, around thirty metres.

I have illustrated this using radio waves because it was low frequencies that were being discussed in the origin of the question.

One question that has always intrigued me  is "Is  bandwidth a fundamental property of photons?"  That is is a photon originating from a broad band or rapid process at a particular frequency fundamentally different from one originating from a narrow band process.  for example broadband and narrowband processes occur at all frequencies radio light and gamma rays a broadband photon is emitted very quickly and has very few waves in its wave packet but a narrow band one may have many orders of magnitude more waves in its wave packet.  look up mossbauer effect where gamma ray sources and detectors can be "tuned" in to each other using mechanical motion and the gravitational red shift due to the earth's gravitational field measured in the laboratory.

[lightarrow]

Light arrow the "size" of a photon is approximately the wavelength multiplied by the reciprocal of the fractional  bandwidth of the frequency of the photon over which the observation is made.  Say for example I was observing radio signals at 100Mhz,  this is around the frequency of FM radio,  the wavelength is around 3 metres and, if I observed the signal with a receiver with a bandwidth of 10MHz, that is one tenth of the frequency.  The receiver therefore needs about ten waves to respond.  So the "size" of the photons being observed is about ten wavelengths, that is, around thirty metres.

I think that you are very optimistic in the possibility of making such kind of considerations, but maybe, who know, you could be right.
In this moment there is one thing that doesn't convince me: you say that the receiver would need n wavelenghts to respond, but such kind of "delay" has never been observed experimentally, for what I know: photons can be detected at every instant of time.

[litespeed]

All this stuff about wave lengths is fascinating, and brings up memories of tuning or 'trimming' antennae for enhanced reception for specific wave lengths. As I recall, you could have full wave length antennae, 1/2 wave, 1/4 wave etc.

In the context we are now discussing this is very weird. Specifically, either the antenna could absorb a single photon or it could not. If it required more then one photon to complete 'the wave length' how were they stored up?

I have a guess. My guess is that individual radio frequency photons can be absorbed entirely by antennae of various lengths, but the output of the antenna is largely a function of the RESONANT effect between the frequency of the photon and the natural frequency of the antenna.

Any thoughts on this.......

[lightarrow]

All this stuff about wave lengths is fascinating, and brings up memories of tuning or 'trimming' antennae for enhanced reception for specific wave lengths. As I recall, you could have full wave length antennae, 1/2 wave, 1/4 wave etc.

In the context we are now discussing this is very weird. Specifically, either the antenna could absorb a single photon or it could not. If it required more then one photon to complete 'the wave length' how were they stored up?

I have a guess. My guess is that individual radio frequency photons can be absorbed entirely by antennae of various lengths, but the output of the antenna is largely a function of the RESONANT effect between the frequency of the photon and the natural frequency of the antenna.

Any thoughts on this.......

A photon don't have a frequency by itself. It's the electromagnetic radiation associated with the photon/photons which have a frequency.

[Soul Surfer]

All photons must have a fundamental frequency because that is what defines the energy in that photon.  The energy in any photon is always Planck's constant multiplied by its frequency.  that is one of the most fundamental laws of quantum physics and has been proved many times over

[JP]

Lightarrow and Soul Surfer,

I think you're both right, from different points of view.  What we classically consider frequency is the number of oscillations per second of a classical monochromatic electromagnetic wave that pass a given point.

A photon has a frequency that determines its energy from E=hf, where E is energy, h is Planck's constant, and f is frequency.  In addition to determining the energy, this frequency appears in the description of the photon according to quantum electrodynamics.  The mathematics of a photon look similar to the mathematics of a quantum harmonic oscillator, where the photon has a frequency just like a harmonic oscillator has a frequency.  However, the photon is not modeled by a nice classical wave that oscillates a certain of number of times per second. 

The two types of frequency are related to each other, however.  If you add up photons of a given frequency in the right way (called a coherent state) they should sum up to give what looks like a classical wave with that frequency, although this wave will have quantum noise present.  A classical wave has high enough amplitude that the quantum noise is negligible and the classical model holds.

Read this for an overview: http://en.wikipedia.org/wiki/Coherent_state  The first figure on the right demonstrates how a collection of photons can form a classical wave.

[lightarrow]

Lightarrow and Soul Surfer,

I think you're both right, from different points of view.  What we classically consider frequency is the number of oscillations per second of a classical monochromatic electromagnetic wave that pass a given point.

A photon has a frequency that determines its energy from E=hf, where E is energy, h is Planck's constant, and f is frequency.  In addition to determining the energy, this frequency appears in the description of the photon according to quantum electrodynamics.  The mathematics of a photon look similar to the mathematics of a quantum harmonic oscillator, where the photon has a frequency just like a harmonic oscillator has a frequency.  However, the photon is not modeled by a nice classical wave that oscillates a certain of number of times per second. 

Exactly, infact it's modeled by *nothing*.

The two types of frequency are related to each other, however.  If you add up photons of a given frequency in the right way (called a coherent state) they should sum up to give what looks like a classical wave with that frequency, although this wave will have quantum noise present.  A classical wave has high enough amplitude that the quantum noise is negligible and the classical model holds.

Read this for an overview: http://en.wikipedia.org/wiki/Coherent_state  The first figure on the right demonstrates how a collection of photons can form a classical wave.

From which you can infer that a photon is not a coherent state and so it couldn't have a single frequency even if it were an electromagnetic pulse, but it's not...

[Geezer (OP)]

Exactly, infact it's modeled by *nothing*.

Lightarrow - are you saying there is no model of a photon? I'm not sure I understood your point. Thanks, G

[lightarrow]

Exactly, infact it's modeled by *nothing*.

Lightarrow - are you saying there is no model of a photon? I'm not sure I understood your point. Thanks, G

Exactly.

[JP]

Exactly, infact it's modeled by *nothing*.

Lightarrow - are you saying there is no model of a photon? I'm not sure I understood your point. Thanks, G

Exactly.

Lightarrow, I think I see what you're getting at, but I don't really agree with your statement that a photon is modeled by "nothing."  There is a perfectly good model for photons via quantum electrodynamics (as a Fock state containing 1 photon).  They certainly aren't simple particles zipping between sources and detectors, and the position representation of the photon isn't clear to me (I've browsed over some books that do define it, or make approximations so that a photon can be treated over space, but I'm not well-versed in these techniques).  However, photons can be modeled and the models appear to be extremely accurate. 

I think the best way to talk about photons is in terms of them being tiny packets of energy that can be emitted or absorbed at a point, but do something strange (although we can model it) in between emission/absorption. 

[lightarrow]

Lightarrow, I think I see what you're getting at, but I don't really agree with your statement that a photon is modeled by "nothing."  There is a perfectly good model for photons via quantum electrodynamics (as a Fock state containing 1 photon).  They certainly aren't simple particles zipping between sources and detectors, and the position representation of the photon isn't clear to me (I've browsed over some books that do define it, or make approximations so that a photon can be treated over space, but I'm not well-versed in these techniques).  However, photons can be modeled and the models appear to be extremely accurate. 

Ok, but when a non-specialist (as me) asks about a model of the photon, he/she usually mean "something like a particle, made (or not) of other particles" and I think it's important to remark the fact such a kind of model for a photon doesn't exist.

1. We don't know which shape has a photon (if it has one).
2. We don't know how big is a photon (if it has dimensions).
3. We don't know if it is a corpuscle travelling from source to detector, on the contrary, it seems that we are not allowed at all to say it's such a kind of thing...

Photons are Much more complicated objects than what non-specialists usually think it is, and often even for specialists...

[JP]

Then it sounds like we agree.  []

[yor_on]

This is a very nice thread

[yor_on]

Although. Thinking of it Are you saying that you can explain the the photoelectric effect by waves now?

[JP]

Could you explain a bit more, Yor_on?  Which post here makes you think that a classical explanation of the photoelectric effect is being proposed?

[yor_on]

"The photoelectric effect posed a significant challenge to the study of optics in the latter portion of the 1800s. It challenged the classical wave theory of light, which was the prevailing theory of the time. It was the solution to this physics dilemma that catapulted Einstein into prominence in the physics community, ultimately earning him the 1921 Nobel Prize."

That one states quite clearly that there is a fundamental difference between photons and waves as I understands it? And I got the feeling that the consensus here was that 'photons' are a misconception rereading it? Then there has to be a equivalent wave definition for the photoelectric effect. So I looked but didn't find it? And no JP, it wasn't a reply to you particularly, just a question of mine.