A photon is a quantum of electromagnetic energy. The energy E of a photon is given by Planck's equation as E=hf, where h is Planck's constant and f is the frequency of the radiation. The velocity of the photon c=fl, where l is the wavelength of the radiation. The alternating electric and magnetic vectors of the photon lie in the plane transverse to the direction of propagation, are at right angles to each other, and are in phase with each other. In the course of traveling a distance of one wavelength the electric and magnetic vectors of the photon alternate one complete cycle. When the photon's energy is converted to another form, the photon no longer exists. A photon exists only while it travels at the velocity of light (c). The value of c depends on the medium through which the photon travels. When not otherwise specified c is taken as the velocity of light in a vacuum (space).

When a wave propagates in water, peaks and valleys of the water propagate away from the source that provided the energy for the waves. A wave in water is a propagating disturbance of the water, that is, of the medium supporting the wave motion. The wavelength is the distance between successive similar positions of the water, such as peaks or valleys. It is the water that does the waving.

In order to explain the wavelike phenomena of light discovered in the 18th and 19th centuries, scientists assumed that some kind of supporting medium had to do the waving. That medium, called the ether, was supposedly at rest with respect to the "fixed" stars. The famous Michelson-Morley Experiment, begun in 1881, tried to detect the orbital velocity of the earth around the sun relative to that stationary ether. Their optical equipment was accurate and sensitive enough to detect one-tenth the known velocity. They could not detect any motion of the earth relative to the ether. The ether idea was subsequently abandoned.

Thus, the photon is not thought of as a wave supported by a medium. A photon does not exist as successive peaks and valleys. Its electric and magnetic vectors alternate periodically as it travels; but, it is not extended in the direction of that travel. The time for one period is 1 / f. The distance through which it travels during one period is called its wavelength; but, at no instant during its travel through space does a photon have a wavelength. A photon is not a wave.

Its wavelength only appears as a result of its interaction with matter. A 1.0 megacycle photon is not a 100 meters long thing moving through space. A photon has only its velocity, its period, and the oscillation of its electric and magnetic vectors. (I have omitted the photon's polarization since it is not relevant to the present line of inquiry.) A photon, while it is travelling, does not have a wavelength to be stretched by the expansion of space.

Considering all the above, and since the velocity of light in space is constant, the only property of a photon that can affect the distance traveled by a photon during one period is the time elapsed during one period. To produce a red shift of the wavelength of the observed (no longer moving) photon, the time duration of its period would had to have been increased prior to its observation. Since a photon is not extended in the direction of its propagation and since wavelength is not intrinsic to a photon, how can the expansion of space increase the time of a photon's period during its travel through space?

Good question! I imagine there is a more formal GR/BB explanation for this supposed wavelength stretching effect, but it's likely for mathematically inclined types only.

Just recently I have been asking similar kind of questions in "Let's create steady state theory" thread, and based on that discussion I assume that nobody knows the answer for your question. It seems that the mechanism for the redshift is not actually known even for the Big Bang theory.

Do we have a slightly better model for the photon than a "packet of energy that's like a bit of a wave"? Because what that guy just said makes me realise that it's a pretty poor model.

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The problem is that no one has ever really been able to explain how light can be a wave phenomenon and a particle phenomenon at one and the same time.

In the thread Ari is referring to I pointed out that nothing actually happens to the photon as it flies through space. Because the photon is travelling at c, it experiences infinite time dilation and Fitzgerald-Lorentz contraction: so, from the photon's point of view, its creation, entire existence and destruction all take place in the same instant in time at the same point in space. There isn't any time for the photon to experience any intrinsic change.

Matter has two forms of energy: rest mass and kinetic energy. Rest mass is intrinsic, but kinetic energy is relative to the observer. Photons have no rest mass. All their energy is relative to the observer. It's we who see the light redshifted. In reality the light is just as it was when it was created (billions of years ago from our point of view, now from the light's POV).

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It certainly does, and that is exactly what is happening. You have not demonstrated even a single fact to indicate the contrary. Rather, you simply assume, a-priori, that the photon cannot have a wavelength, because it is a particle. But that is the root of the great wave-particle duality. The photon is, simultaneously, both a particle and a wave, and will appear at any measurement, to be either of those, whichever the experiment is designed to detect. It's no use complaining that "it can't be", because "it is", whether we like it or not.

But this doesn't answer the question of how the photon is supposed to have its wavelength stretched. It might make sense for the expansion of spacetime to stretch out light waves if spacetime were the medium for light. Is anyone making that argument?

Absolutely.

I'm aware of Einstein's statement that a gravitational ether could be acceptable, so long as it's not the classical, Michelson-Morley optical ether. So I assume you don't mean spacetime is that sort of classical ether. But then what sort of medium is it?

I think the wave-particle duality is the most fascinating aspect of matter. Diffraction of electrons - remarkable!

That was an absolute revelation in high school.

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When it became evident that light was made of waves, after Young's double slit experiments (1801 as I recall), the only waves anybody knew anything about were all mechanical waves (sound waves, ocean waves, & etc.) They were mechanical vibrations. So everyone just assumed that light waves were also mechanical vibrations in something they called the "lumeniferous ether". But the speed of a mechanical wave depends on the stiffness of the medium, stiffer media supporting faster waves. So the ether had to be really weird, stiff enough to support amazingly fast light waves (far stiffer than any known material), yet allowing Earth to gently glide through. Nobody could ever make physical sense of this ether stuff, but they couldn't do without it either, since all waves were known to be mechanical vibrartions.

But Maxwell killed the ether by showing that light was a wave of electromagnetic field, not a mechanical vibration. The ether became instantly unnecessary. And since nobody ever was able to figure out how anything with such strange properties could exist anyway, they were not hesitant in doing away with it altogether. The waves of electromagnetism could propagate through the "emptiness" of space without the need for mechanical support.

Einstein killed the idea of "empty" space by imbuing space (or space-time) with "physical" properties, such as geometry and a limited ability to sustain energy per unit volume (a black hole is a symptom of the "mechanical" failure of space-time due to energy overload).

And quantum mechanics came right behind Einstein, in granting the "emptiness" of space the power to exercise zero-point energy, vacuum fluctuations & virtual particles. So now we think of the "emptiness" of space as an energetic & dynamic environment. In fact, "empty" is simply no longer an acceptable description of space (or space-time).

So, what kind of medium is space? I don't think anyone can answer the question in a literal sense. The best we can do is to describe its behavior, as implied by the various physical theories, like quantum mechanics & general relativity. Curvature is both mathematical and physical (it is mathematically described and has physically observable consequences). The expansion of the universe causes the curvature of space (as measured, say, at a given point) to be time-variable. So, as we see it, the space through which the photon propagates stretches out as the photon moves through, stretching the photon (relative to us), a necessary consequence of the fact that "space" is the basis for coordinate measure.

Ah, so you might ask, if the photon's energy depends on its frequency, and we lower the frequency (by making the wavelength longer), where does the energy go? The answer is nowhere. All you have to do is speed up towards the photon, adding an extra blueshift, and voilá, you measure a photon with just the same energy it had when it left the distant galaxy (if you can speed up enough). The energy of a photon depends on the reference frame of the observer who measures it, and not on the photon itself.

There is a great lesson to learn from relativity and quantum mechanics: It's all about observers & reference frames.

Cheers.

Thanks, Tim. If we were to summarize, I suppose it would go like this. Maxwell weakened the light ether hypothesis, and the Einstein killed it with SR (plus negative Michelson-Morley result). Then Einstein brought back a different kind of medium, which we aren't supposed to call an ether anymore. We don't know exactly what the medium is, but we can equate it with Einsteinian spacetime. And if the medium expands then the waves superimposed on it will expand likewise. This makes sense when you put it this way, but I was under the impression that conventional physics would not describe light as waves in 'spacetime' so bluntly. Don't they prefer to dodge this question, as it just gets us back to the old, classical ether again?

To Tim Thompson and ExpErdMann:

cyreks reply:

When TT says light transmission uses the EM fields for its movement through space, I agree.
However, photons do not use space in any way as a medium for its travels except where the EMF's occupy space.

Plancks formula reduces light to bits (photons) rather than continuous
waves. These photons are generated by electron transitions only.
See my explanation in 'Hubble Double Trouble'.

I can understand Quantum theory well enough.
I cannot understand 'space time' because I cannot 'visualize' it.

There is a Planck constant. There is a Newton gravitational constant.
Is there a spacetime constant?

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The maistream seem to use the term "Quintessence' now.
http://physicsweb.org/article/world/13/11/8
Michelson-Morley result does not take into account than the Earth could drag a part of the Ether in his rotation that could explain the negative result.

Alex W. asked: Eroica wrote: Tim Thompson implied that a photon while traveling does have a wavelength to be stretched. And he wrote:Thank you all for the thought provoking comments that led me to clarify my ideas. Let me first respond to Tim Thompson's last quoted statement.

Assume for a moment that two observers in different reference frames could measure the energy of the same photon. Which of the two energies would the photon have? Wouldn't it be more correct to state that the energy of a photon is independent of the observer, but that the energy measured by an observer depends on the reference frame of the observer?

Perhaps my interpretation (model) of a photon will answer some of your questions. We are all familiar with the notion of a point on the x-y plane describing a circular motion around the origin at a constant angular velocity. The y-displacement of the point's coordinates plotted as a function of time yields a wavelike sinusoidal curve. We can assign a period and wavelength to the curve. However, we would not call the circularly moving point itself a wave.

A photon is analagous to that circularly moving point. A photon's electric and magnetic vectors are at a right angle to the photon's direction of motion. Assume a photon traveling in the x-direction with its electric vector lying on the x-z plane and its magnetic vector lying on the x-y plane. A plot of the electric vector intensity as a function of distance (or time) traveled gives us a wavelike sinusoidal curve. The same is true for the magnetic vector. We can assign a period and wavelength to those curves. However, we should not call a photon itself a wave.

Quantum theory defines the photon as a particle with zero rest mass and spin 1. I would imagine that the spin has two components of 1/2 associated with each of the two vectors. I take the sinusoidal nature of the time functions of the vector amplitudes as evidence of those spins.

Now we investigate whether a photon is in any way extended in space. Your microwave oven gives the answer. The openings in the viewing shield are small enough to prevent passage of microwave photons but large enough to allow passage of visible photons. Diffraction experiments with pinholes or slits give similar evidence. Therefore, a photon has extension at right angles to its direction of travel. Indeed, Maxwell's equations imply that a photon's electric and magnetic vectors have displacements only in the plane transverse to the direction of travel.

If you were the photon you would see everything else rushing by. You would also see your electric vector stretching out above you and below you and coming back. Similarly you would see your magnetic vector stretching out sidewise from you and coming back. However, they would not make the least attempt to move in front of you or behind you. If you didn't look at the other things it would be as if you were standing still and merely extending and retracting your four arms.

If a photon passes the edge of an opaque material and one of its fields touches that edge, the photon will be pushed off course. After all, those fields are force fields. That is a simple explanation for the physical basis of diffraction. Add to it the periodic nature of the fields and you can understand why a quantum (particle) of electromagnetic energy can display wavelike behavior. If my memory serves me, a collimated beam of hot metal ions has also been diffracted.

Since the electric and magnetic vectors of a photon are in phase with each other, they must have zero extension when they have zero amplitude and a maximum extension when they have maximum amplitude. The extension of the electric field occurs on both sides of the line of travel and similarly for the magnetic field. The fields expand at the velocity of light. The extension of a field on one side of the line of travel after 1/4 of a photon's period is 1/4 of its period times the velocity of light. That is the farthest it could travel in that time. For the two sides, then, the total extension of a field is 1/2 the period times the velocity of light. That is the maximum transverse extension attained by a photon.

One half period after attaining the above maximum extension a photon has a second similar maximum transverse extension, differing from the first only by polarity. Now 1/2 the period times the velocity of light is called 1/2 the wavelength of light. Thus, a photon is never extended a full wavelength. It is only extended a half wavelength for two brief instants of its period.

The extension of a photon's fields takes place as the photon moves in its direction of travel. Thus, as its fields extend from zero to 1/2 a wavelength, a photon travels forward a distance equal to 1/4 of its wavelength during the first 1/4 of its period. After traveling a distance equal to 1/2 wavelength its fields have zero extension. When a photon travels a distance equal to 3/4 of its wavelength its fields are again extended to 1/2 a wavelength, but with opposite polarity. When a photon has traveled a distance of one wavelength, it again has no extension of its fields.

The motion of a photon in the x-direction takes place at the velocity of light in a vacuum, c. The extension of its fields in the y-direction or z-direction also takes place at the velocity of light in a vacuum. Thus, the maximum transverse extensions of the photon's fields and the distance the photon travels during its period are both functions of only the duration of the period of the photon and the velocity of light. The period of light and the velocity of light determine the wavelength of light.

An increase in the wavelength of light requires either an increase in the velocity of light to extend a photon's fields faster, and hence farther in a 1/4 period, an increase in its period to allow more time for the extension of its fields, or both. The cosmological red shift hypothesis makes no claim that expanding space alters the velocity of light. The only remaining property of a photon that could be altered to increase its wavelength is its period. If the proponents of that hypothesis cannot produce a reason that expanding space should increase the periods of photons traveling through it, the cosmological red shift hypothesis would seem to be implausible.

Here is a bonus idea to think about. Where is the photon's energy when both its electric and magnetic vectors simultaneously have zero amplitude? (Maxwell's equations say they are in phase with each other.) Is that energy then stored as "the energy of a vacuum" or might it imply that electromagnetic radiation is a propagating disturbance of a gravitational field?

A gravitational field can both store and supply energy. Gravitational fields are everywhere in space. There is a gravitational line of force between every object that is a source of radiation and every object that will react with that radiation. Is the gravitational field the required 'ether' at the heart of 18th and 19th century developments in optical knowledge? I think it is the gravitational field that does the waving. (P.S. I published these last ideas in 2001) Thanks again for your interest and your comments.

Both energies. Since the energy of a photon depends on the frame of the observer, each observer would see a different energy.

Since we can't know a photon's energy until it is measured (and that energy is dependent on the observers frame), how do you propose to determine what that independent energy is?

Then you imagine wrong. Spin is associated with polarization and spin 1/2 and spin 1 particles have four components.

Check out Special relativity and momentum.

Current understanding is that gravity is simply the curvature of spacetime. No waving is required. Unless you are talking about Gravitational waves, but I don't think you are.

Published? Where was this published?

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The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

Tensor, I'd like to hear your explanation of what's causing a photon's reddening in the BB model. Tim's explanation that photons are waves within the spacetime medium seemed unorthodox to me. Is it the general opinion nowadays that light waves require a medium to be propagated? I thought this was all heretical.

cyrek1 wrote: I do not think that was Tim Thompson's meaning when he wrote: The EM field is not responsible for the motion of the photon. Aristotle thought that a moving object had to be continually pushed in order to stay in motion. Newton taught us otherwise. In the same way that a mass object moving in space does not have to be pushed (or push itself) to maintain its motion, no action is required on the part of the photon to keep itself moving. That accords with the equivalence of energy and mass.

My model of a photon is not an a priori conception. It is a synthesis of prior knowledge and experimental observations. I hope these few additional comments will give all of you a more complete understanding of my interpretation.

A photon has three intrinsic properties: its energy, the periodic transverse extension of its EM field, and its forward/propagation velocity. Therefore, we should expect to observe three distinct kinds of effects from a photon.

The first effect is dependent on the energy of the photon. We observe the energy of a photon by its heating effects (the pleasant warmth of the autumn sun), by the photoelectric effect, and by fluorescence effects, to name a few.

The second effect is dependent on the periodic transverse extension of the field of the photon. We observe the transverse extension of a photon by the striking efficacy of a half-wave dipole antenna when broadside to a received signal, by diffraction effects, and by transmission aperture effects, as examples.

The third effect is dependent on both the periodic transverse extension and the forward velocity of the photon. The relationship of the periodic transverse field extension and the distance traveled is what we call the wave nature of the photon. We observe the wave nature of the photon as interference effects and standing wave effects, as examples.

Since we can't know a photon's energy until it is measured (and that energy is dependent on the observers frame), how do you propose to determine what that independent energy is?

Tensor: You claim that we can't know the energy of a photon until we measure it. In this thread we have not been considering an isolated photon in space. We are really dealing with observations of photons from celestial objects when we speak of red shift. Dont we know from spectrographic evidence, and especially from the relative location of Fraunhofer lines, exactly which atoms and which of its quantum states is reponsible for a given spectal line? We are not measuring the energy with a bolometer, for example. We are measuring the wavelength. And if we can identify the quantum state responsible for the photon when it was emitted, doen't that tell us the energy of the photon without having to directly measure its energy?

I happily stand corrected about the spin interpretation. Thanks.

Sorry for the delay in answering, things are kinda crazy around here right now.

I happen to agree with and really can't improve on Tim's explanation. I think your confusion comes from thinking that EM radiation is a mechanical wave (like water or sound waves) requiring something physical to move. But, EM waves are not mechanical, nothing is really waving. It's simply the changing electric and magnetic fields producing properties similar to that of a mechanical wave. I also believe (and I may be wrong, if so I apologize), from reading your posts, that you still aren't really fully reconciled with the idea of photons having wave and particle aspects at the same time.

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Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

It appears you saying a photon has mass.

It helped, but I have a few questions.

It also has spin, but since it doesn't affect your arguments below, we can ignore it for now.

You mention half-wave antenna in the second and then state the third is the wave nature of the photon. I'm not quite clear on this. Wouldn't you need to have the wave nature to affect a half-wave antenna?

I was just responding to this:

It appeared to me that you were talking of a single photon here.

The frequency (f) of a photon is equal to c divided by the wavelength (L) and the energy (E) of a photon is its frequency times Planks constant (h). Mathematically f = c/L and E = fh or through sustitution E = ch/L. Since c and h are constants, when you measure a photon's wavelength, you are measuring it's energy. Also, don't you need to take a mesurement to determine the quantum state?

You're welcome.

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Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

An observer may categorize a photon by two 4-vectors, k and epsilon. The 4-vector k is the energy-momentum 4-vector of the photon. The time-component of k is the frequency. If we multiply this by Planck's constant we obtain the energy for that observer. If we multiply the spatial components by h/c we get the momentum vector of the photon for that observer. The 4-vector epsilon is the polarization 4-vector of the photon.

Now two 4-vectors means 8 numbers, but an electromagnetic field has 6 components, so not all of these numbers can be independent. First of all, k^2 must equal zero for all observers because the photon is a massless particle. Second, by convention the polarization has a magnitude of -1 (time-like metric) or +1 (space-like metric), so there are six independent numbers. But wait a minute! By convention, k and epsilon are orthogonal 4-vectors, so there are only 5 independent numbers. Fortunately everything is OK, because there are really only 5 independent components for the electromagnetic field of a photon. The electric field and the magnetic field are both 3-vectors (6 numbers), but for a photon they are orthogonal, so there are only 5 independent quantities.

There is no intrinsic energy of any particle, massless (like a photon) or massive. Observers record different energies and momenta, but all agree that E^2 - (pc)^2 = (m*c^2)^2 where m equals zero for a photon or the rest mass in the case of a massive particle.

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The bar has been lowered for the promotion of ATM ideas; the bar for the acceptance of ATM ideas must remain high.

I'm aware that the general view is that EM waves do not require a medium, they simply propagate through space. However, Tim was suggesting that they are waves in the spacetime medium. If the latter were true it could explain why photons could be stretched out in the Big Bang model. If it weren't true, it's hard to see why such stretching would occur. I am just curious about how the theorists look at this.
It seems the wave-particle duality just tells us that we don't know much about light. I'm OK with it, in the sense that it highlights our ignorance about it. But I wonder whether you are using wave-particle duality to dodge the tricky question of why photons redden during the BB. My understanding is that we can't say the reddening is straight Doppler (recessional) or straight gravitational. The spacetime expansion seems to be needed. In that case what is it that tacks down the photons to spacetime? If the reddening occurs through spacetime stretching, they have to be tacked don't they? If you say "No, photons are particles too, so they don't need a medium", then what causes them to redden? Somehow I don't think wave-particle duality could be at the core of the Big Bang model. For one thing, Einstein would have complained!

To all above:

cyreks comment:

Quantum theory portrays light as a 'quanta'. The quanta is a photon. It is
not a wave. It is a 'black body pulse' generated by electron transitions at different energies depending on which orbits the electron transits from.

All physics books explain this. Give Bohr credit for this.

The photon moves through space as a disturbance in the EMF generated by the electrons transition. It does not need spacetime and is not intertwined with space. The EMF is its carrier.
That could be the reason why Einstein was at odds with the quantum theory.
The Quantum Theory has proven itself time and time again.

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Newbie question:
I read that experiments have been done to slow down light and the managed to slow it down to as little as 40mph. What I would like to know is if it is possible to slow it down to a halt. If so would you get a "lump" of matter? Would it have some unusual properites?

Can I buy some on Ebay?

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Sometimes it takes a fool to be a genius

Care to explain the interference fringes that arise in the double-slit experiment? :wink:

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Microsoft is over if you want it.

The bar has been lowered for the promotion of ATM ideas; the bar for the acceptance of ATM ideas must remain high.

The best diagram I've seen has the photon as a sort of "packet" of a wave, a little bit that raises in amplitude, peaks in the middle, and lowers at the other end.

Not great, but it does have that particle-wave duality about it...

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Celestial Mechanic wrote:

Care to explain the interference fringes that arise in the double-slit experiment?

cyreks reply:

From what I understsand,
These experiments use a series of light pulses. A lot of photon pulses will trace out a wave pattern.
When the light is reduced to a single pulse, only a spot appears on the screen.
They refer to this as a 'wave collapse'.

It has been a long time since I read about these experiments.

__________________
aka Michael Cyrek

ExpErdMann,
I'm not ignoring you, due to my wife being in the hospital, I probably won't get to your questions until this weekend, Sorry.

__________________
Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth

No problem, Tensor. I hope it's not serious.