# Does a photon propogate faster than c ?

A typical Photon packet consisting of a self propagating electromagnetic pulse is described often nowadays something like this picture

http://superstruny.aspweb.cz/images/fyzika/foton.gif [Broken]

and it is propagating as typically described by this animation ..

The thing is that the electromagnetic stuff of which the wave is embedded has to move further that the overall pulse speed because it is deforming and reshaping to fit a Sinusoidal curved path , and not just the simple straight line path as could be and often is used in the extreme to describe the motion of light .

So while the pulse itself is moving as measured from a point A -- to a point B at said speed/velocity of Vc ... the waveform moment itself is propagating significantly further in the same time (covering an amplitude dependent scalar of the linear distance AB) .. i.e the velocity of the propagating wave is simply greater than c = 3x10^8 m/s .

Looking at this generalised wave form pattern , we can see the point in fact .. the pulse can be seen to travel a wavelength , but a propagating point following the 'ripple' itself is clearly traveling a considerably longer distance (amplitude dependent) in exactly the same time .

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## Answers and Replies

mgb_phys
Homework Helper
It's the difference between phase and group velocity and is the basis for many faster than light inventions. The important point is that the motion you describe cannot transmit any information - so doesn't violate relativity.

George Jones
Staff Emeritus
Gold Member
A typical Photon packet consisting of a self propagating electromagnetic pulse is described often nowadays something like this picture ...

I'm sorry, but I don't understand your post.

In an anomalously dispersive medium, even group velocity can be greater than c. For a nice animation, see (let it build up)

http://gregegan.customer.netspace.net.au/APPLETS/20/20.html.

Photons move with speed c

As mgb_phys said, information cannot be transmitted with speed larger than c.

Just out of interest, how are you defining "information".

mgb_phys
Homework Helper
Pretty much anything you didn't know before the message arrived.

I can't find a good non-mathematical explanation of group and phase velocities and why you can have a phase velocity faster than light.

You seem to be under the impression that a light wave IS a photon moving up and down in a sine-like waveform (as well as forward). This is not the case. There is nothing physically moving in an electromagnetic wave. The only thing that is 'waving' is the electromagnetic field strength, there is no particle 'riding the wave' moving up and down with it.

You seem to be under the impression that a light wave IS a photon moving up and down in a sine-like waveform (as well as forward). This is not the case. There is nothing physically moving in an electromagnetic wave. The only thing that is 'waving' is the electromagnetic field strength, there is no particle 'riding the wave' moving up and down with it.

Is the electromagnetic field strength:
a) constant within the wave shown, or
b) gradually (linear or non-linear) decreasing along with the amplitude and
b1) zero outside the wave, or
b2) zero only at infinitely far away.

You seem to be under the impression that a light wave IS a photon moving up and down in a sine-like waveform (as well as forward). This is not the case. There is nothing physically moving in an electromagnetic wave. The only thing that is 'waving' is the electromagnetic field strength, there is no particle 'riding the wave' moving up and down with it.

I understand your mention , and i agree that is to some degree correct .

but in fact the point is that the distortion in the emf that is created by the photon wave ripple is in fact distorting the background emf field at a speed greater than the linear speed of light .

Even if the particle aspect is completely disregarded the propagating "ripple" in the emf's space-time is being constructed/deconstructed significantly faster than light .

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It's the difference between phase and group velocity and is the basis for many faster than light inventions. The important point is that the motion you describe cannot transmit any information - so doesn't violate relativity.

well , technically it does convey information . It conveys information regarding the shape of the wave form .. ie it conveys a distinctly different quality of information that the midline straight plotted line (group vector) does carry ..

this is actually demonstrated on the 2d graph .. the line describing the wave form definately conveys more information that is contained in the straight midline curve representing group velocity .

We may 'choose' to accept that the wave form itself -- the phase vector does in fact contain information . imagine the example whereby light velocity (group velocity) has been well described , but that as yet no mention or observation of any wave like behavior has been observed or described . Then one day the wave vector (the phase vector) is observed .. Does describing the phase vector add any information to the science - Yes it does .. the recognition of the Wave form vector (the Phase velocities vector) contains a rich amount of information as compared to the straight midline vector .

After all the phase vector is the basis of the Photon/electron quantum Double Slit Experiment .

It's the difference between phase and group velocity and is the basis for many faster than light inventions. The important point is that the motion you describe cannot transmit any information - so doesn't violate relativity.

ahh but it does , it conveys information regarding the underlying nature of the EMF field ,, information regarding the photons sharing of these emf characteristics and information regarding the manner in which the emf field can be distorted .. all factors of which the Scince of Physics alone would be missing if description of the wave form vector (the phase vector) was unknown or undescribed .

It provides information in terms of physics answers and also in terms of physics questions .. both very high forms of information .

I'm sorry, but I don't understand your post.

In an anomalously dispersive medium, even group velocity can be greater than c. For a nice animation, see (let it build up)

http://gregegan.customer.netspace.net.au/APPLETS/20/20.html.

Photons move with speed c

As mgb_phys said, information cannot be transmitted with speed larger than c.

Yes that is an excellent graphic .. Yet overall phase shift pulse build up of that type so well conveyed in the first link is a quite different scalar from that we are looking at with the moment of Emf distortion .

The moment of the phase vector is conveying information regarding a number of things ,, for a start it is constantly changing yet remaining bound to a reasonably well defined pattern ..
That is the wave form moment at any finite point of consideration is holding information regarding the interaction of that moment to the EMF field itself and the resonant distortions of the EMF space time that is occurring .
These are not only direct information's in terms of real data because they are also leads to and part solutions of larger meta information questions that can be derived from them .. ie certain characteristic natures of the EMF field itself .

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russ_watters
Mentor
well , technically it does convey information . It conveys information regarding the shape of the wave form .. ie it conveys a distinctly different quality of information that the midline straight plotted line (group vector) does carry ..
And that information propagates at C.

Regardless of the correctness or incorrectness (incorrectness) of your concept of the wave-particle duality of light, you are misunderstanding the concept of the speed of a wave - any wave. The speed of a wave is not the speed of the individual particles that make up the wave. Sound waves, water waves, a wiggling jump rope, the speed of individual particles that make up the wave has very little to do with the speed of the wave.

 I think some in the beginning of the thread may have misinterpreted: this isn't a question about group velocity vs phase velocity. This quote in the OP contains the error in straightforward form:
Looking at this generalised wave form pattern , we can see the point in fact .. the pulse can be seen to travel a wavelength , but a propagating point following the 'ripple' itself is clearly traveling a considerably longer distance (amplitude dependent) in exactly the same time .
There is no "propogating point following the ripple" in any wave.

Is the electromagnetic field strength:
a) constant within the wave shown, or
b) gradually (linear or non-linear) decreasing along with the amplitude and
b1) zero outside the wave, or
b2) zero only at infinitely far away.

No answers yet... dumb question?

the Clothes do maketh the man ..

And that information propagates at C.

Regardless of the correctness or incorrectness (incorrectness) of your concept of the wave-particle duality of light, you are misunderstanding the concept of the speed of a wave - any wave. The speed of a wave is not the speed of the individual particles that make up the wave. Sound waves, water waves, a wiggling jump rope, the speed of individual particles that make up the wave has very little to do with the speed of the wave.

 I think some in the beginning of the thread may have misinterpreted: this isn't a question about group velocity vs phase velocity. This quote in the OP contains the error in straightforward form: There is no "propagating point following the ripple" in any wave.

Yes , the reason i wanted to draw attention to the Sine Curve path was to make clear the fact that the Curved surface itself draws a line that is longer than the straight line path .

It is the very nature of the propagating wave packet itself that is the real subject . We can refer to the angular moments of set locations about the EMF waveform while not seeing these moments as being separate or divisible parts in the mechanical sense .

And there is a distinction between this aspect and the "Group" "Phase velocity" as referred to above .
The "Phase Velocity" http://gregegan.customer.netspace.net.au/APPLETS/20/20.html" [Broken] as being faster than light , can in some very real sense even be considered to be an optical illusion .

But in this case the Scalar is quite different .

In this example it serves us to focus on the shape of the Wave packet in a single 'frozen' moment . And while taking note of the shape and pattern of what we see there , consider the actual mechanics of the displacement of the EMF field in which it is evident .

Further, then to actually determine in a formal mathematical manner how quickly this Wave pattern is being established over any given moment .
That is how much change has occurred in the EMF medium in order for the wave form to have established itself in the next frozen space-time moment .

The point is that there is a quality of the Photon packet energy that determines the shape of the Waveform .. and this quality of energy itself has the ability to construct and shape a distinct pattern in the fabric of the EMF medium that is longer than the straight line median path that is referred to as Vc - the linear Velocity of Light .

Simply put the length of the curved surface is longer than the 'straight' linear displacement .

And this Curved surface is being continually constructed and deconstructed over every single moment of the photons propagation through the EMF medium .

That is to say there is a part of the interaction between the Photon and the EMF field that displays a mechanical ability to run rings around c , the speed of light .

Like an invisible seamstress who is ever about and stitching new sets of fine clothing for the Emperor over every measured movement forward he makes .. And Yes , even when the King does run as fast as he possibly can run this Seamstress never misses a single beat and weaves clothes that are ever new ..

How is the fabric of the EMF field reworked as the Photon packet propagates .. ?

How is even one single definable thread reworked .. ?

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Staff Emeritus
Simply put the length of the curved surface is longer than the 'straight' linear displacement .

You seem to be under the impression that a light wave IS a photon moving up and down in a sine-like waveform (as well as forward). This is not the case.

No, a photon doesn't propagate faster than c, a photon is defined as something that moves at c.

You seem to be under the impression that a light wave IS a photon moving up and down in a sine-like waveform (as well as forward). This is not the case.

russ_watters
Mentor

Yes , the reason i wanted to draw attention to the Sine Curve path was to make clear the fact that the Curved surface itself draws a line that is longer than the straight line path....

Simply put the length of the curved surface is longer than the 'straight' linear displacement .
Yes, I know.
And this Curved surface is being continually constructed and deconstructed over every single moment of the photons propagation through the EMF medium .
"This curved surface" does not physically exist. What you are saying is wrong.

This thread violates forum guidelines prohibiting personal theories and is therefore closed.