[General] On photon momentum

[Chip Akins]

Hi Dr. Grahame Blackwell

 

I too am finding little use in this discussion.  

 

For me the evidence that a photon carries momentum is quite compelling.

 

Not only the evidence from the reaction of matter to light, but also the clear and definable results we can get for the mass of a particle if propagating energy carries momentum.  And if we conclude that propagating energy in a particle has momentum, then it is likely that any propagating energy also has momentum.

 

I get the feeling you don’t like the concept of momentum in light.  While it may be interesting to suggest that momentum doesn’t happen until the light hits a massive particle, the results of that assumption are at odds with the creation of mass from energy.

 

I have no problem with you holding the opinion that light does not carry momentum.  I just can’t accept that premise myself without a host of strong arguments which illustrate how the pieces can fit if propagating energy has no momentum. Those arguments would of course need to be simpler and more robust than the alternative.

 

So unless you have such a complete explanation I am finished with this momentum debate.

 

Chip

 

 

From: General [mailto:general-bounces+chipakins=gmail.com at lists.natureoflightandparticles.org] On Behalf Of Dr Grahame Blackwell
Sent: Monday, January 30, 2017 9:45 AM
To: Nature of Light and Particles - General Discussion <general at lists.natureoflightandparticles.org>
Subject: Re: [General] On photon momentum

 

Thanks Chip,

 

But you still haven't answered my question!

 

Understanding momentum isn't a problem - the issue is getting a clear unequivocal definition that relates to photons - without it being a circular definition.

 

I'm equally well able to look up a textbook as anyone else; as I said before, such definitions refer to massive objects - which light isn't.  So your first sentence doesn't move the issue one step forward (I am of course extremely well conversant with the concept of impulse, which is reciprocally defined in terms of momentum - this doesn't actually add anything).

 

Note that I'm not asking this question because I'm naive scientifically - on the contrary, I'm asking it because I believe that the scientific community as a whole is using a concept that they don't actually understand and are glossing over the fact that they don't understand it by trotting out the same old equations without clearly defining the terms in those equations.  In this I'm attempting to prod physics into moving forward rather than going round & round in the same old circles.

 

Yes of course p=Ft is a more appropriate expression (not term) for momentum of a photon - so now can you identify what F and t are, quantatively, for a photon?  I.e. can you define the force applied by a photon and the duration for which that force is applied - without simply referring back to momentum or impulse, thus making it a circular definition?  [Personally I think it should be more generally expressed in terms of integral(p dt) , since I know of no evidence that the 'force' applied to a particle by a photon is constant - indeed it's unlikely to be, if the particle is accelerating over that duration.]  I don't know of any source for such information, indeed I question whether that information has ever been experimentally verified (or even whether it could).

 

Again (for the last time - I'm beginning to lose the will to live on this one!): can you, or anyone, provide a clear-cut definition for momentum that can be directly applied to a photon (rather than implicitly applied to a photon by association with impulse - which itself is implied by the transfer of momentum - which is ... ad infinitum).  If such a definition is not possible, then I (again) question the very concept as an intrinsic property of a photon.  If you need more than three lines for your definition then it's unlikely to be a definition and more likely to be a mathematical rationale.

 

As Richard Feynman famously said: "If you can't explain a concept in terms that a layman [or woman] can understand, then maybe you need to think very carefully about whether you truly understand it yourself."  I'm not a lay-person, scientifically speaking - so if there is a comprehensible definition, then I think I should hopefully be able to understand it.

 

In hope (but steadily fading...),

Grahame

 

 

----- Original Message ----- 

From: Chip Akins <mailto:chipakins at gmail.com>  

To: 'Nature of Light and Particles - General Discussion' <mailto:general at lists.natureoflightandparticles.org>  

Sent: Monday, January 30, 2017 1:38 PM

Subject: Re: [General] On photon momentum

 

Dr Grahame Blackwell

 

Textbook definition.

[Conventional momentum is the product of mass and velocity of a massive object p=mv. Dimensionally momentum is equivalent to Impulse, which is defined force through time. So p=mv and p=Ft]. 

 

For a massless wave in space p=Ft is the more appropriate term. 

 

Nothing strange or ill-defined about momentum. It is real, and easy to understand.  

 

Photons do not have mass in the conventional sense, but they do display the property of momentum (impulse) when they interact with matter.  

 

Why try to understand the momentum of a photon?  Simple. If waves propagating through space have momentum, and they also have momentum when confined to form fermions, then we have the reason why E=mc^2, and we have the reason for inertial mass.

 

My reasoning for exploring this…

If we don’t have a clear definition of an item which seems to be responsible for properties we observe, then we should explore the possibilities, rather than just forming opinions without further inquiry. 

 

Chip

 

From: General [mailto:general-bounces+chipakins=gmail.com at lists.natureoflightandparticles.org] On Behalf Of Dr Grahame Blackwell
Sent: Monday, January 30, 2017 6:15 AM
To: Nature of Light and Particles - General Discussion <general at lists.natureoflightandparticles.org <mailto:general at lists.natureoflightandparticles.org> >
Subject: Re: [General] On photon momentum

 

Dear Chandra, Chip, John D, everyone,

 

I really appreciate all your responses, truly I do.

BUT - NOBODY appears to have answered, or even addressed, my question.

(With the exception of Chandra, Vladimir and John D, who actually question the fundamental premise - as I do.)

 

My question was a request for a 'text-book' style DEFINITION OF MOMENTUM APPLICABLE TO PHOTONS (not just how it's contained, how it propagates, whatever).

 

Without having a clear definition of what momentum IS (in a form applicable to photons), IMO it's utterly meaningless to try and explain/discuss/dispute how it propagates or anything else about it.  I don't see such a definition in ANY of the flood of emails following my question.

 

If the answer is that no such text-book definition exists, then we have to question whether momentum itself exists as an intrinsic property of momentum.  In this I'm at one with Chandra that momentum is rather a characteristic that emerges in reactions involving photons or massive particles - nothing more or less.  I'm also with John D that momentum is well characterised by the motion of a wave, and that energy & momentum aren't two different things - momentum is an emergent consequence of energy.

 

[So, Chandra, contrary to your first response, I actually DO like your opinion!  It's the only one that makes sense to me.]

 

I'd further agree 100% with Chandra and Vladimir that 'energy flow' through the ether is rather simply a self-propagating excitation of that ether (Chandra's CTF, Vladimir's node array) - like plucking a (3-dimensional) guitar string and seeing the vibration travel along the string: the vibration, the photon, the material particle, is NOT a physical 'thing' in its own right, it's simply a state-of-being of the ether, one that is self-sustaining (as a point of detail, in my early book I liken a material particle to a whirlpool in a stream - it's not the water, it's the self-sustaining pattern IN the water).

 

So unless we have a clear definition of momentum applicable to photons, I see absolutely no point in trying to explain how it happens - how can you discuss how something happens when you can't even define what it is?!

 

Again: any offers?

 

Best regards to all,

Grahame

 

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