They say photons have momentum, but momentum has dimension kgm/s. Isn't it paradoxial to heve something with no mass having a property with dimension where kg occurs in?

They say photons have momentum, but momentum has dimension kgm/s. Isn't it paradoxial to heve something with no mass having a property with dimension where kg occurs in?
Not at all. Consider the more general definition of momentum in the context of 4vectors
Now calculate the magnitude of this 4vector
This is equivalent to
For a photon ( m=0 ) this just becomes
as expected. The momentum in this case has dimension of N*s, and is not defined in terms of kg.
N is still defined as kgm/s^2 and replacement is valid, so logically you still sit with "kg" in the definition.
Or, more specifically its lack of "inverse time" (whatever that is). [Edit: I suppose inverse time is frequency and so ....]
This would appear to be the physics equivalent of the etymological fallacy; just because something appears in the dimensional analysis, doesn't mean it exists.
Last edited by Strange; October 17th, 2012 at 04:37 AM.
See wikipedia re Proposed future definitions of the kilogram:
"Such a definition would theoretically permit any apparatus that was capable of delineating the kilogram in terms of the Planck constant to be used as long as it possessed sufficient precision, accuracy and stability. The watt balance (discussed below) may be able to do this...
So ? Momentum is a general feature of all particles, regardless of whether they have mass or not. Momentum is just a form of energy, and photons do carry energy. Remember that all forms of energy are equivalent. I don't understand where the issue, let alone the paradox, is ?
Not at all. Did you not follow my maths in post 2 ? The momentum of a photon is just
which is simply a measure of the photon's energy. In fact energy, momentum and stress are all manifestations of the same thing, which is why they can all be unified into just one object, the stressenergymomentum tensor :
Stress
Even more generally, momentum can be defined via the Lagrangian of a given system through
All of this is applicable to massive as well as massless particles  there are no paradoxes anywhere.
Study the equations and see what they are saying. Reread what Markus has told you several times, now. If you open your mind, you will see that they are not at all "very different things." As one hint, notice that the last equation you copied, above, has an equals sign; on the left is energy, on the right is momentum, with a scaling factor of c. Think about what that means. Are energy and momentum truly such different things?
Well, I guess the same way it is sustained that energy and mass are equivalent as a consequence of the E=m.c^2 formula, isn't it?
This way energy, mass, momentum and who knows what else are all the same thing.
I prefer to think they are different things and that's why I agree in that the problem of dimensions for a photon could be seen as a "paradox"...
At the end in all those formulas appear the c factor wich is a velocity (even if with constant value) so it has sense for me to consider them as different things.
Light is a waves, and when waves hit something: it transfer momentum. Just like a collision. Waves on a rope and waves at sea can collide and transfer momentum.
Imagine this:
when an emitter produce magnetic & electric component (of light wave) some internal force will be created* and this changes the momentum of the emitter, and when the light wave hit a reciever then the receiver receive the opposite momentum (caused by the same magnetic & electric interaction) and the total momentum (of both emitter & receiver) is Zero (0). Momentum is conserved & a 'collision' happened.
*force created due to electron's interaction with magnetism. ie: Only conductor with free electrons could create & reflect lightwave.
Last edited by msafwan; October 18th, 2012 at 12:28 AM.
The way I picture photon momentum is like this. Picture yourself as operating in Planck time, that is everything operates from one moment to the next. A photon flies toward an electron at close to the speed of light, but still, within your frame of reference, it looks like a low frame rate movie. Upon collision with this electron there is a moment where the velocity of the photon begins to change, and thus there is a moment it is "at rest." For this incredibly short period of time, it is possible to measure the photon's mass, and therefore view it as "containing momentum."
The photon will disappear (absorbed by electrons) and then reappear (reemitted back by electron) in the opposite direction. So you can't see any acceleration and it certainly doesn't have acceleration. Electronphoton interaction explain alot of things, from how lightwave is produced to how lightwave could exert momentum.
Photon is a carrier of force. It doesn't have acceleration, mass or momentum, and it only 'carry' force*.http://en.wikipedia.org/wiki/Force_carrier
*It is a WAVE. All wave behave like this.
Last edited by msafwan; October 18th, 2012 at 12:33 AM.
Photons have relativistic mass which gives them relativistic momentum, however the technical term for this is 'momenergy'.
Mass and energy are equivlanet therefore they have a relative 'kilograms' I wonder what the weight of a photon would be as it falls from the photon sphere to the singularity. A relative energy weight?
I have a solution to the problem under the following interpretation:
The photon can cause momentum onto an electron it collides with, and it can cause this momentum because it carries energy. Note I use the word "cause" and not "transfer". In this view the photon does not carry momentum.
In this view the photon must carry a direction (as we see with laser light) and energy (since p is a vector).
Isn't collision with a particle the only way to measure a photon's "momentum"?
I absolutely 100% agree. The energy from incoming photons can create a momentum on the target electrons. Magnetic and electric field of photon will definitely (100% sure will) cause motion on any electron(s), no doubt.
This don't violate law of momentum conservation because both the emitter & the absorbing electrons experience the exact opposite motion. Everything involving photonelectron 'collision' is absolutely 100% basic physic (can't possibly be wrong).
I'm afraid that's not correct. The conservation of momentum does not just apply to the end result of an interaction, but to all intermediate stages as well. You cannot have one electron where momentum suddenly vanishes, then a photon which does not carry momentum, and then another electron where momentum suddenly reappears. This would constitute a clear violation of the momentum conservation law, both under classical mechanics and under quantum physics.
If there are still doubts whether a photon actually carries momentum, please refer here :
Photon  Wikipedia, the free encyclopedia
That doesn't make semiclassical explaination wrong. In fact, semiclassical photon can explain all alot of phenomenon like diffraction, dispersion, scattering, light in gravity potential, heinsenberg uncertainty, and everything else.
While more advanced interpretation of photon seems to loose all resemblance to observable world, like interpreting creation of photon from pertubation of quantum field and interaction with virtual photon of arbitrary property (like possibility of having mass or being able to manifest gravitational field). In such case, then "momentum" doesn't make sense anymore because it might as well be a derivative propery (like how "mass" is not really a 'mass' but derivative of somesort of force field).
There's no wrong thing here.
Well Energy has the dimensions of kg*m^2/s^2 And I don't see any meters in energy either!
Your paradox isn't a paradox. Momentum is the cumulation of force over time. In the case of light, this was a quantum jump, hence a quantum of energy. Yet still a change in force. It feels intuitively wrong. Just like it feels as if air has no weight. yet, a filled balloon is heavier then an empty one. If I'd have a perfectly reflective surface on my hand. And a strong laserpulse shining on it. You can still feel the light press with twice the momentum it has. (also it will actually feel a little colder)
Its not an approximation. Photon is a real wave and is created by real field (electric & magnetic field), and it exhibit wave property like all wave would do.
Quantum property is NOT a property photon alone. Everything will exhibit wave & particle property in quantum level and they all became indistinguishable from each other at this level.
Semiclassical explaination for photon is accurate. Not an approximation
Last edited by msafwan; October 26th, 2012 at 03:00 AM.
Intermediate steps doesn't violate the natural world. See here: Lenz's law  Wikipedia, the free encyclopedia .It happen.
Yes, that's right.
Yes, that is right.A photon is a wave and is subjected to quantization.
Semiclassical means that part of a system is treated quantum mechanically, and part of it classically. Mathematically this is achieved by developing the governing relations in power series, usually in terms of Planck's constant. By considering higher and higher powers you can achieve an arbitrary degree of accuracy, but it does remain an approximation until all powers are accounted for, in which case it becomes full quantum mechanics. If only the first power is taken into account you get a classical description. See also here :
Semiclassical physics  Wikipedia, the free encyclopedia
Im a layman mathematically...but would this not perhaps fall into the realm of isotropy and could even be cashed out in Euclid? If not just ignore.
No it is exactly the same. In fact it is safe to say that Energy is more real then a photon. A Photon is a quantization of the electromagnetic field. And the ideal photon (a point particle) would have an infinitetly broad fourier spectrum, meaning a completely undefinded energy.
Energy however is measurable, and electromagnetic waves are quantized in energy.
Also a photon isn't a particle. As it cannot interact with its own kind. It can only cause interference. There is no scattering of photons on photons.
Also, considering that everyone is fucking up this energy business lets solve it once and for all; E=mc^2 is for particles in rest. Considering light is, well, light. It must therefore travel at the speed of light. And this means that it isn't 'in rest' . In that case we need to use the Klein Gordon equation: E^2 = m^2*c^4 + p^2*c^2.
Since a photon doesn't have mass; E^2 = p^2*c^2. Or more simple p=E/C since E is the energy the momentum of a continous wave would be hf/c or p= h/labda where labda is the wavelength.
However, we are speaking of a photon. Which is a summation of several different kinds of modes from the spectrum. Since it scales linearly with the spectrum of energy, it inherits is Gaussian nature. In other words, Photon's have Gaussian momentum distribution. Either way. They have momentum. And, it can be seen. In fact I have one of those mirror fans in my window. It turns fast when it is sunny, slow when it is dark.
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