
#1
Jun2004, 09:14 AM

P: 236

Hi,
I recently came across a proof for the De Broglie wave equation in a book, which went as follows: E of photon = mc2 = m*c*c = (m*c)c = (p)c ( ie  momentum*speed of light) = (p)(f*lamda) Therefore, hf = p(f*lambda) Therefore, p = hf/ f*lambda = h/lambda. Therefore, Lambda (ie wavelength) = h/ mv. However, I'm not sure if I agree with this, so I wanted to ask a few questions. Firstly, it is true that the energy of a photon = hf, but I dont think that it should equal mc2. After all, this Einsteinian equation literally means energy produce = mass destroyed*speed of light. So, what mass has been destroyed to create the energy of the photon? Furthermore, how can we take mc to be the momentum of the photon, if this is mass destroyed, not mass of the photon? As slightly different question, what exactly is the wavelength of a particle? If the wavelength of a wave is "the distance between two crests or adjacent points", then what is the wavelength of a particle? Thanks is advance. :D 



#2
Jun2004, 09:27 AM

Sci Advisor
PF Gold
P: 2,226

Really tou need the equation:
[tex]E^2 = m_0^2c^4 + p^2c^2[/tex] As a photon has zero mass: [tex]E = pc[/tex] In your book they've defined m as relativistic mass. 



#3
Jun2004, 02:46 PM

P: 236

I've had a think and could you say that, since if you accelerate and object and it gains XJoules of KE it will gain X/c2 kg of mass, that in creating the photon you have "destroyed" some mass and thus that the photon must have mass? It must do so that when it gives its energy to something else, that thing will gain energy AND mass.




#4
Jun2004, 02:53 PM

Sci Advisor
PF Gold
P: 2,226

The De Broglie Wave Equation
The photon doesn't have mass as mass is usually defined as the rest mass.




#5
Jun2004, 08:08 PM

P: 24

The verificiation of the De Broglie formula (I am lead to believe) comes from considering a de Broglie wave:
[tex]$\psi ({\bf{r}},t) = Ae^{i({\bf{k}}.{\bf{r}}  \omega t)} $ [/tex] If you assume the relationshup [tex]E = \hbar \omega [/tex] holds for material particles you then write [tex]E = \hbar \omega = \frac{{m_0 c^2 }}{{\sqrt {1  \beta ^2 } }}[/tex], use this in the group velocity formula and obtain the required result. However to show that [tex]E = \hbar \omega [/tex] is a consitent step, you show the invariance of the quantity [tex]kr  \omega t [/tex] in two inertial frames S and S'. 



#6
Mar1809, 03:13 PM

P: 29





#7
Mar2109, 05:41 AM

P: 64

Just consider that all physics equations : E = mc2 ; E = hf ; Gij = Tij; can be expressed in a single way depending on th scale. This single law is :
extensity flux = diffusivity * extensity concentration gradient. 



#9
Mar2109, 01:15 PM

P: 1,504

Do Photons Have Mass? http://www.physicsforums.com/showthread.php?t=104715 Also, you can have a look here: http://www.physicsforums.com/showthread.php?t=300384 



#10
Mar2109, 03:24 PM

P: 29

The mass of a photon is: m=hn/CC; planks constant times the frequency divided by the speed of light squared.
Mass is mass; mass times accelleration is force but for a photon accelleration is defined as CC ie. C squared. Thus F=mCC. 



#11
Mar2109, 03:38 PM

Sci Advisor
HW Helper
P: 4,739

Now, WHERE and WHY should the acceleration of a photon be defined in that way? There is no reason and meaning to that, c^2 does not even have the units of acceleration. sammy, there equation is [itex]E=m c^2[/itex], where [itex]m = \gamma m_0[/itex] and [itex]\gamma = 1/\sqrt{1v^2/c^2}[/itex]. The equation [itex]E=m c^2[/itex] can also be written as [itex]E^2 = (pc)^2 + (m_0c^2)^2[/itex]. The energy of a photon is [itex]E = \hbar \omega[/itex] Now, try to fit the photonenergy equation to make [itex] \hbar \omega = mc^2 [/itex]... and you will find: [itex]E = \hbar \omega = mc^2 = \gamma m_0c^2[/itex] now the rest mass of the photon is zero, and \gamma for the photon is infinite, since the photon moves at c. So what happened to the photon mass? we have that [itex]m = m_0 \gamma = 0 \times \infty [/itex] .. nice BUT this is nice, since m_0 = 0: [itex]E^2 = (pc)^2 \rightarrow [/itex] [itex]p = E/c[/itex] 



#12
Mar2109, 03:46 PM

P: 64

I agree with you, cc has not the unit of acceleration. But cc/l with "l" being wave length has the unit of acceleration. it is then easy to make de broglie and einstein equations equivallent.




#13
Mar2109, 03:50 PM

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P: 4,739





#14
Mar2109, 03:58 PM

P: 64

They are the same mathematical description of the quantic reality. And you can not imagine how Sammy is close to the final solution when he writes F = m cc !!




#15
Mar2109, 04:05 PM

Sci Advisor
HW Helper
P: 4,739

Force is instantaneous change momentum also, so there is in the first place no need for an equation of the kind F = mc^2, since F = dp/dt will work too, using p = E/c for a photon. 



#16
Mar2109, 04:17 PM

P: 64

ok I stope. But there is no fault when you are doing true calculations directly. You do it yourself F = dp/dt... I can do for example E/V = P (Pa) = mc2/V = mc2/S.l et c2/l (m.s2) = S/m * P



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