# The Naked Scientists Forum

### Author Topic: How does an Electro-magnetic wave conserve energy?  (Read 2279 times)

#### MikeS

• Neilep Level Member
• Posts: 1044
##### How does an Electro-magnetic wave conserve energy?
« on: 12/05/2012 09:17:25 »
Light is an electro-magnetic wave.  E is the electric field vector, and B is the magnetic field vector of the EM wave.  For electromagnetic waves E and B are always perpendicular to each other and perpendicular to the direction of propagation.  The direction of propagation is the direction of E x B.

The electric field vector and the magnetic field vector are normally considered to be in phase.  For every cycle there are two null points.  Could it be that a photon is that part of the electro-magnetic wave between two null points?  If so, there would be two of them per wavelength.  Does this explain why light propagates in discreet packets and exhibits a wave like/point like duality?

The electric field vector and the magnetic field vector are normally considered to be in phase.  There is a problem with this view of them being in phase.  At the null points of the wave, energy would appear not to be conserved.  To conserve energy the two components of the wave would need to be either (1) 180 deg. out of phase, or, (2) in some sense the wave needs to be travelling in both directions of time simultaneously.

(1) If the two components of the wave are 180 deg. out of phase then it is difficult to explain why light comes in discrete packets and why it has a wavelike/point like duality.

(2) The simplest explanation therefore is that photons propagate in both directions of time simultaneously.  They do that by propagating backward in time with the same time dilation factor that time flows forward.
This is not the same as photons not experiencing time although the effect is similar.  Neither is it saying that they actually travel backward in time.  They only travel backward in time with the same dilation factor that time flows forward.

Light is an electro-magnetic wave.  Electromagnetic waves can have any wavelength λ or frequency f as long as λf = c.

If you consider photons as not experiencing time or distance then both wavelength and frequency become meaningless.

If you consider photons as traveling backward in time with the same time dilation factor that time progresses forward then they (photons)  have both frequency and wavelength.

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: How does an Electro-magnetic wave conserve energy?
« Reply #1 on: 12/05/2012 17:19:59 »
Light is an electro-magnetic wave.  E is the electric field vector, and B is the magnetic field vector of the EM wave.  For electromagnetic waves E and B are always perpendicular to each other and perpendicular to the direction of propagation.  The direction of propagation is the direction of E x B.

The electric field vector and the magnetic field vector are normally considered to be in phase.  For every cycle there are two null points.  Could it be that a photon is that part of the electro-magnetic wave between two null points? ....

You asked the question How does an Electro-magnetic wave conserve energy? The answer is found here. http://home.comcast.net/~peter.m.brown/em/momentum_of_radiation.htm

See Eq. (25). That is the equation of conservation of EM momentum.

You asked the question How does an Electro-magnetic wave conserve energy? The answer is found here. http://home.comcast.net/~peter.m.brown/em/momentum_of_radiation.htm

See Eq. (25). That is the equation of conservation of EM momentum.

Regarding your speculations above, that requires quantum electro dynamics (QED) which is outside my knowledge base. If I were you I'd be asking myself "What is the magnitude of the electric and and magnetic fields of a photon?" I recall I did think about questions in thi area a long time ago but never got answers to myquestions. However I think there is a PF file someonewhere on the web under the Nobel Prize Web Site. Look under Feynman's information. I think his acception speech had to do with QED.
« Last Edit: 12/05/2012 17:26:21 by Pmb »

#### MikeS

• Neilep Level Member
• Posts: 1044
##### Re: How does an Electro-magnetic wave conserve energy?
« Reply #2 on: 13/05/2012 06:58:44 »
Pete

Unless I am missing something that still does not explain:
"The electric field vector and the magnetic field vector are normally considered to be in phase.  There is a problem with this view of them being in phase. At the null points of the wave, energy would appear not to be conserved.  To conserve energy the two components of the wave would need to be either (1) 180 deg. out of phase, or, (2) in some sense the wave needs to be travelling in both
directions of time simultaneously."

As the electric field vector and magnetic field vector increase from zero at the null point to maximum and then decrease to zero at the next null point then energy does the same.  The waveform does not conserve energy throughout its length.  To do that the electric field vector and the magnetic field vector would have to be 180 deg. out of phase.  If the vectors were out of phase then as one decreases so its energy is transferred into the other and so on. They are always shown as being in phase.
http://electron9.phys.utk.edu/phys135d/modules/m10/emwaves.htm

The point being:
"To conserve energy the two components of the wave would need to be either (1) 180 deg. out of phase, or, (2) in some sense the wave needs to be travelling in both directions of time simultaneously."

#### The Naked Scientists Forum

##### Re: How does an Electro-magnetic wave conserve energy?
« Reply #2 on: 13/05/2012 06:58:44 »