# Reality of E and B fields?

## Main Question or Discussion Point

Does the physical significance of the E and B fields in classical electromagnetism only come into physical significance when you measure the light's intensity?

Just like how the wave function of QM come into physical significance when you take the square of it and it becomes the probability density of the particle.

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I realised that E and B contribute to a force on an atom so in that way, they are more physically significant than the QM wave function.

However, what are the similarities between E and B waves compared to QM psi wave besides the fact that they are both waves?

You can measure E and B. Beyond that I'm not sure how much you can gauge their reality.

ZapperZ
Staff Emeritus
There is a HUGE "reality" of E and B fields in particle accelerators. It is not mere coincidence that many of these are "RF accelerating structures", meaning they use EM fields as the accelerating mechanism. Accelerating cavities having TM01 mode, for example, clearly indicate the geometry of the EM fields that is being excited and responsible for accelerating particles.

Zz

vanesch
Staff Emeritus
Gold Member
Philosophically, "reality" is always a matter of hypothesis, so one should make one that is as useful as possible (however, if for some or other reason, you don't like that, as many things in philosophy, you can change your mind about it and do something else).

You have to realize that the question "is xxxx real", is a philosophical question, not a purely scientific one (which doesn't mean that it doesn't have any importance!). Now, only the scientific method can eventually give clear answers to only scientific questions, and hence there will never be a purely scientific answer to a question "is xxxx real".

Now, the closer the theory at hand is to daily common sense experience, the less this question comes to the foreground, because we have a strong intuition for "what is real, and what not", and that intuition is compatible with most things in basic Newtonian mechanics.

However, the more the theory at hand describes phenomena which are removed from our daily experience, the less one should rely on one's intuition to declare something "real", because doing so can put you in (unnecessary) trouble. Electromagnetism is already quite remote from our daily experience, and the concepts used in there are thus more open to pondering as to whether to take them real, depending on one's ontological a priori.

An extra difficulty comes in when two empirically equivalent formulations of the same physical phenomenon, with different formal elements, exists. For instance, it is possible to reformulate EM without EM fields, but just using retarded effects of one charge onto another. In that (rather clumsy) formulation, there are no fields present, and in as much as one assumes that all EM radiation found its origin in some movement of a charge (and was not an "initial condition" of the field itself), both are empirically equivalent. So one has to pick one's personal choice in setting up one's "reality hypothesis". One can debate over other people's choices ad infinitum.

"What are the similarities between E and B waves compared to QM psi wave besides the fact that they are both waves (hence can interfere with itself when diffracted)?"

I notice that if you don't use the complex number formalism of QM you get two waves, sine and cosine just like in E and B.

There is a HUGE "reality" of E and B fields in particle accelerators. It is not mere coincidence that many of these are "RF accelerating structures", meaning they use EM fields as the accelerating mechanism. Accelerating cavities having TM01 mode, for example, clearly indicate the geometry of the EM fields that is being excited and responsible for accelerating particles.

Zz
What is RF?

What is RF?
Radio Frequency (I think it means that here, not sure)

ZapperZ
Staff Emeritus
Radio Frequency (I think it means that here, not sure)
That's correct. The only caveat here is that the name is being used loosely even if the frequency is outside of the range of what would be called "radio frequency". The name "RF" has stuck due to historical reasons.

Zz.

vanesch
Staff Emeritus
Gold Member

"What are the similarities between E and B waves compared to QM psi wave besides the fact that they are both waves (hence can interfere with itself when diffracted)?"

I notice that if you don't use the complex number formalism of QM you get two waves, sine and cosine just like in E and B.
The electromagnetic complex potential plays the role of a single-photon wavefunction. As such, one can see the EM potential as a kind of psi wave function.

But there are in general quite a number of differences. First of all, the EM fields live over 3D, while the psi wavefunction lives over configuration space.
It is only for single-particle wavefunctions that both domains are 3-D spaces.

Also, except for the MWI interpretation, in most views on quantum theory the "reality" of the wavefunction is put in doubt, which is much less the case for the EM field.

What is MWI? What is electromagnetic complex potential? Is it a potential energy function?

I remember reading that Schrodinger himself thought that his psi function when sqaured represented the charge density of the electron (which is closely related to EM but not quite since E=N/c not c by itself with c=coulomb which represents total charge). Curiosly he never accepted the standard Born intepretation of it being a probability density function.

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Another similarity is that the intensity of EM field is given by E^2+B^2. This gives the energy density and the higher it is implies more photons.

The psi function sqaured gives the probability of finding the particle in a location. But for only one particle. Whereas EM is for many, many of photons. However, if you shoot many electrons over some time, the higher prob density location will mean more electrons landing there. In this way the intensity of E and B and psi squared give very similar intepretations.