0
$\begingroup$

If an electromagnetic wave is constituted by a continually accelerating charge, then how can a photon be neutral?

$\endgroup$
2

3 Answers 3

1
$\begingroup$

Following the history,

in 1864 Scottish mathematical physicist James Clerk Maxwell, proposed a comprehensive theory of electromagnetism, now called Maxwell's equations. Maxwell's theory predicted that coupled electric and magnetic fields could travel through space as an "electromagnetic wave". Source Wikipedia

Years before it was observed, that an electric current in a coiled wire induces a magnetic field as well as a magnet, moving forth and back in a coil induces a current.

Maxwells identified the EM radiation as oscillating electric and magnetic field components, in vacuum both perpendicular oriented on each other and in addition both perpendicular to the direction of propagation:

https://de.wikipedia.org/wiki/Elektromagnetische_Welle

After finding the explanation for the photoelectric effect it turned out that the EM radiation consists of quanta, later called photons.

In 1900, while studying black-body radiation, the German physicist Max Planck suggested that the energy carried by electromagnetic waves could only be released in "packets" of energy. In 1905, Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect.

You ask what's usually asked, but the question doesn’t contain the full picture:

... an electromagnetic wave is constituted by a continually accelerating charge

The charge you are asking for is the electron and electrons have two properties we should take in consideration: Electrons have an electric charge and a magnetic dipole. Both intrinsic (existing independent from outer circumstances qualities) properties are used for induction processes:

Electromagnetic induction has found many applications, including electrical components such as inductors and transformers, and devices such as electric motors and generators.

Knowing and taking this into account it is less surprisingly that the radiation from accelerated electrons contain both - an electric and a magnetic - components.

Furthermore the difference in motion of two particles can be reached by manipulating one of the particles with photons. A plausible example is the technique of laser cooling:

Laser cooling techniques rely on the fact that when an object (usually an atom) absorbs and re-emits a photon (a particle of light) its momentum changes.

So a photon acting on the electron disappears (at least for a little while) and at the same time causes a moment change. Being familiar with this fact it should be less surprising that accelerated electrons emit photons. BTW emitting photons the electron loses moment and decelerates. For example a coiled wire has a higher resistance when a wire of the same length (because now beside the Ohmic resistance an inductive resistance appears).

Long story short

then how can a photon be neutral?

It is what was observed. Furthermore electric charges or magnetic dipoles smaller the electron as freely moving particles are not observed. Never an electron loses subelectric nor submagnetic particles.

$\endgroup$
1
  • $\begingroup$ Thank you so much everyone! $\endgroup$ May 20, 2019 at 17:23
1
$\begingroup$

A charged particle under acceleration loses energy and emits EM waves. The photon replaces waves in particle model of light. EM waves or photons are not electrically charged.

$\endgroup$
1
$\begingroup$

An electromagnetic wave is created by an accelerating charge, but it does not consist of an accelerating charge.

Suppose you have some antenna consisting charges that you let vibrate up and down at a certain frequency $\nu$. The oscillation of these charges will induce oscillating electric and magnetic fields. At sufficient distance from the antenna (for a car antenna, just a few millimeters) these oscillating magnetic and electric fields will take the shape of electromagnetic waves.

That does not mean that the field is charged, it means that it is generated by a charge.

Incidentally, the photons you mention then come into play, since the electromagnetic field is quantized in terms of them. But please don't take this as a proof that photons are electrically neutral, merely as evidence that they might be. Classical electrodynamics is formulated without any regard or even mentioning of photons, and works fine without reference to them.

Griffith's Introduction to Electromagnetics contains a relatively elegant derivation of the maths governing this radiation in chapter 10.

$\endgroup$
3
  • 1
    $\begingroup$ So called light waves are made from billions of photons and ignoring them does not make them go away. You couldn’t have a wave without them. $\endgroup$ May 19, 2019 at 21:28
  • 1
    $\begingroup$ I’m not talking about what is or is not true in the strictest sense of the word. I’m talking about what works and what doesn’t. You can build up all of classical electrodynamics without ever touching upon photons; CED says nothing at all about whether photons exist and what they do. That makes it only confusing to discuss them when talking about CED derivations. A student should be able to understand the concept of radiation without thinking about quantized light, so for brevity I would really recommend ignoring photons. $\endgroup$ May 20, 2019 at 21:08
  • $\begingroup$ ??Radiation is photons $\endgroup$ May 20, 2019 at 21:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.