Is Electromagnetic Propulsion Like Radiation Pressure propulsion?

Question

while browsing for new propulsion methods I found the following interesting:

https://en.wikipedia.org/wiki/Radiation_pressure

"Radiation pressure (also known as light pressure) is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field.

This includes the momentum of light or electromagnetic radiation of any wavelength that is absorbed, reflected, or otherwise emitted (e.g. black-body radiation) by matter on any scale (from macroscopic objects to dust particles to gas molecules)."

So my question is: If a solenoid and a metal plate are attached together and they replace the light with an electromagnetic field from a solenoid, is it possible to have net thrust as shown in picture below? (Eddy current Repulsion Magnetic field from Solenoid and Metal Plate):

Perhaps it is better to Consider 2 cases :

Case 1 : Source of Light and Reflector plate attached together . Will there be Net Unidirectional thrust ?

Case 2 : Source of Magnetic field or Antenna and Metal reflector plate attached together . Will there be Net Unidirectional thrust ?

Answer 1

tl;dr: Think "antenna" and "beam of photons" rather than "pushing a plate with an electromagnet".

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If your spacecraft accelerates (in your frame of reference) because of a force you are generating, you are constantly changing its momentum in that frame. Since momentum is conserved you must also find a way to be constantly imparting the opposite momentum on to something else.

for every action there is an equal an opposite re-action.

The above invokes the 2nd and 3rd of Newton's laws of motion

In your case if you want your electromagnetic propulsion system to accelerate your spacecraft in one direction, you must constantly generate photons going in the other direction. Instead of a stream of fast and massive rocket exhaust carrying away momentum, you need a stream of photons.

The drawing below shows a modestly effective photon generating system.

You make a resonantly-tuned loop antenna for the frequency you're transmitting, which efficiently converts the time-changing magnetic field into a propagating electromagnetic field instead of an evanescent time-dependent magnetic field.

Standing in free space alone, it would radiate bidirectionally so there would be no net thrust. You need a reflector element which could be a 2nd, parasitic loop of a slightly different size to reflect the radiation going one way back the other, using constructive interference, or in this case a flat conducting plane (sheet of metal or mesh of wire).

Now you've made a "radio flashlight" that beams momentum-carrying photons in one direction so that your spacecraft is propelled in the other direction.

Just remember that an electromagnet that's switched on and off doesn't necessarily make much useful electromagnetic radiation in the far field. Unless $\mathbf{B}(\text{t})$ and $\mathbf{E}(\text{t}) \propto d \mathbf{B}(\text{t}) / \text{dt}$ are generated in the right ratio, the field is evanescent and won't produce your beam of photons in the far field.

So think "antenna" rather than "electromagnet".

Side view of a resonantly-tuned loop antenna in front of reflector, spaced at a quarter wavelength. The reflector can be a sheet of metal or a wire grid. The quarter wavelength spacing produces constructive interference (radiation to the left) because the process of reflecting from a conducting surface produces a phase shift of about $\pi$, equivalent to a half-wavelength shift.