[shuntress]

So, thermal radiation slowed the Pioneer spacecrafts. 'Thermal' is a type of Electromagnetic (EM) radiation, right?

Doesn't that make it an EM Drive?

Why the cone apparatus? What is preventing the intentional design of a spacecraft that uses thermal radiation (or Thermal Recoil Force) similar to the Pioneer spacecraft?

e: The replies indicate the answer is that this could be built but the effect is to weak to be practical. Which I find to be a completely acceptable answer, thanks.

[mikeash]

That makes it a photon rocket, which is nothing new. It's a logical consequence of the fact that light carries momentum, which is part of Maxwell's electromagnetic theory from 150 years ago. It's not very practical unless you have an extremely dense power source (nuclear fission won't cut it, you need fusion or antimatter) so it's more of a theoretical toy at this point.

What's interesting about the EmDrive's claims is that it supposedly produces far more thrust for the power that goes into it than would be possible from a photon rocket.

(The Pioneer anomaly wasn't because nobody realized that thermal radiation would act as a photon rocket, but rather that the effect wasn't correctly calculated. The theory is pretty simple, but it gets complex when you apply it to a real spacecraft with complex shapes, non-blackbody materials, and temperature gradients.)

[Dylan16807]

Light has effective mass. It's well known that you can shoot light in one direction for a tiny amount of thrust the other way.

The device under consideration is special because if it works it is a reactionless drive. Nothing is coming out of it.

[triplepoint217]

Light actually has momentum but not mass. Calling it effective mass isn't far off, but sometimes my pedantic physicist kicks in.

[Dylan16807]

The proper term I'm used to is "relativistic mass", but I didn't want to make the discussion complicated. Would you not phrase it that way?