Direct conversion of electricity into thrust

LeSving wrote:

Well then explain why radars and lasers have no thrust that ever has been discovered/measured.

Look at the magnitude of the effect in the example above…

As for experimental verification, @DavidS hit the nail on the head with the Compton effect. If you pick up any book on special relativity, one of the first chapters will have the relationship between energy, momentum and mass for relativistic particles: E^2 = p^2 c^2 + m^2 c^4. For a massless particle (photons are massless), that reduces to E = pc. This has all been known and tested for a century.

And also explain why this new “engine” does not work, and never will according to the vast majority of physicists

That’s because the article about this new engine is full of complete BS, like laser beams traveling faster than the speed of light. This “engine” is some kind of sci-fi wishful thinking, like cold fusion. If you look at my post above, I calculated for you how much radiation thrust you can expect from 30 kW without violating the laws of physics.

jmuelmen wrote:

For a massless particle (photons are massless), that reduces to E = pc. This has all been known and tested for a century.

I know that (even from high school). But I have never seen anything about conservation of momentum between photons and real matter (not particles). 0.1 mN is easy enough to measure, so it is strange that nobody (as far as I know) has ever measured this even in a single experiment.

Well then explain why radars and lasers have no thrust that ever has been discovered/measured.

They do have thrust and it has been measured. At least with lasers. See here: https://en.wikipedia.org/wiki/Photonic_laser_thruster

Peter wrote:

I thought cold fusion does exist, but again at extremely small levels.

Some kinds of “cold fusion” are a theoretical possibility, but none has, to our present level of knowledge, the potential to generate more power than it consumes. For example this process here: https://en.wikipedia.org/wiki/Muon-catalyzed_fusion

And a general remark regarding photons and their seemingly contradicting properties: There are no photons in the real world. The photon is a concept of physicists that allows the formulation of equations that realistically describe some observed effects of the electromagnetic radiation (https://en.wikipedia.org/wiki/Electromagnetic_radiation ). The properties of photons are set by (human) definition not by nature. And therefore, by definition, a photon has no mass, yet it is attracted by gravity, always moves at the speed of light, has no charge or polarity and carries momentum. Photons are well suited for setting up and solving equations, but they can’t explain anything.

Peter wrote:

I thought cold fusion does exist, but again at extremely small levels.

Depends on what you mean by “exists” There are all kinds of situations in which the photon momentum matters (they just happen to not be in GA). There is no situation in which cold fusion matters.

Fusion requires overcoming the electrostatic energy barrier between two protons. The barrier is about 0.1 MeV (I got that number from wikipedia). How likely is it that a particle will have that much kinetic energy at room temperature? The probability of having energy E is proportional to exp(-E / kT) — this is called the Boltzmann distribution. kT at room temperature is 25 meV. exp(-1e5 / 25e-3) = 0 to any level of precision that matters. “Matters” is determined by how often you get the reaction from natural background processes like cosmic rays.

As for how often you get a laser beam traveling faster than the speed of light, that’s up there with violation of charge conservation, momentum, energy, … — if it’s ever observed, we’ll have to rethink things in a pretty fundamental way.

what_next wrote:

There are no photons in the real world.

Would you say the same thing about electrons?

Think about a process like radioactive gamma decay. The decay of one atom gives you one photon. If you have a big lump of radioactive material, it looks like a continuous process, but simply by diluting the emitter sufficiently, you can tune the emission rate so that you only get a photon at a time (i.e., per micro-, milli-, second interval, whatever the resolution of your detector is), most of the time.

Many physics undergrads do experiments with single-photon sources in their lab classes nowadays. A favorite is sending single photons through a double slit and seeing if they diffract. (They do.)

@LeSving,

here’s an instance where the atom doesn’t recoil after emitting a photon: Mössbauer effect

It was surprising enough that it resulted in a Nobel prize.

Would you say the same thing about electrons?

I live in the town where the “most beautiful experiment in physics” (voted by Physics World which requires a subscription, but the list can be seen elsewhere, e.g. here: http://physics-animations.com/Physics/English/top10.htm ) was first conducted: The double-slit electron diffraction. So obviously, electrons and other forms of matter are not the little particles that we imagine them to be. There are observable effects for which a “photon” or “electron” can provide an explanation, but that does not mean that such a particle must really exist.

…you can tune the emission rate so that you only get a photon at a time

Yes, but that only tells us that electromagnetic energy is quantised. The universe does not need “real” particles to produce quantisation.

what_next wrote:

Yes, but that only tells us that electromagnetic energy is quantised.

Yes, I agree with you about that. The photon is a mode of the quantum photon field, and the electron is a mode of the quantum electron field.

But if it quacks like a single particle… why not think of it as a single particle? If it leaves tracks like this: bubble chamber tracks, I don’t see the problem with treating it as a particle. Hence particle physicists call what they’re doing particle physics.

I agree that other times it quacks like a wave, and then it’s easier to think about it as a wave.

But if you insist on using the full quantum field theory formalism (where there is no distinction between particle and wave) all the time, then you make your life difficult for no reason, and probably lose a lot of intuition along the way.