Comment by gus_massa | Hacker News Reader

Comment by gus_massa | Hacker News Reader

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All the information about this is very confusing, and in some press report it looks like a perpetual motion machine. I’ll try to explain what I had understood trying to find an explanations that don’t break the physics laws.

You must use some particles to carry the momentum. It’s not clear, but the main candidates are photons (aka light). The quantization of the electromagnetic field is the cause of the Casimir effect, and the name of the particles that arrays due to the quantization of the electromagnetic is photons. Some other explanations talk about a moving mirror, and an oscillating mirror produces photons.

The number of photons is not conserved, so they can be created or destroyed “out of thin air”. (Really you need another particle like and electron to carry the difference of energy and momentum. The electron then bounce against the ship to accelerate it forwards, and picks a little of energy to compensate the energy that used to create the photon. It’s plenty of electrons, and you can reuse them.)

So this is equivalent to pointing a laser backwards, so the spaceships accelerate forwards. The discussion should be about efficiency. Is this more efficient than a laser?

(Well, it’s not necessary to use a laser, a simple light bulb with a mirror that reflect the light that tries to go forward will work. If it’s not directional you will lost a lot of energy because the light that goes to the sides doesn’t accelerate the ship (I don’t want to do the integrals now, but I guest 1/3 of efficiency.) With a more directional setting, like the mirrors in a flashlight the efficiency will improve.)

This is inconsistent with the claimed "specific force performance of 0.1N/kW". The ratio a photon's momentum to energy is fixed at 1/c, which is 3.3E-6 N/kW.

Which is why using the vacuum fluctuations makes sense – these are particle/antiparticle pairs created randomly in the vacuum. Since these do have finite masses, you can accelerate them to larger momenta than would be possible with photons (which can also carry arbitrarily large momenta ℏω, but creating these high-frequency photons is difficult).

That doesn't solve anything, it only shifts the violation somewhere else? Of course you can't keep the virtual particles, that's a net creation of mass-energy.

"which can also carry arbitrarily large momenta ℏω,"

With proportionally large energy. That's the concern here, not photon count.

(For anyone confused, it's ℏω/c in conventional units. Some unit systems define c=1 for convenience)

See: https://en.wikipedia.org/wiki/Woodward_effect#Propellantless...

The claim appears to be that the momentum is transferred first to the local stress-energy tensor, and then to celestial bodies. It almost sounds feasible at a glance, but, well, it's quantum gravity.

I would say it certainly seems like there's something to be learnt by this, though. A quantum effect with measurable gravitational consequences is... almost unheard of.

There is not a complete Quantum Gravity Theory, so all what I will say are only educated guess, supposing that the final theory will mix gracefully with the current theories for the other forces.

> [...] thereby producing a propulsive force thereon without having to expel propellant from the object. [...] Local momentum conservation is preserved by the flux of momentum in the gravity field that is chiefly exchanged with the distant matter in the universe.

The gravity field should be quantized, and the associated particles are the gravitons. So in this effect they are using a jet of gravitons instead of a jet of photons (like in my example) or a jet of atoms/ions (like in a conventional rocket).

The exact quote can be used to describe how the “laser motor” works:

(modified quote) [...] thereby producing a propulsive force thereon “without” having to expel “propellant” from the object. [...] Local momentum conservation is preserved by the flux of momentum in the electromagnetic field that is chiefly exchanged with the distant matter in the universe.

The correctness of this phrase depends on if you count the photons/gravitons as propellant or not.

The graviton shold be massless, because the gravity force has unlimited range. The relation of the energy-momentum in a graviton should be a cuadrivector, or in more simple terms, the formula in my other comment should be valid, with a “rest mass” m = 0.

So the formula can be simplified and you get the same p = E/c relation of the photons, with is very small as throwaway_yy2Di noted.

The generalization of the well known E=mc^2 equation is E^2 = p^2 c^2 + m^2 c^4 (where p is the momentum and m is the “rest mass”).

So: p = Sqrt( (E/c)^2 - (mc)^2 )

Therefore, for massive particles, you get less moment for the same energy. (An easy way to get a general idea this without calculation is that part of the energy goes to create the mass, and the other part goes to create the moment. But I prefer calculations.)

So the best Moment/Energy ratio is for massless particles, like the photon. And the photon is the easiest particle to create and direct.

But as throwaway_yy2Di said, the ratio with photons is 30000 smaller than the ratio they claim.

So the only consistent explanation is that they are measuring something in a wrong way.

To clarify an important point: this is including rest mass as part of the energy budget, as E=mc^2. In most discussions the conclusion is the opposite: you get vastly higher moment/energy for massive particles, ordinary molecules like hydrogen, because engineers aren't counting the energy required to create hydrogen from nothing -- only the energy to accelerate it! The moment/energy of light is actually terribly low for practical purposes.

Yes, I should have stated that clearly.

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