Peter wrote:
It is still really amazing that this can work.
It can’t. It’s not the first time this has been in the news. There have been a few experiments giving uncertain data which could by some stretch of imagination be interpreted as thrust being produced.
There are any number of ways that even honest errors can creep into the measurement of such very small effects.
The general theory of relativity has held up very well for over 100 years. GPS as it is implemented wouldn’t work if GR was wrong. Until there is positive evidence to the contrary, I see this EmDrive thing as pure wishful thinking.
LeSving wrote:
The problem is of course that photons (or electromagnetic waves) have no mass. They are pure energy and cannot possible move an object (with mass) by hitting it or radiating from it.
Certainly they have mass! They don’t have rest mass, but that’s irrelevant as they are always moving at the speed of light and thus have relativistic mass. Radiation can exert pressure on objects and this has also been demonstrated to work in practice.
Never heard of such a thing. Solar winds are ions (particles with mass), and solar sails could utilize these “winds” through basic physics. Relativistic mass is of no use moving objects.
Photons have no mass, but momentum. This can generate thrust or reactive force. Solar radiation pressure is routinely used to steer satellites by adjusting the angle of their solar panels.
About 100 years ago, rocket science pioneer Eugen Sänger proposed the “photon rocket” as a means of achieving high velocities in space. This is still a valid concept, although nobody has yet found a way to implement it in actual hardware: https://en.wikipedia.org/wiki/Photon_rocket
what_next wrote:
Photons have no mass, but momentum. This can generate thrust or reactive force
How? It can be used to increase the energy in atoms, change electron quantum states, but how can that be used to create thrust?
what_next wrote:
olar radiation pressure is routinely used to steer satellites by adjusting the angle of their solar panels.
These are ions, not photons. Totally different concept altogether. Ions are ordinary mass.
These are ions, not photons.
No. Look here: https://en.wikipedia.org/wiki/Solar_sail
(In my previous life before becoming a pilot I had specialised in the “space” bit of aerospace technology. However, I have never done own research work in the field of electrical propulsion, they only taught me the basics.)
That article is full of misconceptions.
The solar wind.
The solar wind is a stream of energized, charged particles, primarily electrons and protons, flowing outward from the Sun, through the solar system at speeds as high as 900 km/s and at a temperature of 1 million degrees (Celsius). It is made of plasma.
Photons most certainly do have momentum, of magnitude E/c where E is their energy and c is the speed of light.
Another expression for it is h / wavelength where h is Plank’s constant.
As a vector, we usually write p = h-slash k
where h-slash is Plank’s constant divided by 2 pi, and k is the wave vector.
The first few pages of any book on Quantum Mechanics will give more details, usually with a discussion of Compton Scattering
LeSving wrote:
These are ions, not photons. Totally different concept altogether. Ions are ordinary mass.
Did you read the linked Wikipedia article on radiation pressure?
According to Maxwell’s theory of electromagnetism, an electromagnetic wave carries momentum, which can be transferred to a reflecting or absorbing surface hit by the wave.
