Electromagnetic Momentum

[Vitul]

Hello,

 

Considering that lasers can create significant radiation pressure, what are some applications you propose for utilizing the radiation pressure of electromagnetic waves?

 

Also, what are your opinions on the Abraham-Minkowski controversy?

[swansont]

One resounding success of radiation pressure is the laser cooling of atoms. By tuning lasers below the transition frequency you can get scattering that is velocity-dependent and whose rate increases for atoms moving toward the laser. The momentum imparted by the laser is in one direction, and the momentum from the photons emitted by the atoms is symmetric, so it averages to zero. Thus, there's a net force.

 

By using multiple lasers, you can ensure the atom feels a slowing force no matter which direction it travels.

 

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

[Vitul]

I was reading up on optical tweezers, and I found them very fascinating!

 

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

 

Do you think reactionless drives are a possibility in the near future?

[swansont]

Do you think reactionless drives are a possibility in the near future?

That's a completely separate issue, but no. There's no reason to think that conservation of momentum can be violated when it's supposed to apply.

[Vitul]

That's a completely separate issue, but no. There's no reason to think that conservation of momentum can be violated when it's supposed to apply.

 

Going back to laser cooling, have we done any experiments to show that the recoil from photon emission is significantly lower than the original momentum (in the direction that makes Doppler cooling possible)?

[swansont]

Going back to laser cooling, have we done any experiments to show that the recoil from photon emission is significantly lower than the original momentum (in the direction that makes Doppler cooling possible)?

The momentum is basically the same — the frequency is only shifted by the Doppler effect of one absorption, which is small.

[Vitul]

The momentum is basically the same — the frequency is only shifted by the Doppler effect of one absorption, which is small.

If the momentum is indeed the same, how does cooling work? I assumed that the decrease in momentum would reduce the overall kinetic energy of the atom, which is a decrease of temperature.

[swansont]

If the momentum is indeed the same, how does cooling work? I assumed that the decrease in momentum would reduce the overall kinetic energy of the atom, which is a decrease of temperature.

 

 

Momentum is a vector. All of the laser impulses come from the same direction. The emission is symmetric; for every photon emitted in one direction, on average there will be another in the opposite direction one to cancel it. So the net momentum change from emission is zero. Thus the atoms slow down.

[Enthalpy]

Do you think reactionless drives are a possibility in the near future?

A push by light isn't reactionless. Photons carry momentum which is transferred to the absorber or reflector.

But it does bring the advantage of a push created without expelling material carried in the spacecraft.

 

Solar sails work that way. Used for decades to stabilize satellites, being demonstrated for the propulsion. Huge potential, big challenges, feasible.

 

If the craft carries the energy source to produce light it looks bad, because of the energy needs, including with fission or, if it works some day, fusion.

 

Sending light to the craft, whether man-made or concentrated from the Sun, seems impossible with present technology, until someone has a better idea (I mean: numerically provable).