The acceleration of mass using light

**myrhinobutt2** · Nov23-09, 09:59 PM

Just curious, is it possible for light to strike an atom in such a way as to accelerate it? I know that the energy can be absorbed in the atom, which would cause it's energy levels to rise, but could the acceleration be controlled and directed?

**russ_watters** · Nov23-09, 10:29 PM

Yes! http://en.wikipedia.org/wiki/Solar_sail

**Eximus** · Nov24-09, 09:15 AM

Slightly, very small pressures. The force exerted by photons from the Sun on the Earth is less than 141 pounds over our entire cross-sectional area.

http://en.wikipedia.org/wiki/Radiation_pressure

**myrhinobutt2** · Nov24-09, 10:31 AM

Would it then be possible to take a laser and point it at an atom and accelerate it to near light speed?

**Eximus** · Nov24-09, 11:04 AM

I am not certain, but intuition tells me this is possible in a vacuum using photons of energies lower than the atom's ionization energy.

**HallsofIvy** · Nov24-09, 12:32 PM

Originally Posted by myrhinobutt2

Would it then be possible to take a laser and point it at an atom and accelerate it to near light speed?

Yes, although you would have to keep that laser point directly at that atom for a very long time while it moves away from you!

**myrhinobutt2** · Nov24-09, 01:11 PM

So the application of accelerating a rocket into space probably wouldn't work.

**f95toli** · Nov24-09, 01:55 PM

Originally Posted by myrhinobutt2

So the application of accelerating a rocket into space probably wouldn't work.

There are quite a few people working on that, you can probably find a few Youtube videos if you search.
It is certainly possible to "lift" small shiny objects using a laser a few meters (maybe a bit more) over the ground if you have a powerful laser.
The problem is of course that you would need an incredibly powerful laser (much, much more powerful than any existing laser, maybe with the exception of some one-shot models) than to lift even a small satellite into orbit. You also have the problem of attenuation/dispersion due to the air.

Hence, there is nothing wrong with the basic physics but it is HIGHLY doubtful that it would ever become a practical way of lifting things into space.

**Redbelly98** · Nov25-09, 12:37 AM

Originally Posted by myrhinobutt2

Would it then be possible to take a laser and point it at an atom and accelerate it to near light speed?

Lasers have already been used to slow atoms down from a few hundred m/s to a near standstill, and accelerating them is possible to.

To near light speed? Just one problem is that, as the laser accelerates, the wavelength it absorbs will change due to the Doppler shift. One would need to compensate for this, either by changing the laser wavelength or doing something to change the atom's absorption wavelength. But, near light speed the required shift would become larger than can be compensated for practically.

**Bob S** · Nov25-09, 11:13 AM

Originally Posted by Redbelly98

Lasers have already been used to slow atoms down from a few hundred m/s to a near standstill, and accelerating them is possible to.

To near light speed? Just one problem is that, as the laser accelerates, the wavelength it absorbs will change due to the Doppler shift. One would need to compensate for this, either by changing the laser wavelength or doing something to change the atom's absorption wavelength. But, near light speed the required shift would become larger than can be compensated for practically.

Very good point. As an atom accelerates away from the light source, the incident light is red-shifted. The light interacts entirely with the electrons via scattering (e.g., Rayleigh scattering), atomic excitation, photoelectric effect, deep core photoemission, Thomson scattering, etc.), and near zero with nuclear scattering. As the light is red shifted, many of these channels are below threshold, and no longer open. So the ability for the target to absorb momentum decreases.
[added] Also the momentum carried by red-shifted photons is less.
Bob S