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Physics/Solar sails and light bulbs


QUESTION: Dear Mr. Nelson,      I have a question concerning solar sails. If a solar sail was between solar systems where there was not any concentrated light from a sun, and on the solar sail there were light bulbs held by an arm a certain distance from the sail but attached to the sail. If these light bulbs where lit up and shinning on the sail and let's say the bulbs were getting power from a nuclear reactor ; would the sail move just like the sun were shinning on them and react just like a radiometer would in a vacuum?

ANSWER: First, let me address the radiometer comparison.  Radiometers don't actually work by reflection of light.  They're actually quite complicated.  A full discussion of how they work may be found here:  The common misconception is that they actually work on the pressure exerted by photons, but that effect is far too weak.

To your direct question, any directional light source (what you're describing is essentially an enormous flashlight, when you think about it) will exert a force on its source and conserve momentum.  The momentum of photons is very weak, and in this case you'd be better off with powerful lasers than bothering with the unnecessary solar sail, but as a concept question it does work this way.  There's no net thrust on the light bulbs, assuming they are omnidirectional, but the physics of the solar sail remains unchanged.  This is a horribly inefficient way to get thrust, though.  You've far better off using the energy from the reactor to ionize and expel some minimal amount of matter with mass, and you're still better off with lasers (much smaller and lighter than what you propose) than light bulbs and a solar sail.

---------- FOLLOW-UP ----------

QUESTION: Dear Mr. Nelson,      I recently asked a question concerning wether light bulbs could be used for solar sails.  My new question is if you could change the light bulbs to magnetrons which emit microwaves , and have the magnetrons emit in all directions microwaves like the light bulbs emitting in all directions with light, would you produce any force on the sails? The magnetrons would be attached to the solar sail just like the light bulbs with an arm extending out a certain distance. Would this also be like the microwave engine that NASA is testing right now, only in this case only one surface is being struck by microwave radiation?

So you know, I remember your original question, but the system also posts the original above follow-ups when you ask them here.

The microwave system you propose is exactly like the system you proposed with light bulbs, just operating at different wavelengths.  The amount of momentum in a beam of light depends directly on the amount of energy in that beam, no matter the wavelength (p=E/c).  So it would work exactly the same way.

It would not work the same as claims of propellantless propulsion (still not a confirmed thing, and I doubt that it works).  Those devices are supposed to bounce microwaves around inside an asymmetric resonant cavity and just produce thrust with no photon emission.  That violates conservation of momentum.  On top of that, the forces they're trying to measure are 1) really tiny and 2) very easily produced by other effects.  I've seen no attempt to eliminate external electric and magnetic fields from the Earth in these experiments in the couple of papers I read, which would definitely interfere.  I suspect a small external influence creating these effects, since it's way easier to believe than it is to believe these guys actually violated conservation of momentum.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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