The discussion centers on the feasibility of using contained light as a form of propulsion, specifically through the concept of a mirrored sphere containing light. Participants unanimously conclude that this approach violates the conservation of momentum, as the sphere and light form an isolated system that cannot generate propulsion without ejecting mass. The idea of using a terajoule flashlamp within a perfectly reflecting sphere is debated, with some arguing that while photons carry momentum, they do not provide sufficient force to propel the sphere. The consensus is that light cannot be contained in a way that would allow for propulsion in space.
PREREQUISITESPhysicists, aerospace engineers, and anyone interested in advanced propulsion concepts and the fundamental principles of light and momentum.
PAllen said:Of course the only reason I proposed the mythical perfectly reflecting sphere was that the thread title is "contained light as a form of propulsion" and I wanted to argue that it was, indeed, possible in principle, if not in practice. Also, while perfectly rigid rods violate physical laws, perfectly reflecting surfaces violate no laws that I know of; they just don't exist (if they do violate some law, please educate me).
If you just want a photon propulsion system, no containment required, and not required to be light, that is much easier (in imagination, anyway). Just have magnetic bottles of positrons and electrons feeding into a reaction chamber that reasonably reflects gamma rays with an output nozzle. Voila, a mostly photon rocket. Fantasy, yes, but it seems consistent with physical laws.
PAllen said:Light has energy and momentum and participates in conservation of both; contained, it would also gravitate. If really contained, I think it would act the same as a mass of E/c^2, though I think there are some subtle issues here that have been discussed in other threads.
NoDoubt said:As far as I know, Light cannot be contained, So my answer is No, the sphere will not move. But again I could be wrong.
I would like to hear the answer from someone expert.
Even if the light source goes off, light can be trapped for a little while by bouncing it back and forth in a mirrored optical cavity. The time it would be trapped would still be too short to see with your eyes though--this book says that for a 1-meter cavity, a laser inside would decay in intensity by a factor of 1/e (to about 37% of its initial value) in about 30 microseconds, during which time the photons make 5000 trips back and forth between mirrors.Nakisima said:Light can bend because of the gravity of a black hole, I don't know if it actually contains it. Once the light's source is blocked, it stops, simply put.
JesseM said:Even if the light source goes off, light can be trapped for a little while by bouncing it back and forth in a mirrored optical cavity. The time it would be trapped would still be too short to see with your eyes though--this book says that for a 1-meter cavity, a laser inside would decay in intensity by a factor of 1/e (to about 37% of its initial value) in about 30 microseconds, during which time the photons make 5000 trips back and forth between mirrors.
Well, which scenario are you asking about? If the container containing the sound or light waves is closed, then conservation of momentum applies in both cases, the center of mass of the container + waves inside will never move. If there's a hole so that waves can escape in one direction, then by conservation of momentum the container will move a little bit in the opposite direction, though the effect would be small and both the sound and light waves would die out from inside the container quickly unless there's a source continually generating new waves.NoDoubt said:You mean, like sound waves? (echo in room), So sound source can also be used instead light in a sphere. Right? Or will it fail, because sound waves travel in all directions?
JesseM said:Well, which scenario are you asking about? If the container containing the sound or light waves is closed, then conservation of momentum applies in both cases, the center of mass of the container + waves inside will never move. If there's a hole so that waves can escape in one direction, then by conservation of momentum the container will move a little bit in the opposite direction, though the effect would be small and both the sound and light waves would die out from inside the container quickly unless there's a source continually generating new waves.
That's actually a very low reflective index, even if only 1% of photons are absorbed each time they hit the sides, they are bouncing back and forth very frequently since they move at light speed (unless you're talking about a sphere light-years across or something). The book I linked to earlier was discussing mirrors with a reflectivity of 99.99%...NoDoubt said:Thanks, I got my answer, I was talking about the OP's scenario. (Sphere with a hole)
I know there is no perfect sphere ever made to trap the light.
Let us say reflective index of a closed sphere is 99%. only 1% of the light is being absorbed,
Total number of photons shouldn't matter, if the reflectivity of the sphere is 99% then each time the photons bounce back and forth between walls, 1% of the remaining photons should be absorbed.NoDoubt said:Now if the light source inside the sphere suddenly increases it's brightness to extreame level. what will happen? Will 1% impurity in the sphere be enough to absorb all the extra photons?
Light pressure from the kind of light sources we have on Earth is extremely weak. And even if you could withstand the intensity of light that would be seen by someone right next to the surface of the Sun, the pressure on a human-sized object would still be pretty weak!* I don't know what you mean when you say the light source increases to an "extreme level" but unless the source is absurdly powerful (or the material making up the sphere absurdly thin) it's not going to be enough to break it apart.NoDoubt said:or will the sphere explode? like a bottle with a small hole in it, and water rushing into it with high preasure.
JesseM said:even if only 1% of photons are absorbed each time they hit the sides, they are bouncing back and forth very frequently since they move at light speed
Yes, the thought experiment helped clarify things immensely.Zade said:Perhaps I should have referred to this as a thought experiment?
I don't see how the law of conservation of momentum applies to what I said because it refers to regular matter. Matters attraction would bring them together but how could confined light work the same? It seems to me like only that matter on the outside would move directly toward the center of gravity, which would move the sphere, which, I assume, would move the photons... Does it really still apply? Why wouldn't it work like the gravitation tractor thing I was talking about (I found it http://en.wikipedia.org/wiki/Gravitational_tractor)?