Could you use contained light as a form of propulsion?

[PAllen]

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.

If this photon rocked consumed 1/2 gm per second each, of electrons and positrons, it would produce 300,000 Newton's; quite respectable. Just a few practical problems to solve ;) .

[Edit] To mention just one of those practical problems (besides acquiring and containing the positrons), the proposed reaction rate would be equivalent to detonating a couple of Hiroshima scale bombs / second.

 

[Zade]

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.

Yeah, I know it's impossible... it's just... I don't get why. Ohh well, **** it.

 

[Nakisima]

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.

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]

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.

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.

 

[NoDoubt]

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.

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]

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?

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]

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.

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,

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?

or will the sphere explode? like a bottle with a small hole in it, and water rushing into it with high preasure.

 

[JesseM]

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,

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%...

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?

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.

or will the sphere explode? like a bottle with a small hole in it, and water rushing into it with high preasure.

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.

*According to the equation here the intensity of sunlight as a function of radius R is given by 3.2x10^25(1/R^2) (watts/m^2), and the radius at the surface is about 6.955 * 10^8 meters, so at the surface of the Sun the intensity would be about 6.6 * 10^7 watts/square meter, or 6.6*10^7 joules of energy per second on each square meter, with 1 joule = 1 kg*m^2/s^2. This is a lot of energy, but for photons the momentum is given by E/c with c=299792458 m/s, so this only works out to about 0.22 kg * m/s of momentum change per second for each square meter of surface area. So if you were exposed to light this intense in deep space with no gravity (so the light was the only force on you), then if your body has 0.7 m^2 of surface area exposed to the light source and absorbs all the photons hitting it, and you weigh 70 kg, then you will only be accelerated at a rate of 0.22*0.7/70=0.0022 m/s per second, so for example after 60 seconds your speed has only changed by 0.132 m/s, a change of a little under 0.3 miles/hour (if you instead reflected all the incoming photons, this would only double the radiation pressure according to this page).

 

[NoDoubt]

Thanks Jesse;
The part I was missing was:

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

I think I have figured it out now,

If we have a sphere of radius .5 metre, with reflective surface of 99.9% and light source capable of producing daylight inside this sphere, then:

Number of photons hitting the surface per second = 1.2 * 10^22
Number of photons getting absorbed per second = 3.6 * 10^23

Light source needed to crack open the sphere = 30 times brighter than a sunny day.

I'm not very good in math, may have miscounted some zeros, :-)

 

[Phrak]

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)?

Yes, the thought experiment helped clarify things immensely.
The light in the container is no longer uniform in the presence of the external massive object off to one side, but is blue shifted on the side of the container closest to the mass. The light energy is proportional to the frequency. This makes the energy density of the light larger on the side with the massive object.

As light bounces off the interior surface, it exchanges momentum with container wall. Without the external mass, there is no net change in momentum to the container. But with the addition of the external mass, the blue shifted light exchanges more momentum with the container wall closest to the obect. The change in momentum is also proportional to the frequency. So it is not necessary that the container be at all massive in order for the contained light to be attracted to the massive object.