Could you use contained light as a form of propulsion?

[Zade]

I was reading about how an astronaut came up with an idea to change the trajectory of meteors by bringing a spacecraft close to them where it's gravitational field would eventually move it slightly. It made me wonder if you couldn't use light like that.

So say (hypothetically of course) you you contained a large amount of light within a mirrored sphere but on the outside the weight wasn't distributed equally. In space would it move?

 

[bcrowell]

No. That would violate conservation of momentum.

 

[NoDoubt]

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.

 

[berkeman]

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.

That would be in post #2

 

[VanOosten]

No. That would violate conservation of momentum.

how so, light has momentum so it bouncing off the mirror would transfer some momentum to the sphere, no?

 

[berkeman]

how so, light has momentum so it bouncing off the mirror would transfer some momentum to the sphere, no?

The sphere and the light inside it are an isolated system. It can sit there and wiggle back and forth, but its center of mass isn't going to move. So no propulsion. You need to eject something (like rocket exhaust) to cause the ship to move...

 

[NoDoubt]

how so, light has momentum so it bouncing off the mirror would transfer some momentum to the sphere, no?

If this were the case, then Sun would have pushed the Earth away long time ago, So no,

 

[VanOosten]

The sphere and the light inside it are an isolated system. It can sit there and wiggle back and forth, but its center of mass isn't going to move. So no propulsion. You need to eject something (like rocket exhaust) to cause the ship to move...

right i don't believe it could be used as propulsion but there is movement of the sphere in space without breaking conservation laws

If this were the case, then Sun would have pushed the Earth away long time ago, So no,

the sun does put a force on the Earth but its so small we don't feel it...

 

[PAllen]

If you had perfectly reflecting sphere around a terajoule flashlamp that flashed, and you brought it to outer space, and then opened a hole in it, it should move - just as if it contained air.

 

[NoDoubt]

If you had perfectly reflecting sphere around a terajoule flashlamp that flashed, and you brought it to outer space, and then opened a hole in it, it should move - just as if it contained air.

You gota be kidding, right? if not, then we will let the flashlamp glow for some time, until sphere gets packed with a kilogram of photons, then open the the hole, it should shoot like a rocket" lol.

 

[Zade]

I figured the weighted side would be attracted to the light's gravitational field at the center of the sphere so it would move toward it. But since the light would still remain at the center it would still be attracted. I know it would violate the laws of conservation and therefore won't work, I'm just wondering why.

As far as I know, Light cannot be contained, So my answer is No

I always thought saying something was hypothetical meant you didn't have to worry about if it was possible, practical, known yet, ect?... I keep on running into this with math experts. What is the correct way to ask someone to disregard something in favor of considering something else?

 

[bcrowell]

You gota be kidding, right? if not, then we will let the flashlamp glow for some time, until sphere gets packed with a kilogram of photons, then open the the hole, it should shoot like a rocket" lol.

Are you saying that his analysis is wrong, or that it's just clearly impractical? I think his analysis is right, but it's clearly impractical.

 

[NoDoubt]

Are you saying that his analysis is wrong, or that it's just clearly impractical? I think his analysis is right, but it's clearly impractical.

I ment it's impractial, because he said "it should move - just as if it contained air." And because of the following reasons:
Perfect sphere?
Photons have no mass.

I,m skeptical about the analysis, because photons put no preasure on the light source while accelerating to c, This is the only reason for the constant speed of light, If there was any kind preasure on the source, then that preasure will vary with the motion of the source, and so will vary the speed of light.

So in keeping the above reason in mind, trillions and trillions of photons can leave the sphere, but sphere won't buzz.

 

[NoDoubt]

Are you saying that his analysis is wrong, or that it's just clearly impractical? I think his analysis is right, but it's clearly impractical.

I ment it's impractial, because he said "it should move - just as if it contained air." And because of the following reasons:
Perfect sphere?
Photons have no mass.

I,m skeptical about the analysis, because photons put no preasure on the light source while accelerating to c, This is the only reason for the constant speed of light, If there was any kind preasure on the source, then that preasure will vary with the motion of the source, and so will vary the speed of light.

So in keeping the above reason in mind, trillions and trillions of photons can leave the sphere, sphere won't buzz.

 

[bcrowell]

Photons have no mass. [...] I,m skeptical about the analysis, because photons put no preasure on the light source while accelerating to c, This is the only reason for the constant speed of light, If there was any kind preasure on the source, then that preasure will vary with the motion of the source, and so will vary the speed of light.

http://en.wikipedia.org/wiki/Momentum#Momentum_in_electromagnetism

 

[1MileCrash]

Now I'm a bit confused. The existence of radiation pressure is certain, and light certainly carries momentum in this form, but does the light source "kick back" in the same way that a firearm "kicks back" when shooting a bullet with momentum?

Because that's what I'm getting from this. I'm not skeptical at all, I've just never thought about it.

 

[D H]

I,m skeptical about the analysis, because photons put no preasure on the light source while accelerating to c, This is the only reason for the constant speed of light, If there was any kind preasure on the source, then that preasure will vary with the motion of the source, and so will vary the speed of light.

The only reason for the constant speed of light is that photons are massless.

Just because they are massless does not mean they do not have momentum. They do. Pulsing that terajoule flashlamp results in a force on the flashlamp opposite the direction of the beam. You don't need a sphere to contain the photons. (Good luck with that!) You just need a hefty power source to power that flashlamp.

How powerful? The energy and momentum of a photon are related by E=pc, or p=E/c. Multiply this by the number of photons emitted per second and you get F=P/c, where F is the reaction force and P is the power of the photon stream. Flashing your terajoule flashlamp twice per second requires more energy than the entire world's electric energy production (and that is ignoring energy losses). The amount of thrust from flashing your flashlamp twice per second? 6,670 Newtons. Compare that to the 66,700 Newtons produced by a single RL10 rocket (which is rather old technology).

 

[1MileCrash]

Pulsing that terajoule flashlamp results in a force on the flashlamp opposite the direction of the beam.

That answers my question!

 

[NoDoubt]

The only reason for the constant speed of light is that photons are massless.

Just because they are massless does not mean they do not have momentum. They do. Pulsing that terajoule flashlamp results in a force on the flashlamp opposite the direction of the beam. You don't need a sphere to contain the photons. (Good luck with that!) You just need a hefty power source to power that flashlamp.

How powerful? The energy and momentum of a photon are related by E=pc, or p=E/c. Multiply this by the number of photons emitted per second and you get F=P/c, where F is the reaction force and P is the power of the photon stream. Flashing your terajoule flashlamp twice per second requires more energy than the entire world's electric energy production (and that is ignoring energy losses). The amount of thrust from flashing your flashlamp twice per second? 6,670 Newtons. Compare that to the 66,700 Newtons produced by a single RL10 rocket (which is rather old technology).

Good explanation! but it leads to more questions.From this one should be able to calculate the speed of flashlight if it is left powered on (And there is no shortage of energy)
It should reach half the speed of light at one point, because it is being propled by photons. And then photons will also be traveling half of there actual speed. Am I right? If not what am I missing?

Oh yes, being electronics guy, I can create a voltage regulator for this flashlight that will serve the purpose of gas paddle on a car.
If we feel the need for speed, Just crank the knob, it will increase the brightness and result will be increase in speed. How about that?

 

[Drakkith]

I don't know about your specific example, but they do have solar sails.

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

 

[NoDoubt]

I don't know about your specific example, but they do have solar sails.

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

Good example though, but propulsion tests are not very successfull as of writing this post, And who knows later down the road they will find out that it was being drifted because of something unknown today.

 

[bcrowell]

It should reach half the speed of light at one point, because it is being propled by photons. And then photons will also be traveling half of there actual speed. Am I right? If not what am I missing?

http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_theory_of_relativity

 

[NoDoubt]

http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_theory_of_relativity

Well Bcrowell, You always point me to 100 year old formulas and theories, Of course, if we keep reading these old theories and formulas, it's not gona yield any thing, we need come up with something new, something different, We have to come up with our own ideas, only and only then we will be able to understand/invent better.

There is a simple question out there that still need to be understud/explained (Atleast to me), I have read many wiki pages and went through hundreds of fourms, Nothing seemed to have helped, every artical or fourm just create more questions than it solves.

Why the speed of light is always c to all the observers regardless of there speed/direction etc. etc? If you have the answer, please bring it on, Will really appreciate that.

 

[bcrowell]

Why the speed of light is always c to all the observers regardless of there speed/direction etc. etc? If you have the answer, please bring it on, Will really appreciate that.

FAQ: Why is the speed of light the same in all frames of reference?

The first thing to worry about here is that when you ask someone for a satisfying answer to a "why" question, you have to define what you think would be satisfying. If you ask Euclid why the Pythagorean theorem is true, he'll show you a proof based on his five postulates. But it's also possible to form a logically equivalent system by replacing his parallel postulate with one that asserts the Pythagorean theorem to be true; in this case, we would say that the reason the "parallel theorem" is true is that we can prove it based on the "Pythagorean postulate."

Einstein's original 1905 postulates for special relativity went like this:

P1 - "The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion."

P2 - "Any ray of light moves in the 'stationary' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body."

From the modern point of view, it was a mistake for Einstein to single out light for special treatment, and we imagine that the mistake was made because in 1905 the electromagnetic field was the only known fundamental field. Really, relativity is about space and time, not light. We could therefore replace P2 with:

P2* - "There exists a velocity c such that when something has that velocity, all observers agree on it."

And finally, there are completely different systems of axioms that are logically equivalent to Einstein's, and that do not take the frame-independence of c as a postulate (Ignatowsky 1911, Rindler 1979, Pal 2003). These systems take the symmetry properties of spacetime as their basic assumptions.

For someone who likes axioms P1+P2, the frame-independence of the speed of light is a postulate, so it can't be proved. The reason we pick it as a postulate is that it appears to be true based on observations such as the Michelson-Morley experiment.

If we prefer P1+P2* instead, then we actually don't know whether the speed of light is frame-independent. What we do know is that the empirical upper bound on the mass of the photon is extremely small (Lakes 1998), and we can prove that massless particles must move at the universal velocity c.

In the symmetry-based systems, the existence of a universal velocity c is proved rather than assumed, and the behavior of photons is related empirically to c in the same way as for P1+P2*. We then have a satisfying answer to the "why" question, which is that existence of a universal speed c is a property of spacetime that must exist because spacetime has certain other properties.

W.v.Ignatowsky, Phys. Zeits. 11 (1911) 972

Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51

Palash B. Pal, "Nothing but Relativity," http://arxiv.org/abs/physics/0302045v1

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html
[edit]

 

[PAllen]

The only reason for the constant speed of light is that photons are massless.

Just because they are massless does not mean they do not have momentum. They do. Pulsing that terajoule flashlamp results in a force on the flashlamp opposite the direction of the beam. You don't need a sphere to contain the photons. (Good luck with that!) You just need a hefty power source to power that flashlamp.

How powerful? The energy and momentum of a photon are related by E=pc, or p=E/c. Multiply this by the number of photons emitted per second and you get F=P/c, where F is the reaction force and P is the power of the photon stream. Flashing your terajoule flashlamp twice per second requires more energy than the entire world's electric energy production (and that is ignoring energy losses). The amount of thrust from flashing your flashlamp twice per second? 6,670 Newtons. Compare that to the 66,700 Newtons produced by a single RL10 rocket (which is rather old technology).

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]

Well Bcrowell, You always point me to 100 year old formulas and theories, Of course, if we keep reading these old theories and formulas, it's not gona yield any thing, we need come up with something new, something different, We have to come up with our own ideas, only and only then we will be able to understand/invent better.

There is no reason to discard old formulas until they conflict with data. Formulas based on Newton's gravity are still very heavily used - whenever their error is acceptable (400 year old formulas). People do experiments all the time to look for violations of accepted laws, but no clear violations of SR have yet been found. Unless that happens, these formulas are as good now as they were a 100 years ago. Even if some SR violating theory proves true (there are quantum gravity approaches that predict SR violations), these formulas would remain accurate over a huge range of energy and distance.

 

[NoDoubt]

FAQ: Why is the speed of light the same in all frames of reference?

The first thing to worry about here is that when you ask someone for a satisfying answer to a "why" question, you have to define what you think would be satisfying. If you ask Euclid why the Pythagorean theorem is true, he'll show you a proof based on his five postulates. But it's also possible to form a logically equivalent system by replacing his parallel postulate with one that asserts the Pythagorean theorem to be true; in this case, we would say that the reason the "parallel theorem" is true is that we can prove it based on the "Pythagorean postulate."

Einstein's original 1905 postulates for special relativity went like this:

P1 - "The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion."

P2 - "Any ray of light moves in the 'stationary' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body."

From the modern point of view, it was a mistake for Einstein to single out light for special treatment, and we imagine that the mistake was made because in 1905 the electromagnetic field was the only known fundamental field. Really, relativity is about space and time, not light. We could therefore replace P2 with:

P2* - "There exists a velocity c such that when something has that velocity, all observers agree on it."

And finally, there are completely different systems of axioms that are logically equivalent to Einstein's, and that do not take the frame-independence of c as a postulate (Ignatowsky 1911, Rindler 1979, Pal 2003). These systems take the symmetry properties of spacetime as their basic assumptions.

For someone who likes axioms P1+P2, the frame-independence of the speed of light is a postulate, so it can't be proved. The reason we pick it as a postulate is that it appears to be true based on observations such as the Michelson-Morley experiment.

If we prefer P1+P2* instead, then we actually don't know whether the speed of light is frame-independent. What we do know is that the empirical upper bound on the mass of the photon is extremely small (Lakes 1998), and we can prove that massless particles must move at the universal velocity c.

In the symmetry-based systems, the existence of a universal velocity c is proved rather than assumed, and the behavior of photons is related empirically to c in the same way as for P1+P2*. We then have a satisfying answer to the "why" question, which is that existence of a universal speed c is a property of spacetime that must exist because spacetime has certain other properties.

W.v.Ignatowsky, Phys. Zeits. 11 (1911) 972

Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51

Palash B. Pal, "Nothing but Relativity," http://arxiv.org/abs/physics/0302045v1

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html
[edit]

Thank you, It look like I have to do lot of reading. So I'm on it...

 

[NoDoubt]

There is no reason to discard old formulas until they conflict with data. Formulas based on Newton's gravity are still very heavily used - whenever their error is acceptable (400 year old formulas). People do experiments all the time to look for violations of accepted laws, but no clear violations of SR have yet been found. Unless that happens, these formulas are as good now as they were a 100 years ago. Even if some SR violating theory proves true (there are quantum gravity approaches that predict SR violations), these formulas would remain accurate over a huge range of energy and distance.

Thank you for this piece of advice. I'll keep it in mind next time.

 

[Zade]

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)?

 

[PAllen]

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)?

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.

 

[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?