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Solar Sails - may not work
- Thread starter GregLocock
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- #22
GregLocock
Automotive
I give up.
Cheers
Greg Locock
Cheers
Greg Locock
Anyways, the photons do not have exactly the same frequency before and after. Reflection is an absorbtion/reemission process, so the the emitted photons will be Doppler shifted.- Thread starter
- #24
GregLocock
Automotive
Um, so a stationary sail won't get accelerated?
Cheers
Greg Locock
Cheers
Greg Locock
Yes, a stationary sail will be accelerated. The Doppler shift I'm talking about is the incremental one from the velocity picked up just after an absorbtion.
Another way of looking at this is that all of a photon's momentum can be an absorbed by a massive object. Its energy, however, cannot all be turned into the kinetic energy. The energy-momentum dispersion relations are fundamentally different for massive and massless (photons) particles. The excess energy is put into atomic orbitals and such. These excited orbitals are unstable, so they will tend to radiate (this is reflection).
Of course the emitted photons must carry momentum as well as energy, so the emitting atom must be further accelerated in response to this momentum "creation." This takes energy though. Not all of the orbital energy goes into a photon. Some goes into the photon, and the rest goes into the atom's kinetic energy. All these processes nicely balance.
If you look at things in the reference frame of the atom, the exchanges are a bit different, but you can convert using the Doppler effect. The external inertial frame is the one you really care about anyways.
Another way of looking at this is that all of a photon's momentum can be an absorbed by a massive object. Its energy, however, cannot all be turned into the kinetic energy. The energy-momentum dispersion relations are fundamentally different for massive and massless (photons) particles. The excess energy is put into atomic orbitals and such. These excited orbitals are unstable, so they will tend to radiate (this is reflection).
Of course the emitted photons must carry momentum as well as energy, so the emitting atom must be further accelerated in response to this momentum "creation." This takes energy though. Not all of the orbital energy goes into a photon. Some goes into the photon, and the rest goes into the atom's kinetic energy. All these processes nicely balance.
If you look at things in the reference frame of the atom, the exchanges are a bit different, but you can convert using the Doppler effect. The external inertial frame is the one you really care about anyways.
CusterWilson
Aerospace
You all need to read Gregory Landis on this.
And Gold IS senile -- but brilliant. Note his claim that ALL oil wells are in Mountains, which he forced Scientific American to treat opponents badly for.
Re the Light/heat you will find that heat energy from the Sun is about 3000 times the momentum transfer used for Sails -- thus every loss through Redshift barely affects the total energy. Sorry for the vagueness, but you can easliy see it when you look at the Halley Comet Solar Sail & Solar-electric versions: the Sail makes the Solar wing look TINY.
And Gold IS senile -- but brilliant. Note his claim that ALL oil wells are in Mountains, which he forced Scientific American to treat opponents badly for.
Re the Light/heat you will find that heat energy from the Sun is about 3000 times the momentum transfer used for Sails -- thus every loss through Redshift barely affects the total energy. Sorry for the vagueness, but you can easliy see it when you look at the Halley Comet Solar Sail & Solar-electric versions: the Sail makes the Solar wing look TINY.
Colvindesign
Automotive
Not an engineer, but has anyone seen the experiment where a saucer shaped reflective disc is launched into the air by a high powered laser? While the height it reached was only like 20-30 ft, it was from ground level.
Anywho, are there any estimates on acceleration?
Probability of tears or fragmenting that would make it less effective?
Anywho, are there any estimates on acceleration?
Probability of tears or fragmenting that would make it less effective?
The bottom line is radiation pressure (solar or otherwise) has been observed in accordance with the current theory in many ways, many times. As I said before, when one designs a spacecraft, extra stationkeeping propellant must be added to offset the deltaV from solar radiation pressure.
The more accurate orbit propagators include radiation pressure in them.
Laser cooling via radiation pressure....
The Casimir effect...
Particle physics...
Arises out of maxwell, relativity, and quantum mechanics...
Certainly more...I'm sure it has been measured directly, the measurement of the Casimir force is almost a direct measurement of radiation pressure (albeit, from virtual photons).
If Dr Gold were right it would pretty much trash the last 100 yrs of physics. Or it calls into question the ultimate applicability of thermodynamics (an even older branch of science/engineering)
The more accurate orbit propagators include radiation pressure in them.
Laser cooling via radiation pressure....
The Casimir effect...
Particle physics...
Arises out of maxwell, relativity, and quantum mechanics...
Certainly more...I'm sure it has been measured directly, the measurement of the Casimir force is almost a direct measurement of radiation pressure (albeit, from virtual photons).
If Dr Gold were right it would pretty much trash the last 100 yrs of physics. Or it calls into question the ultimate applicability of thermodynamics (an even older branch of science/engineering)
- Thread starter
- #31
GregLocock
Automotive
stingc seems to be getting close to a believable answer, but I still scratch my head.
1 Proposal
A photon strikes a stationary perfectly reflecting surface, and is reflected at the same frequency.
Therefore it is emitted with the same energy as it arrived with, so no energy transfer can take place with a perfect mirror.
Observations - a mirror does not change the frequency of incident light.
2 Counterproposal
The photon is absorbed momentarily and then re-emitted, at the doppler shifted frequency of the now moving body.
3 Objection
The thrust due to this would be a maximum as the sail approaches the speed of light, since that would maximise the doppler shift
Note that Gold is not saying that sails won't work (depsite my title for this thread), he is saying (roughly) they should be black, not reflective.
That mission should have flown by now, how closely did the acceleration match the model? Was it possible to break out the proportion of the thrust obtained by photon reflection rather than absorption of photons and other particles?
Cheers
Greg Locock
1 Proposal
A photon strikes a stationary perfectly reflecting surface, and is reflected at the same frequency.
Therefore it is emitted with the same energy as it arrived with, so no energy transfer can take place with a perfect mirror.
Observations - a mirror does not change the frequency of incident light.
2 Counterproposal
The photon is absorbed momentarily and then re-emitted, at the doppler shifted frequency of the now moving body.
3 Objection
The thrust due to this would be a maximum as the sail approaches the speed of light, since that would maximise the doppler shift
Note that Gold is not saying that sails won't work (depsite my title for this thread), he is saying (roughly) they should be black, not reflective.
That mission should have flown by now, how closely did the acceleration match the model? Was it possible to break out the proportion of the thrust obtained by photon reflection rather than absorption of photons and other particles?
Cheers
Greg Locock
The Casimir effect has been measured and agrees within the error limits of the experiment. It involves the reflection of virtual photons by conducting sheets. Basically is the radiation pressure from the virtual photons on a conducting surface. They cannot be absorbed by the atom permanently (violation of Heisenberg uncertainty principle; conservation of energy).
QED is the most accurately and precisely verified theory in existence(period!)....by experiments involving radiation pressure from virtual photons.(the precise spin of atomic electron orbitals, etc)
QED is the most accurately and precisely verified theory in existence(period!)....by experiments involving radiation pressure from virtual photons.(the precise spin of atomic electron orbitals, etc)
wolfkeeper
Aerospace
We know solar sailing works anyway- some satellites use it for attitude control, and the size of the effect is consistent with photonic pressure. Solar particles are a much smaller effect it turns out.
btrueblood
Mechanical
Greg wrote a long while back:
"2 Counterproposal
The photon is absorbed momentarily and then re-emitted, at the doppler shifted frequency of the now moving body."
Interestingly, this is the modern view of reflection, at least per what I've read of Quantum Electrodynamics theory. Whether a "reflection" occurs or absorption depends on how long the delay is between absorbtion and re-emission of the photon. See Richard Feinman's books "QED" and "7 Not-so Easy Pieces" for a very readable, layman's-terms description.
The problem with making a sail black is that the emitted radiation (as thermal Stephan-Boltzman law radiation) now leaves from both the "front" and "back" sides of the sail, yielding no net thrust or momentum change.
No doppler shift (red or blue) occurs on a stationary mirror because no velocity change occurs of the mirror relative to the observer.
Radiation or photon pressure does exist, as IRstuff has pointed out in his posts.
Finally, the use of Carnot law is impossible with photons, because they travel at a fixed velocity. Gas kinetics theory states that temperature is related to the average kinetic energy of the gas particles, which is related to the average (or rms) velocity. Since velocity is unchanged, you can't state what the "temperature" of a photon is. You measure the energy content of a photon by its frequency or wavelenth, since its speed is constant. And that is relative, as Einstein pointed out.
Reflective sails do obey the 1st & second laws, on an energy basis, when the freq. shift of the incident radiation is considered. Losses occur within the sail material due to incomplete reflection of incident radiation and resulting heat being generated (and then radiated away). And, since the sail gets its highest energy photons when at rest relative to the source, the thrust of the sail drops as its relative velocity increases away from the source (sun).
Yeah, too bad about the sail mission. Gonna be awhile before somebody spends any more money on it.
"2 Counterproposal
The photon is absorbed momentarily and then re-emitted, at the doppler shifted frequency of the now moving body."
Interestingly, this is the modern view of reflection, at least per what I've read of Quantum Electrodynamics theory. Whether a "reflection" occurs or absorption depends on how long the delay is between absorbtion and re-emission of the photon. See Richard Feinman's books "QED" and "7 Not-so Easy Pieces" for a very readable, layman's-terms description.
The problem with making a sail black is that the emitted radiation (as thermal Stephan-Boltzman law radiation) now leaves from both the "front" and "back" sides of the sail, yielding no net thrust or momentum change.
No doppler shift (red or blue) occurs on a stationary mirror because no velocity change occurs of the mirror relative to the observer.
Radiation or photon pressure does exist, as IRstuff has pointed out in his posts.
Finally, the use of Carnot law is impossible with photons, because they travel at a fixed velocity. Gas kinetics theory states that temperature is related to the average kinetic energy of the gas particles, which is related to the average (or rms) velocity. Since velocity is unchanged, you can't state what the "temperature" of a photon is. You measure the energy content of a photon by its frequency or wavelenth, since its speed is constant. And that is relative, as Einstein pointed out.
Reflective sails do obey the 1st & second laws, on an energy basis, when the freq. shift of the incident radiation is considered. Losses occur within the sail material due to incomplete reflection of incident radiation and resulting heat being generated (and then radiated away). And, since the sail gets its highest energy photons when at rest relative to the source, the thrust of the sail drops as its relative velocity increases away from the source (sun).
Yeah, too bad about the sail mission. Gonna be awhile before somebody spends any more money on it.
Hi folks, I just joined. It was interesting to read this post that has gone on so long. Back in the 1980s I was a volunteer with the World Space Foundation that was trying to launch a solar sail spacecraft, and helped build one of the sail prototypes.
Neither they nor their neighbor in Pasadena, the Planetary Society, seems to have had much success so far. Considering how much effort NASA and others have spent making spacecraft to confirm certain of Einstein's predictions, it sure would be nice to get a proof of concept launch finally done for potentially practical solar sail technology.
Advanced propulsion never seems to get much priority, unfortunately. It took decades before NASA finally flew a solar electric mission, Deep Space 1 [] (which went quite well).
One recent development is Space Services' announcement they are working on a solar sail spacecraft to provide (from a libration point) a space weather observatory and communications relay for Antarctica []. They apparently have a preliminary NOAA contract. This is a really good idea but they don't seem to have much information available at this point so it's not clear if this will come to much.
Solar sails are a great idea, but will have difficulty seeing any practical use until the concept is proved. NASA used to do a lot of engineering proof of concept missions (e.g. Gemini, the Application Technology Satellite series) but hasn't seemed very interested in these since the early 70s, which probably has a lot to do with why we keep spinning our wheels with not very aggressive programs. No one has enough confidence in unproven new technologies to risk billions on them.
Neither they nor their neighbor in Pasadena, the Planetary Society, seems to have had much success so far. Considering how much effort NASA and others have spent making spacecraft to confirm certain of Einstein's predictions, it sure would be nice to get a proof of concept launch finally done for potentially practical solar sail technology.
Advanced propulsion never seems to get much priority, unfortunately. It took decades before NASA finally flew a solar electric mission, Deep Space 1 [] (which went quite well).
One recent development is Space Services' announcement they are working on a solar sail spacecraft to provide (from a libration point) a space weather observatory and communications relay for Antarctica []. They apparently have a preliminary NOAA contract. This is a really good idea but they don't seem to have much information available at this point so it's not clear if this will come to much.
Solar sails are a great idea, but will have difficulty seeing any practical use until the concept is proved. NASA used to do a lot of engineering proof of concept missions (e.g. Gemini, the Application Technology Satellite series) but hasn't seemed very interested in these since the early 70s, which probably has a lot to do with why we keep spinning our wheels with not very aggressive programs. No one has enough confidence in unproven new technologies to risk billions on them.
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