h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Sadly, yes.

If one way mirrors actually worked 'as advertised' they would breach one of the laws of thermodynamics (the zeroeth law I think). They would allow a body in space to absorb radiation without re-emitting it and so permit reverse entropy.

F

Will try to post more tomorrow, just wanted to dig this out of the basement meantime

F S

Are you folks aware that the Planetary Society (of which I am a member) has built a Solar sail in RL and plans to launch it this year?
details at http://www.plantary.org/solarsail/index2.html

Are you folks aware that the Planetary Society (of which I am a member) has built a Solar sail in RL and plans to launch it this year?
details at http://www.planetary.org/solarsail/index2.html

please ignore post 23 I left a letter out of the URL

Thanks Shagbark, quite interesting. Please keep us informed on the progress and success of the mission.

F S

Sorry to be so long getting back, but I wanted to wait until I had time for a proper rely.

Xanatic

Perhaps my use of one-way mirrors is a simplification. However the idea is essential to proper Solar Sailing. Say we with to sail from here to Alpha Centari, for simplicity we will say both stars are equal magnatude. When we start we will spread our sails and the light from the Sun will push us into space. If our sails catch light equally on both sides, at the halfway point the light from AC will equal the light from Sol, unless we furl all of the sails we come to a stop. Indeed at the 1/4 point the light from AC will begin to overpower our push from Sol and we will cease to accelerate and begin to slow down.

If you can only use your sails for 1/4 of the trip you are not Solar Sailing you are just Solar Drifting, not much different that what we are doing today with Pioneer. However if can find a Semi Permeable membrane to light, ie a substance or effect that will allow light to pass freely in on direction while reflecting from the other, we can sail all the way with complete control. Just as the sailing ships of old could sail into port or even into a dock, we can put our ship into a docking orbit around our destination.

You are quite correct that one-way mirrors used for security require a darkened room for full efficiency. I could tell you a cute story about my Wife getting dressed for our wedding in an office that overlooked a gift shop, but this is a serious scientific discussion. However all we need is slight refection from one side and full transparency from the other. As for sails being damaged from space dust, it is no different that cloth sails being torn by the wind, or being destroyed by wear and rot. Some sails will have to be replaced during the voyage and spare sail sections must be carried.

Fathom

I disagree, nothing is being absorbed, it is reflected by one side and passes cleanly through the other, as I said above a semi-permeable membrane. The leaves of plants are similar with water. Moisture passes through the surface into the plant, but it can not flow the other way.

F S

Hi FloridaSailor,

"I disagree, nothing is being absorbed, it is reflected by one side and passes cleanly through the other, as I said above a semi-permeable membrane.2

Sorry F/S but the concept of a one-way mirror, even a partially effective one, is a thermodynamic impossibility.

Imagine your one way material surrounding an object in space. Light (and consequently energy) can pass through the material from the universe to the object but cannot pass the other way. The object would steadily heat up from the absorbed light. In normal circumstances the object would only heat up until it reached the temperature of its surroundings when it would be radiating as much energy as it received. The one-way screening prevents this equilibrium being reached and the object would heat up indefinitely. This is a case of negative entropy and would allow for perpetual motion as the energy could be made to do work. Physics, regrettably, does not permit this to exist.

I know your spacecraft does not use its solar sails in this way but my example is simply to demonstrate that the one-way material cannot be made.

F

F

I think I see your thinking now, the sails are never laid against the sides of my ship as you are correct they would trap the energy force, unless all were turned reflective side out, I was envisioning the as sticking out from the ship and turned parallel the front to back axis.

Can we at least agree that there are widows which light can pass through, mirrors that reflect light, and semi permeable membranes exist, at least for some substances?

I find this interesting and I am not trying to force any idea down anybodies throat, this is how problems can be overcome.

>. Physics, regrettably, does not permit this to exist.<

Correction please - . Physics, as we undersand it today, does not permit this to exist.

Another small point;

>...would allow for perpetual motion as the energy could be made to do work.<

How do you explain sailboats if this is true?

I know for a fact that sailboats work.



F S

Hi F/S,

Sailboats work because they extract energy from the wind to move the boat. Solar sails should also work because they can extract energy from the solar wind in a similar way.

One way mirrors can't be made - you'll have to trust me on this - because they violate an immutable law of physics. No, I realise we do not know everything there is to know about the laws of physics but thermodynamics is one area where what we do know is so firmly grounded it is used as the benchmark for everything else. If one-way mirrors could be made a device could be constructed which would be equivalent to a black hole for energy, without the gravity gradient. This would have the effect of concentrating energy into one place, from which it could be extracted for use - unlike a black hole from which nothing can be extracted. If an insulated, windowless box was divided into two by one of these mirrors one side would cool down, as infra-red radiation passed through the mirror, and the other side would heat up. You would have the perfect energy efficient refrigerator. If you took the heat from the warm side and conducted it back to the cold side it could be made do do work in the process - thus you would have perpetual motion.

Reflecting solar sails are not the best way to extract energy from the solar wind, however they may be the most practical to use. As explained earlier in this thread photons can only transfer momentum (and energy) to the sails if they are absorbed and black sails are the best way to absorb photons. Black sails will of course heat up and would be unusable too close to a star (say around 100 million miles away which is about where we are now). It may be better to use reflective sails to reduce the heat build up and rely on the movement of ionised particles in the solar wind to provide the thrust. It may be possible to make a one-way sail material using the ionised particles for thrust if it had, say, millions of tiny valves in it. This would even be possible with a sailboat but would be of dubious practical value.

F

Recent update:

I still say it won't work.

http://news.bbc.co.uk/1/hi/sci/tech/3555048.stm

F

It looks pretty though.