What is the most efficient photon producing compound currently in use? Would it be an LED?
How much thrust does an average laser produce?
What is the most efficient reflecting compound currently in use?
–Tim
Have no idea. I just wanted to keep the thread alive till someone who does know can answer.
-Beeblebrox
“I am so amazingly cool you could keep a side of meat in me for a month. I am so hip I have difficulty seeing over my pelvis.”
Lasers aren’t a very efficient method of producing light. Lasers are useful because they are very coherent (i.e. close to pure sine wave) and collimated (narrow, tight beam). LEDs are much more efficient, but I don’t know if they are THE most efficient. Fluorescent lights aren’t bad either.
As for reflective material, I believe silver is the winner at about 98% reflectivity. But reflective coatings can improve this even further. Reflective coatings consist of alternate layers of high and low density material, like SiO2 and TiO2. If the thickness is 1/2 the wavelength, the reflection from the first boundary will be in phase with the reflection from the second boundary, so they interfere constructively. With many such layers, 99.9999% reflectivity mirrors are now possible.
The technology is similar to **anti-**reflection coatings which you find on eyeglasses and camera lenses. Except these are designed so the reflections from the layers cancel each other out.
though I didn’t read the whole thing, perhaps this press release can be of some assistance:
MIT researchers create a ‘perfect mirror’
As far as LED’s, those little “photon light” things are really fuckin bright. I used my friend’s at school a few times to read late at night…unbelievable brightness
What have you got up your sleeve? Is it legal? 
My guess would be a laser diode. They’re similar in principle to a photo diode (i.e., an LED), but produce coherent light.
But they are much more efficient than incandescent and fluorescent lights (or explosives, either). For a high-intensity photon source, nothing beats a laser. The question is really “what’s the most efficient laser?”.
If my back-of-the-envelope calculation is correct, the thrust of a laser is T = P/c. T is the thrust in newtons, P is the power in watts, and c is Einstein’s constant. Hint: lasers have a very small thrust.
scr4 has answered the reflective question well. Let me add that the special coatings are highly reflective (or anti-reflective) only in a limited bandwidth of frequencies. The higher the reflectivity, the more limited the number of frequencies affected. But that’s quite all right if you’re designing mirrors for a particular laser.
Well, your average incandescent lightbulb is close to 100% efficient at producing photons - it’s just that most of them are no use to us for seeing with! They’re pretty good if you can see in the infra-red though…
For visible light, I think the most efficient source is the luciferin oxidation used by fireflies, although I’m willing to be corrected on this.
Re photons for thrust:
Pleonast: my back-of-the-envelope agrees with your back-of-the-envelope: -
for v = frequency of a photon, h = Planck’s constant:
energy of a photon E = hv
momentum of a photon = h/wavelength = hv/c = E/c
thrust = rate of change of momentum = E/ct = power/c
So the thrust of a photon source = 0.34 grams per megawatt. That’s apallingly inefficient with regards to energy, but it’s as efficient as you can get with respect to reaction mass.
For comparison, the space shuttle main engine get around 430 lbs of thrust for each pound of stuff it throws out the back per second. For 430 lbs thrust, a photon rocket would throw only 6 milligrams of photons out the back per second. So you have to carry much less stuff to throw out the back, which is good, but you have to be able to convert it into photons, which is tough. It is also a 575 gigawatt photon source, which is really the sort of thing to point at people you don’t like.
Most lasers don’t turn out to be enormously efficient light sources. There is however a kind of laser called a free electron laser that generates photons by accelerating a beam of electrons and feeding it through a set of deflecting magnets which make it “wiggle”. Wiggling electrons emit photons, and you can retrieve much of the energy expended by looping your electron beam round and accelerating it again. Theoretical percentage efficiencies are in the high 90’s if you use superconducting magnets for your wiggler and accelerator.
If you want to use photons for propulsion, a laser isn’t really the best way to go. Whatever energy source you’re using, you do better by throwing out the spent fuel as reaction mass than carrying it around with you. E.g. if you’re using nuclear fusion to generate power, you get more thrust using the power to throw the resulting helium out the back than by using it to run a 100% efficient laser. Of course, you’re throwing mass away but what would you want to keep that helium for?
For photon propulsion, what you want is matter-antimatter anihilation. There is no “spent fuel” - you go straight from matter to energy in the form of photons.
Electrons and positrons are one solution, but you have the problem that the anihilation produces gamma-ray photons directly and it’s hard to make them all go in one direction so you go in the other. They’re also bad for you. If you could get round these problems however, you’d have the most reaction-mass efficient rocket possible and a bad-ass gamma ray cannon in one package.
The hydrogen-antihydrogen reaction is more manageable in that it produces pions as an intermediate, and they are charged so you can make them go in one direction using electromagnetic fields. They decay into a shower of gamma photons rather quickly, but by that time they should be a way behind you. You lose a lot of the reaction mass efficiency of the pure photon rocket however.
Okay, if you must know, this is what I was thinking.
Create a massive LED array at the end of a cyndrilical perfect mirror, and use more perfect mirrors to focus and narrow the beam into a laser (I realize this isn’t the commonly accepted idea of a laser, but deal with it).
The focus this laser against a concave mirror that reflects right back to the cylinder again, around the perimeter of the focused laser, so that the light pours back into the cylinder again to be refocused.
Turn it on, crank up the juice, and wait for the rocketship to blast off.
This would only work when resisting against a theoretically immobile object (read: planet) in it’s current iteration, but perhaps further design alterations can make it work once off-planet.
What do y’all think?
–Tim
PS Thanks for the leg-up, Zaphod.
So you want two mirrors, one on the spaceship and one on earth, with light bouncing back and forth to provide thrust? Interesting idea, but I’m afraid there are a few difficulties. First, to make a tight collimated beam of light, you need either a light source that produces one directly (i.e. laser), or a very small point source at the focus of a parabolic mirror. If you use a large (non-point) source, like an LED array or fluorescent light, there is no way to collimate it or focus it into a small point. If you could focus all the light from a fluorescent light into a point, that point would be hotter than the light source - which violates the laws of thermodynamics.
OK, that problem can be solved by using a real laser. The next problem is that the light source itself will get in the way of light bouncing between earth and the rocket. If you’ve ever stood between two parallel mirrors, you know that you can almost look into an infinite series of reflections. Almost but not quite, because the reflection of your head gets in the way. In the same way, if you place a laser between two mirrors, the laser will obscure some of the reflection.
Still, the idea of using a laser to push spacecraft has been suggested by many people. A simple light sail pushed by a big laser is common in science fiction - Rocheworld by Robert L. Forward has a pretty rigorous treatment of this issue, including the idea of using a detaching ring sail to provide reverse thrust. Another idea is to use the laser to heat up a propellant. The spacecraft carries its own propellant or, in case of a launch vehicle, use the surrounding air. But it does not carry the machinery needed to burn or heat up the propellant. Instead, a laser beam from the ground focuses on the spacecraft and heats up the propellant. It can achieve higher temperature than chemical reactions (and thus more specific impulse) because the ground laser station is not limited by weight or availability of power.
So forget about focusing the light into a narrow beam. You should get the same amount of thrust from a broad beam as you would from a tight beam if you’re shooting the same amount of photons. So use a wide (but still relatively narrow) focus to spray photons into a concave mirror. Set up the mirror so that it reflects back to a dish on the perimeter of the light beam and funnel that light back into the mirrored cylinder to be shot back into the concave ground mirror again.
Alternatively, focus multiple laser beams directly at each other, create a wall of laser beams focused on a wall of laser beams.
Shouldn’t methods like these get you pretty damn near 50% the speed of light?
–Tim
Within certain limits, in space, any amount of thrust will eventually allow you to approach the speed of light. It’s a matter of how much thrust and how long you’re willing to wait. The exhaust speed is not the speed limit.
Small nitpick: you can focus a large light source to a small point. (Imagine focusing the sun with a magnifying glass on an ant.) You are right that it won’t be hotter, but you can approach the temperature. And the image is only small near the focus plane. For most practical uses, it’s much easier to start with a small, bright source. Use a laser!
I can remember seeing a Discovery Channel (or TLC maybe) show that demonstrated a small laser-powered platform. Imagine a disk (spinning for stability) with a big concave mirror on the bottom. A high-power laser on the ground shoots pulses at the disk. The mirror on the bottom focuses the laser pulse to a point right underneath the mirror. This superheats the air there, which expands and pushes up on the spinning disk. They could get the disk to several hundred meters with this method. Eventually, they hope to put up small satellites (with a megawatt laser–not easy to come by). I wish I could find a source for you.
BTW, using reflected photons gives twice as much thrust per photon as emitted photons. But collimation will be always be a limiting factor.
Homer: It’s actually a pretty cool idea.
You need a fiercely intense beam to overcome gravity of course, but in your scheme you can just let the intensity build up with time, assuming your mirrors are perfect. It won’t work in an atmosphere though - the beam would heat the air to plasma, which would then get in the way. You could give yourself a good push off the Moon!
The idea of riding a light beam for propulsion isn’t new - scr4 mentioned Rocheworld, which I haven’t read, and there’s The Mote in God’s Eye by Jerry Pournelle and Larry Niven, which I have. Both of these use powerful lasers to accelerate distant craft which have huge reflective “sails”. In your system the same light is used over and over again, which would give a lot more acceleration for the same amount of power. (You don’t get something for nothing though - the photons bouncing off the moving mirror lose energy, changing “colour” to a lower frequency. Eventually they are redshifted out of existence.) I doubt it’s practical to do over any appreciable distance (hard to stop photons missing the mirrors as you get further away) and there are a host of technical difficulties, but it’s interesting.
True, I meant you can’t focus all of the light from a source into an area smaller than the source. Thanks for the correction.
Anyway, you can’t make a parallel beam (even a wide one) from a large source. A parallel beam is thermodynamically (?) equivalent to a point source, because one can be easily converted into the other with a parabolic mirror. Or think of it this way: a parabolic mirror creates a parallel beam from a point source. An extended light source is basically a collection of point sources. When combined with the mirror, each part of the extended source creates a parallel beam going in different directions. There is no optical system that can correct for this.