Slashdot Mirror
Optical Levitation, Space Travel, Quantum Mechanics and Gravity
An anonymous reader writes "Light doesn't just make things brighter; it can also push things around. Normally this "radiation pressure" force is so small you don't notice it. But if you get a really big mirror then you could use it to power a space ship to the stars. This is the idea behind solar sails. The impact of light is more obvious on small things. Scientists are thinking about levitation of a mirror that would be large enough to see with the naked eye. If this turns out to work, the motion of the floating mirror could be used to probe the physics that connects quantum theory and general relativity."
There's nothing new about the idea of spacecraft being propelled by light pressure. There was an Arthur C. Clarke story published in "Boy's Life" in the early 60's about sunlight powered "sailing yachts" in a race from Earth orbit to the Moon. Or the Niven story "The Fourth Profession", in which an alien trading ship arrives at Earth, wanting humanity to build a launching laser to send the crew on the next leg of their journey.
And it's been 30 years since Niven & Pournelle published "Mote In God's Eye" in which an interstellar probe riding a the combined beam of battery of laser cannons arrive in human space.
So if actual human physicists are finally going to get around to proving the concept, so much the better!
There are two sorts of solar sail, those that work off photons (and, no, you don't need a mirror, since you can't afford the extra mass) and those that work off ionized particles being emitted from the sun. Ionized particles have much more momentum and are generally considered superior.
A solar sail that is 50 Km in diameter, attached to a 5 Kg probe, would accelerate that probe to 25% light speed by the time you reached the edge of the solar system.
If you built a car whose headlights could accelerate the car in reverse with photonic pressure, the headlights would vaporize a considerable chunk of the planet in front of you. You can do the calculation yourself. The equations are at http://www.physicsforums.com/s...
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
A trip to the nearest stars with the speeds attained by Voyagers I and II and then some, at the limits of our technology, would take on the orders of 70,000 Earth-years. Should it be done? Hell yeah! Do you expect to be alive? Hell no! But with huge solar panels living off of starlight per person carried, you could take a human colony into orbit to the nearest stars, and they could live there for millions of years. Mind you written history is like 8000 years old (Egyptian Pharaoh's and hyeroglyphs, or 11-13000 if you go with the Sanskrit writings), and modern humans (Omo remains) have only been around for 200,000 years, so a 70,000 year trip is quite a trip. But, you never know, humans on Earth may develop an AI that kills everyone, or a super intelligent genetically modified biotech photosynthetic microorganism that kills everyone and everything including trees, grass, lions, deer, fish.. and emerges as the "winner" back home as the only surviving species, well, at least these folks far away would be safe, at least for a while, till this super intelligent life form chases them down too and eats them too. But at least they get some time to figure out how to defend and stand a chance, possibly by creating their own, friendly superintelligent artificial intelligence silicon/organic android microbe that's better than themselves so it could kill them, but luckily it doesn't, but like a good guard dog, protects them from the attacks of the Earth based microbe, that come after them in say, 1000 years (if they have figured out a 70x faster ways to make the same trip.) See a thousand years is a long time to think things through.
The propulsion system should use extra-solar system harvested hydrogen atoms (they are like 1 H atom per cubic mile or something like that), accelerating it to near speed of light through special cyclotrons, then as the relativistic mass takes over and things get out of sync, special coiled linear accelerators continuing it, and you can get almost any kind of mass out of each atom, and get a great propulsion kick, impulse out of each, for rotation and speed control, or for further accelerating, being mindful that halfway through the trip you have to start decelerating, and such propulsion would still beat the simple light propulsion by orders of magnitude, because the impulse per energy expanded ratio is much better than with simple light. It's true that you're creating mass out of energy as you build the mass of each proton up, and shooting off pure energy as mass is equivalent to shooting off pure light as mass, so there is an optimum velocity, optimum ejection speed dependent on the economics of harvesting each atom from the really thin galactic vacuum vs. economics of not building up too much relativistic mass into it and wasting energy as mass, as in case of a light propulsion. You may have to resort to pure light propulsion in case you cannot find any hydrogen atoms whatsoever within 100 cubic miles or so, such as intergalactic space.
In closer quarters, on rotating cylinder space modules near Earth orbit or Lunar orbit, such propulsion, including light propulsion is pure absolute economic waste, compared to specific impulse gained per size (mass, volume) of the drive, as we have plenty of matter to waste, if nothing else, solar wind close to the Sun is pretty matter rich, visible with things like Aurora Borealis. In particular even a cyclotron drive on a rotation cylinder station may not be the economic optimum to align solar panels and control orientaton, rotation speed and orbit, but instead a liquid oxygen/calcium metal energy cash could be used near the Moon and on the Moon's surface. See life down here on Earth uses ATP (high energy adenosine tri-phosphate) as the energy cash, and all processes within all lifeforms respect the resource limit of energy cash, and all processes either generate ATP with food or light energy from ADP (low energy adenosine di-phosphate) plus P, phosphoric acid, APP + P + energy ---> APPP, or APPP ----> APP + P + energy. So, similarly, near
Also, providing the oxygen surface to the electrolyte is not that complicated, as the calcium ion dissolving electrolyte might react directly with gaseous oxygen, with the surface scraped, the electrolytic potential harvested as Ca++ concentration difference between the metal and oxygen zones, as in concentration batteries. Yeah, you don't need a second electrolyte bs for oxygen reaction.
With a 70,000 year trip corrosion is a big issue, so there may have to be a lot of gold used in the construction of the ship, and as the ship has to be quite a sizable thing considering psychological factors of having a society, and minding that all the gold ever mined on Earth fits into like a 2 km cube, there are issues here. Probably something like a 20 walled ship is needed, with vacuum pumped down to absolute vacuum by the 5th wall, and then corrosion is a nonissue there, and the inner 5 layers have to constantly be renewed/replaced, at least once every 1000 years. Also, once the pioneers/settlers arrive, and there's gotta be floating debris or planets around each star even if there is no liquid water bearing planet, how will they know what to do, would they still have the skills to fabricate things? While traveling there, there may be some outer solar system objects we call comets that have crystallized ice on them, going in the same direction with the same speed as the ship, therefore capturable if sufficient steering is available (what a minuscule chance for such a thing), and then skill can be kept up on the way there to dismantle/process "stuff" found in the vast emptiness of outer space, stuff that should be very dense once arrived near that star. Even in absence of that, practice material and laboratories, and mechanical workshops would have to be kept up, with possibly repair spacewalks, leading to a sizable ship. As there is always a chance of hitting small meteorites at say 30-200,000 km/s, and some units leaking to full vacuum, many completely isolated units would have to be maintained, say even 20 or 50, and ending up with only one surviving by the trip end able to carry and sustain the whole crew would have to be considered. If the different 20 walls rotate with respect to each other, then a small meteorite hole may create a leak into the interstitial wall spaces, but as the holes are not lined up, the vacuum pumps might be able to keep up and scavenge the escaping air, even calcium of vacuum tube barium or liquid helium traps that capture any atoms striking them could be a way to save stuff from leaking in the outer layers. The tighter the spacing between the walls, the less the leakage, and as soon as a leak is detected, there could be a certain preprogrammed rotation misaligning the hole away from each other to maximum distance, then a sudden crash, a halt of the relative rotation of the walls to each other, until the holes are patched, and sliding rotation can be restarted. I don't know what the economic optimum number of walls is.
As nitrogen is scarce, but hydrogen and helium are abundant in outer space, diluting oxygen harvested from comet rocks could be done, but not with hydrogen that forms an explosive mixture with oxygen, but with helium. The helium might have to be fusion generated from the harvested hydrogen, if nothing else, through cyclotron or energy inefficient portable neutron generator bombardment. And everyone would get used to the chipmunk sound of helium balloon inhalation you can hear down here on Earth.
Also, communication with the speed of light would take a few years to go back and forth, to exchange hello's, draining quite a bit of power from the ship for dish/antenna use, and in case the crew on this ship messes up and ends up in deep doo doo sending out an SOS to us, we can reply to them with the phrase/video transcript from Mad TV's Dolla Bill Montgomery's Real Motherf****in Talk Mother's Day episode, "Talk to the hand, you're on your own, motherf****!" https://www.youtube.com/watch?... We can give them advice, but not much else, advice they have to wait 4-8 years for to arrive. For psychological reasons, a reality show transmitted from them and programming transmitted to them would be neat, each without waiting for a reply to arrive, at least not reacting to one for the few years it takes to transmit the message. Their internet ping timeouts would have to be set to the corresponding few years, if
Ironic Isnt it?
No, it's not.
systemd is Roko's Basilisk.
The summary (and the headline) unnecessarily highlights space travel as a usage for radiation pressure and delegates the most interesting part as a footnote-ish last line. The /. crowd as usual starts shouting pros and cons of space travel, as if every comment on this page is not saying what has already been said a million time around here, and nobody to talk about the interesting part.
I wish someone with the right background in physics posted something more interesting about the fact that a group of researchers have come up with prediction of how a non-quantized spacetime (gravity) would look in the presence of quantized matter/energy. Apparently this would look different than a quantized background with quantized foreground (IANAP, so I don't know what is this all about) in a measurable way. If they can levitate a tiny but macroscopic mirror using light and balance it then giving it a gentle push would create a pendulum with no friction slowing it down. By probing the frequency evolution one can potentially get closer to actually knowing whether a quantum theory of gravity is the right way to unify QM and GR.
It's fascinating that such things are possible even in principle with existing technology. I wish someone would explain something more related to this.
Oops, made a mistake there, the magnesium/o2 battery is only 2789 Wh/kg, while calcium/o2 is 2990 Wh/kg, according to the wikipedia article, with oxygen included, it's only better if oxygen comes from the atmosphere, like down here on Earth, so no, magnesium is not better in the energy density respect weight wise, only ease of vaporization-wise, but as calcium is so reactive, and also very soft, it is most likely easy to ignite in the solid form with an iridium (or maybe tungsten(burns) or platinum(melts)) resistance wire with oxygen present in the reaction chamber, and also easier to squeeze because of it's softness (though it's harder than lead, and can be cut with a knife only with difficulty.) and if this softness/vaporization becomes an issue, sodium/oxygen may be good, as sodium is even softer and easier to vaporize/liquefy, but the energy density now drops to 1677 Wh/kg, or half.
With a 70,000 year trip corrosion is a big issue
The biggest issue is that after your 70,000 year trip, you arrive in a star system that humans have inhabited for 69,000 years already. Thats a full stop right there.
"His name was James Damore."
You like talking to yourself?
Four replies to oneself. I think it's a new slashdot record for a non-AC poster.
More importantly, you need to review some of the things that you wrote, because they don't pass the smell test. An example is the suggestion of using beryllium. There are less than 100 kt of mineable beryllium in the world, and most of that is needed for other applications (nuclear, ceramics, etc.). Beryllium exposure also does very nasty things to human biology. 0 points for practicality.
"Politicians and diapers must be changed often, and for the same reason."