Solar sails suck.
In a 2002 paper, Laser Elevator: Momentum Transfer Using an Optical Resonator (available at your local school/library, possibly electronically — J. of Spacecraft and Rockets 2002), Thomas R. Meyer et. al. talk about a neat way to get a lot more speed out of light reflection than with a regular solar sail. The basic physics are pretty simple, and it’s a fun subject to think about.
When a photon hits a solar sail, it gives the sail momentum. If the photon has momentum P and bounces off a stationary sail, it looks like this:
Think of where the energy is in this system. Before it hits, the photon has energy E. After it bounces, the photon still has roughly energy E. But the sail’s moving, so where did it get its kinetic energy? (Remember, energy — unlike momentum — has no direction.)
The answer lies in the word “roughly”. The photon loses a tiny fraction of its energy to Doppler shifting when it’s reflected, but only a tiny fraction. It is this tiny fraction that goes into pushing the sail. This is a phenomenally small amount of energy — far less than a percent of what the photon has. That is, not much of the photon’s energy is being used for motion here.
This is why solar sails are so slow. It’s not that light doesn’t have that much energy, it’s that it has so little momentum. If you set a squirrel on a solar sail and shone a laser on the underside, do you know how much power would be required to lift the squirrel? About 1.21 gigawatts.
This is awful. If we were lifting the squirrel with a motor, railgun, or electric catapult, with 1.21 gigawatts we could send it screaming upward at ridiculous speeds.
This is where Meyer and friends come in. They’ve point out a novel way to extract momentum from the photon: bounce it back and forth between the sail and a large mirror (on a planet or moon, perhaps).
With each bounce, the photon loses a little more energy and adds another 2P to the sail’s momentum. The photon can keep this up for thousands of bounces — in their paper, Meyer et. al. found that with reasonable assumptions about available materials and a lot of precision, you could extract 1,000 times the momentum from a photon before diffraction and Dopper shifts killed you. This means you only need 1/1,000th the energy to levitate the squirrel — a mere megawatt.
This isn’t too practical for interstellar travel. It requires something to push off from, and probably couldn’t get you up to the necessary speeds. It may, they suggest, be useful for getting stuff to Pluto and back, since (somewhat like a space elevator) it lets you generate the power any old way you want (a ground nuclear station, solar, etc). But more importantly, it’s kind of neat — it helped me realize some things about photon momentum that I hadn’t quite gotten before. It’s like Feynman says, physics is like sex — it may give practical results, but that’s not why we do it.
Now we’ll let things get sillier. I spent a while trying to brainstorm how to use this with a solar sail (that is, using the sun). I imagined mirrors catching the sun’s light and letting it resonate with a sail.
But you really need lasers for this — regular light spreads out too fast. Maybe a set of lasing cavities orbiting the sun …
Supplemented by a Dyson sphere …
And since by this point we’ll probably have found aliens …
Why settle for interstellar communication when you can have interstellar war? And we could modulate the beam to carry a message — in this case, “FUCK YOU GUYS!”
xkcd 
“1> The planet’s going to move.
2> What happens when you’ve slowed down all the photons and changed the speed of light?”
1. What’s the mass of your ship compared to 6e21 tons? Conservation of momentum, it’s not going to go anywhere fast.
2. You don’t. Photons always move at c. Their momentum and energy are determined by their wavelength.
“I wouls suggest mining to be a much better aplication for the “Death Ray” melt the surface of a planet so materials are easier to exract”
You’d have to be precise enough not to vaporize everything you’re trying to mine. Otherwise you’d have a belt of gas around where the planet used to be.
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Dyson sphere w/2 rays coming out – isn’t that pretty much a quasar?
@Gen. casey: If we shoot at them, they’ll be hostile eventually; it all works out.
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MacLain, nowhere did I say my “death ray” would be suitable for interstellar use (although I guess the original post suggests it would be suitable, I wasn’t going there). However, in-system (or even near-system), it’s quite lethal.
As for requiring massive amounts of energy to sustain the Dyson sphere – good thing it’s parked around a star, eh? I wonder if we can borrow a cup o’ sugar, love? No? Perhaps some energy from your gigantic nuclear furnace, then?
We’re about eight light-minutes from Sol. Think about the proverbial boy with the magnifying glass, setting things alight on a sunny, summer day. Now extend that to anything within a few light-hours of Sol, and you’ve got a fairly effective area denial device, even if it isn’t suitable for warring with the Alpha-Centaurians. For reference, Neptune is just over 4 LH away, and Pluto (along with most of the Kuiper Belt) sits about 5.5 LH away.
If we can stop an alien horde 5 LH away, I’d say we’re still doing OK. The problem, of course, is SEEING them there, so we know where to shoot. Since we’re in the realm of Sci-fi now, I’ll leave it to someone else to devise a FTL communications device to solve that problem.
Notable here is that Alpha Centauri isn’t really the “nearest star”. A red dwarf called Proxima Centauri is only about 4.2 light years away from us. In another 8,500 years or so, the gentrified Velox Barnardi (Barnard’s Star, also known as Barnard’s Runaway due to its remarkable speed) will be closest dwarf star, perhaps around 3.8 light years. These dwarves might be good candidates for Dyson Spheres, as they’re much smaller than our own sun, and unlikely to be missed by any interstellar neighbors.
[Mind if we take your life-giving star for our experiments? You need that? Really, you don’t say…]
NB: Most distances from http://www.haydenplanetarium.org/universe/duguide/app_light_travel_time_dista.php
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“A red dwarf called Proxima Centauri is only about 4.2 light years away from us.”
True, but usually they’re counted as the same since Proxima and Alpha are essentially part of a ternary system.
“If we can stop an alien horde 5 LH away, I’d say we’re still doing OK. The problem, of course, is SEEING them there, so we know where to shoot.”
Well, assuming they’re in a nice compact fleet. I certainly hope they’re smart enough to spread out if they spot a Dyson sphere with a lasing cavity on it. Plus, they’ll probably attack before we get a chance to build one.
“These dwarves might be good candidates for Dyson Spheres, as they’re much smaller than our own sun, and unlikely to be missed by any interstellar neighbors.”
Except they’re also more likely to have no planets, so where are we going to get the materials?
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Can you (or someone, or me if I can use the picture) make the squirrel / laser picture into a tee shirt? please?
Can I use the picture for a shirt?
Thanks your awesome!
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> I certainly hope they’re smart enough to spread out if they spot a Dyson sphere with a lasing cavity on it.
Which is why we’ll paint a big smiley face on ours, right? There’s that, sorted.
> Except they’re also more likely to have no planets, so where are we going to get the materials?
We’ll have to truck them in … but perhaps there are materials around, in a belt similar to our Kuiper belt*. It’s unlikely that they’ll resolve at this distance, but once we get better “eyes” for interstellar exploration, we might get lucky.
* So we’ll have to figure out how to build a star-encompassing structure out of big chunks of rock and ice. I admit, at this point my ideas are rather tenuous. Again, we’re in the realm of SF at this point.
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Isn’t it Alpha CentAuri?
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i have only a basic high school knowledge of physics so bear with me, but based on the information gathered on this page and in the comments, it would seem like a dyson sphere would be impossible because if the colossal thermal energy released from the star didn’t blast the structure outwards, wouldn’t the energy transferred by the photons constantly bouncing off the inner surface do the same thing?
man, what the hell i hated physics last year but i found myself engrossed in this little page and all the comments. i’m kind of freaked out.
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okay. I didn’t read all 200+ comments, but what if you used this method to first propel way stations into space, basically giant refocusers, to increase the range of either the ship or the death beam (not at the same time). Being only a physiphilliac, I can’t cite anything that would make this possible, but having multiple transfer points seems much more practical than just one station.
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It is imperative to be able to lift squirrels. Cooking the squirrels is necessary. The travel through deep space should keep them frozen until they reach their destination. We wish the aliens who receive them on Alpha Centauri to have a decided preference for squirrel meat over human if and when they reach Earth. Our next urgent priority, keep squirrels from becoming extinct. Looking in my back yard, unlikely to be a problem. I seem to have a planet colonization breeding population living in the trees in my backyard, in a Chicago sized lot.
This suggests that the Chicago squirrel population is in the range of 4.5 Billion +/- 3000% Exceeding the politi, I mean rat population substantially. Possibly.
(NB: I was a Philosophy Major, math is not my strong side, so I just estimate any number between Being – Nothing and Less than Nothing (ie., Paris Hilton (or, if you prefer, Nano-Hilton if you want something really small)).
(EG., His I.Q. was measured in nanohiltons….)
Would threatening someone with proximity to certain celebrities be likely to propel them into space? Just a thought.
When I was 7 or 8 years old, 1967 or 1967 I had the idea that if you could take a mirrored ball and make the mirrors really tiny and make them face slightly odd angles, that is, not just reflect back and forth at each other and have the mirrors on the inside instead of the outside of the ball, that somehow that you might be able to construct a ball where the light could come in but never come out Until you opened the exit.
My theory, and an age 7 theory it was, was that as you poured more light, Superman heat ray (this is well before the Star Wars era and my reading was still limited to DC, Marvel, Harvey and the kids section of the library), whatever, the constant reflections of the light with no exit would allow it to build up more and more heat energy until you finally opened the one tiny opening through which light could emit (come out in age 7 terms) and that this light would be your ultimate heat-ray, laser sort of thing.
After thinking I realized that if you made this right, you might not need Superman, just really good mirrors that would not melt and would keep the light in, concentrating it until you let it loose.
Of course, there are lots of physics problems with this that I’ll leave to the experts, but that makes me wonder about the idea of a photon trap. Suppose your photon goes through strips or layers of one-way material in this sail that capture the energy in the photon then focus the heat out vents in the side, to in converting the light into motion and heat, uses the heat as propulsion, (a miracle occurs!) and more of the energy is somehow used in the whole McGuffin.
My family keeps interrupting.
No one in deep space gets in between me and my Kelvins….
I think what I was getting at, was the idea that there might be some way to use reflections, absorption, the cold of deep space and the fact that light can be transformed as it goes through layers – polarized, made coherent, converted to heat, channeled somehow, to make a space sail more efficient than we have really even begun to think about.
By “We”, I don’t really mean to include me, per say, but thinking that we can’t ever get more than messages out there ever, makes me feel a little, well, squirrelly.
Peter, Editor, Chief Spelling Wrecker, and Science Like Writer (I Like Science and Write About it Sometimes)
The Peter Files Blog of Comedy, Jokes, Satire, Humor, Commentary and Videos
http://thepeterfiles.blogspot.com
Now featuring two comics from XKCD with attribution and suggestions for readers to come here and read more
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On the other hand, we could just say, the heck with this, let’s go down to the pub and design some working x-ray specs. Now there’s something really useful.
Rats, that’s not really Physics is it? I think I just descended into Engineering. (:-)>
Peter
http://thepeterfiles.blogspot.com
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OMG! My Emoticon was decapitated in the last post!
(:- )> “Here I come, I’m on my Way!” %=
(:- )> %=
(:- )> %=
(:-|)>%= At this point Mighty Mouse applies SUPER DUPER GLUE!
(:-)> I Never Even Got a Chance to Thank Him! =%
This emoticon drama brought to you by The Peter Files Blog of Comedy, Original Comedy with no Nutritional Value whatsoever. This blog makes no money to speak of either.
http://ThePeterFiles.blogspot.com
Sorry for the repeated remarks, but seeing my truncated emoticon demanded some kind of comedic response. I hope you understand and/or can ignore it if it annoys you.
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Darn the carefully placed spaces in the last post that were supposed to show the head being put back together were eliminated by the Eidor. In the first line, there were 9 spaces between the top and bottom of the head, in line two, five spaces, line 3, 3 spaces, line 4 no spaces just the sealant mark. and finally the whole head and beard are together again. The effect of MM pushing the bottom of the head across the screen is lost entirely. Sigh.
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fantastic post
as a high school student with a geeky girl physics thing going on, i would love it so much if you guys did a textbook
my monotone classes are hard, and start to stifle the passion
xkcd is brilliant and a textbook would be the ultimate in awesomnivity
i’d sell myself for a copy, hope you guys go for it
also – death ray born of dyson sphere = my future occupation 🙂
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What say you put nicely crafted lips on the edge of the mirror of the photon never leaves the mirror
____
/ MIrror B
.
____/ Mirror A
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Is no one else seeing the possibility of making a giant game of Pong using the photon bouncing back and forth and a moving bottom mirror?
It would be ridiculous… And awesome!
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What about instead of using a mirror on the bottom, using a series of fiber-optic ‘U’ shapes.. would you still incure the -2P force from such, or could a single Photon be used to move you for quite a distance that way? Obviously, it’s not “perpetual motion”, as the photon is losing some small amount of momentum each pass…
Just a thought.. =D
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“i have only a basic high school knowledge of physics so bear with me, but based on the information gathered on this page and in the comments, it would seem like a dyson sphere would be impossible because if the colossal thermal energy released from the star didn’t blast the structure outwards, wouldn’t the energy transferred by the photons constantly bouncing off the inner surface do the same thing?”
Actually, no. All you have to do is have a strong enough material. The photon pressure on opposite sides balances out, as long as you can hold under the stress. Otherwise, you can just make most of it transparent (most of the time).
“It is imperative to be able to lift squirrels. Cooking the squirrels is necessary. The travel through deep space should keep them frozen until they reach their destination. We wish the aliens who receive them on Alpha Centauri to have a decided preference for squirrel meat over human if and when they reach Earth.”
Interstellar meat deliveries. Hmmm… you wouldn’t need refrigerated cars…
“When I was 7 or 8 years old, 1967 or 1967 I had the idea that if you could take a mirrored ball and make the mirrors really tiny and make them face slightly odd angles, that is, not just reflect back and forth at each other and have the mirrors on the inside instead of the outside of the ball, that somehow that you might be able to construct a ball where the light could come in but never come out Until you opened the exit.”
Actually, this has been done, using structures called Bose-Einstein condensates. Shoot a laser in, change the properties a bit, and it stays there until you change the properties again to let it out. IIRC.
“the constant reflections of the light with no exit would allow it to build up more and more heat energy until you finally opened the one tiny opening through which light could emit (come out in age 7 terms) and that this light would be your ultimate heat-ray”
Well, that’s exactly a laser. Maser, probably.
“On the other hand, we could just say, the heck with this, let’s go down to the pub and design some working x-ray specs. Now there’s something really useful.”
Not so much. If enough X-rays existed on the surface of the Earth to see through solid objects by, you’d die of cancer.
“What say you put nicely crafted lips on the edge of the mirror of the photon never leaves the mirror”
Unless the impact pushes the mirrors sideways and out of alignment.
“Is no one else seeing the possibility of making a giant game of Pong using the photon bouncing back and forth and a moving bottom mirror?”
Except you can’t see the ball.
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i lmao’d at the squirrel.. btw if you plant a squirrel in a platform with any of the machine powering the platform and a gigawatt of juice, the squirrel will be ashes in <1s right?
sorry i’m bad at physics.. you should write more squirrel jokes.. xD
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an xkcd physics textbook would make you the new Bill Nye.
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I would have thought one of you would have mentioned this by now:
You don’t need a station to bounce the lasers off of. Just bounce it off a spaceship that’s going the opposite way. That doubles the usefulness of the photons instantly.
Just think, shooting lasers at someone as a friendly gesture…
There would be some steering problems though, except…
If you do have a station, reflecting both ships’ beams to each other, they would both be able to steer, but the station wouldn’t have control over where it goes, but it can probably fly anyway.
You probably won’t ever throw squirrels into space using solar sails though. Space elevators or railguns work much better for that.
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“but I think the mass requirements of a laser and power plant would exceed the benefits of the extra thrust.”
but assuming a nuclear or fusion power plant (even on a warship) most of the energy would be directed to drives; the ship’s powerplant is made to power the engines, weapons would draw a fractional amount compared to the drive units.
The simplest setup would be three mirrors and lasers arranged around the ship’s front; I’d assumed the emitters would have a little mobility for greater degrees of attitude control if not mounted in a full turret to function as weapons if needed. the beams would be wide enough to cover the whole sail, though it could be focused to account for damaged sail panels.
For greatest efficiency, the mirror could potentially be one-way, allowing any… “conflicting” photons through; though it’s not essential. In fact I think the backsides (forward-facing half) of the mirrors would be armoured and capable of folding against the hull when propulsion isn’t needed.
You could also mount a second set of identical mirrors at the back and turn the lasers around to slow down without making a 180-degree turn; this wold mean that the ship’s back and front sides are reversible; and the mirrors would probablly be modular so if some are damaged you could simply switch some out to keep one side fully functional.
you could also use the rear mirrors to reflect light into the forward mirrors when heading “into the light”
===
other ideas for space-faring vessels:
sub-dermal solar collectors. these are just solar mebranes held under the ship’s plating or a protective sheath and are unfurled in-system to recharge the auxialliary powerplant, something with the endurance of a capacitor but with greater energy storage capacity.
Tachyon FTL communications (or any other negative mass particle). If they exist you could utilise tachyon bursts to send data packages at FTL speeds; and maybe they could be used to travel at FTL speeds.
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dunno, I’m not any physicist or math major, just pure theory
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A former professor of mine has a method for getting more propulsive effectiveness out of that ground-based laser of yours: . No good for deep space travel, of course, but it’ll sling your squirrels into orbit better.
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You know, Dyson spheres are unstable. You need to keep a particular speed to remain in a roughly circular orbit around a star, and only two (one if it’s the equator) circles in the sphere will be able to stay at that speed at any time.
You could have a large set of rings in different orbital angles, but that too is unstable, and they’d tend to attract each other to the same plane.
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Hehe, Niven’s rignworld, (*cough halo cough*)
(*cough Marathoon cough*)
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Hehe, Niven’s RIngworld (*cough halo cough*)
(*cough marathonrocks cough*)
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You make my love for science go back to the time my mind was still innocent and free
thank you
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I can has Centerpoint Station?
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I actually found that very educational. And uplifting.
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what if the laser was mounted on the ship itself? You could open the mirror/door, shoot the laser, and close it real fast. Then the light would bounce back and forth almost infinitely, depending on how well aligned the mirrors are.
Just a thought.
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=what if the laser was mounted on the ship itself? You could open the mirror/door, shoot the laser, and close it real fast. Then the light would bounce back and forth almost infinitely, depending on how well aligned the mirrors are.
Just a thought.=
you’d get zero net acceleration, and the photons would eventually escape, being single particles they can bust through any intervening matter relatively easy, something to do with H-bar and all that
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ROTFMAO reading the post and then the comments… There’s always a very short moment of sadness in those cases, when I realize I cannot immediately share my laughter, because it would probably need half an hour of various explanations for most people to even barely grasp the joke.
The good thing is, my roommate, which is a liberal arts major, is sometimes willing to take that half hour of explanations!
Oh, and BTW, (1+ *textbook-requests*)
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Not if the mirror was separate from the ship. That would work, but you’d have to carry the laser and power supplies with you. Probably not too efficient, even with the increased energy transfer from more bounces.
“You don’t need a station to bounce the lasers off of. Just bounce it off a spaceship that’s going the opposite way. That doubles the usefulness of the photons instantly.”
Raising interesting issues about gravitational deflection. If you’re just traveling radially relative to Sol, or using its own light (possibly lased), it wouldn’t matter because the geodesics are straight. But if you have two ships and light is going between them, and one of them isn’t falling into the Sun, the light might be deflected by the Sun’s gravity more than the ships, therefore bringing them out of alignment. Haven’t done the math, though.
“Tachyon FTL communications (or any other negative mass particle). If they exist you could utilise tachyon bursts to send data packages at FTL speeds; and maybe they could be used to travel at FTL speeds.”
Tachyons are pretty weird… according to Wikipedia, two charged tachyons of opposite momentum would annihilate each other at infinite velocity.
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“Testing was erratic due to the frequent premature departure of the sciurine subject.”
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Great article!
I especially liked the last sentences. 😀
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“two charged tachyons of opposites momentum would annihilate each other at infinite velocity”
O.o; well, that’s what we get for theorizing a particle with negative or imaginary mass, but I wonder, are tachyons produced by existing matter like gravitons, or do they foolow the normal rules for matter with only virtual particles to… make new ones?
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Well, to satisfy conservation rules, if tachyons have negative mass, the existing normal matter would have to gain arbitrarily large amounts of momentum to emit a tachyon. If they have imaginary mass, the normal matter would have to acquire complex momentum, turning it into a tachyon too, unless the emitted tachyon has imaginary velocity, in which case the normal matter might just gain real momentum.
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Weird, I just started rereading The Mote in God’s Eye yesterday. The aliens in it have a huge solar sail.
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1.21 Gigawatts isn’t that much. There are lasers that can lift 1,033,057 squirrels (meaning that we can now have a laser-powered squirrel army). Come to think of it, if we use the principle here to extract every last bit of momentum from that laser, we can have an army of 1,033,057,000 squirrels!
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“The answer lies in the word ‘roughly’. The photon loses a tiny fraction of its energy to Doppler shifting when it’s reflected, but only a tiny fraction. It is this tiny fraction that goes into pushing the sail.”
Can someone explain this part in more detail? As I understand it Doppler would change the frequency of the light differently depending on where you are, and whether the electron is moving towards or away from you. But if that change in frequency changed the actual energy of the particle, wouldn’t that mean that different people observing the same particle of light moving at the same speed in the same direction would find it to have two different amounts of energy?
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I’m so interest by solar, if you like laser products ,you can visit our company website: http://www.pro-laser.cn/.
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You’ll never get that thing around the sun to not orbit.
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Hate to poke another hole in the Dyson sphere laser cooking Alpha Centauri idea, but in order to construct the Dyson sphere, we’d probably need more materials that would be available in our solar system and would have to get more from somewhere else. Next closest source of materials would be… Alpha Centauri. They might notice when planets start disappearing from their solar system.
Plus, given that construction of the sphere would probably take some time, they would most likely notice the waning albedo, and ultimate dissapearance of their closest stellar neighbor, and at least be a little suspicious as to why.
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I (n+x) the request for a textbook, where x is any integer Z between infinity and god.
>Plus, given that construction of the sphere would probably take some time, they would most likely notice the waning albedo, and ultimate dissapearance of their closest stellar neighbor, and at least be a little suspicious as to why.
Ah, but that’s why we start on the opposite side and put the laser cavities in a direct line to alpha centauri from the START. Let them figure it out fast enough to do anything then. 🙂
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Sorry if it has already been asked, but doesn’t a photon have no mass? Given that momentum=v*m wouldn’t the photon have no momentum to transfer at all?
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It turns out, no. Momentum is actually more than the p = v*m formula they give out in highschool physics.
For light, E = pc, that is, the energy of a bit of light is equal to its momentum times the speed of light.
This let’s you conveniently calculate the light pressure of a laser; given a N-Watt laser, it will have a laser pressure of N/c Newtons. So, a 1.21 Gigawatt laser would have a laser pressure of roughly 4 Newtons.
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>Why does the reflecting mirror have to be mounted on a planet? Why not just leave it freely in space? Sure, it will accelerate away from the sail — it picks up 9p momentum, after all — but so what? We can pick it up later.”
Well, that would take energy, if you’re sending off spacecraft regularly. I mean, you don’t want it falling into the sun or crashing into the Moon while launching something.
Who says we’ll need only one sail, -or- even a static system? convection in space, with the combined effects of photic momentums and gravity pulls can allow for a system of sails, proceeding in a pattern like an oval with a core (or, like the surface area of a hotdog with a lengthwise hole). the only mechanics necessary for this situation may be contained in each wing/sail/paddle, making each a nearly autonomous, definitely unmanned space vehicle. mathematic provides us with a formula including all the truly affective forces involved (the specific pull of Mercury’s gravity matters more than, say the Moon’s). the longer the system continues, the deeper the effect of any variables missed in the mathematic, sure. Combining the formula with both a self-learning system to allow the inclusion of ‘newly’ important numbers and a group of specialists in as many variables as people want to specialize in (don’t want the blind leading the blind into the sun, eh?) will seriously help. Oh, and the lasing grid could use the same relation between photic momentum and gravity to maintain the necessary distance/alignment/numbers.
At worst, I’m looking at what looks like a Cooperative Space flight mission, where even common computer addicts aid the cause (imagine videogamers’ skill at target acquisition pitted against targets which would the realtime effects of new variables). “Play to save Earth, free downloads available!”
As for construction, if anyone knows any technique more applicable than plasma vacuum deposition (http://www.patentstorm.us/patents/4882198.html) , plug it in! This is a growing system. I hope it grows on all of you, too!
-dP
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I feel that the solar sail is an interesting idea, but if we surround the sun with a dyson sphere, aren’t we sutting off the earth from, well, light? I feel that we need a funnel of sorts on the opposite end of the sun, still capped with a system of lasing cavities, and a mirror, to bounce the laser(s) off to the required direction. A little less power, but still significant.
Option 2- better message (alphacentauribitchsayswhat!)
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