# IF photaons have no mass, how can a solar sail work? **URL:** \ **Category:** Factual Questions\ **Created:** [May 30, 2003, 6:51pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600 "2003-05-30T18:51:57Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![Muad\_Dib](https://avatars.discourse-cdn.com/v4/letter/m/35a633/32.png) [@Muad\_Dib](https://boards.straightdope.com/u/Muad_Dib)\ **Post date:** [May 30, 2003, 6:51pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/1 "2003-05-30T18:51:57Z")
I keep hearing solar sails being described as working by being pushed by light from the sun or other stars. How is this possible if photons have no mass? Did they mean something other than photons? The sun ejects an awfull lot of fast moving ions that could give a real physical push. So what is the deal with solar sails. PS, you have no idea how many times I had to stop myself from writing “Solar cell”. ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [May 30, 2003, 7:16pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/2 "2003-05-30T19:16:47Z")
Photons don’t have **mass** , but they do have **momentum** which is equal to Planck’s constant divided by the photon’s wavelength. If photons collide with a solar sail, they transfer momentum to it just like collisions with physical objects. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [May 30, 2003, 7:20pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/3 "2003-05-30T19:20:07Z")
The solar wind is ions, not photons anyway, BUT it would at least be theoritically possible to erect sails (obviously made out of a suitable material) and let light push you along. Though photons don’t have “rest mass”, they do have “relativistic mass” and momentum. ---
**Author:** ![Sock\_Munkey](https://avatars.discourse-cdn.com/v4/letter/s/aeb1de/32.png) [@Sock\_Munkey](https://boards.straightdope.com/u/Sock_Munkey)\ **Post date:** [May 30, 2003, 7:55pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/4 "2003-05-30T19:55:05Z")
Do the photons have a slightly lower wavelength after they bounce off? ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [May 30, 2003, 8:16pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/5 "2003-05-30T20:16:10Z")
Well the photon(s) could be absorbed, but if IIRC after a collision the wavelength would be lower, depending on the angle of reflection. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [May 30, 2003, 8:17pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/6 "2003-05-30T20:17:45Z")
When I say, depending on the angle of reflection I mean the new wavelength is a fraction of the old wavelength and that fraction is dependant on the angle. ---
**Author:** ![Isosleepy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/isosleepy/32/479_2.png) [@Isosleepy](https://boards.straightdope.com/u/Isosleepy)\ **Post date:** [May 30, 2003, 8:26pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/7 "2003-05-30T20:26:58Z")
How would you steer a solar sail? Don’t believe the angle of the sail would make a difference. Analog would be regular sailboat without rudder or keel. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [May 30, 2003, 8:34pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/8 "2003-05-30T20:34:15Z")
Your right the angle wouldn’t make any difference, but why do you need to steer it? You should have your exact route planned out already and perhaps only small boosters would be needed to maske course adjustments. Using the solar wind is a one way trip as it goes out from the sun only. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [May 30, 2003, 9:01pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/9 "2003-05-30T21:01:07Z")
Hey, [here’s one](http://www.planetary.org/solarsail/faqs.html) soon to launch. I can’t believe I didn’t know about this! ---
**Author:** ![Balance](https://avatars.discourse-cdn.com/v4/letter/b/ccd318/32.png) [@Balance](https://boards.straightdope.com/u/Balance)\ **Post date:** [May 30, 2003, 9:25pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/10 "2003-05-30T21:25:20Z")
I may be wrong about this, but I would think that you could steer by angling the sail, assuming that it works by reflecting the photons (which is more efficient than absorbing them anyway). Assume that the photon is not travelling perpendicular to the surface of the sail. It gets reflected at an angle equal to its angle of incidence. Wouldn’t the direction of the imparted momentum bisect the path of the photon? That means that the momentum imparted will always be perpendicular to the surface of the sail. In order to start accelerating 45 degrees to port, you would rotate the sail 45 degrees counterclockwise (assuming you started out heading straight away from the light source). Did that make sense? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 30, 2003, 9:40pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/11 "2003-05-30T21:40:20Z")
For a photon sail, you really want to use a mirror-like reflective material, not just an absorbing material. You get double the momentum boost per photon, that way: Reflecting a photon (or anything else) is equivalent to catching it and gaining momentum, and then throwing it off again, and gaining more momentum. And if you do that, you can steer by turning your sail at an angle. If you turn your sail at a 45 degree angle, for instance, you’ll get a forward kick from the photons when you catch them, and then a sideways kick when they bounce back off again. There’s more you can do, as well. You can get control of a sort just by deploying and furling your sail at the appropriate times. In fact, this is necessary to get any use out of a solar sail. If you want to sail from, say, Earth to Jupiter, you’re starting off travelling in approximately the Earth’s orbit around the Sun, which is slightly elliptical. If you pointed your sail at the Sun and just left it there, then you’d go into a slightly more elliptical orbit, with apihelion a little bit further out, but nowhere near Jupiter’s distance. What you want to do, is deploy your solar sail when you’re in the half of the orbit where you’re moving away from the Sun, so you’re not slowing down as much as you get further away, but then furl it as you’re moving towards the Sun, so it doesn’t prevent you from speeding up. You’ll end up making a great many orbits, each one reaching out slightly further than the previous one, until you get to where you’re going. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [May 30, 2003, 9:46pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/12 "2003-05-30T21:46:04Z")
[An excellent website about solar sails.](http://www.ugcs.caltech.edu/~diedrich/solarsails/) Sock Munkey: Whether the photons have a lower or higher wavelength after they bounce off the sail depends on whether the sail is moving towards or away from the source, respectively. This would be a consequence of the Doppler effect. Isosleepy, MC: Actually, the angle at which you hold the sail **would** make a difference. It’s tricky to explain why without diagrams, but let me try: Think of the sail as a big mirror. To help visualize this, suppose the photons from the sun are travelling “North”, and suppose you hold your sail so that it lies on an “East-West” axis, i.e. directly facing the sun. Then the photons would reflect from the sail to the “South”, i.e. back towards the sun, and since momentum is conserved your spaceship (or whatever is attached to the sail) would be propelled “North”, directly away from the sun. Now suppose you rotated your sail so that the light from the sun was hitting the sail at a 45-degree angle; say that you placed it on an axis running from “Northwest” to “Southeast.” Then the photons would bounce off your sail and head “West”. The change in momentum of the photons, though, would point to the “Northeast.” So your sail would always push you away from the sun, right? Well, that’s true, but that doesn’t necessarily mean that you can’t use your sail to get closer to the sun. According to the laws of celestial mechanics, if you’re orbiting about the sun and you push yourself (somehow) in the direction you’re travelling, this will cause your orbit to change, and you’ll end up farther away from the sun. If, on the other hand, you give yourself a push opposite to the direction you’re travelling (i.e. you brake yourself somehow, you will (generally) end up in a lower orbit than you started out in. So you could use a solar sail to get closer to the sun in this fashion. Oh, and one more thing: Solar sails are pretty good for travelling around the solar system, but they’re not so good for interstellar travel (since there’s just not that much light out there.) The figures I’ve seen indicate that you could get up to about 10 kilometres per second, which is about one one-thousandth of the speed of light - not practical to get to the stars. If we could build a laser large enough, though, and shine _that_ on the solar sail, we could bump that up to about one-tenth the speed of light or so - much more practicable. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [May 30, 2003, 11:17pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/13 "2003-05-30T23:17:50Z")
Well MikeS, I was talking about sails which utilize the solar wind, not solar power sails. I don’t think sails which utlize the solar wind would have any manevouribilty as IIRC the solar winds density is only 6 ions per cubic metre, which I doubt would be enough to set up a the required gradient across the sails. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [May 30, 2003, 11:31pm UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/14 "2003-05-30T23:31:27Z")
> [@](#): > > \*Originally posted by ricksummon \* > Photons don’t have **mass** , but they do have **momentum** which is equal to Planck’s constant divided by the photon’s wavelength. If photons collide with a solar sail, they transfer momentum to it just like collisions with physical objects. Not quite true. Photons have no **rest** mass, else they would have infinite mass in their normal state (ie, traveling at the speed of light). ---
**Author:** ![ronbo](https://avatars.discourse-cdn.com/v4/letter/r/ba9def/32.png) [@ronbo](https://boards.straightdope.com/u/ronbo)\ **Post date:** [May 31, 2003, 1:04am UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/15 "2003-05-31T01:04:45Z")
Gravity acts as the keel. The interplay between the wind, the angle of the sail and gravity controls the direction of the spacecraft. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [May 31, 2003, 1:06am UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/16 "2003-05-31T01:06:56Z")
> [@](#): > > \*Originally posted by MC Master of Cermonies \* > **Well MikeS, I was talking about sails which utilize the solar wind, not solar power sails.** IIRC, sunlight carries more momentum than the solar wind. If you deployed any kind of sail in the solar systemn it would work mostly as a photon sail, so you might as well optimize it as a photon sail by using a highly reflective surface and using the sail angle for maneuverability. ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [May 31, 2003, 5:08am UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/17 "2003-05-31T05:08:45Z")
> [@](#): > > Not quite true. Photons have no rest mass, else they would have infinite mass in their normal state (ie, traveling at the speed of light). It depends on your point of view. Some physicists use the term “relativistic mass” which is the rest mass plus the kinetic energy. Others say that “mass” only refers to rest mass and that kinetic energy is a separate thing. Of course, the equations work the same regardless of which definition you use. ---
**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\ **Post date:** [May 31, 2003, 5:16am UTC](https://boards.straightdope.com/t/if-photaons-have-no-mass-how-can-a-solar-sail-work/178600/18 "2003-05-31T05:16:43Z")
I might add, though, that it makes more sense to say “mass” is only rest mass. If you were to accelerate a particle to relativistic speeds, it would never collapse into a black hole no matter how great its “relativistic mass” became because its velocity, and thus its “relativistic mass”, depends on your frame of reference. A particle can’t be a black hole in one frame and not in another. Of course, that goes into general relativity, and I don’t know what actually happens to gravitational attraction in different frames of reference.