Most recent solar sail thread, FYI: http://boards.straightdope.com/sdmb/showthread.php?threadid=187467
So … what does this mean for solar sails? My understanding of solar sails gets foggier with every article I read. I used to think that they were propelled by particles (i.e., matter, not photons) from the solar wind. Then I sort of understood that the sunlight itself could exert a force. Now I have no clue. What’s the deal?
My first impression was that how could this have been overlooked all this time. My second impression is that Gold’s argument doesn’t even make a whole lot of sense.
If you take a look at the following link (yes, it is a weblog but it is easier to read than the source at the sci.space newsgroups) it seems to cover the issue pretty well.
http://www.interglobal.org/weblog/#002736
It points out that Gold has neglected doppler shift as well as many observed examples of solar radiation pressure.
Gold is wrong. A photon reflecting off a moving sail will be redshifted or blueshifted, and lose or gain energy depending upon whether the sail is accelerating or breaking. No rules of thermodynamics are broken - if the “perfect mirror” is free to move, photons can indeed “suffer a drop in temperature”.
Solar sails, somewhat confusingly, are not propelled by the solar wind (particles) but by the pressure of sunlight itself.
Yeah, the biggie that he forgot is Doppler shift. In the reference frame of the sail, there’s no change in photon temperature. Which means that in the reference frame of the sail, the sail doesn’t move. Duh.
What’s more relevant is that in the reference frame of the Sun, the photons do experience a temperature drop, due to the Doppler shift. So relative to the Sun, the sail does accelerate.
It’s interesting to note the proponents of this idea. First, we have Gold, described only as “a physicist”, and without honorifics on his name. Is he a professor? Any expertise in thermodynamics or radiation transfer? My guess is that he’s an undergrad who’s excited to have just finished his first thermo course. And then, we have a fellow from the Hayden Planetarium. Now, I won’t knock planetarium operators; they’re great educators. But do you know how much physics a planetarium operator needs to know? None. The momentum rules for photons work exactly the same way that they do for everything else. If they didn’t, then that would break the laws of thermodynamics (and not just the second one, but also the first).
As for radiometers, the ones you buy at a museum gift shop all rotate opposite the direction you’d expect from photon pressure, but that’s just because the vacuum in them is lousy. Get a really good vacuum in there, and it’ll turn the other way. And yes, this has been done.
Finally, even if all this were true, who says that a solar sail needs to be a mirror? Gold says that a non-reflecting sail would heat up in seconds. But notice that he doesn’t say how hot it’d get? About as hot as the day side of the Moon. We’re quite capable of building materials which can withstand that.
My question on all this would be not moving from “sun outward”, but rather “Inwards to sun”. I can understand the physics involved in sailing into the wind, while on water, but given that the free space between the planets does not have a “resistive” force acting in concert with the propulsive force of the sunlight/photons, I don’t get how one would be able to use this.
For an outbound only trip, fine, but full navigation?
-Butler
Actually, Chronos, Gold is a rather well-known physicist. He is getting on in years, however (but then so am I). I believe he is the one who has had some rather quirky ideas, such as an alternate origin for petrochemicals. But he was a respected physicist. Which does not make him immune to silly mistakes. Anyway, there is no point to ad hominem attacks. His argument is either right or wrong (apparently the latter); his authority is irrelevant.
This much I understood: Full navigation is possible, given lots of time. If you angle the sail, the photons will reflect off at an angle, imparting sideways momentum to the craft. Do this to speed your craft up, and you’ll move to a higher orbit around the sun; do it to slow your craft down, and you’ll move to a lower orbit. Changing your orbit can gradually (after several loops, sometimes) swing you out to wherever you need to go.
butler1850
Remember we’re talking solar orbit here. To get further away from the Sun, you don’t just set your sail square on and let it push you directly away. Instead, you set your sail at 45 degrees to the radial direction and accelerate tangentially, moving in an outward spiral. Or you can set your sail at 45 degrees so as to decelerate tangentially and move in an inward spiral.
Hari Seldon Fair comment, but his mistake was a lot more than just silly.
** butler1850** is correct. A boat can sail to windward because you have two levels (water and air but they could be any two fluids or even the same fluid) moving in relation to each other. Then you have a wing in each (keel and sail) and generate lift and you can move in any direction in relation to the fluids (althoigh with speed limitations)
Now, solar sailing is not like that at all, it is more like a balloon drifting in air and can only be pushed directly away by the solar wind, no matter what the angle of the sail (although it could use rockets for side thrust).
sailor I hate to disagree with you on a sailing topic but isn’t the thrust from a solar sail normal to the surface of the sail?
Yeah, I posted in haste and I’m probably mistaken. I’d have to think about it. never mind. Carry on.
I am too tired to think about it in detail but no matter how you set your solar sail there will always be a component pushing you outwards. Now, whether as matt says, you can slow down and let gravity pull you inwards, I do not know and I would have to do some math, which, as I say, I cannot do now. So, my answer is a qualified “maybe” but I am not totally sure.
What’s interesting about this is that light pressure was first intuited by thermodynamic arguments (the electromagnetic and quantum explanations came later), so it’s pretty bizare if Gold is arguing against it on the basis of ther,modynamic arguments. See R.W. Wood’s book Physical Optics.
As for Gold, rebuttals of some of his ideas have shown up in the pages of Skeptical Inquirer. He may indeed be a respected physicist, but when I see his name, warning bells go off.
Here’s a CV for Thomas Gold , Professor Emeritus of Astronomy at Cornell University; founder and for 20 years director of Cornell Center for Radiophysics and Space Research. Not that I’m saying I agree or disagree, just to give you some info on who he is.
Oops, bad link – here’s a good one.
http://people.cornell.edu/pages/tg21/vita.html
When I was searching the internet for opinions on subterranean silicon-based life he was just about the only scientist whose name came up…
seems the prospect is way off mainstream;
“I’m a doctor, Jim, not a bricklayer!” is about as sophisticated as it gets.
(anyway, here is my attempt).
A response to Gold’s argument has been posted on the Planetary Society website by the Cosmos 1 Solar Sail project director.
[best patriotic indignation]
Well, http://uk.arxiv.org/ is hardly “a rather obscure British web site of “e-print physics archives””; to some of us, it’s the handy UK mirror of http://www.arxiv.org.
Hmph.
[/best patriotic indignation]
Anyway, Gold’s paper, such as it is, was actually originally posted to the latter - it’s here.
Ha! You are quite right of course. Friedman would probably excuse himself with his “I am “only” an engineer” disclaimer.
Is is true though that postings on the archives which have not been submitted to a peer reviewed journal as well are often suspect.
Maybe it is a slight Hijack but
If light can exert pressure, but has no inertia, then why couldn’t you equip the ship with its own light source and shine that on the solar sail to accelerate the spaceship?