Public
May 21, 2015
Flying through space, powered by sunlight
Yesterday a rocket launched from Cape Canaveral in Florida carrying the LightSail into space! It's a small spacecraft with a big shiny screen that's pushed by the light of the sun.
It's just a test - it won't go far. It will fall to the Earth and burn up. But next year there will be a more serious test. And someday, solar-powered space flight may become a force to be reckoned with.
One cool thing is that all this is being paid for private donations, by a Kickstarter campaign!
The LightSail is carried to space in a cute little CubeSat. It looks like a big toaster, and it weighs just 10 kilograms. But it holds a sail 32 square meters in area, made of a shiny plastic called Mylar, just 4.5 microns thick. This unfolds in a clever way - watch the movie! - to form a big square.
The Sun will push on this with a tiny force.
Puzzle: How tiny is this force?
Someone named Bill Russell answered this over on Yahoo. Let me go through his calculation so we can check it.
The momentum of light is given by
p = E/c
where E is the energy of the light, p is the momentum, and c is the speed of light.
In outer space near earth the sunlight provides 1370 watts per square meter - that's energy per area per time. We can use the formula above to convert this to momentum per area per time, better known as force per area... or pressure.
Russell calculates
(1370 watts / meter²) / c = 9.13 micronewtons / meter²
and concludes the pressure is 9.13 micronewtons per square meter. His arithmetic checks out, but I think he's neglecting some physics: when the light bounces back off the mirror its momentum completely reverses, so I think we get an extra factor of 2.
Puzzle 2: Am I right or am I wrong?
The area of the LightSail is about 32 square meters. Russell says this gives a total force of
9.13 micronewtons/meter² x 32 meter²
or about 300 micronewtons. I'd double this and get 600 micronewtons.
Puzzle 3: Once it's out of the box, the LightSail weighs about 4.5 kilograms. How much will it accelerate due to sunlight?
Here we use Newton's good old
F = ma
and solve for the acceleration a. But at this point Russell seems to make a serious mistake. I'll let you see what you think, and fix it if necessary! Here is his calculation:
https://answers.yahoo.com/question/index?qid=20121212091408AA3D606
Yesterday a rocket launched from Cape Canaveral in Florida carrying the LightSail into space! It's a small spacecraft with a big shiny screen that's pushed by the light of the sun.
It's just a test - it won't go far. It will fall to the Earth and burn up. But next year there will be a more serious test. And someday, solar-powered space flight may become a force to be reckoned with.
One cool thing is that all this is being paid for private donations, by a Kickstarter campaign!
The LightSail is carried to space in a cute little CubeSat. It looks like a big toaster, and it weighs just 10 kilograms. But it holds a sail 32 square meters in area, made of a shiny plastic called Mylar, just 4.5 microns thick. This unfolds in a clever way - watch the movie! - to form a big square.
The Sun will push on this with a tiny force.
Puzzle: How tiny is this force?
Someone named Bill Russell answered this over on Yahoo. Let me go through his calculation so we can check it.
The momentum of light is given by
p = E/c
where E is the energy of the light, p is the momentum, and c is the speed of light.
In outer space near earth the sunlight provides 1370 watts per square meter - that's energy per area per time. We can use the formula above to convert this to momentum per area per time, better known as force per area... or pressure.
Russell calculates
(1370 watts / meter²) / c = 9.13 micronewtons / meter²
and concludes the pressure is 9.13 micronewtons per square meter. His arithmetic checks out, but I think he's neglecting some physics: when the light bounces back off the mirror its momentum completely reverses, so I think we get an extra factor of 2.
Puzzle 2: Am I right or am I wrong?
The area of the LightSail is about 32 square meters. Russell says this gives a total force of
9.13 micronewtons/meter² x 32 meter²
or about 300 micronewtons. I'd double this and get 600 micronewtons.
Puzzle 3: Once it's out of the box, the LightSail weighs about 4.5 kilograms. How much will it accelerate due to sunlight?
Here we use Newton's good old
F = ma
and solve for the acceleration a. But at this point Russell seems to make a serious mistake. I'll let you see what you think, and fix it if necessary! Here is his calculation:
https://answers.yahoo.com/question/index?qid=20121212091408AA3D606
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Thanks for the update!44w- Here's the latest news, which is good:
"Our LightSail called home! It’s alive! Our LightSail spacecraft has rebooted itself, just as our engineers predicted. Everyone is delighted. We were ready for three more weeks of anxiety. In this meantime, the team has coded a software patch ready to upload. After we are confident in the data packets regarding our orbit, we will make decisions about uploading the patch and deploying our sails— and we’ll make those decisions very soon. This has been a rollercoaster for us down here on Earth, all the while our capable little spacecraft has been on orbit going about its business. In the coming two days, we will have more news, and I am hopeful now that it will be very good.”
http://www.planetary.org/press-room/releases/2015/contact-restored.html44w - Now for some bad news:
LightSail Falls Silent; Battery Glitch Suspected
After reestablishing contact mere days ago and deploying its solar panels, LightSail has once again stopped sending automated chirps back to Earth. While the full status of the cubesat has not yet been determined, one possible culprit is a glitch in the batteries that is keeping the solar panels from sending power to the battery cells. This problem may resolve naturally as the orbit progresses to "full-sun condition". At such a time, the solar sail will finally be deployed.
Read more: http://buff.ly/1AO320a43w 
+John Baez Of course, we also have to question the rationalization: GR has no c's in its equations (nor electromagnetic theory when done right). Even more, you already have variable c contained in GR without the need to put it in by hand. Consider, for instance, a Minkowski metric where the signature parameter ("a") is simply t. The line element ds^2 = dt^2 - t (dx^2 + dy^2 + dz^2) has no c's in it, neither express nor implied (ds being the proper time).
One need only examine the Lagrangian for electromagnetic theory (L = -k/4 root(|g|) g^mn g^rs F_mr F_ns). The electric field E = (F_10, F_20, F_30) and magnetic field B = (F_23, F_31, F_12) have, as their Lagrangian derivatives (by definition) the induction D = dL/dE = k root(|t|) E and H = -dL/dB = k/root(|t|) B, thus yielding a permittivity epsilon = k root(|t|) and permeability mu = root(|t|)/k (the fact that one must use variable epsilons and mus for the cosmological solution is noted in the literature, but frequently neglected). This, of course, produces a light speed c = 1/root(epsilon mu) = 1/root(t)) as expected.
This metric, of course, is an instance of the FRW metric (the 'radiation dominant' solution). Other interesting properties it has puts to the lie a large number of folklore myths. 1) It is singular at t = 0, nonetheless it is geodesically complete 2) The null geodesics and (most) of the timeline geodesics turn and go the other way at t = 0, so it is causally connected! 3) it is signature changing at t = 0 (with the commoving geodesics passing right through).
All of this would pass by unseen with the convention c = 1.
In contrast, within the broader framework I described, one can even do field theory across the signature boundary. Adopt the ansatz ds = dt + t du (so ds is an inexact 1-form). Then the line element reduces to a quadratic constraint dx^2 + dy^2 + dz^2 + 2 dt du + t du^2 = 0. This metric is actually flat! One can transform (t, u) -> (T, U) such that 2 dt du + t du^2 becomes 2 dT dU. Correspondingly, the proper time becomes a symplectic 1-form: actually a 1-form proportional to T dU - U dT.
The field theory is that inherited from the flat 5-D space. For a Maxwell field, the extra component of the potential 1-form going with the extra coordinate u is none other than the so-called "B-field" 'frequently used in QED; producing the equations that one sees in the B-field formalism.43w- Now for some good news again:
After Series of Setbacks, Spacecraft Prepares to Unfurl Sail
A small experimental spacecraft that uses a shiny sail to reflect sunlight to propel itself came back to life Saturday afternoon after a series of near-death experiences. In a rush to finish the mission before anything else goes wrong, the team behind the craft has scheduled the deployment of the sail for Sunday.
http://www.nytimes.com/2015/06/07/science/lightsail-setbacks-spacecraft-prepares-unfurl-sail.html43w 
https://plus.google.com/117663015413546257905/posts/6L2b1B8r7Xt
The Voynich manuscript is in a few Tamil dialects:
https://twitter.com/alysdexia/status/465916019274301440
http://www.reddit.com/r/IAmA/comments/1yt8cb;/i_am_stephen_bax_researching_the_voynich/chd3wca.
+John Baez unblock my Autymn D. C. account. I left my arguments on why EM radiation pressure can't do net work or propulsion (https://plus.google.com/117663015413546257905/posts/2niejXnkjKu; in short, there is no net displacement of charges or masses in a wave; then I cited how a radiometer depends on atomic propellant to work.); they sat there for a few days; then apparently you deleted them and then me. If this LightSail (https://plus.google.com/117663015413546257905/posts/e6QEbgyJrqa) doesn't work, as I predict, then you shall be compelled to listen to me.
I don't remember which other problematic comments I posted on; but in that month you still posted on black hole trash which I confuted on Quora (http://www.quora.com/What-role-if-any-do-supermassive-blackholes-play-in-the-formation-of-galaxies/answer/Autymn-Castleton), Wikipedia, and Usenet; you solicited the size of a black hole if earth became one, but the size along with other black hole parameters depend on classical (nonrelativistic) expressions that assume celerity is infinite. If special relativity were factored into these, all black holes and singularities should collapse and they'd be seen as the fictional blunder they are, and scientists, experimentalists and theorists alike, and reporters shall be seen as the liars they are.
"the hoax theory is one popular theory; it could explain the low entropy in the text, lower than any European language. But it hasn't been proven."
Theories are proven by definition.40w
