What aspect of the “laws of motion” are you referring to? Photon sails are efficient because the spacecraft doesn’t have to carry the propellant and energy source. There are many technical obstacles to overcome, but that’s true for any means of interstellar travel. I don’t see why photon sails are intrinsically worse than other methods.
Have you figured out how much accuracy could be achieved? I calculated that a one gigahertz beam with an initial spread of 100cm would, after one light year, have a spread of about 3 meters. Do you have conflicting numbers?
Tau Zero by Poul Anderson
Adam Yax: Didn’t Deep-Space 1’s engine fail?
I thought the Voyager spacecraft were the fastest things humans have launched. They’re the only ones with the velocity to leave our solar system and travel intersteller space indefinitely. But maybe my information is out of date.
JOHN HARRISON:
Thank you.
Now, somebody–critique the science in it!
I haven’t heard anything about a failure. The official web site mentions no engine failures, though there were some navigational sensor problems.
Actually, it did fail. That’s what I remembered - the initial news reports of the Ion engine failing. According to that web site, the failure was due to shorting between the plates. But the engineers managed to change the spacing between the grids and start the engine within two weeks.
Ah, that sounds familiar now that you mention it. Still, if an in-flight adjustment fixed it and didn’t result in the loss of the mission then in my book it’s a temporary problem, not a “failure.”
Ryan: I think you’re right, and I’m too pessimistic, but I’m not sure you’re interpreting the numbers correctly. You’re using the right equation:
- spread of momentum x spread of position > h/2pi ~ 3 x 10^-34*,
but if you posit an initial spread of one metre, that means massive uncertainty, does it not? It means that your photons are merely within a 90 degree cone whose apex is the end of the laser?
Positing a spread of 10^-17 in both variables, however, renders the laser useable as described, as you correctly guessed.
scr4 -
Are we talking about the photon sail that is self-propelling? That’s what I’m thinking of.
You’re probably talking DS9 style, which is still probably terribly inefficient in terms of resource cost, but is an idea that can be better translated.
Are you sure they’re not referring to Deep Space 1 (DS-1). It had an ion engine which was basically 10 times more efficient than an equivalent chemical rocket/engine.
As for the laser driven sail the mass of the laser platform would be sufficiently large that compared to the ship its recoil would be negligible.
As for a 10E11 Hz laser spot with an initial waist of 1m I wound up with a beam spread of 6 AUs. Not bad. That’s an angle of .003 degrees over 1 lyr
Now if you use an IR beam (3E14 Hz or 10E-6 m) it drops to .02 AU.
So what kinds of lasers have been proposed for this anyway?
SenientMeat
I guess I multiplied by c where I should have divided. Reevaluating the situation, there would be significant uncertainty, but I wouldn’t characterize it as “massive”. So instead of one gigahertz, how about a laser of 10^-9m wavelength. If the transmitter is 10^3 m and the receiver 10^4 m, then the effective distance, within an order of magnitude, would be (10^3m)*(10^4m)/(10^-9m)=10^16m, or a bit more than a light year. Unless I’ve made another mistake.
I get ~3000 km at 1 lyr. I’ll double check at home
How many people would we have to send on a “generational ship” to ensure that the population could maintain enough genetic diversity? Wouldn’t do us much good to send folks to a planet if by the time they got there the descendants of the original travellers all resembled the banjo player from Deliverance!
Don’t we have the technology for long-term storage of sperm and ova? You can carry a fairly large selection of genes that way. And I’d guess that by the time interstellar travel becomes a reality, it’ll also be possible to sequence a complete set of human DNA. We already know how to read the human genome, and last year someone managed to sequence a virus from scratch.
That we do, but still, how many folks are we gonna need? After all, when the ship arrives, you’re not going to want half the population to be infants or sitting around in storage, waiting to be implanted in a womb. So whilst we can carry millions of sperm and egg, we’re still gonna need a certain number of fleshy humans as well.
I don’t think the Initial population really needs to be very big. I would say maybe 10 males and 50 females, from as many different ethnic populations and different parts of the world as possible (the females are more important in this case IMHO). A male is assigned 5 females and after each 5 have a baby, he is assigned another 5. This then continues until each male has had a child with each female. This would make 500 bloodlines, each with at least half the genes diferent. To increase the genetic diversity even more, the bloodlines can be organised by the mother, so there will be 50 bloodlines. The first generation or two could be limited to only breed with any of the other 49 bloodlines. After the first couple of generations the bloodlines can go back to the standard family type used today.
That’s a lot of “control” over an initmate part of people’s lives. I can’t really see folks agreeing to that, or it lasting for very long, though it’s certainly possible a group or two could manage to do it, but I can’t imagine it working if we’re sending out large numbers of ships with only small groups of colonists on board each one.
Well…I for one would agree to it 
Anyway, I’m sure these people would be exstensivly screened and trained, maybe even have a military background. Plus, I think total control would be much better than merely letting them all handle it themself. How can you even attempt to control the bloodline if a female doesn’t know which of say 5 guys is the childs father?
What male wouldn’t! It’s getting the females to agree that tends to be the problem.
And all that military training goes right out the window the moment someone decides that they’ve fallen in love and that this/that person is the only one for them. And given Murphey’s law, it’ll happen when it’s too late for anyone to do anything about it (like bouncing the members out of the program and replacing them with another pair/set of recruits before launch). What about gays? Whilst some of them would be willing to donate sperm/womb for the mission, others might not. Then, of course, there’s the “problems” that occur if members of the next generation are gay and unwilling to donate sperm/womb. Not to mention what do you do if large numbers of the crew die before reproducing during the mission? That can really screw up your calculations.