Another thread is discussing the problems of getting to Alpha Centauri. This was really close to some disordered thoughts I was having from Netflix's Ascension. Namely, what acceleration do you need to maintain to get to alpha centauri in 100 years?
Turns out, something like 0.004 m /(s^2). One G is 10 m/(s^2), so that's almost nothing.
Somebody tell me if I got my math wrong. Can an acceleration of 0.004 meters per second per second, with a turn around at the mid point, get you to Alpha Centauri (call it 4 light years) in 100 years?
Subscribe to:
Post Comments (Atom)
I think the better question would be: is it possible to carry enough fuel to do that and still make the acceleration you need.
ReplyDeleteA couple tags from the other thread: David Rothfeder , John Powell, Meguey Baker
ReplyDeleteFeel free to tag those others in who were interested. I don't want to threadjack anymore than I have.
The issue is the top speed, not the acceleration. 0.004 m/(s^2) is easy to achieve when your at a relative stop in 0 g, but when you've already been accelerating for a few years, the force to maintain those speeds, even in space are unweildy. Both the rate of acceleration and the maximum velocity are determined by the propulsion method. It seems that riding a series of nuclear pulses has the best theoretical limits for speed and also is quite strong for acceleration, but producing that many matter/anti-explosions is probably beyond our capabilities. I read that conventional nukes could get a craft up to 0.05c or maybe even 0.1c
ReplyDeleteIts a problem of mass, right? So, would starwisps* make it a solveable problem?
ReplyDelete*Starwisps: The smallest viable starship, with sufficiently advanced technology. Instead of carrying around crew, you carry around crew patters or uploaded crew and replicate them on demand. Basically, some computronium, a matter creater (ignore that handwavey physics!), and an engine.
I think your math is off. Accelerating at 0.004m/(s^2) for 50 years gives you a speed of
ReplyDelete0.004m/s * 60 s/m * 60 m/h * 24 h/d * 365 d/y * 50y = ~6M m/s, or 0.02C. That's the max speed you'll reach after 50 years of acceleration.
Cruising at that constant speed will take 60 years to travel 1.2 light years, halfway there. So it will take longer at the max cruising speed to get to that halfway point; your average velocity at constant accel will be half that.
Thanks, Albert Andersen . This is the problem with doing math on paper.
ReplyDeleteOh god, don't get me started on Ascension.
ReplyDelete(SPOILER ALERT)
You can't fool a bunch of 1960's scientists and engineers (and their descendants) that they are on a spaceship rather than in a building on Earth. For. Lots. Of. Reasons.
And given what they were really trying to do - create a hothouse of geniuses to produce new tech and develop psychic powers - they could have just stuck them in a deep underground shelter and convinced them the surface was uninhabitable because of nuclear war, then created a series of resource or geologic crises to keep them distracted and provide motivation to develop new technology.
... psychic powers? That's more spoilers than I knew about, John Powell !
ReplyDeleteDoing an end run around the calculus, you need to travel 1.2ly in 50 years, which means you need an average speed of 0.024c.
ReplyDeleteat constant acceleration, that means you need a top speed of 0.048c.
to hit 0.048c in 50 years, you need:
(0.048/50/365/24/60/60)*300,000,000 = 0.009132420091 m/(s2)
Less mass means less energy needed, but I think the issue is e=mc^2, in that any mass requires exponentially more energy to accelerate. I'm not really an expert on this though, just have been reading what theories are out there for this problem. An interesting thing in the Wikipedia article though says that according to a traveler on the shift, they will observe unlimited acceleration since the time dialation will match the acceleration slow down. If this is true it would save several steps of math since you can calculate the time from the travelers perspective and use that to determine the number of generations needed.
ReplyDeleteHalf of 4.3 lightyears is 2.15 ly. To travel that in 50 years means a constant acceleration of .00167 g or 1.67 thousandth of a g.
ReplyDeleteAscension would be in microgravity for the entire trip. Essentially zero-g.
They'd need a big really big atomic reactor, a lot of reaction mass and some really efficient ion engines to pull this off.
But most importantly, you can't simulate zero g for 100 years on Earth.
Maybe if they put the Ascension in a long orbit through our solar system, then simulated the rest of the flight?
But then you couldn't put folks on and off board.
William Nichols the girl with psychic powers is revealed in Episode 3 as being the actual object of the experiment's author, as opposed to the tech development idea he sold it to the government as.
ReplyDeleteIf we're pretending to have awesome ion engines, let's just pretend we have antigravity too.
ReplyDeleteJohn Powell Thanks for the math; my back of the envelope math was fading fast.
ReplyDeleteI had assumed they were heading out at 1 G, because there aren't in microgravity. But then, we get something on the order of 1,000 light years in 50 years. Is my math horribly wrong? AGAIN?
.. which should mean they have their choice of anywhere to go. That's a ginormous radius.
Here's a good calculator for trying different accelerations for Ascension, scroll down to "Long Relativistic Journeys"
ReplyDeletehttp://www.cthreepo.com/lab/math1/
Ooooh. That's actually really hard to use, but I'm glad it exists!
ReplyDeleteI'll note that due to relativistic effects, you cannot get a 100 yr ship time trip at 1g from this calculator due to the limits of Java. 10,000 lightyears would take less than 18 ship board years, 100,000 ly would take just over 22 years. Of course back on Earth a lot more time will have passed...
ReplyDeleteRead Frederik Pohl's novel Tau Zero for more on what constant 1g gets you when you can use it to get arbitrarily close to the speed of light.
Ion propulsion is theoretically possible with today's technology, as is simulated centrifugal gravity (ok, the latter is pretty simple to create). I think the only reason we don't have the gravity substitute is that we're more interested in the effects of 0 g then making people comfortable in space. I know the reason for not developing ion drive though, their acceleration is so slow that they are impractical for use within the solar system. The only reason they're important for interstellar travel is that they are relatively light and efficient, are sustainable, and have a top speeds much higher than conventional rockets (3 to 7 times faster from what I've read)
ReplyDeleteYeah, if you wanted to sell a bunch of 1960's scientists and engineers to take a 100 year interstellar spaceflight to Alpha Centauri and experience 1g all the way, you'd have to convince them you had alien engines and artificial gravity plates from the Roswell crash or something.
ReplyDeleteJohn Powell Right. They have calculators, and -- being engineers and scientists -- probably a far better intuitive grasp of relativity and calculus then we do. Certainly than I do.
ReplyDeleteJust easier to put them in a fallout shelter. Descension anyone?
ReplyDeleteEva Schiffer They were recruiting top scientists and engineers. You wouldn't be able to convince them of antigravity. Hell, you couldn't have convinced me in high school in the 1980's that we'd developed antigravity. Our knowledge of physics just doesn't allow any workable antigravity technology. (Probably because it isn't possible.)
ReplyDeleteJohn Powell I'm not privy to the original discussion that spawned this, so a huge chunk of this conversation is becoming increasingly inexplicable to me.
ReplyDeleteEva Schiffer John is discussing more and more about the Netflix TV series Ascension. Which has a bunch of 1950's era scientists sent on a space ship to Alpha Centauri. Where they have 1 G gravity. And it'll take 100 years.
ReplyDeleteThe only reasonable way to have 1 G in space is acceleration, which isn't very reasonable at all. And if they do that, you can go a lot further than 4 LY in 100 years.
And then the speed of light punches you until you die.
That's basically the relevant parts, which I think you've got. The rest has to do with particulars of the show, including some spoilers.
William Nichols Also gravity and acceleration differ in one really important aspect - gravity lessens as you get further from the center of the planet.
ReplyDeleteThe feeling of gravity caused by acceleration would be the same for all parts of the ship, but real gravity would be stronger at the bottom than at the top of the ship.
Not really measurable by scales that the Ascension crew might have aboard. But if they were given atomic clocks, or some genius on board developed one, they'd notice it would run slower on the lower decks than on the bridge because the ship was in Earth's gravitational field, rather than accelerating through space.
I still don't buy that you couldn't trick people you were in space if you designed the habitat to resemble a spinng ring to simulate gravity and kept the ceilings low. You might need to do some texting and clever engineering time make the floors appear like they slop up word, but there is a threshold you need to exceed before people would notice.
ReplyDeleteAlso - don't get me wrong. Even though all the science is wrong, I love this show (Mad Men in Spaaaace!) and hope they do another season.
ReplyDeleteAs far as scientists in the 60 ' s, it depends on their specialties. There certainly were a lot of theories of how to reach another star system at that time, so unless a scientist had specific expertise, it is unlikely that they would know the feasibility of the propulsion being used. Some of those theories from the 60 ' s actually have decent chances of reaching alpha centuri within 100 years, so even if a scientist knew the theory, they still might be fooled if they weren't specifically investigating.
ReplyDeleteDavid Rothfeder
ReplyDeleteIf the writers couldn't convince me - an interested amateur who's read sf and took a couple physics courses in college, they aren't going to convince folks who know how to use a slide rule and their genius offspring. It's dumb sf written by English majors.
I was unaware that they claiming normalized gravity from constant acceleration. That is an unreasonable way of simulating the earth's pull. I will say though, that if this was achieved, even the most sensitive instruments would have a hard time distinguishing between the acceleration and normal gravity over distances as small as a space ship.
ReplyDeleteDavid Rothfeder So far as I've seen, they haven't said. It was my assumption that they used a constant acceleration engine -- as how else would they simulate gravity?
ReplyDeleteI'm open to other ideas to simulating gravity in the specific direction.
They didn't claim anything, but there is no reasonable way to design the Ascension so it could simulate Earth's pull, and still have it compatible with the basic premise - they haven't left Earth.
ReplyDeleteConstant acceleration is the most reasonable explanation, and as we've seen, to the physicists and engineers on board, that's going to seem like magic.
ReplyDeleteThe easiest way to simulate earth's gravity is with a constant centrifugal force. That's why in sf space stations usually look like giant rings, the idea is they rotate at just the right speed so that the people on the inside think the outer edge is the floor. There are inherent problems with this approach, mostly that if you travel towards the center of the ring the illusion of gravity diminishes. The other problem is seeing the floor curve can be disorienting. There are other designs that use this same concept, but a ring is generally the most space efficient and consistent. The beginning of 2001 a space odessey does a great job of showing what such a design would like from the iside.
ReplyDeleteDavid Rothfeder Yes. In the referenced show, gravity is down like in an office building. Torch at the bottom, ship on top. There's no spin. Believe me., I looked for it.
ReplyDeleteYes, spin gravity works for zero g, but how do you build a ship designed for spin gravity, leave it on Earth and still convince folks they are in space? It's not doable.
ReplyDeleteYou don't need to build a ship, you just need to build an interior that looks and feels like one. This sort of thing is feasible, but I do not have the know how for the specifics.
ReplyDeleteDavid Rothfeder - in the show they built the ship as a full scale office building sized flight simulator with fake starscape projectors and hydraulic rocker arms. The crew has the run of the simulator from bridge to engineering. They believe they are 50 years into a 100 year voyage to Alpha Centauri. They have lived aboard this simulator for all those 50 years. They produce their own power, grow their own food, recycle air and water.
ReplyDeleteHow are you going to fake a spinning habitat for them?
John Powell Remember, not eveyrone has seen the show. The fakery of the ship makes it a lot harder -- that is, that the crew can't tell. And once you've got geniuses on the ship, its a lot harder.
ReplyDeleteI am not arguing the specifications of the show, just that possibility of the premise.
ReplyDeletepretty sure it would take more earth-to-orbit spaceflights than we've had in our entire history to assemble that huge generational ship ready to launch.
ReplyDeleteas far as acceleration, did they ever explain their propulsion system? my vague recollection is a solar sail could actually get a ship up to speed to reach another star in semi-reasonable time. for values of reasonable over 1000 years. ah, citation: http://www.space.com/22442-solar-sail-starship-interstellar-spacecraft.html
To get back to the central conceit: What is the most feasible method of interstellar travel, assuming no miracles occur? (ie, no FTL or worm holes or stargates)
ReplyDeleteIt seems like you don't want living people onboard, and want to minimize mass -- including reaction mass. If you're fine taking a very long time, you don't need any form of constant acceleration -- and the optimal craft starts look like a microbe.
Failing that -- solar sales? Would you slow down quickly enough as you got to the next star?
yeah that's the problem with solar sails i think. how do you stop after you
ReplyDeletereach .05% of light speed?
the creators just need to fire up sagan's old "cosmos". pretty sure he hit
on it there.
... don't you stop as you did with any sail, by breaking with the wind of the next sun?
ReplyDeleteI actually think that ion drives might be more promising than solar or magnetic sails. Then again there are tons of advantages of off ship laser propulsion, but that has quite a few inherent issues (like what happens if the earth stops powering the lazers)
ReplyDeleteAs I've brought up on several of these discussions before, is anyone taking the relativity effects into their calculations? That is, not the time passed on earth to reaching destination, BUT the relative time of the craft itself and its contents in comparison.
ReplyDeleteYes, it has been discussed. An interesting point I read but didn't verify is that if you assume there is no hindrance on acceleration, you end up calculating time from the craft ' s perspective.
ReplyDeleteSo apparently there are real theories from physicists on warp fields as a means of propulsion and serious research being done on the concept.
ReplyDeleteDavid Rothfeder Yeah. There's now a Warp Theory 101 class at NASA. Thanks in part to this: https://en.wikipedia.org/wiki/Alcubierre_drive
ReplyDeleteunfortunately, that requires creation of exotic matter which ... maybe ... would take more energy to create than there is in the observable universe. Oops.
Hooray for awesome infeasible technologies that support the fantasies of my youth.
ReplyDeleteDavid Rothfeder did we have the same youth? Similar fantasies, at least.
ReplyDeleteWell I did grow up on Asimov and Star Trek The Next Generation.
ReplyDeleteYep. Asimov's robots have a "and then a miracle occurs" to get warp power, but the robots figuring it out and almost going crazy.
ReplyDeleteThat's what I love about the robot series, even the impossible technologies have logical consequences. ..well at least until you get into universal hive minds and nearly all knowing psychic androids.
ReplyDeletewell, R. Daneel Olivaw does have consequences ... like the continuation of humanity. Its just bizarre how it happens.
ReplyDeleteI just don't like how in love Asimov got with psionics later in his life. Liar was great, mostly because the story was about the mystery of a psychic robot, and I did like robots of dawn, but the 2nd foundation and Daniel continuing on to manipulate the galaxy fell flat to me.
ReplyDeleteYeah, David Rothfeder , he was looking for a way to get from the robot-centric books to the Empire novels, which easily turn into the Foundation series. But, the push from robots to Empire was difficult, and even he needed some magic.
ReplyDeleteRobots and Empire wasn't bad, and while it wasn't his best, it did serve to show how the whole robots, earthlings, spacers dynamics fall apart to allow for something new. My bigger issue is with the Foundation prequels and the later books where Daniel comes back.
ReplyDeleteYeah, Daneel pulling the strings behind Seldon really reduced the agency of humanity.
ReplyDelete