Talk:Interstellar travel/Archive for 2008

Structure
I don't like the structuring of this article. Why the division to "slow" and "fast"? What is the definition of "slow" and "fast"? Why the division "sub-light-speed" and light speed travel? I think the section now called "sub-light-speed travel" is the most important part of this article, therefore it should have a chapter of its own. Light speed (or faster) travel is extremely speculative compared to the other propulsion methods presented here, so it should also have a chapter of its own.

I'd propose the following new structure: 1. Reasons for interstellar travel (why would we want to go?) 2. Difficulties of interstellar travel (including distances) 3. Manned missions (the first missions will very likely consist of unmanned probes anyway, but this section would discuss the additional problems of manned missions) 4. Proposed methods for interstellar travel 4.1 Rocket propulsion (chemical, nuclear and fusion rockets) 4.2 Nuclear pulse propulsion (fission and fusion) 4.3 Antimatter rockets 4.4 Interstellar ramjets 4.5 Beamed Propulsion 4.5.1 Beamed particle propulsion 4.5.2 Beamed microwave propulsion 4.5.3 Beamed laser propulsion 5. Further speculative methods 5.1 Transmission 5.2 Warped space-time 5.3 Wormholes 6. Methods for slow manned missions (missions, which either use a slow propulsion method OR are directed to very distant stars) 6.1 Generation space ships 6.2 Suspended animation 6.3 Extended human life span 6.4 Frozen embryos 7. NASA research

I think this structure would be a lot clearer than the old one. It does require some new writing (chapters 1 and 3 plus extended chapter 4) Offliner (talk) 22:07, 26 February 2008 (UTC)

I implemented the new structure (still need to write the new chapter "Reasons for interstellar travel"). I'd still like to rewrite chapter 1.2 (manned missions), and extend chapter 2 (imho the most important chapter of this article). Offliner (talk) 22:32, 26 February 2008 (UTC)

Alpha Centauri movement footnote, and link concerns
I removed the following from the article (which is very cool!). I'll explain why:


 * (*) actually our solar system and Alpha Centauri are moving relative to each other at several kilometres per second, and as far as i can remember, they are moving approximately towards each other, so this should reduce travel time a little. It would still be thousands of years though.
 * This point was precisely addressed in First Ark to Alpha Centauri - a 2004 sci-fi novel concept about sending the first human interstellar ark on a 50,000 year long voyage duration by some unknown author, named A. Ahad . See my footnote below First Ark to Alpha Centauri by A. Ahad. 81.154.164.210 (talk) 19:36, 14 March 2008 (UTC)

''The continents on Earth are moving, too. It doesn't affect our travel plans that much. :-) ''

We don't have footnote capability yet; just use parentheses. Maybe someone can put this back in the article--I don't really want to edit it myself, mainly because I might screw it up. Also, we want to avoid first person. If the reason you use first person is because you're not sure of something, why are you putting it into an encyclopedia article?


 * talk:Time Travel

Having a link to a talk page in the article itself is not a good idea.


 * For information on the technical difficulties of interstellar travel, see Spacecraft Propulsion, Astronomy, Physics, Chemistry, Biology, Psychology, and Economics.

I'm sorry, but only the first of those pages actually contains any information obviously relevant to the technical difficulties of interstellar travel. Either find more specific pages, or, if you want to make a claim, make a claim: "Our understanding of the technical difficulties of interstellar travel is informed by multiple fields: ..." Don't mislead people into thinking that they're going to be able to find information about the difficulty of interstellar travel just by following those links. --LMS

Science Fiction?
The openning statement claims everything below to be largely science fiction. This is completely inaccurate. It would be fair to suggest it is highly theoretical but everything mentioned in the article does have a scientific basis. I believe it could be referred to as Protoscience or Highly Theoretical but it is by no means 'science fiction'.

Furthermore, if there is anything in the article that can be accurately described as 'science fiction' then it should be removed.


 * It's darn near impossible to get a scientifically accurate assessment of interstellar travel. People talk about pushing spacecrafts with lasers -- forgetting the law of diffraction would never allow you focus the beam over such distances.  They draw pictures of antimatter engines that would simply vaporize if they were ever turned on.  There is a pretty solid discussion of the relativisitc rocket equeation in Tayler and Wheeler's book "Spacetime Physics".  But basically contemporary discourse about intersteller travel is like the 19th century discourse on interplanetary travel -- on the level of whacky drawings of Victorian gentlemen firing cannon balls to propel themselves to the Moon. DonPMitchell (talk) 04:52, 11 July 2008 (UTC)

Current Propulsion Speeds
With current spacecraft propulsion technologies, a trip to the moon will typically take about three days.

contradicts

The New Horizons spacecraft took only nine hours to reach the Moon's orbit from New Horizons.

Does this not mean that with current spacecraft propulsion technologies, a trip to the moon will typically take about 9 hours? Abbail 14:32, 8 May 2007 (UTC)


 * Well, yes, sort of. "A trip to the Moon" is not quite the same thing as "to reach the Moon".  There are two points.  The first is that the time to pass the Moon's orbit —never mind whether the actual Moon is there or not—starting from Earth is quite sensitive to the launch velocity.  The second point is that if you are planning to go to the Moon, you probably want to stop there and orbit it (and then land, maybe), not be flying by at 5 km/sec.
 * Several spacecraft bound for the outer Solar System have passed the Moon's orbit in hours; if I recall, the European Ulysses mission took about 10, on its way to Jupiter in 1989. To reach the distance of the Moon at all you have to leave the planet at about 11.1 km/s, and it takes around three days.  This puts you into a long elliptical orbit with apogee near the Moon's orbit, allowing you to be captured or land.  But if you go only a little faster at Earth, say 15 km/s, you will be traveling nearly 10 km/s when you pass the Moon's orbit.
 * The reason is energy -- it has to be conserved. The "kinetic energy", that goes with speed, is proportional to the square of the speed (as long as you stay well below the speed of light).  To escape from Earth needs an energy proportional to 11.22, where 11.2 km/s is Earth escape velocity.  But if you launch at 15 km/s, your kinetic energy is proportional to 15² = 225, and when you escape from the Earth's gravity you will have lost 11.22 of that, and then have quite a lot of energy, 225 - 125.44 = 99.56 (in the right units...) left -- so you will still be moving at the square root of 99.56, or 9.98 km/sec.  This much higher than the minimum needed to reach Mars, and only a little less than what is needed to escape from the Solar System entirely.
 * Voyager 1, the fastest thing before New Horizons, has essentially escaped and is traveling at 17.1 km/s away from the Sun, at which speed it would take about 75,000 years to reach the distance of Alpha Cen.
 * So we really need to go ~1000 times faster than that to make it in a lifetime, which needs 1000*1000 times as much energy—or really twice that, if you want to stop. Which is why it is so tough.  Wwheaton (talk) 02:42, 24 May 2008 (UTC)

"Super light speed" travel
Steve's point above in the "Sub-light travel section" is essentially correct though, and well known to many physicists with some background in high-energy physics or general relativity (GR). (I haven't studied his derivation in detail, but I know the basic result.) I believe Misner, Thorne, & Wheeler's classic tome on GR covers it. The quantity $$(g\tau/c)$$ is known as the rapidity to high-energy (ie, elementary particle) physicists, and is basically just what you would get if you carried an inertial guidance system along with an accelerometer and a clock, and you had never heard of GR and never looked outside your ship, but only reckoned you speed by dead-reckoning from your acceleration and the local clock. The nice thing about it is that it is simply additive, unlike velocity in special relativity (SR). And, it is easy to remember, because (by pure chance) 1g * 1 yr ≈ rapidity of 1, pretty near. (Units would be sort of "light years per ship year", in some screwy sense I haven't quite figured out.)

The key point is that the relativistic time dilation effect effectively and neatly cancels out the effect of the speed limit of $$c$$. Distance traveled is essentially exponential in ship clock time $$\tau$$, known as proper time.

The sad news is that no one has any idea how to build a ship that can maintain an acceleration of 1 g for even a day, let alone a year, let alone 30 years. The engineering problem is exponential in the rapidity, just as for a normal rocket. If you had an ideal photon rocket (ie, complete conversion of matter into a photon exhaust of speed $$c$$, according to $$E = m c^2$$), then to get to rapidity 1 would need a mass ratio of $$e = 2.718$$..., etc, essentially according to the usual rocket equation, with rapidity in place of velocity.

Another curious fact is that if you could, say, rob a bank and then accelerate for a certain time (I think to rapidity = 1, but I am not quite certain about the exact value) then if the cops don't start chasing you by that point, they can never catch you, no matter how much more powerful their police cars may be, as long as you don't turn down your engine and stop accelerating. You actually pass through a kind of event horizon, but in special relativity. (Actually not all that surprising maybe, since the key assumption in GR is the Equivalence Principle, that gravity is physically just like acceleration.)

I think it is quite remarkable that you could go far beyond the visible horizon of the universe by simply accelerating at 1g for my lifetime (66 years last week). Of course, if you do that, "you can't go home again", because the Earth would be trillions of years older, by the usual twin paradox problem. Cheers, I guess!? :) Bill Wwheaton (talk) 07:50, 2 March 2008 (UTC)

I have now gone to my books, and found the reference in Gravitation, by C.W. Misner, K.S. Thorne, and J.A. Wheeler, (1973), in chapter 6, especially section 6.2, page 166 ff. Some of the exercises reproduce results similar to those above by User:71.200.127.222 for us rocketeers, more or less exactly. Wwheaton (talk) 09:34, 2 March 2008 (UTC)

I don't quite understand what you mean, with all this general relativity stuff, I think it needs to be explained simpler. As I understand it, you can't get to the speed of light because of mass dilation, the faster you go the more energy you need to go faster. I'm not sure even infinite energy will get you to light speed, but you may be able to take a shortcut by warping space itself, we've calculated it and it would take astronomical amounts of energy and negative energy (See the physics of star trek book by Laurence Krauss) which we couldn't do.

I understand what you mean by if you could near lightspeed, you would perceive the trip as being shorter, and you wouldn't age, or age slower but as that Nasa link above states you couldn't get near it without supertankers of anti-matter and matter. The snare (talk) 03:19, 20 August 2008 (UTC)

LINAC
Anon editor 71.200.127.222 (apparently a SPA) keeps inserting unsourced information about "LINAC"/"Linear Accelerator Propulsion" into this article. The last time I removed it, I asked the editor via his/her talk page to provide a source before adding the information again. The information has been re-inserted with a reference to the book "Einstein for Dummies." However, said book does not appear to contain any mention of either "LINAC" or "Linear Accelerator Propulsion." Can anyone identify what pages of the claimed source deal with this propulsion method, or any other reliable source to justify its conclusion in this article? PubliusFL 18:15, 19 August 2007 (UTC)


 * I believe Misner, Thorne & Wheeler's standard text Gravitation (1974?) has it all, and I have worked it out myself, so I believe it is well known. I added a section after the "Sub light speed trevel section" above with some additional remarks.  I have not seen 71.200.127.222's linac material (reverted, i guess), and think a linac or ion drive is not a viable means of near-light speed propulsion, for engineering reasons, as discussed above.  Wwheaton (talk) 08:37, 2 March 2008 (UTC)


 * Thanks for helping find additional sources, but the Linear Accelerator Propulsion thing is all I objected to. The aforementioned IP editor kept trying to insert information about that propulsion method to justify a link to (what I presume to be his) web site. PubliusFL (talk) 17:49, 3 March 2008 (UTC)

First Ark to Alpha Centauri by A. Ahad
This sci-fi concept seems to address the issues of interstellar travel,and many seem to think it could be our first defacto blueprint for a conceivable voyage to our nearest stars 81.154.164.210 (talk) 19:39, 14 March 2008 (UTC)


 * The book looks interesting, but it appears to be a self-published work, and self-published works generally aren't considered reliable sources for Wikipedia purposes. PubliusFL (talk) 22:24, 14 March 2008 (UTC)


 * Also, it is not really significantly faster than Voyager 1, which gets to the distance of Alpha Cen in about 75,000 years. This is a case where we should definitely spend 10,000 years on energy and propulsion technology research before committing a crew to a 50,000 year journey.  Assuming human technological civilization survives on Earth at all, I would bet that with 1000 years of technology research, we can reach that distance in less than 1000 more years of trip time. Wwheaton (talk) 02:58, 24 May 2008 (UTC)

Organization
I re-wrote Space colonization#Starship recently to try to separate the long-term speculative ideas from those that seem to be real foreseeable possibilities. This is of course quite a dicy, matter-of-speculative-opinion, but I really believe interstellar travel is so terribly, fundamentally difficult that I would guess no one now living will see it happen, though I trust it will come in due course. Space colonization#Starship summarizes what I think is reasonable within the next few centuries without fantasizing breakthroughs in fundamental physics (which will likely come, but are impossible to predict). Those unfamiliar with the physics might also look at Minkowski diagram for some information on just why it seems so impossible to break the speed limit, which really has to do with our understanding of the nature of time and causality (both controversial—progress possible!), nothing to do with engineering.

Anyhow, I think this article would be greatly improved if it were structured to discuss the nearer-term prospects first, all collected together, and then put the more distant possibilities off into a section at the end (or possibly even a separate article) so it is more clear to a naive reader where we are today and what is conceivable for the future. I don't mean to sniff at the speculative ideas, but I do think it is important to make a distinction between what we can foresee and might actually be able to do based on accepted physics and engineering, and what requires breakthroughs in understanding that we can only imagine and dream about. Wwheaton (talk) 17:40, 23 May 2008 (UTC)

Beamed Propulsion
"A light sail or magnetic sail powered by a massive laser or particle accelerator in the home star system could potentially reach even greater speeds..."

Speeds greater than what? The systems described in the previous two sections are said to be able to reach near-light speed. Further down the article the following is written:


 * They can potentially achieve greater speeds than rocket-based propulsion methods. Offliner (talk)

"...interstellar craft with gigantic sails, propelled by laser light to about one tenth the speed of light. It would take such a ship about 43 years to reach Alpha Centauri..."

Which seems to be (according to the article) just the minimum speed needed to even consider a craft to be capable of "interstellar speed". Anyway isn't this inconsistent with the earlier statement? --70.143.56.3 (talk) 08:32, 15 June 2008 (UTC)


 * I don't think they are inconsistent. The latter statement is Landis' opinion on what kind of speeds beamed propulsion could achieve. But potentially they can accelerate to near light speed (as do antimatter rockets too, it seems.) But I admit the article is a bit clumsily written here. Offliner (talk)


 * Note that because the light beam is red-shifted (seen from the ship) as the vehicle velocity increases, the propulsive effect decreases to zero as c is approached.  The same thing would happen with a particle beam (which has other problems as well).  Also, keep in mind that to do the acceleration locally, in the vicinity of the Sun (within 1000 AU, say) you need crushing accelerations to get near light speed.  The alternative would be to have the beam focused on a small (? >0.1 AU, say ? ~0.1 arc sec seen from Alpha Centauri) spot over interstellar distances, which needs a vast sail and a vast (or likely vaster) beaming antenna.  Note also that very large sails would begin to encounter increasing photon drag from the Cosmic microwave background at speeds greater than about a thousand km/s. Wwheaton (talk) 20:45, 2 July 2008 (UTC)


 * The propulsion systems being proposed are diffraction-limited systems. Light has about 500 nanometer wavelength, a kilometer sale at a distance of a lightyear subtends an angle of 1.0E-13 radians, so your laser has to be 2400 kilometers in diameter.  This (planet-sized) laser must be built to optical tolerances, mirrors flat to 500-nanometer precision or better.  Ain't gonna happen.  DonPMitchell (talk) 05:17, 11 July 2008 (UTC)

Locality?
I have temporarily reverted the insertion by ProfessorToomin of "local" into the "Interstellar distances" section, as I think few readers will understand the precise meaning (which I presume is taken from the "local realistic" descriptive term used in discussions of the interpretation of quantum mechanics), and those that do not understand this issue will simply find it confusing. Perhaps this could be moved to another location, where faster-than-light (FTL) travel is discussed, and more fully explained or referenced there? Wwheaton (talk) 05:49, 28 June 2008 (UTC)
 * After a reversion by ProfessorToomin, I have altered a "given region" to a "flat region", and wikilinked to Minkowski space, which I think addresses the issue at stake here. FTL travel appears to be impossible within a flat spacetime, regardless of size or extent.  Hope this is clearer than "a given region", and satisfactory to all. Wwheaton (talk) 20:26, 2 July 2008 (UTC)

Reasons for interstellar travel
Should we add a chapter about the reasons why people think we should launch interstellar missions? The usually mentioned reasons are 1. studying interstellar space and other star system more closely than it is possible by using telescopes, 2. ensuring the long-term survival of the human race, and 3. insipiring our culture. Does someone happen to know any good sources we could use? Offliner (talk) 18:20, 13 September 2008 (UTC)

Faster-than-light travel
Just reverted an edit by User:172.129.225.80 pushing faster-than-light (FTL) travel as a realistic possibility. Of course we do not know everything, so I cannot say we know for certain that it is absolutely impossible, but anyone with as much as a bachelor's degree in physics should understand that it is so far beyond current day science that it has no place in a serious article about interstellar travel: we just cannot discuss it intelligently, or in a way that could advance a naive reader's understanding of how to accomplish it. The causality problems described in Minkowski_diagram I believe are at the root of the trouble, though of course the light-speed barrier has many other manifestations in relativity physics. The reference given cites controversial claims in work reported before July 2000, which I think have not received much traction since, at least in the context of FTL travel. It's OK to dream, of course (and dreams may sometimes come true, out of all reckoning), but I think we should not let a fantasy distract us from the core problems we need to solve to make interstellar travel a reality, in a serious "encyclopedic" article.

I hope other editors will agree with me that this has no place in our article, and I think it should at least be discussed here for a while before being re-instated. Wwheaton (talk) 07:06, 25 September 2008 (UTC)


 * The material I removed has been restored, apparently by the same editor (now at IP 172.129.225.80). This must be discussed before it can go into the main article.  I request comment by other editors so consensus can be reached.  Thanks -- Wwheaton (talk) 00:59, 26 September 2008 (UTC)
 * I'm not someone who normally edits this page, but I agree completely. There is absolutely no real evidence that there is any problem with the current relativity theorys concerning the acceleration of matter to lightspeed. This person is obviously uneducated, shown in their statement that "energy leaves the sun at light speed but doesn't have infinite mass" - why exactly would energy have ANY mass? This person is just trolling for responses or is very missguided. Keep it out of the article. ATOE (talk) 17:25, 26 September 2008 (UTC)

Sentece fragment
I just reverted an edit by User:Ankank because it was an ungrammatical sentence fragment and I could not determine the intended meaning—which may have been useful, however. If it can be clarified here we can probably get the English straightened out. Wwheaton (talk) 15:35, 10 October 2008 (UTC)

Ankank (talk) 16:19, 16 October 2008 (UTC) Hi, I see my addition to the article has been erased for poor English. Sorry everybody, I'll try again. What I am trying to do is to lay to rest the disincentive-to-travel- because-future-ships-will-overtake-earlier-ones fear that is always mentioned in articles about interstellar travel. If you calculate the rate of travel velocity derived from growth (even exponential growth) for a given destination, it can be shown that there is a clear minimum in the time to destination from now. This is significant because voyages undertaken before the minimum will be overtaken by those who leave at the minimum, while those who leave after the minimum will never overtake those who left at the minimum. For any civilization looking to travel to interstellar destinations, it can look forward to a unique date that is best to leave, and one that is the most efficient in terms of cost and time for it.

Is that clear?

Ankank (talk) 16:19, 16 October 2008 (UTC)
 * OK, thanks. Wwheaton (talk) 18:02, 21 October 2008 (UTC)

Difficulty: Intergalactic > interstellar > interplanetary?
This article makes gross assumptions repeatedly that Intergalactic is much more difficult than interstellar... but who really knows? Maybe more massive gravity sources will prove to be more likely to anchor ends of worm holes, or more easily found and latched on to with artificial technologies. If so then intergalactic could be easier than interstellar in turn easier than interplanetary. This might defy common-sense and current technology but still the articles current claims are without proof. 115.129.5.14 (talk) 09:48, 8 December 2008 (UTC)
 * I've just re-located this section to the end to preserve Wiki's standard last-at-the-end talk page ordering. But re the substance of your suggestion, I think it is simply too speculative to say anything useful about, in the context of this article.  Wormholes are indeed a fascinating possibility, but the conceptual problems they bring in from physics (causality, time travel paradoxes, etc), not to mention the practical issues (ie, How could you find or make one?  Can a material object actually pass through one and survive without being torn into quarks?...) are boggling, and take us several steps beyond our present horizon of practicality.
 * I think we can likely actually make it to the nearest stars in a millennium or less, with trip times of no more than a human lifetime, and do it without magic. Of course our understanding could be turned upside-down in fifty years (and likely will be), but we cannot predict now how that will happen or what it will imply.  It's just a personal inclination and others may differ, but I think this article needs to focus on the most realistic scenarios.  The whole subject is at the edge of madness, even so....  Cheers, Wwheaton (talk) 20:24, 8 December 2008 (UTC)

Possibility of interstellar travel
Is interstellar space travel possible? Large questions of this kind would make interesting articles, keeping in mind to maintain neutral point of view and to make it work with the structure of other articles. So, on with the article.


 * I agree!! This article does not give any space for the alternative viewpoint, which is that interstellar travel is not possible.  I suggest this page as a reference. —Preceding unsigned comment added by 68.52.20.230 (talk) 08:28, 15 September 2007 (UTC)


 * Also agreed. At very least the last sentence of the first paragraph of the article should be removed —Preceding unsigned comment added by 72.42.137.13 (talk) 07:51, 27 October 2008 (UTC)