Talk:Interstellar travel

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)

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.

Interstellar travel in fiction

It is probably worth having a seperate page for treatments of this subject in Fiction. With a few rare, honourable exceptions, fictional treatments of this subject make no attempt to coincide with reality. If you are interested in real world issues on this question, you can safely ignore Star Trek and almost every Hollywood movie about space.

Paragraph headings

Some random headers for paragraphs I will write later if no-one does it first.

possibility of new physics allowing faster than light travel

possibility of reaching other stars by travelling slower than light, faster version

and slower version

Biological vs. technological problem

A couple of thoughts on the topic:

There's a comment, (possibly by the english physiologist J. B. S. Haldane) that might be worth tracking down, that interstellar travel is more a problem in biology than technology: ie. the problem is our short lifespans compared with the travel times...I think it was Haldane (or possibly Olaf Stapledon) who came up with the idea of multigeneration 'arks' for interstellar travel.

First vs. third person voice

This is a very interesting article so far, and thanks to whoever has been working on it!

As a relatively minor point, can I ask you please not to write in the first person? It's rather grating in an encyclopedia article. --LMS

Time dilation and wormholes

Someone (with more physics knowledge than me) should talk about time dilation, which as I understand it means that while it will still take more than one year to travel one light-year, the slowing down of time for those on board the ship as its speed approached the speed of light would mean that those on board would experience the trip as taking as little time as one wants. (Of course, there is a limit here due to the amount of time it takes to accelerate at a rate humans could survive in comparison to the length of the trip...)

Someone should also point out that general relativity may allow the shrinking of the distance between the origin and the destination of the trip, by bending space or creating wormholes.

-- Simon J Kissane

Travel by humans vs. travel by robots

Interstellar travel by humans is a fantasy. Humans will be replaced by robots long before interstellar travel becomes possible.

I don't think that's a consensus opinion, you haven't stated any reasons why you think that's the case, and interstellar travel

by robots or extra-terrestrials is worth discussing in any case. Robert Merkel

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 [1]. 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

Fast interstellar travel methods from Robert Zubrin

Robert Zubrin's (the guy behind the Mars Society) "Entering Space", where he discusses possible interstellar travel given currently known physics. He comes up with a couple of seemingly feasible systems - fusion powered ships, lightsails powered by lasers, and "magsails" (which would be used in combination with the other systems for braking at the other end). With these systems, one-way trips to Alpha Centuri within a current human lifetime (and, remember, greatly extended human lifespans are entirely feasible given progress in medical research) are at least plausible. Antimatter-powered starships could theoretically permit travel at near-lightspeed (modulo the shielding problem from any stray atoms in the road) but production of antimatter seems likely to be a considerable challenge in economics, to say the least.

Zubrin makes a persuasive and seemingly scientifically reasonable case (but then, I'm not a physicist). Do people think it's worthwhile for me to add some material based on (obviously not copied from) his book? Robert Merkel

Sounds like it would be a good contribution to the article. It could always go in the biography article on him if people dont want it here anyway Astrokey44 05:24, 26 September 2005 (UTC)

Bussard ramjets

Something on Bussard ramjets, as a possible way of getting to high enough speeds that time dilation comes in?

unfortunately, it turns out that the density of interstellar hydrogen is an order of magnitude less than it was thought to be when Bussard first proposed the design. But I guess it should still be mentioned, as an idea which didn't pan out. BD

Suspended animation

Maybe a discussion of the possibilities and difficulties of suspended animation? Vicki Rosenzweig

Cleanup

I have rewritten and removed many sentences and paragraphs that were probably written with an essay in mind, not an NPOV article. Due to the large amount of changes some copyediting is still needed, and as I'm not a physicist so the paragraphs about relativity should be verified, I have noticed some of them tend to be a bit over-simplyfied. Cheers -- Rotem Dan 17:12 24 Jun 2003 (UTC)

Relativity questions

There's a couple of questions I've been wondering about on this subject:

1. What happens if something travelling near the speed of light is heading toward a source of Gravity? Given that gravity affects all masses equally regardless of how heavy they are (or how heavy they're made based on how fast they're going), could something break the speed of light via being accelerated by gravity as it was moving toward the gravity's source?
2. And if it could be accelerated to at least the speed of light, what would happen at the point that it was very close to the speed of light? There would theoretically be a point where the mass of the object would be greater than the rest of the universe. If that's the case, does it start exerting huge amounts of gravity on the rest of the universe, pulling it all toward the object?
3. As far as I can see, time wouldn't actually go backwards if you went faster than light. The equation for the perception of time with speed is t = t0 / sqrt(1 - (v^2 / c^2)). This would mean that at a speed greater than light, the 1 - (v^2 / c^2) part would go negative, meaning that the sqaure root of a negative number would be taken. Under complex number maths, this means that the number becomes imaginary, so beyond the speed of light the time would be some sort of imaginary number time.
4. So what sort of time would the 'imaginary number' time be? Hypertime?

Daveryan 12:53 28 Jun 2003 (UTC)

Things are not quite so simple. The real answer to your question is "understand relativity". But I'll try to be helpful.

First of all, general relativity (GR) is not a correct theory: we know of phenomena it can't handle. Various attempts at quantum gravity have not yet met with success. But for this kind of problem it is the best theory we havem and it seems to be fairly accurate. So everything else I'll say will be assuming general relativity holds.

Both special and general relativity require you to change how you think about basic physics in order for you to understand what's going on. This can be difficult, but good books exist. You have to be careful, though, because words like "time", "mass", "gravity" and "speed" don't mean what you think they mean.

The idea that gravity accelerates all objects equally, independent of mass, is more subtle than it looks. But perhaps the simplest answer to your first question is that gravity is not independent of velocity in GR. (It's very difficult to be precise here without talking about curvature of spacetime and so on). Just as a light beam gets no faster as it nears a heavy object (although it may experience redshift) an object very near the speed of light does not get much faster either --- although what seems to happen depends on how you measure its velocity. In fact, objects travelling very near the speed of light behave more and more like photons as they get faster.

If you decide to try to accelerate an object up to near the speed of light, you can definitely increase its mass. But it takes energy to do this - in fact, the increase in mass is exactly the same as the energy input divided by c². This holds true for all velocities, even very low ones, but when you throw a baseball, its kinetic energy is tiny compared to its mass times c², so the increase in mass is also tiny. So just by pushing an object faster, you could indeed significantly increase its mass - but that extra mass has to come from somewhere.

The formulas for time dilation and suchlike just don't make sense for objets moving faster than the speed of light. To understand what's going on, you need to look at spacetime as a whole. To simplify things, let's ignore the starship itself and look only at events: the starship leaves, and the starship arrives. Special and general relativity point out that the times of these two events are different for different observers. If you're moving quickly compared to me, a clock you carry with you will read a different time between the two events than a clock I carry. If we pick two arbitrary events, it might happen that you see one event happening first and I see the other happening first. But (in special relativity) if you can get from one event to the other without travelling faster than light, then we will always agree on the order. If you can't though, then we may disagree on the order in which the events happen. This is a problem if one of the events is supposed to cause the other, like starting the starship engines is supposed to cause arriving at their destination. If you see that spaceship happening faster than light, then some observer who's moving fast enough will see that spaceship arrive before it leaves. But the principle of relativity says that the same laws of physics work for this other observer, so the spaceship can't leave before it arrives.

General relativity still says that time travel and faster-than-light travel are the same, but it describes (purely theoretical) ways to do both. --Andrew 03:54, Apr 17, 2004 (UTC)

Best Place I could find to mention that I've removed the line which stated that upon return to Earth, time dialated travelers would experience the full duration (Earth relative) of the trip. This statement was at odds with the classic "twin paradox," wherein one identical twin will in fact age slower over the entirety of a round trip at c-fractional velocities. --Icelight 29 June 2005 21:59 (UTC)

Actually, what that sentence was referring to was that the traveller would arrive to find that more (local) time had elapsed than they'd perceived subjectively in-flight. I've modified that passage to make this clearer.--Christopher Thomas 29 June 2005 23:06 (UTC)

Okay, that's much clearer than it was. --Icelight 30 June 2005 00:18 (UTC)

"Unphysical"

Is "unphysical" a word? It's not in any of my dictionaries.

It's pretty commonly used in physics jargon; google will turn up many examples. It's shorthand for either "this idea is consistent with the math, but (probably) isn't something that actually means anything in the real world", or "this idea is completely out to lunch, but I'm going to phrase it a bit more politely than that", depending on context. --Christopher Thomas 07:26, 31 May 2005 (UTC)

Interstellar distances

I have some problems with the following paragraph. I'd like someone to check my work before I try to modify it, though.

Even this possibility still leads to very long travel times without the use of exotic physics. For a lengthy voyage, the spacecraft cannot accelerate at much more than one Earth gravity, since its acceleration will provide artificial gravity for the passengers, and the passengers cannot long tolerate high gravity. This means that if the ship accelerates throughout the voyage, accelerating on the way out and decelerating on the way back, in a year of ship time, the ship can travel half a light year. Because the ship can accelerate for longer, in two years the ship can go four times as far, or two light years. In three years, the ship could reach Alpha Centauri. More than five years will have passed on Earth. This is a long voyage, but still not much worse than ancient sailing voyages.

First, assuming that a ship accelerates at 1G for half a year, then decelerates at -1G for another half a year:

Distance covered during the deceleration phase will be equal to the distance covered during the acceleration phase.

Distance covered during the acceleration phase = (1/2)* a * t^2

a = 9.8 m/s^2

t = half a year = 60 * 60 * 24 * 365.25 * 1/2 = 15,778,800 seconds

t^2 = 248,970,529,440,000 seconds^2

d = 1,219,955,594,256,000 meters

The above distance is for the first half of the voyage. Total distance is twice that: 2,439,911,188,512,000 meters covered in one year

A light year is 9,460,730,472,580,800 meters, give or take, so in a year you would travel 0.258 light years. This is closer to a quarter light year than the half light year mentioned in the paragraph. That means that in two years the ship could go four times as far, or a little over one light year.

There's another problem beyond that, though. One would reach the speed of light in less than a year accelerating constantly at 1G. The author admits this by saying that one could reach Alpha Centauri in three years, even though Alpha Centauri is over four light years away.

I propose scrapping the above paragraph, as (beyond the problems in the math) acceleration at 1G is not practical for any length of time useful for interstellar travel. I feel a little silly doing all this math and then coming to this conclusion, but it's only through doing the calculations that I came to this. TomTheHand 17:55, Feb 15, 2005 (UTC)

I've gone ahead and deleted the paragraph, but it's archived here if I'm wrong about it. TomTheHand 15:05, Feb 23, 2005 (UTC)

Well, I think you were right to scrap the paragraph. But your arguments aren't compelling. Arguing about the practicality of different methods of interstellar travel is a bit silly, since they're all impractical in one way or another. 1G constant is doable without exotic physics (exotic engineering, yes): just take an asteroid-sized chunk of matter and an asteroid-sized chunk of antimatter along as fuel. And it *still* takes years.

As for the relativity stuff, it turns out you can ignore the relativistic time dilation if you're looking at proper time, that is, shipboard time. Looked at in an inertial frame, the time dilation and the decrease in acceleration are the same.

Anyway, it is a bit of a silly paragraph; I don't think it actually clarifies the issue any. Perhaps some real figures from the Starwisp idea would do... --Andrew 12:59, Feb 25, 2005 (UTC)

Interstellar communication

I've removed the bit in bold from

"However, it would be slow for the people on Earth interested in the results of the mission (and possible long-distance communication would also be slow)."

because this is misleading - slow due to the time dilation or the transit-time of the communication?

• 2 people separated by a large distance, stationary with respect to each other - there is significant delay between sending a message and recieving a reply, but everybody is talking at normal speeds. This already happens to some extent talking to people on the other side of the earth, and on the Appollo missions.
• 2 people fairly close together, moving fast relative to one another - there is a different problem here. The time dilation will make people appear to talk slowly, so a 20-second reply could be recieved over the course of a few minutes.

Maybe this could be expanded into a section of the article, or deserves one of it's own? SeventyThree 15:50, 25 Feb 2005 (UTC)

Recent edits need work before merging

The following was recently added to the article. I took it out temporarily until some serious problems can be resolved.

The nearest star to the Sun is the triple system Alpha Centauri. Light radiating from that star takes more than 4.2 years to reach Earth.

A trip with the current standard spaceship, the Space Shuttle, which travels at 28500 km/u, would take 160,000 years.

First of all, what's a km/u? Standard units here would be km/s or fractions of c. Second, giving a speed for a spaceship is highly misleading, as they do not have top speeds. The exhaust velocity does give a practical order-of-magnitude upper limit if no special tricks (gravitational assist, planet-based launch assistance and so forth) are used, but it's by nomeans an upper limit - most current spacecraft spend their working lives travelling faster than this.

An little bit faster would be an unmanned probe, like the Voyager 1, travelling at 61200 km/u, making a one-way journey 74066 years.

The currently fastest man-made object, the Helios 2, has set a speed record of 252800 km/u.

A journey to Proxima Centauri would even then take 4269 years.

But at the current stage of space technology, the longest space missions that have been initiated are expected to have an operational lifetime of about 40 years before failure of key components is likely to happen. So the ship would run out of power even before coming close to it's target. Power is not actually the main problem - we can build very long-lasting Radioisotope thermoelectric generators. Materials durability is more serious (it's very difficult to build even a terrestrial machine that will function reliably unattended for 40 years).

One might note that science's current best theoretical propulsion system, VASIMR, would be achieving speeds up to 300km/sec, or 1,080,000 km/h.

This would shorten the journey to 999 years.

But that is still beyond the current lifespans of both man and machine.

VASIMR is by no means the best theoretical system - see spacecraft propulsion. Depending on what you're willing to consider viable, anything from ion thrusters to warp drives can be considered the theoretical best. --Andrew 13:32, May 2, 2005 (UTC)

But another proposed method of propulsion, the Nuclear photonic rocket, which has the patential to exceed $c$, would shorten the journey to proxima centauri at about 4.217 years and only needs a few more reserech before being feasable. So andrew; just trying not to be mean but it dosn't matter what kind propulsion you use. It's the amount of energy you use. Fquantum ^talk 14:29, 4 June 2007 (UTC)

• I think most of the text of these edits isn't useful for the article. A more appropriate version would be a short table listing the specific impulse or exhaust velocity for a variety of engines, a reasonable maximum cruising speed, and a flight time to Alpha Centauri, if the information is included at all. A fair bit of the interstellar travel article could stand editing; if I find myself with time on my hands, I may do a rewrite, but for now, the current version looks fine. --Christopher Thomas 00:22, 3 May 2005 (UTC)

Good! I have been waiting to put a table on this article to improve it.

I have just added the table you suspected, but even it was removed. I'm pervasively Suspicios about the removal, but I do have to exept it. Someone else intrested in this article has to reundo and modify this chart and search for citations.

No need for suspicion. That was me, and I explained my actions below. I didn't remove the table from the article, I just commented it out so that it could be worked on in place. PubliusFL 21:00, 27 July 2007 (UTC)

I also think that all(now most) text is not usful to the entire article. That is why I have partially rewritten the Interstellar distances section.Justin Forbes 20:31, 1 May 2007 (UTC)

research

Not trying to stress anyone; but I was here to reasearch this article for a probe Im brainstorming on a journey to Procyon, not to mess it up. If anyone can tell me where I can find a similar article, that has cleaned up sources, than I would be partially proud of the research I have found. — Fquantum ^talk 18:41, 9 September 2007 (UTC)

"Forces which have never been measured or calculated"

I've backed out this line from "difficulty of interstellar" travel because as far as I know, it's just plain incorrect. Astronomers have known the approximate composition of the interstellar medium for many decades, and forces experienced with in it are pretty straightforward to calculate (based on the medium's composition, the craft's speed in it, and the influences of nearby stars). If anything, it's more hospitable to a probe than the environment near stars; it presents problems mostly because you're moving at a very good clip while within it, and various creative shielding mechanisms have been proposed to deal with the resulting concerns.--Christopher Thomas 15:09, 30 May 2005 (UTC)

Good, it looked bogus to me too. I'll find some place for a link to interstellar medium however since it seems relevant. 81.86.225.23 19:21, 30 May 2005 (UTC)

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.

Time travel and faster-than-light travel

I was told by a physicist (and also read elsewhere on Wikipedia, don't remember where) that both time travel and faster-than-light travel are now generally agreed to be possible, under bizarre conditions that don't violate the laws of physics. At least, they don't violate the modern theories of physics, which have come a long way since Einstein. I was also told that the amounts of energy involved are so absurd as to render these completely impractical.

Can anyone else confirm this? Xezlec 20:08, 1 April 2006 (UTC)

Time travel and faster-than-light travel are equivalent under special and general relativity; if you can accomplish one, you have the means to accomplish the other. There are several proposed methods of doing this. Whether they are "possible" or not depends on your assumptions. Several spacetime geometries allow you to move a patch of spacetime at FTL speeds relative to the universe around you, but these involve negative mass. Similarly, you can create shortcuts (wormholes being the most famous) that have properties that allow time travel and very rapid transport, but stabilizing them requires negative mass for most geometries studied. You can also spin a very dense object (a Tipler Cylinder or a Kerr black hole) in a way that might allow time travel, but this requires conditions extreme enough that it's not likely to be done as a result of human actions any time soon. There may be other methods of time travel and FTL travel that I'm not aware of, but the ones I'd heard of were variants on the ones above. The more interesting ones attempted to reduce the amount of matter involved by using very thin, dense shells. It's a neat topic, but still one that scentists are reluctant to make blanket statements about. The more conservative view is that anything that can produce closed timelike curves (allowing time travel) is forbidden by the laws of nature (either as a consequence of the ones we know about, or as a consequence of the features of a theory of everything unifying gravity, the other forces, and quantum mechanics). The less conservative view is that it may be possible given conditions that involve assumptions (like the existence of negative mass) that we don't know the validity of yet. --Christopher Thomas 22:14, 1 April 2006 (UTC)

Cool, thanks. I believe that's the same basic story I had heard. So, maybe someone should remove or revise the line that says "Current theories of physics indicate that it is impossible to travel faster than light". Would changing "impossible" to "impossible or astronomically difficult" be an option? I leave this decision up to the experts. Xezlec 20:21, 29 May 2006 (UTC)

FTL travel requires negative mass? Not imaginary mass?

The exotic matter page briefly summarizes both, and doesn't mention FTL speeds in the negative mass subsection, but does refer to tachyons in the imaginary mass part.

$E={\frac {mc^{2}}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}}.$

For a $v$ to be greater than $c$, $m$ must be an imaginary number (not a negative number) to keep $E$ real... --HantaVirus 13:17, 26 July 2006 (UTC)

under sublight

in the FTL section sublight it states

"NASA is studying methods of extracting energy from empty space. The Casimir force has been proposed to be a force coming from the vacuum energy of virtual particles in empty space. The Casimir force has been measured and proved to be a real phenomemon and recently, tiny amounts of energy have been extracted by devices that work on the Casimir force. Many have put forth the idea that the vacuum energy of empty space and the energy of virtual particles in space is much bigger than Casimir calculated as a result of the "false bottom" effect. There is no proof of a false bottom and a much deeper energy well filled with virtual particles nor has there been proof that empty space contains virtually limitless amounts of energy and gravitiational mass as a result of the equivalence of mass and energy posited by Einstein. Devices that generate energy from the Casimir effect may power nanocircuitry on long voyages on board starships, but they will never provide a source of power for propulsion."

doesnt that seem to need some ciation

SI

Why isn't the International System of Units (SI) upheld in this article?

Wikipedia is international. —Preceding unsigned comment added by 83.108.240.129 (talk) at 10:28, 7 October 2006

Interstellar distances section

Why was the example changed from Alpha Centauri to Epsilon Eridani? Alpha Centauri is much closer, and therefore a better example for illustrating the minimal difficulty of interstellar travel. It may be that Epsilon Eridani is the closest planet known at present to have a planetary system, but that's not saying much, because all we can really detect at this point is gas giants. Alpha Centauri is more likely to have terrestrial planets than gas giants, and all things considered is more likely to be the first target of interstellar travel (unless we are actually able to detect terrestrial planets around some other star system in Sol's neighborhood). Also, what spacecraft is capable of making 77 km/s? To the best of my knowledge, Helios 2 was the fastest spacecraft ever built, and depending on what reference you look at, it was only capable of somewhere around 70 km/s. PubliusFL 01:06, 14 February 2007 (UTC)

I herd there was the first earthlike yet discovered. We are likely to to find alot more in the following year. – Fbs. 13 17:10, 28 April 2007 (UTC)

well.. are any of you going to improve this article, or are you going to leave it lacking sources. I am a pervasive expert on this article and does not like anyone to lack sources on true science. I am also an expert on faster than light travel. ..and if any one, even another expert tries to lack inline citations, I do not want to ignore taking it seriously but you know what might happen.Fbs. 13 00:42, 14 May 2007 (UTC)

interstellar distances section (answer)

I am designing a spacecraft. Fbs. 13 19:43, 23 March 2007 (UTC)

I think the fastest propulsion system may make more sence, the propulsion System you may be looking for may be a magnetoplasmadynamic thruster. Fbs. 13 22:45, 27 March 2007 (UTC)

The problem is not what type of thruster you use. The problem is the energy per unit weight of your fuel, as this gives your maximum possible specific impulse, and so your maximum cruise velocity. --Christopher Thomas 02:58, 28 March 2007 (UTC)

Needs current interstellar spaceflight section

This article would benefit more readers if it provided some "grounding" in reality. Specifically, it would be great to discuss those spacecraft which are currently on interstellar trajectories. Discuss how far they've made it, how long it takes to communicate with them (are they yet light-days away?), etc. This is made even more evident by the redirect of Interstellar probe to this article. Sdsds 23:18, 24 March 2007 (UTC)

Perhaps I was wrong. Maybe it is better to leave this article covering the topic it currently covers, and create content at Interstellar probe to cover current spacecraft and spacecraft planned for the near future. I've removed the redirect and put some preliminary content at Interstellar probe to move in that direction. Sdsds 05:54, 31 March 2007 (UTC)

That makes sense to me. Good work. PubliusFL 06:28, 31 March 2007 (UTC)

I've edited Interstellar probe (edit | talk | history | links | watch | logs) for minor factual corrections, and to distinguish between probes that are expected to reach other stars and ones that aren't. It will be very difficult to keep this article grounded in reality (expect lots of edits from people adding fictional references, among other things). If it can be kept encyclopedic, though, it has potential.--Christopher Thomas 08:49, 31 March 2007 (UTC)

The nearest star (interstellar distances section)

The nearest star may not be proxima centauri, which will take 5,043 years with current technology. Since Nemesis may be the closest star to the sun, lying at about 7,140 AU, which may be an L7-type brown dwarf, current technology may send it in just about 247 years, only twice that of a human lifspan! I have also theorized that it may be close enough to have been part of our solar system and, in theory, it may be a more likely place to harbor an earthlike planet about 2.7 earth masses. —The preceding unsigned comment was added by Fbs. 13 (talk • contribs) 02:40, 1 April 2007 (UTC).

If the Nemesis hypothesis is proven, we should definitely change the interstellar distances section. Until then, it's very hypothetical. And what do you mean that you have theorized that Nemesis is "close enough to have been part of our solar system"? The idea that Nemesis is part of our solar system is the whole point of the hypothesis -- otherwise it wouldn't explain cyclical mass extinctions (I'm assuming you aren't Daniel Whitmire or Albert Jackson). PubliusFL 03:44, 1 April 2007 (UTC)

Here is a probable theory that may explain everything: A second shockwave hit Our solar system. It sliced about 13% of the nebulial disk. The partial nebula was gliding through the Interstellar medium and stoped at about 7,145 AU and formed a dark (cool) L7 red dwarf planetary system with two planets –- one was a jovian planet about 13 earth masses; the other (which you may have realized in the top sentence) was an earthlike planet about 2.7 earth masses. The meteors that caused most of the cyclical mass extinctions where probably small OCO impacts on Nemesis c (the earthlike planet) that caused pieces to sling out of the planet (this may be similar to some of the impacts of Mars). This theory may be difficult to understand but you may soon. — Fbs. 13 01:37, 3 April 2007 (UTC)

We understand the concept behind the Nemesis proposal. It's just that the vast majority of astronomers consider the existence of this companion star unlikely, and the contents of Wikipedia reflect that (per WP:NPOV). If the existence of Nemesis becomes accepted as likely - or even a reasonably plausible possibility - then that change will eventually be reflected in this article. Until then, no. --Christopher Thomas 01:51, 3 April 2007 (UTC)

Insert paragraph on von Neumann probes?

I think it would be useful to add a para which refers to von Neumann probes: manned exploration would be very risky unless we know what's at the other end of the journey, and the smallest objects we can currently detect in other solar systems are hot Jupiters; in theory von Neumann probes could explore the whole galaxy in a few million years.

What do other contributors think? Philcha 20:56, 13 April 2007 (UTC)

Make sure it's given space in proportion to references cited. While the idea is well-known (google for "far edge party"), devoting a huge chunk of the article to it would be overkill. The majority of the article should be the same either way (as the timeframes involved are similar whether or not we're on the ship itself, making the technical challenges similar). --Christopher Thomas 22:17, 13 April 2007 (UTC)

Impossible reaction mass requirements of current technology

NASA used to have a page http://www.grc.nasa.gov/WWW/bpp/bpp_INTERSTELLAR.htm (now a 404, alas) which said that a vehicle the size of the Space Shuttle, using the Space Shuttle engines, would need more fuel than the mass of the known universe to reach the nearest star in 1,000 years (in fact it said propellant required = 10¹¹⁹kg, universe = about 10⁵⁵kg). Does anyone know where to find this now, or something similar? Philcha 21:26, 13 April 2007 (UTC)

Not off the top of my head, but that's actually exactly backwards to how you'd want to draw such a table. Once delta-v gets above exhaust velocity, the amount of mass needed grows exponentially, quickly resulting in silly numbers. What you'd do in practice is assume a best possible mass fraction (95% fuel, 99% fuel, etc), and calculate the delta-v you'd get for any given propulsion method. This in turn gives you the shortest practical cruising time you'd get with any given propulsion approach. I got on the order of 1000-4000 years for fission- and fusion-based drives the last time I worked this out. Your mileage may vary. --Christopher Thomas 22:21, 13 April 2007 (UTC)

I like your explanation of how one would go about designing a mission given a particular propulsion tech. But I was looking for a dramatic illustration of why our current "standard" propulsion tech is unsuitable, with citation. Any ideas? Philcha 10:36, 18 April 2007 (UTC)

Go for one that's accurate, not dramatic, is my best suggestion. Find references that say how long a trip would take using a given mass fraction and drive type, and find other references (or ideally the same references) that say what the maximum lifetime of manned and unmanned craft are expected to be. Numbers I've heard are that around 30-ish years is the maximum design lifetime anyone's willing to give to an unmanned probe, and that's only if it has no moving parts (uses RTEGs or similar generation methods). Pointing out that trip time is a hundred times longer than designed lifetime should be sufficient for your purposes.

I don't have references that do this in front of me, and am too busy to search for them, but they'll be out there if you want to handle this yourself.--Christopher Thomas 19:09, 18 April 2007 (UTC)

Fusion drive question

So, the article states that a fusion drive would be capable of attaining around 10% the speed of light. Why is this? Would this be fast enough that the interstellar medium would produce sufficient drag to limit further acceleration? Some other reason? Whatever the reason, it should be stated or at least cited. Harley peters 03:02, 25 April 2007 (UTC)

Mass ratio. Cited here and in linked articles. Michaelbusch 03:09, 25 April 2007 (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)

A trip to the farthest planet

The distance from Earth to the other planets of the solar system ranges from about three light minutes to about five and a half light hours. Depending on the planet and its alignment to earth, for a typical unmanned spacecraft these trips will take a few monthes to a little over a decade.

contradicts

Pluto is five and a half light hours and is now classified as a dwarf planet.

Neptune, now the farthest planet, is about 4.7 light seconds away. So if neptune and earth are lined in a temporal constant motion, Than an unmanned spacecraft, using current technology, such as the magnetoplasmadynamic thruster, will get to Neptune in only 2.7 years, depending on hou much energy per thrust you use.

so If this does not get edited into the article, than I will edit it my self and I will cite it. Fquantum ^talk 21:11, 29 May 2007 (UTC)

Good point about Neptune being the farthest planet. Neptune's about 4.2 light hours from the Sun at aphelion, so I changed the sentence to say "about four and a third light hours." I left the "few months to a little over a decade" time frame because it's talking about a typical unmanned spacecraft, with currently used propulsion technology. Magnetoplasmadynamic thrusters are hardly in common use yet. PubliusFL 14:13, 30 May 2007 (UTC)

Sub-light travel section

This whole section appears to be wrong according to this website

http://www.nasa.gov/centers/glenn/research/warp/scales.html

As it states, no method for sub light travel, even antimatter would accelerate you that fast without vast amounts of fuel, not sure about solar sails though. The snare 07:30, 2 June 2007 (UTC)

Interstellar Travel

If we suppose that we eventually have the ability to harness enormous resources, but do not uncover new laws of physics, then it will always take individual humans years to travel between the stars. The problem is that we can't accelerate faster than our bodies can survive. So, if we assume that the passengers want to experience the journey at an acceleration of 1 g, then how much travel time do they experience on an interstellar journey?

The difficulty that we have to work through is that the traveler isn't in an inertial frame of reference. That is, v keeps changing. The traveler starts at rest and undergoes a constant rate of acceleration g (in the traveler's frame of reference). What is the traveler's velocity (relative to the original frame of reference) at any time?

Let's define some coordinates. The position of the traveler in the original frame of reference is (x, t). (Here I'm using "position" to refer to both space and time.) The velocity of the traveler as measured in the original frame of reference is v. (The traveler sees the same velocity, but has a different distance scale...) The cumulative elapsed time that the traveler has experienced is τ. We want to define the relationship between these coordinates. To do so, we define two additional sets of coordinates: The coordinates in the traveler's inertial frame of reference are (x1, t1). The traveler doesn't really have an inertial frame of reference, since he/she is accelerating constantly, but this is the inertial frame that the traveler would be in if the acceleration were turned off briefly. The traveler is at position x1 = 0. When we envision turning off the acceleration briefly, we will take that moment to correspond to t1 = 0. At that moment, we will also want to consider another set of coordinates (x0, t0) in the Earth's inertial frame of reference. These coordinates are defined by the Lorentz transformation:

(1) x1 = γ (x0 – v t0) t1 = γ (t0 – v x0 / c2)

x0 = γ (x1 + v t1) t0 = γ (t1 + v x1 / c2)

where γ = √[1 – v2 / c2] –1 There will be an offset between x and x0 and between t and t0, but while we imagine the acceleration to be turned off, the offest is constant. That is, dx = dx0 and dt = dt0. Similarly, there is an offset between τ and t1, but dτ = dt1.

To make the physics perfectly clear, let's consider the acceleration to be a series of discrete boosts in velocity. That is, the traveler instantaneously shifts from the frame (x1, t1) into another frame of reference (x2, t2). The change in velocity dv1 happens at intervals dτ, such that dv1 = g dτ = g dt1. So, the relative velocity of the coordinate system (x1, t1) with respect to the coordinate system (x0, t0) is v, and the relative velocity of the coordinate system (x2, t2) with respect to the coordinate system (x1, t1) is dv1, but what is the relative velocity of the coordinate system (x2, t2) with respect to the coordinate system (x0, t0)?

We find

(2) x2 = γ1 (x1 – v t1)

=  γ1 γ (1 + v dv1 / c2)   [  x0  –    (v + dv1) c2 

---

c2 + v dv1 t0 ]

t2 = γ1 (t1 – v x1 / c2)

=  γ1 γ (1 + v dv1 / c2)   [  t0  –    v + dv1 

---

c2 + v dv1 x0 ]

where γ1 = √[1 – (dv1 / c)2] –1 Now we can recognize that this must also be a Lorentz transformation. Therefore, we must have

(3) v2 = (v + dv1) c2

---

c2 + v dv1

γ2 = γ1 γ (1 + v dv1 / c2)

where γ2 = √[1 – (v2 / c)2] –1 A little painful algebra verifies that the third equation follows from the first two. If dv1 is actually an infinitesimal change in velocity, then we only care about the first order term.

(4) v2 ≈ v + (1 – v2 / c2) dv1 dv = (1 – v2 / c2) dv1

Since dv1 = g dτ, we can integrate to find the velocity at any time τ.

(5) ∫ dv

---

1 – v2 / c2

  =   ∫ g dτ 

 

c tanh–1 (v / c)

  =  g τ 

 

v

  =  c tanh (g τ / c) 

This gives the velocity in terms of the traveler's elapsed time. From here it is a simple process to integrate this equation and find the position as a function of time. It is not, however, a trivial process, because the velocity is not equal to dx / dτ. The velocity is equal to dx / dt, so we need to determine the relationship between dt and dτ before we can integrate. We want to consider this relationship from the point of view of the traveler at x1 = 0, since that is where Equation 5 applies. Looking at Equation 1, we see that dt0 = γ dt1. Since dt = dt0 and dτ = dt1, we find

(6) dt = γ dτ

Now we can use Equation 5 to write γ = cosh(g τ / c), and it is trivial to integrate Equation 5.

(7) ∫ dx = ∫ v γ dτ

x = ∫ c sinh (g τ / c) dτ

=  (c2 / g) [cosh (g τ / c) – 1] 

This is very close to the formula that we want. We want to know the value of τ when the traveler has made it halfway to the destination, because then the deceleration starts. If the total distance is X, then the total travel time T is given by

(8) X / 2 = (c2 / g) [cosh (0.5 g T / c) – 1]

T = (2 c / g) cosh–1 (1 + 0.5 g X / c2)

If X = 4.3 light-years, then T = 3.6 years. Dozens of stars could be reached in five to six years. In fact, a traveler could even go the Andromeda galaxy in under 29 years if a constant acceleration could be maintained.

steve

That's all interesting, but your bit about LINAC still has no sources to allow verifiability. Please provide a reliable source before adding that information again. PubliusFL 18:39, 18 August 2007 (UTC)

Well, that math is definitely over my head, I think wikipedia is supposed to be written in layman's terms, you might want to submit that to a scientific journal instead, I have no idea how right or wrong it is, but........are you talking about the perceived trip, correct? We can't get to "dozens" of stars in a few years with sub-light speeds The snare 02:52, 19 August 2007 (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 $E=mc^{2}$), 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)

A reference to gravity control

There is something I directly herd of called a "Central deflecter fuse". It works by spinning the ship on its side; just like a center fuse. Another, called a "synthetic mass generating cover", which lies at, for example, the back of the saucer depth, generates/transports mass to power the sauser it's self.

If there is any reference to the information I revealed, I barely doubt that it has to be cited soon. Fquantum ^talk 21:26, 15 June 2007 (UTC)

Generation ships

I propose to remove all but the first sentence of the first para of section "Generation ships" (i.e. leaving only the definition), because the remainder duplicates later content which is better described and referenced.Philcha 23:16, 13 July 2007 (UTC)

Done.Philcha 23:26, 27 July 2007 (UTC)

Table of "Propulsion methods that could possibly reach the nearest star"

I have commented out this table because it really needs to be discussed here before going live. What sources is it based on? Without sources it smacks of original research. For one thing, the maximum velocities given are based on unstated assumptions about things like mass fractions. PubliusFL 17:30, 20 July 2007 (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)

removed citations broken and cleanup tag

if any of you saw templates removed then look on the history page for the latest edit of my account for the article. you might know that I have removed them because the tags were needed a while ago. now the article might not need them because the tags are out of date.-- Fquantum ^talk 13:57, 3 November 2007 (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 [2] 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#The speed of light as a limit 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:

"...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)