Talk:Interstellar travel

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Contents 1 Possibility of interstellar travel 2 Interstellar travel in fiction 3 Paragraph headings 4 Biological vs. technological problem 5 First vs. third person voice 6 Time dilation and wormholes 7 Travel by humans vs. travel by robots 8 Alpha Centauri movement footnote, and link concerns 9 Fast interstellar travel methods from Robert Zubrin 10 Bussard ramjets 11 Suspended animation 12 Cleanup 13 Relativity questions 14 "Unphysical" 15 Interstellar distances 16 Interstellar communication 17 Recent edits need work before merging 18 "Forces which have never been measured or calculated" 19 Science Fiction? 20 Time travel and faster-than-light travel 20.1 FTL travel requires negative mass? Not imaginary mass? 21 under sublight 22 SI 23 Interstellar distances section 24 interstellar distances section (answer) 25 Needs current interstellar spaceflight section

[edit] 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.

[edit] 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.

[edit] 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

[edit] 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.

[edit] 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

[edit] 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

[edit] 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

[edit] 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.

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

[edit] 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)

[edit] 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

[edit] Suspended animation

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

[edit] 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)

[edit] 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)

[edit] "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)

[edit] 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)

[edit] 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)

[edit] 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 RTGs. 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)

• 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)

[edit] "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)

[edit] 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.

[edit] 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)

[edit] 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.

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)

[edit] 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

[edit] SI

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

Wikipedia is international. —The preceding unsigned comment was added by 83.108.240.129 (talk • contribs) at 10:28, 7 October 2006.

[edit] 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)

[edit] 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)

[edit] 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)