Talk:Interstellar travel/Archive for 2005

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)

"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? 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)
 * 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.

"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)

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.

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

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