# Physics - space travel



## VickiW (Jan 27, 2013)

I know travel to distant worlds is not possible because they are too far away we can never come close to reaching the speed of light, but I want to write a novel about space travel. Most SF writers use a variation of the wormhole theory and folds in space, but I read somewhere that you can reach a pretty good velocity by constant acceleration (I'm not sure if this is the right term). I have little knowledge of physics beyond high school and my own reading. My question: Is this a viable  or believable method or would the g-force effect rule it out?
If the ship could travel this way and have a generational crew, would they be able to travel, say, 12  light years? I'm projecting a journey taking about two hundred years.


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## Terry D (Jan 27, 2013)

To travel 12 light years in 200 years your craft would only need to travel at about 6% of light speed (on average).  A couple of ways to achieve that are shown here-- Bussard ramjet - Wikipedia, the free encyclopedia -- and here -- Solar sail - Wikipedia, the free encyclopedia.


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## movieman (Jan 30, 2013)

VickiW said:


> I read somewhere that you can reach a pretty good velocity by constant acceleration (I'm not sure if this is the right term). I have little knowledge of physics beyond high school and my own reading. My question: Is this a viable  or believable method or would the g-force effect rule it out?



It's perfectly viable, so long as you have enough fuel. Fusion can realistically get you to around 10% of the speed of light with technology and materials we largely understand today, or 5% if you have to stop at the other end. Antimatter could do quite a bit better, but you'd need to generate the energy to produce it first.

Accelerating at 1g, you'd only be accelerating for about a year to reach 10% of the speed of light.


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## VickiW (Jan 30, 2013)

movieman said:


> It's perfectly viable, so long as you have enough fuel. Fusion can realistically get you to around 10% of the speed of light with technology and materials we largely understand today, or 5% if you have to stop at the other end. Antimatter could do quite a bit better, but you'd need to generate the energy to produce it first.
> 
> Accelerating at 1g, you'd only be accelerating for about a year to reach 10% of the speed of light.



Thank you, Movieman, and Terry for your help. You got me on the right track. Here's my conclusion: it's the acceleration that causes damage to the body, not the speed. If a vessel constantly accelerates at 1g, it wouldn't be a problem. 
About slowing down at the other end, The ship could stop its engines halfway and coast—that would save fuel—and use bursts of reverse thrust to stop. I never go into great technological detail with my science fiction, but I should at least know enough to make it plausible. It's the people and events that are important, not the hardware.
I've seen SF movies and read novels that are absurd because the authors cannot conceive of leaving behind their pet Earth-side habits. These are things that are not an option on long space journeys: eating meat, smoking, coffee, and you can probably think of more. Even firing weapons inside a habitat could cause a disaster.


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## movieman (Jan 30, 2013)

VickiW said:


> If a vessel constantly accelerates at 1g, it wouldn't be a problem.



Yes. The difficult part would be when you switched the engines off; most likely you'd then want artificial gravity from rotation, so you'd be weightless for a while as that spun up, and what had been a wall during the initial acceleration might become the floor as your rotation axis would probably be the same as the thrust axis of the engines.



> About slowing down at the other end, The ship could stop its engines halfway and coast—that would save fuel—and use bursts of reverse thrust to stop.



Yes. The main point is that any kind of engine we currently understand that has to carry its own fuel gives you X amount of velocity change for a given amount of fuel, so if you have to stop at the other end you'll only be able to reach a max speed of X/2 and have to keep the rest in reserve to stop. Ramjets could be more effective, but last I read they seemed to have a fairly limited useful velocity range (not enough fuel at low speeds, too much drag at high speeds). There are other techniques like using a slingshot around the sun to gain velocity, but that only really helps if you're traveling at a tiny fraction of the speed of light.


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## archer88iv (Feb 19, 2013)

The only reason to worry about the physics is if they are going to be important somehow--and even then, you need only worry about the physics to the extent that they're going to be important. 

Of course, even then, there is no reason any of it would need to be able to stand up to scrutiny. Actually, there's no reason you should present enough "evidence" for someone to try scrutinizing it. In the end, after your bestseller reaches enough nerds, plenty of people will be stumbling all over themselves to explain how *your* version of space travel works for you without you ever having lifted a finger.


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## moderan (Feb 19, 2013)

VickiW said:


> I've seen SF movies and read novels that are absurd because the authors cannot conceive of leaving behind their pet Earth-side habits. These are things that are not an option on long space journeys: eating meat, smoking, coffee, and you can probably think of more. Even firing weapons inside a habitat could cause a disaster.


While the first line is undoubtedly true, and the third, I have some issues with the second. Those aren't absolutes. If you're talking about a universe ship, or something large-enough scale that you're using plant life to help feed people/generate oxygen, there's no reason not to have coffee. You're going to be recycling the wastewater anyway. Eating meat...well, yeah. I can't see how a ship could be big enough to provide grazing land. You could move a planet that way, or work with a Dyson sphere or somesuch, but then you're really into macroengineering. Someone with a good grasp of scientific principles could make a believable case for any of the above.
The most glaring example to me is the orientation issue-best covered by Niven and Pournelle in their Mote novels. There is no up and down when you're talking about zero gee.


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## VickiW (Feb 19, 2013)

moderan said:


> While the first line is undoubtedly true, and the third, I have some issues with the second. Those aren't absolutes. If you're talking about a universe ship, or something large-enough scale that you're using plant life to help feed people/generate oxygen, there's no reason not to have coffee. You're going to be recycling the wastewater anyway. Eating meat...well, yeah. I can't see how a ship could be big enough to provide grazing land. You could move a planet that way, or work with a Dyson sphere or somesuch, but then you're really into macroengineering. Someone with a good grasp of scientific principles could make a believable case for any of the above.
> The most glaring example to me is the orientation issue-best covered by Niven and Pournelle in their Mote novels. There is no up and down when you're talking about zero gee.


I think I threw in coffee because it's such a cliché in fiction, and because I don't like it, although I do believe it requires certain specific growing conditions that may not be available everywhere.


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## VickiW (Feb 19, 2013)

archer88iv said:


> The only reason to worry about the physics is if they are going to be important somehow--and even then, you need only worry about the physics to the extent that they're going to be important.
> 
> Of course, even then, there is no reason any of it would need to be able to stand up to scrutiny. Actually, there's no reason you should present enough "evidence" for someone to try scrutinizing it. In the end, after your bestseller reaches enough nerds, plenty of people will be stumbling all over themselves to explain how *your* version of space travel works for you without you ever having lifted a finger.


You're right up to a point. I used the belong to an SF fan group and one of the points that came up frequently was bad science in science fiction. One of the examples brought up was Star Trek. 
Something that bugs me about SF movies is the sound effects when there are battles in space. Sound does not travel on a vacuum, so you wouldn't hear the explosions and screaming engines. I actually think it would be far more ominous if you saw all the activity—apart from radio communications—in silence; it would be spooky.


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## dolphinlee (Feb 20, 2013)

Actually if you accelerate at 1g it would take you around 35 days to reach 10% of light speed. (V = u + at; v = final velocity, u = initial velocity {call it zero}, a = acceleration, t = time it would take)

The problem with this is that a huge amount of force would have to be generated by the engines and sustained by the ship. (F = ma; F = force, m = mass of the ship and everything on it, a = 9.81 m etres per second per second) 

So theoretically the ship would be accelerated much more slowly over a much longer time. 

Therefore there would be no problem with G forces, except when travelling from the Earth to the orbital station.


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## Outiboros (Feb 20, 2013)

archer88iv said:


> The only reason to worry about the physics is if they are going to be important somehow--and even then, you need only worry about the physics to the extent that they're going to be important.
> 
> Of course, even then, there is no reason any of it would need to be able to stand up to scrutiny. Actually, there's no reason you should present enough "evidence" for someone to try scrutinizing it. In the end, after your bestseller reaches enough nerds, plenty of people will be stumbling all over themselves to explain how *your* version of space travel works for you without you ever having lifted a finger.



I'd have to agree with this. The less details you tell, the less chance you have you get one wrong. Also, if you're telling our story from the viewpoint of, say, an engineer or a captain, it would make sense for them to know the details, but a colonist or anyone else without a technical background might not.


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## moderan (Feb 20, 2013)

VickiW said:


> I think I threw in coffee because it's such a cliché in fiction, and because I don't like it, although I do believe it requires certain specific growing conditions that may not be available everywhere.


Nah, not really. You really should read the Mote books. I grew coffee in the lab at ASU when I was there. Wasn't hard, was good coffee. Takes more to grow good canna.


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## movieman (Feb 27, 2013)

moderan said:


> Eating meat...well, yeah. I can't see how a ship could be big enough to provide grazing land.



We're already beginning to grow meat in vats, so by then you shouldn't need grazing land. But if your ship is going to spend five hundred years traveling from star to star at 1-2% of the speed of light, you'd probably want it large enough that you could grow animals if you wanted to.



> The most glaring example to me is the orientation issue-best covered by Niven and Pournelle in their Mote novels. There is no up and down when you're talking about zero gee.



I seem to remember astronauts saying that they generally stick to a consensus vertical in space to reduce space sickness. If everyone is standing the same way, it's less disconcerting than everyone doing their own thing.


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## moderan (Feb 27, 2013)

It may be less disconcerting but after a while it's useless. When you're talking about lifetimes spent in space, space sickness shouldn't be an issue.


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## edinfresno (Feb 27, 2013)

Re: space travel:

Consider the some of the discoveries made at CERN and at the Large Hadron Collider:

It's known and has been confirmed that, on a quantum level electrons, as well as other particles, are capable (and do) move inter-dimensionally. They do this using infinitesimally small amounts of energy.

The latest theory is that the barrier between dimensions is comprised of the nuclear weak force, the same force that comprises gravity.

Now, imagine that the scientists in your story know and understand what the weak force actually is, what gravity actually is and at what frequency the inter-dimensional barrier oscillates at.

With this knowledge your scientists could, and most likely would, construct a means of traveling inter-dimensionally. They, of course, would also have sent probes to "map" inter dimensional space to the point where a ship could exit present space time, log in a co-ordinate into a navigation computer and re-enter present space time literally billions of light years away within nano-seconds thus making space travel not only instantaneous but relatively inexpensive since (theoretically) the amount of energy required for a trans-dimensional drive would be small compared to something like warp drive or space folding. 

It's just something to consider.


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## edinfresno (Feb 27, 2013)

dolphinlee said:


> Actually if you accelerate at 1g it would take you around 35 days to reach 10% of light speed. (V = u + at; v = final velocity, u = initial velocity {call it zero}, a = acceleration, t = time it would take)
> 
> The problem with this is that a huge amount of force would have to be generated by the engines and sustained by the ship. (F = ma; F = force, m = mass of the ship and everything on it, a = 9.81 m etres per second per second)
> 
> ...



Good points, Dolphinlee but also consider that mass and density of the ship increase relative to increases in velocity once the speed of the ship reaches .25 the speed of light so there's always the necessity of increasing output from the engines to compensate. So, without something like an anti-matter or dark matter (maybe even string-energy) drive to meet the ever increasing  power requirements to compensate acceleration beyond .25 LS would be impossible.


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