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[IndonesiaWarMinister]

You all forgot one massive factor:

Time-relativity.

This means a MAJOR cut-off on the food, water , pop needs

[Sean Mirrsen]

That, or hibernation. Or that PLUS hibernation.

[Servant Corps]

Time-dilation is complicated, but basically, the effects of time-dilation only appear outside of the "spaceship". Inside of the spaceship, 100,000 years will pass, so you need 100,000 years of food.

And I agree with what G-Flex said about hiberation: If you could just hiberate and let the robots deal with running the Colony Ship, why not let the robots colonize the area and cut out the 'middle-man' (the humans)?

[Sean Mirrsen]

Because robots will either do only what is programmed and will be prone to errors and eventually will wind up like WALL-E units (with only one crazy robot remaining), or they will learn, achieve sentience, then return and conquer the puny humans. Also, you got time-dilation wrong. Yes, the time inside the ship does not change relative to the ship. But that means that if you travel for 100k years at such a speed that the time rate is halved, then the ship will have travelled 200k years worth of distance.

[mainiac]

Time-relativity.

It's conceivable that a spacecraft might get up to speeds of .1c or even as high as .3c.  But past that point, acceleration becomes very, very difficult.  Time dilation will result in a significant reduction in the time of the journey at that speed, but not to the extent you are predicting.  Check out the top equation on this page:
http://en.wikipedia.org/wiki/Time_dilation
If we plug .3c (a very, very ambitious speed) into the "v" term, we see that we see the colonists are only experiencing a 10% reduction in time spent traveling.

Time-dilation is complicated, but basically, the effects of time-dilation only appear outside of the "spaceship". Inside of the spaceship, 100,000 years will pass, so you need 100,000 years of food.

Not quite.  From inside the ship, time will appear to flow no differently then before.  (The observer has low speed relative to things inside the ship.)  However, the ships residents will observe dilation on things outside the ship, including lengths:
http://en.wikipedia.org/wiki/Length_contraction
So traveling at .3c, they won't experience the journey as going any slower.  But they will experience it as going a shorter distance.

[Sean Mirrsen]

Why exactly does acceleration become difficult? Presuming you're using a device that can constantly dish out 1g of acceleration with the exhaust speed of C (like a VEEEERY powerful flashlight, yeah ) then what will reduce acceleration (in the ship's space-time) except the relative velocity of the accelerating medium, provided the propulsion still relies on Newtonian principles? In the space-time outside of the ship, yeah, the observed acceleration will decrease just as the time on the ship slows down in relation to ours.

[mainiac]

Why exactly does acceleration become difficult? Presuming you're using a device that can constantly dish out 1g of acceleration with the exhaust speed of C (like a VEEEERY powerful flashlight, yeah ) then what will reduce acceleration (in the ship's space-time) except the relative velocity of the accelerating medium, provided the propulsion still relies on Newtonian principles? In the space-time outside of the ship, yeah, the observed acceleration will decrease just as the time on the ship slows down in relation to ours.

I approve of your quick application of Newtonian physics.  However, Newtonian physics don't work at relativist speeds.  The energy needed to keep accelerating approaches infinity as you get to the speed of light.

However, even getting up to a reasonable fraction of the speed of light requires a huge amount of light.  Do a non relativistic calculation for me for a second, y'know: "kinetic energy is equal to one half mass times velocity squared."  How much energy would it take to accelerate a single kilogram to 30,000 kilometers per second (.1c)?  How much energy does humanity consume in a year?

[Sean Mirrsen]

I'm not even trying to get at feasibility here, just the general concept. For all we know, in twenty years we'll have drives that condense and rotationally accelerate magnetic fields that propel stuff with sheer vortex suction effect.

Anyway. Yes, to the outside observer. And no, not infinity. Since there is no such thing as an infinite amount of energy, then there is no action in the universe that can require infinite energy. The Universe itself was created with a finite amount of energy. Like I said, the only thing that can reduce the force output of a Newtonian reactive drive at relativistic speeds is the requirement that the reactive mass must transfer its energy to the drive. Since the speed of light is finite, whatever is used as reaction mass will be eventually blown away as soon as it exits the drive without a chance to transfer its energy because of the natural speed limit.

[Granite26]

I'm pretty sure that 'speeding up being harder' and 'time/space dilation at relativistic speeds' are roughly on the same order of magnitude. 

The problem isn't power or recyclables (food and water being two).

It's propellant.

[mainiac]

I approve of your quick application of Newtonian physics.  However, Newtonian physics don't work at relativist speeds. 

Since there is no such thing as an infinite amount of energy, then there is no action in the universe that can require infinite energy.

...which is precisely why you can't accelerate an object to the speed of light.  It would take infinite energy.  Therefore, it can't be done.

Light has no mass.  It also does not undergo acceleration.

Sean, you have a nice analytical mind.  You quickly form strong arguments.  But you need to be more careful about the assumptions that you start from.  Especially when you are trying to contradict very well establish theories.

[mainiac]

I'm pretty sure that 'speeding up being harder' and 'time/space dilation at relativistic speeds' are roughly on the same order of magnitude. 

The problem isn't power or recyclables (food and water being two).

It's propellant.

Which is why solutions usually employ a no propellant means of acceleration, like a laser sail.  Also, remember that you only need to accelerate and decelerate once.  There's no drag or maneuvering costs to speak of.  So an engine that expels it's propellant at VERY high speeds might be able to work as well.

[Sean Mirrsen]

Nothing can be created and already posess a speed by virtue of existing. Ripples in a pond posess a certain speed either because that's the speed with which the water contracts itself after being excited by an object, or because that's the speed that the object had upon entering the water. Unless the universe is indeed filled with an energy medium, then you cannot create lightspeed waves by simply exciting electromagnetic fields, you have to take an amount of energy - which posesses mass - and propel it to the speed of light. Or probably greater than the speed of light, with the universe slowing it down according to its own laws about electromagnetic field propagation.

A note on the last ninja-post: short bursts of incredible acceleration are alright for a weapon, but not a means of travel.

Also, I have very little in terms of an actual knowledge base. Which allows me to spew forth theories that sane scientists would find laughable or insulting. And I consider it a positive ability.

[Frelock]

"kinetic energy is equal to one half mass times velocity squared"

If we apply this principle, and apply the idea that light has 0 mass, then light has 0 kinetic energy.  Therefore, it can travel as fast as it wants without a need for energy input, as there is no kinetic energy associated with it.  Your idea about ripples in a pond is true, because the ripples have mass, and therefore need energy to move them.

If you could somehow use energy to "accelerate" light, you'd be effectively destroying energy, as the light has no kinetic energy to put that acceleration energy into.  We know that we can't destroy energy, so therefore, we can't accelerate light.  However, we also know that light travels at a certain speed (don't ask me how).  Since light can't accelerate, it makes sense to say it was always moving at that speed by virtue of its existence.

And as to the assumption that the universe not filled with an energy medium, it is.  Or at least I think it is.  I too have little knowledge on which to base my assumptions.

[Servant Corps]

You know, if any sci-fi story wants to explore the idea of sending ships out in the hope of them returning in 200,000 years, you don't have to explain in detail exactly how it works, just make it reasonable enough so the layman says, "Oh, yeah, that's realistic, it doesn't violate the laws of physics, just common sense! I can surely suspend disbelief!"

[Sean Mirrsen]

Light can't have zero mass and zero kinetic energy. Because it can apply pressure. Therefore it still is energy and posesses mass. While travelling at the speed of light no less.