[Bob-san]

Yeah but I'd be killed by the acceleration/deceleration.

Then choose a more distant target. Perhaps you'd like to visit Regulus at 77 ly away.

[General Battuta]

You'd also need a ton of fuel. It's an ugly engineering problem.

[NGTM-1R]

There's actually a finite limit on the maximum percentage of lightspeed you can reasonably accelerate to with most proplusion types, based on the point at which you start wasting fuel simply to carry more fuel for more acceleration as opposed to, you know, actually going anywhere.

Even an antimatter rocket only makes it to like .24 within those constraints IIRC.

[Herra Tohtori]

You'd also need a ton of propellant. It's an ugly engineering problem.

Fix'd.

Fuel != Propellant.

Propellant can be anything with mass accelerated into the direction opposite to desired acceleration. In chemical rockets, propellant is the exhaust gases from the nozzle; both oxidizer and fuel count as propellant; in solid rocket boosters, there's also usually catalyzing agents, binding and preserving agents and other stuff, which all is ejected from the business end of the rocket and used as propellant. Liquid fuel rockets usually use liquid oxygen as oxidizer and liquid hydrogen or kerosene as fuel; both act as propellant gases as they exit the nozzle.

In more effective thruster configurations, it is possible to derive the kinetic energy of the propellant from something else than a chemical reaction. This is the case with ion engines and the awesome planned NERVA thruster system which basically pumped hydrogen through the core of an active nuclear reactor to heat it to very, very high temperatures, expanding it and giving it ejection velocities higher than those achieved by chemical rockets.

Basically, what you want from your ship is specific impulse, or change of momentum. It is a simple factor of the entire mass of propellant you're carrying with, and the ejection velocity of said propellant. Opposite momentum is gained by the ship (although the acceleration given by the engine increases as amount of propellant carried by the ship reduces).

This means that the higher the ejection velocity, the less propellant you need to carry to achieve desired change of momentum.

So, anyway. Anything that can be shot away from a ship at high velocity is propellant. If you, say, had a ship on decaying solar orbit with disabled engines, but you still had the rotation control system online and enough energy for some serious railgun barrages... you could speed up your orbital velocity and slow down your fall towards the Sun. If you know what I mean. But if either RCS, reactor or gunnery control were off-line, you would be sort of screwed.

(Note for Battuta: This is mostly for the benefit of other people - you just gave me a good opening to go into lecture mode. )

[watsisname]

Yeah but I'd be killed by the acceleration/deceleration.

You'd also need a ton of fuel. It's an ugly engineering problem.

Valid points, but it still doesn't result in breaking the universe and ruining everyone's day.  Maybe just yours, if you were stupid enough to subject yourself to 10+ g's of force.  Or a lot more people if you accidentally smash into an inhabited planet at relativistic speed. :O

* watsisname plays with the impact-effects calculator.

[Scourge of Ages]

Ah, I remember the other FTL discussion like it was three months ago...

Here's what I carried away from it:

1. An inertial reference frame is not necessarily an observer. As near as I can tell, it is any point or object that could make an observation. eg. the same way your left eye has a different reference than your right eye, and even the various sensor cells in your eye all have slightly different references, so one side of a ball would have a different IRF than the opposite side, and I extracted from that that every particle could have it's own IRF, possibly even every geometric point in the universe. So, one does not need to see something for an IRF to exist there.

2. The math we have, and the experiments we've run ("we" being humanity) have all pointed to FTL travel breaking reality by changing a certain value to a negative number, and that negative number causes the universe to fold in on itself or something. Perhaps the math is incomplete, but so far we've seen no evidence that it is.

3. Therefore, I speculated that the only way for every IRF in existence to be able to agree on causality is if the object travelling did so in a way without acceleration, ie. instantaneous teleportation. But even that would break the existing math in some other way, iirc.

4. Regarding fraction of light speed, what about a laser drive? If your propellant is light itself, couldn't you get going pretty fast? Though accelleration would be a beast. Laser drives as seen in the Man-Kzin Wars

[Mika]

One comment about velocities approaching c; as if the earlier difficulties weren't enough, small dust particles along the way might cause serious problems. Not only do the eat away any sort of shielding, but they might also cause a static discharge to build up.

I don't even want to guess how deep a small dust particle penetrates when a spaceship collides with it at relativistic velocity.

Also, if you haven't already seen these, here's some interesting relativistic phenomena simulated in a ray-traced world.

[perihelion]

Borked link is borked.

[The E]

Ah, I remember the other FTL discussion like it was three months ago...

Here's what I carried away from it:

1. An inertial reference frame is not necessarily an observer. As near as I can tell, it is any point or object that could make an observation. eg. the same way your left eye has a different reference than your right eye, and even the various sensor cells in your eye all have slightly different references, so one side of a ball would have a different IRF than the opposite side, and I extracted from that that every particle could have it's own IRF, possibly even every geometric point in the universe. So, one does not need to see something for an IRF to exist there.

The "every geometric point" POV is the only true one.

2. The math we have, and the experiments we've run ("we" being humanity) have all pointed to FTL travel breaking reality by changing a certain value to a negative number, and that negative number causes the universe to fold in on itself or something. Perhaps the math is incomplete, but so far we've seen no evidence that it is.

Your understanding is incomplete. The thing to remember is that if you introduce time travel into a Universe, said universe becomes inconsistent.

3. Therefore, I speculated that the only way for every IRF in existence to be able to agree on causality is if the object travelling did so in a way without acceleration, ie. instantaneous teleportation. But even that would break the existing math in some other way, iirc.

Depends on the method. Wormholes could work. However, always remember that any method of circumventing c is also capable of allowing time travel. As said before, once you allow time travel, paradoxa are only an eyeblink away.

4. Regarding fraction of light speed, what about a laser drive? If your propellant is light itself, couldn't you get going pretty fast? Though accelleration would be a beast. Laser drives as seen in the Man-Kzin Wars

That laser still has to be powered by something. It also has to be cooled, and it has to have a high enough precision to still hit its target at interstellar distances. Not to mention that without a corresponding laser on the other end or onboard fuel, you can't decelerate. Also note that photons are not known for the big amount of energy they carry.