Interstellar travel

[capo]

I've heard it's impossible to accelerate an object to the speed of light. I've heard this is because the amount of fuel (in this example, hydrogen fuel) required would outweigh all the hydrogen fuel available in the universe. Okay, but what say, if you weren't actually trying to reach the speed of light..just getting as close as possible to it. And what say if you had a fuel (and propulsion system) with a bit more of a kick, say something like an anti-matter nuclear fusion hybrid..or whatever.

 

Now, I'm not an expert of course. I'm barely an amateur. So I'm gonna hope that someone who really knows their stuff will step in here and fill in the gaps. First of all, I assume that only effectively massless particles (such as photons) are capable of travelling at light speed anyway, so reaching maximum velocity is outta the question for this reason alone. And as you accelerate an object (like a space ship), you give it more energy..you effectively increase its mass? Am I right?

 

Anyway..anti-matter is created in particle accelerators, right? And now they have methods to store and transport it in special vacuum containers right? And "they" reckon you can kick start a nuclear fusion reaction with a bit of anti-matter to produce a much more efficient propulsion system, right? Well...my questions are this:

 

1. For arguments sake, let's say we take a ship which is the mass of..I dunno, the international space station, and then try to accelerate it to as close to the speed of light as possible using this anti-matter nuclear fusion propulsion drive..at the greatest extremes, just how close to the speed of light could you possible get it to? 0.0001%? 1%? 101%? (queue Dr.Who music) And just how much fuel would you need to do this?

 

2. At this speed, what kind of time-distortion(s) might the people aboard the ship experience? If you travel at the speed of light, would you experience that time normally? Or would you be frozen in time in some way? What about a substantial fraction of light speed, say 50% of it? The journey to Proxima Centauri would take 8.4 years from the perspective of someone watching the ship travelling...but what about those aboard the ship? How long would the journey seem to take to them?

 

Lots of questions if anyone can answer them. I may have made some inaccurate remarks..or may be talking garbage all together, so feel free to point out my faults. But..not too brutally! I find it all very interesting and would like to learn more.

Edited December 21, 2010 by capo

[Janus]

I've heard it's impossible to accelerate an object to the speed of light. I've heard this is because the amount of fuel (in this example, hydrogen fuel) required would outweigh all the hydrogen fuel available in the universe. Okay, but what say, if you weren't actually trying to reach the speed of light..just getting as close as possible to it. And what say if you had a fuel (and propulsion system) with a bit more of a kick, say something like an anti-matter nuclear fusion hybrid..or whatever.

 

Now, I'm not an expert of course. I'm barely an amateur. So I'm gonna hope that someone who really knows their stuff will step in here and fill in the gaps. First of all, I assume that only effectively massless particles (such as photons) are capable of travelling at light speed anyway, so reaching maximum velocity is outta the question for this reason alone. And as you accelerate an object (like a space ship), you give it more energy..you effectively increase its mass? Am I right?

 

Anyway..anti-matter is created in particle accelerators, right? And now they have methods to store and transport it in special vacuum containers right? And "they" reckon you can kick start a nuclear fusion reaction with a bit of anti-matter to produce a much more efficient propulsion system, right? Well...my questions are this:

 

1. For arguments sake, let's say we take a ship which is the mass of..I dunno, the international space station, and then try to accelerate it to as close to the speed of light as possible using this anti-matter nuclear fusion propulsion drive..at the greatest extremes, just how close to the speed of light could you possible get it to? 0.0001%? 1%? 101%? (queue Dr.Who music) And just how much fuel would you need to do this?

 

2. At this speed, what kind of time-distortion(s) might the people aboard the ship experience? If you travel at the speed of light, would you experience that time normally? Or would you be frozen in time in some way? What about a substantial fraction of light speed, say 50% of it? The journey to Proxima Centauri would take 8.4 years from the perspective of someone watching the ship travelling...but what about those aboard the ship? How long would the journey seem to take to them?

 

Lots of questions if anyone can answer them. I may have made some inaccurate remarks..or may be talking garbage all together, so feel free to point out my faults. But..not too brutally! I find it all very interesting and would like to learn more.

 

How much fuel it would take depends of the exhaust velocity of the rocket. I could not find any numbers for the hybrid drive you mentioned, but it is estimated that a anti-matter drive could achieve exhaust velocities of 10,000,000 m/s .

 

With this exhaust velocity, it would take 14 million times as much fuel as the mass of the space station itself to reach just half the speed of light. For a person traveling to Proxima C, the trip would take 7.27 years.

 

But it's worse than that. The above gets you there, but you would just fly through the system at 0.5c. To stop, you would have to use just as much fuel as it takes to reach that speed. But since you don't use this fuel until you are ready to slow down, the mass of the fuel used to slow down has to be added to the mass accelerated up to 0.5c in the first place, this means that you would to start out with about 2 x 10^14 times as much fuel as space station. The space station masses 375,727 kg, so this means you would need about 7.4e+19 kg of fuel. This is about the mass of the Saturn moon Enceladus. ( And remember, a good portion of this would have to be anti-matter.)

[Schrödinger's hat]

Well there are some scifi drives that would do better than that.

To find the absolute best case scenario lets assume we have infinite access to technology, but have to stay within the laws of physics where we know we would violate them.

 

The most efficient drive possible (I have no idea how one would make it or attach it to a ship) with known physics is a small black hole. This is placed at the focal point of a parabolic reflector and fed matter. The hawking radiation is mostly low enough frequency that it can be reflected and you get between 50 and 100% efficiency.

You can cheat again, by putting your fuel on a platform, and aiming the 'thrust' (light beam) at your ship's solar sail. That way red shift is not as much of an issue.

Then your equation would be between:

gMv=pmc where g is gamma and p is the efficiency, m is mass of fuel, M is payload, v is change in velocity and c is speed of light.

and gMv=2pmc

The 2 coming from the reflection of the solar sail. I cannot be bothered doing the integral right now, so lets just assume your ship absorbs all the light rather than reflecting it.

gMv=pmc

Or as a percentage of c

1/sqrt(1+(1/p*M/m)^2)=v

Putting in, say, a 1,000,000 tonnes of fuel, with 1 tonne payload we get:

0.999999999998

Or gamma=500,000

So

Stopping, or turning around would be out of the question, as the rocketeers (or rocketeer with such a small craft) would only be able to accelerate away from earth (or up to a 90 degree angle very slowly if they had a reflective sail).

If they somehow managed to stop they would find only one second had elapsed on their ship for ever 5.8 days back on earth.

Double your mass in fuel would get you to roughly half the speed of light.

 

Of course, as soon as you start making any reasonable assumptions for the type of things we could engineer, things get a lot worse (See Janus' comment). This is just the upper bound for what might be possible.