If we're not alone in the universe, then where is everybody else?

Quote: odiousgambit

To endure multiple G's to get to Earth orbit velocity is one thing, but to endure if for years and years as would be necessary in this scenario just seems to be only theoretically possible. It would be an awful thing to inflict on someone, even if it didnt kill them. [edits]

You woulnd't want to accelerate that fast anyway, it's not fuel efficient.

A rocket lifting off Earth ahs to accelerate quickly, menaing with several times the gee force, because it's facing both air resistance and a strong gravitational field. It gets to orbit by 1) leaving the atmosphere behind and 2) reaching an orbital speed of about 11 kilometers per second. So the faster you accelerate, the sooner you get there.

But once in orbit you have a lot of space, all of it mostly empty, and a lot of time. So why rush?

Ideally you'd accelerate at precisely one gee, giving the spacecraft normal Earth gravity throughout the voyage. At midpoint you'd turn around and decelarate also at one gee. But to do so you'd need a LOT of fuel. So absent a working hydrogen ramscoop, that wouldn't be practical. But still, you have lots of room and time. your best bet would be an ion engine pushing at a fraction of one gee for years, or a nuclear engine, likely fission, doing the same thing.

I don't know how long would it take to reach a respectable fraction of c (light speed) at one gee thrust, though.

Oh, BTW, look up Von Neuman machines while you ponder means of interstellar travel. Then ask why we haven't found any.

Quote: Nareed

I don't know how long would it take to reach a respectable fraction of c (light speed) at one gee thrust, though.

Interestingly, it takes about a year to get to the speed of light.

c = 299,792,458 m/s
g = 9.81 m/s^2

So, it would take 30,559,883.59 seconds at constant g acceleration to reach c. Which is 353.7 days.

In that time, you would travel... 4,580,811,308,546,470 metres. Which is about 0.484 light years.

To summarize, ignoring relativity, it would take a year to accelerate at a comfortable speed to the speed of light, and you would cover 1/2 a light year in distance. The distance covered makes sense.

Quote: dwheatley

Interestingly, it takes about a year to get to the speed of light.

Actually it would take an eternity to get to 99.9999999999..999% of c :)

Seriously, no object containing mass can reach lightspeed. It's a relativity thing.

Quote:

So, it would take 30,559,883.59 seconds at constant g acceleration to reach c. Which is 353.7 days.

Let's call that 99.99% of lightspeed, or 0.9999 c. If you stopped accelerating you'd slow down a bit due to friction with the intertellar medium, mostly scattered hydrogen atoms. And you'd lose the feel of gravity, too.

Anyway, there's no need to accelerate rapidly at all. If fuel is an issue, and it will likely be, you'd perforce accelerate slowly and make use of various planets to pick up speed in slingshot maneuvers. So assuming you can reach 0.9999 c, you'd take a year or so to get up to speed. Then you need to decelerate around your target. You can do this in various ways, including a reverse slingshot maneuver using the three Centauri stars. But you'd still need a lot of fuel to slow down.

Quote: Nareed

But once in orbit you have a lot of space, all of it mostly empty, and a lot of time. So why rush?

Good question. I watched a program that claimed the G's needed would cause this problem. Looks like I should have listened better as to why they thought so.

Quote: odiousgambit

Good question. I watched a program that claimed the G's needed would cause this problem. Looks like I should have listened better as to why they thought so.

Look at it this way:

Getting to orbit is like driving up a steep, muddy road with your car. If you floor the gas pedal you'll use up a lot of fuel, but will get there faster and won't risk backsliding or getting stuck.

Now suppose once you climbed to the top there's a clean, gently sloping road that eventually becomes flat and it goes on for hundreds of miles. You want to reach your car's max speed. You don't have to accelerate fast at all. Just gently press on the gas pedal and you'll eventually reach 160 mph or so, but will do so at a lower cost in fuel.

Quote: the link

If interstellar travel requires traveling at light speed, the situation is not promising. The biggest hurdles against such travel are g-forces and time. G-forces are the force exerted on your body when undergoing high accelerations. Accelerating to the speed of light would have to be done gradually or the body would be torn apart. It would take about 2 ½ months to get to just half the speed of light! This explains the need for inertial dampers-the cosmic shock absorbers created by the writers of Star Trek. Inertial dampers somehow cancel out the force responding to the accelerating force. While this solves the problem for script writers, there is no such tool in the real world of physics.

Naturally I was wondering where this came from, and found this. Better articles are probably out there. Something wrong with our numbers?

PS, now wondering what the problem is with taking 2-3 months. This subject is out there though.

Doing some quick calculations, the amount of energy needed to accelerate 1kg from 0 to the speed of light (give or take a percent) at 10 m/s^2 is around 48 PentaJoules (4.8x10^16). If you could completely turn mass into energy (e=mc^2) you'd need a 500,000 kg mass of fuel.

Or in other terms, the two nuclear cores that I knew while growing up produce about 1,000 MegaWatts of electricty. That's 1,000 Million Joules per second. 1x10^9. So we are looking at 4.8X10^7 seconds to produce enough energy to send that 1kg off : 555 days. Strap a pair of them together and we start to have the sort of power required.

Except they don't weigh just 1kg...

Quote: thecesspit

Except they don't weigh just 1kg...

That's the rocketeer's eternal lament: you have to push the fuel along with the payload.

There's a way around it: don't carry the fuel with you.

For interstellar travel, this means using either 1) a hydrogen ramscoop or 2) a solar sail. they're both iffy, but seem safer than carrying tons of anti-matter.

It's not just the fuel, you also need to carry the engine as well. And even if we want to get to sub-light speeds that mean the trip to Proxima Centauri is in a life time (~40 years) 1/10c, it's still a lot energy per kg.... so we'd have to be sucking up vast buckets of hydrogen to keep going. I'm pretty sure there's not enough free hydrogen in the interstellar voids to keep it all going.

In short... given what we know right now, inter-stellar travel is impractical, and possibly unlikely that it ever will be. This may mean there is nothing out there we can talk to anyways. Anyways, we are decades away (I think) from even escaping the immediate earth orbit with manned flight. Too many problems on spaceship Earth to deal with.