Question about time dilation

[cosmicalstorm]

Im having a hard time wrapping my mind around the concept of time-dilation.

I came across a person stating that if a vessel was traveling at a velocity near c that vessel could cover a distance of several billion lightyears in a human lifetime due to the effects of time dilation.

But I always thought a lightyear meant that light itself would actually need a year to travel that distance, meaning that if I was moving at 0.99c it would take me about 4.3 years to reach Proxima Centauri and that a journey spanning several billion lightyears would actually take several billion years to complete.

If that isn't the case why are people arguing so much about generationships, cryosleep etc?

I realize it would be hard to actually reach these velocities, especially with our current technology, but it's not impossible.

Can somebody straighten this out for me?

[Paolo]

It looks like you're hung up over the difference between the proper time (the time measured by an observer in lockstep respect to a traveller) and the time measured by an observer who witnesses the traveler moving with respect to him at some velocity v. An observer on Earth watching a rocket ship traveling to Proxima Centauri arbitrarily close to the speed of light will measure a transit time of t = 4 years. An observer on the rocket ship, however, experiences only t' = gamma * t, where gamma = sqrt(1 - (v/c)^2). That's called the proper time, and you can see as v approaches c ( or (v/c)^2 approaches 1), the proper time t' = gamma * t approaches 0.

[Paolo]

In the example you posted, an observer on Earth would measure the transit as taking 4.34 years. The passengers on the rocket ship, however, would experience just under seven and a half months travel time.

[Wyrm]

If that isn't the case why are people arguing so much about generationships, cryosleep etc?

I realize it would be hard to actually reach these velocities, especially with our current technology, but it's not impossible.

"It would be hard to actually reach these velocities" is very much an understatement. It takes enormous amounts of energy to propel something that fast. (Getting a one-tonne ship to 99%c would take a minimum of 6.37e20 J, or approximately 152 gigatons.) The only objects man has even launched at such high speeds are subatomic particles.

That's what all the big fuss is about cryosleep and generation ships.

[Surlethe]

But I always thought a lightyear meant that light itself would actually need a year to travel that distance, meaning that if I was moving at 0.99c it would take me about 4.3 years to reach Proxima Centauri and that a journey spanning several billion lightyears would actually take several billion years to complete.

Depends on who's measuring. That's the confusing thing about relativity: although all observers will agree on the value of c, they may not agree on other things that we humans consider basic, like length or time duration. So if you're traveling at some large fraction of c, the trip time you measure is different from the trip time, say, Earth measures. In particular, if you're going at 0.99c, the time someone on Earth measures is slightly less than 4.2 years, while the time you measure is just a little over 7 months.

If that isn't the case why are people arguing so much about generationships, cryosleep etc?

I realize it would be hard to actually reach these velocities, especially with our current technology, but it's not impossible.

It would be incredibly time- and energy-intensive to reach these velocities, and at those velocities the interstellar medium becomes a danger to ships. Moreover, remember that the difficulty to accelerate increases with volume, so the larger the ship, the harder it is to accelerate to near-c speeds.

[Feil]

Note: from the accelerated frame, the length of the journey decreases and all periodic events become slower. As his clock ticks merrily along once per second and he zips along at his relative velocity to the rest of everything, he will watch the earth's movement around the sun slow and slow, and the distance he is traveling will contract. As he approaches c, outside periodic systems approach a frequency of 0Hz and all distances approach 0m.

[cosmicalstorm]

Thanks for those explanations, this is very interesting, and very hard to understand... though I suppose the human brain wasn't exactly made for figuring stuff like this out.

And yeah maybe im a bit too optimistic about what might be possible in the future.

(and sorry if I come of as a bit stupid but I only recently became interested in these things)

[OmegaGuy]

Couldn't solar/laser sails get a ship going that fast via acceleration over a long period of time? I read an article about it

[Ariphaos]

I prefer the idea of using mass drivers, myself, though I think the feasible limit would be closer to 20% or so of c (in order for the ship to slow down), it still means getting to Alpha Centauri in a few decades instead of centuries.

Once there, if you're really cocky you could set up a 'target' mass driver, or perhaps two so one could be used for sending and another for receiving, and you can fire stuff between the systems at decent speeds.

[Feil]

Couldn't solar/laser sails get a ship going that fast via acceleration over a long period of time? I read an article about it

Not really. As you increase distance, the beam is going to spread to the point where a sail large enough to catch enough of it to matter is going to give you more momentum bleed from bumping into interstellar hydrogen than you gain momentum from the laser. As you increase velocity, the beam is going to redshift to the point where the momentum per photon goes down. The only utility of it is that it lets you put some of the ship's mass somewhere else, where size and mass isn't a factor.

[Junghalli]

But I always thought a lightyear meant that light itself would actually need a year to travel that distance, meaning that if I was moving at 0.99c it would take me about 4.3 years to reach Proxima Centauri and that a journey spanning several billion lightyears would actually take several billion years to complete.

It does require that long to complete, to an external observer, just not to you. It may take only a few years to get to Andromeda at some really high fraction of c, but to somebody watching from Earth millions of years will have passed.

If that isn't the case why are people arguing so much about generationships, cryosleep etc?

Because the technical challenges of getting a ship up to near-c velocities are formidable. It'd probably only be feasible with a pure matter-antimatter anninhalation drive using quantities of antimatter measured in tons, and that stuff is very expensive in terms of energy. I once did a calculation that producing 100 tons of antimatter would require the entire output of the world's largest hydroelectric plant for several tens of billions of years. You'd need a solar panel array with a combined acreage of millions of square kilometers to get the energy you need to make the fuel for your fleet of .999 c rockets.

For practical interstellar travel I favor a version of the Valkyrie spacecraft powered entirely by antimatter-fusion hybrid mode. Its AM-fusion hybrid drive still gets an exhaust velocity of .12-.2 c, which is considerably better than most other competing drive systems. And the lightweight design is great for interstellar missions. I once did a little write up for a colony ship of such a design that, using optimistic estimates, would be able to carry several hundred colonists to another star system at a speed of .5 c, probably using much less antimatter than the original Valkyrie design.

Not really. As you increase distance, the beam is going to spread to the point where a sail large enough to catch enough of it to matter is going to give you more momentum bleed from bumping into interstellar hydrogen than you gain momentum from the laser. As you increase velocity, the beam is going to redshift to the point where the momentum per photon goes down. The only utility of it is that it lets you put some of the ship's mass somewhere else, where size and mass isn't a factor.

Another problem with laser propulsion is that it's dependant on a groundside installation, meaning that unless there's a corresponding laser in the target you can't slow down. Needless to say, this makes it useless for a colonial expedition.

[Winston Blake]

Another problem with laser propulsion is that it's dependant on a groundside installation, meaning that unless there's a corresponding laser in the target you can't slow down. Needless to say, this makes it useless for a colonial expedition.

A common misconception.

[FA Xerrik]

Come on, everyone knows the best way to propel a spaceship is to blow up nukes behind it and funnel the explosion into forward momentum. Give up on those sissy lasers.

[Ariphaos]

...what the hell is the point of a combined fusion-antimatter drive, if it can run entirely on antimatter?

As comfortable as people would be sitting atop such an intensely radioactive and volatile drive in the first place. At least a mass driver or laser propulsion system can cut your fuel use in half or better -without- risking what happens when one of your hydrogen flakes collides when it's not supposed to.

[Junghalli]

A common misconception.

Well, if you mean you can use a laser propulsion system for acceleration and something else for decelleration, yeah sure.

...what the hell is the point of a combined fusion-antimatter drive, if it can run entirely on antimatter?

Did you not read my post? Because antimatter takes obscene fuckloads of energy to create. A metric ton of antimatter would require at least 1.8 X 10^23 joules to create. The biggest power plants in the world today only generate something on the order of 1 X 10^10 watts. A pure antimatter rocket would require hundreds of tons of antimatter. And that's with a maximally efficient antimatter factory, with currently feasible technology the efficiency is more like .1%. Do the math for how long it would take our civilization to scrape together enough antimatter for just one .9 c rocket. I guarentee it'll depress the fuck out of you.

By contrast, an antimatter-catalyzed fusion drive probably would need quantities of antimatter measured in grams, not tons. Much less daunting. Such a craft would be slower, but much cheaper, because it wouldn't require you to build a solar panel array with the combined acreage of a small US State to power the antimatter factory.

There's more to building a vehicle than just internal engineering considerations.

[Feil]

Come on, everyone knows the best way to propel a spaceship is to blow up nukes behind it and funnel the explosion into forward momentum. Give up on those sissy lasers.

Whether you're using a laser or a bomb, all you're doing is trying to make your ship go faster by shining a really bright light at the back side. There's no atmosphere to explode in in space.

[Junghalli]

Whether you're using a laser or a bomb, all you're doing is trying to make your ship go faster by shining a really bright light at the back side. There's no atmosphere to explode in in space.

I believe with an Orion the idea is to use the superheated plasma from the exploding bomb as propellant.

[Paolo]

Whether you're using a laser or a bomb, all you're doing is trying to make your ship go faster by shining a really bright light at the back side. There's no atmosphere to explode in in space.

Except you'd have to carry the bombs with you. Not so much the laser.

[Ariphaos]

Did you not read my post? Because antimatter takes obscene fuckloads of energy to create. A metric ton of antimatter would require at least 1.8 X 10^23 joules to create. The biggest power plants in the world today only generate something on the order of 1 X 10^10 watts. A pure antimatter rocket would require hundreds of tons of antimatter. And that's with a maximally efficient antimatter factory, with currently feasible technology the efficiency is more like .1%. Do the math for how long it would take our civilization to scrape together enough antimatter for just one .9 c rocket. I guarentee it'll depress the fuck out of you.

I did read your post. You're being an idiot.

Do the math for a civilization .1% along the way to type II (half a second if you're lazy). Our current power output (~15 terawatts) is no reasonable measure for the amount of power a civilization that is seriously looking at interstellar travel is capable of.

By contrast, an antimatter-catalyzed fusion drive probably would need quantities of antimatter measured in grams, not tons. Much less daunting. Such a craft would be slower, but much cheaper, because it wouldn't require you to build a solar panel array with the combined acreage of a small US State to power the antimatter factory.

Did you even read your own stupid article?

Fusion only provides the energy to get up to about .2 of c for a craft made up of ~63% deuterium fuel. Your antimatter hybridization is picking up the rest of that .92c. Accelerating one tonne of mass to .86 of c requires one tonne of energy, and if you are using antimatter at least 2.5 tonnes of AM + matter. The fusion is adding pittance to this equation - it's nearly two orders of magnitude less efficient.

[Junghalli]

I did read your post. You're being an idiot.

Do the math for a civilization .1% along the way to type II (half a second if you're lazy). Our current power output (~15 terawatts) is no reasonable measure for the amount of power a civilization that is seriously looking at interstellar travel is capable of.

I didn't say it was impossible, I just said that an AM-fusion hybrid drive would be cheaper. I meant I favored it in terms of near future efforts (near future as in early next century or so), not that it would be the best thing ever for all time.

I realize I was a bit snippish and unclear and I apologize, I was in a bad mood today for reasons that had nothing to do with you.

Did you even read your own stupid article?

Fusion only provides the energy to get up to about .2 of c for a craft made up of ~63% deuterium fuel. Your antimatter hybridization is picking up the rest of that .92c.

Yes, I know that. There's no reason you can't use the AM-fusion mode to get higher, it just switches to pure M/AM at .2 c because it's most efficient that way. Using AM fusion would be slower, and require much more fuel yes, but it would be cheaper and could be achieved sooner.

I think the disagreement we're having here is sort of like arguing over whether ICF fusion or NERVA is better. There's no question ICF is better, but it's probably the better part of a century away, whereas NERVA could be built in a matter of years. I think I may have been a bit unclear in that respect, sorry.

[Winston Blake]

A common misconception.

Well, if you mean you can use a laser propulsion system for acceleration and something else for decelleration, yeah sure.

I don't mean that. For the reading comprehension impaired:


Link

[Junghalli]

I don't mean that. For the reading comprehension impaired:

I think I finally found it in the first link you posted, actually.

You learn something new every day. The performance is a lot better than I expected too: .5 c after 1.6 years of acceleration with 3000 tons of mission payload and no hundred ton quantities antimatter required (although a 43 petawatt laser capable of years of continuous firing isn't an engineering achievement to be sneezed at either - that transmitter lens must be pretty big to keep from melting). This thing is nicely in the category of "the sorts of ships that could actually give us something approaching a true interstellar civilization with no FTL, as opposed to just sending out a colony ship or robot probe every once in a while that takes 500 years to get wherever it's going". Fascinating.

[Junghalli]

Ghetto edit: I wonder what the effective range of that laser is...

[Ariphaos]

I didn't say it was impossible, I just said that an AM-fusion hybrid drive would be cheaper. I meant I favored it in terms of near future efforts (near future as in early next century or so), not that it would be the best thing ever for all time.

It's not favorable at all, though. One tonne of deuterium is the equivalent of about 10 kg of antimatter (unless you can trap neutrinos). Meaning, you are getting about 1% of your energy from your deuterium supply, when instead spending twice the effort to get more antimatter cuts your weight by nearly a tonne.

Yes, I know that. There's no reason you can't use the AM-fusion mode to get higher, it just switches to pure M/AM at .2 c because it's most efficient that way. Using AM fusion would be slower, and require much more fuel yes, but it would be cheaper and could be achieved sooner.

No it couldn't. Skip the fusion process in the design, and you get .85c instead of .92c (or so, I'd have to run the math).

I think the disagreement we're having here is sort of like arguing over whether ICF fusion or NERVA is better. There's no question ICF is better, but it's probably the better part of a century away, whereas NERVA could be built in a matter of years. I think I may have been a bit unclear in that respect, sorry.

We're not going to have a significant amount of antimatter that we feel safe containing before we have 'normal' fusion.

In the mean time, mass drivers and light sails run circles around you - they are technologically feasible and far, far safer, while at the same time offering significant speeds.

[Junghalli]

No it couldn't. Skip the fusion process in the design, and you get .85c instead of .92c (or so, I'd have to run the math).

Rockets don't magically stop working when you have a delta V that exceeds the exhaust velocity, they just get much less efficient. It's a question of diminishing returns, not any sort of barrier. I have done the math and (if I got all the numbers right) a rocket with an exhaust velocity of 58,900 km/s (the upper limit for AM-fusion) could get away with a mass ratio of around 200 for a speed of .5 c. It's not great but it's enough to send a small payload (a few hundred tons) to a nearby star within a human lifespan. Yes, I acknowledge that is an optimistic estimate, and yes there are better systems, but it has the virtue of being cheap in the short term. In the near term an AM-fusion is a much lower bar to shoot for than a pure AM rocket, which will require expensive infrastructure investment (an Asimov Array).

Really though, at this point it looks to me like laser sails are the best alternative of any system save Clarketech ones. Higher payload, and while they also require extensive infrastructure investment you get a much higher return on it: all the really expensive pieces of it are infinitely reusable, as opposed to an antimatter drive where you expend very expensive stuff; the infrastructure to send out one laser sail ship will support a small fleet at no extra cost save that of the ships themselves. Plus if you can use the last-stage sail the ship uses to accelerate on its return journey to decelerate other ships every ship past the first one is infinitely reusable, with no expensive disposable parts. Not to mention the virtually unlimited speed; delta V limited only by how long you care to keep the laser trained on the ship. The only really big liability is the limited range.