Traveling at the speed of light and time dilation

[typical guy]

I'm a fan of sci fi books and a favorite theme is that of the "sleeper ship" so to speak. They freeze the crew and later thaw them out when they arrive at their destination. The crew arrives at their destination thousands of years later to find that the planet is already populated by billions of people because humans found some loophole that allowed them to "cheat" the speed of light (wormholes or whatever the idea dejour is). The crew of this "highly advanced " ship now have really old technology and no way to make a meaningful living with their skills.

I think the same kind of argument could be made for near light speed travel. If you're traveling for a few weeks your time to go across the galaxy, the rest of the galaxy is experience a great deal more time and if there is some FTL loophole possible, not matter how remote, it will eventually be exploited given enough time. This journey would end rather sadly for the occupants.

 

[cyberdiver]

I wouldn't call it certainty, though.

 

[DHF]

I like the aspect of the crew waking up to find that their civilization had passed them by. I have seen and read it before and I wanted to model aspects of my story after that. A season one episode of Star trek TNG comes to mind where they find a trio of 20th century folks in cryo status, they revive them, pat them on the back and integrate them into 24th century society. personally I agree with you that it wouldn't be that easy. imagine if you transported someone from the 16th century to our age. the culture shock would be physiologically crippling and I think adaption would be difficult. not just for him but for the current age to accept such a throwback.

as I said I wasnt fond of cryogenics and originally wanted to achieve the same effect by having them skim the speed of light for a few weeks subjectively only to find centuries passed on the outside but as we already covered, traveling at such speeds become very unrealistic unless you are pictching the story millennium down the road and limitless forms of energy are at our disposal.

 

[cyberdiver]

If you want to go relativistic, you could use a Bussard ramjet using CNO-catalysed hydrogen fusion. It's been studied, and is probably possible. IMHO, a story that occurs 300-400 years in the future should be allowed a bit of a fudge factor.

 

[DHF]

thanks, that gives me some ideas. What about Radiation? would this be something I would have to develop a mechanism for? I have heard that as you approach c, radiation impact increases but since radiation is impacting a resting mass from all directions doesn't that mean that as you moved forward, the radiation impacting you from behind would be lessened? would the decrease in radiation from behind you offset the increased energy of the radiation impacting you from the front or would the forward radiation's impact increase greater then it was decreasing from behind?

 

[Drakkith]

Yes, the light in front of you is heavily blueshifted while behind you it is redshifted.

 

[DHF]

Ok good to know.
so because its being blueshifted the frequency increses. how would I calculate how much of an increase in radiation my intrepid crew would be facing? would the blueshifting in radiation make the impact such that the acceleration would be pointless? it seems your speed would be limited not only by the energy needed for propulsion but you would need to devote just as much energy to some form of electro-magnetic shield to divert the radiation.

 

[Enigman]

Ok good to know.
so because its being blueshifted the frequency increses. how would I calculate how much of an increase in radiation my intrepid crew would be facing? would the blueshifting in radiation make the impact such that the acceleration would be pointless? it seems your speed would be limited not only by the energy needed for propulsion but you would need to devote just as much energy to some form of electromagnetic shield to divert the radiation.

And dust too...BTW if you don't take into dust, light and gravitational effects into consideration, you will just need a limited amount of fuel to reach 0.99c after that, you won't need any...except to stop.
Just making sure you know that.
Also you could perhaps kill fire with fire and put some kind of cloaking on the ship which absorbs all incident light (a.k.a. perfect black body) and use energy absorbed to beam a high energy LASER or something in form of a spherical aura which would deviate most of the dust and slow all of them down. For fuel it probably should be cold fusion as antimatter can't be 'mined' and to create some you would need at least double the energy needed for the whole journey-not exactly feasible.
For light and dust you may also use some kind of space-time wrapping machine which can clear the spaceship's path. Again not very feasible, except of course a black hole.

And of course don't go to 0.99c near the Earth or any planet cause the huge mass of spaceship will destroy it(the planet) also stop before going too near to alpha centauri...
And is this just a one-way trip? You know, if you are going to get them to come back 'twin paradox' comes into play...making the ship just one glorified time-capsule.
cheers!

 

[cyberdiver]

Ok good to know.
so because its being blueshifted the frequency increses. how would I calculate how much of an increase in radiation my intrepid crew would be facing? would the blueshifting in radiation make the impact such that the acceleration would be pointless? it seems your speed would be limited not only by the energy needed for propulsion but you would need to devote just as much energy to some form of electro-magnetic shield to divert the radiation.

The blueshift/redshift/time dilation/relative mass increase/Lorentz contraction (or relativistic effects, in short) can be calculated with the following equation:
γ = 1/sqrt(1-v^2/c^2)
Where:
γ = Lorentz factor (the factor of relativistic effects)
v = Relative velocity
c = The speed of light

For example, let's say that you're traveling at 99% the speed of light:
γ = 1/sqrt(1-2.97e8/3e8)
An easier way is:
γ = 1/sqrt(1-0.99^2)
γ = 7.09 (3 significant figures)

Let's say that you were approaching an orange star. According to http://rohanhill.com/tools/WaveToRGB/ handy wavelength-to-RGB calculator site, that gives us a wavelength of about 620 nm.
620/7.09
= 87.4 nm (3 significant figures)

The star's orange light would be blueshifted into extreme ultraviolet (all the wavelengths can be looked up on Wikipedia). On the other hand, 4 mm microwaves would be blueshifted into the red part of the visible spectrum. BTW, a magnetic field wouldn't help much to stop UV and X-rays. It shields against particle radiation, like the stuff in a solar storm. Have the dust shield double as a UV shield and put ice between the crew and the front.

EDIT: http://www.projectrho.com/public_html/rocket/slowerlight.php
This site should help with designing your spacecraft .

 

[DHF]

Thanks that site is a good read and hopefully give me some ideas.
I had not considered the idea that cell damage would occur during the suspended animation trip. though I think I can get around that by putting the crew in a deep hypotherrmic state instead of cryostasis, the crew would not be frozen but their metabolisms would be slowed to 1% of normal. Adding in a nano-tech immune and repair system should keep the crew alive during the trip. still not sure which propulsion system, though I am leaning towards the laser assisted solar sail and antimatter combo drive.

 

[Varun Bhardwaj]

to cross the barrier means traveling a distance in zero time. if you do it than your 1 dimension will loss (you are three dimensional) . And you will not stop anywhere. To do it your mass can be zero or energy can be infinite.

 

[phinds]

to cross the barrier means traveling a distance in zero time. if you do it than your 1 dimension will loss (you are three dimensional) . And you will not stop anywhere.

Huh ?

 

[OCR]

I like the aspect of the crew waking up to find that their civilization had passed them by.

Yes, it might be a big surprise... lol


http://en.wikipedia.org/wiki/Planet_of_the_Apes_(1968_film)

http://en.wikipedia.org/wiki/Planet_of_the_Apes_(novel)





OCR

 

[DHF]

Yes OCR that had come to mind :) I actually quite like the idea of exploring what cizilization comes after our own but that won't be in the cards for this tale :)

I know giving them a 1G acceleration would be out of the question, the mass ration would be be almost 900:1 and that just won't fly. I was thinking about giving them an acceleration of 0.10%G or even 0.05% G.

I got around the whole keeping the crew alive thing by ditching a living crew. The current crew will be androids with AIs based on human astronauts back on Earth. In this way the life support can be ditched and the mass of the ship can be further reduced because they won't have to worry about a return flight. Once the ship arrives, the crew will be stationed in the Alpha Centuri Star system permanently, intending to explore and run experiments while transmitting the results back home. The drama will arrive when certain members of the crew start to realize that their programming has changed during the flight, they come to the conclusion that they are self aware and tensions arise when members of the crew question the idea of being abandoned light years from home.

 

[Ryan_m_b]

Yes OCR that had come to mind :) I actually quite like the idea of exploring what cizilization comes after our own but that won't be in the cards for this tale :)

I know giving them a 1G acceleration would be out of the question, the mass ration would be be almost 900:1 and that just won't fly. I was thinking about giving them an acceleration of 0.10%G or even 0.05% G.

To work out how much fuel you need you have to have a speed in mind, not just acceleration.

I got around the whole keeping the crew alive thing by ditching a living crew. The current crew will be androids with AIs based on human astronauts back on Earth. In this way the life support can be ditched and the mass of the ship can be further reduced because they won't have to worry about a return flight. Once the ship arrives, the crew will be stationed in the Alpha Centuri Star system permanently, intending to explore and run experiments while transmitting the results back home. The drama will arrive when certain members of the crew start to realize that their programming has changed during the flight, they come to the conclusion that they are self aware and tensions arise when members of the crew question the idea of being abandoned light years from home.

Seems like a cool idea. If you want some similar fiction to get inspiration from Saturns Children and Neptune's Brood (the better of the two IMO) by Charles Stross are great books. They feature a future time when humanity has gone extinct leaving behind robots, based on humans, to continue civilisation. The technology of their bodies makes them much more durable meaning they can do away with pesky portable ecosystems or having to travel at high speed. They just go as fast as they can with their energy budget (usually 0.01C) and crank their clock rate right down to make the journey seem subjectively quick.

 

[DHF]

Well theoretically using an acceleration of 0.05G they could reach up to 0.43c before having to decelerate. This would get them to their destination in under 20 years. 24 years would be an acceptable amount of time for the scientific community on Earth to wait before receiving data however I am afraid that going that fast would again blow the mass ratio out of the water. having them travel at 0.10c would keep the mass of the ship down but it woul make the trip take close to 100 years, its a pretty long term investment for Earth, even in a sci fi future I am sure politicians would balk at funding a project that neither they nor their constituents have any hope of seeing a return of investment on. I toyed with the idea of Anti matter propulsion but I don't think 200 years is enough time for Earth to have reached a point where we can cheaply mass produce Gobs an gobs of anti hydrogen. Even if they did, then I would have to increase the mass to account for a complex containment system and heavy shielding least a stray Toyota sized rock strikes the fuel tanks on the way there an goodbye ship.

P.S. Thanks for the references, I will certainly look them up this weekend, they sound very interesting.

 

[DHF]

Ok so having fully settled on a Fusion based engine. I am trying to cement how long it will take them. I am having a hard time pinning down their maximum speed. when researching fusion as a method of propulsion, most material I have read marks upper limits on how fast you could go with a particular type of fusion (between 8-11%c) If my characters were utilizing constant acceleration wouldn't speed just be a matter of fuel? or is there some other mechanism that would create a cap on speed? if there is a cap on the speed then how does that work for my characters? Since they are going to alpha Centuri, and using .10G acceleration. theoretically they could reach .57c and be there in a little over 13 years. but if they are limited to a maximum velocity of say 10%c then would they just accelerate for a year then coast for a few decades then decelerate for another year?

 

[Ryan_m_b]

Fuel determines maximum speed in the sense of how fast you can accelerate to before you don't have enough fuel to slow down. It also affects how fast you can accelerate because fuel obviously has mass. If your engine can put out 1MN of thrust and your payload masses 100 tonnes then your maximum acceleration is 1G. But you have to add your fuel on top. If that amounts to 900 tonnes (for example) then your maximum initial acceleration is 0.1G.

If you don't have fuel for constant acceleration (and it's unlikely you will) then yes your craft will have to coast for a large portion of it's journey.

 

[DHF]

OK that works for me. so the only cap on speed is the amount of fuel you can carry. So if they wanted to go .12c its simply a matter of: can you carry enough fuel to accelerate to that speed and still have enough fuel to decelerate? I was under the impression that there was a theoretical maximum exhaust speed depending on the type of fusion reaction you used.

If I was wrong about the Theoretical maximum then I will have the engine use a D + 3He reaction. they can accelerate for a year up to about 12%c, coast for about 35 years then decelerate. Total trip time: 37.7 years. I can deal with that.

would the ship be able to communicate with Earth while they are in transit or would communications be distorted from them moving away at relativistic speeds?Thanks for the help :)

 

[phinds]

Also, keep in mind that one thing science fiction generally ignores is that at even low relativistic speeds, when you hit dust particles, or worse yet something even a little larger than dust, YOU are likely to become dust. So while speed is theoretically limited only in the ways that Ryan pointed out, in the real world, you are going to need shielding of some kind and the faster you got the harder is is to have any effective shielding. I don't know of any current technology that would provide protection at even .1c, but this is not an area that I know much of anything about, I'm just pointing this out from my perspective as a practical engineer.

 

[phinds]

would the ship be able to communicate with Earth while they are in transit or would communications be distorted from them moving away at relativistic speeds?

Communication signals would experience a predictable and continuous down-shift in frequency during the acceleration and then become constantly downshifted while coasting but all that can be compensated for. The biggest issue, I think, would be signal power but you can hand-wave your way out of that pretty easily.

 

[DHF]

Thank you very much Phinds, I was keeping that in mind when designing the ship for the book. when the ship is traveling it employ a shielded dome on the front of the ship and shielded panels covering the sides. while in transit the ship looks like a mushroom with an octagon shaped stem. when the ship arrives it ejects the dome and shielded panels to reduce mass and make travel around the solar system more fuel efficient. each of the eight sides of the ship will extend solar panels to provide auxiliary energy for the ship and crew. power will also come from a second fusion reactor that will utilize the fuel that wasn't spent for propulsion.

Though the front of the ship is protected, the rest of the hull is more vulnerable. while in transit, cosmic rays penetrate the less shielded area of the hull. part of the crew's programming is effected by interaction with said cosmic rays and this leads to uncomfortable thoughts popping up during their mission, like home sickness.

 

[Ryan_m_b]

Phinds is right of course, you'll need to take into account shielding.

OK that works for me. so the only cap on speed is the amount of fuel you can carry. So if they wanted to go .12c its simply a matter of: can you carry enough fuel to accelerate to that speed and still have enough fuel to decelerate?

Not quite, you missed the point of maximum acceleration. If you want to go to speed X in a given time frame you have to have enough fuel to accelerate to it and slow down and a powerful enough engine.

Some back of the envelop calculations (feel free to check this by using the rocket equation yourself, it's not my field by a long way):

If we assume the craft masses 1000 tonnes (small but I'm handwaving here) and the exhaust velocity of a fusion rocket is 500kmps (taking a rough average from these estimates)) then to get to .12c would require ~8*10³⁷ kg of fuel (much more than the mass of the solar system). And that's just to reach the speed! Not even to slow down again.

(You have to bump exhaust velocity way up before the numbers become reasonable. At 10,000kps the fuel mass would be 40 thousand tonnes of fuel. At 100,000kps it's just 440 tonnes but that implies you have an exhaust velocity of 1/3rd the speed of light! At that speed you'd have to pump in ~5e15 joules per kilo of fuel, 300x global energy usage.)

How much thrust you produce is worked out by your mass flow x exhaust velocity. Essentially how much mass exits the back of the ship (assuming it does so perfectly collimated). For the 500kmps ship to accelerate at 1G would take a mass flow of 1.6e33 kgps, again more than the amount of mass in the solar system.

So you'll have to do some playing with the numbers to decide on a reasonable exhaust velocity, a reasonable payload mass and a reasonable mass flow. Once you have that you can begin to work out what acceleration and top speed is feasible.

would the ship be able to communicate with Earth while they are in transit or would communications be distorted from them moving away at relativistic speeds?

I imagine that should be relatively easy to compensate for, it's just Doppler shift.

Thanks for the help :)

No worries.

 

[DHF]

then to get to .12c would require ~8*1037 kg of fuel (much more than the mass of the solar system). And that's just to reach the speed! Not even to slow down again..

augh! damn you reality with all your numbers!

the heck with it, I am just going to put them in a trash can and give them a good shove. they should get there in a few million years, they are androids, they won't care.

there, mass and fuel and all that nonsense solved ;)

or I might just have a dragon take them there. that should help me avoid any further calculations :D

 

[Ryan_m_b]

augh! damn you reality with all your numbers!

Lol! IMO it's entirely possible to write hard SF to get round this, generally there just has to be one piece of super-technology proposed.

the heck with it, I am just going to put them in a trash can and give them a good shove. they should get there in a few million years, they are androids, they won't care.

To be fair that isn't a bad idea. I've read stories before where androids traveled interstellar at very slow speeds. It might have taken them millennia but that wasn't a problem because the crew just cranked their clock rate right down as a form of stasis (speeding back up regularly to conduct maintenance) and anyone not essential was kept backed up on a computer ready to download into a body constructed on arrival.

 

[Ryan_m_b]

I just remembered another way (from an SF story) you could tap dance past this problem and keep it within the realm of acceptabley hard science fiction. You could propose that the ship is a type of Starwisp, essentially a giant solar sail, and give it a small payload of around 1 tonne. The ship could be accelerated by laser arrays orbiting the sun, when it comes to slowing down it could release a large part of its sail which doubles up as a mirror bouncing the beam back to the small payload. The payload itself could be some hypothetically super-compact fab lab, a number of small robots, a computer and some source of energy. AI could be downloaded into the robots and sent out to set up the fab lab, gather resources and build infrastructure. It might take a very long time but assuming no catastrophe (a big assumption admittedly) those initial machines could build up factories for mass producing stored copies of their crew.

 

[DHF]

I know: The ship will be powered by unobtainium. there, done!

 

[Ryan_m_b]

You could switch from fusion to some sort of antimatter rocket. Getting back to the energy requirement of 5e15 joules per kilo for an exhaust velocity of 1e8mps: annihilating a kilo of matter gives 9e16 joules (18 times more than the energy requirement). If you suggest that antimatter can be made in bulk then you could up your fuel requirement by 5% and have half of that be antimatter. That extra 5% of matter and antimatter could be annihilated to produce the energy to accelerate the fuel but you'd need a hand wavy answer to suggest how.

 

[DHF]

I had initially shied away from antimatter because I felt that it might realistically be thousands of years before we can use it in a practical manner. however the more we crunch numbers, I just don't think Fusion will do the job if I want the crew to get there in a human lifetime. Ironically I move away from having a living crew to escape the confines of human life spans but then I realized the mission would still need to operate within human life spans because humans funded the mission and I don't think the government or a private company woul spend hundreds of billions of dollars on a venture that would take centuries to unfold.

so getting back to anti-matter. I suppose if it is the only way I can make it work I will go that route, but I feel that we would sooner perfect proton - proton fusion first. Speaking of which, it is not as powerful as matter/anti-matter but if the characters had access to such perfect fusion, what kind of speeds an acceleration could I expect?

 

[Deleted member 498135]

It's impossible for objects with mass to attain the speed of light. You're question is therefore unanswerable because it essentially asks "if we ignore the laws of physics what do the laws of physical say?"

Very true... Too bad :( if it was real, hypothetically speaking, would time stop?