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Thread: Travelling at percentages of light speed

  1. #1 Travelling at percentages of light speed 
    kaz
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    So...We've all read about this sort of thing before.
    Its theoretically possible to go to percentages of light speed and all that and the relatavistic effects of this would be really rather interesting with the people on the starship aging considerably less and all that.
    Its all been done before.

    One thing I'm wondering about though....
    How fast would it actually be possible for a starship to have a comfortable ride?

    I'm not a physicist but acceleration normally causes a form of artifical gravity right? Surely if you're going at a huge percentage of light speed the people inside the ship would be promptly squashed back towards the engine right?
    How fast would a ship be going to get 1G?


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  3. #2  
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    Experience at low speeds suggests that it is acceleration that will squash you into your seat, whereas travelling at a constant speed a traveller in an enclosed space cannot deduce speed by any known means without an external reference. For a ship to experience 1G it would need to accelerate at a constant 9.8 m/s^2


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  4. #3  
    Administrator KALSTER's Avatar
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    I read somewhere that 4G would be the most someone could get used to since (someone on this forum said) your retina would become detatched at any greater acceleration.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  5. #4  
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    Quote Originally Posted by KALSTER
    I read somewhere that 4G would be the most someone could get used to since (someone on this forum said) your retina would become detatched at any greater acceleration.
    Put three people of your own weight on your shoulders and see for how long you can stand it!

    You can only withstand high G force for short periods and only then if you are strapped down (eg like a racing driver 3 to 5 G or fighter pilot -3 to +9g whilst wearing a G suit).
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    Administrator KALSTER's Avatar
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    Put three people of your own weight on your shoulders and see for how long you can stand it!
    You can only withstand high G force for short periods and only then if you are strapped down (eg like a racing driver 3 to 5 G or fighter pilot -3 to +9g whilst wearing a G suit).
    How about increasing the acceleration over a period of, say, a year?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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  7. #6  
    kaz
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    Doh, that's true. I had 9.8 m/s^2 floating around in my head off mechanical maths but I didn't think to apply it here.

    hmm...makes things interesting, longer distances really will be reached in a much shorter relative time then just x times the short distance.



    Now I can get onto really working out time it'd take to reach stars...acceleration being the limiting factor isn't mentioned much.
    Ack this is well hard...must be a easier way.
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  8. #7  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by kaz
    Doh, that's true. I had 9.8 m/s^2 floating around in my head off mechanical maths but I didn't think to apply it here.

    hmm...makes things interesting, longer distances really will be reached in a much shorter relative time then just x times the short distance.



    Now I can get onto really working out time it'd take to reach stars...acceleration being the limiting factor isn't mentioned much.
    Ack this is well hard...must be a easier way.
    Here you go :

    T = 2 acosh(DA/2c² + 1) c/A

    D is the distance you want to travel.
    A is the acceleration as measured by the ship
    c is the speed of light in a vacuum.
    and
    T is the time in ship time to travel distance D, assuming that you accelerate for half the distance and turn around and decelerate for the second half in order to bring you to rest at your destination.
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    very interesting math I would love to see where it was derived from.

    Anyways. I think that another way to look at the whole acceleration problem would be every couple of hours you could accelerate at say 3Gs for like 2 minutes or something and this would get you to your terminal speed a bit faster. tell the passengers just like in an airliner they need to return to their seat or else lol.
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  10. #9  
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    Accelerating at 1G for just under 12 months would result in travelling at the speed of light, however this is using newtonian mechanics which would actually fail somewhere along way preventing such a speed from being obtained.


    t= v/a v= 300,000,000 (c) a = 9.8m/s giving t = 30,612,224 seconds
    1 year = 31536000 seconds.
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    Th ereal limiting factor in all this isn't really passenger comfort, but fuel.

    Here's how you find out how much fuel it would take to travel to a destination, assuming that you accelerate at 1 g for half the trip and decelerate for the other half, using the most efficient drive theoretically possible.

    Mf = e^( 2 acosh(0.52D+1)) -1

    D is measured in light years and Mf is the mass of the fuel in units of ship mass. For instance, if Mf = 4, then for every kg of ship mass you would need 4 kg of fuel (80% of the fueled ship's mass will be fuel).

    For a trip to Alpha Centauri, 4.3 ly away, it works out to 38, meaning that over 97% of your ship would have to be fuel.

    Now consider that some of that remaining +2% has to be the propulsion system. It is easy to see that at some point there isn't a large of enough percentage of mass remaining for the propulsion system to be powerful enough to accelerate the ship at 1g even if the ship is nothing but engine and fuel.
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    Well your obviously talking about one certain kind of fuel and propulsion. But didn't nasa develop an Ion propulsion system for one of their outer solar system exploratory satellites?

    What I am getting at is before we plan a trip we would obviously need to develop better propulsion systems. I don't think that jet fuel is the answer. I know that this is far fetched but maybe some kind of solar powered vessel would be the system that finally gets us to the stars. It carries no fuel.
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  13. #12  
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    Quote Originally Posted by GenerationE
    Well your obviously talking about one certain kind of fuel and propulsion. But didn't nasa develop an Ion propulsion system for one of their outer solar system exploratory satellites?

    What I am getting at is before we plan a trip we would obviously need to develop better propulsion systems. I don't think that jet fuel is the answer. I know that this is far fetched but maybe some kind of solar powered vessel would be the system that finally gets us to the stars. It carries no fuel.
    Yes, I'm talking about one certain kind of fuel and propulsion: The best kind even theoretically possible. You get the best fuel to final velocity rato with the highest exhaust velocity. The highest possible exhaust velocity is c, so that is what I used to calculate the fuel.

    For comparison, the best Ion propulsion drive would take more mass as fuel than than exists in the entire universe to make the same trip to Alpha Centauri.
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    Quote Originally Posted by Janus
    For a trip to Alpha Centauri, 4.3 ly away, it works out to 38, meaning that over 97% of your ship would have to be fuel.

    Now consider that some of that remaining +2% has to be the propulsion system.
    Perhaps more importantly, your remaining +2% has to include your fuel tanks. That usually takes up much more mass than the engines.

    But of course we're talking about a hypothetical engine that can magically turn the mass of your fuel into energy and thrust with perfect efficiency, so maybe worrying about the mass of the fuel tanks is silly.
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    Forum Bachelors Degree Shaderwolf's Avatar
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    Hey man, we'll just need to harvest a couple of tons of antimatter... and store it safely... and then use it effectively... crap! this is harder than I thought! :wink:
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    Quote Originally Posted by Shaderwolf
    Hey man, we'll just need to harvest a couple of tons of antimatter... and store it safely... and then use it effectively... crap! this is harder than I thought! :wink:
    Oh dear, oh dear oh dear oh dear....

    What next dilithium crystals? :-D
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  17. #16  
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    Nope, those aren't real :wink: (at least in this sence)

    I have a question about that though. They produce extremely small amounts of antimatter in labratories for testing each year. Have they figured out a way to contain them? It is a conundrum that I cannot solve. How could you realy safely store antimatter? It can't touch matter, and so the only possible container would be antimatter. That would blow to though. A strong magnetic field would do it... but It would have to be so powerful that I don't see what the use would be... Where am I mistaken?
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    Quote Originally Posted by Shaderwolf
    Nope, those aren't real :wink: (at least in this sence)

    I have a question about that though. They produce extremely small amounts of antimatter in labratories for testing each year. Have they figured out a way to contain them? It is a conundrum that I cannot solve. How could you realy safely store antimatter? It can't touch matter, and so the only possible container would be antimatter. That would blow to though. A strong magnetic field would do it... but It would have to be so powerful that I don't see what the use would be... Where am I mistaken?
    Like you said, they use magnetic fields. Of course, the magnetic trap always weighs much, much more than the antimatter you are storing.

    Why is everyone so hot to go to other star systems, anyway? There are loads of perfectly interesting/exploitable planets, moons, asteroids, and "minor planets" in this solar system. Even if you get bored with all the major planets and their many moons, there are at least 21 "dwarf planets" like 2005 FY9 or 2003 EL61 floating around with diameters over 700km! That gives them over twice as much surface area as the country of France; if you sent a few thousand colonists there it would be a loooong time before you had to worry about it getting crowded. More than enough room to grow your own civilization with many millions of people.

    Heck, there are even 16 asteroids floating around out there with diameters greater than 240km!

    And all this can be easily reached in just a few months or weeks using nuclear engines that we have on the drawing board today – there’s no need to speculate about hypothetical starships that are 97% fuel and are driven by magical engines that we don’t have any idea how to build.

    My point here is that there’s a ton of stuff out there in the solarsystem waiting to be colonized/mined/studied/whatever, and it’s all realatively easy to get to (or at least it will be whenever we decide to make a serious effort to do it). It seems kind of silly to spend all this time and effort trying to figure out how to get to other star systems when we haven’t even begun to utilize our own, and when there’s so much interesting stuff out there in our backyard.
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    Forum Bachelors Degree Shaderwolf's Avatar
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    I think for now, even Mars is a long way off (no matter how much the science channel supports the Idea) Thoughts like this are a good way to learn though. I learned allot about physics from Janus's informative explanation.
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  20. #19  
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    Quote Originally Posted by Shaderwolf
    I think for now, even Mars is a long way off (no matter how much the science channel supports the Idea) Thoughts like this are a good way to learn though. I learned allot about physics from Janus's informative explanation.
    It's a "long way off" because no one wants to spend the money to build the ships to take us there. We more or less kow how to build the ships; it's just a matter of actually doing it.
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  21. #20  
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    Quote Originally Posted by KALSTER
    I read somewhere that 4G would be the most someone could get used to since (someone on this forum said) your retina would become detatched at any greater acceleration.
    G-forces also occur (in the opposite direction) with rapid deceleration.
    The person known to have survived the greatest ever G-force was a formula one driver who momentarily experienced 190G as his formula one car went from 130mph to zero in less than half a metre.

    And as for acceleration, astronauts re-entering the earths atmosphere commonly experience anything up to 15G's
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  22. #21  
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    HI Leo, long time no chat!


    Acceleration and decceleration are the same force ie rate of change of motion.

    If you were in a closed container and were simply pressed against one side you coould not tell whether you were just subject to normal gravity, accelerating or deccelerating.

    Rember when you stand up, it is the NET force of gravity which pulls you (almost) vertically downwards, the amount of the earth directly underneath you is very very small, most of the erath's mass is below you and to the sides, front and rear, each part of the earth is 'pulling' you.
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  23. #22  
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    Quote Originally Posted by KALSTER
    I read somewhere that 4G would be the most someone could get used to since (someone on this forum said) your retina would become detatched at any greater acceleration.
    Studies have shown that 1.5g is about the limit for prolonged exposure. People have been subjected this for seven days with no ill effects. However, after only 24 hrs at 2g, significant fluid imbalances started to show themselve.
    The thing about g force effect on the human body is that it isn't just magnitude that is an issue, duration also counts.

    G forces you could endure for a fraction of a sec with no problem would kill you if maintained for a few seconds. Likewise, g forces you could endure for a few seconds would kill you after a minute.
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    There are tricks that can theoretically be used to allow people to withstand acceleration much better. If you submerge someone if a fluid with about the same density as your body, for example, you can withstand high accelerations much better because the fluid will apply pressure under acceleration and prevent your body fluids from pooling where they shouldn't. If the fluid is breathable (so that it can fill up your lungs), you can supposedly withstand over ten Gs for a long time; the only limit then is the fact that your body doesn't have a uniform density, so eventually things will try to rearrange themselves inside you to your detriment.
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  25. #24  
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    nice
    Solomon Grundy
    In 1944, this creature rose from the swamp, with tremendous strength and some dormant memories that for example allowed him to speak English, but not knowing what he was, and not remembering Cyrus Gold or his fate. Wandering throughout the swamp, he encountered two escaped criminals, killed them, and took their clothes. When they asked him his name, he simply muttered that he had been born on Monday. Reminded of an old nursery rhyme about a man born on Monday, the thugs named the creature "Solomon Grundy".
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