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Thread: Sci-Fi Musings

  1. - Top - End - #31
    Ettin in the Playground
    Yakk's Avatar

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    Nov 2006

    Default Re: Sci-Fi Musings

    Quote Originally Posted by brian 333 View Post
    You don't need to get anywhere near .99 lightspeed for relativistic effects to kick in. However, if you could, elapsed time for a trip would be virtually instantaneous from the ship's PoV, and 1 year per light year from the PoV of the launch point.

    Note the difference in ship PoV elapsed time using different thrust. That's because the ship is achieving a higher percentage of light speed. The trip duration from the launch point PoV is only marginally different. Basically, 1 year per light year plus time to get up to fractional light speed.
    There is a mathematical shortcut to doing all of the relativistic math. It happens that if you have a 500 ly journey and you want to travel it in 12 years ship-time using a uniform acceleration ship you can solve for the thrust required while ignoring relativity.

    The result matches up with the relativistic result. You don't actually hit the super-luminal speeds, but the thrust time and ship-time match up. Time dilation and space contraction from relativity cancel out.

    So to go 500 ly, you just work out the uniform accelleration required to go 250 ly in 6 years. This is easy to work out - d = 1/2 at^2, 250 ly = 1/2 a (6 years)^2, 500 ly / (6 years)^2 = a, a =~ 132 m / s^2, or about 13 gravities.

    132 m/s^2 times 6 years gives a "no relativity" speed of 83 times the speed of light. That doesn't actually happen, of course, because relativity kicks in. Time dilation factor is 1/sqrt(1-v^2/c^2)

    50% speed of light: 15% time dilation (x1.15)
    75% speed of light: 50% time dilation (x1.5)
    90% speed of light: 130% time dilation (x2.3)
    95% speed of light: 3.2x time dilation
    99% speed of light: 7.1x time dilation
    99.9% speed of light: 22.4x time dilation

    with distance being contracted (by the travelers) by an identical factor.

    "Near instant" travel requires speeds very close to c, which are both energy intensive and basically impossible for macroscopic objects in the current epoch of the universe (the interstellar medium is too thick).

    The energy budget of a perfectly efficient drive happens to match the time dilation factor - if you want to hit 50% of the speed of light, your perfectly efficient energy budget is 15% of your rest mass converted to energy. And no drive system is perfectly efficient (as the exhaust is going to contain energy); the SF 'perfect' drive is one that grabs the entire universe and pushes it backwards (in the limit as the mass of your exhaust reaches infinity, the efficiency of your drive approaches perfection).

    In short: you aren't going to have short interstellar trips without converting the mass of planetoids into energy, even from the perspective of the people on the ship.

    But hitting 0.005c is plausible in a hard-SF universe. At 0.005c you could have an ship the mass of an asteroid decelerate and deliver a payload the size of a pop can, and accelerate using laser sails that use a fraction of the launching star's output during launch. That pop can would then bootstrap production in the target system as a self-replicating probe. When the target system industrializes it can then build a catcher's mitt (a laser braking system) and you can get far, far larger payloads (like, the size of a car) loaded with data, which you can then print out in the destination system.

    (If it takes 10,000 years for a system to industrialize and be able to launch 1 probe every 1000 years for an average of 10 cycles, the probes have a 25% success rate, they travel at 0.005c, that means number of probes double every 15k years or so. There are 100,000,000,000 stars in the milky way, so that is 37 doublings or 555,000 years to have as many probes as stars. The milky way is 100,000 ly in diameter, so crossing it takes 20 million years at 0.005c; so colonizing the entire milky way this way requires on the order of 20 million years, as after a small fraction of that every local star will be industrialized, and a wave of probes traveling at 0.005c will build up heading to new uncolonized stars with multiple redundancy. The milky way is 13 billion years old, so this could complete in 0.15% of the milky way's age (an eyeblink).)

    This also means that if there is more than 1 intelligent species reaching the stars at the same time in our galaxy it has to be because of an external effect; a single species could colonize the entire galaxy with a modest initial investment in a period much shorter than the galaxy has existed. A simple solution to this is that it indeed happened, and humanity (and other intelligent species) are actually the colonists; the alien species sent probes that engaged in uplift, turning life into intelligent technological life.

    Humans start being unique in the geological record on the order of 10,000 to 1 million years ago with our tool use. So you could imagine a galactic 'seeding' event, where systems with life where uplifted using probes, and other systems where used to launch more probes, before the galactic colonization system skuttled itself.

    It still doesn't explain how intelligent life interacts much over interstellar distances, or gives you anything like space battleships.

  2. - Top - End - #32
    Firbolg in the Playground
    Join Date
    Dec 2010

    Default Re: Sci-Fi Musings

    Really the answer to interstellar travel in hard scifi is to just get everyone in the civilization to slow their brains (and the rest of their biology) down by a factor of 1000 or more. Now a trip that would take 1000 years and be enough time for nations to rise and fall is just one perceptual year.

    Granted, this is going to cause some problems for diplomats, sports fans, ...

    I guess you'd have to live in very cold, low gravity (or buoyant) environments where very slow motions would still be sufficient to move around and interact and do things.

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