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Interstellar travel and Special Relativity

  1. Jan 11, 2016 #1
    I always ask myself this question, and I think it is better to ask people that really know about this.
    Due to the impossibilities that Special Relativity poses, such as any object with mass not being able to travel at the speed of light. If we don't manage to at least scratch that speed, I highly doubt any human is leaving our solar system. What do you guys think? What other types of travel can make Interstellar travel a reality?
  2. jcsd
  3. Jan 11, 2016 #2
    Near c travel is not impossible, it's just difficult. But even if we could travel at light speed, it would still take years to get to the nearest star.
  4. Jan 11, 2016 #3


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    You are a master of understatement.
  5. Jan 11, 2016 #4
    I mean to say it's not physically impossible. It might be technologically impossible though.
  6. Jan 11, 2016 #5


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    Science fiction provides two "easy" approaches -- suspended animation or a generation ship in which multiple generations are born en route. Then you just have the technical challenges of actually succeeding with either approach.

    There are also the more exotic approaches. You can accelerate at one gee [by some unexplained means] for about ~10 subjective years, decelerate at the same rate for another ~10 subjective years and wind up almost anywhere you please due to the benefits of time dilation. Or build yourself a wormhole using exotic matter.

    Or one could "beam me up, Scotty" and transmit the blueprints for a human being onto a specially constructed receiver. Arranging to deliver the receiver is a challenge but communication with the receiver is not. Simply beam yourself onto a CD-ROM and ship that along with the receiver.
  7. Jan 11, 2016 #6


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    To add to this, the primary difficulties are:

    - energy
    - interstellar space is dangerous.

    For energy, even for reaching only .9c, you would need to supply the energy of the largest H-bomb to each kilogram of ship, as directed energy. For .99c, you would need 7 of the largest H-bomb's worth of directed energy per kg of ship.

    For .9c, deep space would not be too dangerous, but time dilation effects would also not be great. If you want e.g. 100 fold time dilation, you find interstellar space increasingly dangerous. Molecules become dangerous, let alone dust. Faster still, and the CMB becomes dangerous, and also applies major drag, so coasting is not possible.
  8. Jan 11, 2016 #7


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    I've thought about this a lot, and while near light speed travel is not impossible, the energy requirements are so great that it seems technologically beyond reach. I think the only practical way to reach the stars is to acknowledge that the journey will be at speeds far below light speed, on the order of 0.01c. At these speeds the energy requirements are large but not insurmountable, and the dangers that PAllen alludes to are manageable. You just have to plan on a journey measured in the hundreds of years. I see two ways to make this palatable:

    (1) Large generation ships housing thousands of people in a comfortable environment. If we imagine future large space habitats that are comfortable places to live, one of these could be launched to the stars and arrive many generations later.

    (2) Robot-crewed ships carrying fertilized eggs that are brought to maturity centuries later after the ship arrives at its destination. Of course, this would require autonomous robots far beyond anything we could build today.
  9. Jan 11, 2016 #8
    I'm optimistic for a couple of reasons:

    1] The general motion and progress of science and technology has been on the side of figuring out things that can be done more than those that can't.

    2] Of all the various scientific things discovered, only a few particular things that are of interest and desired get developed into technological wonders (cars, planes, telecommunications, computing, etc.). If we want to do interstellar travel, it falls on the "want list" of technologies to scientifically develop.

    3] Throughout scientific history the consensus was that what was known was generally correct and the scope and span of what was known pretty well covered everything. This sense grew more firm with each subsequent generation despite an embarrassing amount of revision and overhaul of previous ideas... each present moment seems always accompanied by a little too much confidence. When one compares old and new science, the shocking advancement should not be just a comforting suggestion that we are "right", but an acknowledgement that as time moves forward, someday we will have very likely been found to have really been overconfident and "wrong" (or incomplete, or approximately correct for restricted or reduced conditions, or in need of revision, etc...).

    4] Life on Earth started after about .6Byears, then it took 3Byears to go from molecule to microscopic worm, but only 1Byears to go from worm to man... things are speeding up and we may be more amazing than we can imagine in the future.

    5] The Earth is a late comer to the universe; there are systems that have billions of years of head start over us... maybe they figured this out and will clue us in on how to do it.

    6] Patience! Only 100 years ago we had yet to discover the neutron and thought all the universe was the Milky Way Galaxy. I don't think we have exhausted discovering everything else yet.
  10. Jan 11, 2016 #9


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    Only 115 years ago we had yet to discover heavier-than-air flight.
    Been a good century...
  11. Jan 11, 2016 #10


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    Of course progress will surprise us, and things no one thought plausible will occur. That in no way means that everything considered implausible will be achieved. It is easy to forget the counter examples:

    - gravity has been understood in some form since antiquity, understood well enough for almost all purposes since Newton, and the idea of antigravity is ancient. Progress is nil.
    - controlled fusion has actually been considered plausible from the get go, yet always seems 50 years away, in practice.
    - Conservation of energy and the understanding that perpetual motion machines are 'impossible' is old, and there has been not the slightest shift in this.
    - friction has been an annoyance since antiquity and stories of frictionless drives or devices have figured in imagination for centuries. Progress is nil.

    Near lightspeed travel strikes me as most like controlled fusion, except much, much harder. So, I wouldn't be surprised if it is achieved in some form in a thousand years, if we are still around. However, there have been analyses to the effect that if very near light speed travel was achieved anywhere in our galaxy, it would leave very distinctive signatures, which have not been observed. So this implies it might be even harder than I think.
  12. Jan 11, 2016 #11


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    Skepticism about heavier than air flight was always silly and never a universal belief. All you need do is look at birds. The counter view that it was perfectly plausible goes back millenia and Da Vinci certainly thought it plausible, though none of his solutions would have quite worked (though some had valid concepts).
  13. Jan 11, 2016 #12
    However skepticism about interstellar travel is not silly, unless we really think that building generation(s) ships is a viable project.
    Even at much less than light speed, collisions with unforeseen dust clouds could be catastrophic.
  14. Jan 13, 2016 #13
    I see one more problem that should be considered in the discussion. Based on the most fundamental principles on physics (action-reaction), the only way we can conceive to accelerate from one place to another in empty space is to "throw" something in the opposite direction. Unfortunately in space there is no air, nor anything one can use to "swim into".

    This seems to me a much more profound problem, and somewhat different from the simple "technological challenge". Namely we can imagine that in the next 100 years (or 1000 or 10000) new forms of energy will be discovered or old ones will be under control (fusion for example), but still, even if you have a large source of energy, what are you going to "throw in the opposite direction" with that energy, in order to accelerate further? Of course it would be different if one could reach somehow the max velocity (say 0.01c or 0.1c or even 0.99c) in a short amount of time and then just go by inertia... still you are going to need a lot of whatever "fuel" in order to "manoeuvre" around, and I don't know how conceivable this is and what solutions can be thought to solve the problem...
  15. Jan 13, 2016 #14


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    That isn't true. Let's clean it up to make it physically possible and say "near light speed". Then interstellar travel doesn't have to take years, it can take an arbitrarily short amount of time due to length contraction - Months? weeks? days? Probably, we'd be limited to months due to acceleration effects on the human body.

    See, SR both tells us that we can't exceed light speed and tells us that that doesn't prevent interstellar travel...as long as you're comfortable with never seeing your friends back on earth again...
  16. Jan 13, 2016 #15


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    This can all be summed as:
    Any known propulsion technology would require a prohibitive amount of reaction mass to achieve relativistic velocities. We will have to wait for advances in propulsion technology in order to reach the stars.
  17. Jan 13, 2016 #16


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    Rather than suggest that relativistic travel is "impossible" to a hypothetical and unknown "sufficiently advanced technology", I will suggest that it is probably easier just to extend the human lifespan so we are not in so much of a hurry, and use lower velocities. There's also a lot of obvious side benefits to this idea of a longer lifespan- well, if the side effects of greatly prolonged life don't result in overpopulation and the collapse of human civilization, that is.

    Not too many people seem to want to think of the idea that way, I suppose the idea of prolonging human life seems more exotic - or perhaps just more understandable - than the mastery of energy required to implement relativistic travel.
  18. Jan 13, 2016 #17


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    Provided said humans don't go stir crazy and kill everyone on board during the decades or centuries of being cooped up in a spaceship... :nb)
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