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99% of Light speed

  1. Aug 17, 2011 #1
    Is it possible to build a vehicle that can accelerate to .99 C?

    How would we go about doing something like this?

    If at all possible, how many centuries or millennia is it before this can be accomplished?

    Could a Bussard scramjet do it?

    How large would a laser/maser/gaser have to be to get the craft up to such speed?

    How long would it take from the crew's perspective to cross the galaxy?

    How big would the craft be? I imagine a lot of fuel would be required even if you could gather propellant along the way - Bussard scramjet. What kind of fuel would be needed?

    Would such a vessel help establish a galactic civilization? Or would time dilation make implausible?

    What would it look like?

    Are any sort of warp drives completely impossible? If so, which of the following would best for interstellar travel: beamed propulsion, matter/antimatter reaction, Bussard scramjet, or something else?
     
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  3. Aug 17, 2011 #2

    Ryan_m_b

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    Theoretically yes.
    No time soon, it would require fantastic advances in technology that don't look to be anywhere on the drawing board.
    Same answer as above.
    I take it you mean "ramjet" and no, http://en.wikipedia.org/wiki/Bussard_ramjet#Discussions_of_feasibility".
    http://en.wikipedia.org/wiki/Beam-powered_propulsion" [Broken].
    At .99c time dilation is 0.07 minutes for every minute for an observer at rest. To cross from Earth to the far side of the galaxy (~70kly) would take 4962.168 years from the perspective of the crew. From one side of the galaxy to the other (~100kly) would take 7088.821 years.
    That question can't be answered because it depends on technologies that haven't been invented yet e.g. how much mass do you need to hold the necessary stable ecology, industry and society. As linked above ramjets would not work.
    In addition to such a vessel you would also need to be able to terraform otherwise you're not going to have a civilisation. Whether or not it could work as a civilisation is anyone's guess, see the http://en.wikipedia.org/wiki/Fermi_paradox" [Broken] for further discussion.
    The ship or civilisation? Either question is as unanswerable as a Neanderthal trying to envision Facebook.
    On the subject of warp drives http://arxiv.org/PS_cache/gr-qc/pdf/9905/9905084v5.pdf" [Broken]. The "trick" is to change the warp bubble so that it's exterior radius is microscopic yet the interior radius is large enough to accommodate your vehicle (essentially making a warp bubble that's bigger on the inside than on the out). Apparently this would greatly shrink the amount of energy needed to manageable levels. They don't outline how exactly a shell could be build around a ship in such a fashion nor how the ship could leave.

    However neither of these approaches fixes the other problems of a warp bubble such as requiring the construction of an exotic matter shell, superluminal signalling to steer/control the bubble and the huge amount of radiation a warp drive subjects you to. There are some interesting (but technical) objections in http://arxiv.org/PS_cache/gr-qc/pdf/0406/0406083v2.pdf" [Broken] that apparently show that a warp drive would only be capable of very low velocities as well as highlighting other problems.

    In summary unless you have a way of making speculative impossible exotic matter, it's not going to happen (and might not even if you could).
    None of them. For manned interstellar travel beamed propulsion wont be strong enough, M/Am rockets are far too dangerous (your "vessel" is a weapon so powerful that it could easily annihilate the entire surface of Earth countless times over) and the Bussard ramjet won't work.
     
    Last edited by a moderator: May 5, 2017
  4. Aug 17, 2011 #3
    wow ok alot of questions about 1 thing. First I think its theoretically possible I guess. If we were to have a ship that ran on our current fuel and managed to build it, it would probably take a thousand years to even get to 99%C and also an almost infinite amount of energy. Space is huge. It would take many generations of people to travel even to the closest stars. If we ever discover a wormhole (or if they even exist), that would be a much better bet. ^ I agree with the warp bubble and have heard stuff about it, but who knows exactly how much faster that would be.
     
  5. Aug 18, 2011 #4

    Ryan_m_b

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    It's theoretically possible but that doesn't mean much. It's theoretically possible to rearrange every star in the galaxy, doesn't mean we'll ever have the knowledge, resources and will to do it.
    If you want people to survive you also have to be able to build:
    • A stable ecology
    • An industry capable of building and recycling nearly everything
    • A social, legal, political and economical model that allows the society on the ship to survive harmoniously for thousands of years
    All of these are extremely http://en.wikipedia.org/wiki/Nontrivial" [Broken].
    Wormholes suffer from the same problem as warp drives, they require probably impossible http://en.wikipedia.org/wiki/Exotic_matter" [Broken].
     
    Last edited by a moderator: May 5, 2017
  6. Aug 18, 2011 #5
    Please don't forget that, as you get close to the speed of light, tiny specks of the interstellar medium will impact your ship with the kinetic energy of 20 megaton bombs.
     
  7. Aug 19, 2011 #6
    Doesn't this violate relativity? It seems to imply that certain speeds are the universal norm, and others are inherently fast. I thought the universe was supposed to look the same from any vantage point.
     
  8. Aug 19, 2011 #7

    Ryan_m_b

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    It is. From the perspective of the guy on the ship the speck is travelling at 0.99c, the speck sees the ship doing the same. The energy is the result of the kinetic energy that would be the same regardless of who was travelling at that speed.

    However only one of them is experiencing time dilation because one of them has been in an accelerating reference frame and the other in an inertial.
     
  9. Aug 19, 2011 #8
    If you started out from Earth using an ion drive that slowly increased acceleration given enough time it would reach .99c.

    You have to also remember time dilation and length contraction for the ship would mean by the time say it arrived at Proxima Centauri it would of aged differently and moved through a shorter space than it would appear to you as an observer who is relatively stationary on Earth.
     
  10. Aug 19, 2011 #9

    Ryan_m_b

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    It wouldn't matter if it increased its acceleration, it just needs to have a constant acceleration but there are problems with this:

    A) Supplying it with enough fuel to get to 0.99c - taking a http://en.wikipedia.org/wiki/VASIMR" [Broken] as the best speculative ion drive with a specific impulse of 30,000 seconds you would need ~1000 parts kg fuel for every 1kg of ship

    B) Having enough thrust to reach 0.99c in a reasonable time - taking VASIMR again if we assume a 1 tonne ship with 1000 tonnes of fuel then with a thrust of http://en.wikipedia.org/wiki/Ion_thruster#Comparisons" the ship will be capable of an average acceleration of 1e-6 and would reach 0.99c in roughly a million years.

    C) All the other engineering issues of energy, waste heat etc

    What do you mean move through a shorter space? From all reference frames the ship would have travelled the same distance.
     
    Last edited by a moderator: May 5, 2017
  11. Aug 19, 2011 #10

    PAllen

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    This isn't quite true. Each would perceive time for the other running slow, while the ship is in motion. What is true is that when the ship is co-moving again with typical stars again, it will have aged less, and the rocket and a nearby planetary observer agree on this. A good terminology for this is time dilation (relative) versus differential aging (invariant).
     
  12. Aug 19, 2011 #11

    Ryan_m_b

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    Yeah I get this, what was wrong with my explanation? (Not criticising, genuinely interested).
     
  13. Aug 19, 2011 #12

    PAllen

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    You said: "However only one of them is experiencing time dilation ". That is misleading.

    Also, it is, in fact true, that:

    - while moving, the ship perceives the distance to a star as much smaller than appeared before they were up to speed (length contraction; which is why they still measure light from the star as moving at c: shorter distance, smaller elapsed time, compared to earth bound measurement of lightspeed from the star).

    - If you introduce a concept of odometer, there is a perfect symmetry between length and time. Time dilation is relative and once to equivalent clocks are comoving, their rates are the same, but their accumulated time reflects their path through spacetime. Similarly, length contraction is relative, but an odometer measuring integral of apparent distance traveled will be remain different and dependent on path through spacetime. The rocket will perceive it never traveled faster than c, and that it traveled 10 light years in one year because its odometer says it really only traveled e.g. .9 light years.
     
    Last edited: Aug 19, 2011
  14. Aug 19, 2011 #13

    Ryan_m_b

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    I see, of course. I should have explained my point more. Thanks.
     
  15. Aug 19, 2011 #14
    I wasn't talking about the time dilation, but the "20 megaton bombs." At one speed you get hit by them, and at another you don't. This seems to me to point to a "proper speed of the universe"
     
  16. Aug 19, 2011 #15

    Ryan_m_b

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    I don't really see why. The energy has nothing to do with time dilation or relativity, it's simply kinetic energy.
     
  17. Aug 19, 2011 #16

    PAllen

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    No, ryan_m_b is completely correct. Speck says rocket moving very fast, hits hard; rocket says speck moving very fast, hits hard.

    There is, of course, a frame for any region of space such that the total momentum of matter is zero, and one may describe speed in that frame as speed relative to average matter of the region. However, that is completely irrelevant to the principle of relativity.
     
  18. Aug 19, 2011 #17
    So there are no galaxies headed for earth at 99% the speed of light? And no particles? It would only be space ships?
     
  19. Aug 19, 2011 #18

    PAllen

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    Most likely, it will be nothing ever.

    [Edit: missed the reference to particles. Particles hit earth every day with .9999999c. They are called cosmic rays. Occasionally, they are so energetic that one proton carries as much KE as a thrown baseball.

    If a 1 gram object had the speed of the fastest cosmic rays, it would hit earth with an energy of about 1 hundred million gigatons of TNT
    ]
     
    Last edited: Aug 19, 2011
  20. Sep 10, 2011 #19

    For Q1 and Q2 below, imagine two objects traveling on a perfect head-to-head collision course.

    Q1. Is it possible to calculate the energy released if two 1,0 gram objects, each traveling with the speed of the fastest cosmic rays, would hit each other?

    Q2. What would happend if two 1 gram objects, one traveling with the speed of .9999999c and the other at 1c, would colide with each other? Is it possible to calculate the released energy in this impact?

    Thanks :)
     
    Last edited: Sep 10, 2011
  21. Sep 10, 2011 #20

    Ryan_m_b

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    I'm pretty sure (but not absolutely sure) you could just calculate the energy by each and add it up. So;

    1 gram object travelling at 0.9999999c (299792428m/sec) would release 44,937,749,943,067.6 joules of energy or ~45 petajoules. So the collision of two grams would be ~90 petajoules
     
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