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The Limitations of Intergalactic Travel

  1. Aug 16, 2003 #1
    It seems to me that the limitation of human space travel isn't time but energy. Applying Einstein's theory of relativity, we see that the time dilation effect would allow humans to travel to virtually anywhere in the universe within their own lifetime.

    t' = t * gamma,

    where, gamma = [1 - (v/c)2)]-1/2.

    The problem of course is finding the limit of how much energy is needed to transport a human at high enough speeds for the length of the journey to become reasonable. I suppose that if there were a means of efficiently converted mass into energy then the limit is simply:

    E = mfuel * c2.

    The energy needed to move the ship transporting the human would of course be:

    KE = mship * c2 * [gamma - 1].

    So, E > KE tells us how much fuel would have to be consumed.

  2. jcsd
  3. Aug 17, 2003 #2


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    But I think there won't be much of an universe there when they arrive. The time they experience themselves will to t, not t'.
    t' refers to the amount of time the observer (with relative velocity v) would see the ship experience with each second of their own time - in this case, the ship would be going slower than the speed experienced by the crew.

    And c is still a speed limit relative to whatever destination they are looking for.
  4. Aug 17, 2003 #3
    Sure there would be plenty of universe left to see. Remeber that although the traveller's time goes to zero, the stationary observers time is still only distance divided by the speed of light in a vacuum.

    For example, in the extreme limit that a spaceship travels to Alpha Centari at the speed of light (which of course would consume an infinite amount of energy), the trip would be instaneous for the traveller but about 4.3 years for the stationary observer.

    If you had a spaceship that could travel at speeds nearing the speed of light for extended periods of stationary observer time, I think the best strategy would be to look for baby solar systems and hope that by the time you get there some type of life will have evolved. Of course, the down-side to this is that you would never have the chance to see your friends or family again, as all of humanity as you knew it would be deceased before you even could think about it.

    Last edited by a moderator: Aug 17, 2003
  5. Aug 17, 2003 #4
    No, c is constant in all reference frames.

  6. Aug 17, 2003 #5


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    On topic, but slightly off-topic, I don't think that near-c velocities will EVER be reached by any type of machine ever to be made by humans. Too much development time and way too high an energy requirement.

    But, to keep the conversation going, I do think that the "effect" of c or c+ travel will be accomplished if we survive a few hundred thousand years or so. In December, 1903 the Wright brothers made their flight. Less than 66 years after that we landed on the moon. The rate of technological advancement was definitely exponential in the 20th century. Imagine going back in time to MIT in 1970 with a battery powered laptop computer. It would have easily sold for several million dollars.

    If and when c+ travel is accomplished, I am convinced that it will be by some method not yet conceived, not even grazing black holes or through wormholes. The movie DUNE may not be too far off as to c+ travel. It will be by space-time "warping", folding, teleportation or some other odd method instead of building a neat ship and cranking up the power.

    Any other ideas????
  7. Aug 17, 2003 #6


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    Actuallly, the last formula you gave is just basically a modification of the formula from which E= mc² was originally derived:

    E = mc²/(1-v²/c²)

    Thus mc²(1/(1-v²/c²) -1) gives the value of the kinectic energy of an object moving at v.

    The rub is, that in order to actually accelerate your ship through space you have to make use of a action-reaction engine.

    In which case, you need to use the relativistic rocket equation

    v = c *tanh(Ve/c * ln(MR))

    In this case, Ve is the exhaust velocity and MR is the mass ratio (mass of the fueled ship/ mass of unfueled ship)

    For a pure matter to energy conversion ship this means that we convert the fuel to photons, which we direct backwards to provide forward momentum.

    To determine how much fuel we need to attain any given velocity, we re-arrange the formula to read

    MR = etanh-1(v/c) * c/Ve

    If Ve = c and we measure v is units of c we can reduce this to:

    MR = etanh-1v

    To reach .6c you would need a mass ratio of 2 (1 gram of fuel for every gram of payload.)

    for .9c, a mass ratio of 4.259
    .99c ---------------------- 14.1
    .999c--------------------- 44.7
    .9999c------------------- 141.4


    And that's assuming 100% efficiency; every photon produced in the reaction captured and directed straight backward.
  8. Aug 18, 2003 #7


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    Another point:
    The mass ratios given in my last post only concern achieving the given velocity in the first place. You will also need to decelerate once you get to your destination.

    That mean's in order to come to a stop from .999c you need 140.1 g of fuel for every gram of ship and cargo.

    This compounds the problem, because this fuel is part of the payload you have to accelerate up to .999c in the first place. This means it actually takes 19628 g of fuel for every g of payload you actually want to deliver to the end point of the trip, if you are not just planning on doing a fly-by at .999c.

    For actual intergalactic travel, consider the following example:

    Andromeda is the nearest galaxy at 2,000,000 ly. Let's assume a 3 yr trip. (2yrs accelerating and decelerating and one year coasting.)

    This means you would need to attain a velocity of 0.9999999999998749999999999921875c
    for the coasting period.

    To attain this Delta v you would need a mass ratio of 4000000. This is the mass ratio you would need to decelerate at the end.

    Thus you would need 1.6*1013 g of fuel per gram of payload to complete the trip, or just about the mass of Deimos for every 100 kg of payload ( including the empty mass of the ship itself).
  9. Aug 18, 2003 #8
    if mf = mass of the fuel and ms = the mass of the ship, arent you forgetting that the fuel needs to be accelerated? seems like you need a dms/dt in there somewhere, but I ran through it anyway:
    mfc2 > msc2(γ-1)
    say you were to accelerate mass ms which must include mf at 9.8 m/s2 for a year (3.1563E7 sec)
    Alexander's equation for finding relativistic velocity under constant acceleration was:
    v = c tanh (at/c)
    tanh a combination of exponentials of (at/c)
    I found v=2.322E8 m/s or 60% c after 1 year of acceleration, γ = 1.5793
    so the mass of the fuel has to be at least 58% the mass of the fuel + ship by
    mf = ms(γ -1) if all the mass of the fuel is converted into energy.
    and it gets worse from there (infinitely) as you approach c.
  10. Aug 18, 2003 #9
    No, I didn't forget that.

    mship = mfuel + munfueled ship

    I was just presenting some very general equations, and I didn't feel like typing out all the details.

    Last edited by a moderator: Aug 18, 2003
  11. Aug 18, 2003 #10
    Okay, that's the number I was looking for.

  12. Aug 18, 2003 #11
    Does anyone know to what speed a ship could be accelerated if one were to slingshot the schwarzchild radius of a black hole?

  13. Aug 18, 2003 #12


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    For a body traveling with respect to, say your Black Hole, there are 2 possible non capture orbits. You are either parabolic or hyperbolic, in either of these the velocity of approach = velocity of exit. You gain NO velocity simply by passing near something.

    Ok, what is the slingshot that we hear about near Jupiter. The velocity a satilite picks up as it passes near Jupiter is Jupiters ORBITAL velocity. This is the slingshot, not the mere fact that you pass nearby. Thus, the exit velocity of something passing near a BH would depend on the velocity of the BH, with respect to what?
  14. Jan 28, 2009 #13
    "If and when c+ travel is accomplished, I am convinced that it will be by some method not yet conceived. It will be by space-time "warping", folding, teleportation or some other odd method instead of building a neat ship and cranking up the power"

    I tend to agree with you. I am also thinking that while superluminal is one prerequisite for inter planetary or inter galactic travel, perhaps the nature of our bodies and spacecrafts needs modification. While most physicists will probably laugh at what I am about to suggest, I think we need to convert our physical forms into energy or some type of zero mass substance before long range interstellar travel can take place. That will not only take out the kinetic energy problem as we approach c, but also drastically increases our life span to perhaps a few million years.

    I have come across the "Negative Energy" phenomenon postulated by Paul Dirac but didn't really understand it. I wonder how a body of negative energy will behave at c or near c. Perhaps the physicists among us can shed some light on this matter?
  15. Jan 28, 2009 #14
    I think the only way that c will be exceeded is by sidestepping it. Ie bending space, so the distances become closer, hyperspace sounds a little out there but can we bend space so much it "breaks"? if we could get a light year down to a thousandth or a millionth of a light year by bending the space in between, then maybe we'll effectively travel distances faster than would be possible at c or greater. Mind you what do I know, we may in 1 million years just beam ourselves there. Who really knows...
  16. May 12, 2011 #15
    well ive been toying with some ideas Quartz when put under pressure generates electricity so all you need to do is have a big chamber with lots of quartz stalagtights placed really close together with just enough space for air to be around them. then you fill this chamber with compressed air. and use the suns gravity (magnetic field) calculate the polarization and duplicate it and broadcast it tword the sun like polls push with no friction and no interfieance once the magnet is shut down it should continue its rate of travel until stopped using another targeted sun. the only thing i struggle with is how to navigate or how to survive the acceleration
  17. May 12, 2011 #16
    The limit on human space travel definitely is not time. As you said, if you managed to achieve the relativistic speeds necessary to reach distant stars and galaxies, your time frame would be slow enough to make it possible. Most likely, the journey would be a one-way trip due to the fact that the human race may not exist by the time you got to your destination, not to mention by the time you returned.

    The energy considerations discussed here seem to focus only on fuel requirements from rocket-engine technologies. There are theoretical engines that utilize solar energy to achieve sub-luminal speeds, and require pretty much no stored fuel at all. A ship would have to orbit the sun closely and build up speed for many years to achieve its target goal, but at least it can be done (in theory). And as for slowing a ship down, you could certainly attempt the same method of deceleration on the other end (you'd have to be sure you were nearing another star that was capable of providing the needed energy). Perhaps even a giant parachute could be used, trapping interstellar and star dust to slow the ship down. Again, this is probably theoretical at best, but it doesn't require a moon's worth of fuel on the ship.

    To me, the hazards of interstellar space provide another huge hurdle to overcome as well. Imagine running into space debris at 99.9999% the speed of light? The space debris would shred your ship to pieces. Even a molecular gas cloud could theoretically create immense amounts of friction on the ship and tear it apart. Obviously, a huge energy shield of some sort needs to be created to avoid head-on collisions with space debris at near-light speed. And of course, such debris makes a giant parachute a difficult task as well (I assume it would be designed to be destroyed anyway, but still).

    If humans could ever manipulate space itself, near-light-speed travel would be much safer. The ship could move slowly, but the space folded in front of it could make it travel many light years without moving through space faster than 15 MPH. Space debris would not pose nearly the same problem in this situation.

    We can dream, right?
  18. May 12, 2011 #17
    not solar power or orbiting the sun read what i wrote its a quartz generator powering an electromagnet the sun is a giant magnet what happens when you put two north magnets together
  19. May 12, 2011 #18
    and a parachute would be pointless at those speeds one it will be torn apart two what is it going to drag against in a vaccum
  20. May 12, 2011 #19


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    kkassinger and mjacobsca this thread is years old. It's against the forums rules to necropost. If you want to talk about this topic I would advise starting a new thread.
  21. May 14, 2011 #20
    Our fastest manmade craft is New Horizons on its was to Pluto. It is travelling at about 47,000 mph or 412,002,000 miles/yr. One of the closest stars is Alpha Centauri at 2.566E13 miles away. It would take New Horizons 62300 yrs to reach it at this rate. Maybe with larger rocket engines or a constantly accelerating Ion engine could faster speeds be reached but deacceleration must be perfect. Finally there is the problem of interstellar gas and debris that could destroy a spacecraft travelling at thousands of mph.
    Last edited: May 14, 2011
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