Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why can't we go to the center of the galaxy?

  1. May 20, 2004 #1
    why aren't we spending more money on developing nuclear-powered rockets?

    an interesting article about nuclear rocket propulsion

    "...Nuclear-reactor rockets, like the ones that would be used in the Bimodal Nuclear Thermal Rocket, conduct nuclear fission reactions -- the same kind employed at nuclear power plants -- in which uranium atoms are split apart, releasing tremendous volumes of energy. In a nuclear thermal rocket, this energy is used to heat hydrogen propellant, which is stored aboard the rocket as liquid in supercooled fuel tanks..."

    they're talking about creating fuel to travel to mars but wouldn't it be merely an extrapolation to design an even more powerful rocket capable of traveling swiftly to the center of the galaxy? time dilation would be an inevitable hazard but i don't think that should prevent us from traveling a long way in space...maybe our space ships could even search around for nice planets to colonize...or look for extra-terrestrial civilizations in the fertile center of the galaxy...

    i don't get why were not working harder at this...
     
  2. jcsd
  3. May 20, 2004 #2
    The center of the galaxy is very, very far away. Even if we could create a sattelite that travelled at the speed of light, it would take thousands of years to get there.
     
  4. May 20, 2004 #3
    the "speed of light" relative to what? to earth? relative to earth, yes it would take a very long time but remember time dilation and the principle of relativity...as far as the crew of the space ship is concerned there is no speed limit...

    the time it would take to get to the center of the galaxy is only limited by the gradient of acceleration...the gradient of acceleration is only limited by the thrust of the rockets and the mass of the ship, if we can make nuclear thrust technology very efficient i think the journey is a perfectly plausible one...
     
    Last edited: May 20, 2004
  5. May 20, 2004 #4

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To travel to the center of the Galaxy at 1g of acceleration (accelerate for half the distance and decelerate for the remainder) would take 12 yrs by ship time.
    (and more than 33000 yrs by Earth time.)

    Even if you had a ship that used matter-antimatter conversion to produce pure photons directed backwards at 100% efficiency for your reaction mass, (the absolute limit of efficiency for propulsion) You would have to start off with over 8 billion kg of matter and antimatter for every kg you want to deliver to the center of the galaxy.

    Consider that the command/service modules of the Apollo mission massed 28,000 kg and only had to support 3 men for about 8 days, and you can imagine how large a ship you would need to support a crew for 12 years (Even with recycling of resources).

    Multiply that by 8 billion and you have a rough estimate of how much matter-antimatter you'll need.

    Now consider the fact that 100% efficiency is not attainable. If you drop it down to 90% then you need 101 billion kg per kg payload.

    Also consider that half of that has to be antimatter, and that antimatter doesn't exist naturally in any significant amount. It would have to be made, and would take more energy to make it than you would get out of it.

    The cost in natural resources would be astronomical, and this is for the most efficient propulsion system possible.

    Remember, it is the stay at homes that have to expend these resources, and they couldn't even know if the mission succeeded or not for over 66,000 yrs, let alone derive any benefit from it.
     
  6. May 21, 2004 #5
    What distance to the centre of the Galaxy did you use to calculate this time? Most estimates range around 8 kpc. If you input that value into the equations for a rocket accelerating at 1g for half the distance (and then decelerating at 1g for the remainder) then you should get approximately 19.25 years, at least according to my calculations. I might be wrong, though.
     
  7. May 21, 2004 #6

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    at near light-speed it would take 50,000 years to reach the center of the galaxy, by local time. the crew may not age much, but, they would be waiting a very long time to hear back from mission control once there.
     
  8. May 21, 2004 #7
    The speed of light is a constant relative to anything. That's one of the postulates of special relativity.

    Yes there is; the speed of light limits the speed at which they can travel.
     
  9. May 21, 2004 #8
    If we went to the center of the universe, or even far away, it would not benefit anyone. No one would be able to follow, no message could be sent or recieved. It would be a waste of time and money.
     
  10. May 21, 2004 #9

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If you consider the numbers given by Janus you will see that there is insufficient mass in the solar system to power this journey. Such a trip is pretty much impossible according to the current state of Physics.
     
  11. May 21, 2004 #10
    Janus's answer to every question is the same. Not enough fuel. Then we need to find a different way to power ships. The question was "why don't we" not "can we". We can't do any of the things in most of these posts, but it is fun to think about.
     
  12. May 21, 2004 #11

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Some of us are constrained by the physical universe we live in. Others are not, thus we have science fiction.
     
  13. May 21, 2004 #12
    12 years to travel to the centre which is thousands of light years away? are you sure.. if so....
    that means as far as the travelers are concerned, they are moving towards the centre faster than light. It's a funny bit of SR. first you get your head round the idea that nothing can travel faster than light so you'd think it'd take thousands of years even from the travels point of view, then you realize space contraction effect cancels it out so things can appear to travel faster than light, in this case, the centre.

    Does this case work for any old particle flying towards the earth?
     
  14. May 21, 2004 #13
    There is most likely not a center of the universe
     
  15. May 21, 2004 #14
    couple things...

    janus - i'm not following your calculations - are you taking relativistic mass dilation into account?


    Lonewolf -

    forgive me but this is baloney, your two statements are contradictory...

    in a sense, the speed of light does indeed limit your speed, if you measure you're speed against the speed of the light travelling past you, your speed will always be 0....but this has nothing to do with your speed relative to the earth or relative to the centre of the galaxy...if there was something limiting your speed then that would mean there is an absolute reference frame against which to measure your absolute motion - completely in violation of the principle of relativity!

    or think of it like this - if your mass increased in proportion to your speed relative to an observer on the earth, then the earth would be the center of all absolute motion in the universe! the mass increase is a relative effect, it applies to the observers on the earth, not to the spaceship...

    kokain - as far as this hypothetical trip not being of any benefit i beg to disagree...the people on the spaceship would certainly derive some benefit...whether or not we earthlings would be willing to undertake such a massive effort knowing we could not guage its success is an interesting sociological question...if we could find a way to send a dozen spaceships to the center of the galaxy would we not feel some satisfaction that we have quite possibly furthered the seed of the human race? it's interesting to think about...


    meemoe_uk -

    exactly, many people misunderstand the limits SR applies to space travel...as long as the travelers are measuring distance from the earth and time from the spaceship, there is no limit to how fast they can travel...of course now we are using measurements from two different frames so a purist can say - well, you're not really going faster than light at all...in fact, if you then measure your speed as a fraction of local light speed, you will find that you're not actually in motion at all...

    the old 'mass increases as you approach the speed of light' routine only works when you are observing an object in a different inertial reference frame, so it applies when people on earth are watching our very fast ship as well as when scientists are watching very fast particles in accelerators...
     
  16. May 21, 2004 #15

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Your right, I went back a re-checked my time calculation, and I get an answer in the same range as yours , I used a slighty larger number for distance, which gave me an answer of 20 yrs.) The MR calculation still checks out okay.
     
  17. May 21, 2004 #16

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm using the Relativistic Rocket Equations as found at:

    http://www.ksc.nasa.gov/facts/faq04.html

    Which take time and length dilation.

    These actually help you.

    If you were to do the same calculation using the non-relativisitic equations for distance, time and acceleration, and plug the same exhaust velocity(c) into the classic rocket equation, you find that the same trip at 1g would take 242 yrs and use more more fuel/reaction mass than exists in the entire visible universe. ( something in the order of 10178 kg of reaction mass for every kg of payload.
     
  18. May 21, 2004 #17
    janus - absolutely, time and length effects are critical...i was just checking because there was some confusion on the SR, GR forum about whether or not to apply mass dilation to the inertial reference frame of the ship...

    the numbers are very interesting...a 20 year journey could take you to the center of our galaxy, 1g acceleration/deceleration, but you'd need something like 8 x 10^9 kg of fuel-mass even for 100% energy conversion...

    if we are talking about a journey of that duration/speed we cannot obviously store enough mass in a rocket...but perhaps we could convert spacebound energy into fuel, stellar radiation, cosmic rays, vacuum energy...vacuum energy seems a bit far-fetched...cosmic rays have a lot of kinetic energy but probably too sparse and too difficult to harness, stellar radiation would be a boundless energy supply if we could figure out how to use it efficiently...cold fusion of waste supplies would be handy...

    even with all of this, perhaps a 20 year journey is indeed unrealistic...perhaps we should be thinking about generational ships...or, firstly, how about closer destinations - alpha centauri would be nice, i here it's nice in the summer, if a little touristy...

    i wonder how long it would take us to travel to alpha centauri using nuclear thermal rocket technology...
     
  19. May 21, 2004 #18
    travelling 20 lightyears using rocket propulsion is out of the question
    rocket propulsion can never acheive the relativistic velocities needed

    it will also never work using nuclear propulsion

    nor is it possible with any current or near term propulsion technology

    not even matter/antimatter anihilation would be enough (as a source of thrust)

    however matter/antimatter anihilation as an energy source to drive an electromagnetic coil large enough to warp space is a possibilty
     
  20. May 21, 2004 #19

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    All these sources are too dilute to provide any kind of thrust worth mentioning.

    There is one possible energy source you could make use of: interstellar hydrogen. The idea is to scoop up the hydrogen (possibly with a large magnetic field), funnel it to the ship, fuse it, and exhaust it out the back. The good part is that the faster you go, the more hydrogen you collect.

    This idea was first suggested a few decades ago and is called a "Bussard ramjet".

    The down side is that all that hydrogen you are scooping up creates drag on your ship, eventually the drag equals the thrust of your engine and you quit accelerating. Since nuclear fusion doesn't generate exhaust velocities greater than 10% of c, this is pretty much the limit of how fast a Bussard ramjet can go.
     
  21. May 21, 2004 #20
    interesting, Janus, thank you for your illuminative reply, as always...the hydrogen collection idea is fascinating..

    one thing i don't understand though...

    were you trying to suggest that 10% of c is the speed at which hydrogen 'drag' counteracts thrust to further acceleration? or are you suggesting that the fusion rocket's thrust itself is limited by a speed?

    energia -

    why? please be more specific - perhaps you meant to indicate a time frame as well? or do you have a different take altogether?



    this follows logically from our argument if you're talking about conventional fuelled propulsion but could you be a little more specific?



    why exactly would that never work? perhaps some figures might help - are you still talking about a journey of 20ly? in what time frame? what are the limitations? (i'm sure there are limitations to nuclear propulsion but your statements don't elucidate them very well)



    i don't see that this statement really means anything in particular...



    again, you're going to have to be a lot more specific if we are going to be able to judge the merits of your arguments...
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?