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Traveling at the speed of light and time dilation

  1. Aug 16, 2013 #1

    DHF

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    hey guys,

    I need some tech support for a spaceship in a story that is currently in the works :)
    Assuming the characters could get their ship up to the speed up light in real space (no hyper drive or warp drives) What would the effects of time dilation be on the crew? Time slows down as you approach the speed of light but what happens when you actually reach that barrier? does time stop for the crew? A trip to Alpha Centuri would take 4 years to the outside world but to the crew how much time will have passed?


    Thanks for the help
     
  2. jcsd
  3. Aug 16, 2013 #2

    phinds

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    You don't reach it.
     
  4. Aug 16, 2013 #3

    Ryan_m_b

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    It's impossible for objects with mass to attain the speed of light. You're question is therefore unanswerable because it essentially asks "if we ignore the laws of physics what do the laws of physical say?"
     
  5. Aug 16, 2013 #4

    micromass

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    You can't go the speed of light or faster.
    That said, you can get to Alpha Centauri arbitrarily fast if you go close enough to the speed of light. So if you go fast enough, then you can get there in just a minute or less. Of course, to people on earth, it will have taken years.
     
  6. Aug 16, 2013 #5
    The best books

    I've read some science fiction

    You may want to stick with some of the science fiction I've read. Ships (and the people in them) have an almost instant trip time (t1) while traversing the distance of 4 light years (d) as they move 4 years (t2) into the future.

    Hard to comprehend. It's not a standard way of thinking.
     
  7. Aug 16, 2013 #6

    DHF

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    ok good to know. so reaching the speed of light would never happen but the crew could reach 99.99% the speed of light, in which case from their perspective they would only be in transit for a few minutes.
     
  8. Aug 16, 2013 #7

    Bandersnatch

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  9. Aug 16, 2013 #8

    Ryan_m_b

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    I first encountered this in Ken MacLeod's Engines of Light trilogy. Ships essentially have a jump drive but rather than being instant like most other science fiction universes it takes exactly 1 second to jump 1 lightsecond but is instant from the perspective of the crew.

    Actually at 99.99% 4 lightyears would take about 3 weeks. Many science fiction worlds use near-light speed travel but you have to consider things like how these speeds are achieved (try plugging in 0.99c into the relativistic rocket equation and you'll quickly realise how unrealistic the fuel requirements are), what protects the crew from radiation (which gets worse and worse the faster you go) etc etc.
     
  10. Aug 16, 2013 #9
    If you DID (somehow) reach the speed of light, γ = ∞, which means your mass would be m*∞, the light in coming toward you would be blueshifted by a factor of ∞, the shipboard time would be slowed by a factor of ∞...you get the idea (everything changes by a factor of ∞ relative to whatever you choose to call stationary.)
    This website should explain why it's impossible for any matter object to reach the speed of light: http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/Special_relativity_adding/index.html
     
  11. Aug 16, 2013 #10

    DHF

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    yeah that might not be so cool. I wanted to give them a method of travel without warp bubbles or any of the other classic sci-Fi devices but as you all pointed out, traveling in real space would be packed with obsticals. not just the radiation but I imagine the sadness they would have if they passed through a dust cloud.

    thanks for the info and calculators. I will try to figure an alternate method of travel for my intrepid crew.
     
  12. Aug 16, 2013 #11
    Relativistic travel can be quite fun. For example, there's Stephen Baxter's short story Pilot available in the public domain here which explores the real extremes of relativistic travel.

    Edit: The dust problem can be solved with a dust shield, but a dust shield can only erode so much.
     
  13. Aug 16, 2013 #12

    Ryan_m_b

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    What's the general plot of the story? Perhaps we can help come up with a suitably scientific plot device. For example; if the story takes place in another system you could have them getting there very slowly (i.e. in a ship that's only boosted up to 1% light speed) whilst travelling in some form of suspended animation.
     
  14. Aug 16, 2013 #13

    DHF

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    the plot of the story is about the first manned flight to Alpha Centuri. I haven't set a date for the tale yet but I figured it was several hundred years from now. The story will center around the characters's reactions to being separated from their native time. the concept of only a few days passing for them yet years or decades have escaped their notice on Earth. I didn't want warp drives or other magical forms of tech because I didn't feel we would be there yet. I wanted to give Earth advanced tech by our standard but still grounded by our current understanding of physics. That being said, thanks to the Calculator Bandersnatch provided (thanks man you are always helpful) I realize the utterly absurd amounts of energy required, so unless the crew has a ship powered by their own personal sun, I don't think my original idea will work.

    Two possibilities I can think of involve :
    - pushing the ship into an alternate dimension during travel where it would not be subjected to the laws of physics.
    - Wrapping the ship in some some sort of energy field that gives the ship 0 mass during the acceleration period.

    Both of those ideas however dance happily into the realm of magical technology so I wanted to avoid that.
    Taking your advice Ryan_m_b I could put them in suspended animation and move them at 2% of c. that would get them there in about 400 years. it would require a near sentient computer system to pilot the ship on such a long journey but I think that is a lot more feasible then getting a ship to the speed of light.
    Although if I go that route I would have to figure if the government of the time would be willing to spend the money and resources for a venture that would not benefit their nation for centuries. Of course it also opens the door for the crew to spend 400 years in stasis and when they wake up, Humanity is already their, having invented faster means of travel while the astronauts slept.

    possibilities.
     
  15. Aug 17, 2013 #14
    Perhaps the government could be motivated by some threat in Sol, and so start throwing together colony ships to spread humans around.
     
  16. Aug 17, 2013 #15
    dust cloud

    dust... The ship would sort of be in a time bubble. Anyone looking at the ship might see it as a small, fast moving black hole. Maybe since the ship is near light speed the ship density would be altered to allow it to plow through anything non-relativistic. Or maybe it would just be a very quick trip into oblivion. I'd like to see how a ship near c would decompose on impacting an object. May just look like what happens in atom smasher, not just vaporized but decomposed to destabilized particles of energy. How would you ever turn with that much momentum? What power source could possibly produce the energy needed? You'd have to carry tons of extra mass as fuel even if you could get every last proton of the material to convert to energy. You couldn't see anything, let alone dust, because most of the visible light would be reduced to a little point in the front of the ship from lensing.
     
  17. Aug 19, 2013 #16

    DHF

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    that's an interesting idea, The ship moving at near light speed would have near infinite mass but does mass translate into density? and the dust and particles they plowed through,seeing as how they would be impacting the hull at near light speed relative to the ship, would those particles impact them with the same amount of energy as they were spending to move forward?
     
  18. Aug 19, 2013 #17
    The ship would have 'near-infinite' mass relative to a stationary observer. About the particles, probably not (1 hydrogen atom/cm^3).
     
  19. Aug 19, 2013 #18
    If they are successful at someday building this "warp drive": [http://www.newscientist.com/article...ising-a-starship-warp-drive.html#.UhLUIWT70pc ], wouldn't this counteract the effect of time dilation to some extent? I've always thought of time dilation as related to the reduction of space-time density as speeds increase, but since this drive would compress space-time in front of the ship, seems like it would reduce the time dilation effect to some extent.

    That would seem to modify the amount of information in the universe during the duration of the flight (for reasons that are too complicated for me to go into, but if you get it, you get it), so seems like there's some special circumstance math that would have to be included in order to calculate time dilation effects for this type of spacecraft.

    Another thing that bugs me about the idea of warp drives like this are that the density of space-time is correlated with mass. If they are compressing space-time in front of the ship, are they circumventing the "infinite mass" problem?
     
    Last edited: Aug 19, 2013
  20. Aug 19, 2013 #19
    Just realized another thing that bugs me about this particular warp drive design.

    If it compresses space-time in "front" of the ship, and decompresses "behind" the ship, I think this would actually propel the ship *backwards*. Not only does nature abhor a vacuum, but so does space-time. It demonstrates this by propagating the information that represents coherent structures (like spaceships and atoms) from denser regions of space-time to less dense regions, across vast distances. This attribute of information propagation through the switching fabric of space-time is often referred to as Gravity.
     
  21. Sep 3, 2013 #20

    DHF

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    Thanks CyberDriver,

    That was a good read. It did a good job of trying to stick to realistic concepts. One of the weak spots I noticed however was fuel. The author accounts for this by noting that the fugitives are cannibalizing their planet for fuel but the missile on the other hand has no such fuel source so how does it continue to pursue them?

    The other inconsistency I noted was their acceleration. They say the missile is using an Earth made drive which limits it to 1 G acceleration. the missile later updates itself to 2Gs, we can accept this because the dialog suggests the missile is reinventing itself beyond its original limits. but then our fugitive heroes casually accelerate to 2 Gs to match the missile, later there is talk that they are traveling at 1000 Gs acceleration. You might say that they can afford such acceleration because they have massive amounts of fuel but that leads me back to my question about how the missile can keep up.
     
    Last edited: Sep 3, 2013
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