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Time dilation and space travel

  1. Dec 8, 2009 #1
    I read that the Sun will become a red giant and destroy the Earth in about 5 billion years. If this is correct, is there any possible way that a vastly advanced alien civilization that exists in a galaxy 6 billion lights year away could ever reach the Earth and make contact with human civilization in the present or in the Earth's future? If the aliens had a spaceship that could travel at near light speed, would time dilation make such a journey possible or would time only slow down for the spaceship and its occupants and not the destination of the spaceship, Earth?

    If somehow the aliens could go faster than light speed which I know isn't possible according to our present understanding of physics, is there any relationship between time slowing down for the spaceship and its ability to reach Earth before the Earth is destroyed?

    My understanding is that time dilation wouldn't affect the Earth and thus, it's a true statement that no alien could ever reach the Earth unless it started traveling to Earth from a distance of less than 5 billion light years? If that's a true statement, could it be a feasible explanation as to why human civilization hasn't made contact with any alien life-forms?

    As a follw up, if the universe is expanding at any accelerating rate, then can we also conclude that the number of galaxies within 5 billion light years of the Earth will continually decrease so that the more time passes, the less likely it is that Earth will be visited by advanced alien life forms?
    Last edited: Dec 8, 2009
  2. jcsd
  3. Dec 8, 2009 #2


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    What if they left a billion and one years ago?
  4. Dec 8, 2009 #3
    Assume they didn't leave 1 billion years agoo or assume the distance is 7 billion light years. Assuming a trip began in the past avoids the main issues in my quetstion.
  5. Dec 8, 2009 #4


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    So, your question is: if they left their planet in what is our "now", and they were 5Gly away, is there any way they could arrive here before our Sun burns out?

    Do I have it right?
  6. Dec 8, 2009 #5
    Yes, that's exactly right.
  7. Dec 8, 2009 #6


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    Firstly, It's important to understand that "now" here has nothing to do with "now" there.

    The alien race leaves on Day 1 of their "now", and travels for 5 billion years, just as we might do of we left today and travelled somewhere else for 5 billion years. The only difference is that, for the aliens, the universe is only ~8 billion yeas old, whereas for us, it's ~14 billion years old.

    Another clarification question: is your question about the simultaneity of these events, or is your question more specifically about the time dilation issue of the passengers?

    If the latter: if the alien ship traveled at very near c, the alien occupants might experience a much shorter time than 5 billion years. They might get here in only a few generations, or a few years.
    Last edited: Dec 8, 2009
  8. Dec 8, 2009 #7
    I'm asking about time-dilation issue. I don't understand how you came up with the different ages of the universe for the alien planet versus Earth.

    How can the age of the universe be different depending upon what part of the universe one is in. When I read that the universe is calculated to be 8-9 billion years old, doesn't that apply to the entire universe and not just the Earth?

    I assume the age of the universe is the same as the amount of time that has elapsed since the Big Bang. If Earth scientists measure the time since the Big Bang as 8.4 billion years ago, then why would observers in a different part of the universe measure the time that has elapsed since the Big Bang differently?

    Maybe I don't understand this enough to be asking questions about it. :(
    Last edited: Dec 8, 2009
  9. Dec 9, 2009 #8


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    It's not about the part of the universe you're in, it's about velocity; in relativity there is something called the relativity of simultaneity which says different frames of reference moving at different velocities will disagree on which events happened simultaneously. For example, if at age 20 we start to move apart at a constant velocity of 0.6c, then in your frame the event of you turning 30 happens simultaneously with the event of my turning 28 (so in your frame I have aged at 0.8 the rate you have aged), while in my frame the event of my turning 28 happens simultaneously with the event of your turning 26.4 (so in my frame you have aged at 0.8 the rate I have aged). Of course this disagreement only applies to events which happen at different points in space, if any two observers get together to compare ages at a single location they'll all agree on how old they are at the moment they meet (see the twin paradox).

    Anyway, to answer your question, time dilation only applies to the rate a clock seems to be ticking in a given frame, it doesn't change the fact that if something is moving at 0.6c in your frame, then it will only be able to cover 0.6 light years of distance in 1 year of time as measured in your frame (of course different frames measure distance and time differently), so nothing moving slower than light will be able to cover 5 billion years of distance in 7 billion years of time as measured in the Earth's frame (the aliens may well measure a much shorter time to reach the Earth, but all frames will agree on what age the Earth is at the moment the aliens arrive). The same principle applies to hypothetical FTL travel, if an object is moving at 2c in a given frame it'll cover 2 light years of distance for every 1 year of time in that frame, so it could travel 7 billion light years in only 3.5 billion years.
  10. Dec 9, 2009 #9


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    My bad. I was simply drawing a parallel between their civilization and ours. When they left 5 billion years ago, the universe was only 8 billion years old. Equivalent to here on Earth, the day we leave on our similar trip, but it's 5 billion years later.

    Don't worry about it. It's a spurious answer.

    Your issue is about the dilation.
  11. Dec 9, 2009 #10


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    I see.

    Time dilation will only affect the occupants. The trip will still take 5 billion years by Earth measurement.

    Right idea, wrong scale. The number of galaxy clusters in our sky decreases as they slip out of our observable universe. Untold trillions of years in the future, our sky will be sparsely popluated with galaxy clusters.

    But our galaxy is not affected by expansion. Its gravity easily overcomes the expansion and will do so effectively for eternity.

    A point of order if I may:
    You are concentrating on intergalactic distances for your concern over alien contact. This is whimsical in the extreme. Our galaxy alone contains a half trillion stars. It is from these stars that alien visitation will occur if it occurs, not from halfway across the universe.
    Last edited: Dec 9, 2009
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