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Gravity offsetting Velocity's Time Dialation? Trouble Understanding

  1. Oct 24, 2012 #1
    Hi All,

    I am new to these forums. I've been studying relativity in my down time at work and I am having a bit of trouble understanding why gravity doesn't offset velocity in the popular "aging/time travel" theory/scenario. I’ll explain:

    What I think I understand:
    The speed of light is constant everywhere. That is, if I am accelerating at the speed of light, the speed of light from that frame of reference would still move away from me at the speed of light. Conversely, if I am standing completely still, the speed of light will again, move away from me at the speed of light. Because this breaks the laws of physics regarding the relationship between time and distance, the faster you approach the speed of light, the slower time will move to offset your current velocity to ensure the speed of light remains constant from all frames of reference. This is why the theory of someone on a rocket ship travelling at the speed of light or close to it can return to Earth in, say, 24 Earth hours for example, but find years have passed on Earth in that time.

    Here’s what bothers me:
    I’ve heard the following things: 1) Velocity slows down time for the reasons I’ve explained above. 2) Time in space passes more quickly where there is no gravitational pull nearby. 3) Gravity slows down the passage of time.

    So, my question: If both gravity and velocity SLOW the passage of time, then why (in theory) when I return to Earth after 24 "Earth hours" travelling at the speed of light, would I find that everyone has aged many Earth years, if BOTH my velocity slows the passage of time AND Earth’s gravity slows the passage of time? Both effects slow down time, so why does everyone standing still under gravity’s influence age significantly faster than I do under velocity’s influence if BOTH variables have a slowing effect on time?

    I would think the only logical way (assuming the points I’ve heard are correct) I could age faster than Earth’s population would be if I was floating in space, far away from any type of gravity source, as I would then have neither the variable of velocity or the variable of gravity to slow the passage down.

    Any insight is very much appreciated.
  2. jcsd
  3. Oct 24, 2012 #2


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    You pretty much have everything correct except that you should never say "accelerating at the speed of light" because accelerating is not a speed but also because, as you point out later, you should say traveling close to the speed of light.

    But the reasons that when you travel at high speed away from earth and return to find the rest of us having aged more than you, even though we experience time dilation, ours is very slight whereas yours can be very large if you are traveling very fast.

    But even your idea of floating in space, far away from any type of gravity source will require you to travel quite fast so that you don't die before you get there and so it may not be enough to permit you in the long run did age faster than those of us remaining on earth.

    Actually, if you just want to age faster than most other earthlings, all you have to do is live on the top of the highest mountain but you really won't be able to notice much difference.
  4. Oct 24, 2012 #3


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    You are correct. The person who would be the oldest is the one that drifted slowly far away from any massive body. The gravitational time dilation on earth is a few parts per billion, so it is insignificant compared to velocity time dilation for typical "twin paradox" scenarios.
  5. Oct 25, 2012 #4
    Thanks guys.

    So then, with that confirmed, If I made the following statement, would it be correct?

    Hypothetically (and somewhat fictionally, using fabricated numbers) speaking, if I floated in space away from any mass for 1 "Earth Year," then had Scotty instantly beam me back to Earth (and for example's sake, ignoring the speculated effects that Scotty's instant beam could have on time), I would find that, due to the effects of gravity on time on Earth, I would theoretically be travelling back to the past from my perspective. In other words, I would find that, while I spent 1 Earth Year floating far away from gravity, someone else that stayed on Earth from the time I began my drift to the time I was beamed back, may have only witnessed, say, five months time passage.

    Actually, speaking of Star Trek, special relativity would seem to blow a huge hole in the plot. If warp 1.1 is fictionally equivalent to the speed of light, and the "latest" enterprise can do warp 9.975 (nearly 10 times the speed of light), then every time a starship returns to space dock or Starfleet on Earth, depending on the duration of the voyage, everyone would be dead or very old.
  6. Oct 25, 2012 #5


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    I said the amount of time dilation for those of us remaining on earth is very slight--DaleSpam said it is insignificant. So why are you speaking of a [STRIKE]nine[/STRIKE] seven month difference? We're talking about a difference so slight that you would have a very difficult time measuring it.

    And you just can't ignore the other aspect that I talked about with magic instantaneous transportation that wipes out the aspects of time dilation due to velocity. You're doing fiction which is not science and you can't ask for a scientific validation of anything that ignores or contradicts reality.
    With science fiction, you can do the impossible. It only matters whether your audience can forgive you and still pay attention to your product. This is not the place to ask such questions.
    Last edited: Oct 25, 2012
  7. Oct 25, 2012 #6


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    You asked about traveling back to the past from your perspective. When two people age at different rates due to their travels taking them along different paths and they reunite to discover that they have aged differently, do you consider one of them to have traveled to the past of the other one? As I said earlier, if you lived at the top of a high mountain, you would age faster than those of us living at lower elevations. The atomic clocks located in Boulder Colorado at about one mile elevation tick faster than the similar ones near sea level in Greenwich England. Do we think of the inhabitants of Boulder visiting the past when they go to Greenwich? I know the difference is not noticeable but even if it were, I think it would be more like the nuisance we have to deal with now in establishing a coordinated time that is not based on our location so that we can all call our aging the same, even if it isn't.

    Thankfully, time dilation is such a slight effect that no matter what any of us do, we never have to be concerned about it. We can let the experts provide us with GPS time standards and be completely oblivious to how they do it. None of us will ever trek to another star so we can leave it to the science fiction writers to entertain us with what can never happen.
  8. Oct 25, 2012 #7
    Yes. This is why I said using "fabricated" numbers. The popular scenario seems to suggest one travelling at close to the speed of light for X amount of Earth Years (1, for example's sake) would return to Earth to find it's population either very old or dead. This is why I suggested that perhaps my drifting for one year without the influence of gravity might only be five months time passage difference from an Earth frame of reference, as opposed to potentially hundreds of years in time passage difference when comparing velocity time dialation to gravitational time dialation, from that same Earth reference. I was simply fabricating the numbers to suggest that time on Earth would pass more slowly than it would in deep space drift. I have no way of measuring the exact variation of the relationship between non-gravity drift-time passage and gravitational (Earth) time passage.

    I'm not. I did not use that example to explain away the effects of time dialation due to velocity. I used that example because I do not want someone to simply say, "it doesn't matter because you'd be long dead by the time you returned to Earth, assuming you were far away enough to not be influenced by any sort of mass's gravitational field and just "drifting." And that is precisely the answer I would have gotten, and did get, had I not introduced an idea of being able to leave and return to Earth instantaneously. Apparently I need to apologize for my approach of making it fun with a Star Trek reference.

    I didn't ask a question. I surmised an observation based on my understanding of time dialation and your confirmations, and applied it to something in pop culture, thus even further disproving any realistic credit that it could potentially posess. Again, just trying to make it a little fun.

    Regardless, I have my answers, and I thank all who have contributed. Farewell. In the future, maybe try taking a nicer approach with people brand new to physics and relativity, if you want to attract new members to your forums.
  9. Oct 25, 2012 #8


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    DaleSpam said that the difference is a few parts per billion. Since there are 31 billion milliseconds in a year, that means that the difference is way less than one second, no where near several months.

    Gravitational time dilation is not like time dilation due to traveling at a high speed. The only way you can increase the gravitational time dilation is to go to a different planet but any planet big enough to matter will probably be too hot to live on.
    Even if you had all the resources and technological advances to be able to accelerate at any rate and get to any speed you wanted to instantly, you still could not make a difference more than a fraction of a second to the question you asked about. But you can't move instantly from one location to another. Even light can't do that.
    I was not aware that anyone might not have been nice on this thread--until now.
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