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

The speed of light, the term light year , and reference frames.

  1. Dec 10, 2011 #1
    The speed of light, the term "light year", and reference frames.

    Hi everyone.

    This is my first post, and I post out of desperation. A friend of mine and I were casually discussing Time Dilation, interstellar travel, etc. when we came to a point we fundamentally disagreed upon. Neither of us being a physicist it's a fundamental disagreement on what is accepted as truth in the physics community and nothing more.

    The term light year (I'm going to go ahead and get this out of the way so noone feels the need to state the plainly obvious in a response) is a measurement of distance. That's great. As I understand it, the distance that light would travel in one year of time.

    Having said this, here is the root of our disagreement. We were discussing Kepler 2B and the excitement generated by it. The planet exists roughly 352 light years away from Earth if I read correctly, so naturally I state that as exciting as the prospect is, ever getting there would be impractical. Travelling at the speed of light, it would still take 352 years to arrive there.

    His rebuttal is that it wouldn't take 352 years to arrive due to Time Dilation.

    Again I'm not a physicist, but I am familiar with the documented effects of Time Dilation. The question seemed to rightly come down to frames of reference. But from what frame of reference is the term "light year" taken? If you were riding on a light beam would it take you 352 years to arrive at Kepler 2B, or would that be what a relatively stationary observer on Earth would perceive?

    How much time would the traveler note had passed? I would assume that the term "light year" indicates a year of time passing in the reference frame of light (YES I KNOW IT'S A MEASURE OF DISTANCE PLEASE GOD), but as I think about it, I can see his point as well. The problem is that no matter how much we researched it, it's difficult to find a source that very solidly states "the distance of a light year is covered in X time from light's perspective and Y time from a relatively stationary frame". I have looked for an answer but I can find none. So can someone clearly state the matter?

    Also, understand that I'm trying to avoid theoretical proclivities that would render the argument moot on the grounds that from light's frame of reference the trip would be instantaneous, or would take an infinite amount of time, or undefined time. We really have no experience from which to draw what would seem to be errant conclusions, so if we really must, let's assume the speed of travel to be .99C.

    Thanks in advance for the answer.
     
  2. jcsd
  3. Dec 10, 2011 #2
    Re: Light speed, the term "light year", and reference frames. Question

    I'm no physycisist either, but time dilation would indeed make your trip take less time than 352 years, even though for an earth-observer it would take ~352 years. If you traveled "on a light beam" then time would dilate so much that the trip would be instantaneous.

    I don't know what the formula is and I'm too hung-over to figure out how much your time would dilate at .99c, however, I do know that if you could accelerate at just 1g, you could potentially cross the entire universe in your lifetime. Of course, the earth would no longer exist by the time you did this.

    Edit: at .99c your trip would take 50.15725 years, but for earth observers it would seem like 355.5556 years :cool:

    I didn't calculate it, but I found this application on the internets which might be relevant to your interests: http://www.walter-fendt.de/ph14e/timedilation.htm
     
    Last edited: Dec 10, 2011
  4. Dec 10, 2011 #3

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: The speed of light, the term "light year", and reference frames.

    Correction: Kepler-2b is about 1044 light years, or 320 parsecs, away, not 352 light years.

    Your friend is right. If a spaceship could be constructed that could get to 0.999999999992 times the speed of light, you could hop in that spaceship and travel to the Andromeda galaxy (M31 for short) in just ten years even though M31 is 2.5 million light years away.

    M31 is 2.5 million light years away as we see it. It's a tiny bit less than 10 light years away to our relativistic traveler.

    Distance is relative.

    Some misconceptions in your post:
    • It's impossibly to travel at the speed of light. With enough energy one can get arbitrarily close (relative to some other observer), but the speed will never quite reach c.
    • Distance is relative. There's no universal yardstick. Your 2.5 million light years is just 10 light years to our relativistic traveler.
    • There's no such that as "the reference frame of light." The concept doesn't make sense. We have an FAQ on this: [thread]511170[/thread]. (Note: This particular FAQ is still a work in progress.)
     
  5. Dec 10, 2011 #4
    Re: Light speed, the term "light year", and reference frames. Question

    As I understand it, it's not at all certain that a photon would travel at C.

    Regardless, is Lsos correct? Is this the generally accepted view? That the term a distance of 1 light year would not take light one year to traverse from the perspective of light?
     
  6. Dec 10, 2011 #5
    Re: The speed of light, the term "light year", and reference frames.

    Thank you D H for your help in the matter. I'll gladly inform my friend.
     
  7. Dec 10, 2011 #6

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Light speed, the term "light year", and reference frames. Question

    The concept of "from the perspective of light" doesn't make sense. Read my previous post.
     
  8. Dec 10, 2011 #7
    Re: Light speed, the term "light year", and reference frames. Question

    I don't know if it makes sense or not (which I why I put quotes around "travel on a light beam"). However, we can still address OPs question by pointing out that as you go faster, one light years can take less than a year to cross. It can take half a year, it can take a week, it can take a second, or it can be almost instantaneous as you get to almost the speed of light.

    The closer you get to the speed of light the less time it takes. The point I'm making is that if we drew a graph of velocity vs. time, it is interesting to note that it would extrapolate to 0 time to cross an infinite distance at the speed of light.
     
  9. Dec 25, 2011 #8

    phinds

    User Avatar
    Gold Member
    2016 Award

    Re: Light speed, the term "light year", and reference frames. Question

    This is absolutely true mathematically but keep in mind that from a practical point of view it is meaningless. The energy required to approach the speed of light is just the first of a long list of practical problems that have been expounded on in numerous other threads on this forum.
     
  10. Dec 26, 2011 #9
    Re: The speed of light, the term "light year", and reference frames.

    Distance and time is relative, it's 352 light years away from our frame of reference. But in another frame of reference it will be a different distance. a light year is just the measure of how far light moves in a year from our frame of reference. If you are moving very close to the speed of light, you can calculate time dilation and length contraction and see how long it will take someone on the ship to get there. They can get there in far less than 352 years from there frame of reference. But on the earths it will take them slightly longer than 352 years.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: The speed of light, the term light year , and reference frames.
  1. Light years (Replies: 26)

  2. Speed of light (Replies: 17)

  3. Speed of light (Replies: 16)

Loading...