Time Dilation -light vs. spaceship

  1. Feb 16, 2010 #1
    Background: I work as an engineer with someone who is an "engineer" by trade only (not by training or formal education). Un/fortunately, before asking difficult questions, he researches information, or just hears about it online and tries to muddle his way to an understanding of it. He recently heard a discussion on time dilation and how it takes an object a longer amount of time to travel 4 light years than light.

    Quandary: I am unable to explain in lay-terms why this time dilation happens. This, of course, means that I do not a full understanding of the matter. I would like to be able to explain, and would like help. I am not a physicist, but I will try my best.

    Simply put, he wants to know if an object (say a spaceship with a man inside), if traveling at the speed of light and a strong beam of light (perhaps a very strong laser) were aimed at the moon, would they arrive at the moon at the same time? He thinks that the man in the ship could look out the window and see little particles of light "growing" alongside the ship (even with the ship as well).

    It is my understanding that as the object's velocity increases, so would it's mass; and that the object would experience contraction. How do I explain that it is not "as the acceleration increases" that the objects mass increases to a lay person?

    Also, further help in explaining this (to me) would be greatly appreciated, so that I may understand fully, and be able to explain the answer to his question.

    Thanks much!
  2. jcsd
  3. Feb 16, 2010 #2


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    A massive object can never travel at the speed of light, so the question is ill-posed and deserves no further thought.
  4. Feb 16, 2010 #3


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    That's not time dilation. That's just a different velocity. :smile:

    Massive objects can't move at the speed of light. The rocket can move relative to the moon at any speed v<c, but not at v=c. If a rocket moving at v=0.999999c passes (let's say) the space station in the direction towards the moon, and at the same time the space station fires a laser towards the moon, they will get to the moon almost at the same time according to someone on the space station, because the rocket is moving almost at the speed of light. There's no funny stuff there.

    The funny stuff happens on the rocket. The pilot will disagree with the space station about how far away the moon is, at the event where the rocket passes the space station, and therefore also about how much time it takes to get there. To him, the trip will be almost instantaneous.

    You will always see light move at speed c, regardless of your velocity relative to the light source. (And a different matter, which doesn't have anything to do with relativity, is that you don't "see" particles of light. Sometimes a bunch of photons will hit you directly in the eyes, and your brain will interpret that as "seeing" the object that reflected the photons toward you. You can't ever see anything that doesn't scatter light into your eyes, and light won't do that).

    Almost no one here uses the concept of "relativistic mass". When we say mass, we mean "rest mass". The object's (kinetic) energy would increase when it speeds up, because of the work performed by the force that's accelerating it. The calculation is here

    Not sure it can be done. If he understands some calculus and the concept of "work", you can show him that calculation. I'm not sure it's really significant here though, unless what you want to show is that the energy required to accelerate an object to speed v goes to infinity as v goes to c.

    The videos mentioned in this thread might be a good start. (I have only seen small parts of them, so I can't guarantee that they're all good).

    The best way by far to learn special relativity is to learn about spacetime diagrams. This way you can understand simultaneity, length contraction and time dilation, almost completely without math.
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