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B When I lit a laser ray for 10 seconds from a distant planet

  1. Aug 14, 2016 #1
    I am on Alpha 4 light years away from earth. On 1st Jan, 2016 I lit a powerful laser light towards earth for 10 seconds and then switched it off. My friend on earth knows that he has to detect that light after 4 years on the night of 1st Jan, 2020 from a space station using a very powerful telescope.

    Ignoring the facts that the light will be too faint/not-recognizable etc., assume that the photons travelled smoothly through vacuum space and my friend should be able to detect it because of the powerful laser light and powerful telescope on that night. Will he see the laser beam for 10 seconds in year 2016 coming from that 4 light-years distant planet? Though the original source laser lit only for 10 seconds four years ago?

    If yes then what actually happened the moment I turned off the laser after 10 seconds 4 years ago? Is it that 10 seconds * 0.3 million km= a continuous light ray (EM wave) of 3 million km tall travelled for 4 years to reach the lens of my friend's telescope?

    The EM wave was detached from all the connections with the original laser light residing on the planet when I switched off. So is it that the distance between the tail of the light ray and the planet increased for 4 years till the ray hit the telescope lens of my friend for 10 seconds? OR is it that although laser was turned off after 10 seconds in 2016 but the total length of the light ray(EM wave) increased continuously with its tail always on planet alpha and moving towards earth so finally maintained a continuous length of 4 light-years before hitting the telescope lens of earth?

    Actually I am curious about the life and length of the 10 sec ray.
    Last edited by a moderator: Aug 14, 2016
  2. jcsd
  3. Aug 14, 2016 #2


    Staff: Mentor

    Let's say, instead of 10 seconds he left it lit for 1 year (doesn't change the concepts, just makes the units easier). On 1 Jan 2016 you start the laser, all that year there is a "column" of light whose head is moving away from Alpha at c and whose tail is on Alpha. On 2017 he turns off the laser and there is a detached column 1 light year long, both ends moving towards Earth at c. At 2018 the head of the column reaches the halfway point, and the tail is one light year from Alpha. At 2020 the head reaches Earth and the tail is 1 light year away. Etc.
  4. Aug 14, 2016 #3
    Thanks Mr. Dale. So I understand a continuous light ray of 1 light-year long or 10 sec long should exist based on the duration of the laser light. If I keep it lit for 4+ years then on earth in 2020 I have the head of ray at my telescope and the tail at Alpha . That means I am virtually touching a string whose one-end is on earth and another end is on alpha. Can I assume in this case EM wave is continuous without any break if it was not absorbed/reflected by any object in its journey through vacuum? For all the lights coming from distant stars (not dead yet) are we connected virtually by this way? Is there any way to know which stars are dead now and which are alive at this moment?
  5. Aug 14, 2016 #4


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    Staff: Mentor

    Sure. We know a great deal about the life cycle of stars and essentially every star you can see within our galaxy has not had enough time pass between the emission of its light and that light's arrival here on Earth for it to have ceased existing. This is especially true for stars under about 8-10 solar masses (the overwhelming majority of stars) since they do no undergo supernova, instead slowly collapsing into white dwarfs and then cooling off over billions of years.
  6. Aug 15, 2016 #5


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    Science Advisor

    Perhaps you can think of a mad scientist sitting near a remote star with his finger on a detonator button. He flips a coin. Heads, he will blow up the star. Tails, he will go elsewhere to further his evil plans. No, there is no way that you can know whether he has pushed the button until the news of the explosion (or lack thereof) has reached you at the speed of light. Signals never travel faster than light speed.

    Cause and effect propagate only within a so-called "light cone" proceeding forward in time and outward in space from where you are toward what you can affect. Similarly, they proceed forward in time and inward in space to you from things that can affect you. In between the future cone (which we might call the absolute future) and the past cone (which we might call the absolute past) there is a region about which we can have no immediate knowledge. Signals from that area have not had time to reach us yet. What we choose to call "right now, over there" lies somewhere within this region. We are free to choose a particular [hyper-]plane of simultaneity and define all events on that plane to be occurring "right now". Someone else is free to define his hyper-plane of simultaneity differently.
    Last edited: Aug 15, 2016
  7. Aug 15, 2016 #6
    As Dale said :There is a detached column one light year long with both ends moving toward earth at speed c. Is there any chance the length of this column will be Lorentz contracted moving at such a high velocity?
  8. Aug 15, 2016 #7


    Staff: Mentor

    The beam of light has very little in common with a string. I would not recommend that analogy at all.
  9. Aug 15, 2016 #8


    Staff: Mentor

    The 1 light year length is the length in the Earth's reference frame (with Alpha considered at rest relative to the Earth). In other frames the length will be different, but you will need to use the Lorentz transform instead of the length contraction formulas, since the length contraction formula does not apply here.
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