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Quickie physics question

  1. Nov 20, 2003 #1
    Hi !

    Imagine for a second a spotlight. A big one. A really really big one. Now, make is 100 times bigger. Now super size it. Is it really huge yet ? Good. Now make it a million times bigger. And replace the light bulb with The Sun !
    Wooohoo. That's one big spotlight !

    Okay.. Now.. Imagine a dusty room. You know that room where you can see the sun beams in the air because of the dust ? Cool. Remove the walls, the celing, the floors. Now we have us some dust. Spread that dust all over our solar system. Don't be shy. Really spread it out.

    Now.. place the Sun Powered Spotlight on Pluto. Aim the spotlight somewhere far. Turn off the sun for a minute.

    Now... Turn on the spotlight !




    If light moves at speed X. And we can see the light(because it's so really really bright)(and because it's going through thin dust, letting us see the beam itself).

    (here comes the big question)

    Will we see a huge beam racing through the sky ?

    If not, then what ?

    PS - If unclear, I'm a physics newbie, and pretty much don't know alot about it.
  2. jcsd
  3. Nov 20, 2003 #2
    I'm guessing that you wouldn't really see a beam racing through the sky simply because light moves too fast for us to see that. Instead, it would simply be like -- all of the sudden, there was light all around us! Instantaneous. (or so it seems to us)
  4. Nov 20, 2003 #3
    Probably not

    The first light that we see will be the light reflected off the dust towards us. A better experiment would be to send a laser down a smoke filled tube surrounded by an area in a vacuum.
  5. Nov 21, 2003 #4
    That's why I suggested to put the beam on Pluto and to aim it "somewhere far". So that you would be able to actually see the light "move", if that's possible.
  6. Nov 21, 2003 #5


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    Of course. Do you believe there's some reason
    it shouldn't ?
  7. Nov 21, 2003 #6
    Ummm.. Well, every person I asked thinks that you wouldn't see a beam of light going through the sky. And everyone gives me a strange reason why that would happen.

    Also - Even though it's logical to belive that it would be so(a beam of light racing through the sky), it is very hard to(belive). IE shouldn't light be and move in a different way then other "things" ?
  8. Nov 21, 2003 #7


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    Why not ? I don't get it.
    The beam will move and each of the particles that's
    "hit" by a photon will radiate. Some of that radiated
    light will reach you and you'll be able to "see"
    how the beam moves through the solar system.
    (For such distances, of course, you also have to
    consider its "cross-section"'s spacial expansion.)

    Live long and prosper.
  9. Nov 21, 2003 #8


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    BTW, there are certain materials in which light speed is
    actualy slower than that of an ant - so you can see it move
    and even win a race against it. (Of course the
    light still moves at the same speed, it's just "delayed"
    when absorbed by the atoms and then reemitted as well
    as bounced around a lot.)
  10. Nov 21, 2003 #9


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    Here is another aspect of the question you might find interesting:

    If we place the spotlight in the center of the solar system where the sun is (by building a reflective mirror behind and a focusing lense in front of the sun), we could rotate the spotlight, like a lighthouse. We could set this "lighthouse" to complete one revolution every 24 hours, like the earth on its axis.

    In place of Pluto, we could build a wall that goes all the way around the solar system, occupying Pluto's entire orbit. The spotlight projects a big, circular bright spot on the wall. This bright spot races along the wall like a race car on an oval track, completing one lap every 24 hours. The distance the spot has traveled is something in the neighborhood of 24 billion miles (don't know the exact circumference of Pluto's orbit, but that is pretty close). This means the spot of light is traveling at one billion miles per hour. The spot is travelling faster than c!

    Looking down from solar north, (and with all that dust to make the beam visible) we would see not a straight beam of light pointing out into space, but an expanding the spiral.

    And then we would get hypnotized .
  11. Nov 21, 2003 #10
    So you are saying that the beam would be visible ? If so you could actually see an end of a beam of light ?

    That's quite cool.
  12. Nov 22, 2003 #11
    Red-shift reality

    Hi Arkady,
    If you want "really cool" read the following:

    Count yourself extremely lucky! You didn’t ask, in particular, about the century-long but yet unrecognized “real cause” of the “Red Shift”. However, the denizens of this string collectively deserve Nobel nomination for showing that neither “tired light” (the invalid idea that light loses energy because of passing through many attractive gravitational fields on its travel here?) nor “Recession Doppler” (based on the invalid notion that movement away from the earth by a distant stellar entity somehow sucks energy from any emitted light) credibly addresses the phenomenon revealed (to me) when I first witnessed the invisibility of a laser beam (a colleague had invented a split-beam “VISAR”). The beam was a “pencil” cylinder of parallel-directed photons that could be witnessed only by their interference with particulate Brownian matter (so named by Einstein) that only became visible by reradiating, isotropically, an infinite subset of energy-reduced photons that resulted from the absorption of an infinite number of impinging photons.

    To those who might rightfully challenge that the self-shadowing of a radiating particle statistically minimizes the population of shifted-photons that remain in the parallel status of the beam, it should be noted that secondary particles in the beam absorb shifted-photons that are reradiated from a non-parallel direction and that do not self-shadow in the parallel direction. In the case of the laser beam, say, one emitting green photons, the shift is only in the direction toward the weak end of the visible spectrum, which for any specific spectral line merely broadens that line.

    Comparison of the Earth-based laser beam model with that of a natural stellar beam model is allowed because the photons from the stellar entity must be parallel when the beam hits the reflector mirror or lens of the telescope. The monochromatic and coherency aspects of the laser beam that mitigate its invisibility nonetheless do not invalidate the premise that the shift toward the red is caused by the existence, in universal space, of real or anti- particulate matter. That this has been proven to be factual is shown by the comparative observations of the direct and absorption spectral lines that not only reveal the interference of matter in the beam but also identifies the constituency of that matter.

    It is really a wonderment to me that the logic revealed in this really amateur string (including me) has, for so many decades, not been picked up on by the experts that continue, to the present day, to battle inanely over gravity vs recession velocity as the source of red-shift. It is noteworthy that there are still experts that have reverted to “distance alone causing shift” modeling and in a way this modeling, in an indirect way, is as close as the experts have come to reality – i.e., more distance contains more particulates.

    This is logical, don’t you agree! Thanks for your audience, Jim
  13. Nov 23, 2003 #12

    I asked on a thread last month "Where does the energy go for red shifted photons?" and I must admit, the answers seemed a little unclear (NB Not the quality of the answers, just the Physics!)

    There is also a 'Red shift anomoly' thread that raises some real interesting points.

    I think there is room for some work on this to clarify the whole subject.
    Last edited: Nov 23, 2003
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