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Random Thoughts On Dissappearing Planets

  1. Oct 4, 2007 #1
    I was doing some thinking as to whether aliens are really out there. Then I figured, they probably don't want us to find them if they are out there, so they are probably hiding (and maybe a little even embarrassed by us as a species).

    Then I thought, "How exactly would one go about hiding a planet from humans?".

    The answer I came up with is, is to make the planet travel faster than the human eye can see. This can be done by either speeding up the planets rotation on its own axis (probably easiest) or by increasing the velocity of its orbit around whatever is it is orbiting.

    Anyways, assuming this is possible (I dunno, maybe the aliens are ethereal and don't really need to bother with gravity), I was left with a final question. Exactly how fast does an object need to travel before it vanishes from human sight?

    Immediately I thought to myself, "Hmmm...what moves so fast I can't see....Oh yeah, magician slight-of-hand tricks....Hmmm, exactly how fast are those magician's hands moving?" I remember reading somewhere that the fastest baseball pitched was clocked at around 100 miles per hour, so I assume the magicians are moving their hands around that speed.

    Then I thought, hmmm...."why is it that I can't see a magician hands moving at 100mph yet, I can see a Boeing jet pretty clearly moving 550mph".

    Putting two and two together, I concluded, that that velocity at which an object must travel so that is dissapears from human sight is somehow proportional to the mass of that object. Thus, a planet would probably need to travel REALLY fast to disappear (unless its a tiny planet).

    Any thoughts?
  2. jcsd
  3. Oct 4, 2007 #2
    Umm, what?

    Where did you get the idea that moving fast enough will make an object vanish?

    A magician has nothing to do with speed, if you knew where to look, you could easily catch the movement. It has to do with focusing the attention somewhere else.

    If you don't believe me, how fast do you think the moon is moving?
  4. Oct 4, 2007 #3
    Hmm, possibly. You know though, I've fired and have watched people fire a lot of guns at the firing range, and I don't think my eyes were ever once quick enough to see that bullet. But, yes, perhaps I wasn't looking in exactly the right place (although, admittedly, this could also be due more to my own faulty sense of perception).
    Last edited: Oct 5, 2007
  5. Oct 5, 2007 #4
    The reason you don't see a bullet fired from a nearby gun is that it leaves your field of vision too quickly, and once it's far enough away it will get lost in the background.

    You can have a tracer round (you may have seen them as "glowing" bullets fired from planes or antiaircraft guns) that travels at bullet speed, yet is still visible to the naked eye because it does not get lost in the background.

    If you had a perfectly uniform white background and were standing far enough from the bullet being fired, you would be able to see it.

    On the other hand, you could simply use a high-speed camera and see the bullet.
  6. Oct 5, 2007 #5


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    Wait a sec! Before you put two and two together... you should make sure you really do have two and two! You had several thoughts in your argument; can you think of any ways you could test to see if they are right or wrong?
  7. Oct 5, 2007 #6
    I think this is the point the OP is trying to make: its speed makes it imperceptible. A bullet in motion at normal bullet speeds is effectively invisible to the naked eye.

    Due to its very high contrast with the background, and also persistance of vision a tracer bullet is perceptible, but as a streak of light, not as a bullet.

    This is the most important factor in our ability to see something which is moving fast: our distance from it. The farther away we are the less of an angle it subtends in our visual field per unit time and the easier it is to focus on.

    Up close a light stick swung in circles can almost seem a solid hoop of light. The lights of an aircraft a mile away still seem like individual lights even though they may be going faster than the light stick. We aren't physiologically equipped to pick out and focus on a thing that subtends too large an angle in our field of vision per unit time. Without extreme contrast a close object moving quickly is effectively invisible to the naked eye.
  8. Oct 5, 2007 #7
    So, generalizing, the eye has a limit based on the angular velocity of the moving object.

    [itex]\omega=v/R [/itex]

    Since we would expect any alien planet to be a considerable distance R away from Earth, the speed at which it would have to travel to reach the limit would be orders of magnitude greater than the speed of light.
  9. Oct 5, 2007 #8
    So, paranoid aliens speed up their planet's rotation to avoid detection?

    Can't make friends that way...
  10. Oct 5, 2007 #9


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    All planets (except nine in our own solar system) are invisible to our sight. :grumpy:

    We can only detect planets indirectly, through various things such as wobbling parent stars. We have yet to visually resolve an extrasolar planet. So this whole business of "too fast to see" is completely moot.
  11. Oct 5, 2007 #10


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    Dearly Missed

    You are right, your ideas WERE quite random, scattered in all directions by the activity of some type of cerebral centrifuge, I suspect..
  12. Oct 5, 2007 #11
    I think that's correct.

    I wonder, too, about the tracer bullet effect. An Einstein gedanken train with simultaneously flashing lights at the front and rear, going some appreciable fraction of c at some distance from the observer, might actually look like a streak of light to the naked eye: considerably longer than it is in its own inertial frame, not shorter. One photon released from the front and rear simultaneously in the train frame might demonstrate relativistic length contraction, but any conventional flash of lights would take more time and be smeared into a streak.
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