As Light Travels

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As Light Travels

Postby cedrec » Tue Feb 24, 2004 7:34 am

Does light ever die? By that, I mean, if light had to travel enough distance, would it eventually keep dimming to the point of non existence? I don’t think it does but who knows. I think it keeps dimming because it’s forced to spread out farther and farther but will never reach 0, to non existence. For example, if u were in an open field pitches black and someone flashed a flashlight at you a mile away, it is or does not seem as bright as it is if they flashed it right in front of you. Is there some equation or theory stating how far the light can travel before it “dies” or something relating how much light travels and how it dims?
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Postby Geosphere » Tue Feb 24, 2004 12:31 pm

If it never died, how come the sky isn't grey at night, with the light from billions of stars?
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Postby cedrec » Wed Feb 25, 2004 6:00 am

Depends on if you believe the universe is infinite or not.

If you think it is:
It is because the edge or wall of the universe does not exist, therefore not allowing light to reflect off of it giving the "sky" a gray color.

If you think it is not:
If there is a wall to the universe, it could be that it is so far away from us that the light from stars has yet to reflect off of it and reach Earth. Say that the distance from the first light source ever and the wall of the universe is Y light years away, and the star is X years old. If X=Y, the light has just bounced off the wall and has another Y years before that star will see its own light. If X=2Y than the star is now seening its own light and to the star, the “sky” is now white ~ gray. Therefore, if the light produce by the first light source ever travels enough distance to reflect off of the wall of the universe and reach Earth, we too will see the “sky” as white ~ gray.
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Postby Aale de Winkel » Wed Feb 25, 2004 12:13 pm

Strange picture of finite universe, as said elswhere the universe is finite, expanding from the big-bang period and probably somewhere in the distant future crunching back into th big crunch.

For me there isn't some wall which might reflect star-light an give the same star at two different points by reflection. This is an Euclidian point of view. Space is (for me at least) bent in a Riemanian fashion, a bit hard to grasp perhaps. Due to this bending a light ray pointed forward might shine us in the back, however due to the fact that a photon travels at finite speed it simply isn't observable.

For me this bending is entirely within our own universe, which in principle doesn't have to be embedded into some higher dimension, though it might well be, we'll simply never know.

Is the Ant moving on Eschers Mobiüs belt, aware that his flatland is in 3d-space, I don't know.
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Postby RQ » Thu Feb 26, 2004 2:41 pm

I thought a flashlight's reflectors in front of the lightbulb made it spread out.
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Postby Jay » Thu Feb 26, 2004 7:17 pm

Light never dies out. The intensity of light is given by I = 1/d[sup]2[/sup], where I is the intensity and d is the distance from the source. If you look at the graph of this, you'll see that the farther you get from the source, the dimmer the light gets. However, it can never reach zero, even when the distance is infinite.

And even if there was some "wall" at the edge of the universe, light would never bounce off of it b/c the universe is expading at the speed of light. So starlight would never reach the wall.
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Postby Geosphere » Thu Feb 26, 2004 9:01 pm

Jay wrote:Light never dies out.


This is wrong.

Due to the fact that reality has things like, oh, dust, gravity, refraction, absorption and such, light most definitely dies out. In theory, what you say is true, but that theory cannot be used to prove something in the actual universe.
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Postby RQ » Fri Feb 27, 2004 3:34 am

I think ur thinking of a certain point of view geo, when photons maybe slowed down, but no particle can reach absolute zero. If u are referring to a certain point, then depending indeed on the "lux" or intensity of light and the size of the particle (it it is bigger than the wavelength of light). Although each photon would have its own course, it would hit that point of space in theory.
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Postby pat » Fri Feb 27, 2004 5:33 pm

For all of the below, I'm assuming a dust-free universe that contains only my light source and my one-meter square panel.

Just want to add here that the dimming of light as the square of the radius is a consequence of the fact that a sphere's surface area increases with the square of its radius. Imagine you have some large, but finite, number of photons that you release all at once from a point. If you count the number of photons that hit a one-meter square panel placed one meter from the source, there are going to be a great number more than would hit that same panel placed three kilometers away. And, in fact, if you got far enough away, you may actually find spots that don't get any photons. There are only a finite number of them.

But, very few light sources emit all of their photons in one burst. So, even if you were very, very far away, you'd likely get a couple of photons every now and again.
Last edited by pat on Fri Feb 27, 2004 8:09 pm, edited 1 time in total.
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Postby Geosphere » Fri Feb 27, 2004 7:56 pm

RQ, are you implying that there is no such thing as absolute darkness?
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Postby LoD » Sat Feb 28, 2004 1:31 am

I think it doesn't die, but dims and dims until it cannot be seen anymore, but it is still there.
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Postby whiteonriceboy » Sat Feb 28, 2004 10:02 pm

i agree w/ Geosphere. i think none of this applies unless there is no mass for light to bump into. even if light does go on forever, it is almost constantly getting turned around and redirected. not to mention, light is diminishing so quickly, that it is preposterous to think that a star, even how bright it is, would affect the sky much, not to mention, turn the whole thing grey. don't you agree?
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